PHOTOSYNTHESIS

Photosynthesis is a crucial physiological process where chloroplasts in green plants synthesize sugars by using water (H2O) and carbon dioxide (CO2) in the presence of light. The word “photosynthesis” literally means synthesis with the help of light. During photosynthesis, plants convert light energy into chemical energy, which is stored in organic matter, primarily carbohydrates.

Photosynthesis Equation

The general equation for photosynthesis is represented as follows:

6CO₂ + 12H₂O → Light → C₆H₁₂O₆ +6O₂

This equation represents the process where carbon dioxide and water are used by the plant, in the presence of light and chlorophyll, to produce glucose (C6H12O6) and release oxygen (O2) as a by-product.

Energy Conversion in Photosynthesis

During photosynthesis, light energy is converted into chemical energy and stored in the form of organic matter, mainly carbohydrates. For example, one molecule of glucose contains about 686 K Calories of energy. CO2 and water are the raw materials for this process, while oxygen and glucose are the by-products.

• Stephen Hales (1727) first explained the relationship between sunlight and plant leaves.
• Sachs (1887) demonstrated that starch was the visible product of photosynthesis.

Photosynthetic Apparatus

The chloroplast in green plants constitutes the photosynthetic apparatus. In higher plants, the chloroplast is typically discoid in shape, measuring 4-6 µm in length and 1-2 µm in thickness. It is bound by two unit membranes, each approximately 50 Å thick, consisting of lipids and proteins.

Internally, the chloroplast is filled with a hydrophilic matrix called the stroma, which contains grana (stacks of disk-shaped structures called thylakoids). Each grana consists of 5-25 grana lamellae, which are stacked like coins. The thylakoid membrane consists of alternating layers of lipids and proteins, and the thylakoids are connected by stroma lamellae, or fret membranes.

Chlorophyll and other photosynthetic pigments are confined to the grana, and they are the sites of photochemical reactions.

Photosynthetic Pigments

There are three main types of photosynthetic pigments:

1. Chlorophylls (green pigments)
2. Carotenoids (yellow or orange pigments)
3. Phycobilins (red and blue pigments)

Chlorophylls

Chlorophylls are magnesium porphyrin compounds. They consist of a porphyrin ring structure with four pyrrole rings joined by CH bridges. A long chain of carbon atoms, called the phytol chain, is attached to the porphyrin ring. Chlorophyll a and chlorophyll b are the two major types:

• Chlorophyll a: C₅₅H₇₂O₅N₄Mg
• Chlorophyll b: C₅₅H₇₀O₆N₄Mg

Chlorophyll a and b differ slightly, as chlorophyll b has a -CHO group at the 3rd carbon atom in pyrrol ring II, while chlorophyll a has a CH3 group at this position.

Chlorophyll is formed from protochlorophyll in light. Protochlorophyll lacks two hydrogen atoms at the 7th and 8th carbon atoms in pyrrol ring IV.

Carotenoids

Carotenoids are divided into two categories:

1. Carotenes: Hydrocarbons with the molecular formula C₄₀H₅₆.
2. Xanthophylls (carotenols): Similar to carotenes but contain oxygen atoms (C₄₀H₅₆O₂).

Carotenoids help absorb light energy and transfer it to chlorophyll a molecules. They also play a crucial role in preventing photodynamic damage caused by reactive oxygen molecules (O₂).

Phycobilins

Phycobilins are water-soluble pigments found in cyanobacteria and red algae. They contain four pyrrole rings but lack magnesium and the phytol chain.

Location of Photosynthetic Pigments

The photosynthetic pigments are located in the grana portions of the chloroplast, specifically in the thylakoid membrane. The thylakoid membrane consists of both lipid and protein layers. Chlorophyll molecules, being hydrophilic, are embedded in the protein layer, while the hydrophobic phytol tail is embedded in the lipid layer. Other pigments are also associated with chlorophyll molecules.

Distribution of Photosynthetic Pigments in the Plant Kingdom

Light

The primary source of light energy for photosynthesis is the sun. The solar radiation (or solar energy) reaches Earth in the form of electromagnetic radiation with varying wavelengths. The electromagnetic spectrum includes gamma rays, ultraviolet rays, visible light, and infrared rays. The wavelengths range from 280 nm to 1000 nm.

• Below 280 nm: X-rays, Gamma rays, Cosmic rays
• 280-390 nm: Ultraviolet radiation
• 400-510 nm: Blue light
• 510-610 nm: Green light (Visible Light)
• 610-700 nm: Red light
• 700-1000 nm: Far red light (Infrared)

Photosynthetic pigments mainly absorb light in the visible part of the spectrum, which is known as photosynthetically active radiation (PAR).

Absorption Spectra of Chlorophyll

Chlorophylls absorb maximum light in the violet-blue and red parts of the spectrum. The absorption peaks are:

• Chlorophyll a: 410 nm (blue) and 660 nm (red)
• Chlorophyll b: 452 nm (blue) and 642 nm (red)

Carotenoids absorb light primarily in the blue and blue-green parts of the spectrum.

