respiration

• What are autotrophs?

  They have the ability to harness light energy from the sun and convert it to chemical energy, stored in the form of carbohydrates and other organic compounds formed during photosynthesis.
• What are heterotrophs?

  They depend on autotrophs as the source of energy, have the digestive systems to break down the complex organic compounds obtained from food into simple, soluble molecules for absorption into cells, which are further broken down via cellular respiration to obtain energy.
• What is cellular respiration?

  The sequence of enzyme-controlled steps in which chemical energy in an organic molecule, usually glucose, is released by oxidation. The energy released is trapped in the form of ATP.
• What is the main respiratory substrate?

  Glucose (for many organisms).
• What is ATP?

  Universal energy carrier, functions as an instant, readily available energy that a cell uses to carry out cellular processes, in all living organisms.
• Describe the structure of ATP.

  Adenosine triphosphate, which is made up of a ribose sugar, nitrogenous base adenine and 3 phosphate groups.
• How are phosphate groups added?

  Either inorganic phosphates or phosphate groups found on other molecules/substrates.
• What are the characteristics of ATP?

  Small and soluble, able to diffuse and be transported to different parts of the cell where it is required. Hydrolysis/De-phosphorylation of ATP to ADP releases energy for the cell to do work. (yields 30.5kJ/mol for removal of one phosphate groups from ATP) ADP can be re-phosphorylated into ATP via cellular respiration, recycling the ATP, a condensation reaction. (requires 30.5kJ/mol per mole of ATP formed)
• What is the ATP-ADP cycle for?

  Serves as the link between exergonic (hydrolysis) and endergonic (condensation) reactions of a cell.
• What is the overall equation of aerobic respiration?

  C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP
• What are the four stages of aerobic respiration?

  Glycolysis, link reaction, Krebs cycle, oxidative phosphorylation.
• Where is the site for glycolysis?

  Cytoplasm.
• What is the definition of glycolysis?

  Sequence of enzymatic reactions that breaks down 1 glucose molecule (6C) into 2 pyruvate molecules (3C), with the production of 2 net ATP, as well as the formation of 2 molecules of reduced NAD.
• Is O2 required for glycolysis?

  No, it can also occur in the absence of O2 during anaerobic respiration.
• What are the main stages of glycolysis?


  Phosphorylation of sugar, lysis, oxidation via dehydrogenation, substrate-level phosphorylation.
• Describe the phosphorylation of sugar.

  Glucose (6C) enters the cell via facilitated diffusion and is phosphorylated twice to form fructose-1, 6-bisphosphate (6C). For each phosphorylation step, the phosphate group is donated by 1 ATP molecule. 2 molecules of ATP are used.
• Is there any enzyme involved in glycolysis?

  Enzyme phosphofructokinase (PFK) catalyses addition of the 2nd phosphate group, which is the rate determining step of glycolysis. It can be inhibited by high levels of ATP, an allosteric/end-product inhibition.
• Describe lysis.

  Fructose-1, 6-bisphosphate (6C) is lysed/split into 2 molecules of 3C sugar phosphates known as triose phosphate (TP).
• Describe oxidation via dehydrogenation.

  TP is oxidised via the removal of hydrogen atoms via dehydrogenase enzymes. Protons and electrons of the hydrogen atoms are transferred to coenzyme NAD (nicotinamide adenine dinucleotide), which becomes reduced. A total of 4 hydrogen atoms are reduced, forming 2 reduced NAD (NADH) per glucose. An inorganic phosphate is also added to convert each TP to 1, 3-bisphosphoglycerate.
• Describe substrate-level phosphorylation.

  1, 3-bisphosphoglycerate is dephosphorylated to form 3-phosphoglycerate (PGA). PGA is dephosphorylated again to form pyruvate (3C). The 2 phosphate groups removed are transferred to 2 ADP to form 2 ATP. As 2 molecules of 1, 3-bisphosphoglycerate are formed from each glucose molecule, a total of 4 ATP are formed per glucose.
• What is the net ATP per glucose oxidised?

  Since 2 ATP were utilised to phosphorylate glucose in the beginning of glycolysis, thus the production is 2 net ATP per glucose oxidised.
• Is there any enzyme involved in substrate-level phosphorylation?

