what is the meaning of my photosynthesis

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Photosynthesis

What is photosynthesis.

It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word ‘photosynthesis’ is derived from the Greek word phōs, meaning ‘light’ and synthesis meaning ‘combining together.’

Jan Ingenhousz, the Dutch-born British physician and scientist, discovered the process of photosynthesis.

Where does Photosynthesis Occur

Photosynthesis takes place mainly in the leaves of green plants and also in the stems of herbaceous plants as they also contain chlorophyll. Sometimes it also occurs in roots that contain chlorophyll like in water chestnut and Heart-leaved moonseed. Apart from plants, photosynthesis is also found to occur in blue-green algae.

What Happens During Photosynthesis

It involves a chemical reaction where water, carbon dioxide, chlorophyll, and solar energy are utilized as raw materials (inputs) to produce glucose, oxygen, and water (outputs).

Stages of the Process

Photosynthesis occurs in two stages:

1) The Light-dependent Reaction

• Takes place in the thylakoid membranes of chloroplasts only during the day in the presence of sunlight
• High-energy phosphate molecules adenosine triphosphate ( ATP ) and the reducing agent NADPH are produced with the help of electron transport chain

2) The Light-independent or Dark Reaction ( Calvin cycle )

• Takes place in the stroma of chloroplast in the absence of light that helps to fix carbon
• ATP and NADPH produced in the light reaction are utilized along with carbon dioxide to produce sugar in the form of glucose

Factors Affecting the Rate of Photosynthesis

• Intensity of Light: The higher intensity of light increases the rate of photosynthesis
• Temperature:  Warmer the temperature, higher the rate of photosynthesis. The rate is highest between the temperatures of 25° to 35° C, after which it starts to decrease
• Concentration of Carbon dioxide: Higher concentration of carbon dioxide increases the rate of photosynthesis until it reaches a certain point, beyond which no further effects are found   

Although all the above factors together interact to affect the rate of photosynthesis, each of them individually is also capable of directly influencing the process without the other factors and thus called limiting factors.

Importance of Photosynthesis

It serves two main purposes that are essential to support life on earth:

• Producing food for organisms that depend on others for their nutrition such as humans along with all other animals
• Synthesizing oxygen by replacing carbon dioxide in the atmosphere

Ans. Photosynthesis is an endothermic reaction because it absorbs the heat of the sun to carry out the process.

Ans. The oxygen in photosynthesis comes from splitting the water molecules.

Ans. Chlorophyll is the main light-absorbing pigment in photosynthesis.

Ans. The role of water is to provide oxygen in the form of oxygen gas to the atmosphere.

Ans. Sunlight is the source of energy that drives photosynthesis.

Ans. The easiest way to measure the rate of photosynthesis is to quantify the carbon dioxide or oxygen levels using a data logger. The rate of photosynthesis can also be measured by determining the increase in the plant ’s biomass (weight).

Ans. Photosynthesis is an energy-requiring process occurring only in green plants, algae, and certain bacteria that utilizes carbon dioxide and water to produce food in the form of carbohydrates. In contrast, cellular respiration is an energy-releasing process found in all living organisms where oxygen and glucose are utilized to produce carbon dioxide and water.

Ans. Glucose produced in photosynthesis is used in cellular respiration to make ATP.

Article was last reviewed on Tuesday, April 21, 2020

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• Biology Article

Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

• What is Photosynthesis?
• Site of photosynthesis

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words  phōs  (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις   means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

• Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae.  All green parts of a plant, including the green stems, green leaves,  and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read:  Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f,  which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the  plant cell   and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a  and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 ,  chlorophyll- c2 ,  chlorophyll- d and chlorophyll- f .

Also Read:   Biological Pigments

Process Of Photosynthesis

At the cellular level,  the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for  growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
• Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
• Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

• Dark reaction is also called carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
• In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read:  Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

• Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
• Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

Frequently Asked Questions

1. what is photosynthesis explain the process of photosynthesis., 2. what is the significance of photosynthesis, 3. list out the factors influencing photosynthesis., 4. what are the different stages of photosynthesis, 5. what is the calvin cycle, 6. write down the photosynthesis equation..

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Please What Is Meant By 300-400 PPM

PPM stands for Parts-Per-Million. It corresponds to saying that 300 PPM of carbon dioxide indicates that if one million gas molecules are counted, 300 out of them would be carbon dioxide. The remaining nine hundred ninety-nine thousand seven hundred are other gas molecules.

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Photosynthesis

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Photosynthesis is a vital biological process through which green plants, algae, and certain bacteria convert light energy into chemical energy. Using sunlight, these organisms transform carbon dioxide and water into glucose and oxygen, substances crucial for their growth and the sustenance of life on Earth. This process not only fuels the organisms themselves but also supports life on the planet by providing oxygen and forming the base of the food chain. Understanding photosynthesis is essential for comprehending how life thrives on Earth, influencing fields ranging from agriculture to energy production.

Definition of Photosynthesis

Photosynthesis is a fundamental biological process through which green plants, algae, and certain bacteria convert light energy into chemical energy. This transformation occurs primarily in the chloroplasts of plant cells where chlorophyll, the pigment responsible for the green color of plants, captures sunlight. The captured light energy is then used to synthesize glucose from carbon dioxide (CO2) and water (H2O), releasing oxygen (O2) as a byproduct. This process not only fuels the plant’s own cellular activities but also provides the base of the food chain for other organisms.

Where Does Photosynthesis Occur?

Photosynthesis primarily occurs in the leaves of plants, although it can also take place in any parts of a plant that contain green pigments, typically in the stems and young branches. The leaves are the main site of photosynthesis due to their structure and accessibility to sunlight.

Photosynthesis Process

Photosynthesis stands as a crucial biological process through which plants, algae, and certain bacteria convert sunlight into chemical energy, fueling their activities and growth. This process not only supports the organisms performing it but also sustains life on Earth by producing oxygen and forming the basis of the food chain.

Key Stages of Photosynthesis

Photosynthesis occurs primarily in two stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).

Light-Dependent Reactions

These reactions take place in the thylakoid membranes of the chloroplasts. Here, sunlight drives the process, initiating the flow of electrons through a series of proteins known as the electron transport chain. Plants absorb sunlight using pigment molecules, with chlorophyll being the most prominent. This absorption energizes electrons, which the chloroplasts then use to convert water (H₂O) into oxygen (O₂). As a result, this stage releases oxygen as a byproduct and generates ATP and NADPH, energy carriers that the next stage of photosynthesis uses.

Location: Thylakoid Membranes

Within the chloroplasts, thylakoid membranes house the light-dependent reactions. These membranes are rich in pigments like chlorophyll that capture light energy, crucial for water photolysis and energy molecule production.

Steps in Light-Dependent Reactions

• Photon Absorption : When photons strike the chlorophyll in the photosystem II (PSII) complex, embedded in the thylakoid membrane, they excite electrons to a higher energy state.
• Water Splitting (Photolysis) : To replace these excited electrons, enzymes split water molecules into oxygen, protons, and electrons. The process releases oxygen as a byproduct.
• Electron Transport Chain (ETC) : The electrons travel down an electron transport chain, which comprises a series of proteins within the thylakoid membrane. The movement of electrons through the chain helps pump protons from the stroma into the thylakoid space, thereby creating a proton gradient.
• ATP Formation : This proton gradient enables ATP synthase to synthesize ATP from ADP and inorganic phosphate through chemiosmosis, as it uses the flow of protons back into the stroma.
• Photosystem I (PSI) : Upon reaching PSI, the electrons get re-energized by more absorbed light. These re-energized electrons then reduce NADP+ to NADPH, which serves as a critical electron and hydrogen carrier in the Calvin cycle.
• Output of Light-Dependent Reactions : ATP and NADPH, produced during these reactions, enter the Calvin cycle to assist in carbon dioxide fixation into glucose. Meanwhile, the oxygen produced during water splitting exits as a waste product.

