Section 4: Metabolic pathways and cellular respiration

Follow where each product goes next: pyruvate, acetyl coenzyme A, citrate, NADH, then the electron transport chain and ATP synthase.

A lesson for this section

Section 1 style: multiple choice

Question 1Section 1 style

In a cell, substance P is converted to substance Q by enzyme 1 in a reversible reaction, and Q is converted to substance R by enzyme 2. A non-competitive inhibitor of enzyme 2 is added. Which describes what happens?

Question 2Section 1 style

Oxygen released in 5 minutes. 1% hydrogen peroxide: 2 discs, 4.0 cubic centimetres. 2%: 3 discs, 9.0. 3%: 4 discs, 13.6. 4%: 5 discs, 17.5.
Hydrogen peroxide (%)	Number of discs	Oxygen released (cm $^{3}$ )
1	2	4.0
2	3	9.0
3	4	13.6
4	5	17.5

A student placed potato discs, which contain catalase, in hydrogen peroxide of four concentrations and measured the volume of oxygen released in 5 minutes. She used a different number of discs at each concentration. 1%: 2 discs, 4.0 cm

^{3}

of oxygen. 2%: 3 discs, 9.0 cm

^{3}

. 3%: 4 discs, 13.6 cm

^{3}

. 4%: 5 discs, 17.5 cm

^{3}

. Which statement about the results is correct?

Question 3Section 1 style

Which statement about membranes is correct?

Question 4Section 1 style

What is meant by induced fit?

Question 5Section 1 style

In a cell, all of the coenzyme NAD has been converted to NADH, and none is converted back. Which stages of respiration stop because their dehydrogenase enzymes can no longer work?

Question 6Section 1 style

A substance blocks the channel of ATP synthase in isolated mitochondria, so that nothing can pass through it. Which describes the effect?

Question 7Section 1 style

In a cell, substance X is converted to substance Y in a reversible reaction catalysed by one enzyme. Substance Y is then converted to substance Z by a second enzyme. Which change would increase the rate of conversion of X to Y?

Question 8Section 1 style

Which row describes a competitive inhibitor?

Question 9Section 1 style

The graph shows the rate of an enzyme reaction at different substrate concentrations with no inhibitor, with inhibitor X and with inhibitor Y. With no inhibitor the rate is 30 units at a substrate concentration of 5 units, 36 at 10, 39 at 20, and 40 at 30 and 40. With inhibitor X it is 12 at 5, 20 at 10, 30 at 20, 36 at 30 and 39 at 40. With inhibitor Y it is 15 at 5, 18 at 10, 19.5 at 20, and 20 at 30 and 40. Which type of inhibitor is each?

Question 10Section 1 style

Which describes feedback inhibition?

Question 11Section 1 style

In a bacterium, substance A is converted to B by enzyme 1, B to C by enzyme 2, and C to D by enzyme 3. B can also be converted to E by enzyme 4. Each step is irreversible. When D reaches a critical concentration it inhibits enzyme 2. What happens once D reaches this concentration?

Question 12Section 1 style

In an experiment with catalase, the rate of oxygen production was

0.8\ \text{cm}^{3}\ \text{min}^{-1}

with 1% hydrogen peroxide and

2.0\ \text{cm}^{3}\ \text{min}^{-1}

with 3% hydrogen peroxide. What is the percentage increase in the rate?

Question 13Section 1 style

Rate of reaction. Low substrate concentration: 10 units without the inhibitor, 4 with it. High substrate concentration: 30 units without the inhibitor, 29 with it.
Substrate concentration	Rate without inhibitor (units)	Rate with inhibitor (units)
low	10	4
high	30	29

The rate of an enzyme reaction was measured with and without an inhibitor at a low and at a high substrate concentration. At the low substrate concentration the rate was 10 units without the inhibitor and 4 units with it. At the high substrate concentration the rate was 30 units without the inhibitor and 29 units with it. Which conclusion is correct?

Question 14Section 1 style

Where in a cell does glycolysis take place?

Question 15Section 1 style

What is ATP used for during the energy investment phase of glycolysis?

Question 16Section 1 style

Why is there a net gain of ATP from glycolysis?

Question 17Section 1 style

In the citric acid cycle, what does the acetyl group from acetyl coenzyme A combine with?

Question 18Section 1 style

Which row shows where glycolysis and the citric acid cycle take place?

Question 19Section 1 style

Which of the following statements are correct? I Dehydrogenase enzymes act in glycolysis. II Dehydrogenase enzymes act in the citric acid cycle. III Dehydrogenase enzymes remove hydrogen ions and electrons from NADH, forming NAD.

Question 20Section 1 style

During aerobic respiration, in which of these stages is carbon dioxide released?

Question 21Section 1 style

How does ATP synthase produce ATP?

Question 22Section 1 style

What is the role of oxygen in aerobic respiration?

Question 23Section 1 style

A poison stops electrons passing along the electron transport chain in a muscle cell. Which describes the effect of the poison?

Question 24Section 1 style

Which row describes fermentation in yeast?

Question 25Section 1 style

A cell breaks down 40 glucose molecules a minute by aerobic respiration, with a yield of 30 ATP per glucose molecule in this cell. When oxygen runs out, it switches entirely to fermentation, which gives a net yield of 2 ATP per glucose molecule. How many glucose molecules a minute must it now break down to make ATP at the same rate?

