Metabolism is the overall process of chemical reactions involving changes in energy and enzymes in the body of living things. Based on energy requirements, the metabolic process is divided into two, namely:

  1. Catabolism, namely the energy produced when the body digests complex molecules becomes simpler.
  2. Anabolism, which is the energy required to form compounds from simple to more complex ones.


Is a process of breaking complex molecules (containing high energy) into simpler molecules (containing lower energy). The catabolism process aims to produce the energy contained in a compound. Based on the presence of oxygen, the catabolic process can be divided into two, namely respiration and fermentation. Respiration is a catabolic process that occurs in sufficient oxygen (aerobic) conditions, while fermentation is a catabolic process that occurs in conditions of no oxygen (anaerobic).

1. Aerobic Respiration

Repiration is a process that produces energy using oxygen from complex organic compounds to become simpler compounds. Aerobic respiration occurs in four stages, namely:

  1. Glycolysis is the process of breaking down glucose into pyruvic acid.
  2. The oxidative decarboxylation of pyruvic acid, namely the breakdown of pyruvic acid into acetyl Co-A.
  3. The citric acid cycle, which is a cycle that produces energy by breaking down acetyl Co-A into an electron acceptor.
  4. Electron transfer, which is a mechanism for the formation of energy and produces a byproduct in the form of water.

2. Fermentation (Anaerobic Respiration)

Fermentation is a process of glucose change that occurs in anaerobic environment, including glycolysis and formation of NAD. Fermentation can be divided into two, namely:

  1. Alcohol fermentation, is the process of breaking down pyruvic acid into acetaldehyde and then into ethanol.
  2. Lactic Acid Fermentation, is the fermentation of glucose into lactic acid.

alcoholic fermentation

Alcohol Fermentation
Image source: Campbell, NA, et al. (2009).lactic acid fermentation

Lactic Acid Fermentation
Image source: Campbell, NA, et al. (2009)

Catabolism Relationship between Carbohydrates, Proteins and Fat

Catabolism is a series of breakdown of carbohydrates that involve many stages. The first stage is glycoslysis. Glycolysis is a process that produces pyruvic acid from the breakdown of carbohydrates in cells. In the process, glycolysis is assisted by several digestive enzymes. Private acid will enter the second stage, namely the oxytadif decarboxylation stage which produces the final product in the form of Acetyl Co – A. Acetyl Co – A will then enter the krebs cycle to produce NADH and ATP. Thus the carbohydrates found in the body can be used as a source of energy because they have been broken down into simple molecules (ATP).

  • The Relationship between Carbohydrate and Protein Catabolism

Proteins contained in the body are composed of complex molecules so that they cannot be used as energy. Therefore, protein must first be digested into amino acids. With the help of enzymes, amino acids can enter the series of glycolysis processes, thereby increasing the production of pyruvic acid which is continued into the krebs cycle. Meanwhile, amino acids that do not enter the krebs cycle, will experience deamination and form NH3. NH3 that is not absorbed in the body will be excreted in the form of urine.

  • The Relationship between Carbohydrate and Fat Catabolism

Fat contains many hydrogen atoms so fat is the main energy source in the body. However, to be used as energy, fat must be broken down into simpler molecules. Fat hydrolysis is the stage to break down fat into glycerol and fatty acids and in this form new fat can be used as energy. Glycerol can enter the glycolysis series in the form of glyceraldehyde 3 phosphate (G3P) so that pyruvic acid production will increase, besides that fatty acids can also enter the Krebs cycle after experiencing beta-oxidation to become Acetyl Co-A. Thus the source of the Krebs cycle material increases and more ATP is used as energy.protein and fat catabolism relationship

The Relationship of Carbohydrate, Protein, and Fat Catabolism
Image source: Campbell, NA, et al. (2006).


