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Auxins: Plant growth Hormones.

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Biology Revision Notes Auxins: Plant growth Hormones Auxins are hormones and are "chemical messengers" The have many roles in plants including: 1. Growth of cells by cell elongation 2. Prevention of side shoot development Growth of cells The auxin in the cells makes the cells stretchy and this is called cell elongation without it the cell would just continue to multiply Auxins allow the cellulose cell walls of young plant cells to become stretchy Because the cells contain sugars and salts, they will take up water (by osmosis) and expand. Light influences the movement of Auxin within a plant i.e. the Auxin MOVES AWAY FROM THE LIGHT. This causes the pants to grow towards the light The reason that it does this is because the auxin elongates the cells on the side that it is on (the side opposite the light) and elongates the cells making the plant longer on one side so naturally the plants bends towards the light Summary * Auxin causes cell elongation and hence growth * Auxin is produced in the tip of a shoot * Auxin is a water soluble chemical which can be absorbed into agar Prevention of side shoot development The apical bud is the source of Auxin .If the bud is present it will prevent side shoot development =Apical dominance This means that it is a tall plant to out compete all other plants for light. ...read more.


2. The have stomata controlled by guard cells which allow the diffusion of gases into and out of the leaf. 3. Most of the chlorophyll is packed into chloroplasts in the palisade cells which are just below the upper epidermis for maximum light absorption. Action of the guard cell In the light when the plant is photosynthesising, it produces Glucose which increases the concentration of the cell sap and causes it to absorb water by osmosis. This increases the pressure in the guard cells and causes them to stretch and bend, thus opening the stoma. In the inflated state , the cells are said to be Turgid In the dark , photosynthesis stops producing glucose, and the glucose already present is used up by respiration. The guard cells may lose some water by osmosis to the surrounding cells , so they become limp or flaccid. This is the equation for photosynthesis: Light energy Carbon dioxide + water glucose + oxygen Chlorophyll Limiting factors of photosynthesis The rate at which photosynthesis occurs is limited by the following factors: 1. Water- this has and an indirect affect. Lack of water causes the stoma to close and this reduces the amount of carbon dioxide the leaves can absorb Extreme water shortage kills the plant. 2. Carbon dioxide- increasing the carbon dioxide levels leads to increased photosynthesis up to a certain point after which further increase in carbon dioxide levels has no beneficial effect. ...read more.


Regulates the amount of light going into the eye. BLINDSPOT/OPTIC NERVES: the optic nerve leaves the eye at the blind spot and carries all of the sensory nerves to the brain FOVEA; part of the retina opposite the lens with the highest concentration of light receptors for the most detailed images. The heart Arteries :carry blood to tissues Veins: carry it back to the heart Capillaries: allow exchange of chemicals with tissues e.g. glucose + O2 Artery - Carrie sblood away from the heart - blood is under high pressure - therefore they have a tough outer coat to prevent damage - Elastic fibres to smooth out pressure - Circular in cross section Veins - Blood towards heart - under low pressure - Valves to prevent back flow - Irregular in cross section Capillary - the vessels in which chemicals are carries in blood stream are exchanged with living tissue - Link arteries to veins - One cell thick - to speed up diffusion - No valves Blood 1. Red blood cells * Shape inportant- the shape of the red blood celss have a large surface area for maximum uptake of oxygen * RBC's filled with heamoglobin( even the nucleus is excluded to make more room) Heamoglobin (hb) reacts with oxygen but can but can give up that oxygen in the tissues whereit is being usedup i.e. Lungs Hb +o2 Hbo2 Tissues Oxy heamoglobin ...read more.

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