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.
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.
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.
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.
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.
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.
Non-cyclic photophosphorylation is the continuous, one-way flow of electrons from water to NADP+, coupled with the synthesis of ATP. It requires the cooperative effort of both Photosystem II (PS II) and Photosystem I (PS I).
Light Absorption & PS II Excitation: PS II absorbs light energy at 680 nm (via reaction center P680). This excitation drives an electron to a high energy state, leaving P680 oxidized (P680+).
Photolysis of Water (Oxygen Evolving Complex - OEC): To replace the lost electron in PS II, water is split on the inner side of the thylakoid membrane by the OEC (requiring Mn2+,Cl−, and Ca2+ cofactors):
2H2O→4H++4e−+O2↑
Electron Downhill Transport: The excited electron is captured by the primary acceptor Pheophytin (Ph), then transferred down an electron transport chain containing:
Plastoquinone (PQ): A mobile lipid-soluble carrier that also pumps H+ from the stroma to the thylakoid lumen.
Cytochrome b6f complex: An integral membrane complex that facilitates the proton pump.
Plastocyanin (PC): A mobile, copper-containing peripheral membrane protein that delivers electrons to PS I.
ATP Generation: The movement of electrons through the Cytochrome b6f complex pumps protons into the thylakoid lumen, generating a proton gradient that drives ATP synthesis via chemiosmosis.
PS I Excitation & NADPH Production: Simultaneously, PS I (P700) absorbs light at 700 nm, excites its electrons, and passes them to a primary acceptor (A0, an iron-sulfur protein). P700 receives the incoming electron from PC to return to its ground state.
Ferredoxin & Reduction: The excited electrons pass from the primary acceptor to Ferredoxin (Fd) (a soluble iron-sulfur protein) and then to the enzyme Ferredoxin-NADP⁺ Reductase (FNR), which reduces NADP+ to NADPH+H+ in the stroma.
The Breath of Life
Every breath you take comes from the photolysis of water during the light reactions of photosynthesis. Plants don't produce oxygen to help us; it's simply a waste product from splitting water to get electrons!
Cyclic photophosphorylation occurs under specific conditions (e.g., when carbon dioxide fixation is limited, when there is a high demand for ATP relative to NADPH, or when light wavelength is longer than 680 nm). It only involves Photosystem I (PS I).
P700 Excitation: PS I absorbs light energy, exciting electrons from the P700 reaction center.
The Cyclic Path: The excited electrons are accepted by the primary electron acceptor (A0) and passed to Ferredoxin (Fd).
Re-routing to PQ: Instead of being transferred to NADP+, the electrons are re-routed to the Plastoquinone (PQ) pool.
Return to P700: From PQ, the electrons flow through the Cytochrome b6f complex and Plastocyanin (PC), returning back to the oxidized reaction center P700+.
Net Yield: No water splitting occurs, no oxygen is evolved, and no NADPH is synthesized. The cycle solely generates a proton gradient across the thylakoid membrane, powering the synthesis of ATP.
The chemiosmotic hypothesis explains how ATP is synthesized during photosynthesis.
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).
This proton gradient is a form of potential energy.
The protons flow back into the stroma through a channel in the ATP synthase enzyme.
The energy released by the flow of protons is used by ATP synthase to synthesize ATP from ADP and inorganic phosphate.
The biosynthetic phase of photosynthesis, also known as the Calvin Cycle (or Light-Independent Reactions), occurs in the stroma of the chloroplast. It does not require light directly but depends on the assimilatory power (ATP and NADPH) produced during the light reaction to reduce CO2 to carbohydrates.
Significance: The most crucial step where gaseous carbon dioxide is fixed into a stable organic intermediate. Since RuBisCO has both carboxylase and oxygenase activities, its function depends on the ratio of CO2 to O2.
Significance: Utilizes 2 ATP for phosphorylation and 2 NADPH for reduction per molecule of CO2 fixed. Six turns of the cycle are required to produce one molecule of glucose (6C) from six CO2.
Regeneration of RuBP:
Reaction:5×G3P (3C)⟶Sugar rearrangements3×RuBP (5C)(requires 1 ATP per RuBP regenerated, i.e., 3 ATP for 3 RuBP).
