• 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 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

Uses of energy in the human body

• Humans need this energy to do the following things:
  • Contract muscle
  • Synthesise proteins
  • Cell division (to make new cells)
  • Grow
  • Enable active transport to take place
  • Allow nerve impulses to be generated
  • Maintain a constant internal body temperature

   

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 involve the use of a coloured indicator such as the experiment described below
• 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 proportional to the rate of respiration of the yeast cells in the suspension.

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 the rate of aerobic respiration.

Controlling other variables

• It is important when investigating one variable to ensure that the other variables in the experiment are being controlled
  • Volume of dye added: if there is more dye molecules present then the time taken for the colour change to occur will be longer
  • Volume of yeast suspension: when more yeast cells are present the rate of respiration will be inflated
  • Concentration of glucose: if there is limited glucose in one tube then the respiration of those yeast cells will be limited
  • Temperature: an increase or decrease in temperature can affect the rate of respiration due to energy demands and kinetic energy changes. The temperature of the dye being added also needs to be considered
  • pH: a buffer solution can be used to control the pH level to ensure that no enzymes are denatured

   

Results

• A graph should be plotted of temperature (x-axis) against time (y-axis)
• As the temperature increases, the rate of photosynthesis also increases so the time taken for the solution to go colourless reduces
  • This means hydrogens are released by the reactions more quickly, hence the DCPIP accepts electrons/hydrogens more quickly until all molecules of DCPIP are reduced. This means that it will take less time to turn from blue to colourless

   

Graph showing the effect of temperature on the rate of respiration