Question 26Section 1 style

After a sprint, lactate that built up in muscle cells is converted back to pyruvate when oxygen is available again. Why can yeast not convert ethanol back to pyruvate in the same way?

Question 27Section 1 style

What is the role of ATP in cells?

Question 28Section 1 style

A student investigates dehydrogenase activity in yeast. DCPIP is a blue dye that becomes colourless when it accepts the hydrogen ions and electrons removed by dehydrogenase enzymes. Tube 1 contains yeast, glucose and DCPIP. Tube 2 contains boiled yeast, glucose and DCPIP. What is the purpose of tube 2?

Section 2 style questions

Discs that riseSection 2 style · 7 marks

A student soaked small filter paper discs in a catalase solution. Each disc was dropped into a beaker of hydrogen peroxide; it sank, then rose to the surface as oxygen bubbles formed on it. She timed how long each disc took to rise, with three discs at each of five concentrations of hydrogen peroxide. The table shows her results. 1%: 40 s, 44 s, 42 s, mean not given. 2%: 22 s, 20 s, 24 s, mean 22 s. 3%: 15 s, 14 s, 16 s, mean 15 s. 4%: 13 s, 12 s, 14 s, mean 13 s. 5%: 12 s, 13 s, 11 s, mean 12 s.

Time for a disc to rise, three discs at each concentration. 1%: 40, 44 and 42 seconds, mean shown as a question mark. 2%: 22, 20, 24, mean 22. 3%: 15, 14, 16, mean 15. 4%: 13, 12, 14, mean 13. 5%: 12, 13, 11, mean 12.
Hydrogen peroxide (%)	Disc 1 (s)	Disc 2 (s)	Disc 3 (s)	Mean (s)
1	40	44	42	?
2	22	20	24	22
3	15	14	16	15
4	13	12	14	13
5	12	13	11	12

(a) Calculate the mean time for a disc to rise in 1% hydrogen peroxide, in s. [1]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Calculate the percentage decrease in the mean time between 1% and 3% hydrogen peroxide. [1]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(c) Identify one variable, other than the concentration of hydrogen peroxide, that should be kept constant, and explain why it must be kept constant. [2]

0 words · a sentence or two

(d) Describe a suitable control for this experiment. [1]

0 words · a sentence

(e) Explain why the mean time hardly changes between 4% and 5% hydrogen peroxide. [2]

0 words · a sentence or two

Slowing an enzymeSection 2 style · 7 marks

The rate of an enzyme reaction was measured at different substrate concentrations with no inhibitor and with an inhibitor at a fixed concentration. The graph shows the results. With no inhibitor the rate was 30 units at 2 mmol l⁻¹ of substrate, 40 at 4, 44 at 6, 46 at 8, 47 at 10 and 48 at 12. With the inhibitor it was 12 at 2, 22 at 4, 30 at 6, 36 at 8, 41 at 10 and 44 at 12.

(a) Calculate the percentage decrease in the rate caused by the inhibitor at a substrate concentration of 4 mmol l⁻¹. [1]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Which type of inhibitor is it? [1]

0 words · a sentence or two

(d) A different inhibitor binds away from the active site. Describe how it reduces the rate of reaction, and the effect of increasing the substrate concentration on this inhibition. [3]

0 words · two or three sentences

A dye that loses its colourSection 2 style · 6 marks

DCPIP is a blue dye that becomes colourless when it accepts hydrogen ions and electrons removed by dehydrogenase enzymes. Malonic acid inhibits a dehydrogenase enzyme of the citric acid cycle. A student set up four tubes, each with the same volume of yeast suspension, glucose solution and DCPIP at 30 °C, and added malonic acid at a different concentration to each. The table shows the time for the DCPIP to become colourless: 0 mmol l⁻¹ malonic acid, 5.0 minutes; 1 mmol l⁻¹, 7.5 minutes; 2 mmol l⁻¹, 10.0 minutes; 4 mmol l⁻¹, 16.0 minutes.

Time for DCPIP to become colourless. Malonic acid 0 mmol per litre: 5.0 minutes. 1: 7.5 minutes. 2: 10.0 minutes. 4: 16.0 minutes.
Malonic acid concentration (mmol l $^{-1}$ )	Time for DCPIP to become colourless (minutes)
0	5.0
1	7.5
2	10.0
4	16.0

(a) Calculate the percentage increase in the time taken for the DCPIP to become colourless when the malonic acid concentration rose from 0 to 4 mmol l⁻¹. [1]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Which conclusion do the results support? [1]

0 words · a sentence or two

(d) Describe a control tube for this experiment, and state what it would show. [2]

0 words · a sentence or two

Mitochondria in a sealed chamberSection 2 style · 8 marks

Mitochondria were extracted from liver cells and placed in a sealed chamber with a respiratory substrate. An oxygen probe recorded the oxygen concentration of the liquid. At 6 minutes an inhibitor that stops electrons passing along the electron transport chain was added. The graph shows the results: 8.0 mg l⁻¹ at 0 minutes, 7.1 at 2 minutes, 6.2 at 4 minutes, 5.3 at 6 minutes, 5.3 at 8 minutes and 5.3 at 10 minutes.

(a) Calculate the average rate of oxygen uptake by the mitochondria between 0 and 6 minutes, in mg l⁻¹ min⁻¹. [1]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Explain why the oxygen concentration stopped falling after the inhibitor was added. [2]

0 words · a sentence or two

0 words ·