Photosynthesis in Higher Plants Class 11 Biology Chapter 11 Notes

Photosynthesis in Higher Plants Class 11 Biology Chapter 11 Notes

Photosynthesis and Its Significance

• All animals, including humans, rely on plants for food.
• Green plants synthesize their own food through a process known as photosynthesis.
• Photosynthesis is the conversion of light energy into organic compounds.
• Green plants are autotrophs because they can make their own food.
• All other organisms that depend on green plants for food are heterotrophs.
• Photosynthesis is a physico-chemical process carried out by green plants.
• Sunlight is the primary source of energy for all living forms on Earth.
• Photosynthesis is essential for two main reasons:
  1. It is the primary source of all food on Earth.
  2. It releases oxygen into the atmosphere, crucial for breathing.
• The chapter focuses on the structure of the photosynthetic machinery and the reactions involved in converting light energy into chemical energy.

Experimental Evidence for Photosynthesis

Experiment 1: Starch Formation in Variegated Leaves

• Tested starch formation in two types of leaves: variegated leaf and a leaf partially covered with black paper.
• Exposed both leaves to light.
• Results showed that photosynthesis occurred only in the green parts of the leaves in the presence of light.

Experiment 2: The Role of CO2 in Photosynthesis

• Divided a leaf into two parts.
• One part was enclosed in a test tube containing KOH-soaked cotton (which absorbs CO2).
• The other half was exposed to the air.
• Placed the setup in light for some time.
• Afterward, tested for the presence of starch in both parts of the leaf.

Results:

• The exposed part of the leaf tested positive for starch.
• The portion enclosed in the tube tested negative for starch.

Milestones in Understanding Photosynthesis

Joseph Priestley’s Experiments (1770):

• Priestley discovered the essential role of air in the growth of green plants.
• He observed that a candle or a mouse in a closed space consumes air.
• Placing a mint plant in the same environment showed that the mouse survived, and the candle continued to burn.
• Hypothesis: Plants replenish the air with what animals and candles consume.

Jan Ingenhousz’s Sunlight Experiment:

• Ingenhousz placed plants in light and darkness.
• Showed that sunlight is essential for a process that purifies air fouled by burning candles or breathing animals.
• Bubbles, identified as oxygen, formed around green plant parts in sunlight.
• Demonstrated that only the green parts of plants release oxygen.

Julius von Sachs’ Glucose and Chlorophyll Discovery (1854):

• Sachs provided evidence for glucose production in growing plants, typically stored as starch.
• Identified chlorophyll (green pigment) in plant cells’ special bodies called chloroplasts.
• Green plant parts, containing chlorophyll, are where glucose is made and stored as starch.

T.W. Engelmann’s Prism Experiment:

• Engelmann split light into its spectral components using a prism.
• Illuminated a green alga in the presence of aerobic bacteria to detect sites of O2 evolution.
• Bacteria accumulated mainly in the blue and red light regions, forming the first action spectrum of photosynthesis.

Cornelius van Niel’s Light-Dependent Reaction Theory:

• Van Niel studied purple and green bacteria.
• Demonstrated that photosynthesis is a light-dependent reaction in which hydrogen from an oxidizable compound reduces carbon dioxide to carbohydrates.
• In green plants, water (H2O) serves as the hydrogen donor and is oxidized to produce O2.
• Some organisms use different donors, like H2S in sulfur bacteria.
• The O2 in green plants comes from water, not carbon dioxide, later confirmed with radioisotope techniques.
• The complete equation for photosynthesis is a multistep process: 6 CO2 + 6 H2O + Light → C6H12O6 + 6 O2.

Location of Photosynthesis

Photosynthesis primarily occurs in the green leaves of plants, but it also takes place in other green parts of the plant. Some other parts where photosynthesis may occur include:

1. Stems: In some plant species, green stems have chloroplasts and can carry out photosynthesis.
2. Flower Petals: Some flowers have chloroplasts in their petals and can perform photosynthesis to some extent.
3. Unripe Fruits: While the primary function of fruits is not photosynthesis, unripe fruits may contain chloroplasts and perform limited photosynthesis.

