4.1 Photosynthesis in Higher Plants

Introduction to Photosynthesis

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process is the primary means of autotrophic nutrition on Earth, forming the base of most food chains.

Equation of Photosynthesis:

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

Site of Photosynthesis

Photosynthesis occurs in specialized organelles called chloroplasts, which are primarily found in the mesophyll cells of leaves.

Chloroplast Structure

• Outer and Inner Membranes: A double membrane encloses the chloroplast.
• Stroma: A dense fluid-filled space containing enzymes, ribosomes, and DNA. The biosynthetic phase (Calvin cycle) of photosynthesis occurs here.
• Thylakoids: A system of interconnected membranous sacs.
• Grana (singular: granum): Stacks of thylakoids. The photochemical phase of photosynthesis occurs in the thylakoid membranes.
• Lumen: The fluid-filled space inside a thylakoid.

Pigments Involved in Photosynthesis

Photosynthetic pigments are molecules that absorb light energy.

• Chlorophyll a: The primary photosynthetic pigment. It is bluish-green in color.
• Chlorophyll b: An accessory pigment that is yellowish-green in color. It absorbs light at different wavelengths than chlorophyll a and passes the energy to it.
• Carotenoids: Accessory pigments that are yellow, orange, or red. They include carotenes and xanthophylls. They protect the chlorophyll from photodamage.
• Xanthophylls: A type of carotenoid that is yellow in color.

Differences between Chlorophyll 'a' and 'b'

Absorption and Action Spectra

• Absorption Spectrum: A graph that shows the amount of light absorbed by a pigment at different wavelengths.
• Action Spectrum: A graph that shows the rate of photosynthesis at different wavelengths of light. The action spectrum of photosynthesis closely matches the absorption spectrum of chlorophylls, indicating that chlorophylls are the primary pigments involved in photosynthesis.

Phases of Photosynthesis

Photosynthesis is divided into two main phases:

1. Photochemical Phase (Light-Dependent Reactions): Occurs in the thylakoid membranes. It involves the absorption of light energy, splitting of water, release of oxygen, and formation of ATP and NADPH.
2. Biosynthetic Phase (Light-Independent Reactions/Calvin Cycle): Occurs in the stroma. It involves the fixation of carbon dioxide and the synthesis of glucose, using the ATP and NADPH produced during the photochemical phase.

Photochemical Phase

Pigment Systems (Photosystems)

Photosystems are functional and structural units of protein complexes involved in photosynthesis. They are located in the thylakoid membranes.

• Photosystem I (PS I): The reaction center is P700, which means it absorbs light of 700 nm wavelength most effectively. It is involved in both cyclic and non-cyclic photophosphorylation.
• Photosystem II (PS II): The reaction center is P680, which means it absorbs light of 680 nm wavelength most effectively. It is involved only in non-cyclic photophosphorylation.

Photophosphorylation

Photophosphorylation is the process of synthesizing ATP from ADP and inorganic phosphate using light energy.

Non-Cyclic Photophosphorylation

1. PS II absorbs light energy, and its reaction center P680 gets excited.
2. Electrons are released from P680 and are accepted by a primary electron acceptor.
3. The electrons pass through an electron transport chain (ETC) to PS I.
4. As electrons move down the ETC, their energy is used to pump protons into the thylakoid lumen, creating a proton gradient.
5. The splitting of water (photolysis) occurs, releasing electrons, protons (H+), and oxygen. The electrons replace those lost by P680.
6. PS I absorbs light energy, and its reaction center P700 gets excited.
7. Electrons are released from P700 and are accepted by a primary electron acceptor.
8. The electrons are passed to NADP+, which is reduced to NADPH.

Cyclic Photophosphorylation

1. Only PS I is involved.
2. The electrons released from P700 are cycled back to P700 through an ETC.
3. This process generates ATP but not NADPH.
4. It occurs when the cell has a high demand for ATP or when NADP+ is not available.

