A complete summary of Photosynthesis (part of Module 5) for OCR A A-Level Biology, written by a current medical student who achieved an A* in all 4 A-Levels, including Biology. Condensed to 4 pages, easy to read and very clear layout. Written directly from the official textbook to ensure no inaccur...
PHOTOSYNTHESIS
PHOTOSYNTHESIS
- Plants, algae, some bacteria.
- Autotrophic nutrition - use of chemical energy to synthesise large organic molecules from smaller
ones (e.g. water and carbon dioxide).
- Photoautotrophs - photosynthetic organisms, or producers (first trophic level).
- Requires energy from photons (light particles containing a quantum of energy).
- Form of carbon fixation (conversion of CO2 to sugars), which requires energy and electrons
(endothermic and reduction) and regulates CO2 concentration in atmosphere/oceans.
RESPIRATION
- Exothermic oxidation of photosynthesised molecules to release chemical energy.
- Heterotrophs - non-photosynthetic organisms which obtain energy by digesting complex organic
molecules into smaller molecules, used as respiratory substrates.
COMPENSATION POINT
- Plants respire continuously, but only photosynthesise during daylight, and compete for light.
- Light intensity must be high enough to replenish stores lost in respiration.
- Compensation Point = photosynthesis and respiration in plant occurring at the same rate.
(time taken to reach this point is the compensation period).
- Sun plants take longer than shade plants to reach compensation point, as they require a
higher light intensity to reach optimum photosynthetic rate.
CHLOROPLASTS AND PHOTOSYNTHETIC PIGMENTS
- 2-10 micrometres long, double membrane (envelope), permeable outer membrane, grana
(connected by intergranal lamellae / thylakoids).
- 3 distinct membranes (outer, inner, thylakoid).
- 3 separate internal compartments (intermembrane space of width 10-20nm, stroma (fluid-filled
matrix) and thylakoid space).
Grana
- Site of light-dependent stage (stage 1).
- Contains up to 100 thylakoids (folded thylakoid membrane).
- Large SA for:
- distribution of photosystems containing photosynthetic pigments.
- electron carriers and ATP synthase enzymes to convert light energy into ATP.
- Proteins embedded in thylakoid membranes hold photosystems in place.
- Surrounded by stroma, so products of light-dependent stage can easily pass into stroma for use in
light-independent stage.
Stroma
- Contains enzymes for light-independent stage (stage 2), starch grains, oil droplets, small
ribosomes, DNA (codes for photosynthetic proteins, assembled at ribosomes).
Photosynthetic pigments (chlorophyll and accessory pigments)
- Funnel-shaped photosystems containing photosynthetic pigments are in the thylakoid membranes.
- Absorb light of particular wavelengths and reflect others. Energy associated with wavelengths
funnelled to primary pigment reaction centre at base of photosystem, consisting of chlorophyll.
- Mixture of pigments with similar molecular structure (porphyrin group which is a magnesium atom
and long hydrocarbon chain).
- Chlorophyll a (2 forms, blue-green) at centre of photosystems.
- Accessory Pigments - Carotenoids, Xanthophylls and Chlorophyll b.
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