Photosynthesis is the physico-chemical process by which green plants use light energy to synthesize organic compounds (food) from carbon dioxide and water. This makes green plants autotrophs, forming the base of most food chains on Earth, as all other organisms are directly or indirectly dependent on them for food.

Photosynthesis is essential for two main reasons:

1. It is the primary source of food for virtually all life forms on Earth.
2. It is responsible for the release of oxygen into the atmosphere, which is necessary for respiration by most living organisms.

This chapter explores the photosynthetic machinery and the processes involved in converting light energy into chemical energy in higher plants.

What Do We Know?

Based on simple experiments, we already know that several components are essential for photosynthesis:

• Chlorophyll: The green pigment found in leaves. Experiments with variegated leaves (parts are green, parts are non-green) show starch (a product of photosynthesis) is formed only in the green areas exposed to light.
• Light: Essential for the process. Demonstrated by covering part of a leaf with black paper and exposing the plant to light; starch forms only in the exposed part.
• CO$_2$: Carbon dioxide is required as a raw material. An experiment placing part of a leaf in a tube with KOH (which absorbs CO$_2$) while the rest is exposed to air shows starch formation only in the part exposed to air.

Early Experiments

Our understanding of photosynthesis has developed gradually through simple but significant experiments:

• Joseph Priestley (1770): Showed the essential role of air. Observed that a burning candle or a mouse in a closed jar "damages" the air (consumes something essential). But when a mint plant was placed in the same jar, the mouse survived, and the candle could continue burning. He hypothesized that plants "restore" to the air whatever breathing animals and burning candles "remove". He discovered oxygen in 1774.
• Jan Ingenhousz (1730-1799): Using a similar setup to Priestley's, he demonstrated that sunlight is essential for the plant to restore the air. In an experiment with an aquatic plant (*Hydrilla*), he showed that small bubbles (later identified as oxygen) were formed only around the green parts in bright sunlight, not in the dark. This confirmed that only the green parts of plants release oxygen.
• Julius von Sachs (1854): Provided evidence for the production of glucose during plant growth. He showed that glucose is usually stored as starch in green parts of plants and that the green substance (chlorophyll) is located in special bodies (chloroplasts) within plant cells.
• T.W. Engelmann (1843-1909): Performed an elegant experiment using a prism to split light into its spectrum. He illuminated a green alga (*Cladophora*) placed in a suspension of aerobic bacteria (used to detect oxygen). The bacteria accumulated mainly in the regions of blue and red light, indicating maximum oxygen evolution and thus maximum photosynthesis occurred in these parts of the spectrum. This experiment provided the first action spectrum of photosynthesis.

By the mid-19th century, the basic understanding was that plants use light energy to convert CO$_2$ and water into carbohydrates, releasing oxygen. The initial empirical equation was: CO$_2$ + H$_2$O $\xrightarrow{\textsf{Light}}$ [CH$_2$O] + O$_2$.

• Cornelius van Niel (1897-1985): Based on studies of purple and green sulfur bacteria (which use H$_2$S as a hydrogen donor and produce sulfur, not oxygen), he proposed that photosynthesis is a light-dependent process where hydrogen from a suitable oxidizable compound (H$_2$A) reduces CO$_2$ to carbohydrates. His general equation was: 2H$_2$A + CO$_2$ $\xrightarrow{\textsf{Light}}$ 2A + [CH$_2$O] + H$_2$O.

He inferred that in green plants, H$_2$O serves as the hydrogen donor (H$_2$A) and is oxidized to oxygen (A). This hypothesis that oxygen comes from water was later confirmed using radioisotopic techniques.

The correct overall equation for photosynthesis in oxygen-evolving organisms is: 6CO$_2$ + 12H$_2$O $\xrightarrow{\textsf{Light}}$ C$_6$H$_{12}$O$_6$ + 6H$_2$O + 6O$_2$.

Here, C$_6$H$_{12}$O$_6$ represents glucose. The 12 molecules of water on the reactant side are used, and 6 molecules of water appear on the product side because water is both consumed and produced in the overall complex process.

Where Does Photosynthesis Take Place?

Photosynthesis primarily occurs in the green parts of plants, most notably in the leaves, specifically within the chloroplasts located in the mesophyll cells.

Other green parts of the plant, such as green stems or young branches, can also perform photosynthesis.

Chloroplasts in mesophyll cells are typically numerous and align themselves along the cell walls to maximize light absorption. They align with their flat surfaces parallel to the walls when light intensity is high to avoid photodamage, and perpendicular to the incident light when light intensity is low to maximize light capture.

