Respiration in Cells (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

• Respiration is a chemical process that involves the breakdown of nutrient molecules (specifically glucose) in order to release the energy stored within the bonds of these molecules
  • Respiration is enzyme-controlled
• Respiration can take place with oxygen (aerobically) or without oxygen (anaerobically).
  • Much less energy is released for each glucose molecule broken down anaerobically compared to the energy released when it is broken down aerobically
• Respiration occurs in all living cells; most of the chemical reactions in aerobic respiration take place in the mitochondria
• Humans need the energy released during respiration carry out many processes
  • Muscle contraction
  • Protein synthesis
  • Cell division (to make new cells)
  • Growth
  • Active transport across cell membranes
  • Generation of nerve impulses
  • Maintaining a constant internal body temperature

The energy released during respiration is used to fuel many processes in the human body

The Effect of Temperature on the Respiration of Yeast Cells

• There are several different experimental methods that can be used to investigate the rate of respiration in organisms
• Some methods, such as the experiment described below, involve the use of a coloured indicator
• An indicator can be used to investigate the effect of temperature on the rate of aerobic respiration in yeast
• Methylene blue dye is a suitable indicator
• This dye can be added to a suspension of living yeast cells because it doesn't damage cells
• Yeast can respire both aerobically and anaerobically, though in this experiment it is their rate of aerobic respiration that is being investigated
• The time taken for the methylene blue to discolour (lose its colour) is a measure of the rate of respiration of the yeast cells in the suspension
  • The faster the dye changes from blue to colourless, the faster the rate of respiration

Apparatus

• Yeast suspension
• Glucose solution
• Test tubes
• Stopwatch
• Methylene blue
• Temperature-controlled water bath(s)

Methylene blue is added to a solution of aerobically respiring yeast cells in a glucose suspension. The rate at which the solution turns from blue to colourless gives a measure of the rate of aerobic respiration.

Independent and dependent variables

• The independent variable is the variable that is changed on purpose
  • Here the investigation studies the effect of temperature on respiration rate in yeast, so the independent variable is temperature
  • Different temperatures are achieved using water baths
• The dependent variable is the variable that is measured, i.e. the variable that depends on the independent variable for its outcome
  • In an investigation into the effect of temperature on the rate of respiration in yeast, the rate of respiration is the dependent variable
  • The rate is measured here by recording the time taken for methylene blue dye to change from blue to colourless

Controlling other variables

• It is important when investigating the effect of one variable on another to ensure that any other variables that might influence the dependent variable are being controlled, e.g.
  • Volume/concentration of dye added: if there are more dye molecules present then the time taken for the colour change to occur may be longer
  • Volume/concentration of yeast suspension: if more yeast cells are present then more respiration will be occurring and the dye will change colour more quickly
  • Concentration of glucose: if there is limited glucose in one tube then the respiration of those yeast cells will be limited
  • pH: pH can influence enzyme activity, and enzymes are involved in the reactions of respiration, so pH can therefore influence the rate of respiration
    • A buffer solution can be used to control the pH level to ensure that no enzymes are denatured

Results

• A graph should be plotted that shows 'temperature' (x-axis) against 'time for colour change' (y-axis)
  • It is also possible to convert 'time for colour change' into a unit of reaction rate; this has been done in the graph shown below
• As the temperature increases up to 40 °C, the rate of respiration increases so the time taken for the solution to become colourless reduces
  • Raising the temperature of a solution gives the molecules in the solution more kinetic energy, so they move around more and the enzymes and substrates involved in respiration collide with each other more frequently
• As temperature increases above 40 °C, the rate of respiration decreases so the time taken for the solution to become colourless increases
  • Increasing the temperature above a certain point causes the enzymes involved in respiration to denature; the shape of their active site changes and they can no longer form enzyme-substrate complexes

The time taken for methylene blue to change colour can be converted into 'rate of respiration' and plotted on a graph. Note that a graph of temperature against 'time for colour change' will look different to the graph shown here.