Tagged: biology

Commentary on Zondle GCSE Biology Revision challenge 1 questions

I hope that those of you who played the GCSE Biology revision challenge this afternoon enjoyed the process.  I would welcome comments on this blog post along the lines of www (what went well) and ebi (even better if)….

The questions were grouped into several topic areas.

Questions 1 to 4 were on thermoregulation.  Understanding vasoconstriction, vasodilation and sweating are the key things here and if you haven’e done already, I would read my blog post on this topic.

https://pmgbiology.wordpress.com/2014/05/29/skin-a-understanding-for-igcse-biology/

Questions 5 – 9 were on plant transport and these were well answered by almost all players.  Remember that phloem sieve tubes move sucrose and amino acids around the plant.  Water and minerals are transported in xylem vessels of course, but the other distractor answers included various polymers (starch and proteins) that are made in photosynthesis in the leaves but which are too large and insoluble molecules to be transported in phloem.

Questions 10-15 were all on the bacteria in the Nitrogen cycle.  This is a tricky topic but one that rewards patient work by candidates to master it. In reality the Nitrogen cycle is not difficult to understand but it is easy to muddle the names and roles of the four types of bacteria involved.  Again there are a couple of blog posts on Nitrogen cycle that I would encourage you to read….

https://pmgbiology.wordpress.com/2014/04/12/bacteria-of-the-nitrogen-cycle-a-understanding-for-gcse-biology/

https://pmgbiology.wordpress.com/2014/04/12/nitrogen-cycle-for-igcse-biology/

Questions 16-23 were on digestive systems.  These were generally well answered although many players didn’t appreciate that peristalsis doesn’t just happen in the oesophagus: it is the process that moves the food along the entire length of the gut tube from top of oesophagus to the end of the rectum.  The role of the lacteal in transporting fatty acids and glycerol away from the villi in the small intestine is also one of the trickier topics here.  Amino acids and sugars diffuse into the blood capillaries in the villus but fatty acids and glycerol (the products of digestion of lipids) don’t go into the capillaries but instead into a separate vessel called a lacteal. This forms part of the lymphatic system and the liquid formed ends up back in the blood but effectively bypasses the liver, preventing the cells in the liver being overloaded with fatty acids following a fatty meal.

Questions 24-29 were on the heart and circulation.  There were quite a few incorrect answers here but perhaps this was because enthusiasm levels were dropping….  The flow of blood through the heart is an important topic to appreciate – into RA through vena cava, then into RV through right AV valve, then into PA through semilunar valve, then to lungs, back from lungs in pulmonary veins, into LA, through Left AV valve into LV, then into aorta through the aortic semilunar valve…..

The heart strings in the heart (chordae tendinae) are commonly misunderstood.  They play no role at all in opening or closing the AV valves (this is done simply by the balance of blood pressures in atrium and ventricle) but do provide tension to stop the valve “blowing back” and thus opening when the ventricle contracts.  Have a look at pictures of a real heart dissection to see that these tendons attach to the valve flaps and ensure they cannot blow open when the pressure in the ventricles rise during ventricular contraction.  Ask me for more detail if this doesn’t make sense.

Question spotting for IGCSE Biology paper 2

Trying to guess what might come up in paper 2 of public exams is a dangerous business……  But I think it is sensible for Y11 students sitting IGCSE Biology to now focus their remaining revision on topics that have yet to be tested.  You are now two thirds of the way through your exams and a final push to paper 2 might just get you across the A-A* boundary.  Every mark is vital in any exam so keep working hard!

Here are some PMG tips for topic areas that seem a better than average bet for coming up in paper 2:

  • Variety of Living Organisms (5 Kingdom Classification, Viruses)
  • Biological Molecules, especially Enzymes – (graph interpretation question?, effect of temperature and pH on rates of reaction?)
  • Photosynthesis and Respiration (perhaps a question testing bullet points 2.40 and 2.43 on gas exchange in plants over 24 hour period?)
  • Role of White Blood cells in Immunity (perhaps linked in with viruses above, vaccination, memory cells etc.?)
  • Coordination in Humans (nerves, reflex arcs, the eye, homeostasis in the skin, hormones)
  • Reproduction in Flowering Plants (asexual mechanisms plus insect/wind pollination)
  • DNA structure (including mutations), Chromosomes and Cell Division
  • Carbon, Nitrogen and Water Cycles
  • Pollution (atmospheric, water pollution and climate change)
  • Fish Farming (surely they can’t leave this out?…..)

