What is the difference between the light-dependent and light-independent reactions?

The light-dependent reactions require light and occur in the thylakoid membranes. They produce ATP and reduced NADP by photolysis of water and electron transport. The light-independent reactions (Calvin cycle) do not directly require light; they use ATP and reduced NADP to fix carbon dioxide into glucose in the stroma. The Calvin cycle can occur in light or dark, but it depends on the products of the light-dependent reactions.

Why do plants appear green?

Plants appear green because chlorophyll, the main photosynthetic pigment, absorbs red and blue light most efficiently and reflects green light. The absorption spectrum of chlorophyll shows peaks in the blue-violet and red regions, while green light is poorly absorbed and thus reflected, making leaves look green.

What are limiting factors in photosynthesis?

Limiting factors are environmental conditions that can restrict the rate of photosynthesis. The main ones are light intensity, carbon dioxide concentration, and temperature. According to the Law of Limiting Factors, the rate is limited by the factor that is in shortest supply. For example, at low light, increasing CO2 has no effect; light is the limiting factor.

How is ATP produced in photosynthesis?

ATP is produced during the light-dependent reactions via chemiosmosis. Light energy excites electrons in chlorophyll, which pass through an electron transport chain. This pumps protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient. Protons flow back through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate.

What is the role of RuBisCO in the Calvin cycle?

RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyses the fixation of carbon dioxide in the Calvin cycle. It combines CO2 with RuBP (ribulose bisphosphate) to form an unstable 6-carbon compound that quickly splits into two molecules of GP (glycerate 3-phosphate). RuBisCO is crucial but inefficient, and it can also catalyse photorespiration if oxygen levels are high.

How does temperature affect photosynthesis?

Temperature affects the rate of photosynthesis primarily through its impact on enzyme activity (e.g., RuBisCO). As temperature increases, the rate increases up to an optimum (usually around 25-30°C for temperate plants). Above the optimum, enzymes denature, and the rate decreases. Temperature also affects the rate of diffusion of CO2 and the fluidity of membranes.

Photosynthesis

OCR

A-Level

Photosynthesis is the process by which light energy is harvested and used to drive the production of ATP and the synthesis of large organic molecules from inorganic precursors. This topic covers the structure of the chloroplast, the light-dependent and light-independent stages, and the factors that limit the rate of photosynthesis.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This topic is central to OCR A-Level Biology because it underpins the flow of energy through ecosystems and the production of organic molecules essential for life. You will explore the light-dependent and light-independent reactions, the structure of chloroplasts, and the factors affecting the rate of photosynthesis. Understanding photosynthesis also links to topics like respiration, energy transfer, and climate change, making it a key concept for your exams.

Photosynthesis occurs in two main stages: the light-dependent reactions (which require light and produce ATP and NADPH) and the light-independent reactions (the Calvin cycle, which uses ATP and NADPH to fix carbon dioxide into glucose). The process takes place in the chloroplasts, specifically in the thylakoid membranes (light-dependent) and the stroma (light-independent). You need to know the roles of pigments like chlorophyll a, chlorophyll b, and carotenoids, as well as the electron transport chain and chemiosmosis. Mastering these details will help you explain how plants harness sunlight and why factors like light intensity, carbon dioxide concentration, and temperature affect photosynthesis rates.

This topic is not just about memorising equations; it requires understanding how energy is transferred and how environmental factors influence biological processes. In exams, you may be asked to interpret graphs of limiting factors, calculate rates, or explain the adaptations of plants to different environments. Photosynthesis also connects to practical skills, such as using a colorimeter or measuring oxygen production. By grasping the mechanisms and their applications, you'll be well-prepared for both multiple-choice and long-answer questions.

Key Concepts

Core ideas you must understand for this topic

→The light-dependent reactions: photolysis of water, electron transport chain, chemiosmosis, and production of ATP and reduced NADP.
→The light-independent reactions (Calvin cycle): carbon fixation, reduction of GP to TP, and regeneration of RuBP.
→Limiting factors of photosynthesis: light intensity, carbon dioxide concentration, and temperature, including their interaction (Law of Limiting Factors).
→Structure of chloroplasts: thylakoids, grana, stroma, and the role of photosynthetic pigments in absorbing light energy.
→The absorption spectrum of chlorophyll and the action spectrum of photosynthesis, explaining why plants appear green.