Chloroplast Orientation:

The chloroplasts within mesophyll cells of leaves are aligned along the walls to optimize light absorption. Chloroplasts will be:

• Aligned with flat surfaces parallel to the walls when light is abundant and plants can efficiently capture sunlight for photosynthesis.
• Perpendicular to the incident light when light is less intense, allowing them to capture light from various angles.

Chloroplast Structure and Function:

Within the chloroplast, there is a membranous system consisting of grana (stacks of thylakoid membranes), stroma lamellae (connecting thylakoids), and the matrix stroma (the fluid-filled space). These components have specific roles in photosynthesis:

• Membrane System (Grana): Responsible for trapping light energy and synthesizing ATP and NADPH during the light reactions (photochemical reactions).
• Stroma: Enzymatic reactions in the stroma synthesize sugars (glucose) from carbon dioxide and water, which is the basis of the dark reactions (carbon reactions). These reactions depend on the products of the light reactions, namely ATP and NADPH.

It’s essential to note that dark reactions are not light-independent; they are indirectly dependent on the products of the light reactions, even though they don’t directly use light energy. The terminology “dark reactions” can be misleading and should not be interpreted as occurring in complete darkness.

Types of Pigments in Photosynthesis

In photosynthesis, there are several pigments involved, and they each have specific roles. These pigments are responsible for the various shades of green seen in plant leaves. Through paper chromatography, four main pigments can be separated from plant leaves:

1. Chlorophyll a: This pigment appears as bright or blue-green in the chromatogram.
2. Chlorophyll b: It appears as yellow-green.
3. Xanthophylls: These pigments appear yellow.
4. Carotenoids: Ranging from yellow to yellow-orange in color.

Roles of Various Pigments in Photosynthesis:

• Chlorophyll a: It is the primary pigment associated with photosynthesis. Chlorophyll a absorbs light at specific wavelengths and plays a central role in capturing light energy to initiate the process of photosynthesis.
• Chlorophyll b: This pigment, although secondary to chlorophyll a, also absorbs light and transfers energy to chlorophyll a. It expands the range of wavelengths of incoming light that can be utilized for photosynthesis.
• Xanthophylls and Carotenoids: These pigments, known as accessory pigments, serve several functions:
  • They absorb light at different wavelengths, extending the range of light that can be utilized for photosynthesis.
  • They help protect chlorophyll a from photo-oxidation, preventing potential damage from excessive light exposure.

Absorption and Action Spectra:

• Absorption Spectrum: This graph illustrates the ability of chlorophyll a to absorb light at different wavelengths. Chlorophyll a shows maximum absorption in the blue and red regions of the spectrum.
• Action Spectrum: It represents the wavelengths at which maximum photosynthesis occurs in a plant. Notably, the regions where chlorophyll a absorbs light (blue and red) also correspond to higher rates of photosynthesis. This indicates that chlorophyll a is the primary pigment associated with photosynthesis.
• Comparison of Absorption and Action Spectra: While there is a significant overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis, it is not a complete one-to-one overlap. Some photosynthesis occurs at other wavelengths in the visible spectrum as well, indicating the contribution of other pigments.

These pigments, including chlorophyll a and the accessory pigments, collectively enable plants to capture light energy efficiently and perform photosynthesis across a broader range.

Light Reaction in Photosynthesis

The light reaction, also known as the photochemical phase, is the initial stage of photosynthesis. It is a series of complex biochemical processes that occur in the thylakoid membranes of the chloroplasts. The primary functions of the light reactions include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, namely, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

Key points about the light reactions:

1. Photosystems: The light reactions involve two photosystems, Photosystem I (PS I) and Photosystem II (PS II). These photosystems are protein-pigment complexes embedded in the thylakoid membrane.
2. Light-Harvesting Complexes (LHC): Within each photosystem, there are light-harvesting complexes (LHC) made up of hundreds of pigment molecules bound to proteins. These pigments serve to capture light energy by absorbing different wavelengths of light. The LHCs are also known as antennae.
3. Reaction Centre: Each photosystem has a reaction centre, where the actual photochemical reactions take place. The reaction centre contains a single molecule of chlorophyll a. In Photosystem I (PS I), the reaction centre chlorophyll a has an absorption peak at 700 nm, and it is called P700. In Photosystem II (PS II), the reaction centre chlorophyll a has an absorption peak at 680 nm, and it is called P680.
4. Sequence of Processes: During the light reactions, the following sequence of processes occurs: a. Light is absorbed by the pigments in the LHCs. b. The absorbed light energy is transferred through the pigment molecules to the reaction centre chlorophyll a. c. In Photosystem II (PS II), water molecules are split into oxygen, protons (H+ ions), and electrons. This process releases oxygen as a byproduct. d. The excited electrons from the reaction centre chlorophyll a are transferred through a series of protein complexes, known as the electron transport chain. e. As electrons move through the electron transport chain, they generate a proton gradient across the thylakoid membrane. f. The proton gradient is used to generate ATP through a process called chemiosmosis. g. Electrons are ultimately accepted by Photosystem I (PS I) and are re-energized by absorbing more light. h. The re-energized electrons are then transferred to NADP+ (nicotinamide adenine dinucleotide phosphate) to form NADPH.

