There are several specification points in the iGCSE syllabus that mention controlled experiments. For example in the E Michaelmas work, there are two:
- describe how to carry out simple controlled experiments to illustrate how enzyme activity can be affected by changes in temperature
- describe simple controlled experiments to investigate photosynthesis showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll
So what exactly do they mean by a controlled experiment?
In an investigation you need to know what is meant by the independent variable and the dependent variable. To put it in the simplest terms, the independent variable is the thing you are altering; the dependent variable is the thing you are measuring.
In any experiment there will also be a range of other variables that might affect the dependent variable. For example in the first bullet point above, enzyme-catalysed reactions are not only affected by changes in temperature. They can be affected by pH, by the concentration of enzyme and by the concentration of substrate. A controlled experiment is one in which these other variables – now called control variables – are kept constant to ensure it is a fair test. So if you were devising an experiment to investigate the effect of temperature on an enzyme reaction, make sure the pH is kept constant by using pH buffers and that the enzyme concentration and substrate concentration are exactly the same in every experimental set up.
It’s as simple as that…….
I hope the paper goes really well this morning and all the hard work you have done pays off. This is the culmination of three years of work and there is little to be gained from any last minute cramming this morning. So I would try to relax and summon up your concentration and will for this one last exam.
- Have a proper breakfast.
- Make sure you are properly hydrated – fruit juice this morning and water in the exam (if allowed)
- Read the question carefully – every word in every question. If there are data given in the question, take your time to make sure you understand what the data means before starting to answer any questions.
If there is a risk for you today, it is that it is easy to get too blasé and slapdash in the final exams in a set. You have sat many papers in the last few weeks and you can forget the importance of the exam technique and question-answering skills you had honed in the middle of May. Please don’t let that happen to you. Focus for the entire 60 minutes, think before writing and don’t waffle.
- Any question with more than 3 marks should be answered with bullet points.
Enjoy this morning, enjoy showing the examiner your mastery of iGCSE Biology and then enjoy a long, well-deserved summer holiday.
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.
The topic of plant transport can appear quite complicated but you will see from your past paper booklets that the questions examiners tend to set on it are much more straightforward.
The key piece of understanding is to realise that there are two transport systems in plants, learn their names and what they transport.
- Xylem vessels move water and mineral ions from the roots to the leaves.
- Phloem sieve tubes move sugars, notably sucrose, and amino acids around the plant. Both of these molecules are made in photosynthesis in the leaves and so can be transported from the leaves to the areas in the plant where they are needed.
Water is needed for photosynthesis of course in the leaves (remember that rain water cannot enter leaves directly because of the waxy cuticle on the surface of the leaf). All the water that is used in photosynthesis is absorbed in the roots from the soil and moved up the plant in the xylem vessels. Minerals such as nitrate, phosphate and magnesium ions are also required in the leaves for making amino acids, DNA and chlorophyll respectively. These minerals are moved up the plant along with the water in the xylem.
How does water enter the roots from the soil?
Water molecules can only enter root hair cells (and indeed can only cross any cell membrane) by one mechanism and that is OSMOSIS. If you understand the mechanism of osmosis that is great but don’t worry too much about it at this stage. You need to know that osmosis is a net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane.
How do mineral ions enter the roots from the soil?
Minerals are pumped into the root hair cells from the soil using ACTIVE TRANSPORT. This a process that uses energy from respiration in the cell to move ions against their concentration gradient (so from a lower concentration in the soil to a higher concentration inside the cell cytoplasm.)
What do we know about xylem vessels?
The cells that water and minerals are transported in are called xylem vessels. They have some interesting specialisations for this function. They are dead cells that are empty with no cytoplasm or nucleus. The end walls of these cells break down to provide a continuous unbroken column of water all the way up the plant. The cell walls of xylem vessels are thick and strengthened and waterproofed with a chemical called lignin.
What causes the water to move up the xylem?
Clearly it will take energy from somewhere to move water against gravity all the way up a plant from the roots to the leaves. The key question here is what provides the energy for this movement? There is no pumping of water up the plant and indeed the plant spends no energy at all on water movement. The answer is that it is the heat energy from the sun that evaporates water in the leaves that provides the energy for water movement. When you combine this with the fact that water molecules are “sticky” – they are attracted to their neighbours by a type of weak bond called a hydrogen bond – you can see that the water evaporating into the air spaces in the leaf can pull water molecules up the continuous column of water found in the xylem. The proper adjective for this stickiness is cohesive and you should know the name for the evaporation of water in the leaves (Transpiration)
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.
There are just a couple of days left for the boys of bD1:4 to accumulate “Zollars” for the star prize of a lunch at Zero 3 next week. Just to remind you, I will buy lunch (whatever you want) for the boy in my division with the most zollars at the start of our first Biology lesson of the new term.
If you haven’t yet signed up to Zondle, please do so as my revision games should be useful. It is free, you don’t need to add your real name (although please choose an id that I can tell who you are…..) and get playing. Use the class code I emailed you so you can see exactly what I want you to do. You get zollars for playing games but also for the scores you get and you can play each game as many times as you like.
Just for your information, there are 11 boys with zollars already on the board and four players with over a 1000 zollars to their name……