Cellular respiration: Definition, steps, and importance


The essential metabolic process through which our cells turn nutrients, notably glucose, into usable energy in the form of adenosine triphosphate (ATP), is known as cellular respiration. All living things need this process to survive and function properly.

Steps in cellular respiration:


Glycolysis, which happens in the cell’s cytoplasm, kicks off the process of cellular respiration. A molecule of glucose is split into two molecules of pyruvate during the process of glycolysis. A small amount of ATP and NADH, which transport highly energetic electrons, are produced by this process.

Phase 1: Energy Investment:

 The first half of glycolysis involves an energy investment. Here, glucose is activated by two molecules of ATP and divided into two molecules with three carbons each, known as glyceraldehyde-3-phosphate (G3P).

 Phase 2: Energy Payoff

The energy payback phase is the second part of glycolysis. When G3P molecules are transformed into pyruvate, high-energy electrons are released. In the process, two molecules of NADH (nicotinamide adenine dinucleotide) and four molecules of ATP are produced.

The Importance of Glycolysis

It acts as the body’s main source of energy, particularly when there is a shortage of oxygen. Cells can swiftly create a modest quantity of ATP by glycolysis under anaerobic conditions, such as those present during vigorous exercise.

Additional Products of Glycolysis Besides ATP

In addition to ATP and NADH, glycolysis yields the following important byproducts:

Pyruvate: Pyruvate, the glycolysis byproduct, is not only required for the Krebs cycle’s subsequent energy production but also acts as the precursor to fermentation in the absence of oxygen.

ATP: Glycolysis initially uses up two ATP molecules during the energy investment phase, but ultimately produces a net gain of two ATP molecules. Then, during the energy payoff phase, four ATP molecules are produced, yielding a net gain of two ATP molecules for every glucose molecule.

The Krebs Cycle work:

The Krebs cycle is a chain of metabolic processes that occur in a cell’s mitochondria. Its main objective is to release high-energy electrons from carbon-based molecules, producing ATP and decreasing coenzymes like NADH and FADH2 in the process.Pyruvate molecules exit glycolysis and enter the mitochondria, where the Krebs cycle (the citric acid cycle) occurs. Here, pyruvate is further broken down, producing additional ATP, NADH, and FADH2 and releasing carbon dioxide.

The Krebs Cycle Steps :

The Krebs cycle involves several sequential steps that work together to extract energy:

  1. Entry of Pyruvate

Each molecule of pyruvate produced by glycolysis passes through a series of processes to change it into acetyl-CoA before the Krebs cycle can start. The Krebs cycle then receives this acetyl-CoA.

2. Oxaloacetate and Acetyl-CoA

Oxaloacetate, a four-carbon molecule, and acetyl-CoA combine to form citrate, a six-carbon substance. The cycle starts with this phase.This step initiates the cycle. 

3. Energy Harvesting

 As citrate undergoes a series of reactions, it releases carbon dioxide and transfers electrons to NADH and FADH2, which shuttle these electrons to the electron transport chain for ATP production. 

4. Regeneration of Oxaloacetate

Oxaloacetate regenerates at the end of the cycle, allowing the next one to start. The Krebs cycle must continue after this stage in order to function properly.

The Krebs Cycle’s Relevance

An essential link between glycolysis and the electron transport chain is the Krebs cycle. It produces energy-dense molecules like ATP and NADH as well as the high-energy electrons required by the electron transport chain to fuel the generation of even more ATP.

Beyond ATP: Other Products of the Krebs Cycle 

Aside from ATP and NADH, the Krebs cycle produces other significant molecules: –

FADH2: This molecule, similar to NADH, carries high-energy electrons to the electron transport chain.

Carbon Dioxide: Carbon dioxide, a waste product of the cycle, is released into the bloodstream and eventually expelled from the body through respiration. 

  The Krebs cycle is carefully regulated by the availability of substrates and the activity of enzymes. Feedback mechanisms ensure that the cycle operates at a rate that meets the cell’s energy demands.

Electron Transport Chain: Powerhouse of ATP Production

A series of protein complexes embedded in the inner mitochondrial membrane transfer the high-energy electrons carried by NADH and FADH2. This transfer sets up a flow of protons across the membrane, creating a proton gradient. The flow of protons back through the membrane via ATP synthase generates a surge of ATP, the primary energy currency of the cell.

The Chemical Equation of Cellular Respiration

We can represent the overall chemical equation for cellular respiration as:

 Glucose + Oxygen → Carbon Dioxide + Water + ATP

In this equation, the reactants are glucose and oxygen, and the products are carbon dioxide, water, and ATP. The main result of cellular respiration, the transformation of glucose and oxygen into carbon dioxide, water, and energy in the form of ATP, is summarized by this equation.


Cellular respiration is a prime example of how complicated life is at the cellular level. Our cells use the energy from nutrition to power a variety of biological processes through a series of carefully planned stages. Each step, from glycolysis to the Krebs cycle and the electron transport chain, helps produce ATP effectively, supporting life as we know it.

Frequently asked questions:

  1. What is cellular respiration?

Cellular respiration is the process by which cells convert nutrients into energy, particularly ATP, through a series of biochemical reactions.

2. What are the main steps of cellular respiration?

The main steps include glycolysis, the Krebs cycle (citric acid cycle), and the electron transport chain.

3. What is the role of ATP in cellular respiration?

ATP (adenosine triphosphate) is the primary energy currency of the cell, produced during cellular respiration and used to power various cellular processes.

 Why is cellular respiration important?

 Cellular respiration provides the energy necessary for cells to perform essential functions, including growth, repair, and reproduction. 

5. How does cellular respiration differ from photosynthesis?

 Cellular respiration involves the breakdown of glucose to produce energy, while photosynthesis converts light energy into glucose

6. What is the Krebs cycle?

The Krebs cycle, also known as the citric acid cycle, is a series of reactions that extract energy from carbon compounds, generating ATP and reducing coenzymes.

7. Where does the Krebs cycle take place?

The Krebs cycle occurs within the mitochondria of cells.


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