SBI4U
Multiple Choice
Energy and Metabolism – Practice Answers
1.
When substrate concentration increases, in a reaction catalysed by an enzyme, why does
the rate of reaction increase?
A. The substrate molecules collide more frequently with the active site.
B. There are more active sites to catalyse the reaction.
C. The substrate molecules are moving faster.
D. There are more substrate molecules to catalyse the reaction.
2.
What determines the specificity of an enzyme for its substrate?
A. The temperature at which it is operating
B. The optimum pH of the enzymes
C. The concentration of the substrate
D. The structure of the enzyme molecule
3.
Which statement(s) about anaerobic cell respiration is/are correct?
I. ATP is produced
II. Oxygen is required
III. Glucose is converted into pyruvate
A. I only
B. III only
C. I and III only
D. I, II and III
4.
What property of enzymes is shown when their substrate concentration changes?
A. The activity of an enzyme increases constantly with an increase in substrate concentration
B. Enzyme activity decreases with increased substrate concentration
C. Enzyme activity increases to an optimum substrate concentration and then decreases
D. Enzyme activity increases with increased substrate concentration until all the active sites
are occupied
5.
What is acetyl (ethanoyl) CoA?
I. An intermediate in carbohydrate metabolism under aerobic conditions
II. A product of the oxidation of fatty acids in lipid metabolism
III. An intermediate in carbohydrate metabolism under anaerobic conditions
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
6.
What is an active site?
A. The part of an enzyme that binds only to the product molecules.
B. The sequence of amino acids responsible for the catalytic activity of enzymes.
C. The sequence of amino acids responsible for the structure of an enzyme.
D. The specific area responsible for the activity of all proteins.
7.
What is the “link reaction” in eukaryotic respiration?
A. Pyruvate joining with coenzyme A to produce CO2 and NADH + H+
B. Oxidation of NADH to yield electrons and protons
C. Acetyl coenzyme A combining or joining with a C4 compound to give C6 + coenzyme A
D. Passage of acetyl coenzyme A through the mitochondrial membrane
Long Answers
1.
(a) Draw and label a mitochondrion as seen in electron micrographs. [4]
(b) Outline the cellular locations of different named processes in cell respiration. [4]
Glycolysis occurs in the cytoplasm, Krebs Cycle occurs in the
mitochondrial matrix, the Electron Transport Chain and
Chemiosmosis occurs in the inner membrane and there is a link
reaction between the cytoplasm and the inner matrix.
2.
Explain the formation of ATP by chemiosmosis in cellular respiration [8]
1. Electrons are delivered to NADH dehydrogenase by NADH.
2. The electrons reduce a series of protein complexes in the inner
membrane.
3. NADH dehydrogenase, Cytochrome b-c1 complex and cytochrome
oxidase all pump H+ in to the intermembrane space, using the energy
from the excited electrons. (2)
4. 2e have enough energy to pump a H+, so each pair of electrons move
3H+ into the intermembrane space.
5. Since H+ can only return to the matrix by way of a transport channel
and not through the membrane, an electrochemical gradient is created
on either side of the inner membrane.
6. As the H+ flow back into the matrix though ATP synthase, the
electrochemical energy is used to drive the formation of ATP by
allowing ADP and Pi to react.
7. The electrons combine with oxygen and H+ to form water and the
process repeats with new NADH delivering electrons.
3.
Enzymes that work inside cells are sometimes affected by non-competitive inhibitors.
Explain how a non-competitive inhibitor affects the activity of an enzyme. [3]
Non-competitive inhibitors attach to the allosteric site of enzymes,
changing the shape of the active site temporarily and inhibiting the
activity of the enzyme, since the substrate molecules can no longer
fit in the space.
Data Analysis Question
At the start of glycolysis, glucose is phosphorylated to produce glucose 6-phosphate, which
is converted into fructose 6-phosphate. A second phosphorylation reaction is then carried out,
in which fructose 6-phosphate is converted into fructose 1,6-bisphosphate. This reaction is
catalyzed by the enzyme phosphofructokinase. Biochemists measured the enzyme activity of
phosphofructokinase (the rate at which it catalyzed the reaction) at different concentrations of
fructose 6-phosphate. The enzyme activity was measured with a low concentration of ATP and a
high concentration of ATP in the reaction mixture. The graph below shows the results.
1.
Using only the data in the above graph, outline the effect of increasing fructose
6-phosphate concentration on the activity of phosphofructokinase, at a low ATP
concentration. (2)
Increasing the substrate concentration increases enzyme activity
until it plateaus and no longer increases, even with an increase in
concentration.
2.
Explain how increases in fructose 6-phosphate concentration affect the activity of
the enzyme. (2)
Increasing the concentration increases the number of substrate
molecules available. At a certain point, however, all of the active
sites are occupied and an increase in substrate concentration no
longer has any effect.
3.
Outline the effect of increasing the ATP concentration on the activity of
phosphofructokinase. (2)
Increasing ATP concentration leads to a less rapid (more gradual
slope) increase in enzyme activity as the concentration of the
substrate increases. This is an example of end-product inhibition,
since there is plenty of ATP, less needs to be synthesized.