Question

1. Describe two types of benefits derived from separating the reactions of glycolysis in the cytosol from those that occur during the citric acid cycle in the mitochondrion.

Answer 1

Answer : Two types of benefits that are derived from separating the reactions of glycolysisin the cytosol from those that occur during the citric acid cycle in the mitochondrion are:

1. Glycolysis is a metabolic pathway and an anaerobic source of energy that has evolved in nearly all types of organisms. Glycolysis is oxygen independent process. It can happen in anaerobic conditions also and hence during such conditions Glycolysis can keep the body functions going for some time by fulfilling basic energy (ATP) requirements. If glycolysis had taken place in mitochondrion like Citric Acid Cycle or TCA Cycle, then it would also have stopped yielding energy immediately during anaerobic conditions.
Glycolysis is a metabolic pathway which creates ATP without the use of oxygen but can occur in the presence of oxygen as well. In cells which use aerobic respiration as the primary source of energy, the pyruvate formed from the pathway can be used in the citric acid cycle and go through oxidative phosphorylation to be oxidized into carbon dioxide and water. Even if cells primarily use oxidative phosphorylation, glycolysis can serve as an emergency backup for energy or serve as the preparation step before oxidative phosphorylation.

   This owes to evolutionary reason also. Before the endosymbiosis of an aerobic bacteria, few billion years ago, which is now thought to be modern mitochondrion, this was the only way that our ancestors could produce energy.

Also the cells without mitochondrion like in RBCs or Erythrocytes , glycolysis keeps up the normal cell functionalities by providing basic energy requirements. In erythrocytes, lactic acid fermentation is the sole source of ATP for these cells have no mitochondria, and once mature, the red blood cells have little demand for ATP. Another part of the body which relies entirely or almost entirely on anaerobic glycolysis is the lens of the eye, which is devoid of mitochondria to prevent light scattering.

During heavy exercise also glycolysis plays important role. Though skeletal muscles prefer to catalyze glucose into carbon dioxide and water during heavy exercise where the amount of oxygen is inadequate, the muscles simultaneously undergo anaerobic glycolysis along with oxidative phosphorylation.

2. Second type of benefit this separation provides is that Citric Acid Cycle is a complex pathway and needs proper functioning of so many enzymes and if due to any reason a single enzyme is absent or limited at the required time in mitochondrion, then glycolysis happening in the cell cytosol can continue to provide ATPs or energy to the cell. Also since most of the enzymes required in glycolysis are present in high concentrations in cytosol therefore it increases the efficiency of glycolysis being in cytosol.

   Hence it is beneficial to the body that the two metabolic pathways are separated in nature.