Chapter 12: Respiration

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Chapter 12: Respiration

Internal versus external:

  • Internal respiration is the enzyme-controlled release of energy from food
  • External respiration is the exchange of gases with environment

Aerobic versus anaerobic:

  • Aerobic respiration is the enzyme-controlled release of energy from food using oxygen
  • Anaerobic respiration is the enzyme-controlled release of energy from food without the use of oxygen

Aerobic respiration occurs in the mitochondrion.

Structure of the mitochondrion


Aerobic respiration
Two stage process:

  1. Glycolysis
  2. Krebs cycle & Electron Transport Chain

Stage 1: Glycolysis

  • Oxygen-independent (can occur in presence or absence of oxygen)
  • Occurs in cytosol
  • Glucose (a 6-carbon molecule) is changed into 2 three-carbon molecules (pyruvate)
  • This breaking down of glucose releases high energy electrons and protons – they are captured by NAD+ to become NADH
  • Glycolysis also produces two molecules of ATP directly

Stage 2: Krebs cycle & Electron Transport Chain

  • Oxygen-dependent (can only occur in the presence of oxygen).
  • Occurs in the lumen of the mitochondrion (matrix).
  • Pyruvate enters the mitochondrion and is converted to two-carbon molecule (acetyl-coA) with release of NADH and carbon dioxide.
  • The acetyl-coA then joins with four-carbon molecule from the previous Krebs cycle to form six-carbon molecule.
  • The six-carbon molecule is then broken down into five-carbon molecule with release of carbon dioxide and NADH.
  • The five-carbon molecule is then broken down into four-carbon molecule with release of ATP, carbon dioxide, and 2NADH.
  • The four-carbon molecule goes into the next Krebs cycle.
  • The ATP goes to power metabolism (chemical reaction in cells).
  • The NADH is an energy carrier that goes to the electron transport chain where its energy is used to power the production of three ATP molecules.
  • The electron transport chain is located in the inner membranes of the mitochondrion (cristae).


Anaerobic respiration
Anaerobic respiration occurs when no oxygen or limited oxygen is present.
Two stages:

  1. Glycolysis
  2. Lactic acid fermentation/Alcohol fermentation

Mandatory Experiment: to prepare alcohol using yeast

  • Yeast
  • Glucose
  • Deionised water
  • Beakers
  • Conical flasks
  • Vegetable oil
  • Fermentation lock
  • Limewater
  • Incubator
  • Filter paper
  • Funnel
  • Potassium iodide
  • Sodium hypochlorite
  • Water bath


  • Dissolve 50 g glucose in 500 ml deionised water and bring to the boil (to exclude oxygen).
  • Separate into two conical flasks (250 ml glucose solution in each).
  • Once cooled add 5 g yeast to one of the conical flasks and label this “TEST’ – the other flask remains as the control (without yeast).
  • Slowly pour a layer of vegetable oil on top of each glucose solution (keeps the oxygen out).
  • Place a stopper in each and attach fermentation locks each containing limewater.
  • Leave in a 25˚C incubator for 2 days.

Test for alcohol (Iodoform test):

  • Filter the solutions obtained from each conical flask.
  • Place 3 ml of each in two separate test tubes.
  • Add 3 ml of a potassium iodide solution and 5 ml of a sodium hypochlorite solution to each test tube.
  • Warm the test tubes in a hot water bath and observe any changes.


  • Appearance of pale yellow crystals in the ‘TEST’.
  • No appearance of pale yellow crystals in the control.


  •  Yeast produced alcohol by anaerobic respiration in the ‘TEST’.