12. Respiration

Question 1

Define aerobic respiration

Answer 1

Requires oxygen and produces carbon dioxide, water and much ATP

Question 2

Define anaerobic respiration

Answer 2

Takes place in the absence of oxygen and produces lactate (in animals) or ethanol and carbon dioxide (in plants/fungi) but only a little ATP

Question 3

Describe the stages of aerobic respiration

Answer 3

1. Glycolysis-the splitting of the 6-carbon glucose molecule into 2 3-carbon pyruvate molecules.
2. Link reaction- the 3-carbon pyruvate molecules enter a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule.
3. Krebs Cycle- 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.
4. Oxidative phosphorylation- the use of the electrons, associated with reduced NAD and FAD, released from the Krebs cycle to synthesise ATP with water produced as a by-product.

Question 4

Describe the stages of glycolysis

Answer 4

1. Phosphorylation of glucose to glucose phosphate- glucose made more reactive by adding phosphate, reactions powered by ATP
2. Splitting of the phosphorylated glucose into 2 3-carbon molecules, triose phosphate
3. Oxidation of triose phosphate- hydrogen removed from each of the triose phosphate molecules and transferred to hydrogen carrier molecule known as NAD to form reduced NAD
4. Production of ATP- triose phosphate is converted into another 3-carbon molecule, pyruvate, generating 2 molecules of ATP in the process.

Question 5

What is the overall yield from one glucose molecule undergoing glycolysis?

Answer 5

• 2 molecules of ATP (4 molecules produced, but 2 used up in the initial phosphorylation of glucose)
• 2 molecules of reduced NAD
• 2 molecules of pyruvate

Question 6

Give the overall equation for the link reaction

Answer 6

pyruvate + NAD + CoA —> acetyl CoA + reduced NAD + CO2

Question 7

Summarise the Krebs Cycle

Answer 7

• The 2-carbon acetylcoenzyme A from the link reaction combines with a 4-carbon molecule to produce a 6-carbon molecule.
• In a series of reactions this 6-carbon molecule loses carbon dioxide and hydrogen to give a 4-carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation.
• The 4-carbon molecule can now combine with a new molecule of acetylcoenzyme A to begin the cycle again.

Question 8

For each molecule of pyruvate, what do the link reaction and Krebs cycle produce?

Answer 8

• Reduced coenzymes such as NAD and FAD. These have the potential to provide energy to produce ATP molecules by oxidative phosphorylation.
• 1 molecule of ATP
• 3 molecules of carbon dioxide

Question 9

What is a coenzyme?

Answer 9

Despite their name coenzymes are not enzymes. They’re molecules that some enzymes require in order to function.

Question 10

Give examples of coenzymes

Answer 10

• NAD, important throughout respiration
• FAD, important throughout the Krebs cycle
• NADP, important in photosynthesis

Question 11

Why is the Krebs cycle important?

Answer 11

• It breaks down macromolecule into smaller ones- pyruvate into carbon dioxide.
• It produces hydrogen that are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation. This leads to the production of ATP (providing metabolic energy for the cell)
• It regenerates the 4-carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate.
• Source of immediate compounds used by cells in the manufacture of other substances: e.g. amino acids, fatty acids and chlorophyll.

Question 12

State how many carbon molecules there are in a single molecule of pyruvate

Answer 12

3

Question 13

Name the 2-carbon molecule that pyruvate is covered to during the link reaction

Answer 13

Acetylcoenzyme A

Question 14

State precisely in which part of the cell the Krebs cycle takes place?

Answer 14

Matrix of mitochondria

Question 15

Describe the structure of a mitochondrion

Answer 15

Bound by a smooth outer membrane and inner one that is folded into extensions called cristae. The matrix contains proteins, lipids and traces of DNA.

Question 16

Which cells would you expect to see numerous mitochondria?

Answer 16

Metabolically active cells such as muscles, liver and epithelial cells, which carry out active transport.

