• Revision:Photosynthesis OCR Notes

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Photoautotrophs - Make organic molecules from non-organic molecules using sunlight (energy from sunlight), such as green plants.

Chemoautotrophs - Use chemical energy to make organic molecules from non-organic molecules, such as nitrifying bacteria.

Photosynthesis is the fixation of carbon dioxide and it's reduction using hydrogen from water.

nCO2 + nH2O ---> n(CH2O) + nO2

Since starch and hexose sugars are normally formed:

6CO2 + 6H2O ---> C6H12O + 6O2

Light Dependent Stage

Only takes place in the presence of pigments which absorb certain wavelengths of light.

Light is necessary for the splitting of water to hydrogen and oxygen which is given off as a waste product.

Energy is also needed for the formation of ATP used in the light independent stage.

1. Light is trapped by photosynthetic pigments which trap different wavelengths of lights. These are known as chlorophylls and caretenoids. Chlorophylls absor mainly red, blue and violet wavelengths, whereas caretenoids absorb blues and violets.

2. The absorption spectra can be viewed graphically and the general shapes should be known. Visible light 400nm - 700nm.

Click here to see graph.

3. The light absorbed is converted into chemical energy. The light absorbed excited electrons in the pigment molecules.

4. The photosynthetic pigments fall into two groups; primary pigments and accessory pigments. Primary pigments are two versions of chlorophyll a, each of which have a slightly different peak absorbance. (700nm and 680nm). The accessory pigments are various forms of chlorophyll a, chlorophyll b and caretenoids.

5. The accessory pigments are arranges into light absorbing clusters called photosystems. The light energy trapped by the accessory pigments is passed to the primary pigment which acts as a reaction centre. Photosystem I is generally known as P700 and photosystem II is generall known as P680.

6. The reactions that take place in the reaction centre are the production of ATP via photophosphorylation, the splitting of water by photolysis into O and hydrogen ions and the reduction of NADP to NADPH2.ATP and NADPH2 are then passed into the light independent stage.

Cyclic Photophosphorylation

Involves only photosystem I. Light is absorbed by photosystem I and passed to P700. Electrons in P700 become excited to a higher energy level and is emitted from the chlorophyll molecule. It is then captured by an electron acceptor and passed back to P700 via a chain of electron carriers. This reaction gives out enough energy to create ATP from ADP + Pi, and the ATP passes into the light independent reaction.

This type of phosphorylation takes place mainly in lower light levels.

Non-Cyclic Photophosphorylation

1. Light is absorbed by each photosystem and excited electrons are emitted from P700 and P680.

2. These electrons are absorbed by electron acceptors and pass along a chain of carriers, leaving the photosystems positively charged.

3. P700 absorbs electrons from photosystem II. P680 receives electrons from the photolysis of water.

4. ATP is synthesised as the electrons lose energy passing along the carrier chain.

Photolysis of Water

1. Photosystem I has an enzyme capable of splittling water into 2H+ and O.

H2O ---> 2H+ + 0.5O2

2. Oxygen is a waste product. Hydrogen ions combine with electrons from photosystem I and NADP to give NADPH2.

2H+ + 2e- + NADP ---> NADPH2

3. This then passes into the light independent reaction and is used for the fixation of CO2.

Accessory Pigments

Accessory pigments such as chlorophyll b (P680) and carotenoids are in photosystemII to maximise the amount of light wavelengths absorbed. The green light is reflected by cholorophyll a and b, which causes the green colour in leaves. In the autumn these leaves turn red/orange as the leaf breaks down the chlorophyll a and b to preserve the Mg in the molecule when the leaf falls of the tree/plant until the next spring. The remaining pigments are carotenoids which are not broken down and reflect red wavelenghts causing us to see the leaves as red.


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