Structure of Chloroplast (Figure 13.2):

Within the chloroplast, there is a division of labor:

• Membrane system (Grana, Stroma lamellae): Responsible for the light reactions (photochemical phase). These reactions directly depend on light energy for absorption, water splitting, oxygen release, and synthesis of energy-carrying molecules (ATP and NADPH).
• Stroma (Matrix): The fluid-filled space within the chloroplast, outside the membrane system. This is where the dark reactions (carbon reactions / biosynthetic phase) take place. These reactions are not directly driven by light but depend on the products of the light reaction (ATP and NADPH) to synthesize sugars from CO$_2$ and water using enzymatic reactions. It's important to note that 'dark reaction' is a misnomer, as these reactions are dependent on ATP and NADPH produced during the light reaction, and cannot occur indefinitely in total darkness.

How Many Types Of Pigments Are Involved In Photosynthesis?

The green color of leaves is not due to a single pigment but a mixture of pigments. These pigments can be separated using techniques like paper chromatography.

Chromatographic separation reveals four main pigments in green leaves:

1. Chlorophyll a: Appears bright or blue-green. It is the chief pigment directly involved in the light reaction.
2. Chlorophyll b: Appears yellow-green.
3. Xanthophylls: Appear yellow.
4. Carotenoids: Appear yellow to yellow-orange.

Pigments are molecules that absorb light at specific wavelengths.

Chlorophyll a shows maximum absorption in the blue and red regions of the visible spectrum (Figure 13.3 a). The action spectrum of photosynthesis (rate of photosynthesis at different wavelengths, Figure 13.3 b) roughly matches the absorption spectrum of chlorophyll a, with peaks in the blue and red regions, confirming chlorophyll a's primary role.

While chlorophyll a is the main light-trapping pigment, chlorophyll b, xanthophylls, and carotenoids are called accessory pigments. They absorb light at different wavelengths than chlorophyll a and transfer the energy to chlorophyll a. This expands the range of light wavelengths that can be used for photosynthesis. Accessory pigments also have a protective role; they protect chlorophyll a from photo-oxidation (damage by excessive light energy).

What Is Light Reaction?

The Light Reaction (Photochemical phase) is the initial stage of photosynthesis, occurring in the thylakoid membranes of the chloroplast. Key processes include:

• Light absorption by photosynthetic pigments.
• Water splitting (photolysis of water).
• Oxygen release as a byproduct.
• Formation of high-energy chemical intermediates: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate, reduced form).

Pigments are organized into two distinct Photosystems (PS) within the thylakoid membrane: Photosystem I (PS I) and Photosystem II (PS II).

Each photosystem consists of:

• A Light Harvesting Complex (LHC) or Antennae: Composed of hundreds of accessory pigment molecules (chlorophyll b, carotenoids, xanthophylls) bound to proteins. These pigments absorb light energy at various wavelengths and transfer it to the reaction center.
• A Reaction Centre: A single molecule of chlorophyll a. This molecule receives energy from the antenna pigments and undergoes oxidation, releasing an electron. The reaction center chlorophyll a is different in the two photosystems:
  • In PS I, the reaction center chlorophyll a absorbs light optimally at 700 nm and is called P700.
  • In PS II, the reaction center chlorophyll a absorbs light optimally at 680 nm and is called P680.

The Electron Transport

The movement of electrons during the light reaction involves electron transport chains associated with PS I and PS II (Figure 13.5).

Mechanism of Electron Flow (Non-cyclic photophosphorylation or Z-scheme):

1. Light energy absorbed by PS II (P680) excites an electron to a higher energy level.
2. The excited electron is captured by a primary electron acceptor molecule.
3. The electron is then transported downhill through an electron transport system (ETS) composed of cytochrome complexes. Energy is released during this downhill movement.
4. As electrons move through the ETS, protons are pumped across the thylakoid membrane, contributing to a proton gradient (essential for ATP synthesis).
5. The electrons are passed from the ETS to the reaction center of PS I (P700).
6. Simultaneously, light energy absorbed by PS I (P700) excites an electron to a higher energy level.
7. The excited electron from PS I is captured by another primary electron acceptor molecule (different from PS II acceptor).
8. This electron is then transported downhill again, this time to the molecule NADP$^+$.
9. NADP$^+$ accepts the electron and also picks up a proton (H$^+$) from the stroma, reducing it to NADPH + H$^+$.

