Cell Respiration: Skills (SL IB Biology)

Variables affecting the rate of cell respiration

• The rate of cell respiration may vary depending on the following:
  • How metabolically active the cell is
    • e.g. muscle cells will have a higher rate of cell respiration than adipose cells because of their higher energy needs
  • Size of the organism
    • e.g. smaller organisms will have a higher surface area : volume ratio than larger organisms, so they will have a higher rate of respiration to compensate for higher heat loss
  • The oxygen supply
    • When oxygen availability is low, cells will respire anaerobically
  • Supply of respiratory substrates
    • e.g. glucose availability is of particular importance, since it is the main respiratory substrate
    • The lower the supply of these substrates, the lower the rate of respiration will be
  • Temperature
    • The rate of respiration will increase up to the optimum temperature of the enzymes catalysing the reactions, whereafter the rate will drop as the enzymes denature 
  • pH
    • Carbon dioxide released during respiration will decrease the pH of cells and tissues, which may also denature enzymes involved with respiration

Determining the rate of respiration

• Respirometers are used to measure and investigate the rate of oxygen consumption during respiration in organisms
• The experiments usually require live organisms such as seeds or invertebrates
  • Use of animals should be minimised when seeds can provide excellent data
• There are many different designs of respirometers, though they all have certain features in common
  • A sealed container containing live organisms and air
  • An alkaline solution (e.g. potassium hydroxide) to absorb CO₂
  • A capillary tube connected to the container and set against a graduated scale (a manometer)
• The organisms respire aerobically and absorb oxygen from the air
• The CO₂ they release is absorbed by the alkali
• This reduces the air pressure inside the sealed chamber
• The manometer fluid (shown in red below) moves towards the organisms because of the pressure drop inside the chamber
• The respirometer must be kept in very temperature-controlled conditions because slight fluctuations in temperature can affect the air pressure
  • A thermostatically controlled water bath is the best way to maintain a constant temperature
• Repeat readings should be carried out for each set of experimental conditions, in order to identify and eliminate anomalies
  • Repeat readings give a reliable mean

Analysis

• • Respirometers can be used in experiments to investigate how different factors affect the rate of respiration of organisms over time
    • E.g. temperature – using a series of water baths

Use of technology to measure rate of respiration

• Technological devices can automate and make the measurement of respiration rate easier
  • Not to be confused with breathing rate
• Oxygen sensors and CO₂ monitors can measure oxygen and CO₂ concentration in real-time
  • Without the need to expose the subject to hazards such as strong alkalis
• Dataloggers can record data over a period of time for analysis later

Respirometer Diagram

The typical set-up of a respirometer

The equation for calculating a change in gas volume

• The volume of oxygen consumed (mm³ min^-1) during respiration can be worked out using the radius of the lumen of the capillary tube r (mm) and the distance moved by the manometer fluid h (mm) in a minute using the formula:

πr²h

• The volume of oxygen consumed can then be used to determine the average rate of respiration per unit time