CELLULAR RESPIRATION: Definition, Types, Combustion vs. Respiration, ATP and cells


The term “Respiration” refers to one of a living being’s vital systems. Respiration is one of the universal features that all living beings share as signs of life.

During respiration Foods (carbohydrates, proteins, and fats) are chemically burned /oxidized to produce energy (ATP), carbon dioxide, and water. It is referred to as an amphibolic (catabolic and anabolic) process because it involves both the breakdown of food and the creation of energy (ATP) and other substances.

The term “respiration” was first used to refer to animal breathing, but it was later expanded to include all metabolic events in which organic compounds such as carbohydrates, lipids, and proteins are broken down to release CO2, water, and energy in all living things.

Respiration oxidizes high-energy molecules, which are referred to as respiratory substrates. In respiration, glucose is the most readily available substrate (carbohydrate). Fats and proteins are respiratory substrates as well, but they come in second. Glucose is stored as a high molecular weight polymer in plants and animals, such as starch and glycogen. Bacteria produce both forms of polysaccharides. To meet the energy need, these molecules release glucose.

Types of Respiration

Respiration is one of the most important critical processes that occur in all living cells. There are two types of it, aerobic and anaerobic

Aerobic respiration

1. O2 is used in aerobic respiration.

2. Aerobic respiration is a series of enzyme-controlled processes that allow living organisms to use the energy accumulated in carbohydrates and lipids during photosynthesis.

3. Energy is emitted in the range of 3000 kJ mol-1.

4. The general equation for aerobic respiration is

 C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy (673 Kcal.)

Anaerobic respiration (without oxygen)

1.O2 is not required in anaerobic respiration.

2. Yeasts and certain other bacteria need anaerobic respiration.

3. The energy produced by aerobic respiration is more than that produced by anaerobic respiration.

4. In humans, anaerobic respiration occurs when muscle endures intense contraction, such as during vigorous exercise.

5. Only around 10% of the energy released in the entire oxidation of glucose is produced.

Respiratory biochemical reactions can be written as:

Anaerobic– C6H12O6 ——-2 C2H5OH + 2 CO2 + energy (28 Kcal.)

Combustion vs. respiration

Combustion (the burning of coal and other fuels) is an oxidative process, but it is not the same as respiration. Oxidation/chemical burning is used in both procedures. Combustion, like the burning of fuels and firewood, is a non-enzymatic, uncontrolled process. Adenosine triphosphate (ATP) is not made. In an uncontrolled manner, energy is released as heat and light.

Respiration is a well-regulated biochemical activity that is conducted by enzymes and is a key feature of living cells. The energy is stored as a biomolecule (ATP), which releases it as needed by the cell. The following metabolic stages are part of the respiration process:


Glycolysis is the initial step in the non-oxygenated breakdown of glucose in respiration. In this mechanism, glucose is converted to pyruvate. It’s a ten-step enzyme-mediated process in which one molecule of glucose yields two pyruvate molecules, resulting in a net gain of 2ATP and 2 molecules of decreased Co-factor (NADH) in the end. It occurs in both aerobic and anaerobic species and takes place in the cytosol. Though it is an energy-producing activity, it first consumes two ATP molecules for each glucose molecule used, but later creates four ATP molecules.

Krebs’ Cycle/Citric Acid Cycle /Tricarboxylic Acid Cycle –

Pyruvate, which is generated during glycolysis, enters mitochondria. The enzyme Pyruvate translocase catalyzes its entry. Pyruvate undergoes oxidative decarboxylation in mitochondria, yielding the crucial intermediate chemical Acetyl-CoA. Acetyl-CoA goes through a cyclic process to produce reduced cofactors including NADH+ H+, FADH2, GTP/ATP, and CO2. Krebs’ cycle is the name given to this cycle. In aerobic respiration, this cycle is the primary source of ATP-generating molecules (NADH, FADH2, and GTP). In this cycle, acetyl-CoA produced by oxidative decarboxylation of pyruvate reacts with oxaloacetate to create a variety of organic acids and other metabolically important molecules.

3. Electron Transport System – The electron transport system is a series of redox events that take place in the inner membrane of the mitochondria. This process comprises protein complexes ordered in ascending order of their Reduction potentials oxidizing reduced Cofactors NADH & FADH2 (electron & proton donor). These protein transporters carry electrons from decreased cofactors. This electron transfer proceeds through electron carriers (Cytochromes) organized in various protein complexes, reducing to O2 in the end (ultimate electron acceptor). Because ATP is produced in mitochondria during this process (oxidative phosphorylation), mitochondria are also known as the cell’s powerhouse.

ATP and cells

Energy is released when adenosine triphosphate (ATP) is hydrolyzed to adenosine diphosphate (ADP) and phosphoric acid (Pi) (it is an exergonic reaction). Some chemical reactions in cells necessitate the use of energy. This energy can be obtained by ATP hydrolysis processes. This requires energy, which can be obtained by the exergonic reaction of glucose oxidation. Energy from glucose oxidation is not used directly to drive chemical reactions in the cell for two reasons:

1. When compared to glucose oxidation, ATP hydrolysis releases a modest amount of energy in a controlled manner. 2. The hydrolysis of ATP releases energy instantly, however, the oxidation of glucose takes time

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