Tagged: oxygen

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.



2.46 Alveoli and Gas Exchange: A* understanding for iGCSE Biology

The primary function of the lungs is to allow gas exchange to occur.  Oxygen gas can diffuse into the blood from the air in the lungs.  Oxygen of course is needed for the process of aerobic respiration that is happening in every cell all the time.  Aerobic respiration produces carbon dioxide as a waste product.  Carbon dioxide diffuses out the blood in the lungs into the air in the lungs.  Hence the name gas exchange – one gas (oxygen) diffuses in, another (carbon dioxide) diffuses out.469521_1311326813_large

This diagram above shows the bronchial tree – the branching network of tubes that carry air into the lungs.  The trachea at the top branches into the right and left bronchi, then each in turn branch into smaller bronchi and finally into the smallest tubes called bronchioles.  Bronchioles carry air into a cluster of tiny airsacs called alveoli (not ravioli as AZB told his F division today…)

Diffusion is the passive movement of molecules of a liquid or gas from a high concentration to a low concentration.  So the first question is what ensures that there is an appropriate concentration gradient for each gas to diffuse?


In order to understand this, you have to remember that the blood going to the lungs is deoxygenated.  The right ventricle pumps deoxygenated blood to the lungs in the pulmonary arteries.  The tiny alveoli are then covered with capillaries and these join together to form the pulmonary veins.   The pulmonary veins carry the oxygenated blood back to the left atrium of the heart.  So the blood coming to the lungs will have a low oxygen concentration but a high carbon dioxide concentration.

How are the structure of alveoli adapted for efficient gas exchange?

  1. The alveoli in total provide a large surface area for the diffusion of oxygen and carbon dioxide.  The total surface area of the alveoli in humans is approximately 90 m2 – the equivalent of two tennis courts…..
  2. The walls of the alveoli are very thin.  The alveolus is lined with a single layer of cells, and of course the capillaries are also only one cell thick. So the distance for the diffusion of oxygen and carbon dioxide is very small (hence the rate of diffusion is very fast)
  3. The alveoli have a rich blood supply.  Alveoli are lined by many capillaries.
  4. The surface of the alveolus is moist.  Gas exchange surfaces are always moist as oxygen and carbon dioxide will diffuse more rapidly if they are dissolved in water.
  5. Alveoli also contain a cell that secretes surfactant.  This molecule reduces the surface tension in the film of water that lines the alveolus, allowing air to move in and out more smoothly.


Respiration: A* Understanding for iGCSE Biology 2.33 2.34 2.35

I can’t believe that it is over year since I started posting about iGCSE Biology misconceptions and yet I have never written about Respiration.  If there is one topic that students misunderstand more than any other (apart perhaps from genetics), this must be it….  So I am going to try to explain in a straightforward way what respiration is and why it is so important for life.

Life requires energy.  Living cells are constantly doing things that use up energy: pumping molecules across their cell membranes, moving organelles around the cell, cell division, nerve cells sending electrical impulses around the body, muscle fibres contracting etc. etc.  In every case, this energy comes from a metabolic process called Respiration.  It is a series of chemical reactions, catalysed by enzymes and in some way, it happens in all cells.

So let’s start with a good definition.  [Examiners are simple souls and often start questions with the classic “What is Respiration?”]

Respiration is a series of chemical reactions that happens inside cells in which food molecules (for example glucose) are oxidised to release energy for the cell.

My definition has to be a little vague because although glucose is found in all the equations for respiration, other food molecules can certainly be respired.  And oxygen is only used in aerobic respiration.  Many organisms can only respire without oxygen (anaerobic respiration) and some, such as humans can switch between aerobic and anaerobic depending on the conditions.


Aerobic Respiration happens for the most part in tiny organelles in the cytoplasm called Mitochondria.  The diagram above shows the structure of a mitochondrion (I wouldn’t worry about learning it but perhaps you should be able to recognise the characteristically folded inner membrane?)

What are the differences between aerobic and anaerobic respiration in humans?

Well we have mentioned two already and there are others…..:

  • Aerobic respiration requires oxygen, anaerobic does not.
  • Aerobic respiration takes place in mitochondria, anaerobic only occurs in the cytoplasm.
  • Aerobic respiration produces much more energy per glucose molecule than anaerobic – it is a more complete oxidation of the glucose, so much more energy is released.
  • Anaerobic respiration produces lactic acid as a waste product (in humans) whereas in aerobic, carbon dioxide and water are the products

The summary equations for the processes are different as well.