Significance: The primary carbon dioxide acceptor RuBP must be regenerated to sustain continuous carbon fixation. This step requires 1 ATP per turn.
Photorespiration is a light-dependent metabolic process occurring in C3 plants that consumes oxygen and releases carbon dioxide without producing ATP, NADPH, or sugars. It is initiated when the enzyme RuBisCO binds to O2 instead of CO2 under conditions of low CO2 concentration and high O2 concentration.
Glycolate Formation: The 3-PGA molecule enters the Calvin cycle. The 2-phosphoglycolate (2C) is dephosphorylated into Glycolate by a specific phosphatase. Glycolate is then transported out of the chloroplast into the peroxisome.
In the Peroxisome:
Oxidation: Glycolate is oxidized to Glyoxylate by Glycolate Oxidase, producing hydrogen peroxide (H2O2) as a toxic byproduct. H2O2 is immediately split into water and oxygen by the enzyme Catalase.
Transamination: Glyoxylate undergoes transamination to form the amino acid Glycine (2C), which is then transported into the mitochondrion.
In the Mitochondria:
Condensation & Decarboxylation: Two molecules of Glycine (2×2C=4C) undergo oxidative decarboxylation and deamination to form one molecule of Serine (3C).
Byproducts: This reaction releases one molecule of CO2↑ and one molecule of ammonia (NH3↑), and generates one molecule of NADH from NAD+.
Return and Recovery:
In the Peroxisome: Serine is transported back to the peroxisome, transaminated to Hydroxypyruvate, and reduced to Glycerate (3C) using NADH.
In the Chloroplast: Glycerate returns to the chloroplast, where it is phosphorylated by Glycerate Kinase using 1 ATP to form 3-PGA (3C), which can now enter the Calvin cycle.
Carbon Loss: Approximately 25% of the carbon fixed by the light reaction is lost as CO2.
Energy Waste: It consumes ATP (in the chloroplast during glycerate recovery) and utilizes reducing power (NADH) without generating any chemical energy.
Nitrogen Loss: Nitrogen is lost as ammonia (NH3) and must be refixed in the chloroplast (requiring further ATP and reduced ferredoxin).
The C4 Pathway is a specialized evolutionary adaptation in certain plants (mostly tropical monocots like maize, sugarcane, and sorghum) to minimize photorespiration and maximize water-use efficiency in hot, dry environments.
C4 plants possess a unique leaf anatomy known as Kranz anatomy (German for "wreath"):
Mesophyll Cells: Outer layer of cells lacking RuBisCO but rich in PEP carboxylase. They are responsible for initial carbon capture.
Bundle Sheath Cells: Inner ring of large cells surrounding the vascular bundle, containing thick cell walls impermeable to gases. They house RuBisCO and lack PS II (minimizing internal oxygen release).
Competitive Edge: Where is RuBisCO?
In C3 plants, RuBisCO is present in the Mesophyll cells. But in C4 plants, RuBisCO is entirely ABSENT in mesophyll cells; it is exclusively restricted to the Bundle Sheath cells. This spatial separation is the key to preventing photorespiration!
Significance: Initial fixation is performed by PEP Carboxylase (PEPCase), which has a very high affinity for carbon dioxide (in bicarbonate form) and zero affinity for oxygen, completely preventing photorespiration at this stage.
Significance: Malate is decarboxylated inside the bundle sheath cells, raising the local concentration of CO2 to very high levels. This saturated CO2 environment forces RuBisCO to act exclusively as a carboxylase in the Calvin cycle, eliminating photorespiration.
Regeneration of PEP in Mesophyll Cells:
Reaction:Pyruvate+ATP+Pi⟶PPDKPEP (3C)+AMP+PPi
Significance: Pyruvate is transported back into the mesophyll cells, where it is converted back to PEP by the enzyme Pyruvate-Phosphate Dikinase (PPDK). This step requires the equivalent of 2 ATP (conversion of ATP to AMP).
Competitive Edge: The Law of the Minimum
F.F. Blackman stated: "If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed."
Example: Even if a plant has optimal Light and Temperature, if the CO2 concentration is very low, the rate of photosynthesis will be low. Here, CO2 is the limiting factor. As soon as you increase CO2, the rate will shoot up until some other factor (like light) becomes the new bottleneck.
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.
BioNotes