Module 5: Section 6 - Respiration

Question 1

What are the four stages in aerobic respiration and where do these stages occur?

Answer 1

1) the four stages in aerobic respiration are glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation
2) the first three stages are a series of reactions. The products from these reactions are used in the final stage to produce loads of ATP
3) the first stage happens in the cytoplasm of cells and the other three stages take place in the mitochondria.

Question 2

Stage one of aerobic respiration: glycolysis makes pyruvate from glucose. Explain in three steps how this occurs

Answer 2

1) glycolysis involves splitting one molecule of glucose (with 6 carbons - 6C) into two smaller molecules of pyruvate (3C)
2) the process happens in the cytoplasm of cells
3) glycolysis is the first stage of both aerobic and anaerobic respiration and doesn’t need oxygen to take place - so it’s an anaerobic process

Question 3

Stage two of aerobic respiration: there are two stages in glycolysis - phosphorylation and oxidation.

First, ATP is used to phosphorylate glucose to triose phosphate. Then triose phosphate is oxidised, releasing ATP. Overall there is a net gain of 2 ATP. What occurs in stage 1/stage 2, phosphorylation and oxidation?

Answer 3

stage one: phosphorylation:

1) glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP
2) this creates 1 molecule of hexose biphosphate and 2 molecules of ADP
3) then, hexose biphosphate is split up into 2 molecules of triose phosphate

stage two: oxidation

1) triose phosphate is oxidised (loses H), forming 2 molecules of pyruvate
2) NAD collects the H ions, forming 2 reduced NAD
3) 4 ATP are produced, but 2 were used up in stage one, so there’s a net gain of 2 ATP

Question 4

Explain in two steps what happens to the products of glycolysis

Answer 4

1) the two molecules of reduced NAD go to the last stage (oxidation phosphorylation)
2) the two pyruvate molecules molecules are actively transported into the matrix of the mitochondria for the link reaction

Question 5

Stage 2 of aerobic respiration - the link reaction converts pyruvate to acetyl coenzyme A. Please explain how the link reaction takes place in the mitochondrial matrix

Answer 5

1) pyruvate is decarboxylated - one carbon atom is removed from pyruvate in the form of CO2
2) NAD is reduced - it collects hydrogen from pyruvate, changing pyruvate into acetate
3) acetate is combined with coenzyme A (CoA) to form acetyl coenzyme A (acetyl CoA)
4) no ATP is produced in this reaction

Question 6

The link reaction occurs twice for every glucose molecule. Two pyruvate molecules are made for every glucose molecule that enters glycolysis. This means the link reaction and the third stage (the Krebs cycle) happen twice for every glucose molecule. So for each glucose molecule:

Answer 6

• two molecules of acetyl coenzyme A go into the Krebs cycle
• two CO2 molecules are released as a waste product of respiration
• two molecules of reduced NAD are formed and go to the last stage (oxidative phosphorylation)

Question 7

Pyruvate produced during glycolysis is transported across the outer and inner mitochondrial membranes via a specific pyruvate-H+ symport, a transport protein that transports two ion or molecules in the same direction, and into the matrix. What happens next?

Answer 7

1. pyruvate is converted to a two-carbon acetyl group during the link reaction
2. the acetyl group is oxidised during the Krebs cycle

Question 8

The link reaction occurs in the mitochondrial matrix. Pyruvate is decarboxylated and dehydrogenated, catalysed by a large multi-enzyme complex, pyruvate dehydrogenase, which catalyses the sequence of reactions that occur during the link reaction. No ATP is produced during this reaction - explain in four steps how the coenzyme NAD becomes reduced

Answer 8

1. the carboxyl group is removed and is the origin of some of the carbon dioxide produced during respiration
2. this decarboxylation of pyruvate together with dehydrogenation produces an acetyl group
3. the acetyl group combines with coenzymes A (CoA) to become acetyl CoA
4. the coenzyme NAD becomes reduced

Question 9

What equation summarises the link reaction for two molecules of pyruvate derived from one molecules of glucose?

