Decomposition & Decay

Decomposition & Decay

• Temperature, water and availability of oxygen affect the rate of decay of biological material
• Decomposition (also known as decay or rotting) is the breaking down and digestion of biological material (waste products and dead organisms) by organisms called decomposers
• Decomposers include microorganisms (bacteria and fungi) and detritus feeders
• Decomposition is a crucial process as it ensures that materials such as carbon and mineral ions are recycled and returned to the environment
  • Remember, there is a finite (fixed) number of elements on Earth, new atoms cannot be created out of nothing!
• The rate of decay is the speed that decomposers break down biological material and is affected by three key factors:

Factors Affecting Rate of Decay Table

Uses of decomposition

• Compost can be used as a natural fertiliser
• Gardeners and farmers try to provide optimum conditions (warmth, moisture and an oxygen supply) for rapid decay of waste biological material (eg. waste plant matter)
• The compost produced is used as a natural fertiliser for growing garden plants or crops
• Once the compost is spread onto the soil, it is broken down further by decomposing microorganisms (bacteria and fungi) and detritivores (eg. earthworms and woodlice)
• This ensures the recycling of minerals (such as magnesium and nitrates) that can then be absorbed by plants to be used for growth (magnesium is used to make chlorophyll, nitrates to make amino acids)

Slowing down decomposition

• Food can be preserved by storing it in conditions which do not allow decomposition e.g. low temperature, no oxygen or no water
  • At low temperatures, such as in a fridge, activity and reproduction of microorganisms and the chemical reactions involved in decomposition occur very slowly
    • In a freezer, this activity is stopped completely
  • The sterile conditions used to create tinned food and the way that the food is sealed, prevents access for microorganisms
  • Vacuum packed food has all the air removed which means that microorganisms can not respire and therefore cannot grow or reproduce
  • Dried foods lack the water required by microorganisms for survival and reproduction
    • Similarly storing food in brine or sugar solutions causes water to move out of microorganisms by osmosis so they do not have the water that they require

Calculating the rate of Decay

• When studying rates of decay, you should be able to:
  • Calculate rate changes in the decay of biological material
  • Translate information between numerical and graphical form
  • Plot and draw appropriate graphs selecting appropriate scales for the axes

Worked Example

A student is investigating rate changes in the decay of milk. They incubated milk at three different temperatures and recorded the pH every 24 hours. Their results table is shown below.

Calculate the rate of change in pH of the milk in each 24 hour period for each of the three temperatures.

Draw a graph of the results.