Courseworks

The general structure of a leaf

The general structure of a leaf is built to do specific, needed functions. However the main function for most leaves is ultimately photosynthesis. Other functions include: water storage/regulation; gaseous exchange; protection (for example – spikes on cacti); and trapping food in some cases (for example Venus flytraps). The annotated diagram I have drawn below shows an example of the main structures inside a dicot leaf:This shows the structure of a general dicot leaf. The xylem and phloem are shown – indicating where the vein (vascular bundle – xylem vessels) is in the leaf cross section. Throughout the leaf the veins branch a lot to distribute water and minerals and therefore the minerals (mainly magnesium and iron) needed for chlorophyll are taken directly and easily fast the palisade mesophyll and therefore each chloroplast can receive the necessary minerals without difficulty. Also nitrogen compounds needed for protein synthesis are brought through minerals transferred by the same process of diffusion through the mesophylls. Advantages of the vascular bundle include the above-mentioned processes.The cells, when fully function, work to an optimum and the plant only uses enough minerals as it need to, to create the required amount of chlorophyll for instance. However when the required amount of minerals is not present the plant receives difficulty because, if it needs that extra chlorophyll to photosynthesise a little more, and it cannot produce the chlorophyll, a chain occurs. The unmade chlorophyll reduces the efficiency of photosynthesis, therefore less ‘food’ is made and the plant finds it difficult to survive.The waxy cuticle is also another adaptation that the plant has made. It has done this to try and stop water loss through transpiration out of the top of the leaf. The wax is ideal for this as there are very few gaps in it and any gaps that are present are not large enough to create a major effect. The wax also repels the water molecules due to them both having the same negatively charged molecules. The waxy cuticle is on both the upper and lower epidermis’s but there is a thicker layer of cuticle on the upper layer of cells. This is because transpiration typically moves in an upward direction.This is because the evaporated moisture is lighter than the air and is always warmer. Therefore the hotter the moisture is the faster it moves to the upper extent. In a leaf this is the roof of the leaf, just under the epidermal cells and above the palisade mesophyll. The advantages of the waxy cuticle are mainly to reduce water loss through transpiration but also to provide a small barrier against water-borne infections/diseases and also to stop damage from rain drops in some ‘heavy rain’ areas. There are no disadvantages of the waxy cuticle that I can see that affects the plant itself.As the plant must be able to control this water exchange the plant has evolved stomatal pores. There are shown on the diagram as stoma, openings in the leaf controlled by ‘doors’ called guard cells. The stoma also control gaseous exchange in the leaf. The plant needs CO2 for respiration and it needs a way of getting rid of the waste product from respiration, O2. This is all controlled by the guard cells which open and close at specific times during the day to regulate all effecting factors of the plant. Below is a diagram taken from www.emc.maricopa.edu that shows simple diagram of a stomatal pore. I have annotated it to show how each area is designed by the plant.This shows the plant can regulate all gaseous exchange and all water storage. During the day the stomata is open as photosynthesis is occurring and the plant must respire. However, once there is no more sunlight present, photosynthesis seizes and the stomata closes as there is no need in it being open as the plant does not need to respire any more. The stomata is able to open because when the guard cells become turgid (full of water) the thicker membrane/cell wall, on the inside, does not stretch to the same degree that the other side of the guard cell. Therefore a oval shape is formed with the same shaped opening in the middle. The main advantage of the stomata closing at night is that it can regulate the water loss to another degree – it not only reduces loss of water through transpiration but it also reduces water loss of general moisture contained at the bottom of the leaf.The disadvantages of the stomata are that the plant has to have the required amount of water to trigger the responses in the guard cells. Therefore in extremes (very wet or very dry), the guard cells can be over stretch in some cases or not open enough in other cases so that the plant cannot respire to a functional level and therefore create a worryingly low amount of energy, eventually leading to a reduction in photosynthesis. This then creates a very big problem for the plant, as it is not producing enough ‘food’ for the plant to survive upon. The plant usually gets round this problem by terminating the leaf and only using the fully functional leaves until the plant can grow a replacement.The epidermal cells themselves act as a barrier. They are mainly to defend the inner tissues from, in some cases herbivores, dehydration or pathogens. The layers are both on the top and bottom of every leaf and are usually one cell thick. These cells do not have chloroplasts, which would seem a reasonable idea for the plant to think about. A secondary function of the epidermal cells is to produce and replace the waxy cuticle in areas, which it may wear thin or get destroyed for one reason or another. It can also produce the waxy cuticle to different thicknesses, which would be sensible in hotter, drier climates and therefore, in theory, reduce loss of water by transpiration to a greater extent. However this may not always be possible in the cases of extremely hot climates, such as deserts.

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