Important Diagrams Photosynthesis in Higher Plants - Biology Class 11 - NEET

The Structure of a Chloroplast

Chloroplasts are double‐membrane bound organelles found in green parts of plants and are the sites of photosynthesis. The main structural features are:

• Outer membrane - selectively permeable boundary.
• Inner membrane - encloses the internal matrix or stroma.
• Stroma - a gel‐like matrix containing enzymes for the Calvin cycle, DNA, ribosomes and starch grains.
• Thylakoids - flattened membrane sacs; membranes contain pigments and protein complexes (PSI, PSII, cytochrome b6f, ATP synthase).
• Granum (grana) - stacks of thylakoids connected by intergranal thylakoids (stroma lamellae).
• Lumen - interior of thylakoids where proton accumulation occurs during light reactions.
• Plastoglobuli - lipid bodies associated with thylakoid membranes.

Functions of major compartments:

• Thylakoid membranes - site of light-dependent reactions (photophosphorylation, water splitting, electron transport).
• Stroma - site of carbon fixation (Calvin cycle) and synthesis of carbohydrates.

Cyclic Photophosphorylation

Cyclic photophosphorylation occurs when electrons excited in Photosystem I (PSI) follow a cyclic path back to the same photosystem rather than reducing NADP+.

• Involves PSI, ferredoxin (Fd), cytochrome b6f complex and plastocyanin (PC).
• Electrons from excited P700 are passed to Fd and then returned via the electron transport chain to P700+, producing a proton gradient across the thylakoid membrane.
• Result: synthesis of ATP only; NADPH and O2 are not produced.
• Occurs when cellular demand for ATP is high (e.g., during Calvin cycle) or when NADP+ is limited.
• Importance: supplies additional ATP needed for carbon assimilation and other metabolic processes.

Non‐cyclic Photophosphorylation

Non‐cyclic photophosphorylation (also called the Z‐scheme) involves both Photosystem II (PSII) and PSI and leads to formation of ATP, NADPH and evolution of O2.

• Light energy excites electrons in PSII (P680). Electrons pass to plastoquinone (PQ), cytochrome b6f complex and plastocyanin (PC) to reach PSI (P700).
• PSII replaces lost electrons by splitting water (photolysis): 2 H2O → O2 + 4 H+ + 4 e−.
• PSI transfers electrons to ferredoxin and then to NADP+ via NADP+ reductase to form NADPH.
• Proton pumping across thylakoid membrane during electron transport creates proton motive force used by ATP synthase to produce ATP.
• Net result: production of ATP and NADPH in stroma and release of O2 into the atmosphere.

Electron Transport System

The electron transport system (ETS) in chloroplast thylakoid membranes links the light‐capturing pigment complexes and generates a proton gradient. Main components and flow:

• PSII (P680) → primary acceptor → plastoquinone (PQ).
• PQ carries electrons to the cytochrome b6f complex, which pumps protons into the thylakoid lumen.
• Plastocyanin (PC) transfers electrons to PSI (P700).
• PSI passes electrons to ferredoxin (Fd) and then to NADP+ reductase, reducing NADP+ to NADPH.
• The proton gradient produced by PQ and cytochrome b6f is used by ATP synthase to synthesise ATP from ADP and inorganic phosphate (Pi).

Chemiosmotic Hypothesis

The chemiosmotic hypothesis, proposed by Peter D. Mitchell, explains how ATP is synthesised using the proton motive force across membranes.

• Light energy drives electron flow through the ETS, causing protons (H+) to be pumped from the stroma into the thylakoid lumen.
• A transmembrane proton gradient (difference in H+ concentration and electric potential) is established across the thylakoid membrane.
• Protons flow back to the stroma through the ATP synthase channel, and this flow provides the energy for phosphorylation of ADP to ATP.
• Thus, chemical energy of ATP is synthesised by using energy stored as an electrochemical gradient (chemiosmosis).

Calvin Cycle (C3 Pathway)

The Calvin cycle (also called the C3 cycle) takes place in the stroma of chloroplasts and fixes atmospheric CO2 into carbohydrates. It comprises three main phases:

1. Carboxylation: CO2 combines with ribulose‐1,5‐bisphosphate (RuBP) catalysed by Rubisco, forming two molecules of 3‐phosphoglycerate (PGA).
2. Reduction: PGA is phosphorylated by ATP and then reduced by NADPH to produce glyceraldehyde‐3‐phosphate (G3P).
3. Regeneration: Most G3P molecules are used to regenerate RuBP so the cycle can continue; a portion of G3P is withdrawn for synthesis of glucose and other carbohydrates.

Stoichiometry and energetic cost:

• Fixation of 3 CO2 yields one net molecule of G3P and consumes 9 ATP and 6 NADPH.
• To synthesise one glucose (C6 sugar), fixation of 6 CO2 is required, consuming 18 ATP and 12 NADPH.

Key enzyme: Rubisco (ribulose‐1,5‐bisphosphate carboxylase/oxygenase) - catalyses carboxylation and oxygenation reactions.

