Cellular Respiration and Metabolic Pathways

Regulating Cellular Respiration and Metabolic Pathways

• Cellular respiration rate depends on supply and demand
• High demand during exercise, low demand when resting
• Basal metabolic rate (BMR) measures energy expenditure at rest, usually measured in oxygen consumption
• BMR varies with age and sex

Regulation of Aerobic Respiration

• Feedback inhibition is a process that allows the cell to control the rate of biochemical reactions

• The product of the last reaction in a pathway inhibits the enzyme that catalyzes the first reaction in the pathway

• This ensures that enough product is available for each reaction.

• Feedback Inhibition of Enzymes in Aerobic Respiration

• Many enzymes in glycolysis and the Krebs cycle are controlled by feedback inhibition of ATP and NADH

• Phosphofructokinase (PFK) is the main control point in glycolysis

• ATP inhibits PFK, signifying that when there is enough ATP, it binds to an allosteric site on the enzyme

• Citrate inhibits PFK, Pyruate is not converted to acetyl-CoA until there's limited citrate

• Feedback Inhibition of Pyruvate Dehydrogenase

• Pyruvate dehydrogenase is inhibited by NADH, slowing the conversion of pyruvate to Acetyl-CoA, and CO2.

Other Factors Affecting Rate of Reaction

• Temperature: Higher temperatures lead to faster reactions.
• Nutrient concentration: Higher concentrations result in faster reactions.
• Availability of oxygen: Critical for cellular respiration efficiency and reaction rate.

Interconnections of Metabolic Pathways

• Mitochondria harvests energy from one glucose molecule
• Carbohydrates that can be converted to glucose enter glycolysis
• Other macromolecules (proteins, lipids) can also be used for energy

Protein Catabolism

• Proteins are broken down into amino acids
• Amino acids can be converted to intermediates of glycolysis or the Krebs cycle before being used.
• Deamination removes amino groups from amino acids.
• Ammonia is created during deamination which can then be converted to urea, excreted by the kidneys
• Transamination transfers amine groups to other amino acids

Lipid Catabolism

• Fat molecules are broken down into glycerol and fatty acids
• Glycerol can be converted to a glucose molecule via gluconeogenesis
• Glycerol is converted into Glyceraldehyde 3-Phosphate, which makes pyruvate
• Fatty acids undergo beta oxidation to form Acetyl-CoA
• Each 2-carbon unit becomes an acetyl Co-A to enter the Krebs Cycle
• 12 carbon fatty acid chain = 6 Acetyl-CoA molecules

ATP Production

• Each cleavage of a fatty acid uses 1 ATP and produces 1 NADH and 1 FADH2
• The number of ATP produced per fatty acid cleavage needs to be determined.
• 18 carbon fatty acid yields 9 Acetyl CoA's.
• Through the Krebs Cycle and electron transport chain, these 9 Acetyl CoAs will contribute to 27 NADH, 9 FADH2, and 9 ATP.