Tagged: respiration

Yeast and Brewing: Grade 9 Understanding for IGCSE Biology 5.5

Humans first discovered how to make beer around 7000 years ago and brewing has been an integral part of human civilisations ever since.  Yeasts are a family of single celled fungi that can use the sugars in fruits and seeds as a source of energy for respiration.  Yeast can respire both aerobically and anaerobically and you should know the equations for these two processes.

Aerobic respiration

Glucose + Oxygen ——-> Carbon Dioxide + Water

Anaerobic respiration (aka Fermentation)

Glucose ——> Ethanol + Carbon Dioxide


So when yeasts respire anaerobically they produce ethanol and carbon dioxide as the waste products.  Ethanol is also known as alcohol and humans learned a long time ago that alcohol is a drug that changes the way you think or feel, often in a pleasurable way in moderate doses.  Making drinks that were alcoholic also helped to kill potentially harmful bacteria and other pathogens in pre-industrial times when drinking water was not readily available.

If you add yeast to a source of sugar in anaerobic conditions, the yeast cells will ferment the sugars into alcohol and carbon dioxide.  In order to make beer, the source of sugar comes from germinating barley seeds.  Hops (dried flowers of a hedgerow plant) are added later to give the bitter flavour beer drinkers seem to like…..


The flow diagram above shows the stages in making beer.  I can’t imagine you would be expected to know the details.  Any question on this topic would presumably focus on the anaerobic respiration of the yeast rather than the details of the brewing process.

In case anyone is interested, I am not a huge fan of beer although I can occasionally be forced by peer pressure into consuming one or two.  My preferred fermentation reactions happen not in the copper tuns of English breweries but in the beautiful Northern Rhone valley in France, where skilled wine-makers can take Syrah grapes grown under the influence of the cooling mistral wind and turn them into beautiful Cote Rotie or Hermitage.  Now there’s a happy thought for a cold November evening……..


Respiration: Grade 9 Understanding for IGCSE Biology 2.34 2.36 2.37 2.38

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….

Characteristics of Life: Grade 9 Understanding for IGCSE Biology 1.1


The iGCSE specification says that all living organisms share the following basic characteristics and then lists 8 bullet points.  This seems unnecessarily unhelpful because every student in the whole word learns MRS GREN for the 7 characteristics of life…

Make sure you understand the exact meaning of each of the following terms:

  • Movement
  • Respiration
  • Sensitivity
  • Growth
  • Reproduction
  • Excretion
  • Nutrition

Not all organisms Move from place to place of course and lots of things move that are not alive.  So that doesn’t make me think that this is a good way to start the whole study of Biology.  It is true that all living things, without exception, Respire.  “Respiration is a series of chemical reactions that happens inside cells in which food molecules are oxidised to release energy for the cell” – good definition that….   Sensitivity means the ability to detect and respond to changes in the environment.  Mammals do this through their nervous and hormonal systems, plants through plant growth substances such as auxin.  Growth either involves a cell getting larger or in multicellular organisms, the two processes of cell division and cell specialisation.  All living things have the potential to Reproduce, to create new individuals of their species.  Excretion is the removal of waste molecules (e.g. carbon dioxide, urea) that have been made inside cells.  Nutrition means either obtaining food molecules by eating another organism or if you are a plant, and I guess none of you are,  by making your own food molecules through photosynthesis.

The people who wrote the specification have added “they control their internal conditions” to the list.  This is actually a better characteristic of life than many above as it is a universal feature of all life.  The term for this process is Homeostasis – the ability to regulate and control the internal environment.

It is a shame that two of the best ways to decide whether something is alive have been left off the list.  All living things on earth are made of cells.  Some organisms are unicellular (Paramecium for example) but many are made of many cells.  And all living organisms have the molecule DNA as their genetic material.  If you get a question on this in the exam, it’s probably better to talk about the 8 characteristics of life the examiner likes… That’s exams for you!

Demonstrating Carbon Dioxide production in Respiration of Yeast 2.39

Yeast is a single celled fungus that can respire aerobically when oxygen is available and anaerobically in the absence of oxygen.

Aerobic respiration in yeast:

Glucose + Oxygen ======> Carbon dioxide + Water

Anaerobic respiration in yeast:

Glucose ======> Ethanol and Carbon dioxide

Both forms of respiration produce carbon dioxide as a waste product so how could this be demonstrated experimentally?


This is the simplest set up that could demonstrate this.  Lime water will go cloudy in the presence of carbon dioxide.  Glucose solution is needed to provide the reactant sugar for the yeast to respire.  The oil layer on the top is to prevent the diffusion of oxygen from the air into the Yeast in Glucose solution, this ensuring anaerobic respiration will occur.

What experimental factors could be altered in this set up?

Well assuming you keep the volume and concentration of lime water constant, the time taken for the limewater to go cloudy could be measured under differing conditions: the faster the time, the faster the rate of respiration.  The experimenter could investigate the effect of changing the temperature, the pH or the concentration of glucose solution used.  Make sure you understand how and why changing each of these factors might affect rates of respiration in yeast.

Respiration experiments: Grade 9 Understanding for IGCSE Biology 2.39

There is a syllabus point in the iGCSE Respiration section that asks candidates to know about an experiment that demonstrates heat production in respiration.  This must be one of the least interesting experiments ever devised but here goes…..

Respiration is the chemical process occurring in all cells in which food molecules are oxidised to release energy for the cell.  Cells need energy for a whole variety of things – active transport of molecules across the cell membrane, muscle contraction, movement of materials around the cytoplasm, cell division, many metabolic reactions etc.   In fact, much of the energy released from glucose molecules in respiration is not “useful energy” for the cell but is given off as heat, a waste product.  In warm-blooded animals such as humans, this heat energy is used to maintain our body temperature at around 37 degrees Celsius.

How can you demonstrate heat production in respiration?


The germinating seeds in the vacuum flask on the left are respiring because they are alive.  The boiled seeds in the vacuum flask on the right will not be respiring because they are dead – boiling will denature all the enzymes needed for metabolism,   The thermometer on the left will show a rise in temperature, the one on the right will stay the same.  The flask on the right with the boiled seeds is a control.  Vacuum flasks are used to insulate the seeds and so prevent heat loss.

The experiment is as simple as that.  If the examiners wanted to ask a question on this, I guess they would give you the set up, ask about the design of the experiment, ask about which variables you might control and perhaps what conclusions could be drawn.

Gas Exchange in Plants – Grade 9 Understanding for IGCSE 2.40B, 2.41B, 2.44B, 2.45B

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.