OCR A2 Biology F214 - Photosynthesis

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Photosynthesis Revision Notes
Autotrophs ­ Organisms that use light energy or chemical energy and inorganic molecules (CO2 + H2O) to synthesise
complex organic molecules.
Chemoautotrophs ­ Use chemical energy.
Phototrophs ­ Use light energy.
Heterotrophs ­ Organisms that ingest & digest complex organic molecules, releasing the chemical potential energy
stored in them.
Photosynthesis ­ The manufacture of carbohydrates (organic molecules) from inorganic substances (water and
carbon dioxide) using light energy: light energy is converted into chemical potential energy, released by respiration.
Photosynthetic Pigment ­ Molecules that absorb light energy. Each pigment absorbs a range of wavelengths in the
visible spectrum and has its own distinct peak, the others are reflected. Located in thylakoid membranes.
Primary Pigments ­ Reaction centres of every photosystem e.g. Chlorophyll a.
Photosystem II ­ Where P680 is found with peak absorption at wavelength 680nm.
Photosystem I ­ Where P700 is found with peak absorption at wavelength 700nm.
Accessory Pigments ­ Absorb different wavelengths of light and pass the energy to the primary pigment in the
reaction centre of photosystems e.g. Chlorophyll b, carotene and xanthophyll.
Absorption and Action Spectra ­ A graph showing the wavelengths of light absorbed by a pigment & the rate of
reaction.
Chlorophyll a - 450-480nm & 660-710nm (blue and red) Pigment appears yellow-green *efficient*.
Chlorophyll b ­ 480-510nm & 630-670nm (blue and red) Pigment appears blue-green *efficient*.
Carotenoids ­ 460-490nm & 510-530nm (blue and green) Pigment appears yellow-orange.
Light Dependent ­ Takes place on thylakoid membranes. Requires light energy to be converted into chemical energy.
Photophosphorylation ­ Formation of ATP in the presence of light energy. Takes place in the thylakoid membranes
of chloroplasts. Two types are cyclic and non-cyclic, during the light dependent stage.
Photolysis ­ Enzyme catalysed reaction where water molecules are split, using light energy. It occurs in photosystem
II during the light dependent stage.
Electron Carriers ­ Molecules that transfer electrons. Reducing agent.
Electron Acceptors ­ Chemicals that accept electrons from another compound. Acts as an oxidising agent.
Oxidation ­ Chemical reactions involving the loss of electrons, gain of oxygen or loss of hydrogen atoms.
Reduction ­ Chemical reactions involving the gain of electrons, loss of oxygen or gain of hydrogen atoms.
Light Independent ­ In stroma and chloroplasts (contains enzymes). No light is required, but is does require two of
the end products of light dependent stage ­ Reduced NADP, ATP and carbon dioxide to make organic molecules.
Chloroplast Structure ­ Phototrophs have specialised organelles within their cells called chloroplasts; where
photosynthesis takes place.
Limiting Factor ­ The factor present in the lowest amount, therefore limits the rate of the overall reaction.
Photosynthesis is the production of organic compounds such as glucose from inorganic compounds like carbon dioxide
and water using energy from sunlight.
6CO2 + 6H2O (+ Light energy) C6H12O6 + 6O2 (Oxygen isn't a waste product as it can be used in respiration.)
The Photosynthesis can be divided into two stages

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ATP and reduced NADP by photophosphorylation and the splitting of water
(photolysis)
light independent stage uses the ATP and reduced NADP from the light dependent stage to convert carbon dioxide
into sugars via the Calvin Cycle.
Light Dependent Stage - generating ATP and NADPH2
Cyclic Photophosphorylation
This uses only photosystem I (P700). The excited electrons pass to an electron acceptor and back to the chlorophyll
molecule from which they were lost.…read more

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Water is a source of:
Hydrogen ions, which are used in Chemiosmosis to produce ATP. These protons are then accepted by a
Coenzyme NADP which becomes Reduced NADP, to be used during the light independent stage to reduce
carbon dioxide and produce organic molecules.
Electrons to replace those lost by the oxidised chlorophyll.
It also keeps the plant turgid, enabling them to function.
Light Independent Stage ­ Calvin Cycle
1.…read more

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The electron carriers are held in position in membranes to allow rapid transfer of electrons.
ATPase enzymes are found in the membranes, these are impermeable to hydrogen ions which accumulate in
the thylakoid compartment.
The hydrogen ions must cross the membranes down their diffusion gradient via the ATPase enzymes, the
energy released by this process is used to generate ATP.
The stroma holds the enzymes for Calvin cycle.…read more

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