Aerobic respiration:

word equation                       Glucose + Oxygen ——> Carbon Dioxide + Water

balanced chemical equation            C6H12O6 + 6O2  ——> 6CO2 + 6H20

Anaerobic respiration in humans:

Glucose —–> Lactic Acid

Anaerobic respiration in Yeast (a single celled fungus):

Glucose —–> Ethanol and Carbon Dioxide

A couple of final points to note:

Anaerobic respiration in muscle cells does not produce carbon dioxide as a waste product (see the equation above…) Lactic acid is the only waste product.  But lactic acid will accumulate in muscles and stop the muscle functioning properly so after a period of intense activity, lactic acid needs to be removed.  How does this happen?

Lactic acid moves from the muscle in the blood and is transported to the liver.  In the liver, the lactic acid is metabolised in an aerobic pathway that uses oxygen.  This is why sprinters will always be breathing fast after the race, even when they are standing still.  Their body needs extra oxygen to oxidise the lactic acid they have produced during the race.  This extra oxygen is termed an oxygen debt and is the oxygen needed in the liver to fully oxidise lactic acid to carbon dioxide and water.

Finally, respiration is not the same as breathing.  Our American cousins sometimes muddle these processes up but in this one case, the British way is much better….  Use the term ventilation for breathing – moving air in and out of the lungs – and reserve respiration for the chemical reactions that happen inside the cells to release energy.


Please leave a comment below if you find this post helpful or ask me about anything that isn’t clear….

Blood part 1 Plasma and RBCs: A* understanding for iGCSE Biology 2.57 2.58 2.59

Blood is a tissue in the body that plays a variety of roles in transport and in defending the body against disease.  It is an unusual tissue since it is a liquid, with many different kinds of cells suspended in a watery solution called plasma. blood-composition

Plasma makes up 55% of the volume of blood and is a solution of many different chemicals in water.  For example, the plasma contains dissolved glucose, amino acids and other products of digestion from the intestines.  It also transports the waste molecule urea from the liver where it is made to the kidney where it is excreted.  Blood plasma contains dissolved carbon dioxide, mostly in the form of hydrogencarbonate ions.  Many hormones (for example testosterone, ADH, adrenalin) are transported in the blood plasma and because the plasma is mostly water, it provides a good way of moving heat around the body from respiring muscles to the skin where it can be lost.


The most common cell in blood are the red blood cells (or erythrocytes).  These tiny cells are adapted for the transport of oxygen.  Each red blood cell contains around 270 million molecules of a transport protein, haemoglobin.  Each molecule of haemoglobin can bind up to four molecules of oxygen in the lungs and then unload the oxygen when the red blood cell passes through a capillary in an actively respiring tissue.


(Don’t worry too much about the structure of the protein – this is A level stuff really…. Just remember haemoglobin is a transport protein for oxygen found in red blood cells)

As well as being packed full of haemoglobin molecules, red blood cells have other adaptations for transporting oxygen.  Red blood cells lose their nucleus during their development as this allows more haemoglobin to be packed into each cell.  Having no nucleus means the red blood cell cannot divide nor repair damage to its structure.  This is why each red blood cell only lives for 100-120 days in the body.

Red blood cells have a characteristic shape.  It is called a biconcave disc and they have an especially flexible shape.  Remember that a capillary is actually smaller in diameter than a red blood cell, so the cells have to squeeze through capillaries in single file…..


3.10 Role of the Placenta – A* understanding for GCSE

There are two syllabus points in bold (only tested in paper 2) that refer to embryonic and foetal development.  The first asks you to understand the role of the placenta in supplying the developing foetus with nutrients and oxygen and the second concerns the role of amniotic fluid in protecting the developing embryo.


The placenta is in many ways a remarkable organ.  It contains a mixture of maternal cells from the uterine lining and embryonic cells, but these cells from two genetically different individuals are capable of sticking together to form the placenta.  The placenta is only present in the uterus once an embryo has successfully implanted a week or so after fertilisation has happened in the Fallopian Tubes.  The placenta is linked to the foetus via the umbilical cord, a structure that contains an umbilical artery and vein carrying foetal blood to and from the placenta.



