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Metabolism Is All the Reactions in the Body

Definition

Metabolism

The sum of all the chemical reactions that take place in a cell or in the body.

  1. Metabolism is the sum of all the chemical reactions in a cell or in the body.
  2. These reactions are controlled by enzymes.
  3. The energy transferred by respiration is used to drive them.

A diagram of metabolism showing how chemical energy from food (carbohydrates, fats, and proteins) is converted into ATP and heat, with carbon dioxide and water released as waste products.

  1. Metabolism includes reactions that build larger molecules and reactions that break molecules down.

A diagram titled Metabolism, showing anabolic and catabolic reactions. An anabolic reaction uses energy to join smaller molecules into a larger molecule. A catabolic reaction breaks a larger molecule down into smaller molecules, releasing energy.

Key Idea

Respiration supplies the energy that powers all the reactions of metabolism.

Building Larger Molecules

Definition

Glycogen

A carbohydrate store in the muscles and liver that can be broken down quickly to glucose for respiration.

  1. Glucose is joined together to make larger carbohydrates such as starch, glycogen and cellulose.

Molecular structures of the monosaccharides glucose, fructose, and galactose, which are the simple sugar building blocks of carbohydrates.

A chemical diagram showing a condensation reaction (dehydration synthesis) between two monosaccharides (glucose and fructose) to form a disaccharide (sucrose). A water molecule (H2O) is released as a glycosidic bond forms between the two sugar rings.

Structural diagrams of three polysaccharides: starch (consisting of amylose and amylopectin), glycogen (highly branched), and cellulose (parallel chains forming a lattice).

  1. A lipid molecule is made from one molecule of glycerol and three molecules of fatty acids.

A chemical diagram of a triglyceride molecule, showing one glycerol molecule bonded to three fatty acid chains.

  1. Glucose and nitrate ions are used to make amino acids, which are joined together to make proteins.
Note

Making bigger molecules from smaller ones is called synthesis, and it needs energy from respiration.

A diagram showing a condensation reaction where two chains of repeating units are joined together by a covalent bond, with the release of a water molecule (H2O). This process is also known as dehydration synthesis.

Breaking Molecules Down

  1. Respiration breaks glucose down to transfer the energy the cell needs.
  2. Excess proteins that the body cannot store are broken down in the liver.
  3. This produces urea, which is carried to the kidneys and removed from the body in the urine.
Example

Eating far more protein than you need means the extra is broken down and lost as urea.

The Building Blocks of Metabolism

  1. Sugars, such as glucose:
    1. Used to build carbohydrates, and broken down in respiration to release energy.
  2. Amino acids:
    1. Made from glucose and nitrate ions, and joined together to build proteins.
  3. Fatty acids and glycerol:
    1. Joined together to make lipids, and released again when lipids are broken down.
Self review
  • What is metabolism?
  • Name three large carbohydrates made by joining glucose molecules.
  • What is a lipid molecule made from?
  • How are amino acids made, and what are they used for?
  • What does the liver make when it breaks down excess protein, and how is it removed?
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4.2.3 Metabolism Revision Guide

  1. GCSE
  2. /Biology
  3. /4.2.3 Metabolism