Diffusion, Active Transport and Osmosis: A* understanding for iGCSE Biology 2.12 2.13 2.14 2.15

This post is going to describe some of the ways molecules can cross the cell membrane.  (For Eton students revising for Trials, diffusion and active transport are found in the F block syllabus, osmosis comes in E Block)

Diffusion is the simplest to understand.  Diffusion does not even need a cell membrane to occur.   In the example below the dye molecules will move randomly in the solution.  As the dye starts in one place, these random movements will mean that slowly spread out until an equilibrium is reached.  This movement of the dye from the region of high concentration to the low concentration is called diffusion.

diffusion_1

When considering diffusion into a cell, if the cell membrane is permeable to a particular molecule then the random movements of the molecule will mean that there will be a net (overall) movement from the higher concentration to the lower concentration down the concentration gradient.

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Key Points about diffusion:

  • Always happens down a concentration gradient (from a high concentration to a lower one)
  • Never requires any energy from the cell – it is a passive process

Active Transport is a process that will move molecules into a cell against the concentration gradient – i.e. from a low concentration to a high concentration.  This “pumping” of the molecules against the gradient requires energy from the cell and of course this energy comes from respiration.

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You can see from the diagram above that active transport is working against the concentration gradient, is using energy from inside the cell (actually a molecule made in mitochondria in respiration called ATP) and that a specific transport protein is involved in the cell membrane.  This protein will have a binding-site that is specific for a particular molecule and the solute molecule to be transported will collide with the transport protein due to random movement.  Energy from the cell can cause the transport protein to change shape such that the solute is released on the other side of the membrane.

Can you think of another area of the iGCSE syllabus which features collisions between a specific binding-site on a protein and a certain other molecule?  Linking ideas is a key characteristic of the A* Biologist!

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Osmosis is the hardest of these processes to understand properly, especially as an iGCSE student when you are often told an over-simplified account that does not make sense….  Let’s try to simplify it in a way that does make sense.

Firstly it is only water molecules that can move by osmosis into and out of cells – never anything else.  Indeed osmosis is the only way water can cross a membrane – it never moves by diffusion or active transport.

Osmosis is a passive process – it never needs any energy from the cell’s respiration and the only energy involved is the kinetic energy of the water molecules.

Osmosis can only occur through a partially permeable membrane.  All cell membranes are partially permeable and this means they let small molecule like water through but prevent the diffusion of the larger solute molecules.

osmosis_1

The water molecules on both sides of the membrane in the diagram above will be moving around randomly.  They will occasionally hit one of the pores in the membrane and so pass across the membrane.  This movement will be happening from left to right and from right to left.

But….

The presence of the sucrose (solute) in the solution on the right means that some of the water molecules on that side of the membrane are less able to move.  This is because they are temporarily attracted to the solute molecules by weak hydrogen bonds.  So their kinetic energy is reduced and this makes them less likely to randomly collide with the pores in the membrane.  The presence of the solute on the right means that water molecules on the left on average are more likely to collide with the membrane than the water molecules on the right and this leads to an overall movement from left to right.  This net movement of water molecules from the dilute solution to the more concentrated solution through the partially permeable membrane is called osmosis.

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This diagram has the two solutions reversed so in which direction will osmosis happen here?  Thats right from right to left.  You can see the hydrogen bonds attracting water molecules to the solute – these are the ones that lower their kinetic energy overall.

You might even have been taught about osmosis with reference to the water potential of a solution.  The water potential of a solution is just a measure of how much kinetic energy the water molecules in a solution possess.  So a dilute solution will have a high water potential, a concentrated solution (with lots of dissolved solute) a lower water potential.

Osmosis is the 

  • net movement of water
  • through a partially permeable membrane
  • from a solution with a high water potential (a dilute solution) to a solution with a lower water potential (a concentrated solution)

Biological examples

Diffusion 

  • Oxygen diffuses from the air in the alveolus into the blood
  • Carbon Dioxide diffuses from the air spaces in the leaf into the palisade mesophyll cells of the leaf
  • Glucose diffuses from the blood into an actively-respiring muscle

Active Transport

  • Nitrates are pumped from the soil into root hair cells by active transport
  • In the kidney, glucose and other useful molecules are pumped from the nephron back into the blood by active transport.
  • In nerve cells, sodium and potassium ions are pumped across the cell membrane to set up the gradients needed for a nerve impulse

Osmosis

  • Water enters root hair cells from the soil by osmosis
  • In the kidney, water is reabsorbed from the nephron by osmosis.
  • In the large intestine, water is reabsorbed from the colon back into the blood by osmosis

There are many many more examples of each process, but this should be enough to be going on with…….

