Question

1. Glucose will react with oxygen in a laboratory setting using a combustion chamber. The reaction is exergonic, with the release of a large amount of energy in the form of heat and the production of carbon dioxide and water according to the following chemical equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O       G = –686 kcal/mol

Glucose can also be incubated with cells that completely oxidize glucose, according to the same net chemical equation and with the same quantity of energy release. However, glucose oxidation in cells occurs during a series of steps, rather than in one step like the combustion reaction taking place in a lab.

a. A coupled reaction is a pair of reactions occurring together in which the energy released by one reaction is used in bond formation in the second reaction. For example, glycolysis and pyruvate processing form a coupled reaction. Explain how coupling reactions allows a cell to capture the free energy of glucose oxidation rather than allow it to escape to the environment in the form of heat. Describe how this process affects other reactions in the cell.

b. Pose a scientific question that a scientist might ask about how these processes are conserved across phyla.

Answer 1

The combustion of glucose releases 686 kcal/mol of energy. It is a single step that involves the instantaneous release of energy. However, aerobic respiration involves the sequential oxidation of glucose molecules via multiple steps. The energy released during this process is coupled to produce ATP from ADP + Pi.

Metabolism = The entire collection of chemical transformations occurring in a living system
It includes two types of reactions.
1. Catabolism:
Degradation reactions
Large molecules are broken down to produce small molecules
Energy is released
Ex: Respiration

2. Anabolism:
Biosynthetic reactions
Small molecules are fused to produce large molecules
Energy is absorbed
Ex: Photosynthesis

Anabolic reactions cannot occur on their own in the cell. However, they are coupled with exergonic catabolic reactions. i.e. All anabolic reactions in the living systems are coupled with ATP hydrolysis that drives them. Enzymes couple the energy released from ATP hydrolysis to drive anabolic reactions.
Example: For the production one glucose molecule by Calvin cycle, 18 ATP and 12 NADPH are required.

Many of the fundamental biochemical pathways are highly conserved across several groups of organisms.
For example, glycolysis is found in almost all living organisms. Some scientists state that these pathways have evolved early during the cellular evolution and retained by all the organisms that have subsequently evolved. The major reason for their conservation is that they are energy efficient and attained near perfection so that they have not tampered in subsequent groups of organisms that evolved later.