A-Level Biology OCR Notes
5.2.1 Photosynthesis
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Photosynthesis
- Photosynthesis is the process in plants, from which energy from sunlight is used to convert inorganic molecules into organic molecules
- The light-dependent reaction occurs in the thylakoids of the grana in chloroplasts
- Photolysis of water requires light energy to break the bonds between oxygen and hydrogen atoms
2H2O -> 4H^+ + 4e^- + O2
- Chlorophyll molecules absorb light energy via photosystem II, exciting a pair of electrons to a higher energy level, leaving the chlorophyll molecules ionized. The electron passes through an electron transfer chain to produce ATP, and reaches photosystem I.
- The electrons replace the electrons lost in photosystem I when it absorbs light to reduce NADP with the protons created from photolysis
- The photoionized chlorophylls electrons in photosystem II are replaced by the electrons from photolysis of water
- Cyclic photophosphorylation only uses photosystem I, where the electrons are passed back to photosystem I rather than NADP via electron carriers, producing small amounts of ATP
- The light-independent reaction occurs in the stroma of chloroplasts
- The Calvin cycle depends on the products from the light dependant stage
- The fixation of carbon dioxide is catalysed by RuBisCo
- 5 out of every 6 TP molecules are used to regenerate RuBP instead of producing hexose sugars
- The rate of photosynthesis is limited by temperature and the availability of carbon dioxide, water & light energy.
- The law of limiting factors states that at any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value
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- Chromatography can be used to separate out photosynthetic pigments, identifying them by their Rf value
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