The overall pathway of electron transfer, from PS II to PS I and finally to NADP$^+$, resembles a 'Z' shape when the electron carriers are arranged on a redox potential scale. This is called the Z-scheme.

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Splitting Of Water

To ensure a continuous supply of electrons to PS II, water molecules are split through a process called photolysis of water or water splitting. This process is associated with PS II and occurs on the inner side of the thylakoid membrane (in the lumen).

The reaction is: 2H$_2$O $\rightarrow$ 4H$^+$ + O$_2$ + 4e$^-$

The electrons released from water replace the electrons lost by P680 of PS II. The protons (H$^+$) accumulate in the thylakoid lumen. Oxygen (O$_2$) is released as a byproduct of photosynthesis.

The electrons passed from PS II through the ETS are ultimately used to replace the electrons lost by P700 of PS I.

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Cyclic And Non-Cyclic Photo-Phosphorylation

Phosphorylation is the synthesis of ATP from ADP and inorganic phosphate (Pi).

Photo-phosphorylation is the synthesis of ATP that occurs in the presence of light (in chloroplasts and mitochondria). In photosynthesis, it's specifically the light-dependent synthesis of ATP.

Based on the electron flow, photo-phosphorylation can be non-cyclic or cyclic:

• Non-cyclic photo-phosphorylation: Involves the combined action of both PS II and PS I, operating in series (as described in the Z-scheme). Electrons flow from water to NADP$^+$. This process generates both ATP and NADPH + H$^+$ (Figure 13.5).
• Cyclic photo-phosphorylation: Occurs when only PS I is functional (Figure 13.6). The electron excited from P700 is captured by the primary acceptor but is then cycled back to the PS I complex through the electron transport chain, instead of being passed to NADP$^+$. This flow of electrons is cyclic. This process takes place in the stroma lamellae, which lack PS II and NADP reductase enzyme. Cyclic photo-phosphorylation results only in the synthesis of ATP, but not NADPH + H$^+$ or oxygen. It occurs when light of wavelengths beyond 680 nm is available.

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Chemiosmotic Hypothesis

The chemiosmotic hypothesis explains how ATP is synthesized in chloroplasts (and mitochondria). ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane (Figure 13.7).

Mechanism of proton gradient development across the thylakoid membrane:

1. Water splitting: Occurs on the inner side (lumen) of the thylakoid membrane. This releases protons (H$^+$) directly into the thylakoid lumen, increasing the proton concentration inside.
2. Electron transport: As electrons move through the electron transport chain (particularly between PS II and PS I), protons are actively transported from the stroma to the lumen. Some electron carriers (like plastoquinone) pick up protons from the stroma while accepting electrons and release them into the lumen when passing electrons to the next carrier.
3. NADP$^+$ reduction: The NADP reductase enzyme is located on the stroma side of the thylakoid membrane. For NADP$^+$ to be reduced to NADPH + H$^+$, it requires electrons (from PS I) and protons. These protons are removed from the stroma.

These processes lead to a buildup of protons in the thylakoid lumen and a decrease in proton concentration in the stroma, creating a proton gradient and a pH difference (lower pH in the lumen, higher pH in the stroma) across the thylakoid membrane.

ATP synthesis:

• The proton gradient represents stored energy. The breakdown of this gradient drives ATP synthesis.
• Protons diffuse back from the lumen to the stroma through specific transmembrane channels in the ATP synthase enzyme.
• ATP synthase (also called CF$_0$-CF$_1$ complex) has two parts: CF$_0$ (embedded in the membrane, acts as a proton channel) and CF$_1$ (protrudes into the stroma).
• The energy released by the movement of protons through CF$_0$ causes conformational changes in the CF$_1$ part, activating the enzyme to catalyze the synthesis of ATP from ADP and Pi.

Chemiosmosis requires a membrane, a proton pump (to create the gradient), a proton gradient, and ATP synthase (to use the gradient energy for ATP synthesis).

The ATP and NADPH produced by the light reaction are released into the stroma and are immediately used in the biosynthetic phase (dark reaction) to fix CO$_2$ and synthesize sugars.

Where Are The Atp And Nadph Used?

The products of the light reaction are ATP, NADPH, and O$_2$. Oxygen diffuses out of the chloroplast. ATP and NADPH are utilized in the biosynthetic phase (dark reaction) of photosynthesis to convert CO$_2$ into sugars.

The biosynthetic phase does not directly require light but is dependent on the supply of ATP and NADPH from the light reaction, as well as CO$_2$ and H$_2$O. This dependency is evident because the biosynthetic process continues for a short time after light is removed but then stops if light is not restored to produce more ATP and NADPH.

