Section 5 - Energy transfer in and between organisms: 12. Respiration

Question 1

What is cellular respiration

Answer 1

The process by which ATP is formed from the breakdown of glucose

Question 2

What is Aerobic respiration

Answer 2

Respiration that requires oxygen, producing CO(2), water and lots of ATP

Question 3

What is Anaerobic respiration

Answer 3

Respiration that occurs in the absence of oxygen, only producing a small amount of ATP
- Animals: Produces lactate
- Plants/Fungi: Produces ethanol + CO(2)

Question 4

What are the 4 stages of Aerobic respiration

Answer 4

1) Glycolysis: The splitting of glucose into 2 pyruvate
2) Link reaction: Pyruvate undergo reactions to form Acetyl-CoA
3) Krebs Cycle: Acetyl-CoA is introduced into a cycle of REDOX reactions, forming reduced coenzymes
4) Oxidative phosphorylation: Electrons released from reduced coenzymes allow for the synthesis of ATP

Question 5

What is the role of glycolysis in Aerobic respiration

Answer 5

It is the initial stage of both Aerobic and Anaerobic respiration, involving the splitting of a 6-Carbon glucose molecule into 2 3-Carbon pyruvate molecules.

Question 6

Where does Glycolysis occur

Answer 6

In the Cytoplasm of the cell

Question 7

What is the process of Glycolysis in (4 main stages)

Answer 7

1) Phosphorylation of Glucose, forming glucose phosphate
- 2 phosphate molecules are added to the glucose, making it more reactive so it can be broken down
- Phosphate comes from the hydrolysis of ATP into ADP
- The energy released lowers the activation energy of the following reactions

2) Splitting of the phosphorylated glucose
- Each 6-carbon glucose molecule splits into 2 3-carbon ‘triose phosphate’ molecules (TP)
- Each TP contains one phosphate molecule

3) Oxidation of triose phosphate
- 2 Hydrogens are removed from each triose phosphate
- The Hydrogen then attaches to NAD (coenzyme), forming reduced NAD

4) Production of Pyruvate (and ATP)
- Enzyme controlled reactions convert each 3-carbon TP molecule into a 3-carbon Pyruvate molecule
- The process results in the formation of 2ATP from each TP (∴ 4 total)

Question 8

What is the total yield for Gylcolysis

Answer 8

• 2ATP (Produces 4, but 2 are required)
• 2 Reduced NAD
• 2 Pyruvate

Question 9

How does the process of Glycolysis provide indirect evidence for Evolution

Answer 9

The process is a universal feature of all living organisms, proving that there is common ancestry for all organisms

Question 10

What is the role of the link reaction in Aerobic respiration

Answer 10

The oxidation of Pyruvate, to form Acetyl-CoA that can be used later in the Krebs cycle

Question 11

Where does the link reaction occur in Aerobic respiration

Answer 11

In the matrix of the mitochondria (Pyruvate moves in by active transport)

Question 12

What is the process of the Link reaction in Aerobic respiration (2 main stages)

Answer 12

1) Pyruvate is oxidised to form acetate
- 3-carbon pyruvate loses 1CO(2) + 2H to form a 2-carbon acetate molecule
- The hydrogen is accepted by NAD to form reduced NAD

2) Acetate combines with Coenzyme A, forming Acetyl-CoA

Question 13

What is the role of the Krebs Cycle in Aerobic respiration

Answer 13

Uses the Acetyl-CoA from the link reaction to form reduced coenzymes that can be used later for the production of ATP

Question 14

What is the equation for the Link reaction in Aerobic respiration

Answer 14

Pyruvate + NAD + CoA → Reduced NAD + CO(2)

(Occurs twice for every glucose molecule, as 2 Pyruvate are released)

Question 15

Where does the Krebs cycle occur in Aerobic respiration

Answer 15

In the matrix of the mitochondria

Question 16

What is the process of the Krebs cycle in Aerobic respiration (3 main stages)

Answer 16

1) The 2-carbon Acetyl-CoA from the link reaction combines with a 4-carbon molecule to produce a 6-carbon molecule

2) Through a series of reactions, the 6-carbon molecule loses 2CO(2) + Hydrogen, forming a 4-carbon molecule
- The hydrogen is accepted by coenzymes, forming reduced NAD and reduced FAD
- 1ATP is produced through substrate level phosphorylation

3) The 4-carbon molecule released can combine with more Acetyl-CoA, allowing the cycle to repeat

Question 17

What is the total yield of the Link reaction and Krebs cycle from 1 Pyruvate molecule

Answer 17

• Reduced coenzymes, for later use in oxidative phosphorylation (FADH and 2NADH)
• 1ATP
• 3CO(2) as a by-product removed by gas exchange

2 molecules of Pyruvate are released from each glucose molecule, so the above yield is doubled if for every glucose

Question 18

What are coenzymes and why are they important in respiration (+photosynthesis)

Answer 18

Coenzymes are molecules that some enzymes require in order to function
- Eg. NAD, NADP (plants), FAD
- Important in respiration as they work with dehydrogenase enzymes that catalyse the removal of hydrogen atoms from substances, then carry the H to where they are need (eg. for oxidative phosphorylation)

Question 19

What is the role of Oxidative phosphorylation in Aerobic respiration

Answer 19

Produces ATP through the use of electrons released from reduced coenzymes provided by previous stages

Question 20

Where does Oxidative phosphorylation occur in Aerobic respiration

Answer 20

In the Cristae (Inner folded membrane) of the mitochondria
∴ There are more mitochondria, with more densely pack cristae in metabolically active cells (eg. muscle, liver, epithelial, etc)