I will post some blog entries on some or all of these topics in the week or two after half term so keep your eyes posted on Twitter or follow this blog.

I am not suggesting that these are the only topics you revise in preparation for paper 2.  That would be very foolish as the examiners can ask questions on anything at all.  I am merely suggesting that you focus your remaining revision time on the topics most likely to come up and the list above might help you to decide what best to do.

Good luck and keep working hard!  Not long to go now……

 

How is energy lost between one trophic level and the next? Grade 9 Understanding for IGCSE Biology 4.8 4.9

Image The diagram above shows how energy moves up the food chain through feeding.  Remember that if you are asked what the arrows represent in a food chain, there is only one possible correct response.  “Arrows in a food chain show the flow of energy from one trophic level to the next”

The big idea here is that not all the energy in one trophic level can ever pass to the next.  The specification suggests that only 10% of the energy is transferred from one level to the next (but in fact the percentage varies between 0.1% to around 15%)

ImagekCal is a unit of energy and the pyramid shows that only 10% of the energy in one level is found in the next.

So there is a big question here – where does all the other 90% of the energy in one level end up?

There are a whole load of different ways energy is lost.  Consider the transfer of energy between mice and owls.

  • The mice use up energy in the process of respiration.  The glucose molecules that mice oxidise to provide the energy to move around are not available to an owl if the mouse is eaten.
  • Not all mice are eaten by owls or other predators. Many die of disease, starvation and exposure and a few might even live long enough to die peacefully in their sleep.  All these “dead mice” will have energy in their bodies that cannot pass up a food chain but instead passes to decomposers.
  • Even the mice that are eaten by owls are not eaten in their entirety.  The owl might only eat the energy-rich parts of a mouse and regurgitate out the bones and fur.  So some  energy is lost as not all the mouse is eaten and digested by the owl.
  • There will be parts of the mouse that even when swallowed and digested are not accessible.  Owl faeces will contain some molecules from the mice eaten that contain energy.   This energy is perhaps found in molecules that the owl digestive system cannot digest.  The energy present in the owl faeces is lost to the food chain and like the example above will pass to decomposers.

This energy adds up to around 90% of the energy in any trophic level.  Ultimately though where does it all go?  All the energy in all the organisms in an ecosystem has the same fate:  it ends up as heat that is dissipated into the system.  Energy can only enter an ecosystem in one way (as sunlight trapped in the process of photosynthesis in producers) and in the end, it all ends up leaving the system as heat energy.  This heat energy is a waste product of respiration.

A Simple Reflex Arc: Grade 9 Understanding for IGCSE Biology 2.90

GCSE Biology students often find the reflex arc a difficult topic in the section on human coordination and response.  This is because it is the only type of response they learn about and doesn’t really fit into a sensible flow of ideas on the various types of behaviours organisms can show.  But it is not too complicated, at least if you restrict yourself to ideas that might be tested in the iGCSE exam.

Prior Knowledge (you need to understand these things before you can appreciate a reflex arc)

  • basic structure of a neurone/nerve cell
  • three different kinds of neurones – sensory, motor and relay – and where they are found in the body
  • nerve impulses are electrical events that travel at up to 100ms-1 along nerve cells but cross synapses much more slowly by diffusion of a chemical called a neurotransmitter

Reflex responses

Most human behaviours are complex and involve millions of neurones interacting in the brain.  Our ability to link stimuli (changes in the environment) with an appropriate response can develop over time, can be modified by past experience and can produce different outcomes depending on the circumstances.  For example if you see a fast moving spherical object moving towards your head, you might head it (football), catch it (cricket), hit it (cricket again), duck out of the way (cricket again) or eat it (flying Malteser)

A simple reflex response is much more straightforward:  the same stimulus always produces the same response.  It does not need to be learned but is innate (you are born with it) and in humans, reflex responses tend to be involved in protecting the body from harm or maintaining posture.  The example we look at is called a withdrawal reflex to a painful stimulus e.g. touching a hot plate on a cooker.

