Definitions

Enzyme - biological catalysts that speed up reactions in the body
Anabolic Reactions - Reactions that build up, the substrate molecules are being added to
Catabolic Reactions - reactions that break down the larger substrate molecules
Metabolism - Sum of all the enzyme catalysed reactions occurring within it. Combination of anabolic reactions and catabolic reactions
Catalyst - speeds up chemical reactions in the body without getting used up in the reaction
Metabolic Pathway - The sequence of enzyme controlled reactions
Specificity - only able to catalyse specific reactions
Substrate - molecules that enzymes work on
Product - molecules produced by the enzyme

Enzymes can be found inside the cell where they are intracellular or outside the cell where they are extracellular.

Examples:

Decarboxylase - removal of carboxyl group in respiration with the formation of CO2 - intracellular, catabolic
Maltase - breakdown of maltose to glucose in digestion - catabolic, extracellular
DNA polymerase - joins nucleotides together in DNA replication - intracellular, anabolic

There are two theories about how enzymes fit together:

Lock and Key - assumes that the active site of an enzyme is rigid in shape.
substrate has a complementary shape and change to active site
substrate binds to active site and forms an enzyme/substrate complex
substrate converted into product
product is no longer complementary shape, so is released.

Induced Fit - suggests that the active site is flexible and only assumes its catalytic conformation after the substrate molecules bind to the site. When the product leaves, it reverts back to its original form.
substrate binds to active site
active site changes shape to accommodate substrate and more bonds form
forms enzyme/substrate complex
change in active site, weakens bonds in the substrate and activation energy is reduced
further shape change of enzyme after products form.

An enzyme reduces the activation energy during a reaction. The reaction rate can be measured by:

rate of reaction = amount of reactant used or amount of product formed / time taken

The energy required to break bonds and start a reaction is known as the activation energy. Without an enzyme, heating could provide this energy. In cells, enzymes reduce the amount of activation energy without raising the temperature of cells. Scientists calculate the rate at which an enzyme works. By determining the quantity of substrate used or quantity of product forms in a given time, rate can be calculated. The initial rate of reaction is used when comparing rate of enzyme controlled reactions.

If you change the concentration of either enzyme or substrate, then they become limiting factors which means the factor is stopping the reaction from preceding at a higher rate. If it is the limiting factor, increasing concentration will increase the reaction rate up to a point, after which any increase will not affect the rate of reaction. This is because it will no longer be the limiting factor and another factor will be limiting the maximum rate of reaction. As a reaction proceeds, the rate of reaction will decrease since the substrate will get used up. The highest reaction rate is the initial reaction rate

Increasing Substrate Concentration:

This increases the rate of reaction as there are more substrate molecules colliding with enzymes so more product is formed. However, after a certain concentration, any increase will have no effect since substrate concentration wont be the limiting factor. The enzymes have been saturated.

Increasing Enzyme Concentration:

Increase rate of reaction as more enzymes are available for substrate collision but this will only effect reaction rate to a certain concentration as the enzyme will no longer be the limiting factor as all the substrates have been used up.

	Created by Sarah . almost 8 years ago

A brief description of some of the topics found in topic 1 and 2 of edexcel biology. I will continue to add to these with more detail.

Biology Topic 1 and 2 Notes