Energy Transfer to Chlorophyll a

All pigments, except chlorophyll a, are called accessory pigments or antenna pigments. They absorb light energy and transfer it to chlorophyll a molecules. This energy transfer, called resonance energy transfer (or Förster transfer), is a crucial part of the primary photochemical reactions in photosynthesis.

Excited States of Chlorophyll Molecules

1. Ground state (Singlet state): The normal state of the chlorophyll molecule.
2. Excited second singlet state: When the molecule absorbs light energy, it is raised to a higher energy level. This state is unstable and lasts for about 10^-12 seconds.
3. Excited first singlet state: The electron moves to an even higher energy level, lasting 10^-9 seconds.
4. Triplet state: The electron moves to a meta-stable state (lifetime of 10^-3 seconds).

At this point, the molecule can release energy by:

• Fluorescence: Emitting light at a longer wavelength than the incident light.
• Phosphorescence: Emitting light even after the incident light is removed.
• Electron transfer: The excited electron may be used in further photochemical reactions.

Quantum Requirement and Quantum Yield

• Quantum requirement refers to the number of photons (quantum) needed to release one molecule of oxygen in photosynthesis.
• Quantum yield is the number of oxygen molecules released per photon of light in photosynthesis.

Quantum Requirement

The minimum quantum requirement for photosynthesis is 4. This is because the reduction of one molecule of CO₂ by two molecules of H₂O requires the transfer of 4 H atoms, and each H atom requires one photon of light.

Quantum Yield

The quantum yield refers to the efficiency with which plants convert light energy into chemical energy. For photosynthesis, it is typically 0.125 to 0.1, meaning that for each photon absorbed, only a small fraction is converted into usable chemical energy.

The mechanism of photosynthesis is an essential process that allows plants to convert light energy into chemical energy, ultimately producing glucose and releasing oxygen as a by-product. This process occurs in two main stages: the light reaction and the dark reaction (also known as the Calvin Cycle). The photosynthesis process is primarily driven by two systems, known as Photosystem I (PSI) and Photosystem II (PSII), and involves several key components and processes.

Photosystems and Their Functions

Photosynthesis is initiated by light absorption, which excites electrons in the photosynthetic pigments. These pigments are organized into two complexes, Photosystem I (PSI) and Photosystem II (PSII), both involved in the light reactions.

1. Pigment Systems:
   • Photosystem I (PSI) contains P700 (a form of chlorophyll a), which absorbs light at 700 nm.
   • Photosystem II (PSII) contains P680 (another form of chlorophyll a), which absorbs light at 680 nm.
   • Both PSI and PSII also contain chlorophyll b, carotenoids, and xanthophylls that help in light absorption and energy transfer.

Discovery of Photosystems and Interactions Between Them

The discovery of the red drop and Emerson’s enhancement effect led scientists to propose that photosynthesis involves two photochemical reactions: one driven by PSI and another by PSII. Red drop refers to the decrease in the rate of photosynthesis when light with a wavelength shorter than 680 nm is used. On the other hand, Emerson’s enhancement effect refers to an increase in the rate of photosynthesis when light of different wavelengths (700 nm and 680 nm) is combined.

These two systems are interconnected by a protein complex called the cytochrome b6-f complex, which facilitates electron transport between the two systems. The electron carriers like plastoquinone (PQ) and plastocyanin (PC) move electrons between the systems.

The Photosynthetic Units and Quantasomes

The quantasomes are the fundamental units of the photosynthetic process. Each quanta (light photon) can excite electrons in chlorophyll molecules. Emerson and Arnold (1932) determined that around 2500 chlorophyll molecules are required to fix one molecule of CO2. This was initially called the chlorophyll unit but later renamed the photosynthetic unit. It was found that 10 quanta are needed to reduce one molecule of CO2 or produce one molecule of O2, meaning each photosynthetic unit contains about 250 chlorophyll molecules.

Action Spectrum

The action spectrum refers to the wavelengths of light that are most effective in driving a specific photoreaction, such as photosynthesis. By comparing the action spectrum of a photoreaction with the absorption spectrum of different pigments, it becomes possible to identify which pigment is responsible for absorbing light and driving the reaction.

• For example, if pigment B has a peak absorption at 660 nm and its action spectrum closely matches, it suggests that pigment B is primarily responsible for absorbing the light energy and driving the photoreaction.

Mechanism of Photosynthesis: Light and Dark Reactions

Photosynthesis consists of two main phases: the light reaction and the dark reaction.

Light Reaction (Primary Photochemical Reaction)

The light reaction occurs in the thylakoid membranes of the chloroplast and is the first stage of photosynthesis. The main functions of the light reaction are to produce ATP and NADPH2, which will be used in the dark reaction.

• Absorption of Light Energy:
  • The photosynthetic pigments absorb light energy, and this energy is transferred from accessory pigments (like chlorophyll b, carotenoids) to chlorophyll a molecules, specifically P700 in PSI and P680 in PSII.
  • When chlorophyll molecules absorb light, they become excited and enter a high-energy state (excited triplet state), expelling electrons.