The response to this is that you contract muscles in your arm to move your hand away from the hot plate.  The key idea is that you will do this before you feel the heat or burn the skin.  The sequence of events is

  • touch the hot plate (pain receptors stimulated in the skin)
  • move your arm away (reflex arc)
  • feel the pain (brain receives the nerve impulses and a conscious sensation of pain is felt

The reason that you move your arm away before you feel anything is that your brain is not involved in this response.  This produces a rapid, involuntary reaction called a reflex response.  The reason the response is so rapid is that at most three neurones are involved in linking the painful stimulus to the response.  The arrangement of these three neurones is called a reflex arc.

ImageThe cell that detects the stimulus is called a sensory neurone.  One end of this cell is a pain receptor in the skin and the other end of this individual cell is found in the spinal cord (see diagram above)  Neurones can be very long cells!  The sensory neurone forms a synapse (junction) with a relay neurone found entirely in the grey matter in the centre of the spinal cord and this in turn synapses with a motor neurone.  The cell body of the motor neurone is on the spinal cord and the other end of this individual cell is a synapse with a skeletal muscle in the arm.

Synapses are the things that slow nerve impulses down and as this whole pathway only includes two synapses (sensory-relay and relay-motor) the response will be as fast as possible.  The response is involuntary as the brain is not involved.

In humans, we can modify most reflex responses using the conscious parts of our brain.  As the sensory neurone synapses with the relay neurone in the diagram, it will also synapse with other neurones carrying nerve impulses up to the brain.  This is why touching a hot plate will hurt (the feeling of pain is in the brain).  There will also be neurones from the brain that can modify the synapse between the relay and motor neurone.  If I told you that I would pay anyone who can touch a hot plate for 2 seconds $10,000 (although of course I don’t have $10,000) many of you would be able to force yourself not to pull your arm away from the hotplate when you touch it.  You could overcome the reflex response with signals from your brain which would know how much fun you could have with $10,000.

 

 

 

 

Experimental Design questions IGCSE Biology

I have been posting comments about the questions that appear year after year on iGCSE Biology papers.  Questions like the one below are found in every past paper we have.  I call these the “Design an experiment to” questions for obvious reasons…..

“Rivers are sometimes polluted by warm water from power station outflows.  This is known as thermal pollution and can affect the growth of plants.  Design an experiment to investigate the effect of water temperature on the growth of plants.  6 marks. November 2010”

As you all know, the mark scheme for this kind of design an experiment question is based around the acronym CORMS.

C – how do you change the independent variable?

The independent variable is the thing you are going to change to see its effect.  In this experiment it is the temperature of the water.  So how are we going to change it?  Well it might appear obvious but you need aquatic plants living in water baths at a range of temperatures, say 10,20,30,40,50,60 degrees.  Try to make your independent variable continuous if it is possible – the range of temperatures above is much better than just one set of plants in hot water, another in cold water.

O – what organisms (or other biological material) will you use?

To get this mark you will need to say something about the plants you will use in your investigation.  For the experiment to produce reliable results, there are many features of the plants that will need to be kept the same in each water bath.  Same species, same age of plants, same starting size, same surface area of leaves etc.  There are other factors too about the plants that need to be controlled.  Can you think of any others?

R – reliability

In order to produce reliable results you will need to set up multiple repeats of each experiment so anomalous readings disappear as you average your results.  How would you do this?  Well in the example above, I would set up 5 identical water baths at each temperature.  We are investigating six different temperatures so we will need 30 water baths.  Don’t worry about this.  For research as vital as this fascinating experiment, no expense should be spared……

M – how are you going to measure the dependent variable?

