Answer to Question #328043 in Molecular Biology for Magodi

Question

Outline the main concepts of entropy and free energy. [10 marks]

2. Describe photorespiration and how its limitations are imposed on carbon assimilation by

the photosynthetic carbon oxidation cycle in C3 plants. [20 marks]

Answer 1

Free energy G is the ability of the system to do work:

$G = U + p∆v - TS$

U – the internal energy of the system;

p – pressure;

v – volume;

T – temperature;

S – entropy.

Entropy S is the part of the system's energy that is dissipated in thermal form and cannot be used to do work at a constant temperature. The change in entropy (∆S) is defined as the ratio of the total value of the heat absorbed by the system (∆Q) to the temperature (T) of the system:

$∆S ≥ ∆Q/T$

For an isolated system with ∆Q = 0, the equation takes the following form:

$∆S ≥ 0$

For reversible processes, the change in entropy (∆S) is zero:

$∆S = 0$

For irreversible processes, the change in entropy (∆S) is positive:

$∆S > 0$

Photorespiration is a metabolic pathway that occurs in photosynthetic organisms such as C3 plants that consume oxygen and release carbon dioxide and do not produce chemical energy or food. The chemistry of photorespiration can be explained with the following equations (before CO₂ formation):

• O₂ + ribulose-1,5-diphosphate → phosphoglycolate + 3-phosphoglyceric acid + 2H⁺ (by the action of ribulose-1,5-diphosphate carboxylase/oxygenase, or Rubisco);
• phosphoglycolate + H₂O → glycolate + phosphate (by the action of phosphoglycolate phosphatase);
• glycolate + O₂ → glyoxylate + H₂O₂ (by the action of glycolate oxidase);
• glyoxylate + glutamate → glycine + 2-oxoglutarate (by the action of glutamate glyoxylate aminotransferase);
• glycine + NADH₂ → serine + NAD⁺ + CO₂ + NH₄⁺ (by the action of glycine decarboxylase complex).

As far as the limitations of photorespiration, which are imposed on carbon assimilation by the photosynthetic carbon oxidation cycle in C3 plants, then Rubisco is the enzyme that performs its function in these two processes at once, which means that these processes cannot occur simultaneously, and as a result, during the day, under the action of ultraviolet radiation, the photosynthetic carbon oxidation cycle occurs, and photorespiration occurs at night.