Chemiosmotic Hypothesis

The chemiosmotic hypothesis explains how ATP is synthesized during photosynthesis.

1. The pumping of protons into the thylakoid lumen during the electron transport chain creates a proton gradient (higher concentration of H+ in the lumen than in the stroma).
2. This proton gradient is a form of potential energy.
3. The protons flow back into the stroma through a channel in the ATP synthase enzyme.
4. The energy released by the flow of protons is used by ATP synthase to synthesize ATP from ADP and inorganic phosphate.

Biosynthetic Phase (Calvin Cycle)

The Calvin cycle is a series of biochemical reactions that take place in the stroma of chloroplasts. It is also known as the C3 cycle.

Steps of the Calvin Cycle

1. Carboxylation: Carbon dioxide combines with a five-carbon compound called ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon compound, which immediately splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). This reaction is catalyzed by the enzyme RuBisCO.
2. Reduction (Glycolytic Reversal): The 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P) in a two-step process that uses ATP and NADPH from the light-dependent reactions.
3. Regeneration: For every six molecules of G3P produced, one is used to make glucose and other organic molecules, while the other five are used to regenerate RuBP, which requires ATP.

Photorespiration

Photorespiration is a wasteful process that occurs in C3 plants when the CO₂ concentration is low and the O₂ concentration is high.

Pathway of Photorespiration

1. RuBisCO can act as an oxygenase, binding to O₂ instead of CO₂.
2. This produces one molecule of 3-PGA and one molecule of a two-carbon compound called phosphoglycolate.
3. The phosphoglycolate is converted to glycolate, which is then transported to the peroxisome and then to the mitochondrion, where it is broken down, releasing CO₂.

Significance of Photorespiration

Photorespiration is considered a wasteful process because it consumes ATP and releases CO₂ that has already been fixed. However, it may have a protective role against photo-oxidative damage.

RuBisCO as an Oxygenase

The active site of RuBisCO can bind to both CO₂ and O₂. The binding of which molecule is favored depends on the relative concentrations of CO₂ and O₂. When O₂ concentration is high, RuBisCO acts as an oxygenase.

C4 Pathway (Hatch and Slack Pathway)

The C4 pathway is an adaptation of some plants to hot, dry climates. It is a mechanism to concentrate CO₂ around RuBisCO, thus minimizing photorespiration.

Kranz Anatomy

C4 plants have a specialized leaf anatomy called Kranz anatomy. The vascular bundles are surrounded by large bundle sheath cells, which are in turn surrounded by mesophyll cells.

Steps of the C4 Pathway

1. In the mesophyll cells, CO₂ is fixed by the enzyme PEP carboxylase, which has a high affinity for CO₂. The product is a four-carbon compound called oxaloacetate, which is then converted to malate or aspartate.
2. The malate or aspartate is transported to the bundle sheath cells.
3. In the bundle sheath cells, the four-carbon compound is broken down, releasing CO₂.
4. The CO₂ is then fixed by RuBisCO in the Calvin cycle.

Differences

C3 vs. C4 Plants

C3 vs. C4 Cycles

Photosystem I vs. Photosystem II

Factors Affecting Photosynthesis

Blackman's Law of Limiting Factors

This law states that the rate of a physiological process is limited by the factor that is in the shortest supply. For example, if a plant has plenty of light and water, but not enough CO₂, the rate of photosynthesis will be limited by the CO₂ concentration.

Factors

• Light: The rate of photosynthesis increases with light intensity up to a certain point, after which it becomes constant.
• Carbon Dioxide: The rate of photosynthesis increases with CO₂ concentration up to a certain point, after which it becomes constant.
• Temperature: The rate of photosynthesis increases with temperature up to an optimum point, after which it decreases.
• Water: Water is essential for photosynthesis. A lack of water can cause the stomata to close, which reduces the intake of CO₂ and thus the rate of photosynthesis.