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  • Process by which food substances are chemically broken down in living cells to release energy, carbon (iv) oxide, water or alcohol.
  • Respiration takes place mainly in the mitochondria. It has two membranes, inner and outer.
  • Inner membrane is folded into projections called cristae. Cristae provide a large surface area for respiratory enzymes. Respiratory enzymes are bound to the cristae.


Practical Activity 1

To investigate the gas given off when food is burnt.

Types of Respiration

  • Aerobic Respiration
  • Anaerobic Respiration.

Aerobic Respiration

  • Process by which food substances such as glucose are broken down in the presence of oxygen to release energy, water and carbon (IV) oxide.
  • The energy is stored in the form of a chemical substance called Adenosine Triphosphate (ATP).
Respiratory Enzymes  

This energy is released in small quantities since a lot of heat energy would burn the body cells.

C6H12O6 + 6O2                                    6CO2       +      6H2O       +     Energy (ATP)

  • Respiration takes place in two phases with each phase consisting of series of reactions.

First Phase (Glycolysis)

  • This takes place in the cell cytoplasm. Oxygen is not required in this stage.
  • Glucose is broken down into a 3 carbon compound called Pyruvic acid through a process called glycolysis.
  • In glycolysis one molecule of glucose gives 2 molecules of ATP.
  • In absence of oxygen Pyruvic acid is broken down into lactic acid in animals and into alcohol (ethanol) in plants.

Second phase (Krebs Cycle)

  • This takes place in the matrix of the mitochondria and involves a series of enzyme controlled reactions that require oxygen.
  • Pyruvic acid formed in the first phase is oxidized by oxygen in a series of enzymatic reactions (Krebs cycle) into energy, water and carbon (IV) oxide.
  • In this phase one molecule of glucose gives 38 molecules of ATP.
  • The following conditions are required in this phase;
  • Cells must be provided with glucose/food.
  • Oxygen must be present.
  • Respiratory enzymes must be present to catalyse the reaction.
  • Temperature must be favourable for efficient functioning of enzymes.
  • End products of the reaction (energy, water and carbon (iv) oxide) must be constantly removed from the mitochondrion.

Practical Activity 2

To investigate heat production in germinating seeds.

Anaerobic Respiration in Plants and Animals

  • This is the process by which food substances are broken down in the absence of oxygen to release energy.
  • The glucose is not completely broken down hence less energy is given out.
  • In plants glucose is broken down into energy, carbon (iv) oxide and ethanol (alcohol).

Glucose                       Ethanol   +      Energy.   +      Carbon (iv) oxide

(C6H12O6)           (2C2H5OH)            (ATP)                             (CO2)

  • Anaerobic respiration in plants is also referred to as fermentation.
  • In animals glucose is broken down into energy and lactic acid

Glucose                       Lactic acid     +      Energy.

(C6H12O6)           (2C3H6O3)              (ATP)

Oxygen Debt

  • This is oxygen required to get rid of the lactic acid that accumulates in the body tissues when the oxygen supply is less than required.
  • Accumulation of lactic acid causes fatigue and muscle crumps.
  • Oxygen debt is paid back by breathing more quickly and more deeply in order to increase oxygen supply such as during recovery period after a race when a person pants.
  • When paying back the oxygen debt, lactic acid is oxidized to energy, water and carbon (iv) oxide or it is taken to the liver and converted into glycogen.

Application of Anaerobic Respiration

  1. Baking industry
  2. Beer brewing and distillery industry.
  3. Dairy industry in the production of yoghurt and cheese.
  4. Production of vinegar citric acid, oxalic acid, butyric acid and some drugs.
  5. Production of power alcohol which is used as a substitute for petrol.
  6. Silage making.
  7. Biogas production.
  8. Making compost manure

Practical Activity 3

To investigate gas produced during fermentation.

Comparison between Aerobic and Anaerobic Respiration

Aerobic RespirationAnaerobic Respiration.
Oxygen is requiredOxygen not required
High amount of energy is released as one molecule of glucose yields 38 ATP molecules (2880 KJ)Low amount of energy is released as one molecule of glucose yields 2 ATP molecules (210 KJ)
There is complete breakdown of the substrate into carbon (iv) oxide and water.There is incomplete breakdown of substrate hence lactic acid or alcohols are produced.
End products are energy, water and carbon (iv) oxideEnd products are energy, alcohol in plants and lactic acid in animals.
Water molecules are produced.Water molecules are not produced.
Over a short period of time, energy is not released fasterOver short period of time, energy is released faster.
Occurs in the cytoplasm and in the mitochondrion.Occurs only in the cell cytoplasm.

Respiratory Substrates

  • These are energy rich foods which when oxidized release energy. They include;
  • Carbohydrates
  • They are the main source of energy mainly in the form of simple sugars such as glucose, fructose and galactose.
  • They produce about 17KJ (2898/mole) per gram when completely oxidized.
  • Fats
  • They produce more energy than carbohydrates or proteins. One gram of fats yields about 38 KJ of energy when completely oxidized.
  • They are however not the main substrate because they are not very soluble in water hence not easily transported to the sites of respiration. It also requires more oxygen to oxidize one gram of fats than one gram of glucose.
  • Proteins
  • They are not normally used in respiration unless in cases of extreme starvation.
  • One gram of proteins yields 22KJ of energy when completely oxidized.


  • Where do plants and animals get the following from;
  • Carbohydrates.
  • Fats
  • Proteins

Respiratory Quotient (RQ) and its Significance

  • RQ is the ratio showing the relationship between the amounts of carbon (iv) oxide used against the amount of oxygen used in respiration.

RQ               =     

  • RQ varies with the type of substrate being oxidized. For example carbohydrates have a RQ of 1.0 when fully oxidized, fats have 0.7 and proteins have 0.9.
  • RQ can therefore be used to indicate the type of substrate being oxidized and also whether aerobic respiration or anaerobic respiration is taking place.
  • RQ is also affected by factors such as age, health of the organism and the temperature.

Factors Affecting the Rate of Respiration

  1. Oxygen concentration. When the amount of oxygen increases, the respiration rate also increases. Decrease in oxygen concentration will lead to decreased respiration rate.
  2. Substrate concentration. Increase in sugar concentration increases respiration and vice versa.
  3. Hormones. Presence of some hormones such as adrenaline and thyroxine in the body increases the rate of respiration.
  4. Surface area to volume ratio (Body size). If the SA/volume ratio is high, the organism would lose more heat energy. As more heat is lost to the surrounding more is required to replace the lost energy hence more respiration.
  5. Age. Young people require more energy because their cells are actively dividing hence respiration rate is higher in them than in older people.
  6. Occupation. People engaged in heavier tasks have higher rate of respiration.
  7. Sex. Generally male’s have faster respiration rate than females due to presence of more muscles in their bodies.
  8. Basal metabolic rate. This is the energy required to maintain normal body functions such breathing, heartbeat, blood circulation etc while at rest.
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