Enzymes
Senior 2 · Biology
Enzymes
Lesson 1 of 4
18 min
Let's begin
Chew a piece of plain ugali or bread for two minutes without swallowing, and it starts to taste slightly sweet. Something in your saliva is turning starch into sugar, at body temperature, in seconds. In a laboratory, breaking starch down that fast would need strong acid and boiling.
That "something" is an enzyme. Without enzymes almost none of the reactions in your cells would happen fast enough to keep you alive.
A catalyst is a substance that speeds up a chemical reaction without being used up or permanently changed by it. An enzyme is a biological catalyst: a protein, made by living cells, that speeds up a particular reaction in a living organism.
For example, the enzyme amylase acts on the substrate starch to give the product maltose:
starch —(amylase)→ maltose
Most enzyme names end in -ase and are based on the substrate they act on.
| Group | Substrate → products | Examples |
|---|---|---|
| Carbohydrases | carbohydrates → simpler sugars | amylase: starch → maltose; maltase: maltose → glucose; sucrase; lactase |
| Proteases | proteins → peptides and amino acids | pepsin (stomach), trypsin (small intestine) |
| Lipases | lipids → fatty acids and glycerol | pancreatic lipase |
| Catalase | hydrogen peroxide → water + oxygen | found in almost all living cells, especially liver |
Enzymes can also be grouped by where they work. Intracellular enzymes work inside the cell that made them, such as the respiration enzymes. Extracellular enzymes are released and work outside the cell, such as digestive enzymes in the gut.
They can also be grouped by what they do. Catabolic reactions break large molecules into smaller ones, as in digestion. Anabolic reactions build large molecules from smaller ones, such as joining glucose into starch or amino acids into proteins.
Question: A student drops a cube of fresh liver into hydrogen peroxide solution and it froths strongly. A cube of boiled liver produces no froth. Explain both results and write a balanced equation.
Solution: Fresh liver contains the enzyme catalase, which rapidly breaks hydrogen peroxide down into water and oxygen; the froth is oxygen gas:
2H₂O₂ → 2H₂O + O₂
Boiling denatured the catalase, a protein, and destroyed its shape, so it could no longer catalyse the reaction. The boiled liver is a control. It shows that the froth is caused by an active enzyme, not by the liver tissue alone. Hydrogen peroxide is a harmful by-product of cell reactions, so catalase protects cells.
Tip
Memory hook: an enzyme is like a matchmaker at a wedding. It brings the partners together quickly and walks away unchanged, ready for the next pair.
Exam technique: "enzymes are biological catalysts made of protein" is worth two marks: catalyst and protein. Never write that enzymes are "living"; they are molecules made by living cells.
Define an enzyme.
Solution: A biological catalyst, a protein made by living cells, that speeds up a specific chemical reaction without being used up.
Name the enzyme and give the products: (a) lipids are digested; (b) starch is broken down in the mouth.
Solution: (a) lipase, giving fatty acids and glycerol; (b) amylase (salivary amylase), giving maltose.
Why does the body need only small amounts of each enzyme?
Solution: Enzymes are not used up in the reactions they catalyse, so each enzyme molecule can be used again and again, thousands of times per second.
Is amylase in saliva an intracellular or an extracellular enzyme? Explain.
Solution: Extracellular: it is made by salivary gland cells but released into the mouth, where it works outside the cells.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
