Respiration releases chemical energy for biological processes

Question 1

Explain the one common feature in all models of life

Answer 1

The one common feature in all models of life is the presence of metabolism

• The term metabolism refers to all the reactions of the organism
• Respiration is a metabolic pathway, which means it is a sequence of reactions controlled by enzymes
• The reactions of respirations are catabolic
• They break down energy-rich macromolecules, such as glucose and fatty acids
• In respiration, C-C,C-H and C-OH bonds are broken and lower energy bonds formed
• The energy difference allows the phosphorylation of ADP to ATP
• ATP does not ‘produce’ energy, but when it is hydrolysed, it releases energy
• This energy is available for use by the cell or is lost as heat

Question 2

There are three types of phosphorylation: Explain oxidative phosphorylation

Answer 2

• It occurs on the inner membranes of the mitochondria in aerobic respiration. The energy for making the ATP comes from oxidation-reduction reactions and is released in the transfer of electrons along a chain of electrons carrier molecules

Question 3

There are three types of phosphorylation: Photophosphorylation

Answer 3

• Which occurs on the thylakoid membranes of the chloroplasts in the light-dependent stage of photosynthesis
• The energy for making the ATP comes from light and is released in the transfer of electrons along a chain of electron carrier molecules

Question 4

There are three types of phosphorylation: Substrate level phosphorylation

Answer 4

• Which occurs when phosphate groups are transferred from donor molecules, e.g. glycerate-3-phosphate to ADP to make ATP in glycosis or when enough energy is released for a reaction to bind ADP to inorganic phosphate e.g in the Krebs cycle

Question 5

Explain how the three groups of organisms are recognised, depending on their respiration

Answer 5

• Most living organisms use aerobic respiration, and break down substrates using oxygen, with the releases of a relatively large amount of energy. These are obligate aerobes
• Some micro-organisms, including yeast and many bacteria, respire aerobically, but can also respire without oxygen; these are facultative anaerobes
• Some species of bacteria and Archaea use anaerobic respiration. They respire without oxygen and cannot grow in its presence. They are obligate anaerobics

Question 6

What are the four distinct stages of aerobic respiration?

Answer 6

1. Glycolysis, which occurs in solution in the cytoplasm and generates pyruvate, ATP and reduced NAD
2. The link reaction, in solution in the matrix of the mitochondrion. Pyruvate is converted into acetyl coenzyme A
3. The Krebs cycle, in solution in the mitochondrial matrix generates carbon dioxide and reduced NAD and FAD
4. The electron transport chain, on the cristae of the inner mitochondrial membrane, in which the energy from protons and electrons generates ATP from ADP and inorganic phosphate

Question 7

What is glycolysis?

Where does it occur?

Answer 7

• It is the initial stage of both aerobic and anaerobic respiration.
• Glycolysis occurs in the cytoplasm, because glucose cannot pass through the mitochondrial membranes.
• But even if it could, the enzyme for its breakdown are not present in the mitochondria and so it could not be metabolised there

Question 8

Explain in detail the stages involved in glycolysis (1)

Answer 8

A glucose molecule is phosphorylated by the addition of two phosphate groups, using two molecules of ATP. making a hexose phosphate called glucose diphosphate. As a result, the phosphorylated glucose is:

• ] More reactive so less activation energy is required for enzyme controlled reactions
• ] Polar and, therefore, less likely to diffuse out of the cell
• The glucose diphosphate splits into two molecules of a triose phosphate, a 3-carbon sugar, glyceraldehyde-3-phosphate
• The two triose phosphate molecules are dehydrogenated, i.e. hydrogen is removed from each of them, oxidising them to pyruvate, also a 3-carbon molecule. The hydrogen atoms are transferred to NAD, a hydrogen carrier molecule, making reduced NAD. These steps release enough energy to synthesise four ATP molecules

Question 9

In glycolysis the ATP is formed by substrate level phosphorylation:

Answer 9

The phosphate from the triose phosphate converts ADP to ATP, without the involvement of an electron transport chain, producing pyruvate

Question 10

Explain in detail the stages involved in glycolysis (2)

Answer 10

• Of the 4 ATPs made by substrate-level phosphorylation, 2 were used to phosphorylate glucose molecule. Therefore there is a net gain of 2 ATPs form each molecule of glucose
• Two molecules of reduced NAD are also produced. If oxygen is available, each has the potential for the synthesis of an additional three molecules of ATP, making six altogether from the electron transport chain
• Some energy is lost as heat but a considerable amount of chemical potential energy remains in the pyruvate. If oxygen is available, some of this energy can be released via the Krebs cycle, in the mitochondria

Question 11

The link reaction links glycolysis to the Krebs cycle.

