Sign up to access the complete lesson and track your progress!
Unlock This CourseDead plants, animals and waste are broken down by decomposers, which are mainly bacteria and fungi. The speed at which this happens is the rate of decay. Decomposers are living things, so decay goes fastest when they have the conditions they need. Three things matter most: temperature, water and oxygen.
Key terms:
Warmth speeds up decay because the decomposers' enzymes work faster and the microorganisms grow and reproduce faster. In the cold, decay is slow. If it gets too hot, the enzymes are denatured and decay slows or stops.
Decomposers need water to live and to carry out their reactions. Damp material decays quickly. Very dry material decays slowly, which is why dried food lasts for a long time.
Most decomposers use oxygen for aerobic respiration, which releases plenty of energy, so they grow fast. With a good supply of oxygen, decay is quicker. Without it, decay is slower.
In every case there is an optimum. Warm, moist and well supplied with oxygen is best. Too cold, too dry, or too little oxygen all slow decay down.
Everyday examples are easy to spot. Food goes off more slowly in a fridge because it is cold. A fallen leaf in a damp, warm garden disappears faster than one on a dry, cold path. Waterlogged soil has very little air in it, so decay there is slow.
Decay with oxygen is aerobic decay. It is quick and mainly releases carbon dioxide. When oxygen runs out, other microorganisms carry on breaking the material down without it. This is anaerobic decay. It is slower, and it produces methane gas.
Key terms:
You find a rate of decay by looking at how much something changes in a set time:
rate = change in mass (or other measurement) ÷ time taken
A 200 g pile of leaves has a mass of 140 g after 6 weeks.
Change in mass = 200 − 140 = 60 g
Rate of decay = 60 ÷ 6 = 10 g per week
To compare conditions, work out the rate for each one. A rate that has gone up means decay is faster.
Two identical piles of grass lose mass over 10 days. At 10°C the pile loses 4 g. At 25°C it loses 12 g.
Rate at 10°C = 4 ÷ 10 = 0.4 g per day
Rate at 25°C = 12 ÷ 10 = 1.2 g per day
Decay is three times faster at 25°C.
You may need to turn a table of numbers into a graph, or read numbers back off a graph. To plot a good graph:
To go the other way, read across from a point on the line to the axis and write down the value. Remember to include the units.
Gardeners and farmers try to give decomposers the best conditions so that waste plant material decays quickly. They keep the heap warm and damp and turn it to let air in. The compost made is used as a natural fertiliser for garden plants and crops.
A biogas generator is a sealed container where waste, such as manure or plant waste, decays without oxygen. The methane that is made is collected and used as a fuel.
Key terms:
Saying that warmer is always faster. Warm conditions speed decay up, but when it gets so hot that the decomposers' enzymes are denatured, decay slows or stops. Also, do not say that decomposers need oxygen to make methane: methane comes from decay without oxygen.
A gardener keeps a compost heap in a sealed plastic bin in a cold shed. The waste in the bin decays very slowly.
(a) Give two reasons why the waste decays slowly. [2 marks]
(b) Suggest how the gardener could make the waste decay faster. [2 marks]
(c) A 500 g sample of waste has a mass of 380 g after 12 days. Calculate the rate of decay in g per day. [2 marks]
(a) It is cold, so enzymes work slowly and the microorganisms grow slowly (1). The bin is sealed, so there is little oxygen (1).
(b) Move the heap somewhere warmer (1). Open the bin or turn the heap to let air in (1). Keeping it damp is also accepted.
(c) Change in mass = 500 − 380 = 120 g (1). Rate = 120 ÷ 12 = 10 g per day (1).
In part (c), write the unit with your answer. Marks are often lost for a correct number with no "g per day".