Question 21

What is the process of Oxidative phosphorylation in Aerobic respiration (5 main stages)

Answer 21

1) Hydrogen atoms released during previous stages combine with coenzymes to give Reduced NAD and reduced FAD

2) Reduced coenzymes donate the electrons from their H atoms to the first molecule of the electron transport chain, in the cristae of the mitochondria

3) Electrons pass along the transport chain, moving between proteins through a series of REDOX reactions
- The electrons release energy as they flow along the chain
- This energy causes protons (H+) from the reduced coenzymes to be transported across the inner membrane, from the matrix to the intermembrane space
- This causes protons to accumulate in the inter-membrane space, forming a concentration gradient

4) This conc. gradient means that the protons move back into the matrix by facilitated diffusion, via ATP synthase channels
- This provides energy for the synthesis of ATP from ADP + Pi
- This is the main production of ATP from Aerobic respiration
- This movement of H+ is called the chemiosmotic theory of oxidative phosphorylation

5) At the end of the electron transport chain, oxygen acts as the final electron acceptor, as it combines with the e- and H+ to form water

Question 22

Why is the energy of the electrons released gradually in the electron transport chain of oxidative phosphorylation

Answer 22

• The greater the energy released in a single step, the more that is wasted (eg. lost as heat)
• ∴ The electrons from the reduced coenzymes are not transferred to the final acceptor in one step
• The electrons pass through a series of carrier molecules (proteins) in the membrane, each with a slightly lower energy level
• ∴ Electron release their energy slowly, causing more H+ ions to be actively transported into the inter-membrane space, so more diffuse through the ATP synthase channel
  (More ATP is produced)

Question 23

How can Lipids be used as an alternative respiratory substance, instead of glucose

Answer 23

• Lipids are first hydrolysed into glycerol and fatty acids
• Glycerol is phosphorylated to form triose phosphate (enters glycolysis → Link → Krebs → Oxidative Phosphorylation)
• Fatty acids are broken down into 2-carbon fragments that are converted into acetyl-CoA (enters Krebs → Oxidative Phosphorylation)
• Lipids can be oxidised, forming 2-carbon fragments and many H+ atoms to be used for ATP synthesis in oxidative phosphorylation

∴ Lipids release more than double the energy of the came mass of carbohydrates

Question 24

How can Proteins be used as an alternative respiratory substance, instead of glucose

Answer 24

• Proteins are first hydrolysed into their constituent amino acids
• Amino acids have their amine groups removed (Deamination)
• They then enter the respiratory pathway depending on how many carbon atoms they have
  3-carbon → converted into pyruvate
  4/5-carbon → converted into intermediate molecules for the Krebs cycle

Question 25

Why can’t Aerobic respiration occur without oxygen

Answer 25

Oxygen acts as the final electron and proton acceptor.
∴ Without it…
- The conc. gradient of H+ can’t from across the inner membrane of the mitochondria, so no ATP will be formed during oxidative phosphorylation
- The electron transport chain would stop
- All coenzymes would eventually be reduced, so no more e- or H+ could be taken from previous reactions to be used in oxidative phosphorylation

Question 26

How can Anaerobic respiration produce energy without the presence of oxygen

Answer 26

• The reduced coenzymes are oxidised by reacting with the pyruvate produced during Glycolysis
  (as there is no O(2) to act as a final electron acceptor)
• The pyruvate accepts the H+, so NAD is released, allowing for more glycolysis to occur, producing ATP (small amount)

Question 27

What is the equation for Anaerobic respiration in plants (and some microorganisms)

Answer 27

Pyruvate + Reduced NAD → Ethanol + CO(2) + NAD

(Anaerobic respiration of yeast = fermentation)

Question 28

What is the equation for Anaerobic respiration in animals

Answer 28

Pyruvate + Reduced NAD → Lactate + NAD

Question 29

How many H+ and e- bind to one coenzyme to form a reduced coenzyme

Answer 29

2H and 2e-

eg:
NAD + 2H + 2e- → Reduced NAD (called NADH)
FAD + 2H + 2e- → Reduced FAD (called FADH)

Question 30

Why does an ‘oxygen debt’ establish after periods of intense exercise

Answer 30

• After periods of anaerobic respiration, the lactate produced results in a lactic acid build up in the muscles, causing cramps and fatigue as acid alters pH (effecting enzymes)
• ∴ lactate must eventually be oxidised back into pyruvate
  (either to release energy, or converted into glycogen in the liver for storage)
• This can only occur when O(2) becomes available again after the exercise, so the oxygen debt is the excess O(2) required to break down the lactic acid, on top of the levels already required for respiration.

Question 31

How is energy released from substrate level phosphorylation in respiration (Glycolysis/Krebs)

Answer 31

A phosphate group is directly transferred from an intermediate respiratory molecule to ADP, forming ATP that can be hydrolysed to release energy

Question 32

How is energy released from oxidative phosphorylation in aerobic respiration

Answer 32

• Chemiosmotic theory
• Energy provided by the movement of H+ down their conc. gradient (established by active transport due to the electron transport chain) allows ATP to be formed from ADP + Pi
  (Most of the ATP made by the cell is produced in this way)

Question 33

How is energy released from anaerobic respiration

Answer 33

• The pyruvate from glycolysis is converted into either ethanol or lactate when it accepts the H+ from the reduced coenzymes
• ∴ Pyruvate is not available to enter the link reaction/Krebs, so Oxidative phosphorylation can’t occur
• ∴ The only ATP produced in anaerobic respiration is from glycolysis