Respiration

Question 1

What is respiration?

Answer 1

The process whereby the chemical potential energy stored in complex organic molecules is used to make ATP in living cells.

Question 2

What is metabolism?

Answer 2

All chemical reactions that occur inside living cells that form an essential part to its survival and the survival of the organism.

Question 3

What are the two main types of reactions that make up metabolism?

Answer 3

1. Catabolism: The breakdown of larger molecules into multiple smaller molecules.
2. Anabolism: The build-up of smaller molecules into larger molecules.

Question 4

What cellular processes require energy from respiration?

Answer 4

• Active transport of substances across a membrane against the concentration requires a lot of energy and is an essential process in living organisms. An example is the mechanism to maintain the resting potential of neurones using Na+/K+ pumps.
• Secretion by exocytosis requires energy and is the primary secretion mechanism for larger molecules.
• Endocytosis also requires energy.
• Some anabolic reactions such as protein synthesis and cellulose synthesis require energy.
• DNA replication and many processes that happen pre-cellular replication require energy.
• Movement involving the cytoskeleton, including flagella/undulipodia, cilia and microtubules all require energy.
• Muscular contractions also requires energy.
• Other types of metabolic reactions such as phosphorylation reactions that are vital for survival also require energy.
• Heat energy released by respiration is used to maintain constant core temperature in endotherms in order to maintain constant rates to enzyme-catalysed reactions.

Question 5

Where does most energy in living things come from?

Answer 5

1. Most energy in living things originate as light energy from the sun.
2. Photoautotrophs (producers) carry out photosynthesis which converts light energy into chemical potential energy stored in large organic molecules such as carbohydrates.
3. These producers are then consumed by consumers that use respiration to break down these large organic molecules into small inorganic molecules, releasing the stored energy as heat and chemical energy stored in ATP.

Question 6

What is the structure of ATP?

Answer 6

ATP (Adenosine Triphosphate) consists of an adenosine group (adenine bonded to a ribose sugar), bonded to 3 phosphate (phosphoryl) groups.

Question 7

What is the role of ATP in living organisms?

Answer 7

• ATP is the universal energy currency in cells.
• It is used in all energy-requiring reactions as a source of energy in all organisms.
• It acts as an intermediate molecule between all energy-releasing processes and energy-requiring processes in a cell.

Question 8

How does ATP store energy from respiration?

Answer 8

Respiration occurs in many small steps that release small amounts of energy each time. This energy is used to phosphorylate ADP into ATP, storing the energy, which can be hydrolysed in an energy-requiring reaction. However, ATP is never stored and rarely imported and each cell only has a small amount, so it needs to be regenerated rapidly by respiration in order to keep up in demand.

Question 9

What are the 4 stages of aerobic respiration?

Answer 9

Common reaction:
1. Glycolysis - Occurs in cytoplasm.
Aerobic respiration only:
2. Link reaction - Occurs in matrix of mitochondria.
3. Krebs cycle - Occurs in matrix of mitochondria.
4. Oxidative phosphorylation - Occurs in cristae, folded inner mitochondrial membrane.

Question 10

What are coenzymes?

Answer 10

Molecules that are not part of the enzyme but are required in order for an enzyme to function properly. This makes them cofactors to enzymes.

Question 11

What coenzymes are involved in respiration?

Answer 11

1. NAD (Nicotinamide Adenine Dinucleotide).
2. FAD (Flavin Adenine Dinucleotide).
3. Coenzyme A.

Question 12

Why are coenzymes involved in respiration?

Answer 12

During most stages of respiration, oxidation reactions occur whereby hydrogen atoms are removed by the process of dehydrogenation. These reactions are carried out by dehydrogenase enzymes. However, the reactions release hydrogen atoms which are particularly unstable and cannot exist on their own, so dehydrogenation reactions cannot occur. Coenzymes NAD and FAD combine with these H atoms to form reduced NAD and FAD. This ensures that dehydrogenation reactions can occur when catalysed by dehydrogenase enzymes, making them coenzymes to dehydrogenase. They also act as carriers of H atoms to the cristae where they are used in oxidative phosphorylation.

Question 13

How many hydrogen atoms can one NAD molecule accept?