The light reactions are the steps in photosynthesis that convert light energy into chemical energy in the form of ATP and NADPH. These high-energy molecules are essential for the subsequent dark reactions (Calvin cycle) of photosynthesis, which use them to convert carbon dioxide into glucose and other organic compounds.

Electron Transport in Photosynthesis

The electron transport chain in photosynthesis is a crucial process that occurs during the light reactions. It involves the movement of excited electrons from photosystem II (PS II) to photosystem I (PS I) and eventually to the reduction of NADP+ to NADPH. This process is known as the Z scheme due to its characteristic shape on the redox potential scale.

Here is a step-by-step explanation of the electron transport chain:

1. Photosystem II (PS II): The reaction centre chlorophyll a in PS II absorbs light at a wavelength of 680 nm, exciting the electrons. These high-energy electrons are then transferred to an electron acceptor and enter the electron transport system.
2. Electron Transport System: The electron transport system consists of cytochromes, which facilitate the downhill movement of electrons along the redox potential scale. As the electrons move through the transport chain, they provide energy for the synthesis of ATP through chemiosmosis but are not consumed in the process.
3. Transfer to Photosystem I (PS I): The electrons continue along the transport chain and are eventually passed to the pigments of photosystem I. In PS I, the reaction centre chlorophyll a absorbs light at a wavelength of 700 nm, exciting the electrons once again.
4. Final Electron Acceptor and NADP+ Reduction: The excited electrons from PS I are then transferred to a different acceptor molecule with a higher redox potential. These electrons, now possessing significant energy, are further transferred to NADP+ to form NADPH + H+ through reduction. NADPH is a crucial energy carrier used in the subsequent dark reactions of photosynthesis.

The Z scheme illustrates the stepwise movement of electrons from PS II to PS I and finally to NADP+ reduction, creating a characteristic shape on the redox potential scale. This process not only generates ATP but also produces NADPH, both of which are essential for the synthesis of organic compounds during the subsequent dark reactions.

Splitting of Water in Photosynthesis

In photosynthesis, the continuous supply of electrons to replace those removed from photosystem II (PS II) is achieved through the splitting of water. This water-splitting process is associated with PS II, and it results in the production of oxygen, which is one of the net products of photosynthesis.

The overall reaction for the splitting of water is as follows: 2H2O → 4H+ + 4e- + O2

This reaction represents the conversion of two water molecules into four protons (H+ ions), four electrons (e-), and molecular oxygen (O2).

Now, the question arises regarding where the protons (H+) and O2 produced in this water-splitting process are likely to be released:

• Protons (H+): The protons released during the water-splitting process are likely to be released into the thylakoid lumen. The thylakoid lumen is the inner space of the thylakoid membrane, and it is the site where the proton gradient is established, which is later used to generate ATP during chemiosmosis.
• Oxygen (O2): Molecular oxygen (O2), which is a product of water splitting, is likely to be released on the outer side of the thylakoid membrane, where it can diffuse out of the chloroplast and eventually be released into the atmosphere as a waste product of photosynthesis.

So, the protons are released into the thylakoid lumen, and oxygen is released on the outer side of the thylakoid membrane. This separation of products is important for the proper functioning of the electron transport chain and the generation of chemical energy in the form of ATP during the light reactions of photosynthesis.

