Define transpiration
Loss of water vapour through the stomata
Describe stomata. How, when, and why do they open and close?
- Stomata are tiny pores on leaves for gas exchange.
- Guard cells control each stoma, swelling to open and shrinking to close it.
- Open during the day to let in CO₂ for photosynthesis and release O₂.
- Close at night or in dry conditions to conserve water.
- Help plants balance CO₂ intake for photosynthesis with water retention.
Light Reactions: Describe photoexcitation, photosystem I (P700) and photosystem II (P680)
Photoexcitation happens when light hits P680 and P700, making them excited and ready to move electrons.
In Photosystem II, P680 gets excited and gives up an electron, making it need more electrons. It takes electrons from water, splitting it and releasing oxygen. This starts the electron transport chain.
In Photosystem I, more light excites P700, and its electrons are passed to another molecule to keep the process going.
Describe chemiosmosis
Chemiosmosis in photosynthesis involves creating a proton gradient across the thylakoid membrane. This is done by:
- Protons are added to the lumen through redox reactions.
- Protons are also added from water splitting inside the lumen.
- Protons are removed from the stroma to make NADPH, lowering proton concentration in the stroma.
This gradient creates a proton-motive force that drives protons through ATP synthase, producing ATP, similar to the process in cellular respiration.
What is the purpose of NADPH & ATP?
- NADPH: Acts as an energy carrier in cells, providing energy and hydrogen ions (H⁺) for reactions like the Calvin cycle in photosynthesis.
- ATP: Main energy source for cells, used in reactions and cell functions. Made during cellular respiration, glycolysis, the Krebs cycle, and photosynthesis.
Describe cyclic electron flow and why it happens
- Photosystem I absorbs light and excites an electron.
- The excited electron is passed to a molecule called ferredoxin.
- Instead of going to NADP⁺, the electron is sent back to a carrier called plastoquinone.
- Plastoquinone moves the electron in a cycle, transferring protons across the thylakoid membrane.
- This creates a proton gradient that drives ATP production.
- No NADPH is made, and no water is split in this process—only ATP is produced.
Why it happens: Cyclic electron transport plays an important role in overall photosynthesis. The reduction of carbon dioxide by the Calvin cycle requires more ATP than NADPH, and the additional ATP molecules are provided by cyclic electron transport.
Non-cyclic electron flow
- Oxidation of P680: Light energy excites the P680 molecule in photosystem II, which then loses an electron to a nearby electron acceptor.
- Oxidation-reduction of Plastoquinone: The electron moves from the primary acceptor to plastoquinone (PQ), which shuttles electrons to the cytochrome complex. PQ also picks up protons (H+) from the stroma and releases them into the lumen, increasing proton concentration there.
- Electron Transfer by Plastocyanin: The cytochrome complex passes electrons to plastocyanin, a carrier that transports them to photosystem I.
- Oxidation-Reduction of P700: When photosystem I absorbs light, the P700 molecule becomes excited and transfers an electron to its primary acceptor. P700 then regains an electron from plastocyanin.
- Electron Transfer to NADP+ by Ferredoxin: The excited electron from P700 is passed to ferredoxin, which transfers it to NADP+, reducing it.
- Formation of NADPH: A second electron and a proton combine with NADP+ to form NADPH, a molecule carrying high-energy electrons. The proton movement during these reactions builds a gradient, which powers ATP production.
This series of steps, known as the linear pathway, results in the production of NADPH and ATP, which are used for energy in the plant.