Answer 13

2

Question 14

What is glycolysis?

Answer 14

The metabolic pathway where each glucose molecule is broken down into 2, 3 carbon pyruvate molecules.

Question 15

Where does glycolysis take place?

Answer 15

In the cytoplasm of respiring cells.

Question 16

Why is glycolysis known as the ‘common’ pathway?

Answer 16

It does not require oxygen and is the only reaction pathway that takes place both in aerobic and anaerobic respiration.

Question 17

What are the stages in glycolysis?

Answer 17

Phosphorylation:

1. 1 ATP molecule is hydrolysed to ADP and Pi. Pi attached to carbon-6 on glucose to form glucose 6-phosphate. This process is called phosphorylation.
2. Glucose 6-phosphate is converted to fructose 6- phosphate.
3. Another ATP molecule is hydrolysed and the Pi attaches to carbon-1 to form fructose 1,6-bisphosphate.
4. Energy released from ATP hydrolysis used to activate molecule to form hexose 1,6-bisphosphate to prevent molecule from leaving cell.
5. 2 ATP molecules are used to form hexose 1,6-bisphosphate.

Lysis of hexose 1,6-bisphosphate:
6. Hexose 1,6-bisphosphate splits into 2 molecules of triose phosphate.

Oxidation of triose phosphate:

7. Dehydrogenase enzymes oxidise each triose phosphate molecule by removing 2 hydrogen atoms from each.
8. Hydrogen atoms combine with NAD molecules to form reduced NAD.
9. ADP and Pi are recombined to form ATP in a process called substrate level phosphorylation.
10. 2 triose phosphate molecules are converted to 2 oxidised intermediate compounds, with the formation of 2 ATP and 2 reduced NAD molecules.

Conversion of triose phosphate to pyruvate:

11. A series of 4 enzyme catalysed reaction convert the intermediate compound into a pyruvste molecule.
12. A further 2 ATP molecules are formed through substrate level phosporylation.
13. 2 intermediate molecules are converted to 2 pyruvate molecules with the formation of 2 ATP molecules.

Question 18

What is the overall process of glycolysis?

Answer 18

Glucose + 2 x NAD + 2 x ATP —> 2 x pyruvate + 2 x reduced NAD + 4 x ATP

Question 19

What is the net gain in ATP through glycolysis?

Answer 19

2

Question 20

What types of cells have many mitochondria?

Answer 20

Cells that characteristically have high energy requirements. Including muscle cells, brain cells, ciliated epithelial cells and PCT epithelial cells.

Question 21

What is the structure of a mitochondrion?

Answer 21

• All mitochondria are enclosed by 2 phospholipid membranes surrounding it in an envelope. An outer membrane and an inner membrane.
• The outer membrane is smooth whereas the inner membrane is folded into cristae to maximise surface area.
• Space between outer and inner membrane called the intermembrane space.
• Matrix surrounded by the inner membrane and is of a gel-like consistency, containing lipids, proteins, enzymes, ring of DNA and mitochondrial ribosomes.

Question 22

How is the matrix adapted to its function?

Answer 22

• Matrix is site of link reaction and Krebs cycle.
• Contains the appropriate enzymes to catalyse these reactions.
• Contains a lot of NAD.
• Oxaloacetate present in number to accept acetate molecules into the Krebs cycle from link reaction.
• DNA coding for vital proteins required by mitochondria,
• Ribosomes to manufacture proteins inside the mitochondria itself.

Question 23

How is the outer membrane adapted to its function?

Answer 23

• Contains carrier proteins that only allow certain molecules to enter and leave mitochondria, controlling movement of substances.
• Enzymes to catalyse certain reactions.

Question 24

How is the inner membrane adapted to its function?

Answer 24

• Inner membrane site of oxidative phosphorylation.
• Folded into cristae to maximise surface area.
• H+ pumps for active transport of H+ ions during chemiosmosis.
• Contains many cofactors and enzymes that form part of the electron carrier chain.
• Many ATP synthase to manufacture ATP from ATP synthase.
• Different composition to outer membrane and is impermeable to small ions.

Question 25

What is the structure of ATP synthase molecules?

