Respiratory Substrates

Define Respiratory Quotient (RQ).

• : The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration. Formula:

RQ = \frac{CO_2}{O_2}

What are carbohydrates used for in organisms and what is its RQ value?

• The primary fuel source. Stored as glycogen (animals) or starch (plants).

  • Energy Value: approx 15.8 kJ g^-1.

  • RQ: 1.0.

When are proteins used in organisms and what is the RQ?

• Only used during starvation. Muscle protein is broken down into amino acids.

  • RQ: 0.9.

What is the approximate RQ value for Lipids (Triglycerides)?

• 0.7.

Why do lipids yield more energy per gram than carbohydrates?

• Lipids possess a greater proportion of Hydrogen atoms (C-H bonds). This generates more Reduced NAD/FAD, leading to a higher yield of ATP through oxidative phosphorylation.

What process must amino acids undergo before entering the respiratory pathway?

• Deamination (removal of the amine group) in the liver.

After deamination, where do the remaining parts of amino acids enter respiration?

• They usually enter as Pyruvate (glycolysis) or Acetyl CoA (Krebs cycle), depending on the specific amino acid.

How are lipids prepared for respiration?

Hydrolysed into Glycerol and Fatty Acids.

• Glycerol: Converted to Triose Phosphate (enters Glycolysis).

• Fatty Acids: Broken down into 2-carbon Acetyl groups (Beta-oxidation) to enter the Krebs Cycle.

What does an RQ value greater than 1 indicate?

• It suggests anaerobic respiration is taking place (or a mixture of aerobic and anaerobic).

Why don't we use proteins as a primary fuel source?

• Proteins are essential for structure/enzymes.

• Deamination produces toxic ammonia/urea, which requires energy to excrete.

A student measures the RQ of a yeast culture as 1.4. Explain what this value suggests about the respiration of the yeast. (3 marks)

• An RQ greater than 1.0 suggests anaerobic respiration is occurring (1).

• In anaerobic respiration, CO₂ is produced (via decarboxylation of pyruvate/ethanal) (1).

• However, no Oxygen is consumed, shifting the ratio significantly higher than 1 (1).

Describe how the structure of a triglyceride molecule makes it a suitable energy store. (3 marks)

• It is insoluble (doesn't affect water potential of cells) (1).

• It has a high proportion of Hydrogen atoms (C-H bonds) (1).

• This allows it to release more energy/ATP per gram (energy dense) compared to carbohydrates (1).