  The formation of ATP in these 2 steps involves an enzyme transferring the phosphate group from 1, 3-bisphosphoglycerate to ADP.
• Describe PFK.

  It has an active site for fructose-6-phosphate and ATP (substrates), in order to catalyse the transfer of a phosphate group from ATP to fructose-6-phosphate. When ATP level is high, it can also bind to allosteric site on the enzyme to inhibit the enzyme. ATP can either be a substrate or allosteric inhibitor of PFK.
• What happens at low ATP concentration? (context of PFK)

  When a cell uses a lot of ATP, the level of ATP in the cell falls. The chance of ATP binding to the allosteric site on PFK and acting as an allosteric inhibitor is low. The 3D conformation of the enzyme remains in the active site and the shape of the active site is complementary to the substrate molecules. Both fructose-6-phosphate and ATP can bind to the active site for glycolysis to proceed. Respiration pathway accelerates and more ATP can be produced in the cell.
• What happens at high ATP concentration?

  The chance of ATP binding to the allosteric site on PFK increases. This changes the 3D conformation of the enzyme to the inactive state. The active site is not complementary to the substrate molecules. Glycolysis is inhibited and respiration slows down.
• What are the other regulators of PFK?

  AMP and ADP can act as allosteric activator of PFK - high levels of AMP and ADP correspond to low level of ATP. Citrate from Krebs cycle also act as an allosteric inhibitor - high level of citrate, aerobic respiration is occuring at a high rate, corresponding to high level of ATP.
• What are the products of glycolysis per molecule of glucose?

  2 reduced NAD, 2 net ATP, 2 pyruvate.
• What are the raw materials needed for one round of glycolysis?

  1 glucose, 2 NAD, 4 ADP and 2 ATP.
• Where is the site for link reaction?

  Mitochondrial matrix.
• What is the key reaction of link reaction?

  Oxidative decarboxylation.
• Why phosphorylate glucose?

  It activates glucose by increasing energy level of glucose, so that more energy can be extracted from it later. Phosphate groups are negatively charged and these cause glucose molecules to be trapped within the cell as they cannot pass through the cell surface membrane. It helps maintain a low concentration of glucose inside the cell as glucose is converted to fructose-1, 6-bisphosphate. This in turn maintains a steep concentration gradient for further uptake of glucose from outside the cell.
• How does link reaction start?

  Pyruvate can diffuse freely across the outer membrane of mitochondria. A transport protein (pyruvate translocase) specifically transports pyruvate across the inner membrane into the matrix of the mitochondria.
• Describe link reaction.

  In the mitochondrial matrix, the pyruvate (3C) undergoes oxidation decarboxylation to form acetyl-CoA (2C). Pyruvate (3C) is first decarboxylated (removal of carbon) via the removal of C to form a 2C fragment. The 2C fragment is then oxidised via dehydrogenation, forming reduced NAD. At the same time, coenzyme A is added, giving rise to a molecule of acetyl-CoA (2C).
• What are the products of link reaction per molecule of pyruvate?

  1 acetyl-CoA, 1 reduced NAD, 1 CO2.
• What are the products of link reaction per molecule of glucose?

  1 acetyl-CoA, 2 reduced NAD, 2 CO2.
• Where is the site for Krebs cycle?

  Mitochondrial matrix.
• What are the key reactions of Krebs cycle?

  Oxidative decarboxylation, oxidation via dehydrogenation and substrate-level phosphorylation.
• What is Krebs cycle?

  Final pathway by which all carbon atoms from carbohydrates, lipids and proteins are oxidised through decarboxylation and dehydrogenation reactions.
• Why is Krebs cycle a cycle?

  Acetyl groups (in acetyl-CoA) are first added to oxaloacetate (OAA) to form citrate. At the end of a series of reactions, OAA is regenerated to allow more acetyl groups to be accepted.
• Describe Krebs cycle.

  Acetyl-CoA (2C) is accepted by oxaloacetate (4C) to form citrate (6C). Citrate then undergoes oxidation decarboxylation twice, forming 3 reduced NAD and 2 CO2 per acetyl-CoA. 1 ATP is formed via substrate-level phosphorylation. After that, 2 additional oxidation reactions in the formation of 1 reduced NAD, 1 reduced FAD and the regeneration of oxaloacetate. This allows Krebs cycle to continue as more acetyl-CoA can be accepted.