Answer 25

• Proton channel part embedded into the inner membrane and allows H+ ions to pass from intermembrane space into matrix.
• Headpiece attached to an axel (stalk) which allows for free rotation.
• Kinetic energy from the H+ ions flowing down concentration into the matrix through proton channel drives rotation of headpiece which allows ADP to join with Pi to form ATP.

Question 26

Where does the link reaction occur?

Answer 26

In the matrix of the mitochondria.

Question 27

What are the steps in the link reaction?

Answer 27

1. Dehydrogenation/decarboxylation - Pyruvate decarboxylase removes one CO2 molecule per pyruvate molecule. Pyruvate dehydrogenase oxidises pyruvate by dehydrogenation, with NAD acting as the hydrogen acceptor. The pyruvate (3C) becomes an acetate molecule (2C).
2. Acetate molecule binds onto coenzyme A (CoA) to form acetyl coenzyme A and is carried into the Krebs cycle.

Question 28

What is the overall equation for the link reaction?

Answer 28

2 pyruvate + 2 CoA + 2 NAD —> 2 acetyl CoA + 2 CO2 + 2 reduced NAD

Question 29

Where does the Krebs cycle take place?

Answer 29

In the matrix of the mitochondria.

Question 30

What are the stages in the Krebs cycle?

Answer 30

1. The acetate is transferred from the acetyl CoA onto a 4C oxaloacetate compound to form a 6C citrate compound.
2. Citrate is decarboxylated and dehydrogenated by enzymes, one molecule of NAD accepts the hydrogen atoms from dehydrogenation to form reduced NAD. A 5C intermediate compound is formed, as well as CO2.
3. the 5C compound is further decarboxylated and dehydrogenated, producing another CO2 molecule and reducing an NAD to reduced NAD, as well as a 4C compound.
4. The 4C compound is changed into another 4C compound through a series of reactions which results in a molecule of ADP being phosphorylated to ATP through substrate-level phosphorylation.
5. The 4C compound is further dehydrogenated, with FAD acting as the electron acceptor to form a molecule of reduced FAD.
6. 4C compound further dehydrogenated to form oxaloacetate and a molecule of NAD is reduced.
7. Oxaloacetate ready to accept more acetyl CoA molecules to repeat cycle.

Question 31

How many turns of the Krebs cycle require per glucose molecule?

Answer 31

2

Question 32

What are the products of the Krebs cycle per molecule of glucose?

Answer 32

4 x CO2, 6 x reduced NAD, 2 x reduced FAD, 2 x ATP.

Question 33

What is oxidative phosphorylation?

Answer 33

Formation of ATP by adding a phosphate group to ADP, in the presence of oxygen which acts as the final electron acceptor.

Question 34

Where does oxidative phosphorylation take place?

Answer 34

On the cristae ( folded inner membrane of the mitochondria).

Question 35

How are reduced NAD and reduced FAD re-oxidised during oxidative phosphorylation?

Answer 35

• Reduced NAD binds onto complex I, the first member of the electron carrier chain embedded into the cristae, also called NADH dehydrogenase. The hydrogen atoms are removed from reduced NAD and split into H+ and e- ions. The NAD is released back into the matrix to be reused.
• Reduced FAD also binds onto dehydrogenase enzymes that re-oxidise them into FAD and splits hydrogen atoms into H+ and e-. H+ ions released into matrix.

Question 36

What is the fate of e- ions?

Answer 36

They are carried along an electron carrier chain in the cristae to oxygen atoms which act as the final electron acceptor. The oxygen reacts with protons and e- ions to form water in the reaction:

4H+ + 4e- + O2 —> 2H2O

Question 37

What is the fate of H+ ions?

Answer 37

• As the electrons flow along the electron carrier chain, coenzymes associated with complex I, III and IV in the chain use the energy from the flowing electrons to pump H+ ions released by dehydrogenation of NADH and FADH from the matrix into the intermembrane space against concentration gradient by active transport.
• A concentration, pH and electrochemical gradient is set up between the matrix and intermembrane space, across the inner membrane. This causes the H+ ions to diffuse into the matrix down multiple gradients.
• Inner membrane impermeable to H+ ions except for channels in the membrane associated with ATP synthase.
• As the H+ ions flow through channel, they rotate the headpiece of ATP synthase which uses this kinetic energy to phosphorylate ADP to ATP. This process is called chemiosmosis.