There is a key idea here that is very important.  There is no mixing of maternal and foetal blood in the placenta.  This would be disastrous for both mother and baby for a whole variety of reasons.  The maternal blood is at a much higher pressure than the foetal blood and if the foetus were connected to the maternal circulatory system directly, its blood vessels would burst.  The foetus and mother can have different blood groups of course and you may now that some blood groups are incompatible and can trigger clotting.  So it is essential that there is never any mixing of blood.  But what happens in the placenta is that mother’s blood empties into spaces in the placenta and babies’ blood is carried by the umbilical artery into capillaries that are found in finger-like projections called villi.  This means there is a large surface area and a thin barrier between the two bloods and so exchange of materials by diffusion is possible.

The main function of the placenta then is to allow the exchange of materials between the foetal and maternal circulations.  The developing foetus inside its mother’s uterus has no direct access to oxygen nor food molecules of course yet both are needed to allow healthy development.  The foetus also needs a mechanism to get rid of the waste molecule, carbon dioxide that is being produced in all its cells all the time.  Until the kidneys mature fully the foetus also has to get rid of urea, another excretory molecule that could build up to toxic concentrations unless removed from the growing foetus.


A few interesting points:

You will see that antibodies are small enough to cross the placenta.  This gives the baby a passive immunity that can protect it for a short time from any pathogens it encounters.

Drugs such as alcohol and nicotine can cross the placenta.  This is why it is so vital that pregnant mothers do not smoke and drink to ensure that the foetus’ development is not affected by these drugs.


Gas Exchange in Plants – A* understanding for iGCSE 2.39 2.40 2.41 2.42

The topic of gas exchange in plants is often tested in exams because it can be a good discriminator between A grade and A* grade candidates.  If you can master the understanding needed for these questions, important marks can be gained towards your top grade.

Firstly you must completely remove from your answers any indication that you think that plants photosynthesise in the day and respire at night.  Even typing this makes me feel nauseous….  Yuk?  Respiration as you all know happens in all living cells all the time and so while the first half of the statement is true (photosynthesis only happens in daytime), respiration happens at a steady rate throughout the 24 hour period.


Although the equations above make it look like these two processes are mirror images of each other, this is far from the truth.

How can gas exchange in plants be measured?

The standard set up involves using hydrogen carbonate indicator to measure changes in pH in a sealed tube.  In this experiment an aquatic plant like Elodea is put into a boiling tube containing hydrogen carbonate indicator.  The indicator changes colour depending on the pH as shown below:

  • acidic pH:  indicator goes yellow
  • neutral pH: indicator is orange
  • alkaline pH: indicator goes purple


a) If the tube with the plant is kept in the dark (perhaps by wrapping silver foil round the boiling tube), what colour do you think the indicator will turn?  Explain why you think this.

b) If the tube with the plant is kept in bright light, what colour do you think the indicator will turn and why?

c) If a control tube is set up with no plant in at all but left for two days and no colour change is observed, what does this show?

In order to score all the marks on these kind of questions, there are two pieces of information/knowledge you need to demonstrate.  You need to show the examiner that you understand that carbon dioxide is an acidic gas (it reacts with water to form carbonic acid) and so the more carbon dioxide there is in a tube, the more acidic will be the pH.  As oxygen concentrations change in a solution, there will be no change to the indicator as oxygen does not alter the pH of a solution.

Secondly you need to show that you understand it is the balance between the rates of photosynthesis and respiration that alters the carbon dioxide concentration.  If rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide so the pH will fall (become more acidic).  If the rate of photosynthesis in the tube is greater than the rate of respiration, there will be a net uptake of carbon dioxide (more will be used in photosynthesis than is produced in respiration) and so the solution will become more alkaline.

So to answer the three questions above I would write:

a) The indicator will turn yellow in these conditions.  This is because there is no light so the plant cannot photosynthesise but it continues to respire.  Respiration releases carbon dioxide as a waste product so because the rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide from the plant.  Carbon dioxide is an acidic gas so the pH in the solution will fall, hence the yellow colour of the solution.

b) The indicator will turn purple in these conditions.  This is because the bright light means the plant photosynthesises at a fast rate.  Photosynthesis uses up carbon dioxide from the water.  The plant continues to respire as well and respiration releases carbon dioxide as a waste product.  As the rate of photosynthesis is greater than the rate of respiration in these conditions there will be a net uptake of carbon dioxide.  Carbon dioxide is an acidic gas so if more is taken from the solution than released into it, the pH in the solution will rise as it becomes more alkaline, hence the purple colour of the solution.

c) This shows that without a living plant in the tube there is nothing else that can alter the pH of the solution.  It provides evidence that my explanations above about the cause of the colour change is correct.