Significance of Light-Dependent Reactions

The light-dependent reactions are vital as they provide the necessary energy carriers (ATP and NADPH) for the Calvin cycle. They also maintain the oxygen level in the atmosphere, which is critical for the survival of aerobic life on Earth.

Calvin Cycle (Light-Independent Reactions)

The Calvin cycle unfolds in the stroma, the fluid-filled space surrounding the thylakoid membranes. It does not require light directly. Instead, it uses the ATP and NADPH from the light-dependent reactions to convert carbon dioxide (CO₂) from the air into organic molecules. During this cycle, the enzyme RuBisCO incorporates CO₂ into an organic molecule, starting a series of chemical reactions that regenerate the starting molecule with the production of glucose and other carbohydrates.

Key Steps of the Calvin Cycle

• Carbon Fixation : The cycle begins when the enzyme RuBisCO incorporates carbon dioxide into a five-carbon molecule, ribulose bisphosphate (RuBP). This reaction produces a six-carbon compound that immediately splits into two three-carbon molecules of 3-phosphoglycerate (3-PGA).
• Reduction Phase : Each molecule of 3-PGA then receives a phosphate group from ATP, becoming 1,3-bisphosphoglycerate. Next, NADPH donates electrons to these molecules, reducing them to glyceraldehyde-3-phosphate (G3P), a sugar. This step consumes the ATP and NADPH generated during the light-dependent reactions.
• Release of One Glucose Molecule : For every six molecules of CO2 that enter the cycle, twelve molecules of G3P form. However, only two G3P molecules leave the cycle to contribute towards forming glucose, while the remaining ten G3P molecules proceed to the next step.
• Regeneration of RuBP : The ten remaining G3P molecules undergo a series of transformations that require further ATP. These transformations regenerate RuBP, enabling the cycle to process new carbon dioxide molecules continuously.

Significance of the Calvin Cycle

The Calvin Cycle is crucial for synthesizing glucose, which plants use as an energy source to fuel various cellular activities and growth. The glucose also serves as a building block for other essential biomolecules such as cellulose and starch. Additionally, this cycle plays a pivotal role in the global carbon cycle, as it is the primary pathway through which atmospheric carbon dioxide transforms into organic compounds within plants.

Importance of Photosynthesis

Photosynthesis is vital for life on Earth. It provides the primary energy source for all ecosystems, where plants form the base of the food web and create biomass from inorganic substances. Moreover, photosynthesis is responsible for the oxygen that makes up a significant portion of the Earth’s atmosphere and supports aerobic life forms.

Photosynthesis Equation

The formula for photosynthesis is central to understanding how plants convert solar energy into chemical energy. Here is the equation:

6𝐶𝑂2+6𝐻2𝑂+𝑙𝑖𝑔ℎ𝑡𝑒𝑛𝑒𝑟𝑔𝑦→𝐶6𝐻12𝑂6+6𝑂26 CO 2​+6 H 2​ O + lightenergy → C 6​ H 12​ O 6​+6 O 2​

Explanation of the Photosynthesis Equation

This equation represents the overall process by which plants, algae, and certain bacteria produce glucose and oxygen from carbon dioxide and water, using energy from sunlight. Here’s a breakdown of each component:

Carbon Dioxide (6CO2)

Tiny pores in the leaves called stomata take carbon dioxide from the air. Carbon dioxide is one of the key reactants in the process.

Water (6H2O)

The roots absorb water and transport it to the leaves, providing the source of electrons and protons necessary for the chemical reactions of photosynthesis. Water molecules split to produce oxygen.

Light Energy

Chlorophyll and other pigments in the chloroplasts capture light energy, converting it into chemical energy in the form of ATP and NADPH. These energy carriers then power the later stages of photosynthesis.

Glucose (C6H12O6)

The plant uses the sugar produced by photosynthesis as an energy source. It can use this sugar immediately, store it, or convert it into other necessary substances for growth.

Oxygen (6O2)

Oxygen releases through the stomata as a byproduct, playing a critical role in the Earth’s atmosphere and supporting the survival of aerobic organisms.

Causes of Photosynthesis

Light energy, especially from the sun, triggers photosynthesis. Pigments in the plant, mainly chlorophyll, absorb mostly blue and red wavelengths of light, crucial for converting ADP and inorganic phosphate into ATP, and NADP+ into NADPH, which are used in glucose synthesis.

Chlorophyll and Other Pigments

Chlorophyll primarily absorbs light. Other pigments like carotenoids and phycobilins help capture energy from sunlight, absorbing light wavelengths that chlorophyll cannot. These pigments are critical in harnessing the light energy required to drive the reactions of photosynthesis.

Water (H2O)

Water acts as an electron donor in the light-dependent reactions of photosynthesis. The process of photolysis splits water molecules, releasing electrons, hydrogen ions, and oxygen. The reactions use the electrons and hydrogen ions to produce glucose, while plants release oxygen as a byproduct.

Carbon Dioxide (CO2)

The stomata in the leaves absorb CO2 from the atmosphere. It is a critical substrate in the Calvin cycle (light-independent reactions), where it fixes into glucose using the ATP and NADPH produced in the light-dependent reactions.

Various enzymes facilitate the chemical reactions involved in photosynthesis. For example, the enzyme Rubisco plays a pivotal role in fixing carbon dioxide during the Calvin cycle, converting it into glucose. Enzymes ensure that the photosynthetic reactions occur efficiently and at a sufficient rate to meet the plant’s needs.

Cellular Structures (Chloroplasts)

Chloroplasts, specialized organelles in plant and algal cells, house the biochemical machinery necessary for photosynthesis. The thylakoid membranes in chloroplasts provide a framework for light-dependent reactions, while the surrounding stroma hosts the Calvin cycle.

Temperature and pH

Temperature and pH levels also influence the rate of photosynthesis. The enzymatic reactions involved are sensitive to temperature and have optimal pH ranges. Deviations from these optimal conditions can slow down or inhibit the process.

What Cells and Organelles Are Involved in Photosynthesis?

Photosynthesis, a critical process through which green plants, algae, and some bacteria convert light energy into chemical energy, occurs largely in specialized cells and organelles designed to maximize the efficiency of light capture and conversion. The primary cells and organelles involved in photosynthesis are mesophyll cells, chloroplasts, and, more specifically, structures within the chloroplasts including the thylakoid membranes and stroma.

Mesophyll Cells

Mesophyll cells in plant leaves primarily conduct photosynthesis. These cells contain high concentrations of chloroplasts, the essential organelles where photosynthesis takes place. There are two types of mesophyll cells:

Palisade Mesophyll

These cells, located directly under the leaf surface, are elongated and densely packed with chloroplasts, primarily absorbing light and conducting photosynthesis.

Spongy Mesophyll

These cells, found below the palisade mesophyll, aid in gas exchange and also contain chloroplasts contributing to photosynthesis.

Chloroplasts

Chloroplasts are the key organelles where photosynthesis occurs. These double-membraned structures contain their own DNA and can replicate independently within the cell. Inside chloroplasts, two major stages of photosynthesis—the light-dependent reactions and the Calvin cycle—take place in different components:

Thylakoid Membranes

These membrane-bound structures, stacked into grana within chloroplasts, contain chlorophyll, essential for absorbing sunlight. The light-dependent reactions of photosynthesis occur here, converting sunlight into chemical energy in the form of ATP and NADPH.

This fluid-filled space surrounds the thylakoid membranes inside the chloroplast. Here, the Calvin cycle, also known as the light-independent reactions, occurs. It uses the ATP and NADPH produced by the light-dependent reactions to convert carbon dioxide into glucose, serving as an energy storage molecule for the plant.

