the water and carbon cycles and decay

• the water cycle


  the continuous process by which water moves from Earth's surface to the atmosphere and back
• the water cycle steps

  evaporation, transpiration, condensation, precipitation
• evaporation


  the change of a substance from a liquid to a gas and occurs when sunlight warms the surface of the water
• transpiration


  evaporation of water from the leaves of a plant
• condensation


  the process where water vapour in the atmosphere cools and changes back into liquid water, forming clouds
• precipitation


  any form of water that falls from clouds and reaches Earth's surface
• the water cycle process

  1. energy from the sun makes water evaporate from the land and sea, turning it into water vapour. water also evaporates from plants - this is known as transpiration

  2. the warm water vapour is carried upwards (as warm air rises). when it gets higher up it cools and condenses to form clouds

  3. water falls from the clouds as precipitation (rain, snow, hail) falls onto land, where it provides fresh water for plants and animals

  4. it then drains into the sea, before the whole process starts again
• the carbon cycle


  the organic circulation of carbon from the atmosphere into organisms and back again
• carbon cycle processes


  photosynthesis, respiration, decomposition, combustion
• carbon cycle steps

  1. CO2 is removed from the atmosphere by green plants and algae during photosynthesis. the carbon is used to make glucose, which can be turned into carbohydrates, fats and proteins that make up the bodies of the plants and algae

  2. when the plants and algae respire, some carbon is returned to the atmosphere as CO2

  3. when the plants and algae are eaten by animals, some carbon becomes part of the fats and proteins in their bodies. The carbon then moves through the food chain

  4. when the animals respire, some carbon is returned to the atmosphere as CO2

  5. when plants, algae and animals die, other animals (called detritus feeders) and microorganisms feed on their remains. When these organisms respire, CO2 is returned to the atmosphere

  6. animals also produce waste that is broken down by detritus feeders and microorganisms

  7. the combustion (burning) of wood and fossil fuels also releases CO, back into the air

  8. so the carbon (and energy) is constantly being cycled - from the air, through food chains (via plants, algae and animals, and detritus feeders and microorganisms) and eventually back out into the air again
• how do organic materials taken in by living organisms return back to the environment?

  in waste products or when the organisms die and decay
• what does decay do?

  puts stuff that plants need to grow (e.g. mineral ions) back into the soil
• decay


  the natural process where dead organic matter (e.g. dead plants and animals) is broken down into simpler substances by microorganisms (e.g. bacteria and fungi)
• decomposition


  a chemical reaction that breaks down compounds into simpler products
• compost


  decomposed organic matter (e.g. food waste) that is used as a natural fertiliser for crops and garden plants
• what produces compost?


  decomposition/decay
• what is responsible for decay?

  microorganisms, such as bacteria and fungi, and detritus feeders
• detritus feeders


  organisms that feed on small pieces of dead organic matter
• what is the rate of decay affected by?

  - temperature

  - water availability

  - oxygen availability

  - number of decay organisms
• how does temperature affect the rate of decay?

  warmer temperatures make things decompose quicker because they increase the rate that the enzymes involved in decomposition work at. if it's too hot though, decomposition slows down or stops because the enzymes are destroyed and the organisms die. really cold temperatures slow the rate of decomposition too
• how does water availability affect the rate of decay?

  decay takes place faster in moist environments because the organisms involved in decay need water to carry out biological processes
• how does oxygen availability affect the rate of decay?

  many organisms need oxygen to respire, which they need to do to survive. the microorganisms involved in anaerobic decay don't need oxygen though
• how does the number of decay organisms affect the rate of decay?

  the more microorganisms & detritus feeders there are, the faster decomposition happens
• optimum conditions for decay

  - warm temperature

  - moist environments

  - plenty of oxygen

  - many decay organisms
• biogas


  a mixture of methane and carbon dioxide produced by bacterial degradation of organic matter and used as a fuel
• how is biogas produced?

  in a simple fermenter (called a digester or generator) with many different types of microorganisms that decay plant and animal waste anaerobically - this type of decay produces methane gas
• what can be used to make biogas on a large scale?

  sludge waste from sewage or sugar factories
• biogas generators


  devices that produce methane for fuel from organic waste - they must be kept at a constant temperature to keep the microorganisms responsible for the decay respiring
• why can't biogas be stored as a liquid?

  because it needs too high of a pressure, so it must be used straight away (for heating, cooking, lighting or to power a turbine to generate electricity)
• batch biogas generator


  these make biogas in small batches and are manually loaded up with waste, which is left to digest, and the by-products are cleared away at the end of each session
• continuous biogas generator


  these make biogas all of the time with waste continuously being fed in, and biogs is produced at a steady rate - these are more suited to large-scale biogas projects
• what must a biogas generator have?

  - an inlet for waste material to be put into

  - an outlet for the digested material to be removed through

  - an outlet so that the biogas can be piped where it is needed
• how can temperature's effect on decay be investigated?

  1. measure out 5 cm³ of lipase solution and add it to a test tube. label this tube with an 'L' for lipase

  2. measure out 5 cm³ of milk and add it to a different test tube

  3. add 5 drops of phenolphthalein indicator to the tube containing milk

  4. measure out 7 cm³ of sodium carbonate solution and add it to the tube containing milk and phenolphthalein. This makes the solution in the tube alkaline, so it should turn pink

  5. put both tubes into a water bath set to 30 °C and leave them to reach the temperature of the water bath. you could stick a thermometer into the milk tube to check this

  6. once the tubes have reached 30 °C, use a calibrated dropping pipette (a dropping pipette with a scale) to put 1 cm' of the lipase solution into the milk tube and start a stopwatch straight away

  7. stir the contents of the tube with a glass rod. the enzyme will start to decompose the milk

  8. as soon as the solution loses its pink colour, stop the stopwatch and record how long the colour change took in a table

  9. repeat the experiment at a range of different temperatures (e.g. 10 °C, 20 °C, 40 °C, 50 °C). make sure you carry out the experiment three times at each temperature, then calculate the mean time taken for the colour change to occur at each temperature

  10. use your results to calculate the rate of decay
• rate of decay formula

  1000/time (units will be s⁻¹ since rate is given per unit of time)