Chloroplasts and the light-dependent stageEdexcel A-Level Biology B: Revision notes
Section 1
Structure of a chloroplast
A chloroplast is about 3 to 10 µm long and is bounded by a double membrane, the envelope. Inside is a fluid stroma containing the enzymes of the light-independent stage, starch grains, lipid droplets, DNA and ribosomes.
The membrane system forms flattened sacs called thylakoids. Stacks of thylakoids are grana (singular granum), and they are joined to one another by intergranal lamellae. This gives a very large surface area for light absorption.
Do not confuse the stroma (light-independent stage) with the thylakoid membranes (light-dependent stage).
Section 2
Role of the thylakoid membranes
The light-dependent stage takes place in the thylakoid membranes. They contain the photosystems (chlorophyll a and accessory pigments), electron carriers that form the electron transport chain, and ATP synthase.
The thylakoid space is where hydrogen ions accumulate, forming a proton gradient across the membrane. Light energy is converted to chemical energy as ATP and reduced NADP, and oxygen is released as a by-product.
Section 3
Non-cyclic photophosphorylation
- Light excites electrons in chlorophyll a (photoionisation), and they leave the chlorophyll.
- The electrons pass along the electron transport chain; the energy released pumps hydrogen ions into the thylakoid space.
- Hydrogen ions flow back to the stroma through ATP synthase (chemiosmosis), making ATP from ADP and phosphate (photophosphorylation).
- Photolysis of water replaces the lost electrons: 2H₂O → 4H⁺ + 4e⁻ + O₂.
- The electrons and hydrogen ions are accepted by NADP, forming reduced NADP.
Products: ATP, reduced NADP and oxygen.
In an essay say where each step happens: in the thylakoid membrane, with the hydrogen ions pumped into the thylakoid space.
Section 4
Cyclic photophosphorylation
In cyclic photophosphorylation the excited electrons from chlorophyll pass along the electron transport chain and then return to the chlorophyll. Only ATP is made, by chemiosmosis.
There is no photolysis, so no oxygen is released, and no reduced NADP is formed. The cycle supplies extra ATP: the Calvin cycle needs more ATP than reduced NADP (3 : 2), so cyclic photophosphorylation tops up the ATP supply.
Cyclic photophosphorylation makes ATP only: no reduced NADP and no oxygen.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chloroplasts and the light-dependent stage
- Palisade mesophyll cells of a leaf were examined with an electron microscope. Each chloroplast was about 5 µm long, was bounded by two membranes, and contained stacks of flattened membrane sacs joined to one another and surrounded by a fluid.Explain how the structure of the grana adapts the chloroplast for the light-dependent stage.2 marks
- In the thylakoid membranes of a chloroplast, light energy excites electrons in chlorophyll a. The excited electrons pass along a chain of electron carriers. In non-cyclic photophosphorylation the electrons are finally accepted by NADP.Explain how the movement of electrons along the chain leads to the production of ATP.2 marks
- A herbicide binds to the first electron carrier of the electron transport chain in thylakoid membranes, so that electrons can no longer pass from chlorophyll to the carrier. Isolated chloroplasts treated with the herbicide were illuminated in a solution containing ADP, phosphate and NADP.Explain why no oxygen is released from the treated chloroplasts.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).