Importance of Each Component

Each part within the mesophyll cells and chloroplasts plays a crucial role in the process of photosynthesis:

• Mesophyll cells ensure that chloroplasts are in the optimal position to receive sunlight and facilitate gas exchange, which is vital for photosynthesis.
• Chloroplasts function as the site of the photosynthesis process, housing all the necessary molecular machinery.
• Thylakoid membranes are critical for capturing light and transforming it into usable energy.
• Stroma provides the enzymatic playground for synthesizing organic molecules from carbon dioxide and water.

Factors Influencing the Rate of Photosynthesis

1. light intensity.

• Impact : The rate of photosynthesis typically increases as light intensity rises, up to a certain point. Beyond this point, the process plateaus as other factors become limiting.
• Explanation : Light provides the energy needed for photosynthesis. More light equates to more energy available to drive the chemical reactions involved.

2. Carbon Dioxide Concentration

• Impact : Increasing the concentration of carbon dioxide can enhance the rate of photosynthesis, until the process is constrained by another factor.
• Explanation : Carbon dioxide is a raw material used in the formation of glucose during photosynthesis. Higher concentrations can increase the rate of carbon fixation in the Calvin cycle.

3. Temperature

• Impact : Photosynthesis is temperature-dependent, with the rate increasing up to an optimal temperature and then rapidly decreasing at higher temperatures.
• Explanation : Enzymatic reactions that drive photosynthesis perform optimally within a certain temperature range. Too high or too low temperatures can denature these enzymes, reducing the efficiency of photosynthesis.

4. Water Availability

• Impact : Water stress can severely limit the rate of photosynthesis.
• Explanation : Water is not only a reactant in the chemical equation of photosynthesis but also essential for the plant’s overall health and turgidity. Lack of water can lead to stomatal closure to conserve water, thereby reducing CO2 uptake.

5. Quality of Light

• Impact : Different wavelengths of light affect photosynthesis differently. Blue and red lights are most effective in driving photosynthesis.
• Explanation : Chlorophyll, the primary pigment in photosynthesis, absorbs blue and red light more efficiently than other wavelengths.

6. Chlorophyll Content

• Impact : The amount of chlorophyll in leaves affects their ability to capture light energy.
• Explanation : More chlorophyll molecules increase the capacity for light absorption, enhancing the photosynthetic rate.

7. Leaf Anatomy and Orientation

• Impact : Leaf structure and positioning can influence light capture and gas exchange, impacting photosynthesis.
• Explanation : Leaves arranged to maximize light capture and minimize overlap can more effectively convert light energy into chemical energy.

Energy Efficiency of Photosynthesis

Measuring energy efficiency.

The energy efficiency of photosynthesis generally refers to the percentage of solar energy that plants convert into the chemical energy of sugars. Solar energy strikes the Earth with a power of about 1000 watts per square meter at noon on a clear day. Plants absorb only a fraction of this energy, primarily using the visible light spectrum.

Factors Affecting Efficiency

Several factors impact the energy efficiency of photosynthesis:

• Pigment Absorption : Chlorophyll, the primary pigment in plants, absorbs blue and red light effectively but reflects green light, which is why plants appear green. This selective absorption limits the range of light energy plants can use.
• Photosynthetic Active Radiation : Only about 45% of the sunlight’s energy is in the form of photosynthetically active radiation (PAR). Plants primarily use this portion for photosynthesis.
• Energy Conversion Process : The complex series of reactions in photosynthesis includes losses due to reflection, respiration, and heat production.

Calculating Efficiency

• Theoretical Maximum Efficiency : Research suggests that the theoretical maximum efficiency of photosynthesis in converting solar energy into biomass is around 11% under ideal conditions. This accounts for the energy absorbed and utilized in the formation of glucose.
• Typical Real-World Efficiency : In real-world conditions, the efficiency is much lower. On average, photosynthesis converts only about 0.1% to 2% of solar energy into biomass. This range varies significantly with the type of plant, environmental conditions, and time of year.

Implications of Efficiency

Despite its relatively low energy efficiency, photosynthesis is incredibly effective in supporting life on Earth:

• Global Scale : Annually, photosynthesis captures approximately 130 terawatts of energy as biomass, more than six times the current power consumption of human civilization.
• Ecosystem and Climate : The process is essential for carbon capture, which helps regulate atmospheric CO₂ levels and mitigate climate change.
• Agricultural Productivity : Understanding and improving the efficiency of photosynthesis could lead to increased crop yields and better food security globally.

Enhancing Photosynthetic Efficiency

Scientists are researching ways to enhance the efficiency of photosynthesis to benefit food production and bioenergy. These efforts include genetically modifying plants to absorb light more effectively, bypassing inefficient steps in natural photosynthesis, and developing artificial photosynthesis systems that could one day surpass the efficiency of natural photosynthesis.

C4 Photosynthesis

C4 photosynthesis is a highly efficient photosynthetic pathway that some plants use to overcome the limitations of the standard C3 pathway, especially under conditions of drought, high temperatures, and limited nitrogen or CO2. In C4 photosynthesis, plants capture CO2 in the mesophyll cells and then transport it to the bundle-sheath cells where the Calvin cycle occurs. This process minimizes photorespiration, an energy-wasting process that occurs in C3 plants. Key enzymes like PEP carboxylase initially fix CO2 into a four-carbon compound, which is why we call it C4 photosynthesis. This adaptation allows C4 plants, such as maize and sugarcane, to photosynthesize more efficiently under extreme conditions compared to C3 plants.

What is Photosynthesis in Short Answer?

Photosynthesis is the process where plants use sunlight to produce energy from water and carbon dioxide, releasing oxygen.

How Do You Explain Photosynthesis?

Photosynthesis transforms sunlight into chemical energy, enabling plants to create glucose and oxygen from water and CO2.

What is the Main Process of Photosynthesis?

The main process of photosynthesis involves converting light energy into chemical energy, which plants use to make glucose and release oxygen.

How to Start Photosynthesis?

Photosynthesis begins when chlorophyll in plant cells absorbs sunlight, initiating energy conversion that powers chemical reactions.

What is the Simplest Definition of Photosynthesis?

Photosynthesis is the process by which plants make their own food using sunlight, water, and carbon dioxide.

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Photosynthesis

Plants are autotrophs, which means they produce their own food. They use the process of photosynthesis to transform water, sunlight, and carbon dioxide into oxygen, and simple sugars that the plant uses as fuel. These primary producers form the base of an ecosystem and fuel the next trophic levels. Without this process, life on Earth as we know it would not be possible. We depend on plants for oxygen production and food.

Biology, Earth Science, Chemistry, Ecology

Photosynthesis

Photosynthesis n., plural: photosyntheses [ˌfŏʊ.ɾoʊ.ˈsɪn̪.θə.sɪs] Definition: the conversion of light energy into chemical energy by photolithorophs

Table of Contents

Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs by converting light energy into chemical energy . Among the endless diversity of living organisms in the world, producers are a unique breed.

Unlike consumers ( herbivores , carnivores , omnivores , or decomposers ) that rely upon other living organisms for their nutritional requirements and nourishment, producers have been distinguished by their ability to synthesize their own food. This is the reason that we call producers “autotrophic or self-reliable” in nature while consumers of all the different categories are called “heterotrophic or dependent” in nature.

Now among producers, there are different categories of producers, i.e. different mechanisms via which they produce their own food.

• Photo-auto-litho-trophs: Since these organisms tend to derive their nutrition by channeling the sun’s light energy, they are termed phototrophic in nature. Also, since they utilize inorganic carbon and translate it into organic carbon atoms, i.e. their means of deriving food becomes autotrophic. Additionally, since the source of electrons (electron donors) here are inorganic compounds, they are specified as lithotrophic . In totality, they can be called photo-auto-litho-trophic in nature. Example : Green plants are nature’s brilliant entities that come under this category. They carry out a photosynthesis cycle by taking in carbon dioxide and fixing it into carbohydrates (energy storage molecule). Some of them also give out oxygen gas that’s vital for the other life forms to survive in the earth’s atmosphere.
• Chemo-auto-lithotrophs: Many of us might be unaware of the fact that there are some autotrophs that don’t utilize sunlight. Rather they derive their energy stored from a different energy source like oxidation of inorganic compounds.

