Cells and organisation · GCSE Biology
Enzyme active site
Explain the lock-and-key model for GCSE Biology: enzymes as protein catalysts, complementary active sites, specificity, and why a changed shape means the reaction stops.
Enzyme = protein catalyst with a uniquely shaped active site. Substrate fits (lock and key). If the shape changes, the enzyme is denatured and the substrate no longer binds.
The important bits
What you need to know
- 1
Enzymes are biological catalysts: protein molecules that speed up reactions without being used up. They lower the activation energy so more collisions are successful.
- 2
Each enzyme has an active site with a specific shape. The substrate has a complementary shape and binds to form an enzyme–substrate complex.
- 3
The lock-and-key model: the active site is the lock, the substrate is the key. One key fits one lock, so enzymes are specific (amylase acts on starch, not on protein).
- 4
Higher-tier papers may mention the induced-fit model: the active site moulds slightly around the substrate, still requiring a close complementary shape.
- 5
Amylase (saliva and pancreas) hydrolyses starch to maltose. Protease (stomach pepsin, pancreatic trypsin) hydrolyses proteins to amino acids. Lipase hydrolyses lipids to fatty acids and glycerol.
- 6
Bile from the liver emulsifies lipids into droplets (larger surface area for lipase) and is alkaline, so it neutralises stomach acid in the small intestine. Bile is not an enzyme.
- 7
Digestive enzymes are produced by specialised cells and secreted; intracellular enzymes (for example in respiration and photosynthesis) work inside the cell that made them.
- 8
If the active site changes shape, the substrate no longer fits, so the reaction rate falls to zero even if substrate is present. That shape change is denaturation, not “the enzyme being killed”.
Quotations worth analysing
Short evidence. Real method.
“Enzymes are biological catalysts that speed up reactions without being used up.”
Include protein if the question asks what type of molecule. Catalyst means not used up, so a small amount of enzyme can process a lot of substrate.
“The substrate fits into the active site (lock and key).”
Say complementary shape, not “the same shape”. After denaturation the fit is lost.
“Amylase breaks down starch into sugars; protease breaks down proteins into amino acids; lipase breaks down lipids into fatty acids and glycerol.”
Name enzyme, substrate and products as a trio. Mixing bile with lipase is a common zero.
Go deeper
What must a lock-and-key diagram actually show?
Draw the enzyme as a shape with a gap (active site), the substrate as the matching piece, then the complex, then two product pieces leaving and the enzyme unchanged. Label active site and enzyme–substrate complex. Complementary means matching, like a jigsaw, not identical. Specificity follows: starch fits amylase’s active site; a protein molecule does not, so amylase cannot digest egg white. If the question gives an unfamiliar enzyme name, still write complementary active site and name the substrate in the stem. Induced fit (Higher) is a small moulding of the site around the substrate; it does not mean any molecule can bind. Collision theory still applies: the substrate must hit the site at the right orientation. More enzyme or more substrate (up to a point) means more successful collisions per second.
Go deeper
How do I write digestion answers without mixing bile and lipase?
Starch → maltose (then glucose) by amylase, made in salivary glands and pancreas, working in mouth and small intestine. Protein → amino acids by proteases: pepsin in the stomach at low pH, trypsin from the pancreas in the small intestine at alkaline pH. Lipids → fatty acids + glycerol by lipase from the pancreas. Bile, made in the liver and stored in the gall bladder, emulsifies fats (physical break-up into droplets, larger surface area) and neutralises acid. Emulsification is not digestion; lipase still has to hydrolyse the bonds. In the exam, “bile digests fat” loses the mark. Write “bile emulsifies fat so lipase has a larger surface area”. Enzymes are proteins, so they themselves can be denatured by boiling — that is why cooked food is not digested by the food’s own enzymes in the same way.
Go deeper
Why does “the enzyme is used up” fail on the paper?
A catalyst provides an alternative route with lower activation energy and is regenerated at the end of each cycle. The same amylase molecule can break down thousands of starch chains. That is why a drop of enzyme solution clears a starch tube. If the enzyme were a reactant, it would run out in a 1:1 ratio with starch and the iodine would stay blue-black once the enzyme was gone. Rate depends on how often substrate hits an intact active site. Using up substrate slows the rate later because collisions become rarer; the enzyme is still there. Denaturation removes active sites; inhibition (Triple) blocks them. Keep those ideas separate: used up (false), denatured (shape gone), substrate limiting (substrate gone).
See the idea in action
Explain why amylase does not break down protein (3 marks). Amylase is a protein with an active site complementary to starch (lock and key). Protein molecules have a different shape, so they do not fit the active site and no enzyme–substrate complex forms. Therefore amylase cannot lower the activation energy of peptide-bond hydrolysis. Protease, with a different active site, catalyses that reaction instead.
Exam technique
Turn knowledge into marks
Always name active site, complementary shape, and enzyme–substrate complex. For digestion, give enzyme + substrate + products. Never write that enzymes are killed or that bile is an enzyme.
Common mistakes
Do not give these marks away
- 01
Saying the substrate has the same shape as the active site, rather than a complementary shape.
- 02
Writing that enzymes are used up, or that they are living organisms that can be killed.
- 03
Confusing bile (emulsifies lipids, alkaline) with lipase (digests lipids).
What happens at the active site of an enzyme?
AThe enzyme is used up and becomes the product
BThe substrate binds because it has a complementary shape, forming an enzyme–substrate complex
CBile emulsifies the enzyme
DDNA is replicated
Show the answer
The substrate binds because it has a complementary shape, forming an enzyme–substrate complex. Lock and key: complementary shapes. The enzyme is not used up. Bile is not an enzyme and does not act on the active site in this way.
Quick questions
If this is the bit you searched
What is the lock and key model of enzyme action?
The substrate has a complementary shape to the enzyme’s active site, so they fit like a key in a lock and form an enzyme–substrate complex.
Are enzymes used up in a reaction?
No. They are catalysts, so they are not used up and can catalyse the same reaction many times.
What does amylase break starch into?
Maltose (a sugar). Iodine stays blue-black while starch is present and turns orange-brown when amylase has broken the starch down.
Is bile an enzyme?
No. Bile emulsifies fats into droplets (larger surface area for lipase) and neutralises stomach acid. Lipase is the enzyme that digests lipids.