• Photolysis of Water and Oxygen Evolution:
  • In PSII, water molecules are split in the presence of light, releasing oxygen (O2), electrons (e-), and protons (H+). This process is called photolysis and provides electrons to replace those lost by the chlorophyll molecules in PSII.
  • 4H2O → O2 + 4H+ + 4e-

• Electron Transport:
  • The excited electrons from P680 in PSII are passed through a series of electron carriers (like plastoquinone, cytochrome b6-f complex) to PSI.
  • The electrons from P700 in PSI are passed through ferredoxin and can either be used for NADP reduction or cycled back in cyclic electron transport.

• Formation of ATP and NADPH:
  • The movement of electrons through the electron transport chain drives the formation of ATP via phosphorylation (photophosphorylation) and NADPH through NADP+ reduction.
  • There are two types of electron transport: cyclic (involving PSI) and non-cyclic (involving both PSI and PSII).
  • Cyclic Electron Transport:
    • In cyclic electron transport, electrons from PSI are recycled back to PSI, producing only ATP, but not NADPH or oxygen.
    • This occurs when PSII activity is blocked or when the plant needs more ATP relative to NADPH.
  • Non-Cyclic Electron Transport:
    • In non-cyclic electron transport, electrons flow from PSII to PSI, leading to the production of both ATP and NADPH, as well as the evolution of oxygen (from the photolysis of water).

Dark Reaction (Calvin Cycle)

The dark reaction, also known as the Calvin Cycle, occurs in the stroma of the chloroplast and does not directly require light, although it depends on the ATP and NADPH produced during the light reaction. In this cycle, carbon dioxide (CO2) is fixed into an organic molecule and eventually converted into glucose.

Summary of the Photosynthesis Process

1. Light Absorption: Light energy is absorbed by chlorophyll molecules in PSI and PSII.
2. Water Splitting: Water is split into oxygen, electrons, and protons in PSII.
3. Electron Transport: Excited electrons move through electron carriers and produce ATP and NADPH.
4. Carbon Fixation (Dark Reaction): ATP and NADPH are used to reduce CO2 into glucose through the Calvin Cycle.

Significance of Cyclic Photophosphorylation

1. No Photolysis of Water or O2 Evolution: In cyclic photophosphorylation, water is not split (photolysis), and oxygen is not evolved. Additionally, NADP+ is not reduced to NADPH.
2. Electron Cycling: The electron released from the P700 chlorophyll molecule in Photosystem I (PSI) cycles back to the same chlorophyll molecule. Thus, it serves both as the donor and acceptor of the electron.
3. ATP Production: ATP is generated at two sites in cyclic photophosphorylation, but it cannot directly drive the dark reactions (Calvin cycle) of photosynthesis, as it does not produce NADPH.
4. Role in Balancing ATP for Photosynthesis: Cyclic photophosphorylation compensates for the ATP deficiency generated by non-cyclic photophosphorylation. The energy-rich ATP produced by cyclic phosphorylation is essential for processes such as starch, protein, lipid, nucleic acid, and pigment synthesis within the chloroplast.

Significance of Non-Cyclic Photophosphorylation

1. Involves Both Photosystems: Non-cyclic electron transport involves both Photosystem II (PSII) and Photosystem I (PSI).
2. Electron Flow: The electron expelled from the chlorophyll in PSII moves through various electron carriers, and it is not cycled back to PSII. The electron in PSI is transferred to NADP+ to form NADPH.
3. Photolysis of Water: Photolysis of water takes place during non-cyclic photophosphorylation, where water is split to release oxygen, protons (H+), and electrons.
4. ATP Synthesis: ATP is synthesized at one point during the electron transport chain in non-cyclic photophosphorylation.
5. Reduction of NADP+: NADP+ is reduced to NADPH, which plays a crucial role in the dark reactions (Calvin cycle) of photosynthesis.
6. Oxygen Evolution: Oxygen is evolved as a by-product due to the photolysis of water.

Comparison of Cyclic and Non-Cyclic Electron Transport and Photophosphorylation

Significance of the Light Reaction

1. Location: The light reaction occurs in the presence of light and takes place in the thylakoid membranes of the chloroplast.
2. Production of Assimilatory Powers: ATP and NADPH2, essential for the dark reactions, are synthesized during the light reaction.
3. Photolysis of Water: Water is split to provide electrons and protons, which are used in NADP+ reduction.
4. Oxygen Evolution: Oxygen is released as a by-product of water photolysis.

Red Drop and Emerson’s Enhancement Effect

• Red Drop: Emerson observed a decrease in the quantum yield of photosynthesis when light with wavelengths greater than 680 nm was used. This phenomenon is called the “Red Drop,” where light in the red part of the spectrum was less effective for photosynthesis.
• Emerson’s Enhancement Effect: Emerson found that if far-red light beyond 680 nm (which was inefficient on its own) was combined with light of shorter wavelengths (such as blue light), the quantum yield of photosynthesis increased beyond the sum of the individual effects. This phenomenon is known as Emerson’s Enhancement Effect, showing the synergy between different wavelengths of light in enhancing photosynthesis.