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What is Metabolism?

Metabolism is the sum of all metabolic pathways within a cell, tissue, or organism.The sum of all metabolic pathways within a cell, tissue, or organism.

Type of Metabolic Pathway

There are three (3) main types of pathways that reactions can occur through in the body:

1) Linear

2) Branch point

3) Cyclic

Types of Metabolism

Of the 3 types of pathways they fall into two (2) categories of metabolic pathways

I. Catabolism

Oxidizing pathways that generate energy.

Large molecules are broken and the energy stored as chemical energy within the bonds are converted into ATP or transferred into other small molecule shuttles such as NAD, FAD, and NADPH.

II. Anabolism

Biosynthetic pathways where small molecules come together to form large macromolecules.

The ATP that was generated during catabolic stages is then used generate the various molecules that are needed for the survival of cell.

"Catabolic and Anabolic pathways are linked with each other so that energy or molecules gained from catabolic reactions can be used for anabolic reactions"

Catabolic and Anabolic pathways are not the reverse of each other.

This would lead to futile cycle where in the end only energy is spent.

To avoid these futile cycle each metabolic pathways contains a committed step.

A committed step makes the pathway unidirectional.

The anabolic and catabolic pathways are usually quite different, which allows for them to be regulated separately.

Catabolic and anabolic pathways are regulated such that they usually take place in different physical environments.

Energy Source for organisms:

Light (energy from photons) => phototroph

Molecules (inorganic or organic) => chemotroph

Carbon sources for organisms:

Inorganic (CO2 or Bicarbonate) => autotroph

Organic (carbohydrates, lipids, etc.) => heterotroph

Basic Steps of Metabolism

1) Macromolecules are broken down into their constituent parts

2) Constituent parts are oxidized to acetyl CoA, pyruvate or other metabolites forming some ATP and reduced coenzymes (NADH and FADH2) in a process that doesn’t directly utilize oxygen

3) If oxygen is available and the cell is capable of using oxygen, these metabolites go into the citric acid cycle and oxidative phosphorylation to form large amounts of energy.

"Oxygen is the final electron acceptor of the electron transport chain because O2 has a high electron affinity for electrons"

The second and third stages, the energy acquiring stages, are called respiration.

If oxygen is used, the respiration is aerobic. If oxygen isn’t used, the respiration is anaerobic.

Aerobic organisms: Glycolysis is the first step and molecule generates most of the energy from the oxidation of organic oxygen to molecular oxygen.

Anaerobic organism: They only do glycolysis and fermentation

Obligatory anaerobes: can only live in enviroments without oxygen and perform only anaerobic respiration

Facultative anaerobes can use both aerobic and anaerobic respiration to survive

Substrate-level phosphorylation is a type of chemical reaction that results in the formation of adenosine triphosphate (ATP) by the direct transfer of a phosphate group to adenosine diphosphate (ADP) from a reactive intermediate.

In cells, it occurs in the cytoplasm (in glycolysis) under both aerobic and anaerobic conditions. Unlike oxidative phosphorylation, here the oxidation & phosphorylation is not coupled.

Metabolism Citations

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