18 - respiration

Question 1

What are the 4 main steps of glycolysis?

Answer 1

1. Phosphorylation- requires 2 molecules of ATP, two phosphates are released from the 2 ATP molecules and are attached to a glucose molecule forming hexose bisphosphate.
2. Lysis- This destabilises the molecule causing it to split into 2 triose phosphate molecules.
3. Phosphorylation- another phosphate is added to each TP, forming 2 triose bisphosphate molecules. These phosphate groups cone from the free inorganic phosphate ions present in the cytoplasm.
4. Dehydrogenation and formation of ATP- The 2 triose bisphosphate molecules are then oxidised by the removal of hydrogen atoms to form 2 pyruvate molecules. NAD coenzymes accept the removed hydrogens- they are reduced- forming 2 NADH molecules.
   At the same time, 4 ATP molecules are produced using phosphates from the triose bisphosphate molecules.

Question 2

How is glycolysis an example of substrate level phosphorylation?

Answer 2

It involves the formation of ATP without an electron transport chain. ATP is formed by the transfer of a phosphate group from a phosphorylated intermediate- Triose bisphosphate) to ADP.

Question 3

Explain the main steps of the link reaction- Oxidative decarboxylation.

Answer 3

1. Pyruvate enters the mitochondrial matrix by active transport via specific carrier proteins.
2. Pyruvate undergoes oxidative decarboxylation- CO2 is removed along with a hydrogen.
3. The H atoms removed are accepted by NAD, NAD is reduced to form NADH.
4. The resulting 2 C acetyl group is bound by coenzyme A, forming acetylcoenzyme A.
5. Acetyl CoA delivers the acetyl group to the next stage of aerobic respiration- Krebs cycle. The NADH is used in oxidative phosphorylation to produce ATP.
6. The acetyl groups are the only things left from the original glucose molecules. The CO2 produced will either diffuse away and be removed from the organism as metabolic waste, or in autotrophic organisms it may be used as a raw material for photosynthesis.

Question 4

Explain the main steps of the Krebs cycle.

Answer 4

1. Acetyl CoA delivers an acetyl group to the Krebs cycle. The 2C acetyl group combines with a 4C oxaloacetate to form a 6C citrate.
2. The citrate molecule undergoes decarboxylation and dehydrogenation producing 1 NADH and CO2. A 5C compound is formed.
3. The 5C compound undergoes further decarboxylation and dehydrogenation reactions, eventually regenerating oxaloacetate. More CO2, 2 more NADHs and one FADH are produced. ATP is also produced by substrate level phosphorylation.

Question 5

What are the importance of coenzymes in respiration?

Answer 5

• Coenzymes are needed to transfer protons, electrons and functional groups between many of the enzyme-catalysed reactions.
• Redox reactions have an important role in respiration so without coenzymes transferring electrons and protons between these reactions, many respiratory enzymes would be unable to function.
• NAD and FAD are both coenzymes that accept protons ad electrons released during the breakdown of glucose in respiration.

Question 6

What are the differences between NAD and FAD?

Answer 6

• NAD takes part in all stages of cellular respiration but FAD only accepts hydrogens in the Krebs cycle.
• NAD only accepts 1 H but FAD accepts 2
• NADH is oxidised at the start of the ETC releasing protons and electrons, whilst FADH is oxidised further along the chain.
• NADH results in the synthesis of 3 ATP molecules but FADH results in the synthesis of 2 ATP molecules.

Question 7

Explain the process of oxidative phosphorylation

Answer 7

1. The hydrogen atoms that have been collected by the coenzymes NAD and FAD are delivered to the ETCs present in the membranes of the cristae of the mitochondria.
2. The hydrogen atoms dissociate into H+ ions and electrons. The high energy electrons are used in the synthesis of ATP by chemiosmosis. Energy is released during redox reactions as the electrons reduce and oxidise electron carriers as they’re flowing along the ETC.
3. This energy is used to create a proton gradient leading to the diffusion of protons through ATP synthase, resulting in the synthesis of ATP.
4. At the end of the ETC, the electrons combine with hydrogen ions and oxygen to form water. Oxygen is the final electron acceptor and the electron chain cannot operate unless oxygen is present. Respiration which involves the complete breakdown of glucose is therefore an aerobic process.

Question 8

Why do the hydrogen atoms released from NAD and FAD not combine directly with oxygen during oxidative phosphorylation?

