8303839-02-photosynth - September 1997 B io Factsheet +...

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1 B io F actsheet September 1997 Number 2 The essential guide to photosynthesis Photosynthesis is the use of light energy from the sun to fix carbon dioxide i.e. turn it into sugars. These sugars can then be converted into other essential substances - fats and proteins etc. - which plants need to live and grow. At GCSE level the process of photosynthesis is represented by the following equation: 6CO 2 + Carbon dioxide, a colourless gas which makes up 0.04% (by volume) of the atmosphere. Enters through microscopic pores (stomata) on leaves. 6H 2 O Root hairs absorb water passively i.e. no energy is required. Water is needed to: 1. Keep plant tissues turgid eg. to maximise leaf surface area for light absorption. 2. As a source of electrons in non- cyclic photophosphorylation (Fig 2). 3. As a solvent (all chemical reactions must occur in solution). The reaction is catalysed by light energy absorbed by chlorophyll contained in chloroplasts in leaves and green stems. C 6 H 12 O 6 + Glucose, which can be used to make other substances such as fats and proteins etc. 6O 2 Oxygen, which may be used by the plant in respiration or which may diffuse out of the leaves via the stomata. The equation is an over-simplification. 1. There is clear evidence that photosynthesis occurs in two stages - one which is light dependant (the LDS) and one which is light independent (the LIS). 2. Glucose is not the first or only useful product. 3. Only visible light - a small part of the electromagnetic spectrum - is used. 4. Certain wavelengths are much more important than others Chlorophyll and light absorption Chlorophyll absorbs light from the visible part of the electromagnetic spectrum. Chlorophyll is made up of a number of different pigments: chlorophyll a, chlorophyll b, chlorophyll c along with other pigments such as carotenoids. Each of these absorb different wavelengths of light so that the total amount of light absorbed is greater than if a single pigment were involved. Not all wavelengths of light are absorbed equally. An absorption spectrum is a graph showing the percentage absorption plotted against wavelength of light (Fig 1). An action spectrum is a graph showing the rate of photosynthesis plotted against wavelength of light (Fig 1). The similarity between the absorption spectrum and the action spectrum shows that red (650- 700nm) and blue (400-450nm) wavelengths, which are absorbed most strongly, are also the wavelengths which stimulate photosynthesis the most. Green light (550mm) is mostly reflected. Fig 1. The absorption and action spectra Structure to function: chloroplast 1. Internal compartmentalisation. LDS and LIS effectively separated, thus allowing rate- determining factors such as pH and enzyme concentrations to be optimized. 2. DNA and ribosomes means chloroplast can code
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8303839-02-photosynth - September 1997 B io Factsheet +...

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