Answer 9

2 pyruvate + 2NAD + 2CoA –> 2CO2 + 2 reduced NAD + 2 acetyl CoA

Coenzyme A accepts the acetyl group and, in form of acetyl CoA, carries the acetyl group on to the Krebs cycle

Question 10

Like the link reaction, the Krebs cycle takes place in the mitochondrial matrix. What is the Krebs cycle?

Answer 10

The Krebs cycle is a series of enzyme-catalysed reactions that oxidise the acetate from the link reaction to two molecules of carbon dioxide, while conserving energy by reducing the coenzyme NAD and FAD (flavine adenine dinucleotide)

Question 11

Please explain the six main steps of the Krebs cycle

Answer 11

1. the acetyl group released from acetyl CoA combines with a four-carbon compound, oxaloacetate, to form a six-carbon compound
2. citrate is decarboxylated and dehyrdogenated, producing a five-carbon compound, one molecule of carbon dioxide and one molecule of reduced NAD
3. this five-carbon compound is further decarboxylated and dehydrogenated, producing a four-carbon compound, one molecule of carbon dioxide and one molecule of reduced NAD
4. this four-carbon compound combines temporarily with, and is then released from, coenzyme A. At this stage, substrate-level phosphorylation takes place, producing one molecule of ATP
5. the four-carbon compound is dehydrogenated, producing a different four-carbon compound and a molecule of reduced FAD
6. rearrangement of the atoms in the four carbon molecule, catalysed by an insomerase enzyme, followed by further dehydrogenation, regenerate a molecule of oxaloscetate so the cycle can continue

for every molecule of glucose there are two turns of the Krebs cycle

Question 12

What are the products of the Krebs cycle and where do they go?

Answer 12

1 coenzyme A - reused in the next link reaction
oxaloacetate - regenerated for use in the next Krebs cycle
2 CO2 - released as a waste product
1 ATP - used for energy
3 reduced NAD - to oxidative phosphorylation
1 reduced FAD - to oxidative phosphorylation

Question 13

What is oxidative phosphorylation and where does it take place?

Answer 13

1) oxidatative phosphorylation is the process where the energy carried by electrons, from reduced coenzymes (reduced NAD and reduced FAD), is used to make ATP.
2) oxidative phosphorylation takes place in the inner mitochondrial membrane

Question 14

Protons are pumped across the inner mitochondrial membrane - but how does oxidative phosphorylation actually work? please tell me in 7 steps….

Answer 14

1) hydrogen atoms are released from reduced NAD and reduced FAD as they’re oxidised to NAD and FAD. The H atoms split into protons (H+) and electrons (e-)
2) the electrons move along the electron transport chain (made up of three electron carriers), losing energy at each carrier. (the electron transport chain is located in the inner mitochondrial membrane. This membrane is folded into cristae, which increases the membrane’s surface area to maximise respiration)
3) this energy is used by the electron carriers to pump protons from the mitochondrial matrix into the intermembrane space (the space between the inner and outer mitochondrial membranes)
4) the concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix - this forms an electrochemical gradient
5) protons move down the electrochemical gradient, back into the mitochondrial matrix, via ATP synthase. This movement drives the synthesis of ATP from ADP and inorganic phosphate
6) this process of ATP production driven by the movement of H+ ions across a membrane (due to electrons moving down an electron chain) is called chemiosmosis (which is described by the chemiosmotic theory)
7) in the mitochondrial matrix, at the end of the transport chain, the protons, electrons and O2 (from the blood) combine to form water. Oxygen is said to be the final electron acceptor

Question 15

What are the two types of anaerobic respiration and what processes do/don’t they involve?

Answer 15

1) anaerobic respiration doesn’t use oxygen
2) it doesn’t involve the link reaction, he Krebs cycle or oxidative phosphorylation
3) there are two types of anaerobic respiration - alcoholic fermentation and lactate fermentation

Question 16

What similarities are there between anaerobic and aerobic respiration?

