Cell Respiration

• 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
• Avoid the common misconception that respiration is breathing!

  • Respiration is a series of chemical reactions that release energy from glucose inside cells.
  • Be careful that you always state that energy is released, it is NEVER made, produce, or created
  • The respiration reactions are all controlled by enzymes. You need to be able to state this in an exam!
• 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
• Yeast test 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
• Methylene blue experiment

  A) methylene blue / DCPIP

  B) suspension

  C) yeast cells

  D) glucose

  E) blue

  F) colourless

  
• 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
• 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 (1)

  • 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
• 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 (2)

  • 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
• 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
• Temperature and the rate of respiration in yeast graph