2.2 Enzymes and the lock and key model
7 exam questions worth 15 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
0 / 15
Enzymes are biological catalysts. What is a catalyst?
Mark scheme
a substance that speeds up a (chemical) reaction without being used up / changed
Model answer
A catalyst is a substance that speeds up a chemical reaction without being used up.
What type of molecule is an enzyme?
Choose one answer.
Correct Not quite: the answer is Protein
Mark scheme
B: protein
Model answer
B: protein. Enzymes are long chains of amino acids folded into a specific shape.
Explain why an enzyme can only catalyse one reaction.
Mark scheme
- the active site has a specific shape (1)
- (only) one substrate is complementary to / fits the active site (1)
- (so) an enzyme–substrate complex can only form with that substrate (1)
Model answer
The active site has a specific shape. Only one substrate has a complementary shape that fits into it, so an enzyme–substrate complex can only form with that substrate.
After an enzyme has catalysed a reaction, it can be used again.
Explain why.
Mark scheme
the enzyme is not changed / not used up in the reaction (so its active site is still the same shape)
Model answer
The enzyme is not changed by the reaction, so its active site keeps the same shape and another substrate molecule can fit.
Describe what happens to an enzyme when it is heated to 70 °C.
Mark scheme
- bonds holding the enzyme in shape break (1)
- the active site changes shape (1)
- the substrate no longer fits / enzyme is denatured (1)
- do not accept "killed"
Model answer
The high temperature breaks the bonds that hold the enzyme in shape. The active site changes shape so the substrate no longer fits, and the enzyme is denatured.
A student says: "Cooling a denatured enzyme makes it work again." Is the student correct? Give a reason.
Mark scheme
- no (1)
- denaturing is permanent / the active site has (permanently) changed shape (1)
Model answer
No. Denaturing is permanent: the shape of the active site has been changed and does not return when the enzyme is cooled.
Use the lock and key model to describe how an enzyme breaks down a substrate molecule.
Mark scheme
- substrate collides with / fits into the active site (1)
- because their shapes are complementary (1)
- enzyme–substrate complex forms (1)
- substrate is broken down into products which are released (1)
- enzyme is unchanged / can be reused (1)
Maximum 4 marks
Model answer
The substrate collides with the enzyme and fits into its active site, because the shapes are complementary. An enzyme–substrate complex forms. The substrate is broken down into products, which leave the active site. The enzyme is unchanged and can be used again.
Found a mistake?