The scope of today’s discussion is limited to photosynthesis and photoautotrophs. So, let’s get started and get to know the answers to these common questions: what is the photosynthesis process, what are the 3 stages of photosynthesis, what does photosynthesis produce, what is a byproduct of photosynthesis, what is the purpose of photosynthesis, is photosynthesis a chemical change, the various inputs and outputs of photosynthesis, which organisms perform photosynthesis , and many other more questions!!!

What is Photosynthesis?

Photosynthesis definition: Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs . In simpler language, photosynthesis is the process by which green plants convert light energy into ‘chemical energy’.

This energy transformation is only possible due to the presence of the miraculous pigment molecule chlorophyll in photosynthesis. The chemical energy as referred to before is the fixed carbon molecules generated during photosynthesis.

Green plants and algae have the ability to utilize carbon dioxide molecules and water and produce food (carbohydrates) for all life forms on Earth. There’s no doubt in the fact that life is impossible and unimaginable without green plants that photosynthesize and sustain the cycles of life.

Let’s give you a brief outline of the topic before we head forward.

• Etymology: The photosynthesis process finds its origin in 2 Greek words, firsts one being “phōs (φῶς)” meaning ‘light’ and the second one being “sunthesis (σύνθεσις)” meaning ‘putting together’ . The process of photosynthesis aids the conversion of light energy to chemical energy in varied forms of carbohydrate molecules like sugar molecules and starches.
• Organisms that perform photosynthesis: The organisms are called photo-auto-litho-trophs or simply photoautotrophs.
• Atmospheric gas consumed: Photosynthesizing organisms utilize carbon dioxide in photosynthesis (CO 2 ).
• Atmospheric gas released by “some” photosynthetic organisms (MIND IT-Not all): Some photosynthesizing organisms convert carbon dioxide and aid the process of producing oxygen gas (O2).
• Examples of photosynthesizing organisms: Green plants, cyanobacteria (earlier termed as blue-green algae), and different types of algae that essentially carry out phytoplankton photosynthesis.
• Why is photosynthesis important? The important function of photosynthesis: Food supply for the organisms on Earth, Oxygen supply for the survival of all organisms.
• Site of photosynthesis: Leaves and green tissues. (So when asked where photosynthesis takes place, we can tell that it is this site.)
• What are the reactants of photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
• Products of photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water

Watch this vid about photosynthesis:

Biology Definition: Photosynthesis is the synthesis of complex organic material using carbon dioxide , water , inorganic salts , and light energy (from sunlight) captured by light-absorbing pigments , such as chlorophyll and other accessory pigments . Photosynthesis may basically be simplified via this equation: 6CO 2 +12H 2 O+energy=C 6 H 12 O 6 +6O 2 +6H 2 O, wherein carbon dioxide (CO 2 ), water (H 2 O), and light energy are utilized to synthesize an energy-rich carbohydrate like glucose (C 6 H 12 O 6 ). Other products are water and oxygen .

• Photosynthesis occurs in plastids (e.g. chloroplasts ), which are membrane-bounded organelles containing photosynthetic pigments (e.g. chlorophyll ), within the cells of plants and algae .
• In photosynthetic bacteria ( cyanobacteria ) that do not have membrane-bounded organelles, photosynthesis occurs in the thylakoid membranes in the cytoplasm .

Etymology: from the Greek photo-, “light”, and synthesis, “putting together” Related forms: photosynthetic (adjective) Compare: chemosynthesis See also: photoautotroph

Types of Photosynthesis

Plant photosynthesis and photosynthetic organisms can be classified under different categories on the basis of some characteristic features. They are:

• Types of organisms that carry out photosynthesis on the basis of “cellular structure” Both prokaryotic and eukaryotic organisms carry out photosynthesis.
• Photosynthetic prokaryotes: for example, cyanobacteria
• Eukaryotic: for example, protists ( diatoms , dinoflagellates , Euglena) and green plants. In particular, algae photosynthesis can be observed in green algae , red algae , brown algae , & land plants, like bryophytes , pteridophytes, gymnosperms , and angiosperms .
• Prokaryotic ONLY (anoxygenic photosynthetic bacteria, green sulfur bacteria and purple bacteria)

Photosynthesis: a two-stage process

Photosynthesis is an example of a metabolic process with 2 stages. Both the stages need light (direct or indirect sunlight). Hence, the long-claimed notion of the 2 processes being ‘absolute LIGHT and DARK reactions’ isn’t apt.

Scientific studies have pointed out that even the 2nd stage of photosynthesis requires indirect sunlight. Therefore, rather than classifying the stages as light and dark photosynthesis reactions, we’ll like to classify the 2 stages as follows:

• Photochemical Reaction Process: Light energy is converted to ATP ; photophosphorylation process (light-dependent reactions)
• Through Calvin cycle: In oxygenic photosynthesis as well as anoxygenic photosynthesis
• Through Non-Calvin cycle: Only is some anoxygenic photosynthesis

Evolution of Photosynthesis Process

It is postulated that the very first photosynthetic beings and photosynthesis evolved quite early down the evolutionary timescale of life.

It is also believed that the first photosynthetic beings would have initially resorted to other available reducing agents like hydrogen ions or hydrogen sulfide in contrast to the modern-day photosynthetic organisms that utilize water as the “prime and only sources of electrons”.

It is believed that cyanobacteria would have appeared on the surface of Earth much later than the first photosynthetic beings. Once appeared they must have saturated the Earth’s atmosphere with oxygen gas and led to its oxygenation. Only after the Earth was oxygenated, the more complex forms of life would have later evolved.

When we compare photosynthesis to other metabolic processes like respiration, we can clearly notice that these two processes are almost opposite to each other. But another point to note is that both the processes in synchrony sustain life on Earth.

You cannot separate respiration from photosynthesis or photosynthesis from respiration and expect life to run normally. It is not possible that way. Let’s try to compare and list some characteristic features of photosynthesis and cellular respiration processes.

Photosynthesis vs. Respiration

• Photosynthesis: Anabolic process
• Cellular respiration: Catabolic process By anabolic, we mean the photosynthesis process “utilizes energy to build biomolecules” like carbohydrates, starch, and sugars. These biomolecules are further utilized by both the plants and the organisms dependent on plants for their nutritional needs. On the other hand, respiration is a catabolic process. This energy is utilized to break down complex molecules to derive nutrition out of them.
• Photosynthesis: In the chloroplasts of the eukaryotic phototrophic cells.
• Respiration: Primarily in the mitochondria of the cell.
• Photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
• Respiration: Glucose + Oxygen
• Photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water
• Respiration: Carbon dioxide + Water +energy (ATP)
• Photosynthesis: Endergonic and endothermic
• Respiration: Exergonic and exothermic Just note that these terms endergonic and endothermic both convey the same meaning of “absorbing heat”. And the terms exergonic and exothermic also convey the same meaning of “releasing heat”. The only difference is that –gonics relates to “the relative change in the free energy of the system” while –thermic relates to “the relative change in enthalpy of the system”.
• Photosynthesis: 6CO 2 + 6H 2 O → C6H 12 O 6 + 6O 2
• Respiration: C 6 H 12 O 6 6 + 6O 2 → 6CO 2 + 6H 2 O

Photosynthetic Membranes and Organelles

When we begin the discussion on this topic, it’s important that we know that no photosynthesis is possible without the pigment molecules that absorb light. The absorption of sunlight is the most vital step of photosynthesis.

We should also note that the energy of photons is different for every light of different wavelengths. And the energy needed for the photosynthesis to be conducted is of “a very specific wavelength range”.

For the absorption of lights of desired wavelengths, phototrophs organize their pigment molecules in the form of reaction center proteins . These proteins are located in the membranes of the organisms. Let’s learn how these pigment molecules reside inside the organism and how they make the membranes photosynthetic in nature.