Question 17

What is oxidative phosphorylation? (simply)

Answer 17

The mechanism by which some of the energy of the electrons within the hydrogen atoms is conserved in the formation of ATP

Question 18

Describe how ATP is produced in oxidative phosphorylation (chemiosmotic theory)

Answer 18

• The hydrogen atoms produced during glycolysis and Krebs cycle combine with coenzymes NAD and FAD.
• The reduced NAD and FAD donate the electrons of the hydrogen atoms they are carrying to the first molecule in the electron transfer chain.
• The electrons pass along a chain of electron transfer carrier molecules in a series of oxidation-reduction reactions. As the electrons flow along the chain, the energy they release causes the active transport of protons across the inner mitochondrial membrane and into inter-membrane space.
• The protons accumulate in the inter-membrane space before they diffuse back into the mitochondria matrix through ATP synthase channels embedded in the inner mitochondrial membrane.
• At the end of the chain the electrons combine with these protons and oxygen to form water. Oxygen is the final acceptor of electrons in the electron transfer chain.

Question 19

Why are the electrons carried by NAD and FAD not transferred in a single step?

Answer 19

Energy is released a little at a time, so more of it can be harvested for the benefit of the organism. Electrons are passed along a series of electron transfer carrier molecules, each of which is at a slightly lower energy level. The electrons move down an energy gradient, allowing them to be released gradually and usefully.

Question 20

Describe the respiration of lipids?

Answer 20

Before being repaired, lipids are first hydrolysed to glycerol and then phosphorylated and converted into triose phosphate which enters the glycolysis pathway and subsequently into the Krebs Cycle. The fatty acid component is broken down into 2- carbon fragments which are converted into Acetylcoenzyme A, which enters the Krebs cycle.
The oxidation of lipids produces 2-carbon fragments of carbohydrate and many hydrogen atoms. The hydrogen atoms are used to produce ATP during oxidative phosphorylation. For this reason lipids produce double the energy of the same mass of carbohydrate.

Question 21

Describe the respiration of protrein

Answer 21

Protein is another potential source of energy. It’s first hydrolysed to it’s constituent amino acids. These have their amino group removed before entering the respiratory pathway at different points depending on the number of carbon atoms they contain.
3-carbon compounds are converted into pyruvate, while 4- and 5- carbon compounds are converted to intermediates in the Krebs cycle.

Question 22

The processes that occur in the electron transfer chain are known as oxidative phosphorylation, explain why this term is used

Answer 22

The movement of electrons along the chain is due to oxidation. The energy form the electrons combined inorganic phosphate and ADP to form ATP= phosphorylation.

Question 23

The surface area of the inner mitochondrial membrane is highly folded to form cristae. State one advantage of this to the electron transfer chain.

Answer 23

It provides a large surface area of membrane incorporating the coenzymes (NAD/FAD) and electron carriers that transfer the electrons along the chain.

Question 24

The oxygen taken up by organisms has an important role in aerobic respiration. Explain this role

Answer 24

Oxygen is the final acceptor of electrons and hydrogen ions/protons in the electron transfer chain. Without it the electrons would accumulate along the chain and respiration would cease.

Question 25

As part of which molecule does the oxygen taken into an organism leave after being respired?

Answer 25

Water

Question 26

Why can’t aerobic respiration happen in the absence of oxygen?

Answer 26

Neither the Krebs cycle nor electron transfer chain can continue because soon all the NAD and FAD will be reduced. No NAD or FAD will be available to take up the H+ produced during the Krebs cycle and so the enzymes stop working. This leaves only the anaerobic process of glycolysis to produce ATP.

Question 27

Give the equation for anaerobic respiration in plants and microorganisms

Answer 27

pyruvate + reduced NAD —> ethanol + carbon dioxide + oxidised NAD

The pyruvate formed at the end of glycolysis loses a CO2 and accepts hydrogen form reduced NAD to produce ethanol.

Question 28

Give examples of where anaerobic respiration occurs in plants and microorganisms

Answer 28

• cells of higher plants (eg root cells under waterloggged conditions)
• bacteria
• fungi (eg yeast)

Question 29

Give the equation for anaerobic respiration in animals

Answer 29

pryruvate + reduced NAD —> lactate + oxidised NAD

Lactate causes cramp and muscle fatigue if accumulated in muscle tissue.

Question 30

Does the Krebs cycle occur in anaerobic respiration?

Answer 30

Pyruvate is converted to either ethanol or lactate, so isn’t available for the Krebs cycle. Therefore anaerobic respiration doesn’t involve either the Krebs cycle or the electron transfer chain. The ATP that can be produced by anaerobic respiration is therefore formed by glycolysis.