Cyclic and Non-cyclic Photophosphorylation in Photosynthesis

Photophosphorylation is the process by which ATP is synthesized from ADP and inorganic phosphate in the presence of light during photosynthesis. Two main mechanisms of photophosphorylation in photosynthesis are cyclic and non-cyclic photophosphorylation:

1. Non-cyclic Photophosphorylation:
   • In non-cyclic photophosphorylation, both photosystem II (PS II) and photosystem I (PS I) work in a series, with PS II functioning first and then passing the excited electrons to PS I.
   • The two photosystems are connected through an electron transport chain, as illustrated in the Z scheme.
   • During this process, both ATP and NADPH + H+ are synthesized as a result of the electron flow.
   • This type of photophosphorylation is crucial for the production of both energy (ATP) and reducing power (NADPH) required for the dark reactions (Calvin cycle) of photosynthesis, which involve carbon fixation and the synthesis of glucose and other organic compounds.
2. Cyclic Photophosphorylation:
   • In cyclic photophosphorylation, only photosystem I (PS I) is functional, and the electron circulates within this photosystem.
   • The phosphorylation occurs due to the cyclic flow of electrons, cycling back to the PS I complex through the electron transport chain.
   • Cyclic photophosphorylation results in the synthesis of ATP but not NADPH + H+.
   • This mechanism is useful when the plant primarily needs additional ATP and does not require NADPH.
   • Cyclic photophosphorylation can also occur when only light of wavelengths beyond 680 nm is available for excitation, as PS II functions optimally with shorter-wavelength light.

Location of Cyclic Photophosphorylation: Cyclic photophosphorylation typically occurs in the stroma lamellae (also known as the “intergranal lamellae”). These are the regions within the chloroplast where the membrane lacks PS II and the NADP reductase enzyme. As a result, the excited electrons do not pass on to NADP+ but are cycled back to the PS I complex, resulting in the synthesis of ATP.

Chemiosmotic Hypothesis in Photosynthesis

The chemiosmotic hypothesis provides an explanation for how ATP is synthesized in the chloroplast during photosynthesis. This hypothesis is based on the concept of a proton gradient across a membrane, similar to the mechanism of ATP synthesis in cellular respiration. However, in photosynthesis, the proton gradient is established across the thylakoid membrane. The following points outline the key steps of the chemiosmotic hypothesis:

1. Proton Accumulation in the Thylakoid Lumen:
   • The first step involves the splitting of water molecules within the thylakoid membranes. This process results in the release of protons (H+) into the thylakoid lumen.
2. Proton Transport During Electron Transfer:
   • As electrons move through the photosystems (PS II and PS I), protons are transported across the thylakoid membrane.
   • The primary electron acceptor of electrons, located on the outer side of the membrane, transfers its electron to an H carrier. This carrier simultaneously removes a proton from the stroma while transporting an electron. When this carrier passes on its electron to an electron carrier on the inner side of the membrane, the proton is released into the thylakoid lumen.
3. Reduction of NADP+ to NADPH+ H+:
   • The NADP reductase enzyme, located on the stroma side of the membrane, uses electrons from the acceptor of electrons of PS I and protons from the stroma to reduce NADP+ to NADPH + H+.
   • As a result, protons are removed from the stroma, causing a decrease in the number of protons in the stroma and an accumulation of protons in the thylakoid lumen. This creates a proton gradient across the thylakoid membrane and a decrease in pH in the lumen.

The proton gradient established across the thylakoid membrane is essential because it is the breakdown of this gradient that leads to the synthesis of ATP. The breakdown occurs as protons move across the membrane to the stroma through the transmembrane channel of the CF0 portion of the ATP synthase enzyme.

The ATP synthase enzyme has two parts: CF0, embedded in the thylakoid membrane, and CF1, protruding on the outer surface of the thylakoid membrane facing the stroma. The energy released during the diffusion of protons back across the membrane activates the ATP synthase enzyme, causing a conformational change in the CF1 particle, which leads to the synthesis of several molecules of ATP.

This chemiosmotic process, known as chemiosmosis, is crucial for providing the energy-rich ATP needed for the biosynthetic reactions in the stroma, which include the fixation of carbon dioxide (CO2) and the synthesis of sugars. Along with NADPH, ATP plays a vital role in these biosynthetic reactions, ultimately resulting in the formation of glucose and other organic compounds.

