Summary Mark Scheme notes for Topic 3 - Enzymes (CIE 9700 A Level Biology/AS Level Biology)

NOTES: Italics indicates NOT from the mark scheme (often means no
mark schemes relating to the spec point). These are taken instead
from the CIE A Level textbook, which occasionally includes extra
detail to help my own understanding. All others are directly copied
from CIE 9700 mark schemes.

Mark scheme abbreviations: I = ignore, A = accept, R = reject. A
semi-colon “;” separates marking points, which are sometimes
numbered. “AVP” = any valid point. See the start of any mark
scheme for full abbreviations.

3. Enzymes

state that enzymes are globular proteins that catalyse reactions
inside cells

(intracellular enzymes)

or are secreted to catalyse reactions outside cells (extracellular
enzymes)



Km (the Michaelis-Menten constant) is the substrate concentration
required to reach ½ Vmax. Lower Km -> higher affinity of enzyme for
substrate



Induced-fit:

1 active site is not (fully) complementary to substrate ;

2 active site, changes shape / moulds around, to fit the substrate ;

A conformational change for shape change

3 enzyme-substrate complex / ESC, forms ;

4 active site returns to original shape on release of product ;

5 change of shape (to give complementary fit) lowers activation
energy / puts strain on bonds



Lock-and-key:

1 (shape of) substrate/elastin, complementary to (shape of) active
site ;

2 lock is active site, and key is, substrate ;