Basic Summary Of Photosynthesis

Biology Notes PHOTOSYNTHESIS (90-93) There are TWO main stages of photosynthesis: - Light-dependent stage, in which water is broken down into hydrogen and oxygen using light energy - Light-independent stage in which the hydrogen reacts with carbon dioxide to form a carbohydrate. Takes place in leaf  green parts  chloroplasts  contain all machinery needed for LDS and LIS. Light-independent takes place  Stroma Light-dependent takes place  Thylakoid membranes Equation: 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H20 Light-Dependent Stage: • Photophosphorylation • Photolysis • Z-scheme Photophosphorylation - Chlorophyll a absorbs light - Energy level of one of its electrons = raised - In this excited stage  electron can leave the chlorophyll molecule and pass to another molecule - Electrons in excited state are passed along a chain of electron carrier molecules called an electron transport system - Each molecule along the chain wants the electron more than the previous one and so its passed on through the chain - As electrons pass down the system in a series of redox reactions they release energy which is used to make ATP. Basically light energy is used to add a phosphate group to ADP.

Photolysis

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This occurs when water molecules are split with the aid of light energy absorbed by chlorophyll a. - Chlorophyll a absorbs light - It then looses an electron = positively charged - In this oxidized state the chlorophyll becomes electron hungry, it needs to replace the electron before reacting again - The enzyme extracts electrons from water to replace each electron lost from chlorophyll a

= water molecules split = electrons, protons and oxygen  produced Oxygen  is given up as waste gas/respiration Protons  used to reduce NADP+  NADPH Z-scheme  Photophosphorylation and photolysis = not isolated events but are linked - The Z shaped pathway (N) show the fact that the energy levels of the electrons is twice boosted by the light absorbed by the photosystems and lowered as they pass through the electron transport system - The final acceptor of electrons = NAD+ which is reduced to NADPH which stores the electrons and protons until they can be transferred to carbon dioxide (LIS) Photosystem 1  P700 Photosystem 2  P680 (first) Non-cyclic Photophosphorylation  involves both photosystems Cyclic Photophosphorylation  involves only photosystem 1 Summary The light-dependent stage of photosynthesis uses energy from sunlight to make ATP and splits water to release protons and electron which reduce NADP+ to make NADPH. The ATP and NADPH are used in the next stage to synthesis carbohydrates from CO2 Light-Independent stage  The Calvin Cycle During this stage CO2 is fixed. It is taken up into a plant by being converted into biochemical products in the plant. Overall  Carbon dioxide reduced to glucose Enzymes convert the carbon dioxide to glucose in a series of reaction – CALVIN CYCLE – these reactions use ATP and NADPH which are the products of the LDS.

Calvin Cycle  Cyclic chain of reactions converting CO2  glucose - CO2 fixation involves a 5-carbon compound called ribulose biphosphate (RuBP) and an enzyme called rubisco (same as above + carboxylase) - With the help of rubisco, RuBP combines with CO2 = unstable sixcarbon compound formed which splits immediately into two molecules of a three-carbon compound; glycerate 2-phosphate (GP) rubisco

RuBP + CO2  2GP 5C

1C

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2 times 3C

This GP is then reduced to a triose phosphate called GALP. This reduction requires hydrogen from NADPH and energy from ATP.

ATP --- ADP + Pi

GP  triose phosphate (GALP) NADPH --- NADP+

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About 1/6 of the GALP is used to make glucose (which can be converted into other carbohydrates e.g. sucrose, starch and cellulose) amino acids, fatty acids or glycerol. The remaining 5/6 = converted back to RuBP = requires phosphate from ATP.

Summary Basically CO2 is converted to glucose, amino acids, fatty acids or glycerol. This conversion is explained in the Calvin cycle and uses RuBP and rubisco. GALP  glyceraldehyde 3-phosphate

GP  glycerate 3-phosphate