Scientists investigated how CO$_2$ is incorporated or 'fixed' into organic compounds. Using radioactive carbon-14 ($^{14}$C) in algal photosynthesis studies, Melvin Calvin discovered that the first stable product of CO$_2$ fixation was a 3-carbon organic acid: 3-phosphoglyceric acid (PGA).

Calvin and his colleagues worked out the entire pathway for sugar synthesis, which was named the Calvin cycle after him. Since the first stable product is a 3-carbon compound, plants that primarily use this pathway are called C3 plants.

Further research revealed that in some other plants, the first stable product of CO$_2$ fixation is a 4-carbon organic acid, oxaloacetic acid (OAA). Plants using this pathway are called C4 plants.

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The Primary Acceptor Of Co2

Scientists initially thought the molecule accepting CO$_2$ in the Calvin cycle must be a 2-carbon compound since the first product (PGA) is a 3-carbon compound (CO$_2$ + C$_2$ $\rightarrow$ C$_3$). However, extensive experiments showed that the primary CO$_2$ acceptor is a 5-carbon ketose sugar called ribulose-1,5-bisphosphate (RuBP).

The reaction is: CO$_2$ + RuBP (C$_5$) $\rightarrow$ 2 molecules of 3-PGA (C$_3$).

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The Calvin Cycle

The Calvin cycle (also known as the C3 pathway) is the main biosynthetic pathway for sugar synthesis and occurs in all photosynthetic plants, regardless of whether they are C3 or C4 plants. The RuBP is regenerated throughout the cycle, allowing it to continue.

The Calvin cycle can be described in three stages (Figure 13.8):

1. Carboxylation: The crucial first step where CO$_2$ is fixed into a stable organic compound (PGA). CO$_2$ is added to RuBP, catalyzed by the enzyme RuBP carboxylase-oxygenase (RuBisCO). This reaction yields two molecules of 3-PGA.
2. Reduction: A series of steps converting 3-PGA into glucose (specifically, triose phosphate, which can be used to synthesize glucose and other sugars). This stage utilizes energy from the light reaction: 2 molecules of ATP are used for phosphorylation, and 2 molecules of NADPH are used for reduction for each molecule of CO$_2$ fixed.
3. Regeneration: Steps that regenerate the CO$_2$ acceptor molecule, RuBP, so the cycle can continue. This stage requires 1 molecule of ATP for phosphorylation for each molecule of CO$_2$ fixed.

Energy requirement per CO$_2$ molecule fixed in the Calvin cycle:

• ATP: 3 molecules (2 in reduction, 1 in regeneration)
• NADPH: 2 molecules (in reduction)

To produce one molecule of glucose (a 6-carbon sugar), the Calvin cycle needs to fix 6 molecules of CO$_2$. This requires 6 turns of the cycle.

Total energy required to make one glucose molecule (6 CO$_2$ fixed):

• ATP: $6 \times 3 = 18$ molecules
• NADPH: $6 \times 2 = 12$ molecules

The cyclic photophosphorylation might be responsible for producing the extra ATP needed (18 ATP vs 12 NADPH) compared to the equal ATP and NADPH produced in non-cyclic photophosphorylation.

The C4 Pathway

C4 plants are adapted to dry tropical regions and exhibit the C4 pathway for initial CO$_2$ fixation. While they fix CO$_2$ first into a 4-carbon acid (OAA), they still use the Calvin cycle (C3 pathway) for synthesizing sugars.

Differences between C3 and C4 plants:

1. Special leaf anatomy: C4 plants have Kranz anatomy ('wreath' anatomy). This involves large bundle sheath cells forming layers around vascular bundles. These bundle sheath cells are characterized by:
   • Large number of chloroplasts.
   • Thick walls impermeable to gaseous exchange.
   • No intercellular spaces.

   Mesophyll cells, located outside the bundle sheath, also contain chloroplasts but differ structurally and functionally from bundle sheath cells.

2. Tolerance to higher temperatures: C4 plants generally thrive at higher temperatures than C3 plants.
3. Response to high light intensities: Show higher rates of photosynthesis at high light compared to C3 plants.
4. Lack of photorespiration: Photorespiration is significantly reduced or absent in C4 plants, leading to higher productivity.
5. Greater biomass productivity: Due to higher efficiency and minimal photorespiration.

The C4 pathway, also called the Hatch and Slack Pathway, involves initial CO$_2$ fixation in mesophyll cells, followed by transport and decarboxylation in bundle sheath cells (Figure 13.9).