• Prokaryotic photosynthetic organisms: These organisms have their pigment systems or photosystems located in the cell membranes or the thylakoid membranes in the cytosol itself. There are no special organelles called chloroplasts in the prokaryotes.
• Eukaryotic photosynthetic organisms (like green plants): These organisms have their pigment systems or photosystems located in the thylakoids of the chloroplast membranes. Eukaryotes have specialized organelles called chloroplasts (chlorophyll-containing plastids) in their cells.

Photosynthetic Pigments

There are 2 types of photosynthetic pigments in the oxygenic photosynthesizing organisms . They are as follows:

• Porphyrin-derivatives (Chlorophyll in plants and Phycobilin)

Carotenoids

Chlorophyll.

Chlorophyll is the green-colored pigment essential for photosynthesis. Let’s try to list its major characteristic features and roles of it.

• Nature: Lipid
• Location: Embedded in the thylakoid membrane
• Types: 9 types as identified by Arnoff and Allen in 1966 (chlorophyll-a, b, c, d, e, bacteriochlorophyll a, b, chlorobium chlorophyll-650,666). Bacteriochlorophylls are present in the anoxygenic photosynthetic organisms.
• Primary photosynthetic pigment: Chlorophyll-a
• Presence: In all oxygenic photosynthetic organisms
• Absorption range: Visible (blue and red) and IR (Infra-red)
• Ion important for its biological functioning: Magnesium ion (Mg 2+ )
• Structure: Chlorophyll-a, b, and d are “ chlorin ” derivatives; c is a “ porphyrin ” derivative.
• Chlorophyll Tail: Oxygenic photosynthetic organisms have a “ phytol ” tail in their chlorophyll; anoxygenic photosynthetic organisms have a “ geranyl ” tail in their bacteriochlorophylls.
• Main pigment for capturing and storing solar energy
• Photochemical reaction (chlorophyll-a is present in the photochemical reaction center i.e. PCRC. Chlorophyll a, b, c, and d play a role in resonance energy transfer.)

Carotenoid is the photosynthetic pigment essential for working in conjunction with chlorophyll. Let’s try to list its major characteristic features and roles of it.

• Nature: Lipid-soluble
• Types: More than 150
• Absorption range: 400-500nm
• Forms: Carotene (simple hydrocarbon, for example, beta carotene) and xanthophyll (oxygenated hydrocarbon, for example, lutein)
• In excitation and resonance energy transfer
• Photo-protection (work as a free-radical scavenger as well as a quencher)

Phycobilins

Phycobilins aren’t present in all the oxygenic photosynthetic organisms. They have a tetrapyrrole structure (no need for magnesium ion).

• Types: Phycoerythrobilin, Phycocyanobilins, Allophycocyanobilins When these pigment molecules combine with a water-soluble protein, they form the pigment-protein complex (phycobiliproteins, like phycoerythrin and phycocyanin).
• Location: Since these phycobiliproteins are water-soluble, they can’t exist in the membranes like chlorophyll and carotenoids. Therefore, phycobilin pigments as their pigment-protein complex aggregate into clusters and adhere to the membrane. These clusters are called phycobilisomes .
• Exceptional Note: These are the only pigments that are associated with protein molecules.
• Role: Resonance energy transfer

Organelle for Photosynthesis

What is chloroplast? In eukaryotes, photosynthesis occurs in chloroplasts as they are the designated organelles for the photosynthesis process. There are nearly 10-100 chloroplasts in a typical plant cell .

Inside chloroplasts are the thylakoids; the very specific site for the light capturing. The structure of this very unique part of the chloroplasts is briefly discussed here.

Thylakoid is a membrane-bound compartment in the chloroplasts of eukaryotic organisms. They are also present as such in the cytosol of cyanobacteria (cyanobacteria don’t have chloroplasts but they have simply thylakoids).

These thylakoids are the “primary site of the 1st stage of photosynthesis. i.e. “photochemical reaction” or popularly called “light-dependent reactions of photosynthesis”. The main components of the thylakoid are membrane, lumen, and lamellae. The chlorophyll molecules are present inside these thylakoid membranes.

Light-dependent Reactions

The first stage of photosynthesis is popularly called “light-dependent reactions” . We choose to call this stage the “1st stage: PHOTOCHEMICAL REACTION STAGE”. It is also called the “thylakoid reaction stage” or “hill’s reaction” .

This stage is marked by 3 essential steps of photosynthesis: Oxidation of water , reduction of NADP + , and ATP formation . The site where these reactions occur is the lamellar part of the chloroplast. The units of light-dependent reactions are quantosomes .

Let’s discuss this stage under some subheadings:

Wavelengths of light involved and their absorption

The white light that reaches Earth has subparts of different wavelengths together constituting the visible spectrum (390-760nm). But the photosynthetic organisms specifically use a subpart called PAR ( P hotosynthetically A ctive R adiation).

PAR ranges from 400-760nm. Blue light is 470-500nm while red light is 660-760nm). The green light (500-580nm) is reflected back by the plants and this is the reason that plants appear green in color. Blue-green light is not used, only blue light is used.

Absorption spectrum and action spectrum

• Absorption Spectrum: This is a pigment-specific entity or terminology. To find the absorption spectrum of a pigment, you need to plot “the amount of absorption of different wavelengths of light by that particular pigment” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “percentage of light absorption” on the Y-axis.
• Action Spectrum: To find the action spectrum of a pigment, you need to plot the “effectiveness of the different wavelengths of light in stimulating photosynthesis process” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “rate of photosynthesis (measured as oxygen released)” on the Y-axis. When you superimpose the action spectrum of photosynthesis with the absorption spectrum of the specific pigment, you can find the contribution of each different wavelength in the photosynthesis rate, photosynthetic efficiency, and photosynthetic productivity.

IMPORTANT NOTE: The absorption spectrum is calculated for any of the many pigments involved in photosynthesis. Contrastingly, the action spectrum is calculated only for the photochemical reaction performing pigment i.e. chlorophyll-a present at the reaction center. We identify the progress of photochemical reactions as the “evolution of oxygen gas” that primarily happens at the reaction center where only chlorophyll-a is present. Since the action is directly correlated to the specific excitation of chlorophyll-a molecule, the action spectrum is scientifically calculated only for this chlorophyll-a.

• Absorption spectrum of chlorophyll- a : 430 nm (blue), 660nm (red) {more absorbance at 660 nm)
• Absorption spectrum of chlorophyll-b: 430 nm (blue), 660nm (red) {more absorbance at 430 nm)

What actually happens in the Light-dependent reaction

Let’s briefly describe what actually happens here.

• 1 photon is absorbed by 1 molecule of the chlorophyll (P680) and simultaneously 1 electron is lost here.
• The electron flow of the photochemical reaction begins here.
• The electron is transferred to D1/D2 protein, then to a modified form of chlorophyll and “pheophytin”.
• After that, it’s transferred to plastoquinone A and then B.
• Initiates an electron flow down an electron transport chain.
• Ultimately aids the NADP reduction to NADPH.
• Creation of a proton gradient across the chloroplast membrane.
• Further on this proton gradient is exploited by the ATP synthase for the generation of ATP molecules.

Water photolysis

Now, if you are wondering how the first electron lost by the 1st chlorophyll is replenished to keep this cycle going, read on. The answer to this query is “photolysis of water molecules” . The chlorophyll molecule regains the lost electron when the “oxygen-evolving complex” in the thylakoid membrane carries out the photolysis of water. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases oxygen.

Many scientists had a doubt about the source of oxygen in photosynthesis. Some speculated the oxygen atom of the CO 2 gas is the source of oxygen post-photosynthesis. But it was the collective contribution of some 4 scientists that gave clarity on this topic.

C.B. Van Niel worked on purple photosynthetic bacteria ( Chromatium vinosum ) and found out that the source of oxygen is the oxidation of water molecules (‘indirect evidence’). While Ruben, Hassid, and Kamen carried out an isotopic study that gave ‘direct evidence’ of oxygen-evolving from H 2 O molecules and not CO 2 molecules.