Answer 8

• They would release energy from the formation of bonds during the production of water.
• However, this energy couldn’t be used to synthesise ATP. Heat released in the exothermic reaction would simply raise the temperature of the cell.

Question 9

What is substrate level phosphorylation?

Answer 9

It is the production of ATP using the transfer of a phosphate group from a short-lived, highly reactive intermediate such as creatine phosphate. This is different from oxidative phosphorylation which couples the flow of protons down the electrochemical gradient through ATP synthase to the phosphorylation of ADP to produce ATP.

Question 10

What are obligate anaerobes?

Answer 10

They cannot survive in the presence of oxygen. Almost all obligate anaerobes are prokaryotes e.g. Clostridium (bacteria that causes food poisoning), although there are some fungi as well.

Question 10

What are facultative anaerobes?

Answer 10

Synthesise ATP by aerobic respiration if oxygen is present, but can switch to anaerobic respiration in the absence of oxygen e.g. yeast.

Question 11

What are obligate aerobes?

Answer 11

Can only synthesise ATP in the presence of oxygen e.g. mammals.
- The individual cells of some organisms e.g. muscle cells in mammals, can be described as facultative anaerobes because they can supplement ATP supplies by employing anaerobic respiration in addition to aerobic respiration when oxygen conc is low. However, this is only for short periods and oxygen is eventually required. The shortfall of oxygen during the period of anaerobic respiration produces compounds that have to be broken down when oxygen becomes available again, so the organism as a whole is an obligate aerobe.

Question 12

What is fermentation?

Answer 12

It is the process by which complex organic compounds are broken down into simpler inorganic compounds without the use of oxygen of the involvement of an ETC.
- The organic compounds such as glucose, arent fully broken down so fermentation produces much less ATP than aerobic respiration. The small quantity of ATP produced is synthesised by substrate-level phosphorylation alone.

Question 13

What are the end products of alcoholic fermentation?

Answer 13

Ethanol and CO2

Question 14

What is the end product of lactate fermentation?

Answer 14

Lactate

Question 15

What is the consequence of there being no oxygen to act as the final electron acceptor at the end of an ETC in oxidative phosphorylation?

Answer 15

• Flow of electrons down the ETC stop. This means that synthesis of ATP by chemiosmosis also stops.
• As the flow of electrons along the ETC has stopped, the NADH and FADH are no longer able to be oxidised as there is nowhere for the electrons to go. This means that NAD and FAD cannot be regenerated.
• This means that the decarboxylation and oxidation of pyruvate and the Krebs cycle comes to a halt as there are no coenzymes available to accept the hydrogens being removed.
• Glycolysis would also come to a halt due to the lack of NAD if it were not for the process of fermentation.

Question 16

Explain the process of lactate fermentation in mammals.

Answer 16

• In mammals, pyruvate can act as a H acceptor, taking the hydrogen from NADH, catalysed by the enzyme lactate dehydrogenase.
• The pyruvate is converted into lactate(lactic acid) and NAD is regenerated.
• This can be used to keep glycolysis going so a small quantity of ATP is still synthesised.
• In mammals in particular, anaerobic respiration in the muscles is often supported by ATP from aerobic respiration, which is still being produced as fast as oxygen can be delivered into other parts of the body.
• Lactic acid is converted back to glucose in the liver, but oxygen is needed to complete this process. This is the reason for oxygen debt after exercise.

Question 17

Why can’t lactate fermentation occur indefinitely?

Answer 17

• The reduced quality of ATP produced wouldn’t be enough to maintain vital processes for a long period of time.
• The accumulation of lactic acid causes a fall in pH, leading to proteins denaturing. Respiratory enzymes and muscle filaments are made from proteins and will cease to function at low pH.

Question 18

Explain the process of alcoholic fermentation in yeast ( and many plants).

Answer 18

• Unlike lactate fermentation, it is not a reversible process.
• Pyruvate is first converted to ethanal, catalysed by the enzyme pyruvate decarboxylase.
• Ethanal can then accept a hydrogen atom from NADH, becoming ethanol. The regenerated NAD can then continue to act as a coenzyme and glycolysis can continue.
• It isn’t a short term process and can continue indefinitely in the absence of oxygen. Ethanol is a toxic waste product to yeast cells and they are unable to survive if ethanol accumulates above approx. 15%. This is allowed to happen during the process of alcohol brewing or wine making.