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Cellular Respiration

Human Biology (Year 11) - Cell Metabolism

Gemma Tueno

Biochemical Processes

Cells complete their work by undergoing chemical reactions. These reactions can be classified into two types: catabolic and anabolic. Catabolic reactions involve the breaking down of large molecules into smaller molecules. An example of catabolic reactions includes the breakdown of glucose into carbon dioxide, water, and energy in cellular respiration.  Anabolic reactions result in the production (or synthesis) of a molecule bigger than the starting molecules. For example, creating proteins involves chemical reactions that join small amino acid molecules (further discussed below). Chemical reactions that occur in the cell are controlled by specific enzymes in the cell.

Cellular Respiration

 Cellular respiration is an important metabolic process that cells undergo to produce energy for the cell’s activities. These activities can include the movement of the cell, the uptake of materials, and the production and secretion of new chemical compounds.

During cellular respiration, glucose (C6H12O6) is combined with oxygen through a series of over 20 sequential catabolic chemical reactions to produce carbon dioxide, water, and energy. Each individual chemical reaction is catalysed by its own specific enzyme and produces an intermediate molecule which is used in the next reaction. This entire process is represented in the following equation:

C6H12O6 + 6O2   6CO2 + 6H2O + energy

60% of the energy produced in cellular respiration is released as heat. The cells can’t utilise heat, but it is still essential to maintain body temperature. The remaining energy is used to form the compound adenosine triphosphate (ATP). ATP is produced by the addition of an inorganic phosphate group to an adenosine diphosphate molecule (ADP). The phosphate groups in these molecules are joined by high-energy chemical bonds.  

Imaged Sourced from:

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The second image above describes how energy is temporarily stored in ATP.  The energy stored in the bond between the second and third phosphate groups in ATP is utilised by the cell to perform tasks. When this bond is broken, ATP loses the third phosphate and becomes ADP and the energy released and immediately used in any of the following operations:

-          Building complex molecules

-          Cell division and growth

-          Movement of cell organelles

-          Movement of whole cell

-          Maintaining cell organisation

-          Active transport

-          Transmission of nerve impulses

The resulting molecule of ADP can be recycled again into another molecule of ATP when the energy is produced by cellular respiration to recreate this bond.

Aerobic v. Anaerobic Respiration


The first phase of breaking down glucose is called glycolysis and occurs in 10 steps, in the cell cytosol. The entire process of glycolysis results in 2 molecules of a substance called pyruvic acid and converts 2 molecules of ADP into ATP. Glycolysis does not require oxygen to be present and is therefore known as an anaerobic process. It is an important form of energy production when the cells do not have access to enough oxygen (such as during vigorous exercise). After glycolysis is complete, if there continues to be a lack of oxygen, the pyruvic acid molecules are converted to lactic acid. Lactic acid accumulates during the absence of oxygen and causes the pain that we feel in our muscles when exercising. Lactic acid can also be taken to the liver (in the bloodstream) and recombined with oxygen to form glucose and then glycogen (a long-term storage molecule for glucose).

When we are exercising, we cause there to be an ‘oxygen debt’ in the body, because we are using more energy than our cells can aerobically produce. Therefore, we breathe heavily after exercise to ‘repay’ this debt and to clear the lactic acid which what is called recovery oxygen.


Aerobic respiration is the complete breakdown of glucose and requires water. The pyruvic acid produced from glycolysis is broken down into carbon dioxide and water. Aerobic respiration occurs in mitochondria. Mitochondria have a folded inner membrane which means there is a large surface area for the enzymes responsible for the reactions in aerobic respiration. To complete glucose breakdown, two pyruvic acids molecules must enter the mitochondrion. They then react with enzymes in 2 series of reactions. The first of these series is known as the citric acid cycle (aka krebs cycle) and results in 2 more ATP molecules. The second is the electron transport chain which produces 34 ATP molecules (from one molecules of glucose). Overall, 38 molecules of glucose are produced between glycolysis, the citric acid cycle, and the electron transport chain.

Organic Compounds

Many of substances involved in metabolism are organic. This means they consist of large molecules and contain carbon. Such substances include carbohydrates, amino acids, proteins, lipids, and nucleic acids. Other molecules involved in metabolism include H2O, CO2 and O2 but these are inorganic because they either don’t have carbon and/or are small molecules.

Carbohydrates also contain carbon (carbo), hydrogen (hydr) and oxygen (ates) and there is always twice as many hydrogen atoms as there are oxygen. Monosaccharides are the smallest types of carbohydrates known as simple sugars or single-unit sugars (examples include glucose, fructose, and galactose). Disaccharides are carbohydrate molecules that consist of two monosaccharides joined together (examples include: sucrose, maltose, and lactose). Polysaccharides are molecules with large amounts of monosaccharides joined together (e.g., glycogen, cellulose, and starch)

Proteins always have carbon, hydrogen, oxygen, and nitrogen and can often include phosphate and sulfur as well. Proteins consist of large amounts of smaller units called amino acids that have joined together to form a chain which is then folded into a 3D shape. There are 20 different amino acids and the bond that forms between them is called a peptide bond. 2 amino acids joined together are called a dipeptide and 10 or more is called a polypeptide. Proteins generally have over 100 amino acids. All enzymes are protein molecules and hence proteins are involved in a lot of chemical reactions in the body.

Lipids contain carbon, hydrogen, and oxygen but have much less oxygen than carbohydrates. Lipid molecules consist of one molecule of glycerol and 1,2, or 3 fatty acid molecules Examples of lipids are fats stored as energy reserves (in molecules called triglycerides), phospholipids in cell membranes, steroids like cholesterol, and sex hormones.

Nutrients are substances in food, such as the ones described above, that are used for growth, repair, and metabolism. There are six groups of nutrients:

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