Photosynthesis in Higher Plants and Respiration in Plants Revision and Important Questions By PW

Understanding photosynthesis and respiration is essential for learning how plants produce food and release energy for cellular activities. Photosynthesis stores solar energy in carbohydrates, whereas respiration breaks down these compounds to produce ATP.

For NEET, important concepts include photosynthetic pigments, chloroplast structure, light-dependent reactions, the Calvin cycle, factors affecting photosynthesis, glycolysis, aerobic and anaerobic respiration, and the respiratory quotient.

Meaning and Importance of Photosynthesis

Photosynthesis is the process by which green plants use light energy to synthesise carbohydrates from carbon dioxide and water in the presence of chlorophyll. Oxygen is released during the process.

General equation:

Carbon dioxide + Water + Light energy → Carbohydrate + Oxygen

The major components and their roles are:

• Carbon dioxide: Supplies carbon for carbohydrate synthesis.

• Water: Provides electrons and hydrogen during photosynthesis.

• Sunlight: Supplies the energy required for the process.

• Chlorophyll: Absorbs light energy.

• Glucose: Stores chemical energy in its bonds.

• Oxygen: Released as a by-product of photosynthesis.

Photosynthesis is a physicochemical and endothermic process because it involves light absorption and requires an energy input. In contrast, respiration releases usable energy from organic molecules in the form of ATP.

Autotrophs and Consumers

Experimental Evidence for Photosynthesis

Several experiments established the conditions required for photosynthesis and the products formed during the process.

• Variegated Leaf Experiment

A variegated leaf is exposed to light and tested for starch using iodine solution. The green regions turn blue-black because starch is present, while the non-green regions do not show the same result.

Conclusion: Chlorophyll is necessary for photosynthesis.

• Moll's Half-Leaf Experiment

Part of a leaf is enclosed in a flask containing potassium hydroxide (KOH), which absorbs carbon dioxide. After exposure to light, the leaf is tested for starch. The part deprived of carbon dioxide does not show starch formation.

Conclusion: Carbon dioxide is necessary for photosynthesis.

• Priestley's Bell Jar Experiment

Priestley observed that a green plant could restore the air affected by a burning candle or a living animal in a closed environment.

Conclusion: Green plants release oxygen during photosynthesis.

• Ingenhousz's Experiment

Ingenhousz observed oxygen bubbles from aquatic plants exposed to light. Oxygen evolution was not observed in the same way in darkness.

Conclusion: Light is essential for photosynthesis and oxygen release.

• Julius von Sachs' Experiment

Sachs demonstrated that green parts of leaves produce starch during photosynthesis.

Conclusion: Carbohydrates formed during photosynthesis can be stored as starch, and chlorophyll-containing regions are involved in the process.

Absorption Spectrum and Action Spectrum

The absorption spectrum and action spectrum explain how different wavelengths of light contribute to photosynthesis.

Engelmann's experiment demonstrated that photosynthetic activity is high in the blue and red regions of the visible spectrum.

Chloroplast Structure and Photosynthetic Reactions

Chloroplasts are the primary sites of photosynthesis in plant cells. Their thylakoid membranes contain photosynthetic pigments and are arranged in stacks called grana. The fluid-filled region surrounding the thylakoids is called the stroma.

Photosynthesis involves two major stages:

The term dark reaction can be misleading. The Calvin cycle does not require darkness; it uses ATP and NADPH generated by the light-dependent reactions.

Photosystems

Photosystems are pigment-protein complexes involved in capturing light energy.

• Photosystem I (PSI): Reaction centre P700.

• Photosystem II (PSII): Reaction centre P680.

During non-cyclic electron transport, Photosystem II functions before Photosystem I, although Photosystem I was discovered first.

Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation involves both photosystems. Electrons move through an electron transport chain rather than returning to the original photosystem.

Key features include:

• Produces ATP and NADPH.

• Releases oxygen through water splitting.

• Involves both PSI and PSII.

• Electrons originating from water ultimately reduce NADP⁺ to NADPH.

Cyclic Photophosphorylation

Cyclic photophosphorylation involves Photosystem I only. Excited electrons return to PSI through an electron transport pathway.

Key features include:

• Produces ATP.

• Does not produce NADPH directly.

• Does not release oxygen.

• Does not involve water splitting.

• Helps meet additional ATP requirements.

Water Splitting and the Source of Oxygen

Photolysis of water occurs in association with Photosystem II. It supplies electrons and protons and releases oxygen.

The electrons replace those lost by PSII. The oxygen released during photosynthesis comes from water, as demonstrated by isotope-tracing experiments using oxygen-18.