Hydrolysis of 2 molecules of water leads to the evolution of 1 molecule of oxygen gas. The photosynthesis equation for light-dependent reactions (non-cyclic electron flow) or the chemical formula for photosynthesis:

2 H 2 O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O 2

The photochemical reaction (or the light-dependent reactions) can be classified as:

• Cyclic reaction: Only 1 photosystem ( PS1 ) is involved. (Photon excites P700 in PS1, electron reaches Fe-S, then Ferredoxin, then Plastoquinone and then Cyt b6f complex and then Plastocyanin). Since in the solo involvement of PS1 here, the electron flow becomes cyclic. And this phosphorylation process is called cyclic phosphorylation. It happens in the stroma lamellae when light beyond 680nm is available.
• Non-cyclic reaction: Both photosystems (PS1 and PS2 ) are involved. (Photon excites P680 in PS2, the electron is lost and transferred to pheophytin, then sent on a roller coaster (Z-scheme). Within the z-scheme, the final redox reaction enables the reduction of NADP+ to NADPH. And the chemiosmotic potential generation via proton pumping proton across the membrane and into the thylakoid lumen ensures ATP synthesis.

Data Source: Akanksha Saxena of Biology Online

Light-Independent Reactions (Carbon-fixation Reaction)

Also called the carbon fixation process, the “light-independent reactions” is a misnomer as Science has now already proved that the second stage of photosynthesis isn’t really light-independent reactions. Though it doesn’t need direct light, indirect light is involved even in this process. We choose to label this stage of photosynthesis as the “2nd stage: CARBON-FIXATION REACTION STAGE ”, which is also called:

• Calvin Cycle or “stromal reaction” as it manifests in the stroma part of the chloroplast
• “C3 Cycle” or the “reductive pentose phosphate cycle”

Calvin cycle

The inputs for the Calvin cycle  in most plants come from the previously occurred photochemical reaction. In this cycle, the carbon dioxide produced is fixed to a glucose molecule. To be very specific, the Calvin cycle directly doesn’t produce glucose, rather it produces glyceraldehydes-5-phosphate (G-3-P). Glucose is formed after these G-3-P molecules move into the cytosol from the chloroplast .

It consists of primarily 3 steps as follows:

• Carboxylation: Acceptance of CO 2 by RuBP which is a 5-carbon compound and the CO2-acceptor). 2 molecules of 3-phosphoglycerate are generated as the result of the carboxylation process.
• Reduction: Generation of 3C/4C/5C/6C/7C molecules.
• Regeneration of RUBP: 3 molecules of RuBP are regenerated.

In totality, 3 molecules of CO 2 produce 1 molecule of G-3-P. This uses 9 ATPs and 6 NADPHs. And, 6 molecules of CO 2 produce 2 molecules of G-3-P which further produce 1 molecule of glucose. This uses 18ATPs and 12 NADPHs.

The main enzyme is RuBisCo . It’s a multi-enzyme complex with 8 large and 8 small subunits. The substrates for this enzyme are CO 2 , O 2 , and RuBP. An essential ion for the biological functioning of this enzyme: Mg 2+ . The role of RuBisCo is that it captures carbon dioxide gas from the atmosphere and utilizes the NADPH from the 1st stage (photochemical reaction/light-dependent reaction stage) to fix the CO 2 .

The equation of dark reaction of photosynthesis/light-independent reaction stage/2nd stage is: 3 CO 2 + 9 ATP + 6 NADPH + 6 H + → C 3 H 6 O 3 -phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H 2 O

The simple carbon sugars formed via the C3 cycle are utilized by the biological systems to form complex organic compounds like cellulose, precursors for amino acids synthesis and thereby proteins, precursors for lipids, and the source of fuel for respiration.

Important Point To Note: It happens in all the photosynthetic organisms as the basic carbon-fixation step.

Carbon concentrating mechanisms

There are many carbon concentrating mechanisms to increase the carbon dioxide levels and the carbon fixation process like C4, CAM, etc.

• Doesn’t happen in all photosynthetic organisms. Rather it happens in conjunction with the C3 cycle in some 4% of angiosperm families.
• Most commonly angiosperm families that witness C4 cycle: Poaceae, Cyperaceae.
• First explained by: Hatch and Slack (hence also called the Hatch and Slack cycle). They worked on the maize plant.
• Role: Endow the ability to efficiently conduct photosynthesis in plants of the semi-arid regions by making them well adapted.
• Mechanism: By separation of photosynthesis stages in 2 types of cells (mesophyll cells and bundle sheath cells). The light reaction is restricted to the mesophyll cells and the CO 2 fixation happens in the bundle sheath cells. This phenomenon is also termed as “chloroplast dimorphism” in C4 plants. The Kranz anatomy is visible here.
• Why does the need arise in the first place? – In semi-arid regions or regions with very hot and dry environmental conditions, plants are forced to close their stomata in order to limit water loss. Under such harsh conditions, the intake of CO 2 decreases during the day as the stomata are forced closed. This might lead to no CO 2 intake and hence no CO 2 fixation (2nd stage of photosynthesis). But the 1st stage of photosynthesis keeps running as it doesn’t depend on stomata opening or closure. This means that a continuous oxygen evolution happens which can lead to oxygen saturation. As we know that RuBisCo enzymes use O 2 gas as substrate too, and this can lead to an increased rate of photorespiration by the oxygenase activity of RuBisCo. This further decreases the carbon fixation. This is a very big issue if not resolved. Hence, for situations like these, carbon concentrating mechanisms have evolved in some families of plants to concentrate and enrich the CO 2 concentration in the leaves of these plants under such conditions.
• Important enzyme for CO 2 concentration: PEP carboxylase
• CO 2 is first added to a three-carbon compound called phosphoenolpyruvate (PEP) in this cycle. This leads to the formation of a four-carbon (4C)  molecule called oxaloacetic acid or malate. This step happens in the mesophyll cells of the leaves.
• After that, these 4C compounds are transferred to the bundle sheath cells where the normal C3 cycle fixes them into glucose molecules.
• This CO 2 concentrating mechanism works on the “principle of separating the RuBisCo enzyme from the O 2 -generating photochemical reactions” in order to reduce the rates of photorespiration and simultaneously increase the rates of CO 2 fixation.
• This increases the photosynthetic capacity of the leaf/leaf photosynthesis.
• When the high light and high-temperature conditions are dominant, C4 plants prove more photosynthetically efficient than C3 plants as they produce more sugar molecules in such conditions.
• Examples of C4 plants: Many crop plants like wheat, maize, rice, sorghum, millet, and sugarcane.
• Number of ATPs required: 12 (for C-enrichment) + 18 (for C-fixation)= 30 ATPS for 1 glucose production
• Number of NADPH required: 18 NADPH for 1 glucose production
• Some plants resort to another mechanism called the CAM cycle in conjunction with the C3 cycle to fix carbon dioxide.
• Examples: xerophytes like cactus photosynthesis, and most succulents.
• Around 16,000 species of plants utilize the CAM mechanism
• Mechanism: Utilize PEP carboxylase to capture carbon dioxide. In contrast to the C4 cycle where there is a “spatial separation of the 2 processes of CO 2 reduction to PEP and PEP fixation to glucose”, CAM plants display a “temporal separation of the 2 listed processes”.

Land plants display different types of photosynthesis based on their requirements and environmental constraints. They are C3, C4 +C3, and CAM+ C3 types of photosynthesis.

Aquatic plants and algae display some extra features in the photosynthetic machinery. These features further refine and define the smooth functioning and efficiency of photosynthesis.