Use of ATP and NADPH in Photosynthesis

ATP and NADPH, the products of the light reactions in photosynthesis, play a vital role in the biosynthetic phase of photosynthesis, which involves the conversion of carbon dioxide (CO2) and water (H2O) into sugars and other organic compounds. The ATP and NADPH generated during the light reactions are used in the following ways during the biosynthetic phase:

1. Carbon Fixation:
   • ATP and NADPH provide the energy and reducing power required for the fixation of carbon dioxide. In this process, CO2 is combined with water to produce sugars, more specifically, (CH2O)n or sugars with a variable number of carbon atoms.
2. Calvin Cycle:
   • The biosynthetic phase involves the Calvin cycle, named after Melvin Calvin, who used radioactive carbon-14 (14C) in algal photosynthesis studies to discover the first product of CO2 fixation.
   • The first stable product identified in the Calvin cycle is 3-phosphoglyceric acid (PGA), a 3-carbon organic acid. PGA is a key intermediate in the carbon fixation process.
   • The Calvin cycle is a series of enzyme-catalyzed reactions that convert CO2 into organic compounds like sugars. It involves several steps, and ATP and NADPH are used to drive these reactions.
3. Carbon Assimilation:
   • Carbon assimilation during photosynthesis can follow one of two main pathways:
     • C3 Pathway: In some plants, the first stable product of CO2 fixation is a 3-carbon acid, PGA. These plants are referred to as C3 plants. The C3 pathway includes plants like rice, wheat, and most trees.
     • C4 Pathway: In other plants, the first stable product of CO2 fixation is a 4-carbon acid, oxaloacetic acid (OAA). These plants are called C4 plants and include species like corn (maize), sugarcane, and many grasses.

Discussion on the Term “Dark Reaction”: The term “dark reaction” has been used historically to describe the biosynthetic phase of photosynthesis because it does not directly depend on the presence of light. However, it is more accurate to call it the “biosynthetic phase” or “Calvin cycle” to avoid any confusion related to the term “dark reaction.”

The reason for this is that even though the biosynthetic phase can proceed in the absence of light, it relies on the products of the light reactions (ATP and NADPH) as essential energy and reducing power sources. The term “dark reaction” may imply that it only occurs in the dark, which is not the case. As mentioned, if light is made available after the biosynthetic phase has started in the dark, the synthesis continues. Therefore, it is more appropriate to refer to it as the “biosynthetic phase” or “Calvin cycle” to accurately describe its characteristics and dependencies.

The Primary Acceptor of CO2

The primary acceptor molecule for CO2 in the Calvin cycle is a 5-carbon ketose sugar called ribulose-1,5-bisphosphate (RuBP). This discovery was quite unexpected and took scientists a significant amount of time and experimentation to confirm.

Scientists initially believed that the primary acceptor for CO2 would be a 2-carbon compound, given that the first stable product of CO2 fixation was a 3-carbon compound (PGA). As a result, they focused their efforts on identifying a 2-carbon compound. It wasn’t until extensive research and experiments that they realized that the actual primary acceptor was the 5-carbon RuBP.

The discovery of RuBP as the primary acceptor was a crucial step in understanding the details of the Calvin cycle, which plays a central role in carbon fixation during photosynthesis.

The Calvin Cycle

The Calvin cycle, also known as the Calvin-Benson cycle, is the biosynthetic phase of photosynthesis and takes place in all photosynthetic plants. It is the pathway through which carbon dioxide (CO2) is fixed into stable organic intermediates, ultimately leading to the synthesis of glucose and other organic compounds. The Calvin cycle can be broken down into three main stages: carboxylation, reduction, and regeneration.

1. Carboxylation:

• Carboxylation is the process of fixing CO2 into a stable organic intermediate. This crucial step is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). RuBisCO facilitates the carboxylation of ribulose-1,5-bisphosphate (RuBP), a 5-carbon sugar, resulting in the formation of two molecules of 3-phosphoglyceric acid (3-PGA). RuBisCO not only catalyzes carboxylation but also oxygenation, although carboxylation is its primary function.

2. Reduction:

• The reduction stage involves a series of reactions that lead to the formation of glucose or other sugars. This phase requires the utilization of ATP and NADPH to convert the 3-PGA molecules into molecules of a higher energy state, such as glyceraldehyde-3-phosphate (G3P). For each CO2 molecule fixed, two molecules of ATP and two molecules of NADPH are used.

