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Enzymes are protein molecules, which can be defined as biological catalysts. They speed up chemical reactions, but they don’t alter throughout the reaction. Enzymes are globular proteins, the enzyme molecules is coiled into a precise three-dimensional shape, with hydrophilic R groups on the outside of the molecule ensuring that they are soluble. They possess and active site, which is a precise shape indented into the enzyme, this acts like a ‘lock’ for the ‘key’ that is attached to the substrate of the enzyme.

The shape of this active site allows the substrate to fit perfectly, and to be held in place by temporary bonds, then for it to be split into other molecules, or 2 or more molecules to be joined together. When the reaction is finished the products leave the active site and the enzyme is left unchanged and ready to receive another substrate molecule. As enzymes are catalysts they increase the rate at which chemical reactions occur.

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Most reactions without the presence of enzymes would occur so slowly that they would not happen at all. In many reactions the substrate will not be converted to a product unless it is temporarily given some extra energy, this is called activation energy. This is important because chemicals do not have to be heated up as much for the reaction to be efficient, in some cases heating the chemicals could cause irreversible damage to certain molecules, especially protein molecules. So enzymes prevent these molecules being damaged.

Collision theory-simplified, if there is a larger number of molecules then they are more likely to collide (higher concentration, also the faster molecules are moving the more likely they are to collide with each other (higher temperature). This means that if they collide more often (an enzyme and a substrate) then the reaction will occur faster. Increase in substrate concentration will affect rate of reaction. As substrate concentration increases the initial rate of reaction increases, as the more substrate molecules there are, the more often an enzyme’s active site can bind with one.

However if we go on increasing substrate concentration keeping the enzyme concentration constant, there comes a point where every enzyme active site is working constantly. If more substrate is added, the enzyme simply cannot work faster, substrate molecules are effectively ‘queuing up’ for and active site to become vacant. Catalase is an enzyme found inside most living cells (in this case potatoes). Catalase catalyses the breakdown of hydrogen peroxide into water and oxygen (which protects the cells from the toxic affect of hydrogen peroxide.

For the above reasons I predict that the amount of gas collected will increase as concentration increases, also as the reaction goes on, the rate of the reaction will decrease and on a graph it will tail off at the end of the reaction. I also believe the initial rate of reaction will increase as concentration increases, this is because the amount of substrate in each reaction begins to vary as the substrate changes to its product, therefore at the beginning of the reaction it is certain there is the same amount of substrate, and less confounding variables than in the middle or near the end of the reaction.

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