C3 and C4 Pathways

The C3 and C4 pathways differ in their initial carbon fixation products, enzymes, cellular locations and adaptations to environmental conditions.

C3 Pathway or Calvin Cycle

The Calvin cycle occurs in the stroma of chloroplasts. In C3 plants, carbon dioxide is initially fixed by RuBisCO, and the first stable product is the three-carbon compound 3-phosphoglyceric acid (3-PGA).

Important points:

• First carbon acceptor: Ribulose-1,5-bisphosphate (RuBP).

• First stable product: 3-phosphoglyceric acid (3-PGA).

• Key enzyme: RuBisCO.

• Location: Chloroplast stroma.

• Energy requirement per CO₂ fixed: 3 ATP and 2 NADPH.

• Requirement for one glucose molecule: 18 ATP and 12 NADPH.

Factors Affecting Photosynthesis

The rate of photosynthesis depends on environmental conditions and the characteristics of the plant.

Major factors include:

• Light intensity and quality.

• Carbon dioxide concentration.

• Water availability.

• Temperature.

• Leaf age, size, number and orientation.

• Genetic characteristics of the plant.

Photosynthetically active radiation (PAR): The wavelength range from 400 to 700 nanometres is used for photosynthesis. Blue and red light are particularly effective.

Blackman's Law of Limiting Factors

Blackman's law states that when a process depends on multiple factors, its rate is limited by the factor that is in the least favourable supply relative to its requirement.

NEET tip: Increasing one factor may not increase photosynthesis if another factor becomes limiting.

Glycolysis

Glycolysis is the breakdown of one six-carbon glucose molecule into two three-carbon pyruvate molecules. It is also called the EMP pathway, after Embden, Meyerhof and Parnas.

Key features of glycolysis:

• Occurs in the cytoplasm.

• Consists of ten enzyme-catalysed steps.

• Occurs in both aerobic and anaerobic conditions.

• Is a common pathway for glucose breakdown.

• Involves partial oxidation of glucose.

Two ATP molecules are used during the energy-investment phase, while four ATP molecules are generated during the energy-payoff phase.

Net Products of Glycolysis

For each glucose molecule, glycolysis produces:

• 2 pyruvate molecules.

• 2 ATP molecules as the net gain.

• 2 NADH molecules.

Phosphorylation helps retain glucose and its derivatives inside the cell and prepares them for subsequent reactions.

Fate of Pyruvate: Fermentation

When oxygen-dependent respiration is not used, pyruvate may undergo fermentation. Fermentation regenerates NAD⁺, allowing glycolysis to continue, but does not generate additional ATP beyond the net gain from glycolysis.

Lactic Acid Fermentation

• Pyruvate is converted into lactate.

• The enzyme involved is lactate dehydrogenase.

• NADH is oxidised to NAD⁺.

• Carbon dioxide is not released during the conversion.

• No additional ATP is produced directly.

Alcoholic Fermentation

• Pyruvate is converted into acetaldehyde, releasing carbon dioxide.

• Acetaldehyde is converted into ethanol.

• The enzymes involved are pyruvate decarboxylase and alcohol dehydrogenase.

• NAD⁺ is regenerated.

• No additional ATP is produced directly.

Key point: Fermentation yields a net gain of 2 ATP per glucose molecule through glycolysis.

Aerobic Respiration and the Krebs Cycle

In aerobic respiration, pyruvate enters the mitochondrion and is converted into acetyl-CoA through the link reaction, also called oxidative decarboxylation.

For two pyruvate molecules, the link reaction produces:

• 2 acetyl-CoA molecules.

• 2 carbon dioxide molecules.

• 2 NADH molecules.

Acetyl-CoA enters the Krebs cycle, also known as the citric acid cycle. Citrate is the first stable product formed after acetyl-CoA combines with oxaloacetate.

Products of the Krebs Cycle

For each acetyl-CoA molecule, the cycle produces:

• 3 NADH.

• 1 FADH₂.

• 1 ATP or GTP.

• 2 carbon dioxide molecules.

Since one glucose molecule produces two acetyl-CoA molecules, the Krebs cycle operates twice per glucose molecule.

Electron Transport System and Chemiosmosis

The electron transport system (ETS) is located in the inner mitochondrial membrane. NADH and FADH₂ donate electrons to the system, and electron transfer releases energy used to pump protons across the membrane.

This creates a proton gradient. As protons move back into the mitochondrial matrix through the F₀ component of ATP synthase, the resulting proton motive force drives ATP formation by the F₁ component.

