OCR Biology A2 Unit 1 Notes-Photosynthesis

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AUTOTROPHS: organisms that use light energy or chemical energy and inorganic molecules (CO2, H2O) to make complex organic molecules.

HETEROTROPHS: organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them.

PHOTOSYNTHESIS: the process whereby light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances. The energy is used by consumers and decomposers.

PHOTOAUTOTROPHS: organisms that can photosynthesise with sunlight as their source of energy-plants, some bacteria and some protoctists.

Photosynthesis: 6CO2 + 6H2O --> C6H12O6 + 6O2

Respiration: C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy (some ATP)

Chloroplasts: vary in size, most disc shaped between 2-10um long, has a double membrane, with an intermembrane space 10-20nm wide, outer membrane permeable to small ions, transport proteins in inner membrane, folded into lamellae, each stack called a granum, intergrated lamellae, two distinct regions-stroma (fluid filled matrix-where the light independent stage of photosynthesis takes place, where the enzymes are located, starch grains, oil droplets, DNA, ribosomes, and grana-stacks of thylakoids, sites of light absorption and ATP synthesis, ligt dependent stage.)

PHOTOSYNTHETIC PIGMENTS: molecules that absorb energy from light. Each pigment absorbs a range of wavelengths in the visible region and has its own distinct peak of absorption. Other wavelengths are reflected. They are in thylakoid membranes, arranged in funnel shaped structures called PHOTOSYSTEMS.

PHOTOPHOSPHORYLATION: the making of ATP from ADP and Pi in the presence of light.

Water is a source of hydrogen ions, used in chemiosmosis to produce ATP, electrons, keeps plant cell turgid.

Light dependent stage: (occurs in the thylakoid membrane-non-cyclic photophosphorylation):

1. Light strikes PS2, exciting 2e- which leave the chlorophyll molecule from the primary pigment reaction centre (PPRC).

2. These electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP.

3. Light has also struck PS1 and a pair of electrons has been lost.

4. These electrons, along with protons produced at PS2 by photolysis of water, join NADP which becomes reduced NADP.

5. The electrons from the oxidised PS2 replace those lost from PS1.

6. The electrons from photolysed water replace those lost by the oxidised chlorophyll in PS2.

7. Protons from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP, in the stroma. They will be used in the light independent stage.

Cyclic photophosphorylation: this uses only PS1. The excited electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost. There is no photolysis of water and no generation of NADPH, although small amounts of ATP are made. This may be used in the LIS or in guard cells to bring in K atoms, lowering the wp and causing…


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