Tagged: Cycles

Deforestation: A* understanding for IGCSE 4.17


Deforestation means the cutting down of mature forest and woodland for non-forestry purposes.  No new trees are planted and so the total area of forest decreases.  Humans have cut down forests for many reasons and have built their economies on exploiting natural resources.  Forests today are cleared to provide wood for logging, to provide land for building homes, for subsistence farmers as well as for commercial growing of crops and cattle farming.


Deforestation happens all the world.  (It is worth noting that the only reason there are no European countries on the list below is because all our forests were cleared very effectively some time ago….)


What are the biological consequences of deforestation?

Wherever deforestation occurs, the biological consequences are the same:

  1. Atmospheric Gases
  2. Soil Erosion
  3. Disturbance to the Water Cycle
  4. Loss of Biodiversity (inexplicably omitted by the people who wrote the specification

Growing trees have a net uptake of carbon dioxide and a net loss of oxygen due to photosynthesis.  Carbon dioxide is a gas that acts as a pollutant in our atmosphere because it is a greenhouse gas.  Carbon dioxide concentrations have been rising over the past century and this is leading to permanent and potentially damaging alterations to the earth’s climate system – a process called climate change.  Oxygen is the gas that almost all organisms require for their respiration.


In many tropical regions, forests protect the sometimes violent tropical storms from hitting the ground. When forest cover is removed, rainfall hits the soil much harder and this can lead to loss of topsoil in a process called soil erosion.  As the water runs through the soil, it will dissolve minerals as it goes, thus leaving the soil that is left denuded of essential minerals for plant growth.  This leaching of minerals makes it difficult to use the land cleared for agriculture and so more forest is cleared.

Deforestation also disrupts the water cycle.  Trees move large volumes of water a year from the soil into the atmosphere in a process called transpiration.  So when trees are lost, less water evaporates from the soil, more water is lost in run-off and so rainfall can be reduced.


The diagram below shows a before and after explanation of how the water cycle is disrupted. (Evapotranspiration is a term for the total water evaporated from a piece of land, combining evaporation directly from the ground and transpiration lost from plants)


There is a final problem with deforestation although the examiners have omitted it from the specification for some inexplicable reason.  Forests provide a habitat for a wide variety of animal and plant species.  So when forests are lost, species become extinct.  This loss of biodiversity is a final terrible consequence of deforestation.  80% of known species live in tropical rainforest so the fact that in the last 50 years, over half of this area has been cleared is a major concern.  The rate of loss of rainforest is around 140,000 square kilometres a year although in some parts of the world, the rate of loss is slowing.

Deforestation is a complex issue and a GCSE revision blog like this is not the place to go into the interesting political and cultural details. I would direct you to the WWF site for more information and indeed some ideas as to what we can do to help.



Water cycle: the simplest topic in iGCSE Biology 4.17 4.8

I am wary of writing a post about the water cycle as I so rarely teach it.  It seems too much like common sense to me to require any elaboration in class, but perhaps writing this post will sooth my guilty conscience for Y10 and Y11 students?


The processes that happen in the water cycle are almost all nothing to do with Biology.  Water evaporates from lakes, streams and the sea.  Evaporation is when thermal energy from the sun changes water from a liquid to the vapour state.  The warmer the day, the more evaporation will occur.  The biological component here is that water evaporates from the above ground parts of a plant.  This process is called transpiration and mostly happens through the stomata (tiny pores in the lower epidermis of the leaves).  Geographers like to combine “transpiration” with the “evaporation” of water direct from the soil to come up with the exciting term “evapotranspiration”.  Water vapour condenses in the atmosphere to form clouds and then water falls as a liquid as rain/snow/hail which can be combined together as precipitation.

That’s the water cycle for you:  couldn’t be much simpler really, could it?

Just to finish, check your A* understanding of transpiration by answering these questions – if you are feeling really digital, why not add the answers as a comment at the foot of this post?

1) When are stomata open in the leaf and when do they close?

2) What four environmental factors can speed up rates of transpiration?

3) What is the name of the experimental set up that can be used to measure transpiration rates?  (Does it actually measure transpiration rate or does it really measure something else entirely?)

4) In what ways would you think of transpiration as a “necessary evil”?


4.10 Nitrogen cycle for IGCSE Biology

I will make a blog post on each of the three “cycles” you need to know about for iGCSE.  By far the most complicated is the Nitrogen cycle, so we might as well start there….

The first bit of understanding you need to is be clear the difference between how energy moves through an ecosystem and how matter (i.e. atoms) are exchanged between organisms and their environment.  Energy in the ecosystems moves in a linear flow:  there is no recycling of energy.  The energy comes in at one end (in the producers through the process of photosynthesis) and is ultimately all lost as heat to the environment through the process of respiration.  There is no possible way energy can be recycled.  The “circle of life” that students like so much from Disney certainly does not apply here….  People find this idea very difficult to appreciate.  All the time students will tell me that the energy in dead plants and animals goes into the soil and is then absorbed through the roots of plants:  “it’s the circle of life sir” they earnestly tell me.  And in the words of the late, great Amy Winehouse, I say “NO,NO,NO”…

Matter on the other hand is recycled through the ecosystem.  The individual atoms that make up your body (H,O,C,N,S etc.etc,) have all been in other organisms and indeed will be again in the future.   You took them in through your food and use these atoms to build the molecules that make up your cells.  But ultimately all these atoms will leave your body either through metabolic processes or when you die and are decomposed.  You could draw up a cycle for any of the atoms that are found in living things but your specification only requires you to understand two.  How are carbon atoms cycled – the Carbon cycle – and how are nitrogen atoms cycled – the Nitrogen cycle.  (You will also look at the Water cycle as well…..)

Nitrogen Cycle diagram

Things to understand about the Nitrogen Cycle:

1) Which molecules in living things contain nitrogen atoms?

Well the answer is fairly simple.  Proteins are polymers of amino acids.  Amino acids all contain an -NH2 group (amine group) and so Nitrogen is found in proteins.  The bases in DNA (Adenine, Cytosine, Guanine and Thymine) are described as nitrogenous bases and so they contain nitrogen too.

2) Where are nitrogen atoms found in the ecosystem other than in the molecules of living organisms?

This is more complicated.  Nitrogen gas makes up 78% of the atmosphere so clearly there is a lot of nitrogen in the air.  In the soil, there will be urea from the urine of animals and urea contains nitrogen.  There are also a range of ions found in the soil that contain nitrogen: the two most important are ammonium NH4+ and nitrate, NO3-.  (I don’t know how to do subscript and superscript in WordPress and so you will have to excuse the rather ugly molecular formulae)

3) How do nitrogen atoms move from the abiotic (non-living) parts of the ecosystem and into the organisms?

There is only one way nitrogen atoms can move from the abiotic environment and into the organisms in an ecosystem.  This is via plants that can absorb nitrate ions from the soil in their roots.  This is a slight simplification but it will do at the moment.  Look at the diagram above and find the arrow that shows assimilation of nitrates from the soil into plants.

4) How many different kinds of soil bacteria are involved in cycling nitrogen?

You need to know about four different kinds of bacterial that live in the soil that play a role in recycling nitrogen.  Use the diagram above and your notes to describe the role each of these organisms play in the cycling of nitrogen atoms in the ecosystem.

  • Decomposers (Putrefying Bacteria)
  • Nitrifying Bacteria
  • Nitrogen-Fixing Bacteria
  • Denitrifying Bacteria

I am off to have my supper: another post about these bacteria will appear here later tonight or tomorrow.  Please don’t read it until you have tried to write down a paragraph on each of the four types of bacteria in the bullet point list above.