Tagged: oxygen

Deforestation: Grade 9 Understanding for IGCSE Biology 4.18B


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.



Alveoli and Gas Exchange: Grade 9 Understanding for IGCSE Biology 2.46 2.48

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: Grade 9 Understanding for IGCSE Biology 2.34 2.36 2.37 2.38








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


  • Aerobic respiration requires oxygen, anaerobic does not.

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:






Blood part 1 Plasma and RBCs: Grade 9 Understanding for IGCSE Biology 2.59, 2.60, 2.61

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…..


Role of the Placenta – Grade 9 Understanding for IGCSE Biology 3.11

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.







How can gas exchange in plants be measured?


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







So to answer the three questions above I would write:


