How does photosynthesis capture light energy and store it as chemical energy in sugars?
Topic 3.5 Photosynthesis: explain how the light-dependent reactions and the Calvin cycle capture light energy and use it to fix carbon dioxide into sugar.
A focused answer to AP Biology Topic 3.5, covering the light-dependent reactions, the electron transport chain, chemiosmosis, the Calvin cycle, and how light energy is converted to the chemical energy of sugars.
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What this topic is asking
The College Board (Topic 3.5) wants you to explain how photosynthesis captures light energy and stores it as the chemical energy of sugars, through two linked stages: the light-dependent reactions (in the thylakoid membranes) and the Calvin cycle (in the stroma). You should be able to track where ATP and NADPH are made and used, and explain chemiosmosis.
The chloroplast and the two stages
The overall result is the conversion of light energy plus carbon dioxide and water into sugar and oxygen.
The light-dependent reactions
In the thylakoid membrane, pigments in photosystems absorb light, which excites electrons to a higher energy level.
So the light reactions produce three things: oxygen (released), ATP, and NADPH (both passed to the Calvin cycle).
The Calvin cycle
The Calvin cycle runs in the stroma and does not directly need light, but it depends on the ATP and NADPH from the light reactions, so it stops soon after the light is removed.
In the cycle, the enzyme that fixes carbon attaches carbon dioxide to a five-carbon acceptor molecule (carbon fixation). ATP and NADPH from the light reactions then provide energy and reducing power to convert the fixed carbon into a sugar (a three-carbon sugar), some of which is used to build glucose and some of which regenerates the acceptor so the cycle can continue.
Try this
Q1. Identify the products of the light-dependent reactions and state where each goes. [3 points]
- Cue. Oxygen (released as a by-product), ATP and NADPH (both passed to the Calvin cycle to drive carbon fixation).
Q2. Explain why the Calvin cycle stops soon after a plant is placed in the dark. [2 points]
- Cue. It depends on ATP and NADPH from the light reactions; in the dark these are no longer produced, so carbon fixation cannot continue.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2018 (style)4 marksSection II (long FRQ excerpt). A plant is moved into the dark. (a) Explain why the light-dependent reactions stop. (b) Predict and explain what happens to the Calvin cycle shortly after the light is removed.Show worked answer →
A 4-point explain-and-predict FRQ on the link between the two stages.
(a) Explain (2 points): (1 point) the light-dependent reactions need light to excite electrons in photosystems and split water; (1 point) without light, no electrons are excited, so no ATP or NADPH is produced.
(b) Predict and explain (2 points): (1 point) the Calvin cycle slows and stops; (1 point) because the Calvin cycle depends on the ATP and NADPH supplied by the light reactions to fix carbon dioxide and reduce it to sugar, removing that supply halts carbon fixation.
Markers reward linking the stages: the light reactions supply the ATP and NADPH that the Calvin cycle consumes.
AP 2022 (style)1 marksSection I (multiple choice). In the light-dependent reactions, what is the source of the electrons that replace those lost from photosystem II? (A) Carbon dioxide. (B) Glucose. (C) The splitting of water. (D) ATP.Show worked answer →
The correct answer is (C).
Photosystem II replaces its excited, lost electrons by splitting water (photolysis), which also releases oxygen as a by-product and hydrogen ions into the thylakoid space. (A) carbon dioxide is fixed in the Calvin cycle; (B) glucose is a product, not the electron source; (D) ATP is a product of the light reactions.
Related dot points
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A focused answer to AP Biology Topic 3.6, covering glycolysis, the Krebs cycle, oxidative phosphorylation, chemiosmosis, the role of oxygen, and fermentation, with the link back to photosynthesis.
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Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)