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
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe how chloroplast structure, seen in electron micrographs and diagrams, relates to what chloroplasts do.
- Explain that the Calvin cycle (the light-independent stage) uses the energy carried as ATP and reduced NADP, made in the light-dependent stage, to build complex organic molecules.
- State that, inside a chloroplast, the light-dependent stage takes place in the thylakoids (thylakoid membranes plus thylakoid spaces), which are stacked into grana, while the light-independent stage takes place in the stroma.
- Describe how the chloroplast pigments carotene, xanthophyll, chlorophyll a and chlorophyll b absorb light in the thylakoids.
- Interpret action spectra for photosynthesis and absorption spectra for the pigments in chloroplasts.
- Describe how chromatography separates and identifies chloroplast pigments, and use the technique yourself, referring to Rf values when identifying each pigment.
- State that both cyclic and non-cyclic photophosphorylation take place in photosynthesis, during its light-dependent stage.
- Explain that cyclic photophosphorylation involves photosystem I (PSI) only, includes photoactivation of chlorophyll and produces ATP.
- Explain that non-cyclic photophosphorylation uses both photosystems, photosystem II (PSII) and photosystem I (PSI); involves photoactivation of chlorophyll; includes photolysis of water, catalysed by the oxygen-evolving complex; and makes both reduced NADP and ATP.
- Explain that in photophosphorylation: energetic electrons give up energy while moving along the electron transport chain (carrier details not needed); this energy pumps protons across the thylakoid membrane; protons return from the thylakoid space to the stroma, passing through ATP synthase by facilitated diffusion, and this provides the energy to make ATP (ATP synthase details not needed).
- Outline the Calvin cycle's three main stages: rubisco catalyses CO₂ fixation, in which CO₂ combines with ribulose bisphosphate (RuBP, 5C) to form two 3C molecules, glycerate 3-phosphate (GP); ATP and reduced NADP are used to reduce GP to triose phosphate (TP); some TP is used, with ATP, to regenerate RuBP.
- State that intermediate compounds of the Calvin cycle are used to make other molecules, limited to: some amino acids made from GP, and lipids, carbohydrates and amino acids made from TP.
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
- › If photosynthesis happens at wavelengths that chlorophyll a absorbs poorly, other (accessory) pigments must be absorbing that light.
- › Remember that chloroplasts produce their own ATP via chemiosmosis on the thylakoid membrane specifically for the Calvin cycle.
- › Attribute pigments and ATP synthase to the 'thylakoid membrane', but assign the accumulation of hydrogen ions to the 'thylakoid lumen'.
- › Distinguish between light 'absorbed' by a sample (biological) and 'absorbance' (the quantity a colorimeter reads).
- › State clearly that accessory pigments absorb different wavelengths of light and pass the absorbed energy to the primary pigment in the reaction centre.
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?
All four pigments absorb light in the thylakoids, each at different wavelengths, which widens the range of light used. Chlorophyll a is the primary pigment in the reaction centres of the photosystems and gives off excited electrons. Chlorophyll b, carotene and xanthophyll are accessory pigments that pass absorbed energy on to chlorophyll a, and carotenoids also protect against excess 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.
Review the material
Read full revision notes on Photosynthesis as an energy transfer process — definitions, equations, common mistakes, and exam tips.
Read NotesMore 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.
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