There are often two possible marks for this and you will see M1 and M2 on the mark schemes.  The key idea is often the same however (there’s a shock)  The first mark is for identifying what you will measure about the plants to measure growth.  There are lots of alternatives depending on what kind of plant you are using.  I am picturing a small floating algae growing in my water baths so I would measure the mass of the plants.  (Dry mass would be better but this would lead to destructive sampling – plants won’t grow further if you dehydrate them completely in an oven before weighing them……)  You could measure the height of the stem of a plant, or the total surface area of water covered.  It doesn’t really matter which thing you choose as long as it is a sensible measure of growth.  What will M2 be awarded for?  Well it is essential you leave all 30 waterbaths for exactly the same length of time between measurements.  How frequently will you measure the growth of your plants?  Every hour would be too often, so perhaps every day would be sensible.  So a statement that says “use a mass balance to measure the total mass of the plants in each water bath every day for a period of 10 days” will be certain to get both M marks…

S – what factors do you need to standardise to make the experiment a fair test?

You will have mentioned some of these “fair test” factors in the mark point O above.  Now it is time to show that you understand what factors other than the temperature of the water will effect the growth of your plants.  Growth of plants is done by photosynthesis so I would be aiming to show you understand the other factors that will effect rates of photosynthesis:  i.e. light intensity. light wavelength and carbon dioxide concentration.  All three should be kept constant and I would say how:  same lamp at the same distance from the water baths, carbon dioxide in water controlled by dissolving same mass of sodium hydrogencarbonate in the water.  There are often two S marks but by stating all three important control variables this should guarantee we get both.

Now I have written this post without looking at the mark scheme.  “Promise….  Honestly Sir I wouldn’t cheat myself like that…..”  But here it is and look we would have got full marks.  Full Marks = A* #result

Image

Motivational advice for D block Biologists

Motivational advice for D block Biologists

I know some of you are working hard at the moment. How do I know? Well I can see that you are reading these blog posts, are playing the revision games on Zondle and producing good scores…..

Revision is a great example of a self-perpetuating process. The hardest thing is to get started but once you are in a routine, I hope the rhythm of revision will see you through to the end.

It is good to have an end-product to show for your work. This is why revision notes or flash cards are such a good idea. You can see them on your desk every day, you can see how they are growing or being used and this can motivate you to keep going! Have a go at working through some of the past paper questions in the booklet. Mark your answers using the provided mark schemes. Can you show evidence to yourself that you are making progress?

Revision can feel like a slog and in some ways it is….. There is no getting round that. But if you can make it fun, you are much more likely to keep going and all the effort will be worth it in the end.

You have a long summer holiday ahead of you. I can just about remember what that particular summer was like for me in 1987. (I know it’s hard to believe but I am so old I didn’t even do GCSEs but their predecessor O levels) I remember dancing to New Order, U2 releasing Joshua Tree and a lot of parties with some very special people. It will be the same for you I’m sure so work hard now so you can enjoy it with not a drop of guilt……

Zondle – does this help your revision?

I have posted a few games on Zondle with questions on the various topics I have written about in the past few weeks.  I don’t know if you can search for my material on Zondle but my username is PMG_Biology.  (Boys in my D division received an email with a class code so they can sign up and see all my material – thanks to the four boys who have signed up so far).  I know that two of you have played some of these games and I can see the scores you got on my tests.  The feedback I also get is which questions you have not scored well on which could be useful in giving me feedback about your learning.

If you think these revision games are useful, please can you comment below or tweet me with your thoughts?  I can easily add more material to this site, but won’t do so unless the games are proving useful.  Over to you…….

Immunity: Grade 9 Understanding for Biology IGCSE 2.63B

The most complicated topic in the human transport topic is certainly immunisation.  In a previous post, I said you should be able to answer the following two questions:

Why is it that the first time your body encounters measles virus, you suffer from the disease measles?  Why will someone who has had measles as a baby (or been immunised against it) never contract the disease measles even though the virus might get into their body many subsequent times?

I thought in this post I should attempt to expand a little so as to provide answers to these two important questions. This understanding is quite complex for IGCSE but you cannot really see how immunity works unless you can work through each stage in the process.

Let’s pretend you are a new born baby and you get measles virus particles into your bloodstream from contact with an infected person.  Remember viruses are not living organisms as they are not made of cells and have no metabolism.  All they are is a tiny particle made of DNA (genetic material) surrounded by a protein coat.