Example: Cyanobacteria photosynthesis – cyanobacteria have carboxysomes  that help in enriching the concentration of carbon dioxide around the RuBisCO enzyme. This directly increases the photosynthetic rates. The distinguished and specially enabled enzyme in the carboxysomes is called “carbonic anhydrase”. The carbonic anhydrase possesses the ability to evolve and release CO 2 from the dissolved hydrocarbonate ions (HCO-). As soon as the CO 2 is released, RuBisCo takes care that it doesn’t go to waste.

Order and Kinetics

There are innumerable reactions and processes involved in the biological mechanism of photosynthesis. Besides the normal flow of photosynthesis, there are some plant-specific and condition-specific additional steps that further complicate the mechanism.

Since every biological mechanism has a lot of enzymes, factors, cofactors, substrates, and entities involved, photosynthesis is no different.

Let’s try to list some kinetics-specific pointers that may help.

As discussed in the overview and starting of this article, the early photosynthetic organisms must have been primarily “anoxygenic” in nature. These bacteria used some other source than water molecules as their primary electron donors. Even the geological evidence aligns with this fact as the early atmosphere of Earth was highly reducing in nature. Some speculated organisms of the early evolutionary phase are :

• Green sulfur bacteria (Electron donor= hydrogen and sulfur)
• Purple sulfur bacteria (Electron donor= hydrogen and sulfur)
• Green nonsulfur bacteria (Electron donor= various amino and other organic acids)
• Purple nonsulfur bacteria (Electron donor= variety of nonspecific organic molecules)

After this, some filamentous photosynthetic organisms are expected to have evolved. This is scaled to be an occurrence of some 3.4 billion years old timeline. It is around 2 million years ago that oxygenic photosynthesis is believed to have evolved.

The modern and more commonly known photosynthesis in plants and most of the photosynthetic prokaryotes= Oxygenic (Electron donor= Water molecules)

Symbiosis and the origin of chloroplasts

There are some animal groups that have the ability to form and establish symbiotic relationships with photosynthetic organisms. By establishing such a relationship, these organisms can directly rely upon their photosynthetic partner for energy and food requirements. Some examples of such animal groups are:

• Sea anemones
• Marine mollusks (example: Elysia viridis & Elysia chlorotica )
• Fungi photosynthesis (Lichens)

When such symbiotic relationships are established, it’s sometimes observed that some genes of the plant cell’s nucleus get transferred to the animal cell . (Observed in some slugs).

Origin of Chloroplasts

Such symbiosis is popularly claimed to be the source of chloroplast evolution. As we notice many similarities between the photosynthetic bacteria and chloroplasts, the evolution of chloroplasts is often hinted to have occurred from these bacteria. Some of the common features between the 2 are:

• Circular chromosome
• Prokaryotic-type ribosome
• A similar set of proteins in the photosynthetic reaction center

It is for all these commonalities the “ endosymbiotic theory ” had been proposed for the evolution of chloroplasts and mitochondria in the eukaryotic cells. According to the endosymbiotic theory, the early eukaryotic cells are believed to have acquired the photosynthetic bacteria by the process of endocytosis). Those early eukaryotic cells after acquiring the photosynthetic bacteria transformed to be self-sustainable and became the “first plant cells”. (Mitochondria photosynthesis is true, they are associated with respiration!)

Photosynthetic eukaryotic lineages

Photosynthetic eukaryotic lineages include:

• Glaucophytes
• Chlorophytes
• Rhodophytes
• Cryptophytes (some clades)
• Haptophytes (some clades)
• Dinoflagellates & chromerids
• Euglenids—clade Excavata (unicellular)

Cyanobacteria and the evolution of photosynthesis

Almost all the prokaryotes carry out anoxygenic photosynthesis in contrast to cyanobacteria, which perform oxygenic photosynthesis. This ability to carry out oxygenic photosynthesis is speculated to have evolved at least 2450–2320 million years ago. The first photosynthetic cyanobacteria might not have been oxygenic as Earth’s atmosphere had no oxygen then.

This topic still requires more scientific study to bring out conclusive results. From the paleontological evidence, it is claimed that the 1st cyanobacteria evolved around 2000 Ma.

For the initial years of the Earth’s oxygen-rich environment (after the oxygen-evolving mechanism evolved), cyanobacteria are claimed to be the “principal primary producers of oxygen”. Even to date, cyanobacteria have been proven vital for marine ecosystems. They’re the primary producers of oxygen in oceans.

Cyanobacteria also fix nitrogen electrons fixation and play a role in biological nitrogen cycles.

Experimental History

We will list the long experimental history in deciphering the extensive photosynthesis process through the ages.

Discovery, Refinements, and Development of the concept

Find out the discovery, refinements, and development of photosynthesis as summarized in the table below:

C3 : C4 photosynthesis research

Several studies were conducted using isotopes of radioactive elements to identify the various aspects of the photosynthetic process. A number of organisms like Chlorella , Stellaria media, Cladophora, Spirogyra, Rhodopseudomonas , sulfur bacteria, green plants like maize, etc have been used to understand the photosynthesis process over the years. Gas exchange studies, isotopic studies, light spectrum studies, radioactive studies, plant anatomical and physiological studies, studies involving roles of carbon dioxide and water, etc have all together opened the gates for our deeper understanding of this topic.

The 3 main factors that directly affect the photosynthesis process are:

• Light irradiance and wavelength
• Carbon dioxide concentration

Temperature

Although there are many more corollary factors, these 3 are the most important ones.

Light intensity (irradiance), wavelength, and temperature

Light is an essential factor for photosynthesis. It directly affects the rate of it. There are 3 different parameters that we should look into:

• Sciophytes : Grow under “diffuse” light. Example: Oxalis
• Heliophytes: Grow under “direct: light. Example: Dalbergia
• Light quality: PAR as previously discussed is the quality or the fraction of light energy that is ‘photosynthetically active’ in nature. It ranges from 400-700nm in wavelength.
• Duration of light: This parameter doesn’t affect the rate of photosynthesis but affects the total photosynthetic output.

Carbon dioxide levels and photorespiration

Carbon dioxide concentration is the major factor in determining the rate of photosynthesis. There is no carbon-dioxide enriching system in C3 plants like the C4 plants. So, if you increase the concentration of CO 2 in the system, the photosynthetic rate of C3 plants will increase as the CO 2 concentration increases. On the other hand, the photosynthetic yield of the C4 plant won’t increase in such a scenario.

• CO 2 Compensation Point: A stage in CO 2 concentration when there’s no absorption of CO 2 by the illuminated plant part.

Featuring… “The curious case of RuBisCO and PEP Carboxylase”

Imagine an equal concentration (50-50%) of the two isotopes of carbon, C-12 and C-13, in the form of 12CO 2 and 13CO 2 , made available to both C3 and C4 plants. Now, can you tell which isotope of the carbon will be fixed more or less by the two types of photosynthetic organisms? Can you guess if there would be a “preferable” isotope between the two? Do you think C3 plants will fix the 12CO 2 and 13CO 2 equally or unequally? Or do you think the 12CO 2 and 13CO 2 incorporation would have a biased ratio in any of the two (C3/C4 plants)????

The answer to this lies in the major carbon fixing enzyme involved.

• C3 plants: Major C-fixing enzyme is RuBisCo and RuBisCo has a “discriminatory ability” to preferably fix 12CO 2 and not 13CO2. Hence, you will find more 12CO 2 fixed than 13CO 2 in the C3 plants.
• C4 plants: Major C-fixing enzyme is not RuBisCo but PEP Carboxylase . PEP Carboxylase has “no discriminatory ability”. So, you’ll find an almost equal proportion of 12CO 2 and 13CO 2 getting fixed in C4 plants. So, in comparison to C3 plants, the chances of getting 13CO 2 fixed are more in C4 plants.

