#143 Nutrient cycles - Carbon and water cycles

Most of the chemicals that make up living tissue contain carbon. When organisms die the carbon is recycled so that it can be used by future generations. Four main processes are involved: photosynthesis, respiration, decomposition and combustion. 

A. Carbon cycle

Carbon cycle

1. Carbon enters the atmosphere as CO₂ from respiration and combustion.

Credit: BBC Bitesize

2. CO₂ is absorbed by producers to make carbohydrates in photosynthesis.

3. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as CO₂ formed during respiration. The animals and plants eventually die.

4. The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as CO₂. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

Common misconception

Plants do not start respiring when they stop photosynthesizing (at night) – they respire all the time, but during the day there is usually a net intake of CO₂ ant out put of O_2. 

B. Water cycle

Water cycle  (credit: BBC Bitesize)

• Energy from the Sun heats the surface of the Earth.
• Plants release water vapour into the air through transpiration.
• Water evaporates from oceans, rivers, lakes and soil.  
• The warm, moist air rises because it is less dense. 
• Water vapour condenses into water droplets as it cools down, forming clouds. 
• Water droplets get bigger and heavier they begin to fall as rain, snow and sleet (precipitation), draining into streams, rivers, lakes and sea. 
• Plant root take up water by osmosis. 
• In addition, animals lose water to the environment through exhaling and sweating, and in urine and faeces. 

Try this

Figure below shows a diagram of the carbon cycle.

a) Copy and complete the cycle by filling in boxes A and B.             [2 marks]

b) On your diagram, label with the letter indicated an arrow that represents the process of: 

i) combustion – C                                                                           [1 mark]

ii) decomposition – D                                                                      [1 mark]

iii) photosynthesis – P                                                                     [1 mark]

iv) respiration – R                                                                           [1 mark] 

Answer

a) A, carbon compounds in plants.

    B, carbon compounds in dead plants and animals.

b) i) C on arrow between fossil duels and CO₂ in the air.

   ii) D on arrow between box B and CO₂ in the air.

   iii) P on arrow between CO₂ in the air and box A.

   iv) R on arrow between carbon compounds in animals (or box A) and CO₂ in the air.