International A Level Biology / Proteins and DNA

Enzymes: biological catalysts

Edexcel International A Level Biology (YBI11), chapter 5: Proteins and DNA, specification point 2.7. Free lesson.

Read the revision notes · International A Level Biology past papers

Enzymes are biological catalysts that speed up reactions without being used up. They lower activation energy and may work inside or outside the cells that make them.

What makes an enzyme a biological catalyst?

A catalyst speeds up a chemical reaction without being used up or permanently changed. This means an enzyme can work again and again. Biological means that an enzyme is a globular protein made by a living cell, so a full definition needs both parts. Enzymes speed up chemical reactions. They are globular proteins made by living cells.

How enzymes lower activation energy

Activation energy is the energy needed to start a reaction. On an energy profile, reaction progress is shown along the bottom and energy up the side. The activation energy is measured from the energy of the reactants to the peak, not from zero on the axis. The graph uses arbitrary units, so its numbers have no meaning on their own. Without an enzyme, reactants must pass over a high energy barrier. At body temperature, few molecules have enough energy to do this, so the reaction is slow. An enzyme provides a route with a lower energy barrier. More molecules have enough energy to react at…

What enzymes do not do

Enzymes do not give substrate molecules extra energy. Instead, they lower the activation energy, which lets more molecules react at body temperature. This is why the reaction becomes faster without the enzyme adding energy. Write that enzymes lower activation energy and increase the number of molecules able to react. Do not say that enzymes supply energy to the reaction.

Where intracellular and extracellular enzymes work

All enzymes are made inside cells, but they do not all work there. Intracellular means inside: an intracellular enzyme catalyses reactions inside the cell that made it. Extracellular means outside: an extracellular enzyme is secreted and catalyses reactions outside that cell. Decide which type an enzyme is by where it works, not where it was made. Intracellular enzymes include catalase, which breaks down toxic hydrogen peroxide into water and oxygen, and enzymes involved in respiration. They are found in the cytoplasm or organelles such as mitochondria. Extracellular enzymes…

More lessons in Proteins and DNA