Photosynthesis

Photosynthesis is the process used by plants to synthesise (make) many of the chemicals they need. Most of the substance of a plant (except the water) was made by photosynthesis. The process is therefore vital for the survival of all the organisms on this planet, including humans which grow plants to eat and to feed to animals. Many chemicals are made by photosynthesis, but the main one we will look at is glucose, which is the main food source for the plant to provide it with energy.

Glucose is made from Carbon, Hydrogen and Oxygen (C6H12O6). Complicated substances like glucose need to be built by an organism, which is why we say that they are organic. If you went to the moon or Venus you would not find any complicated substances like this. You would only find simple substances like Carbon dioxide (CO2) and water (H2O). Plants use the Carbon and Oxygen from CO2 and the Hydrogen from H2O to synthesize molecules of glucose. The Oxygen from the H2O is not needed, so is given off as a waste gas. A lot of energy is needed to convert simple substances into more complicated ones. Plants get this energy from sunlight. This is why glucose (a kind of sugar) contains so much stored chemical energy.

We can summarise photosynthesis with the following word equation (you don?t need to know the chemical equation):

Carbon dioxide + Water + Sunlight 輯font> Glucose + Oxygen

Sunlight is absorbed by a chemical in plants called chlorophyll. Chlorophyll is green, and it is what gives plants their distinctive colour. The reason it is green is because green light cannot be used for photosynthesis (only the red and blue wavelengths of light can be used), so it is reflected.

Like all chemical reactions, photosynthesis works faster in warm conditions than in cold conditions. It also works faster if there is more Carbon dioxide and sunlight and plenty of water. This is why plants grow so quickly in El Salvador in the wet season. If any of these factors is in short supply the rate of photosynthesis will be less and the plant will not grow as fast. Farmers sometimes use greenhouses to increase the temperature, or grow plants on slopes that face the sun, or even pump extra CO2 into their greenhouses to increase the rate of photosynthesis.

Conversely, a lot of photosynthesis can affect the environment, such as aquatic environments and the atmosphere. Most of the Oxygen in the air has come from photosynthesis. The more photosynthesis that takes place, the more Oxygen is produced and the less CO2 remains.

Most photosynthesis takes place in leaves, which are specially designed for the job they do. The flat shape provides a big surface to allow more sunlight to be trapped, but is thin enough to allow CO2 to diffuse in and O2 to diffuse out. The leaves need to have an outer layer of wax, called cuticle, to prevent them from drying out. This is made by the outer protective layer of cells called the epidermis. These cells do not take part in photosynthesis. The water is transported from the roots to the leaves in the xylem vessels, which are found in the veins of the leaf. The veins also contain phloem to transport the glucose (and other products of photosynthesis) to the rest of the plant.

Near the surface of the leaf lie the palisade cells which have large numbers of chloroplasts containing chlorophyll to trap sunlight. The underside of the leaf has small holes called stomata. This is where CO2 and O2 diffuses in and out. The size of the hole can be altered by the guard cells on each side of the stoma. This means that the stomata can be closed to prevent drying out in dry conditions. The diffusion of these gasses around the inside of the leaf is made easier because the spongy mesophyll cells in the lower half of the leaf have air pockets between them to increase the movement of air.

You may remember back to topic 2 where we looked at osmosis. If a cell has lots of dissolved solutes it will tend to absorb more water and could eventually burst. For this reason plants cannot store very much glucose, so they convert it into starch before they store it. Starch is made from about a thousand glucose molecules joined together into a complex chain. The size of the starch molecules means that they cannot dissolve very easily, so they do not cause very much osmosis, which makes them ideal for storage.

You need to remember how to perform starch tests on leaves.
1. Boil leaf in water for one minute to kill the cells.
2. Place the leaf in a boiling tube of alcohol and boil the leaf for 10 minutes (do not use a flame, but place the tube of alcohol in a beaker of boiling water). This removes the chlorophyll and makes the leaf white so that you can see the results of the Iodine test.
3. Dip the leaf in water to soften it.
4. Lay the leaf on a flat surface and add a few drops of Iodine.
5. The presence of a blue/black colour indicates the presence of starch.

We saw how Carbon, Hydrogen and Oxygen are used in photosynthesis to make glucose and other organic substances. Many of these other substances contain other important elements such as Nitrogen. Nitrogen is particularly important to make proteins. However, plants cannot take Nitrogen directly from the air. It first needs to be in the form of nitrate (NO3-) ions. These (and other important ions) are found in the soil, dissolved in the water. When the plant absorbs water through the roots it also takes in the nitrates.