Oxygen acts as the terminal electron acceptor. It combines with electrons and protons to form water.

NEET concept: Chemiosmosis links the movement of protons down their electrochemical gradient with ATP synthesis.

Respiration as an Amphibolic Pathway

Respiration is described as an amphibolic pathway because it participates in both catabolism and anabolism.

• Catabolism: Breaks down glucose, fats and other respiratory substrates to release energy.

• Anabolism: Supplies intermediates that can be used to synthesise fatty acids, amino acids and other biomolecules.

Therefore, respiratory pathways contribute to both energy production and the synthesis of cellular components.

Respiratory Quotient (RQ)

The respiratory quotient is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration.

Formula:

RQ = Volume of CO₂ released ÷ Volume of O₂ consumed

The respiratory quotient helps identify the type of substrate being used during respiration.

Photosynthesis in Higher Plants and Respiration in Plants: Important Questions 

Practise important questions on Photosynthesis in Higher Plants and Respiration to revise key concepts, strengthen your understanding, and prepare effectively for NEET 2026.

Question 1

How many ATP and NADPH molecules are required to produce one molecule of glucose through the Calvin cycle?

(1) 18 ATP and 12 NADPH (2) 6 ATP and 12 NADPH (3) 24 ATP and 18 NADPH (4) 12 ATP and 18 NADPH

Question 2

Which of the following statements about photosynthesis are incorrect?

A. The water-splitting complex is associated with Photosystem I (PSI). B. C₄ plants use the C₃ pathway of CO₂ fixation as the main biosynthetic pathway. C. Photorespiration does not occur in C₄ plants. D. C₃ plants exhibit Kranz anatomy. E. ATP synthesis in chloroplasts occurs through chemiosmosis.

Choose the correct answer from the options given below.

(1) B only (2) A and D only (3) B and C only (4) B and E only

Question 3

Which enzyme is required for carboxylation in the Calvin cycle?

(1) PEP carboxylase (2) RuBP carboxylase-oxygenase (RuBisCO) (3) Carboxypeptidase (4) Hexokinase

Question 4

Which of the following statements is incorrect?

A. Stroma lamellae contain only PSI and lack NADP reductase. B. Grana lamellae contain both PSI and PSII. C. Cyclic photophosphorylation involves both PSI and PSII. D. Both ATP and NADPH + H⁺ are synthesised during non-cyclic photophosphorylation.

Question 5

The reaction centre in Photosystem II (PSII) has an absorption maximum at:

A. 780 nm B. 680 nm C. 700 nm D. 660 nm

Question 6

Which of the following statements about RuBisCO is correct?

A. It is active only in the dark. B. It has a higher affinity for oxygen than for carbon dioxide. C. It is an enzyme involved in the photolysis of water. D. It catalyses the carboxylation of RuBP.

Question 7

Given below are two statements.

Statement I: RuBisCO is the most abundant enzyme in the world.

Statement II: Photorespiration does not occur in C₄ plants.

In light of the statements above, choose the most appropriate answer from the options given below.

A. Statement I is correct, but Statement II is incorrect. B. Statement I is incorrect, but Statement II is correct. C. Both Statement I and Statement II are correct. D. Both Statement I and Statement II are incorrect.

Question 8

What is the role of the large bundle sheath cells found around the vascular bundles in C₄ plants?

A. To protect the vascular tissue from high light intensity. B. To provide the site for the photorespiratory pathway. C. To increase the number of chloroplasts for the operation of the Calvin cycle. D. To enable the plant to tolerate high temperatures.

Question 9

Given below are two statements.

Statement I: The primary CO₂ acceptor in C₄ plants is phosphoenolpyruvate (PEP), which is found in mesophyll cells.

Statement II: Mesophyll cells of C₄ plants lack the RuBisCO enzyme.

In light of the statements above, choose the correct answer from the options given below.

A. Statement I is incorrect, but Statement II is correct. B. Both Statement I and Statement II are correct. C. Both Statement I and Statement II are incorrect. D. Statement I is correct, but Statement II is incorrect.]

Mastering Photosynthesis in Higher Plants and Respiration requires a clear understanding of photosynthetic pigments, light and dark reactions, C3 and C4 pathways, glycolysis, the Krebs cycle, and ATP production. Revise the important concepts, practise NEET-level MCQs, and review key differences to strengthen your preparation. For dedicated guidance, classes, and comprehensive Biology revision, explore PW’s NEET 2026 preparation batches and take a confident step towards your medical entrance exam goals. 

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