Answer 16

1) these two processes are similar, because they both take place in the cytoplasm and they both start with glycolysis (which produces pyruvate)
2) they differ in which organisms they occur in and what happens to the pyruvate

Question 17

Lactate fermentation occurs in mammals and produces lactate - but how?

How does the production of lactate regenerate NAD?

Answer 17

1) reduced NAD (from glycolysis) transfers hydrogen to pyruvate to form lactate and NAD
2) NAD can then be reused in glycolysis
- the production of lactate regenerates NAD. Glyolysis needs NAD in order to take place. This means glycolysis can continue even when there isn’t much oxygen around, so a small amount of ATP can still be produced to keep some biological process going
- our cells can tolerate a high level of lactate (and the coinciding low pH conditions) for short periods of time. For example, during short periods of hard exercise, when they can’t get enough ATP from aerobic respiration
- however, too much lactate is toxic and is removed from the cells into the bloodstream. The liver takes up lactate from the bloodstream and converts it back into glucose in a process called gluconeogenesis

Question 18

Alcoholic fermentation occurs in yeast cells and produces ethanol - how?

-what does the production of ethanol regenerate?

Answer 18

1) CO2 is removed from pyruvate to form ethanAL
2) reduced NAD (from glycolysis) transfers hydrogen to ethanAL to form ethanOL and NAD
3) NAD can then be reused in glycolysis
- the production of ethanol also regenerates NAD so glycolysis can continue when there isn’t much oxygen around

Question 19

Please explain why anaerobic respiration releases less energy than aerobic respiration

Answer 19

1) the ATP yield from anaerobic respiration is always lower than from aerobic respiration
2) this is because anaerobic respiration only includes one energy-releasing stage (glycolysis), which only produces 2 ATP per glucose molecule
3) the energy-releasing reactions of the Krebs cycle and oxidative phosphorylation need oxygen, so they can’t occur during anaerobic respiration

Question 20

Cells respire glucose, but they also respire other carbohydrates, lipids and proteins. What is a respiratory substrate?

Answer 20

any biological molecule that can be broken down in respiration to release energy is called a respiratory substrate

Question 21

Different respiratory substrates release different amounts of energy when they’re respired. Lipids have the highest energy value, followed by proteins, then carbohydrates - why is this?

Answer 21

This is because most ATP is made in oxidative phosphorylation, which requires hydrogen atoms from reduced NAD and reduced FAD. This means that respiratory substrates that contain more hydrogen atoms per unit of mass, cause more ATP to be produced when respired. Lipids contain the most hydrogen atoms per unit of mass, followed by proteins and then carbohydrates

Question 22

What is the respiratory quotient and how can it be calculated?

Answer 22

1) the respiratory quotient is the volume of carbon dioxide produced when the substrate is respired, divided by the volume of oxygen consumed in a set period of time

RQ = volume of CO2 released / volume of O2 consumed

Question 23

Respiratory quotients have been worked out for the respiration of other respiratory substrates. Lipids and proteins have an RQ value lower than 1 - why is this?

Answer 23

Lipids and proteins have an RQ value lower than one because more oxygen is needed to oxidise fats and lipids than to oxidise carbohydrates

Question 24

Why is the respiratory quotient for an organism useful? please give an example

• what do high RQs (greater than 1) mean?
• why do plants sometimes have a low RQ?

Answer 24

1) the respiratory quotient for an organism is useful because it tells you what kind of respiratory substrate an organism is respiring and what type of respiration its using (aerobic or anaerobic)
2) for example, under normal conditions the usual RQ for humans is between 0.7 ans 1.0. An RQ in this range shows shows that some fats (lipids) are being used for respiration, as well as carbohydrates like glucose. Protein isn’t normally used by the body for respiration unless there’s nothing else
3) high RQs (greater than 1) mean that an organism is short of oxygen, and is having to respire anaerobically as well aerobically
4) plants sometimes have low RQ. This is because the CO2 released in respiration is used for photosynthesis (so it’s not measured)