Gas exchange experiments

Hydrogencarbonate indicator is used to show carbon dioxide concentration in solution. The table shows the colour that the indicator turns at different levels of carbon dioxide concentration.

Concentration Indicator turns
Highest Yellow
Higher Orange
Atmospheric level Red
Low Magenta
Lowest Purple

A leaf is placed in a stoppered boiling tube containing some hydrogen carbonate indicator solution. The effect of light intensity can then be investigated.

The table shows some typical results.

Tube 1 Tube 2 Tube 3 Tube 4
Light turned on
Paper on tube Black paper Tissue paper None None
Leaf Living Living Living Dead (boiled)
Indicator colour at the end Yellow Magenta Purple Red
Carbon dioxide concentration Highest Low Lowest Atmospheric level
Respiration
Photosynthesis
  • Tube 4 was a control. The results in tubes 3 and 4 show that the leaf has to be alive for the carbon dioxide concentration to change.
  • Tubes 1, 2 and 3 show the effect of increasing the light intensity. The black paper stopped light reaching the leaf in tube 1, so only respiration could happen.
  • The tissue paper stopped some of the light reaching the leaf in tube 2, and the leaf in tube 3 received the most light.
  • Photosynthesis happened as well as respiration in tubes 2 and 3, so there was a net absorption of carbon dioxide.
  • The rate of photosynthesis was greatest in the leaf in tube 3, and it had the greatest net absorption of carbon dioxide.
Glossary
  1. aerobic respiration Respiration that requires oxygen.
  2. control A part of the experiment in which all the variables except the dependent variable are kept the same. A control lets you observe the effect (if any) of changing the independent variable.
  3. diffusion The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.
  4. flaccid Soft and floppy.
  5. hydrogen carbonate A hydrogen carbonate ion has two negative charges and consists of one hydrogen atom, one carbon atom and three oxygen atoms.
  6. osmosis The movement of water molecules across a partially-permeable membrane from a region of low solute concentration to a region of high solute concentration.
  7. photosynthesis A chemical process used by plants and algae to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis.
  8. respiration Chemical change that takes place inside living cells, which uses glucose and oxygen to produce the energy organisms need to live. Carbon dioxide is a by-product of respiration.
  9. respire To engage in respiration, the energy-producing process inside living cells.
  10. spongy mesophyll The plant tissue in a leaf which has loosely packed cells and air spaces between them to allow gas exchange.
  11. stomata Tiny holes in the epidermis (skin) of a leaf - usually on the undersides of leaves. They control water loss and gas exchange by opening and closing. Singular is stoma.
  12. turgid Having turgor; enlarged and swollen with water.
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