3. Regeneration:

• In the regeneration stage, the CO2 acceptor molecule, RuBP, is regenerated to ensure the cycle can continue. This process involves the phosphorylation of RuBP using one molecule of ATP, thus making RuBP ready to initiate another round of carboxylation.

In summary, for each CO2 molecule entering the Calvin cycle, the following is required:

• 3 molecules of ATP for phosphorylation
• 2 molecules of NADPH for reduction
• 1 molecule of ATP for RuBP regeneration

To produce one molecule of glucose through the Calvin pathway, six turns of the cycle are required. This results in the utilization of 18 ATP molecules, 12 NADPH molecules, and the production of one molecule of glucose.

Here’s a summary of what goes into and comes out of the Calvin cycle for the production of one molecule of glucose:

• In:
  • Six CO2 molecules
  • 18 ATP molecules
  • 12 NADPH molecules
• Out:
  • One molecule of glucose
  • 18 ADP molecules
  • 12 NADP molecules

The Calvin cycle is essential for the synthesis of organic compounds, particularly sugars, and it is a central process in the overall photosynthesis pathway.

C4 Pathway

C4 plants are a group of photosynthetic plants that have evolved a unique carbon fixation pathway, distinct from the standard C3 pathway used by most plants. They exhibit several adaptations that make them well-suited for dry, hot environments and reduce photorespiration. The key difference between C3 and C4 plants is the presence of the C4 pathway for carbon fixation in C4 plants.

Key Characteristics and Adaptations of C4 Plants

1. Kranz Anatomy: C4 plants exhibit a distinct leaf anatomy known as “Kranz” anatomy. In this arrangement, the mesophyll cells and bundle sheath cells around vascular bundles are specialized differently from those in C3 plants.
2. Bundle Sheath Cells: Bundle sheath cells are a unique feature of C4 plants. These cells surround the vascular bundles and have a high number of chloroplasts. They play a crucial role in the C4 pathway.
3. PEP Carboxylase: C4 plants have an enzyme called phosphoenolpyruvate carboxylase (PEPcase) in their mesophyll cells. This enzyme is responsible for the initial fixation of CO2 into a 4-carbon compound, oxaloacetic acid (OAA).
4. C4 Acid Formation: In the mesophyll cells, CO2 is initially fixed as OAA. OAA is then converted into other 4-carbon compounds like malic acid or aspartic acid, which are subsequently transported to the bundle sheath cells.
5. CO2 Release in Bundle Sheath Cells: In bundle sheath cells, C4 acids are broken down to release CO2 and a 3-carbon compound.
6. Calvin Pathway: The CO2 released in the bundle sheath cells enters the Calvin pathway, the same pathway used by C3 plants, for the synthesis of sugars.
7. Reduced Photorespiration: C4 plants exhibit a lower rate of photorespiration because CO2 is initially concentrated in the bundle sheath cells, reducing the chances of oxygenation by RuBisCO. This is advantageous, especially in high-temperature and high-light conditions.
8. Tolerance to Heat and Drought: C4 plants are better adapted to high-temperature and drought conditions due to their reduced water loss and enhanced carbon fixation efficiency.
9. High Biomass Productivity: C4 plants are known for their high biomass productivity, making them suitable for agricultural crops like maize, sugarcane, and sorghum.

Comparison of C3 and C4 Plants

Here’s a comparison of C3 and C4 plants based on various characteristics:

C3 plants primarily use the Calvin cycle for photosynthesis and are common in a wide range of plant species. They are, however, more susceptible to photorespiration, which reduces photosynthetic efficiency, especially in hot and arid conditions.

C4 plants employ the Hatch and Slack pathway for photosynthesis, which is more efficient under high-temperature and high-light conditions. They have specialized leaf anatomy, including bundle sheath cells that concentrate CO2, reducing photorespiration. C4 plants are well-suited for arid regions and are known for their high biomass productivity, making them important in agriculture.