Explain the link reaction

Answer 11

• Pyruvate diffuses form the cytoplasm into the mitochondrial matrix
• The pyruvate is dehydrogenated and the hydrogen released is accepted by NAD to form reduced NAD
• The pyruvate is also decarboxylated, i.e. a molecule of carbon dioxide is removed from it. All that remains of the original glucose molecule is a 2-carbon acetate group which combines with coenzyme A (CoA), making acetyl coenzyme A (AcCoA), which enters the Krebs cycle

Question 12

Summary of the link reaction :

Answer 12

Pyruvate + NAD + CoA —-> AcCoA + reduced NAD + CO2

Question 13

Describe the Krebs cycle

Answer 13

• The Krebs cycle is a means of liberating energy from C-C, C-H and C-OH bonds
• It produces ATP, containing the energy which was held in the chemical bonds of the original glucose molecule
• It also produces reduced NAD and reduced FAD, which deliver hydrogen atoms to the electron transport chain on the inner mitochondrial membrane
• Three molecules of water are used in reactions in the Krebs cycle
• Carbon dioxide is released as a waste product

Question 14

Explain the Krebs cycle

Answer 14

• Acetyl CoA enter the Krebs cycle by combining with a 4-carbon acid, to form a 6 carbon compound, and the CoA is regenerated
• The 6-carbon acid is dehydrogenated, making reduced NAD and FAD, and decarboxylated to make carbon dioxide and regenerate the 4-carbon acid
• The 4-carbon acid can combine with more AcCoA and repeat the cycle

Question 15

What are the two significant types of reaction in the Krebs cycle

Answer 15

1. Decarboxylation happens twice. Decarboxylases remove carbon dioxide from the -COOH groups of Krebs cycle intermediates, as
   6C acid–> 5C acid—>4 acid
2. Dehydrogenation happens happen four times. Dehydrogenases remove pairs of hydrogen atoms from Krebs cycle intermediates. They are collected by hydrogen carriers giving three molecules of reduced NAD and one molecule of reduced FAD

Question 16

In summary, each turn of the Krebs cycle produces

Answer 16

• One ATP produced by substrate-level phosphorylation
• Three molecules of reduced NAD
• One molecule of reduced FAD
• Two molecules of carbon dioxide

Question 17

What is the electron transport ch;ain?

Answer 17

• The electron transport chain is located on the cristae of the inner mitochondrial membranes
• It is a series of protein molecules that are carriers and pumps, which are sometimes called ‘respiratory enzymes’
• The carriers molecules include cytochrome and so the electron transport chain is sometimes called the cytochrome chain
• Cytochromes are proteins conjugated to iron or copper and the metal ions are oxidised and reduced by electron transport
• The reactions they catalyse release energy, which is carried by ATP
• Hydrogen atoms are carried into the electron transport chain by coenzymes NAD and FAD
• NAD feeds electrons and protons into the electron transport chain at an earlier stage than FAD does
• Each pair of hydrogen atoms carried by reduced NAD provides enough energy to synthesise three molecule of ATP, using three proton pumps
• Reduced FAD passes the hydrogen atoms directly to the second proton pump so the carrier system involving FAD has two pumps and produces two molecules of ATP for each pair of hydrogen atoms

Question 18

Explain the passage of electrons:

What happens at the end of the chain?

Answer 18

• The reduced NAD donates the electrons of the hydrogen atoms to the first of series of electron carriers in the electron transport chain
• The electrons from these atoms provide energy for the first proton pump and protons from the hydrogen atoms are pumped into the inter-membrane space
• The electrons pass along the chain of carrier molecules providing energy for each of three proton pumps in turn
  At the end of the chain, the electrons combine with protons and oxygen to form water
  2H+ + 2e- + 1/2 O2 —-> H2O

Question 19

Explain the passage of protons

Answer 19

• The inner membrane is impermeable to protons and so the protons accumulate in the inter membrane space
• The concentration of protons in the inter membrane space becomes higher than in the matrix, so a gradient of concentration and charge is set up, and maintained by the proton pumps
• In the membrane are protein complexes, which are channels through which protons flow back into the mitochondrial matrix. The enzyme ATP synthetase is associated with each channel. Protons diffuse back through these channels, and as they do so their electrical potential energy produces ATP: ADP + Pi–> ATP + H2O
• At the end of the chain, the protons combine with electrons and oxygen to form water

Question 20

The passage of protons: the role of oxygen

Answer 20

Oxygen is referred to a the ‘final electron acceptor’ or the ‘final hydrogen acceptor’ in the electron transport chain. It is essential as it removed protons and electrons: oxygen is reduced by the addition of hydrogen ions and electrons, making water. Cyanide is a non-competitive inhibitor of the final carrier in the electron transport chain. In its presence, electrons and protons cannot be transferred to water. They accumulate, destroying the proton gradient. ATP synthetase cannot operate and the cell dies very quickly

Question 21

For each molecule of glucose entering the Krebs cycle, the electron transport system receives

Answer 21

• 10 NAD red which generates 10 x 3 = 30 ATP

- 2FAD red which generates 2 x 2 = 4 ATP

Question 22

Explain anaerobic respiration

Answer 22

• If there is no oxygen to remove hydrogen atoms from reduced NAD, and make water, the electron transport chain cannot function
• There is no oxidative phosphorylation and no ATP is formed
• Without oxygen, reduced NAD cannot be reoxidised and no NAD is regenerated to pick up more hydrogen
• Consequently, the link reaction and the Krebs cycle cannot take place and only the first stage of respiration, glycolysis is possible
• For glycolysis to continue, pyruvate accepting the hydrogen from reduced NAD
• The only ATP that can be made in the absence of oxygen is by substrate- level phosphorylation