What You Need to Demonstrate

Key skills and knowledge for this topic

Interrelationship between photosynthesis and respiration regarding raw materials and products
Structure of the chloroplast (outer membrane, lamellae, grana, thylakoid, stroma, DNA)
Role of photosynthetic pigments in light harvesting systems and photosystems
Light-dependent stage: production of ATP and reduced NADP via electron carriers, cyclic and non-cyclic photophosphorylation, and the role of water
Light-independent stage (Calvin cycle): fixation of CO2, role of RuBP, RuBisCO, and GP to produce triose phosphate (TP)
Uses of triose phosphate for synthesis of carbohydrates, lipids, and amino acids, and regeneration of RuBP
Limiting factors: CO2 concentration, light intensity, temperature, and water stress (stomatal closure)
Effect of changing limiting factors on levels of GP, RuBP, and TP

Marking Points

Key points examiners look for in your answers

Interrelationship between photosynthesis and respiration regarding raw materials and products
Structure of the chloroplast (outer membrane, lamellae, grana, thylakoid, stroma, DNA)
Role of photosynthetic pigments in light harvesting systems and photosystems
Light-dependent stage: production of ATP and reduced NADP via electron carriers, cyclic and non-cyclic photophosphorylation, and the role of water
Light-independent stage (Calvin cycle): fixation of CO2, role of RuBP, RuBisCO, and GP to produce triose phosphate (TP)
Uses of triose phosphate for synthesis of carbohydrates, lipids, and amino acids, and regeneration of RuBP
Limiting factors: CO2 concentration, light intensity, temperature, and water stress (stomatal closure)
Effect of changing limiting factors on levels of GP, RuBP, and TP

Examiner Tips

Expert advice for maximising your marks

💡Be prepared to interpret graphs showing the effect of limiting factors on the rate of photosynthesis
💡Ensure you can link the Calvin cycle intermediates (GP, RuBP, TP) to the changes in their concentrations when a limiting factor is altered
💡Use precise terminology when describing the role of RuBisCO and the regeneration of RuBP
💡Practice calculating rates of photosynthesis from experimental data provided in graphs
💡Always use the correct terminology: 'light-dependent reactions' and 'light-independent reactions' (not 'dark reactions'). The Calvin cycle can occur in light, so avoid calling it 'dark reactions'.
💡When describing limiting factors, mention the Law of Limiting Factors: the rate is limited by the factor in shortest supply. In graphs, identify the plateau and explain which factor is limiting.
💡For high marks, include details of the electron transport chain and chemiosmosis: describe how protons are pumped across the thylakoid membrane, creating a proton gradient that drives ATP synthesis via ATP synthase.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the products of the light-dependent stage with the reactants of the light-independent stage
Failing to correctly identify the specific site of each stage within the chloroplast
Misinterpreting the effect of limiting factors on the concentrations of Calvin cycle intermediates (GP, RuBP, TP)
Confusing cyclic and non-cyclic photophosphorylation mechanisms
Misconception: Photosynthesis only occurs during the day. Correction: The light-dependent reactions require light, but the Calvin cycle can continue in the dark for a short time using ATP and NADPH produced earlier. However, prolonged darkness stops the cycle.
Misconception: Oxygen produced in photosynthesis comes from carbon dioxide. Correction: Oxygen comes from the photolysis of water, not from carbon dioxide. The oxygen atoms in CO2 end up in glucose and water.
Misconception: The rate of photosynthesis always increases with light intensity. Correction: At high light intensities, other factors (e.g., CO2 concentration or temperature) become limiting, so the rate plateaus.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Cell structure: knowledge of organelles, especially chloroplasts and their membranes.
•Enzymes: understanding of how enzymes work and factors affecting their activity (temperature, pH).
•Energy and ATP: basic understanding of ATP as an energy carrier and how it is synthesised.

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Calculate

Compare

Evaluate

Predict

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Practice questions tailored to this topic

Photosynthesis

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Photosynthesis — OCR A-Level Biology

Examiner Tips for Photosynthesis

Common Mistakes in Photosynthesis

Key Marking Points

Photosynthesis

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between the light-dependent and light-independent reactions?

Why do plants appear green?

What are limiting factors in photosynthesis?

How is ATP produced in photosynthesis?

What is the role of RuBisCO in the Calvin cycle?

How does temperature affect photosynthesis?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules