Photosynthesis Essay

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The leaves are the part of a plant where most photosynthesis takes place. If you cut a leaf in half and look at the cut end, it would look like this:


1. Waxy cuticle: this gives the leaf a waterproof layer, which lets in light.

2. Upper epidermis: provides an upper surface.

3. Palisade cells: contain chloroplasts.

4. Spongy mesophyll: collection of damp, loosely packed cells.

5. Lower epidermis: layer of cells on the lower surface.

6. Air space inside the leaf: allows contact between air and moist cell surfaces.

7. Stoma: a hole in the leaf through which gases diffuse.

8. Guard cells: change shape to close the stoma.

One unique feature of leaves is that they have tiny holes in them to let carbon dioxide and oxygen enter and exit. The hole formed between these cells is called a stoma. A stoma is just a hole. It is controlled by two guard cells, which change shape to either open or close the hole. Something makes water enter the cells by osmosis and so they swell up and change shape, but no one is quite sure of the trigger. The stomata (air holes) on plants are normally open during the day and closed at night. These stomata are found on the undersides of leaves. This is because if they faced the sunlight, some of the plant's precious water could evaporate out of them.

[IMAGE] Guard cells


Open stoma Closed stoma

Photosynthesis is the way that plants make their food using energy from sunlight. This is the word equation:


Plants use the green dye (or pigment) called chlorophyll to pick up the energy from the sunlight. Plants make sugar and use some of it for energy to keep them alive (respiration) but they also use some for growth and repair by making fats and proteins. However, it is not always sunny so plants need to be able to store some of the sugar they make, so they convert it to a storage carbohydrate (starch). Plants could use starch or glucose. Starch is insoluble (it does not dissolve in water) while glucose is soluble. This means that if starch is used, less water is required to keep its food stored. The amounts of water, carbon dioxide, sunlight and temperature can all affect how effectively a plant carries out photosynthesis. The amount of water is effected by how much is taken up through the roots and how much is lost from the leaves. If less water is available in the leaf then photosynthesis will occur more slowly. Similarly, if there is less carbon dioxide around then photosynthesis will occur more slowly. There wont be enough of the fuel (substrate) to get the reaction to work. If there is less sun, which usually means it is cooler too, then there is less energy for photosynthesis and it occurs more slowly. So photosynthesis works best when it is warm and sunny.


The aim of my experiment is to determine whether or not the intensity of light will affect the rate of photosynthesis in a plant. To do this, I am going to observe Canadian pond weed (Elodea) under varying light intensities. The Elodea will be submerged in water. I will count the amount of oxygen given off in this experiment by counting the number of bubbles produced. I used Canadian pondweed because of its unusual ability to emit bubbles of gas from a cut end, when placed in water.


Photosynthesis occurs only in the presence of light, and takes place in the chloroplasts of green plant cells. Photosynthesis can be defined as the production of simple sugars from carbon dioxide and water causing the release of sugar and oxygen. The chemical equation for photosynthesis can be expressed as:


[IMAGE]Carbon dioxide + water sugar (glucose) + oxygen + water

CO2 + H2O C6H2O6 + O2 + H2O

All plants need light in order to photosynthesise. This has been proven many times in experiments, so it is possible to say that without light, the plant would die. The...
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