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Two days to go before the EdExcel IGCSE Biology paper 2

I’m writing this on Friday evening and the 2018 IGCSE Biology paper 2 is on Monday.  So you are almost there…..

What can you do in the final two days?

  1. Look over the topics that can only appear in paper 2 and make sure you understand them as well as you can.
  2. Look at my post with some guesses as to what topics were not assessed on paper 1 and make sure you understand them as well as you can.
  3. One last work through those topics in the specification where you know your knowledge or understanding is not great (don’t waste time this weekend going over things you definitely already understand!)
  4. Rest properly, get two proper nights’ sleep so you are relaxed and ready to THINK in the exam on Monday.  You will need some downtime this weekend to enjoy yourself and switch off thinking about exams…..

Good luck to you all!  Your study of GCSE Biology is almost over… I will have my fingers crossed that the exam paper on Monday allows you to demonstrate the detailed knowledge and understanding you have built up

Genetics follow up: A* understanding for IGCSE Biology

In an earlier post, written for my own Y11 students at a previous school, I asked them to check that they knew the answers to 5 questions before attempting to answer any practice questions involving inheritance and genetics. I think that as this blog is now read by a much wider audience, it is probably time to answer the 5 questions myself! Here goes……

1) What is the difference in meaning between a gene and an allele?

People who I have taught know that there is just one thing that every A* student should memorize for IGCSE Biology and that is the definition of a gene.

“A gene is a sequence of DNA that codes for a single protein”

Genes are located on chromosomes and exist in alternate versions called alleles. So for example in pea plants there is a gene that codes for a protein that determines the height of the pea. This gene exists in two possible versions: a T version that makes the plant tall and a t version that makes the plant short. These alternate versions of a single gene are called alleles.

2). Why does the genotype of a person, plant or rabbit always contain two alleles for each gene?

In order to understand this, you need to understand something about chromosomes. Humans, plants and rabbits are all diploid organisms – this means that they have pairs of chromosomes one inherited from mum, one from dad. Because genes are found on chromosomes, this means that a genotype (combination of alleles) will also have two alleles. Alleles are versions of genes, genes are found on chromosomes and chromosomes come in pairs! Simple…

3) What is different about the genotype of a gamete compared with other body cells? Why are gametes different?

Gametes only contain one allele for each gene. This is because gametes are cells that do not contain pairs of chromosomes like every other cell in the body. Gametes are haploid – they only have one member of each pair of chromosomes. One chromosome per pair means only one version of each gene…. Gametes have to be different because they have a different fate or destiny to every other body cell. (Just typing the word destiny means I can hear the Star Wars theme as clear as anything in my head!). Luke’s destiny was to unite the light and dark sides of the force. A gamete’s destiny is less exciting but it is to fuse with another gamete in the act of fertilisation. If both gametes had pairs of chromosomes like every other body cell, then the act of fertilisation would result in a doubling of the chromosome number in every generation and that is clearly unlikely to do anyone any good!

4) How would you explain what is meant by a recessive allele?

A recessive allele is always given the lower case symbol, for example t. The best way to explain what is meant by a recessive allele is to say that recessive alleles only determine the phenotype (the appearance of the organism) if there are two of them. An individual with two identical alleles for a gene is described as being homozygous so a recessive allele will only determine the phenotype in a homozygous individual. In the example I have used so far, because the dwarf allele, t is recessive, the only genotype possible for a short pea plant is tt.

5) What does it mean if two alleles are codominant?

Codominant alleles are alleles that both contribute to the phenotype in a heterozygous individual. Heterozygous is the adjective used if the two alleles present are different to each other.

A good example is the genetics of the ABO blood group system in humans. There are three possible alleles present for this gene in the human population: IA, IB and IO.

The IA allele is dominant to the IO allele.

The IB allele is dominant to the IO allele.

But if you are heterozygous with the genotype IAIB then you have an intermediate blood group called AB. Both alleles are contributing to your blood group and so you are neither blood group A, nor blood group B but a different phenotype called AB. This is because the alleles IA and IB are codominant to each other.

This is a complex topic (not tested at all in the 2018 Paper 1B) so it is worth trying to get your head around all the jargon here over half term…….

If you have any questions at all, please ask me in the “Leave a Comment” box at the foot of this post.

EdExcel IGSE Biology Paper 2B 2018 predictions

I hope readers of my blog felt that the Biology paper 1B went well earlier this week.  For what it is worth, I thought it was a good but challenging paper but there is nothing at all on there that seemed unfair to me.  I am confident that it gave students a good chance to show the understanding and knowledge you have built up over three years.  But…..   you still have 33% of your marks still available to you, so my thoughts have now turned to paper 2B in a few weeks time.

