5.2 respiration

Question 1

give the two forms of cellular respiration, the conditions under which they occur, and the products formed.

Answer 1

aerobic respiration - requires oxygen, and produces carbon dioxide, water and much ATP.

anaerobic respiration - takes place in the absence of oxygen, and produces lactate (in animals), or ethanol and carbon dioxide (in plants and fungi), and only a little ATP.

Question 2

give the four stages of aerobic respiration.

Answer 2

• glycolysis
• link reaction
• Krebs cycle
• oxidative phosphorylation

Question 3

give the four stages of glycolysis.

Answer 3

• phosphorylation of glucose to glucose phosphate.
• splitting of the phosphorylated glucose.
• oxidation of triose
  phosphate.
• the regeneration of ATP.

Question 4

summarise the events that occur during glycolysis, and state where glycolysis takes place.

Answer 4

• glycolysis is the splitting of the 6-carbon glucose molecule hexose bisphosphate into 3-carbon pyruvate molecules.
• glycolysis occurs in the in the cytoplasm of cells.

Question 5

gives two reasons why glycolysis occurs in the cytoplasm, as opposed to the mitochondria.

Answer 5

• glucose is too large to cross the mitochondrial membrane.
• no transport proteins to move large molecules, such as glucose, across the membrane.

Question 6

what is the overall yield from glucose molecules undergoing glycolysis?

Answer 6

• two molecules of ATP.
• two molecules of reduced NAD.
• two molecules of pyruvate.

Question 7

where in a cell do the Krebs cycle and link reaction occur?

Answer 7

the mitochondrial matrix.

Question 8

summarise the events that occur during the link reaction.

Answer 8

the 3-carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule.

Question 9

during the link reaction, pyruvate undergoes a series of reactions. outline the changes that take place.

Answer 9

• the pyruvate is oxidised to acetate.
• in this reaction, the 3-carbon pyruvate looses a carbon dioxide molecule and two hydrogens.
• these hydrogens are accepted by NAD and used to form reduced NAD, which is later used to produce ATP.
• the 2-carbon acetate combines with coenzyme A (CoA) to produce acetylcoenzyme A.

Question 10

give a summary of the overall equation for the link reaction.

Answer 10

pyruvate + NAD + CoA → acetyl CoA + reduced NAD + CO₂

Question 11

summarise the events that occur during the Krebs cycle.

Answer 11

the introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD.

Question 12

what are NAD and FAD?

Answer 12

• NAD and FAD are coenzymes involved in the oxidation and reduction reactions that occur during cellular respiration.
• they are responsible for accepting ‘high energy’ electrons and carrying them to the electron transport chain, where they are used to synthesise ATP molecules.

Question 13

summarise the events that occur during oxidative phosphorylation.

Answer 13

the use of the electrons associated with reduced NAD and FAD are released from the Krebs cycle to synthesise ATP, with water produced as a by-product.

Question 14

the synthesis of ATP by oxidative phosphorylation involves the transfer of electrons down what?

Answer 14

a series of electron carriers, which together form an electron transfer chain.

Question 15

as the electrons are passed along the electron transfer chain, they release energy. describe what this energy is used for.

Answer 15

the energy released by the electrons causes the active transport of protons across the inner mitochondrial membrane into the inter-membranal space.

Question 16

the protons accumulate in the inter-membranal space before they diffuse back into the mitochondrial matrix. give the enzyme is responsible for this process, and where it is found.

Answer 16

the protons diffuse back into the mitochondrial matrix through ATP-synthase channels, embedded in the inner mitochondrial membrane.

Question 17

which molecule is the final acceptor of electrons in the electron transfer chain?

Answer 17

oxygen.

Question 18

explain why it is important for oxygen to act as the final electron acceptor.

Answer 18

• the importance of oxygen in respiration is to act as the final electron acceptor of the hydrogen atoms produced in glycolysis and the Krebs cycle.
• without its role in removing hydrogen atoms at the end of the chain, the hydrogen ions would ‘back up’ along the chain and the process of respiration would come to a halt.

Question 19

the process of oxidative phosphorylation involves a series of what type of reaction?

Answer 19

oxidation-reduction reactions.

Question 20

during the Krebs cycle, a single molecule of ATP is produced as a result of substrate-level phosphorylation. explain substrate-level phosphorylation.

Answer 20

the 6-carbon molecule citrate loses carbon dioxide and hydrogen to produce the 4-carbon molecule succinate and a single molecule of ATP.

Question 21

how is citrate produced?

Answer 21

the 2-carbon acetylcoenzyme A from the link reaction combines with a 4-carbon dicarboxylic acid to produce citrate.

Question 22

give the overall yield of the link reaction and the Krebs cycle, per one molecule of pyruvate.

Answer 22

• reduced coenzymes NAD and FAD.
• one molecule of ATP.
• three molecules of carbon dioxide.

Question 23

give two reasons why the Krebs cycle is significant in aerobic respiration.

Answer 23

• the Krebs cycle breaks down macromolecules into smaller, more useful molecules - pyruvate is broken down into carbon dioxide.
• it produces hydrogen atoms that are carried by NAD to the electron transfer chain to provide energy for oxidative phosphorylation.

Question 24

describe the chemiosmotic theory of oxidative phosphorylation.

Answer 24

oxidative phosphorylation via the chemiosmotic theory occurs on the inner mitochondrial membrane and requires NADH and FADH₂, from the Krebs cycle, to produce water and many molecules of ATP.

Question 25

give two alternative respiratory substrates, and explain how they are oxidised by cells to release energy for respiration.

Answer 25

lipids - before being respires, lipids are first hydrolysed to glycerol and fatty acids. the glycerol is then phosphorylated and converted to triose phosphate (TP), which then enters the glycolysis pathway, eventually ending up in the Krebs cycle.

proteins - proteins are first hydrolysed to its constituent amino acids. these have their amino group removed before entering the respiratory pathway at different points depending on the number of carbon atoms that they contain.

Question 26

in the absence of oxygen, ethanol and carbon dioxide are formed in plants via anaerobic respiration. explain how ethanol is formed.

Answer 26

the pyruvate molecule formed at the end of glycolysis loses a molecule of carbon dioxide and accepts hydrogen from reduced NAD to produce ethanol.

Question 27

summarise the equation that represents the formation of ethanol during anaerobic respiration in plants.

Answer 27

pyruvate + NADH → ethanol + CO₂ + oxidised NAD

Question 28

in animals, when oxygen is in short supply, NAD from glycolysis can accumulate and must be removed. describe how this process results in the formation of lactate during anaerobic respiration.

Answer 28

each pyruvate molecule takes up the two hydrogen atoms from the reduced NAD produced in glycolysis to form lactate.

Question 29

summarise the equation that represents the formation of lactate during anaerobic respiration in animals.

Answer 29

pyruvate + NADH → lactate + oxidised NAD

Question 29

what happens to the lactate produced during anaerobic respiration in animals once oxygen becomes available again?

Answer 29

the lactate produced is oxidised back to pyruvate. this can then be either further oxidised to release energy, or removed by the blood and taken to the liver to be converted into glycogen.

Question 30

muscle has a certain tolernace to lactate. despite this, explain why it is important for lactate to be removed from the muscle.

Answer 30

• if allowed to accumulate in the muscle tissue, lactate will cause cramp and muscle fatigue.
• as lactate is an acid, it also causes pH changes which affect enzymes.

Question 31

give the two ways in which energy from cellular respiration is derived.

Answer 31

• substrate-level phosphorylation in glycolysis and the Krebs cycle.
• oxidative phosphorylation in the electron transfer chain.