Photosynthesis in Higher Plants: Chapter 13 Revision Notes

Key Terms in Photosynthesis

Photosynthesis

A physio-chemical process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. It is an anabolic, endergonic process.

Action Spectrum

A graph that shows the relative effectiveness of different wavelengths of light in driving a particular process, such as photosynthesis.

Absorption Spectrum

A graph that shows the amount of light absorbed at different wavelengths by a pigment.

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy. It can be cyclic or non-cyclic.

Calvin Cycle

The light-independent reactions of photosynthesis where energy from ATP and NADPH is used to build chemical compounds like sugars. It occurs in the stroma of the chloroplast.

Photorespiration

A wasteful metabolic pathway that occurs when the enzyme RuBisCO acts on oxygen rather than carbon dioxide. It consumes ATP and releases CO2, reducing photosynthetic output.

Kranz Anatomy

The special structure of leaves in C4 plants (e.g., maize, sugarcane) where the vascular bundles are surrounded by large bundle sheath cells, which are then surrounded by mesophyll cells.

Law of Limiting Factors

Proposed by Blackman, it states that if a chemical process is affected by more than one factor, then its rate is determined by the factor which is nearest to its minimal value.

The Two Stages of Photosynthesis

Photosynthesis is not a single-step process but occurs in two distinct, yet interconnected, stages. The first stage is the Light-Dependent Reactions (or Light Reactions), which take place within the thylakoid membranes of the chloroplasts. This phase directly uses light energy to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Water is split (photolysis) to release oxygen as a byproduct, protons (H+), and electrons. These electrons are passed through a series of protein complexes in the thylakoid membrane, known as the Z-scheme, involving Photosystem II (PS II) and Photosystem I (PS I), generating a proton gradient that drives ATP synthesis. The second stage is the Light-Independent Reactions (or Dark Reactions), commonly known as the Calvin Cycle. This process occurs in the stroma of the chloroplast. It does not directly require light but depends on the products of the light reaction, ATP and NADPH. In this cycle, atmospheric CO2 is 'fixed' into an organic molecule by the enzyme RuBisCO. Through a series of reactions, this fixed carbon is reduced to form glucose and other carbohydrates, using the energy from ATP and NADPH. Essentially, the light reactions capture solar energy, and the Calvin cycle uses that energy to make food.

Must-Remember Key Points

• Overall Equation: 6CO₂ + 12H₂O → (in presence of light & chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂.
• Photosystems: PS I (P700) and PS II (P680) are the two pigment-protein complexes. PS II is involved in the photolysis of water.
• Non-Cyclic Photophosphorylation: Involves both PS I and PS II. It produces ATP, NADPH, and O₂. This is the primary pathway (Z-scheme).
• Cyclic Photophosphorylation: Involves only PS I. It produces only ATP and is used when NADPH is abundant. No O₂ is released.
• Calvin Cycle Stoichiometry: To produce one molecule of glucose (a 6-carbon sugar), the cycle needs 6 turns, consuming 6 CO₂, 18 ATP, and 12 NADPH.
• RuBisCO: Ribulose-1,5-bisphosphate carboxylase/oxygenase is the most abundant enzyme on Earth. It has an affinity for both CO₂ (carboxylation) and O₂ (oxygenation).
• C4 Pathway Advantage: C4 plants minimize photorespiration by concentrating CO₂ in the bundle sheath cells, making them more efficient in hot, dry climates.
• Limiting Factors: The rate of photosynthesis is limited by the factor in shortest supply, which could be light intensity, CO₂ concentration, or temperature.

Comparison of C3 and C4 Pathways

The Three Stages of the Calvin Cycle

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2. —
3. —

Exam Traps and Scoring Tips

A common point of confusion is the stoichiometry of the Calvin Cycle. Remember this: For 1 molecule of glucose (6 carbons), you need 6 turns of the cycle. This requires 6 CO₂, 18 ATP, and 12 NADPH. Examiners often ask questions based on producing 1, 3, or 6 molecules of glucose, so be careful with multiplication. Also, clearly differentiate between Cyclic and Non-Cyclic Photophosphorylation. Draw a simple table in your mind: Non-cyclic produces ATP, NADPH, and O₂. Cyclic produces ONLY ATP. Diagrams like the Z-scheme and the Calvin Cycle are high-scoring; practice drawing simplified versions with correct labels for reactants, products, and key enzymes like RuBisCO.

Practice Questions with Solutions

• What are the three end products of the light-dependent reactions (non-cyclic photophosphorylation)? ATP, NADPH, and Oxygen (O₂).
• In C4 plants, what is the primary CO₂ acceptor and where does this reaction occur? The primary acceptor is Phosphoenolpyruvate (PEP). This reaction occurs in the mesophyll cells.
• What is the key difference between the active site of RuBisCO and PEPCase? RuBisCO can bind to both CO₂ and O₂, leading to photorespiration. PEPCase (in C4 plants) binds only to CO₂, making it more efficient at carbon fixation.
• If a plant is moved from optimal light to a dark room, which process of photosynthesis will stop first: ATP production or glucose synthesis? ATP production (part of the light-dependent reactions) will stop almost immediately. Glucose synthesis (Calvin cycle) will continue for a very short time until the supply of ATP and NADPH from the light reaction is exhausted.

Frequently Asked Questions

Frequently Asked Questions