Factors Affecting Photosynthesis

Photosynthesis is a complex process influenced by various internal and external factors. Here’s a brief overview of these factors:

Internal Factors:

1. Number, Size, Age, and Orientation of Leaves: The number and size of leaves on a plant, their age, and their orientation relative to the sun all affect the plant’s overall photosynthetic capacity. Young leaves are often more efficient at photosynthesis.
2. Mesophyll Cells and Chloroplasts: The number and health of mesophyll cells (where chloroplasts are located) within leaves can influence photosynthetic efficiency.
3. Internal CO2 Concentration: The availability of carbon dioxide within the leaf can significantly impact the rate of photosynthesis.
4. Amount of Chlorophyll: The presence and quantity of chlorophyll pigments within chloroplasts are crucial for capturing light energy.

External Factors:

1. Light Availability: Light is a key external factor that influences photosynthesis. An adequate amount of light is essential for photosynthesis to occur.
2. Temperature: Temperature affects the rate of photosynthesis. Photosynthesis generally increases with temperature up to a certain point, beyond which it can be inhibited. Different plants have different temperature optima for photosynthesis.
3. CO2 Concentration: Carbon dioxide is a reactant in photosynthesis, and its concentration can affect the rate of the process. Higher CO2 levels usually lead to increased photosynthesis.
4. Water Availability: Adequate water supply is necessary for the opening of stomata, which allows for the uptake of CO2 and the release of oxygen during photosynthesis.
5. Nutrient Availability: Essential nutrients such as nitrogen, phosphorus, and potassium, as well as micronutrients, can influence plant growth and photosynthesis.
6. Humidity: Relative humidity can affect the rate of water loss from plant leaves, influencing stomatal conductance and photosynthesis.
7. Oxygen Concentration: High oxygen levels can inhibit photosynthesis due to competition with CO2 for the active site of the RuBisCO enzyme.

The Law of Limiting Factors, formulated by Blackman, explains that the rate of photosynthesis is primarily limited by the factor that is nearest to its minimal value. When several factors simultaneously affect photosynthesis, the one that is the most suboptimal will determine the overall rate. For example, even with optimal light and CO2, photosynthesis may be limited by low temperatures.

Understanding these factors is crucial for optimizing agricultural practices, enhancing crop yields, and studying the responses of plants to changing environmental conditions.

1. Light

Light, a critical factor in photosynthesis, can be discussed in terms of light quality, light intensity, and the duration of light exposure. Here’s an explanation of these aspects:

1. Light Quality: Light quality refers to the color or wavelength of light. Chlorophyll, the primary pigment involved in photosynthesis, absorbs light most efficiently in the blue and red regions of the spectrum. Light quality affects the rate of photosynthesis because it determines the type of light energy available for absorption. Plants have adapted to efficiently utilize the available light spectrum.
2. Light Intensity: Light intensity refers to the amount or brightness of light. It has a direct impact on the rate of photosynthesis. At low light intensities, there is a linear relationship between incident light and CO2 fixation rates. As light intensity increases, photosynthesis rates also increase. However, there’s a point of saturation where further increases in light intensity do not lead to higher photosynthesis rates. Other factors, such as the availability of CO2 or enzymes, may become limiting. Light intensity is rarely a limiting factor in natural conditions because it saturates at relatively low light levels.
3. Duration of Light Exposure: The duration of exposure to light influences the total amount of photosynthesis that can occur in a day. Longer periods of light exposure allow for more photosynthesis to take place, provided that other factors like CO2 and temperature are optimal.

It’s interesting to note that light saturation, where photosynthesis reaches its maximum rate, occurs at low light intensities, typically around 10 percent of full sunlight. Beyond this point, additional light can lead to the breakdown of chlorophyll and a decrease in photosynthesis, which can be detrimental to plant health.

2. Carbon dioxide Concentration

Carbon dioxide (CO2) concentration is a significant factor that influences photosynthesis. Here’s an explanation of how CO2 concentration affects photosynthesis, particularly in C3 and C4 plants:

1. CO2 as a Limiting Factor: CO2 is often the major limiting factor for photosynthesis. The natural concentration of CO2 in the atmosphere is relatively low, typically between 0.03% and 0.04%. When CO2 concentration is low, it can restrict the rate of photosynthesis.
2. Effect of Increased CO2 Concentration: An increase in CO2 concentration, up to a certain point, can lead to higher rates of CO2 fixation in photosynthesis. This is because the primary enzyme responsible for CO2 fixation, RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), can more efficiently bind to CO2 when its concentration is higher. The initial response to increased CO2 levels results in increased photosynthesis rates.
3. Response of C3 and C4 Plants: C3 and C4 plants respond differently to changes in CO2 concentration.
   • In C3 plants, such as wheat and rice, increasing CO2 concentration generally leads to increased photosynthesis rates. These plants respond positively to elevated CO2 levels, and saturation of photosynthesis occurs at higher CO2 concentrations, typically beyond 450 µlL-1. As a result, current atmospheric CO2 levels are limiting to the photosynthesis of C3 plants.
   • C4 plants, like maize and sugarcane, show a different pattern. They respond positively to increased CO2 concentration, with saturation occurring at lower levels, typically around 360 µlL-1. However, they are generally less responsive to CO2 than C3 plants.
4. Practical Applications: The ability of C3 plants to respond to higher CO2 concentrations by increasing photosynthesis has practical applications. In some controlled environments like greenhouses, crops such as tomatoes and bell peppers are allowed to grow in a carbon dioxide-enriched atmosphere, leading to higher yields. This demonstrates the potential for manipulating CO2 levels to improve crop productivity.

3. Temperature

Temperature is a significant factor that influences photosynthesis, and it affects both the light and dark reactions of photosynthesis. Here’s how temperature impacts photosynthesis:

1. Effect on Enzymatic Reactions: Photosynthesis involves both enzymatic and non-enzymatic reactions. The dark reactions, which include the Calvin cycle, are enzymatic and highly temperature-sensitive. These enzymatic reactions are controlled by temperature, with the rate of the Calvin cycle generally increasing with temperature.
2. Less Impact on Light Reactions: In contrast, the light reactions, such as the electron transport chain and the formation of ATP and NADPH, are also temperature-sensitive but to a much lesser extent. These reactions do occur at varying temperatures, but their impact on the overall rate of photosynthesis is not as pronounced as the dark reactions.
3. Temperature Optimum: Different plants have varying temperature optima for photosynthesis. The temperature at which photosynthesis is most efficient can vary depending on the type of plant and its adaptation to its environment. C4 plants tend to respond positively to higher temperatures and show a higher rate of photosynthesis, while C3 plants have a lower temperature optimum.
4. Adaptation to Habitat: The temperature optimum for photosynthesis also relates to the habitat and climate to which the plants are adapted. Tropical plants, which are adapted to higher temperatures, tend to have a higher temperature optimum for photosynthesis. Conversely, plants adapted to temperate climates have a lower temperature optimum.
5. Impact of Extreme Temperatures: Extreme temperatures, whether very high or very low, can negatively impact photosynthesis. At extremely high temperatures, enzymes can become denatured, disrupting the photosynthetic process. Cold temperatures can slow down the rate of photosynthesis, primarily by reducing the activity of enzymes.

4. Water

Water plays an important role in photosynthesis, not only as one of the reactants but also due to its indirect effects on the plant and the photosynthetic process. Here’s how water impacts photosynthesis:

1. Direct Role as a Reactant: Water is one of the key reactants in the light-dependent reactions of photosynthesis. During the light reactions, water molecules are split, and the oxygen is released into the atmosphere while protons and electrons are used to generate ATP and NADPH, which are crucial for the subsequent dark reactions.
2. Indirect Effects on the Plant: Water availability can have significant indirect effects on photosynthesis. Water stress, which occurs when a plant experiences a shortage of water, can lead to several consequences that affect photosynthesis:
   • Stomatal Closure: Water stress often causes the stomata (small openings on the surface of leaves) to close. Stomata are responsible for the exchange of gases, including the uptake of carbon dioxide (CO2) for photosynthesis. When stomata close due to water stress, CO2 uptake is reduced, limiting the rate of photosynthesis.
   • Reduction in Leaf Surface Area: Water stress can cause leaves to wilt and reduce their surface area. Smaller leaf surfaces mean less area for light absorption and photosynthesis. Wilting also impacts the metabolic activity of the plant.
   • Disruption of Transport: Water is essential for the transport of nutrients and photosynthetic products within the plant. Water stress can disrupt these transport processes, affecting the availability of resources required for photosynthesis.
3. Affects of Water Scarcity: While water is essential for photosynthesis, excessive water can also be detrimental. Overly wet or waterlogged conditions can limit the oxygen supply to plant roots, impairing root respiration and, consequently, photosynthesis.