Question

Species on Earth all use the same cellular fuel: ATP. ATP can be produced via two cellular respiration pathways, one that occurs in the presence of oxygen and one that occurs in its absence.

Name and explain how these two mechanisms differ. Can an organism use both?

Answer 1

Part 1

In the presence of oxygen, ATP is produced by aerobic respiration.

The first step is glycolysis which occurs in the cytoplasm. It is The breakdown of one molecule of glucose into two molecules of pyruvic acid with generation of ATP and NADH.

The second step is decarboxylation of pyruvic acid to form acetyl coenzyme a. It also generates NADH. It occurs in mitochondria. It is known as oxidative decarboxylation or linking reaction or Gateway reaction.

The third step is tricarboxylic acid cycle which is also known as citric acid cycle or kreb cycle. It occurs in mitochondria. It starts with the entry of acetyl coenzyme A. It generates high energy electron carriers, NADH and FADH2. It also generates GTP and various fatty acid intermediates.

The fourth step is electron transport chain which takes place in inner mitochondrial membrane in eukaryotes and in plasma membrane in prokaryotes. It is the passage of electrons from one electron carrier to another. It leads to generation of proton gradient across the mitochondrial membrane which is responsible for spinning of ATP synthase.

The last step is synthesis of ATP which occurs by the spinning of enzyme ATP synthase. This is known as chemiosmosis.

Part 2

In the absence of oxygen, ATP is produced by anaerobic respiration.

The first step is glycolysis. Same as that of aerobic respiration. This process in common in both the types of respiration.

The second step is fermentation which occurs in cytoplasm. It converts pyruvate to ethanol (alcohol fermentation) or to lactic acid (lactic acid fermentation) along with the production of NAD+. This NAD+ then enters glycolysis or TCA to fuel aerobic respiration.

Part 3

Yes an organism can show both aerobic and anaerobic fermentation. Human beings and other animals show aerobic fermentation normally, but after heavy muscular exercise, when oxygen supply to tissues decreases, aneraobic respiration starts in muscles.

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