Steps in the C4 pathway:

1. Initial Carboxylation (in mesophyll cells):
   • Primary CO$_2$ acceptor: Phosphoenol pyruvate (PEP), a 3-carbon molecule, present in mesophyll cells.
   • Enzyme: PEP carboxylase (PEPcase), present in mesophyll cells but lacks RuBisCO.
   • CO$_2$ (from atmosphere) + PEP (C$_3$) $\xrightarrow{\textsf{PEPcase}}$ Oxaloacetic acid (OAA) (C$_4$), the first stable product.
2. Transport (from mesophyll to bundle sheath cells): OAA is converted into other 4-carbon compounds (like malic acid or aspartic acid) in the mesophyll cells. These C4 acids are then transported to the bundle sheath cells.
3. Decarboxylation and Calvin Cycle (in bundle sheath cells):
   • In bundle sheath cells, the C4 acids are broken down (decarboxylated) to release CO$_2$ and a 3-carbon molecule. Bundle sheath cells are rich in RuBisCO but lack PEPcase.
   • The released CO$_2$ enters the Calvin cycle (C3 pathway), which occurs in the bundle sheath cells of C4 plants. This is where sugars are ultimately synthesized.
   • The 3-carbon molecule is transported back to the mesophyll cells, where it is converted back to PEP, regenerating the primary CO$_2$ acceptor.

The Calvin cycle is common to both C3 and C4 plants, but in C4 plants, it takes place only in the bundle sheath cells, not in the mesophyll cells as in C3 plants.

Photorespiration

Photorespiration is a wasteful process that occurs in C3 plants, creating a significant difference from C4 plants.

Background: The enzyme RuBisCO, which catalyzes the first step of the Calvin cycle (carboxylation of RuBP), is unique because its active site can bind to both CO$_2$ and O$_2$. The binding of CO$_2$ and O$_2$ to RuBisCO is competitive, depending on their relative concentrations at the enzyme site.

Process in C3 plants:

• Under conditions of high oxygen concentration and low carbon dioxide concentration (e.g., when stomata close in hot, dry weather), O$_2$ can bind to RuBisCO instead of CO$_2$.
• When O$_2$ binds, RuBisCO acts as an oxygenase. It catalyzes the reaction of RuBP with O$_2$ to form one molecule of 3-phosphoglycerate (3PGA, a 3-carbon compound) and one molecule of phosphoglycolate (a 2-carbon compound).
• Phosphoglycolate is then metabolized through a pathway called photorespiration, involving chloroplasts, peroxisomes, and mitochondria.

Consequences of photorespiration in C3 plants:

• It results in the release of CO$_2$.
• It does not synthesize sugars.
• It utilizes ATP.
• It does not produce ATP or NADPH.

Photorespiration is a wasteful process that reduces the efficiency of photosynthesis in C3 plants, particularly under hot and dry conditions where O$_2$ levels are relatively high and CO$_2$ levels are low (due to stomatal closure).

Photorespiration in C4 plants:

• Photorespiration is significantly reduced or absent in C4 plants.
• This is due to the C4 pathway mechanism that increases the concentration of CO$_2$ at the site of RuBisCO activity (in the bundle sheath cells).
• The C4 acids transported from mesophyll cells are decarboxylated in the bundle sheath cells, releasing a high concentration of CO$_2$ right where RuBisCO is located.
• This high CO$_2$ concentration ensures that RuBisCO preferentially binds CO$_2$ (acting as a carboxylase) rather than O$_2$ (minimizing oxygenase activity and photorespiration).

The lack of photorespiration contributes significantly to the higher productivity and efficiency of C4 plants compared to C3 plants, especially in environments with high temperatures and high light intensity.

C4 plants also tolerate higher temperatures compared to C3 plants.

Summary of differences between C3 and C4 plants (Table 13.1):