Look up a picture showing the structure of a virus particle in Google.  This one comes from science learn.org.nz

Virus generalised structure

The “spikes” on the surface of the virus particle are proteins that are essential to allow the virus to get inside a host cell.  But they can also act as antigens allowing the immune system to recognise the virus as a foreign object and so mount an immune response to it.

In the body there are hundreds of billions of white blood cells called B lymphocytes.  Each B lymphocyte is able to divide by mitosis over and over again to form a clone of cells called plasma cells.  These plasma cells secrete a type of protein called an antibody which has a shape specific to the shape of the antigen such that it can bind to the antigen and neutralise it.  (Can you think of another example in the specification where the shape of a protein is essential to its function?)

Now here is the first key piece of information needed in understanding immunity. Each B lymphocyte is only able to produce an antibody molecule with one particular shape.  So the reason you need hundreds of billions of lymphocytes is to be able to produce antibodies that have the correct shape to combat hundreds of billions of possible shaped antigens on a lifetime of pathogen exposure.

Go back to your newborn baby exposed to measles virus.  There might be only a handful of B lymphocytes in the babies’ body that just happen to be able to produce a shape of antibody specific to antigens on the surface of the measles virus.  Before any antibodies can be produced, the “correct” B lymphocyte has to come into contact with measles virus particles and be activated.  It then has to divide many times by mitosis to form a clone of plasma cells and the plasma cells have to differentiate and start producing antibodies.  This whole process is called the primary response (first exposure hence primary) and it may take up to 8 days before any antibodies start appearing in the babies’ blood.  What are the measles virus particles doing all this while?  Well they are infecting host cells, damaging them and causing disease.  This is why the baby will suffer from the disease measles.

The second key piece of information for immunity is this:  when the B lymphocyte that has been activated divides by mitosis to form a clone, not all the cells produced form antibody-producing plasma cells.  About 25% of the clone just remain as lymphocytes and are called memory cells.  This is because they are long-lived cells that account for immunological memory.

Let’s pretend the baby gets better from measles due to the antibodies produced in the primary response.  What happens if years later, the child goes to school and meets measles virus again for a second time?  You all know that the child won’t get the disease measles this time.  This is because the immune response is different second time round – the secondary response.  The secondary response to antigen is quicker (no 8 day delay), larger (more antibodies made) and lasts for longer.  This is because in a secondary response there are not just a handful of B lymphocytes in the body capable of making antibodies to combat measles virus.  There are now millions of memory cells left over from the primary response that can all immediately “leap into life” and start making antibodies.  These antibodies will be produced so quickly and in such large numbers that the virus particles will be eliminated before they have time to cause harm and disease.   No harm caused to host cells therefore no disease measles this time round!

Finally, you know that you can have immunity to measles without having had the disease.  This is because everyone in the UK sitting GCSE exams this summer will have been immunised against measles virus as a baby.  You were injected with antigens from the surface of measles virus particles when you were a baby.  These antigens by themselves could not give you measles (why not?) but they did cause a primary response to occur and memory cells to measles antigens be formed.  So now if you do encounter measles virus, your body will mount a secondary response and you won’t get the disease.  #result

Common misconception:

When answering questions on this topic in exams, candidates often think that it is the antibodies produced in the primary response that are left over to stop you getting measles later in life. Look at a graph showing primary and secondary responses to antigen such as the one below.

This graph shows how antibody concentration in the blood changes in the primary and secondary immune response.

This graph shows how antibody concentration in the blood changes in the primary and secondary immune response.

Antibodies are proteins and you can see they have a half-life in the blood of a few weeks.  (The liver breaks down proteins in the blood as one of its many functions)  So all the antibodies from a primary response will have been removed within a few months of the first exposure. Immunity can last a lifetime and this is because memory cells can survive as long as you do.  Unlike antibodies they can hang around in your blood and lymph nodes for the rest of your life.  If you live to be a hundred, you still won’t catch measles more than once.

This is a tricky topic so do please comment on this post if you have any questions.  Work hard at revision – it will be worth it in the end…..  (At least with Biology revision, it is fascinating stuff isn’t it?)