Question 38

What is the theoretical yield of ATP per molecule of glucose?

Answer 38

30-36

Question 39

Why is this yield rarely achieved?

Answer 39

• Some H+ ions may leak back into the matrix from intermembrane space through other part besides ATP synthase.
• Some ATP is used to actively transport pyruvate into mitochondria from cytoplasm.
• Some ATP used to actively transport reduced NAD produced during glycolysis into cytoplasm.
• Some energy is lost as heat during respiration pathway as opposed to being used for phosphorylation.

Question 40

What is the role of oxygen during oxidative phosphorylation?

Answer 40

Acts as final electron acceptor.

Question 41

What processes in aerobic respiration cannot operate without oxygen?

Answer 41

Link reaction, Krebs cycle and oxidative phosphorylation.

Question 42

What is the principle behind anaerobic respiration?

Answer 42

Glycolysis is able to produce 2 molecule of ATP per glucose molecule. However, the reduced NAD produced by glycolysis needs to be regenerated in order for glycolysis to continue, which is what the extra reactions during anaerobic respiration are for.

Question 43

What are the 2 types of anaerobic respiration in eukaryotic cells?

Answer 43

• Lactate fermentation - used by mammalian cells.

- Ethanol fermentation - used by fungi (such as yeast) and some plant cells.

Question 44

What are the stages in lactate fermentation?

Answer 44

1. Pyruvate accepts hydrogens from reduced NAD to form lactate in the presence of lactate dehydrogenase. NAD is regenerated which can be reused in glycolysis.
2. Lactate diffuses into blood where it’s carried to the liver to be stored and broken down when more oxygen is available.

Question 45

What are the stages in ethanol fermentation?

Answer 45

1. Pyruvate molecules are decarboxylated by the enzyme pyruvate decarboxylase to ethanal, releasing a molecule of CO2 in the process.
2. Ethanal accepts hydrogens from reduced NAD to form ethanol in the presence of ethanol dehydrogenase. NAD is regenerated which can be reused in glycolysis.
3. Ethanol diffuses out of the respiring cell into the surrounding environment.

Question 46

How are carbohydrates respired?

Answer 46

• All carbohydrates are converted to glucose and are respired via glycolysis. For this reason, energy value for all carbohydrates are the same.
• Glucose respiration has an efficiency of around 34%, with the rest of the energy being released as heat to maintain body temperature.

Question 47

How are proteins respired?

Answer 47

• Proteins are broken down into amino acids by protease enzymes.
• Amino acids are deaminated to create keto acids that can either enter the Krebs cycle directly, be converted to pyruvate or acetate molecules and then enter the respiratory pathway.

Question 48

How are lipids respired?

Answer 48

• Triglyceride lipids are broken down to glycerol and fatty acids by lipase enzymes.
• Energy from hydrolysis of ATP to AMP required to bind fatty acid onto CoA.
• After being transported into the matrix, the fatty acid-CoA complex is broken down into acetate CoA complexes that can enter the Krebs cycle directly.
• This breakdown happens through the beta-oxidation pathway and produces molecules of reduced NAD and FAD.
• Breakdown of acetate groups in the Krebs cycle also produces reduced NAD and FAD.
• The reduced NAD and FAD are able to enter oxidative phosphorylation to produce ATP.

Question 49

Why do lipids have higher energy values than carbohydrates?

Answer 49

Lipids have more hydrogen atoms per mole than carbohydrates do. This means that more reduced NAD and FAD are produced during its breakdown which can be used to produce ATP during oxidative phosphorylation.`

Question 50

What are the advantages of using ATP as the universal energy currency?

Answer 50

• Using one molecule as source of energy for all processes in a cell makes regulating and coordinating these processes much easier.
• It can be easily hydrolysed to form ADP and Pi, releasing energy in small and manageable amounts of around 30.6KJ/mol. This ensures that it can be controlled easily and won’t damage the cell.
• Hydrolysis of ATP is usually coupled into the reaction pathways of many metabolic reaction including DNA synthesis to ensure that energy can be released and is available when it is required.