Cellular respiratin
Cellular respiratin

Cellular Respiration: Definition, and 4 Steps

The discussion today is we will review about cellular respiration. Before getting into the discussion of cellular respiration, of course, you have to know first about what respiration is.

As we know that respiration in biological sciences is the process of energy mobilization carried out the body of life through the breakdown of high-energy compounds used to perform a life function.

Why do we need to eat? Hungry? Not necessarily. In biological sciences, the reason we eat is to get energy. The foods we eat will be broken down, glucose extracted, and energy converted into ATP. Why should humans breathe? Of course, if not breathing will not survive. Why should humans need air to live? We need oxygen to do cell respiration to gain energy from the food. 

Cellular respiration definition

Cellular respiration is the process of breaking down complex organic molecules that are rich in potential energy into a lower energy waste product (catabolic process) at the cellular level.

In cell respiration, oxygen is involved as a reactant along with organic fuels and will produce water, carbon dioxide, as well as ATP’s main energy products.

ATP (adenosine triphosphate) has an energy for cell activity such as the synthesis of biomolecules from smaller beginner molecules, conducting mechanical work such as in muscle contraction, and transporting biomolecules or ions through the membrane towards a higher concentrate area.

Broadly, cell respiration involves processes called glycolysis, the Krebs cycle or the citric acid cycle, and the electron transport chain. 

Cellular respiration steps

Cellular respiration steps
Cellular respiration steps (Iluustration/RegisFrey / CC BY-SA)

Cellular respiration is divided into four main steps:

  • Glycolysis
  • Krebs cycle (Citric acid cycle or Trikarbolsilat acid cycle). 
  • Electron Transport Chain
  • Chemiosmosis

The reaction of glycolysis occurs in the cytoplasm of the cell, while the Krebs cycle, the electron transport chain, and the chemiosmosis occur inside the mitochondria.

More precisely, the Krebs cycle occurs in the mitochondrial matrix, and the electron transport chain as well as chemiosmosis occurs in the membrane within the mitochondria.

From this cellular respiration reaction of any glucose molecule will be able to produce 30-32 molecules of ATP.


Glycolysis takes place in the cytosol, a glucose molecular breakdown process that has 6 atoms of C into two molecules of pyruvate acid that have 3 atoms C. The reaction taking place on the cytosol results in 2 NADH and 2 ATP.

Krebs cycle (Citric acid cycle or Trikarbolsilat acid cycle).

Like other living creatures, cells need to metabolize to produce energy, one of which is through respiration. Cell respiration can be aerobic, meaning it involves the perfect breakdown of the substrate in the presence of oxygen. Aerobic respiration takes place in the mitochondria cell and produces more energy. One of the steps in aerobic respiration is the Krebs cycle.

The Krebs cycle was discovered by a German physician and biologist, Hans Adolf Krebs.

The Krebs cycle is a series of chemical reactions that occur in living cells to produce energy from acetyl co-A, i.e. A change of pyruvate acid results in glycolysis. The stages of aerobic respiration itself begins from glycolysis, oxidative decarboxylation, the cycle of Krebs, and electron transfer.

Electron transport chain

The electron transport chain takes place on the mitochondrial crest. The principle of this reaction is: each H ion transfer (electron) that is removed from the first two steps between the resulting acceptor energy used for ATP formation.

Each one of the NADH molecules that are oxidized into NAD will release the energy used for the formation of three ATP molecules. While FADH oxidation becomes FAD, loose energy can only be used to form 2 ATP. Thus, a mole of glucose experiencing the respiration process resulted in a total of 38 ATP.

Read also: What Is Hypoxia? Here are The Types, Symptoms, and Treatment


Through the process of cellular respiration, the energy in the food is converted into energy that can be used by the cells of the body. During cell respiration, glucose and oxygen are converted into ATP, carbon dioxide, and water.

Cellular Respiration: Definition, and 4 Steps

Post in | Last updated: May 18th, 2020 |