I have been through the paper 1B carefully to look at which subject areas have been tested in the questions. This might allow you to focus your revision now onto topic areas that have not yet come up.  But the next point is very important:  there is absolutely nothing that says they cannot ask questions on the some of the same topics inboth papers.  Predicting exam questions is a dangerous game (!?!) and there is absolutely nothing in this blog post that should contradict the idea that you need to be fully clued up and ready for questions on every single specification point when you sit your paper 2B.

But there are some things we know about what is likely to appear in the paper 2B.

  • Firstly there are some specification points in bold that cannot be assessed in paper 1 and can only appear in paper 2.  I have a blog post that lists these so I won’t type them all out again.  Have a look here to see a full list of the bullet points in bold….
  • Listed below are some topic areas that were not tested at all in the summer 2018 paper 1B.  I would anticipate that several of these will definitely appear in paper 2B so if I were in your position, these would be my top priorities for revision in the coming weeks:

Variety of Living organisms and 5 Kingdom Classification 1.2

Viruses 1.2

Movement of substances into and out of cells – Diffusion, Osmosis and Active Transport  2.12, 2.13, 2.14, 2.15, 2.16

Transport in Plants – Xylem and Phloem 2.51, 2.52, 2.53, 2.54, 2.55. 2.56

Heart Structure and Circulatory Systems 2.63, 2.64, 2.65, 2.66

Human Nervous Systems 2.83, 2.84, 2.85, 2.86,

The Eye 2.87, 2.88, 2.89,

Hormones 2.90

Reproduction in Flowering Plants 3.3, 3.4, 3.5, 3.6, 3.7

Reproduction in Humans 3.8, 3.9, 3.10, 3.11, 3.12

DNA and Genetics – this must come up!!  3.13, 3.14, 3.15, 3.16, 3.17, 3.18, 3.19, 3.20, 3.21, 3.22

Ecology – Food chains and Webs, quadrats etc. 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7

Carbon and Nitrogen Cycles 4.8, 4.9, 4.10

Selective Breeding 5.10, 5.11

Genetic Modification 5.12, 5.13, 5.14, 5.15, 5.16

Cloning 5.17, 5.18, 5.19, 5.20

The focus of your revision now should be looking at past papers and mark schemes.  I know many of you have done this already for paper 1B.

If you want more past papers and mark schemes, I suggest you download them for free from this website: http://www.physicsandmathstutor.com/past-papers/gcse-biology/edexcel-igcse-paper-2/

Please remember there are posts on my website for every single specification point above.  Just type in the specification code in the search box and it should (?!) take you straight to the relevant page.  Please do ask me questions or leave comments at the bottom of any posts.  It may take a while for them to be visible as I have to “approve” them to remove spam.  You would be amazed at some of the stuff I seem to attract from the depths of the internet…..

Keep working hard and good luck!

 

 

How best to organize your revision time in the Easter holidays: some PMG tips

Tomorrow is the first day of the 2018 Easter holidays and so it is definitely a good time for Y11 students to think about how they are going to make the most effective use of the time available for revision.  I know that everyone works differently but I thought I would write a post to give you something to think about….  Please leave a comment at the end of this post if you find any of this useful.

Starting Points:

Easter holidays are a critical time for Y11 students.  The IGCSE and GCSE specifications contain so much content that the challenge for you is mostly one of being on top of so much material come exam day.  And your exams arrive so early in the summer term that there won’t be much time when you get back to school after the holidays.  So it has to be now! (The summer will be very long and there will be plenty of time for lazy days when nothing at all happens…. )

The hardest thing about revision is getting started.  If you can build up some early momentum, you will be able to keep your energy and enthusiasm up right through the holidays.  So why not start tomorrow…..?

How to organise a revision plan

  • It is vital you have a plan.  A little time this evening or tomorrow morning spent on getting organized will be time well spent!  This would be how I would do it if I could rewind the clock to the summer of 1987….. U2 released The Joshua Tree that year and I thought at the time it was the greatest music ever made….
  • Speak to your parents tonight and ask them to talk you through what’s coming up in the holidays.  What family commitments do you have that will impact on your revision schedule?
  • Count up how many days you will be able to work between now and when school starts again.  For my current students, we have 25 days before term starts.  But I think you need a few days off at times in the coming weeks.  So let’s say, there may be 20 “work” days.
  • This is how I suggest you organise a “work day”…. You always do work in the morning session (9am-11.30am) and then you choose either the afternoon slot (2-4.30pm) or the evening slot (7.30pm – 10pm).  This is an ambitious schedule as 5 hours work in a day is quite a lot.  But let’s aim high!
  • Each session is divided into 5 periods of 30 minutes.  You must work on a different subject in each period in a session.  Have a plan before the session so you know exactly what you are going to achieve.  The next bit is very important.  At the start of the session, switch your phone off and put it in a different room.  Start a kitchen timer (not a timer on your phone obviously!) and set it to 25 minutes.  Work at your revision until the alarm goes off.  Then you get a 5 minute gap (tea/check Snapchat) before the next slot starts.
  • The PMG schedule has two huge advantages:  firstly you have completed half your work for the day by 11.30am (which feels good, believe me) and secondly you can enjoy the time you are not working without feeling guilty…. This is a key component to a successful revision programme. Work when you are supposed to be working but then do other stuff, see friends, do some exercise , watch tv, relax.  Revision isn’t effective if you are tired or bored so both must be avoided at all costs.