Hatch & Slack Cycle in C4 Plants

C4 photosynthesis (Hatch & Slack pathway) is an adaptation to high light intensity, high temperature and low atmospheric CO2. It involves spatial separation of initial CO2 fixation and the Calvin cycle.

• Mesophyll cells: CO2 is initially fixed by PEP carboxylase into oxaloacetate (OAA), which is converted to malate or aspartate.
• Bundle sheath cells: C4 acids are transported into bundle sheath cells where they are decarboxylated to release CO2 near Rubisco; CO2 is then fixed by the Calvin cycle.
• This mechanism concentrates CO2 around Rubisco, reducing photorespiration and increasing efficiency under warm, dry conditions.
• Typical examples: maize, sugarcane, and many tropical grasses; anatomical feature: Kranz anatomy (bundle sheath cells surrounded by mesophyll).
• Cost: additional ATP is required for regeneration of phosphoenolpyruvate (PEP), so C4 pathway has higher ATP demand per fixed CO2 than C3.

CAM Pathway

CAM (Crassulacean Acid Metabolism) is an adaptation in many succulents and some epiphytes for water conservation. It relies on temporal separation of steps:

• Night: stomata open; CO2 is fixed by PEP carboxylase into organic acids (mainly malate) and stored in vacuoles.
• Day: stomata close to conserve water; malate is decarboxylated to release CO2 for the Calvin cycle in the chloroplasts.
• Examples: cacti, pineapple, many succulents.
• Advantage: reduces water loss while allowing CO2 fixation in arid conditions.

Photorespiration

Photorespiration is the process in which Rubisco acts as an oxygenase, fixing O2 instead of CO2, leading to formation of phosphoglycolate which is metabolised through a pathway involving chloroplasts, peroxisomes and mitochondria.

• Occurs when O2 concentration is high and CO2 concentration is low (often at high temperatures and when stomata are closed).
• Results in loss of fixed carbon and energy (ATP and reducing power), making it wasteful for the plant.
• Pathway: phosphoglycolate → glycolate (chloroplast) → glyoxylate/glycine (peroxisome and mitochondrion) → serine → back to Calvin cycle intermediates.
• C4 and CAM mechanisms reduce photorespiration by concentrating CO2 around Rubisco or by temporal separation.

Pigments involved in Photosynthesis

Photosynthetic pigments absorb light and transfer energy to the reaction centres. Major pigments:

• Chlorophyll a - primary pigment in reaction centres; absorbs strongly in blue‐violet (~430 nm) and red (~662 nm) regions.
• Chlorophyll b - accessory pigment; absorbs at slightly different wavelengths (~453 nm and ~642 nm) and broadens the range of light capture.
• Carotenoids (carotenes and xanthophylls) - accessory pigments; absorb blue‐green light, protect chlorophyll from photooxidative damage and transfer energy to chlorophyll.

Difference between absorption spectrum (which wavelengths a pigment absorbs) and action spectrum (relative effectiveness of different wavelengths in driving photosynthesis): accessory pigments extend the action spectrum beyond peaks of chlorophyll a.

Light Reaction

The light (photo) reactions of photosynthesis occur in the thylakoid membranes and convert light energy into chemical energy (ATP and NADPH). Key features:

• Light harvesting complexes (LHC) containing pigments absorb photons and transfer excitation energy to reaction centres (PSII and PSI).
• PSII absorbs light, excites electrons and drives photolysis of water, producing O2, H+ and electrons.
• Electrons flow through the electron transport chain creating a proton gradient; ATP synthase uses this gradient to produce ATP (photophosphorylation).
• PSI absorbs light and transfers high‐energy electrons to NADP+ to form NADPH.
• Products ATP and NADPH are used in the Calvin cycle for carbon fixation and reduction steps.

Light Harvesting Complex

Z Scheme of Light Reaction

Light Effect on Photosynthesis

Light affects photosynthesis in several ways. Important concepts:

• Light intensity: rate of photosynthesis increases with intensity up to a saturation point beyond which no further increase occurs.
• Light quality (wavelength): red and blue wavelengths are most effective for photosynthesis because pigments absorb these wavelengths well.
• Light duration (photoperiod): affects flowering and growth; some plants require specific daylengths.
• Light compensation point: the light intensity at which rate of photosynthesis equals rate of respiration (net CO2 exchange = 0).
• Light saturation point: the intensity at which photosynthesis reaches its maximum rate.

Typical experimental representation: a light‐response curve showing three regions - initial slope (light‐limited), plateau (light saturation), and sometimes photoinhibition at very high intensities.

Summary: Photosynthesis in higher plants involves coordinated structural organisation within chloroplasts and two linked sets of reactions: the light reactions (in thylakoids) that capture light and produce ATP and NADPH, and the Calvin cycle (in stroma) that fixes CO2 into organic compounds. Variations such as C4 (Hatch & Slack) and CAM pathways are adaptations that reduce photorespiration and improve water‐use efficiency under specific environmental conditions.