Photosynthesis as an energy transfer process
Cambridge A-Level Biology (9700) · Unit 13: Photosynthesis · 16 flashcards
Photosynthesis as an energy transfer process is topic 13.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 13 — Photosynthesis , alongside Investigation of limiting factors. In one line: ATP and reduced NADP, produced during the light-dependent stage, supply the energy and reducing power, respectively, needed to convert carbon dioxide into glucose during the light-independent stage (Calvin cycle).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 16 flashcards — 8 definitions and 8 key concepts — covering the precise wording mark schemes reward. Use the 8 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The roles of ATP and reduced NADP in photosynthesis
ATP and reduced NADP, produced during the light-dependent stage, supply the energy and reducing power, respectively, needed to convert carbon dioxide into glucose during the light-independent stage (Calvin cycle).
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain that during photophosphorylation: • energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected) • the released energy is used to transfer protons across the thylakoid membrane • protons return to the stroma from the thylakoid space by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected)
- outline the three main stages of the Calvin cycle: • rubisco catalyses the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound • GP is reduced to triose phosphate (TP) in reactions involving reduced NADP and ATP • RuBP is regenerated from TP in reactions that use ATP
- state that Calvin cycle intermediates are used to produce other molecules, limited to GP to produce some amino acids and TP to produce carbohydrates, lipids and amino acids
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Photosynthesis as an energy transfer process
- › In C4 plants, PEP carboxylase is in the mesophyll cells to fix CO2, while Rubisco is restricted to the bundle sheath cells.
- › Note that if the action spectrum for photosynthesis is higher than the absorption spectrum of one pigment, other accessory pigments must be present.
- › Clearly state that respiration occurs continuously in plants, both in the dark and in the light.
- › When describing a graph where the x-axis is temperature or pH, use 'above/below' or 'at higher/lower values', not temporal language.
- › Remember that chloroplasts produce their own ATP via chemiosmosis on the thylakoid membrane specifically for the Calvin cycle.
Describe the relationship between chloroplast structure and function.
Chloroplasts have thylakoids arranged in grana for light-dependent reactions, maximizing light capture. The stroma contains enzymes for the light-independent reactions (Calvin cycle), providing the necessary environment for carbon fixation.
What are the roles of ATP and reduced NADP in photosynthesis?
ATP and reduced NADP, produced during the light-dependent stage, supply the energy and reducing power, respectively, needed to convert carbon dioxide into glucose during the light-independent stage (Calvin cycle).
Where do the light-dependent and light-independent stages of photosynthesis occur within the chloroplast?
The light-dependent stage occurs in the thylakoid membranes and spaces within the grana. The light-independent stage (Calvin cycle) takes place in the stroma of the chloroplast.
What are the roles of chlorophyll a, chlorophyll b, carotene, and xanthophyll in photosynthesis?
These pigments absorb different wavelengths of light, maximizing the range of light energy captured for photosynthesis. Chlorophylls directly participate in the light-dependent reactions, while carotenoids act as accessory pigments, passing energy to chlorophylls and providing protection from excessive light.
Explain how chromatography is used to separate and identify chloroplast pigments.
Pigments are separated based on their solubility in the solvent and affinity for the stationary phase. The Rf value (distance moved by pigment / distance moved by solvent) is calculated and compared to known values to identify each pigment.
Distinguish between cyclic and non-cyclic photophosphorylation.
Cyclic photophosphorylation involves only photosystem I (PSI), producing ATP but no reduced NADP. Non-cyclic photophosphorylation involves both photosystems I and II (PSI and PSII), producing both ATP and reduced NADP, and also involves the photolysis of water.
Describe the process of photoactivation of chlorophyll during the light-dependent stage.
Chlorophyll molecules absorb photons of light energy, exciting electrons to a higher energy level. These energized electrons are then passed along the electron transport chain, initiating the light-dependent reactions.
What is the role of the oxygen-evolving complex in non-cyclic photophosphorylation?
The oxygen-evolving complex (OEC) is an enzyme within photosystem II (PSII) that catalyzes the photolysis of water. This reaction splits water molecules into electrons (to replenish PSII), protons (contributing to the proton gradient), and oxygen gas (released as a byproduct).
Describe the role of the electron transport chain in photophosphorylation.
Energetic electrons release energy as they pass through the electron transport chain within the thylakoid membrane. This released energy is used to pump protons (H+) from the stroma into the thylakoid space, creating a proton gradient.
Explain how ATP is synthesized during photophosphorylation.
Protons (H+) accumulate in the thylakoid space, creating a high concentration gradient. Protons return to the stroma through ATP synthase via facilitated diffusion. This movement of protons provides the energy for ATP synthase to catalyze the production of ATP from ADP and inorganic phosphate.
Outline the first stage of the Calvin cycle, including the enzyme involved.
The first stage is carbon fixation. Carbon dioxide combines with ribulose bisphosphate (RuBP), a 5-carbon compound. This reaction is catalyzed by the enzyme rubisco, resulting in two molecules of glycerate 3-phosphate (GP), a 3-carbon compound.
Describe the second stage of the Calvin cycle and its products.
The second stage is the reduction of glycerate 3-phosphate (GP). GP is reduced to triose phosphate (TP) using reduced NADP (NADPH) and ATP, which provides the necessary energy and reducing power for the reaction.
Outline the final stage of the Calvin cycle.
The final stage is the regeneration of RuBP. Triose phosphate (TP) is used to regenerate ribulose bisphosphate (RuBP), the initial 5-carbon acceptor molecule, allowing the cycle to continue. This regeneration requires ATP.
What role does rubisco play in photosynthesis?
Rubisco is the enzyme that catalyzes the initial fixation of carbon dioxide in the Calvin cycle. It combines carbon dioxide with ribulose bisphosphate (RuBP) to form glycerate 3-phosphate (GP).
Name the products of the light-dependent reactions that are used in the Calvin cycle.
The light-dependent reactions produce ATP and reduced NADP (NADPH). These are both essential for the Calvin cycle, ATP provides energy, and NADPH provides the reducing power (electrons).
What are glycerate 3-phosphate (GP) and triose phosphate (TP) used for within a plant cell?
GP can be used to produce some amino acids. TP can be used to produce carbohydrates (like glucose and starch), lipids, and also some amino acids, effectively linking photosynthesis to other vital metabolic pathways.
More topics in Unit 13 — Photosynthesis
Photosynthesis as an energy transfer process sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Photosynthesis as an energy transfer process deck
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