16558303-511a - QUESTION SHEET 1 Photosynthesis Before...

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QUESTION SHEET Photosynthesis Before studying this topic in depth, it is necessary to have a sound understanding of the structure of a palisade cell, the structure of a chloroplast as revealed by electron microscopy and the nature of biological molecules from Unit 1. Knowledge of transport across membranes from Unit 1 and transport in flowering plants from Unit 2B is also required. Photosynthesis is the process in which green plants transform light energy into chemical bond energy and results in the building up of organic molecules from the inorganic molecules carbon dioxide and water. Oxygen is produced as a waste product. In green plants, the first stable molecules formed are simple sugars, which may be used as a source of energy or used in the synthesis of other organic molecules. The process can be summarised in the following equation: chlorophyll carbon dioxide + water ———→ carbohydrate + oxygen + water light CO 2 + 2H 2 O (CH 2 O) + O 2 + H 2 O In the process, carbon dioxide is first fixed and then reduced. Leaf structure Photosynthesis occurs mainly in the leaves of green plants, but cortical cells in the stems of herbaceous plants also contain chloroplasts and can carry out the process. In most plants, the leaf is adapted for photosynthesis because: it has a large surface area over which light absorption and gas exchange can occur it is thin so that the diffusion paths for carbon dioxide in, and oxygen out, are small there is good support for the thin lamina from the midrib and veins the network of veins, containing phloem and xylem, enables efficient transport of materials to the photosynthesising cells the mesophyll tissue possesses large numbers of chloroplasts, containing the photosynthetic pigment chlorophyll, which can absorb the light energy. NAS Biology Online Teachers’ Guide Nelson Thornes © 2002 1 1
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shows the distribution of the tissues within the leaf. It can be seen that the palisade tissue, composed of palisade cells and where most of the chloroplasts are located, is situated just below the upper epidermis of the leaf. In this situation, it receives the maximum amount of available light. Palisade cells are adapted to their function in the following ways: they are columnar and packed tightly together in a compact layer just below the upper epidermis because of their orientation, the long sides of the cells are exposed to intercellular air spaces from which carbon dioxide can be rapidly absorbed they contain large numbers of chloroplasts for light absorption they have thin cellulose cell walls and a thin layer of cytoplasm so that diffusion is rapid they have large central vacuoles and their turgidity contributes to the support of the leaf tissues. NAS Biology Online Teachers’ Guide
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This note was uploaded on 12/04/2009 for the course BIOL 230 taught by Professor Gibson during the Spring '09 term at Tennessee Martin.

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16558303-511a - QUESTION SHEET 1 Photosynthesis Before...

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