Question 23

There are two different anaerobic pathways to remove the hydrogen from reduced NAD. Both take place in the cytoplasm:

Answer 23

• In animals, muscle cells may not get sufficient oxygen during vigorous exercise. When deprived of oxygen, pyruvate is the hydrogen acceptor and is converted to lactate, regenerating NAD. If oxygen subsequently becomes available, the lactate can be respired to carbon dioxide and water, releasing the energy it contains
• In various micro-organisms, such as yeast, and in plant cells under certain conditions such as in roots in waterlogged soils, pyruvate is converted to carbon dioxide and to ethanal, a hydrogen acceptor, by decarboxylase. Ethanal is reduced to ethanol and NAD is generated, in alcoholic fermentation. This pathway is not reversible, so even if oxygenated becomes available again, ethanal is not broken down. It accumulates in the cells and can rise to toxic concentrations

Question 24

Aerobic respiration and leakage

Answer 24

• ATP is used to move pyruvate, ADP, reduced NAD and reduced FAD across the mitochondrial membrane
• The proton gradient may be compromised by proton leakage across the inner mitochondrial membrane, rather than passing through ATP synthetase
• Molecules may also leak through membranes

Question 25

On average, how many molecules of ATP are produced per molecules of glucose respired

Answer 25

30-32
- If a mole of glucose is combusted in oxygen, it produces 2880KJ. The energy required to make ATP= 30.6 KJ/mole. If the theoretical maximum is considered, a mole of glucose makes 38 moles of ATP which is equivalent to (30.6 x 38)= 1162.8KJ

Question 26

What is the equation for the efficiency of ATP production?

Answer 26

Energy made available through ATP
—————————————————- x100
Energy released in combustion

Question 27

Anaerobic respiration: Without ATP synthetase

Answer 27

• Without the ATP synthetase associated with the electron transport system, the only ATP formed is in glycolysis, which makes two molecules of ATP per molecule of glucose, by substrate-level phosphorylation
• This is a small amount compared with the 38 molecules of ATP produced during aerobic respiration

Question 28

Anaerobic respiration: Pyruvate

Answer 28

• In anaerobic respiration, pyruvate is not transferred to the mitochondria but is converted, in the cytoplasm, to ethanol in plants or lactate in mammals.
• The 2H released in the conversion of glucose to pyruvate reduce NAD and they are given up again in the formation of ethanol in plant cells or lactate I the cells of mammals
• A huge number of different metabolic pathways have been identified in bacteria and archaea, and many organic and alcohols are produced by their fermentation

Question 29

Alternative respiratory pathway

Answer 29

• The Krebs cycle is sometimes called ‘metabolic hub’ because the metabolic pathways of carbohydrates, lipids and proteins can feed into it and I some situations, fats and proteins can be used as respiratory substrate
• Acetyl coenzyme A is a most significant molecule as it links the metabolism of the three types of macromolecules

Question 30

Alternative respiratory pathway: Lipids

Answer 30

• Fat provides an energy store and is used as a respiratory substrate when carbohydrate in the body, such as glycogen and blood glucose are low
• First, fat is hydrolysed into its constituted molecules, glycerol and fatty acids
• Then the glycerol is phosphorylated with ATP, dehydrated with NAD and converted into 3-carbon sugar, triose phosphate, which enter the glycolysis pathway
• The long chain fatty acid molecules are split into 2-carbon fragments that enter the Krebs cycle as AcCoA
• Hydrogen is released, picked up by NAD and fed into the electron transport chain
• This produces very large numbers of ATP molecules but the precise number depends on the length of the hydrocarbon chain of the fatty acid

Question 31

Alternative respiratory pathway: Lipids- The longer fatty acid chains have:

Answer 31

• More carbon atoms, so more carbon dioxide is produced. Muscles have a limited blood supply and if they respired fat, rather than glucose, they would produce more carbon dioxide than could be removed quickly enough
• More hydrogen atoms, so more NAD is reduced, so more ATP is produced. This explains why tissues with a rich blood supply, such as the liver, respire fat: the large amount of ATP they produce is readily transported around the body
• More hydrogen atoms, so more water is produced. This ‘metabolic water’ is very important for desert animals and explains why they respire fat

Question 32

Explain the function of proteins in the alternative respiratory pathway

Answer 32

• Protein can be used as a respiratory substrate whenever dietary energy supplies are inadequate; the protein component of the food is diverted for energy purposes if carbohydrate and fat are lacking in the diet
• In prolonged starvation, tissue protein is mobilised to supply energy
• Heart muscle and kidney tissue are among the first the body breaks down to release protein
• Protein is hydrolysed into its constituent amino acids, which are deaminated in the liver
• The amino group is converted into urea and excreted
• The residue is converted to acetyl CoA, pyruvate or some other Krebs cycle intermediated, and oxidised