Anabolism is the process of forming or arranging complex compounds from simpler compounds. It takes energy to carry out anabolism. The following are the sources of energy and how this energy is used in Anabolism:

1. Photosynthesis

Photosynthesis is an event of formation of carbohydrates from basic ingredients in the form of water and carbon dioxide. To carry out photosynthesis, energy comes from sunlight. Photosynthesis occurs in chlorophyll in plants. Chlorophyll is a color pigment that can capture energy from sunlight. Following are the reactions that occur during photosynthesis:

photosynthetic reactions

Photosynthesis takes place in 2 reaction stages, namely:

The light reaction is the stage where the chlorophyll found in chloroplasts is captured by light energy (photons). The light reaction can only occur when there is sunlight. The light reaction occurs in the thylakoid membrane. The processes in the light reactions include:

  1. Photons are absorbed by the chloroplast and converted into energy that can move electrons.
  2. The water molecule is broken down which removes electrons, hydrogen and oxygen from the molecule. This process is called water photolysis.
  3. The released hydrogen will combine to form NADP and ATP synthesis occurs.
  4. The NADPH and ATP that are formed are materials that will enter the dark reaction stage. Meanwhile, some of the oxygen is released and used for catabolism.

the light reaction of photosynthesis

Light Reaction in Photosynthesis
Image source: Campbell, NA, et al. (2006).

The dark reaction is an advanced stage of the light reaction. Dark reactions can occur without the help of energy from sunlight. The dark reaction is also known as the Calvin – Benson cycle or the Calvin cycle. The place where the dark reaction occurs is the stroma. The processes for dark reactions include:

  1. The dark reaction begins when carbon dioxide is fikasasi by RuBP (Ribulose BiPhospat) to become 6 carbon compounds.
  2. The 6-carbon compound is broken down to 2 phosphoglycerate (PGA).
  3. PGA accepts the P group from ATP and electrons from NADPH, thus converting to 12 PGAL.
  4. 2 PGAL condensed into 6 phosphate glucose, while 12 PGAL was reduced to 10 PGAL to return to the initial stage to become RuB.
  5. Then, glucose 6 phosphate is used to form the final carbohydrates of photosynthesis.

the dark reaction of photosynthesis

Dark Reaction
Image source: Campbell, NA, et al. (2006).

2. Chemosynthesis

Chemosynthesis is the synthesis of organic compounds from organic compounds through a certain chemical reaction. Organisms that can carry out chemosynthesis are chemoautotrophic bacteria (bacteria that do not have chlorophyll).

The following is an example of chemosynthesis in some living things.

  • Chemosynthesis by Nitrifying Bacteria

Nitrifying bacteria can oxidize ammonia to nitrite. Examples of nitrifying bacteria are: Nitrosomonas, Nitrosococcus, Nitrobacter, and Bactoderma.

chemosynthesis by nitrifying bacteria
  • Chemosynthesis by Sulfur Bacteria

Metal sulfide can be oxidized to sulfur by sulfur bacteria. In addition to metal sulfides, these bacteria can also use sulfur deposits to be oxidized and used as energy. Examples of sulfur bacteria, namely Beggiatoa and Thiospirillum.

chemosynthesis by sulfur bacteria
  • Chemosynthesis by Iron Bacteria

The ability of this banteri is to oxidize ferrous ions into ferric ions. Examples of iron bacteria are Leptothrix, Crenothrix, Cladothrix etc.

2Fe + H2O + O  longrightarrow

2Fe (OH)3 + 4CO2 + Energy

4FeCO3 + O2 + 6H2O  longrightarrow

4Fe (OH)3 + 4CO2 + Energy

  • Chemosynthesis of Hydrogen Bacteria

Bacillus panctotrophus is an example of a hydrogen bacterium. Free hydrogen can be oxidized by these bacteria as an energy source.

  • Chemosynthesis of Methane Bacteria

Methanonas is an example of methane bacteria. The energy source for these bacteria is methane. Methane can be oxidized to carbon dioxide and can be used as an energy source.


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