Characteristics	C3 Plants	C4 Plants
Cell type in which the Calvin cycle takes place	Mesophyll	Bundle sheath
Cell type in which the initial carboxylation reaction occurs	Mesophyll	Mesophyll
How many cell types does the leaf have that fix CO2.	One: Mesophyll	Two: Bundle sheath and mesophyll (Initial fixation in mesophyll, re-fixation in bundle sheath)
Which is the primary CO2 acceptor	RuBP	PEP
Number of carbons in the primary CO2 acceptor	5	3
Which is the primary CO2 fixation product	PGA	OAA
No. of carbons in the primary CO2 fixation product	3	4
Does the plant have RuBisCO?	Yes	Yes
Does the plant have PEP Case?	No	Yes
Which cells in the plant have Rubisco?	Mesophyll	Bundle sheath
CO2 fixation rate under high light conditions	Low	High
Whether photorespiration is present at low light intensities	High	Negligible
Whether photorespiration is present at high light intensities	High	Negligible
Whether photorespiration would be present at low CO2 concentrations	High	Negligible
Whether photorespiration would be present at high CO2 concentrations	Negligible	Negligible
Temperature optimum	20-25$^\circ$C	30-40$^\circ$C
Examples	Most plants (wheat, rice, soybean)	Tropical grasses (maize, sorghum, sugarcane)

Factors Affecting Photosynthesis

The rate of photosynthesis determines plant productivity and yield. It is influenced by both internal and external factors.

• Internal factors: Plant-specific factors like the number, size, age, and orientation of leaves; the amount and health of mesophyll cells and chloroplasts; the internal concentration of CO$_2$; and the amount of chlorophyll. These factors depend on the plant's genetics and growth conditions.
• External factors: Environmental factors including the availability of sunlight, temperature, CO$_2$ concentration in the atmosphere, and water.

When multiple factors influence a process like photosynthesis, the rate is often limited by the factor that is present at the lowest or most suboptimal level. This concept is described by Blackman's Law of Limiting Factors (1905): If a process is affected by multiple factors, its rate is determined by the factor that is nearest to its minimum value, and changing this factor will directly affect the process rate.

Example: A plant in low temperature might not photosynthesize efficiently, even with optimal light and CO$_2$. Increasing the temperature (the limiting factor) would then increase the photosynthetic rate.

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Light

Light affects photosynthesis through its quality (wavelength), intensity, and duration of exposure (Figure 13.10).

• Low light intensity: There is a linear relationship between incident light intensity and the rate of CO$_2$ fixation; increasing light intensity increases the rate.
• Higher light intensity: As light intensity increases, the rate of photosynthesis eventually plateaus. Other factors (like CO$_2$ concentration or temperature) become limiting.
• Light saturation: Photosynthesis becomes light-saturated at relatively low light intensities, often around 10% of full sunlight.
• Excessively high light intensity: Can cause damage to chlorophyll (photo-oxidation) and reduce the photosynthetic rate.

Light is generally not a limiting factor in nature, except for plants growing in shade or dense forests.

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Carbon Dioxide Concentration

CO$_2$ is often the major limiting factor for photosynthesis, as its atmospheric concentration is very low (0.03-0.04%).

• Increasing CO$_2$ concentration up to about 0.05% can increase the rate of CO$_2$ fixation. Beyond this, high concentrations can be damaging over time.
• C3 and C4 plants respond differently to CO$_2$ concentration:
  • Low light conditions: Neither C3 nor C4 plants show increased photosynthetic rates with higher CO$_2$ concentration; light is the limiting factor.
  • High light conditions: Both C3 and C4 plants show increased photosynthetic rates with increasing CO$_2$.
  • C4 plants: Show CO$_2$ saturation at a lower concentration ($\sim 360 \textsf{ µlL}^{-1}$).
  • C3 plants: Continue to respond to increasing CO$_2$ concentration and saturate at a higher concentration (beyond $\sim 450 \textsf{ µlL}^{-1}$).

Current atmospheric CO$_2$ levels are limiting for C3 plants. This is exploited in greenhouses where increasing CO$_2$ concentration leads to higher yields of C3 crops like tomatoes and bell peppers.

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Temperature

Temperature affects photosynthesis because the dark reactions (carbon fixation) are enzymatic processes.

• While light reactions are also temperature sensitive, they are affected to a lesser extent.
• Each plant has an optimum temperature range for photosynthesis.
• C4 plants generally have a higher temperature optimum and show higher photosynthetic rates at higher temperatures compared to C3 plants, which have a lower optimum.
• The temperature optimum also depends on the plant's habitat adaptation (e.g., tropical plants have higher optimum than temperate plants).

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Water

Water is a reactant in the light reaction, but its effect on photosynthesis is more indirect, primarily through its impact on the plant's water status.

Water stress (lack of sufficient water) causes:

• Stomatal closure: This reduces CO$_2$ uptake, limiting the raw material for carbon fixation.
• Wilting of leaves: Reduces the surface area available for light absorption.
• Reduced metabolic activity: Overall decrease in physiological processes, including photosynthesis.

Water availability is a significant limiting factor for photosynthesis.

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