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• Rutherford, A.W., Faller, P. (Jan 2003). “Photosystem II: evolutionary perspectives”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 358 (1429): 245–253. doi:10.1098/rstb.2002.1186. PMC 1693113. PMID 12594932.
• Arnon, D.I., Whatley, F.R., Allen, M.B. (1954). “Photosynthesis by isolated chloroplasts. II. Photophosphorylation, the conversion of light into phosphate bond energy”. Journal of the American Chemical Society. 76 (24): 6324–6329. doi:10.1021/ja01653a025.
• Ehrenberg, R. (2017-12-15). “The photosynthesis fix”. Knowable Magazine. Annual Reviews. doi:10.1146/knowable-121917-115502. Retrieved 2018-04-03.
• El-Sharkawy, M.A., Hesketh, J.D. (1965). “Photosynthesis among species in relation to characteristics of leaf anatomy and CO 2 diffusion resistances”. Crop Sci. 5 (6): 517–521. doi:10.2135/cropsci1965.0011183x000500060010x.
• Earl, H., Said Ennahli, S. (2004). “Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation”. Photosynthesis Research. 82 (2): 177–186. doi:10.1007/s11120-004-1454-3. PMID 16151873. S2CID 291238.

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Last updated on July 15th, 2022

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Photosynthesis - what is photosynthesis?

Photosynthesis enables plants and algae to convert CO2 and water into vital oxygen and sugar. This is why scientists are also working with photosynthesis in the battle against climate change and food shortages.

What is photosynthesis?

When Earth formed from a cloud of gas and dust around the young Sun approximately 4.6 billion years ago, our planet's atmosphere contained no oxygen.

About one billion years later, tiny blue-green bacteria emerged in the oceans and began to produce oxygen. They triggered the development of Earth's oxygen-rich atmosphere, which forms the basis for not only humans, but all animals on the planet to breathe.

Oxygen is produced through photosynthesis.

Photosynthesis is a biochemical process that enables plants, algae, and some bacteria to absorb the inorganic substances of carbon dioxide (CO2) and water (H20) to form the carbohydrate of glucose, which enables plants to grow.

Just as importantly, oxygen is produced as a waste product of the process, making photosynthesis the most important biochemical process on Earth.

The formula for photosynthesis is:

6 CO2 + 6 H2O + light energy --> C6H12O6 (glucose) + 6 O2

How does photosynthesis work?

Green plants use energy from light through photosynthesis. The process converts the CO2 of the air into oxygen and carbon compounds (sugars).

How photosynthesis works

Photosynthesis is a process in which plants use microscopic chloroplasts in leaves to capture the energy of light and utilise it to convert water and carbon dioxide into sugar and oxygen.

Water (H2O) is carried from the plant's roots to the leaves.

Light energy from the Sun splits the water molecules, releasing electrons.

Photosynthesis takes place in the chloroplasts of plant cells.

Carbon dioxide (CO2) is absorbed through stomata in plant cells.

Carbon (C), a building block of the plant, is formed, when electrons released from water split carbon dioxide (CO2).

Oxygen (O2) is released by the plant as a waste product of the process.

When was photosynthesis discovered?

In other words, plants work in the opposite way of humans and animals, which inhale oxygen and exhale CO2 - also known as respiration.

This was discovered by Dutch scientist Jan Ingenhousz in 1771. He documented that plants release oxygen bubbles when exposed to sunlight and release carbon dioxide when it gets dark.

In 1999, Greenlandic researcher Minik Rosing sensationally found evidence of photosynthesis in Greenlandic rock. The evidence came in the shape of carbon deposited by cyanobacteria, also known as blue-green algae, 3.7 billion years ago.

Photosynthesis is undoubtedly the most dramatic thing that has happened to life on Earth. Minik Rosing, Professor of Geology at the University of Copenhagen

Like plants, cyanobacteria convert sunlight into chemical energy and produce oxygen as a waste product. All the oxygen the bacteria produced in the oceans through photosynthesis over the next hundreds of millions of years provided a breeding ground for new and more sophisticated life forms.

Cyanobacteria have existed for 3+ billion years, and their oxygenic photosynthesis forms the basis of all life on Earth as we know it.

In the course of several geological periods, cyanobacteria pumped oxygen into the environment. Initially, the oxygen was chemically bound by substances such as iron, so it did not end up in the atmosphere.

Over time, however, the oxygen from photosynthesis filled the atmosphere. In addition, the ozone layer, which protects life from the Sun's ultraviolet radiation, was formed.

For the past two billion years, Earth has had an oxygen-rich atmosphere, and calculations show that today, 280 billion t of oxygen are produced on Earth every year. 46 % of the oxygen are produced in the oceans by algae and cyanobacteria, while the remaining 54 % are produced on dry land.

This corresponds to 21 % of the atmosphere (the rest of the atmosphere is made up of nitrogen, CO2, and other gases). 46 % of the oxygen are produced in the oceans by algae and cyanobacteria, while the remaining 54 % are produced on dry land.

Video: NASA observes photosynthesis from space

Hundreds of km from Earth's surface, satellites detect the invisible light - fluorescent light - that plants emit during photosynthesis.

Climate change and photosynthesis

For millions of years, the ratio of oxygen to CO2 was in natural balance in the atmosphere, but over the past few decades, the balance has been disturbed.

In 2019, global atmospheric CO2 emissions reached 37 gigatonnes, which is 30 % higher than in the 1970s.

Scientists widely agree that the increase in CO2 emissions is due to the burning of fossil fuels, which explains why natural photosynthesis cannot keep up - and why CO2 levels in the atmosphere are steadily increasing.

Therefore, research is also being done to find new ways to store or capture CO2, so that the concentration in the atmosphere does not continue to skyrocket.

Artificial leaves perform photosynthesis

Artificial leaves that mimic nature's photosynthesis have been invented several times.

Unfortunately, they have all shared the same problem. The leaves can convert CO2 from pressurised tanks in a lab, but they cannot extract CO2 from ordinary atmospheric air. Until now.

In 2019, researchers developed a special membrane that can solve the problem.

The membrane encapsulates the artificial leaf and bathes it in water while it performs photosynthesis. When the sunlight heats the water behind the membrane, it can evaporate through the subsurface of the leaf and absorb CO2 through small slots on the surface of the leaf. The slots are similar to the stomata of natural leaves.

A light-absorbing material captures energy from sunlight, which, together with a number of auxiliary substances, triggers photosynthesis in the artificial leaf.

The artificial leaf converts CO2 into oxygen and carbon monoxide (CO). The oxygen can be captured or released, while the carbon monoxide can be used in synthetic fuel.

How artificial leaf photosynthesis works

Scientists have invented a membrane that envelops artificial leaves in water, enabling the leaves to draw CO2 from the air and hence provide themselves with the raw material for photosynthesis.

Sea slug harvests chloroplasts

The Elysia chlorotica sea slug sucks chloroplasts out of the algae with a straw-like structure, absorbing them into its cells, so it can convert sunlight into energy.

Pea aphids produce pigments

The pea aphid (Acyrthosiphon pisum) produces plant pigments that likely enable the insect to harvest energy from sunlight.

Salamanders parasitise on algae

The spotted salamander (Ambystoma maculatum) is colonised by algae in the embryonic stage, when the egg grows in water. The embryo hoards oxygen and glucose from the algae, as they perform photosynthesis.

Researchers hack photosynthesis

In 2019, a new international research project known as Realizing Increased Photosynthetic Efficiency (RIPE) succeeded in genetically modifying the leaves of tobacco plants to make them much more efficient at photosynthesis.

When plants capture carbon dioxide from the air, they do so via the rubisco enzyme. Unfortunately, the enzyme not only captures carbon dioxide, but also oxygen, producing toxins that the plant requires energy to break down.

This detoxification process means the plant has less energy to convert the CO2 into glucose, which fuels the plant's growth.

The researchers' genetically modified leaves made detoxification in tobacco plants much more efficient, causing the plants to grow faster and up to 40% larger compared to normal tobacco plants.

The next step is to test the method on tomatoes, soybeans, and similar plants, so that plant-based food production can double over the next 50 years, which is necessary to feed a world population expected to hit nine billion by then.