There is a specification bullet point in bold (paper 2 only) about blood clotting and the role of platelets, and students are sometimes not sure to what level of detail is needed for a full GCSE understanding of this topic. Well the good news is that the only questions I can recall are very straightforward indeed. But in this post I will give you a little more detail than the minimum needed for A* answers so that you can be confident you are completely clear on this part of the specification.
Why does blood need to clot?
Capillaries have a very thin wall (one cell thick in fact) so can easily tear and get damaged. This means that damage causes blood to leak into tissues forming a bruise and if the skin is broken, blood can be lost from the body entirely. Blood clotting is the response in the blood that ensures that blood loss is minimised and also that the time micro-organisms have to get into the blood stream is kept as short as possible. The surface of the skin is covered with millions of pathogenic organisms (mostly bacteria) all waiting for the chance to get into the blood stream through a cut or tear.
What are platelets?
Platelets are small fragments of cells found in bone marrow that then get into the blood and are carried round in the plasma. They are not entire cells as they lack a nucleus but they do play an essential role in blood clotting.
How does blood clotting work?
When the lining of a capillary is broken, platelets initially stick to the site of damage. They then trigger a series of reactions in the blood plasma that causes a clot to form. The details of how this works are too complicated to go into here but the basic idea is that in the blood plasma are a whole family of proteins called clotting factors. There is a cascade of reactions such that one clotting factor is activated and in turn, activates the next in the sequence. The final reaction in the clotting cascade is that a soluble protein called fibrinogen is converted into an insoluble fibrous protein called fibrin. Fibrin forms a mesh around the platelet cap covering the site of damage and this mesh traps red blood cells forming the final clot.
The roman numerals on the diagram above refer to clotting factors and each in turn is activated. You can see the final stage of the cascade is that soluble fibrin is converted into the fibrin clot.
Many of you will know of the disease where blood doesn’t clot called haemophilia. The commonest type of haemophilia is a genetic disease where patients cannot produce clotting factor VIII. This means one step in the clotting cascade does not work and so the blood cannot clot normally.
(Extension idea: find out the link between Haemophilia, the British Royal family and the 20th century history of Russia)
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.
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….
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.
The other thing to focus your revision on for paper 2 are the small number of points (in bold in the specification) that could only be tested in paper 2.
IGCSE Biology specification – bullet points in Bold
- 2.10 understand how the functioning of enzymes can be affected by changes in active site caused by changes in pH
- 2.14 understand the importance in plants of turgid cells as a means of support
- 2.23 understand that a balanced diet should include appropriate proportions of carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre
- 2.25 understand that energy requirements vary with activity levels, age and pregnancy
- 2.32 describe an experiment to investigate the energy content in a food sample.
- 2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms.
- 2.40 understand that respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen depends on the intensity of light
- 2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator
- 2.51 describe the role of phloem in transporting sucrose and amino acids between the leaves and other parts of the plant
- 2.61 understand that vaccination results in the manufacture of memory cells, which enable future antibody production to the pathogen to occur sooner, faster and in greater quantity
- 2.62 understand that platelets are involved in blood clotting, which prevents blood loss and the entry of micro-organisms
- 2.88 understand the function of the eye in focusing near and distant objects, and in responding to changes in light intensity
- 2.89 describe the role of the skin in temperature regulation, with reference to sweating, vasoconstriction and vasodilation
- 3.5 understand the conditions needed for seed germination
- 3.6 understand how germinating seeds utilise food reserves until the seedling can carry out photosynthesis
- 3.10 describe the role of the placenta in the nutrition of the developing embryo
- 3.11 understand how the developing embryo is protected by amniotic fluid
- understand the meaning of the term codominance
- 3.33 understand that the incidence of mutations can be increased by exposure to ionising radiation (for example gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (for example chemicals in tobacco).
- 4.8 describe the stages in the water cycle, including evaporation, transpiration, condensation and precipitation
- 4.10 describe the stages in the nitrogen cycle, including the roles of nitrogen fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria (specific names of bacteria are not required).
- 4.15 understand the biological consequences of pollution of water by sewage, including increases in the number of micro-organisms causing depletion of oxygen
- 5.7 understand the role of bacteria (Lactobacillus) in the production of yoghurt
- 5.8 interpret and label a diagram of an industrial fermenter and explain the need to provide suitable conditions in the fermenter, including aseptic precautions, nutrients, optimum temperature and pH, oxygenation and agitation, for the growth of micro-organisms
- 5.16 understand that the term ‘transgenic’ means the transfer of genetic material from one species to a different species.
- 5.20 evaluate the potential for using cloned transgenic animals, for example to produce commercial quantities of human antibodies or organs for transplantation.
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……
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%)
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.
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
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.
The 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.
Cloning is one of the harder topics to understand. Work through the topic making revision notes using the textbook and then have a go at the Zondle quiz – link above.
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
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……
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…….