Many of you have 9 or 10 subjects to revise.  If we assume 10 subjects to revise (which makes the Maths easier) 20 work days in the holidays and 10 periods per day (see schedule above), this means that you have 20 periods per subject in the holidays.  Do the calculation with your numbers so you know how many revision tasks you need to plan per subject.    My example gives you 8 hours 20 minutes per subject – your job is to make the very best use of this time so you gain the most from it.  I wish you all the very best of luck and don’t forget to leave a comment below.  Happy revising!

Revision strategies for IGCSE Biologists

Holiday revision: some PMG tips for Y11 students to maximise the effectiveness of their work 

The post above was written a couple of years ago for my Y11 students embarking on Easter revision.  I know that some of my current 5th formers are working this week so I thought I would re-post it.  Some things have changed since 2015 (notably the sad demise of Zondle) but the key principles remain…. I hope it helps!

Moving on….

The last time I changed jobs was in July 1997 after three hectic but happy years teaching at the Perse School in Cambridge.  But with a new position to take up in January at a different school in Cambridgeshire and having now passed the 20 year mark here, I feel like I am hurtling on my final lap towards a finish line in a couple of weeks.

Things now are very different from the heady days at the end of the Millenium.  I was probably at least two stone lighter in 1997 for a start….  No Facebook, no Twitter, no Snapchat, dial up Internet but no wifi, no Sky Sports, no texting…. What did we do all day?  But the last 20 years have been an amazing experience and I feel privileged to have worked with and indeed taught so many outstanding individuals.   I have made memories in my schoolroom, on the sports pitches, at Queens Club, in Dubai and in tutorials that will last a lifetime and I know I have spent the last two decades in a very special institution and one I will continue to hold dear.

But my overriding thoughts at the moment are most certainly looking forwards, not back and I am massively excited at the thought of a fresh professional challenge in 2018.  There are lots of people I will miss terribly in the New Year but I also know there is a vibrant, friendly and supportive community at my new school and I am looking forward to running a department.  And moving back to live in Northampton full time after 25 years away does feel like coming home……  I can’t wait!!

GCSE Results Day 2017

I wish you all the best of luck on results day.  Be pleased (but not too pleased) if you’ve done better than you expected and don’t be too disappointed if the reverse is true.  GCSE grades are important in determining the next stages of your education but do remember that in 5 years time, no-one will ever again be interested in your news from today….

I think the most important thing to think about today is this.  From around the beginning of April until the end of your final GCSE exam, you have been working independently, working things out for yourself, making notes and using web resources to improve your learning and understanding.   You have decided what to study and how best to achieve your learning goals.  And these are exactly the skills that will be needed to make a success of the next stage of your education!  So the question is “can you keep up these high levels of motivation/determination/organisation when you start your A level courses in September?”

But I hope you will find time for some celebrations before the serious stuff starts again in September!  Enjoy the rest of your summer.

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Well done!

Well done to everyone who has now finished their 2017 IGCSE Biology course.  I hope paper 2 was to your liking (it seemed pretty typical to me) and all the hard work you have put in over two years has finally paid off.  I guess that many Y11 students will be almost finished so it is nearly time for you to have a long, relaxing summer break.  I wish you all the best of luck when the results come out in August.

Can I ask for one small favour before you switch off schoolwork completely?

If you have time, please can you leave a comment below with any tips you have as to things you have done in revision that really helped you.  I imagine you have learned a great deal about how to motivate, organise and maximise your own learning over the past few months so why not leave a short comment to inspire/enthuse/help those that follow you……?

My page settings require me to approve any comment before it appears so don’t expect to see your comment straightaway as it may take an hour or two before it becomes visible.

But I hope you all have a brilliant summer and I wish you the best of luck!

IGCSE Biology 2017 paper 2 predictions

This post starts with a massive proviso of course.  Making predictions as to which topics might appear in a future exam is a very risky business.  The paper 2 you will all sit after half term can test material from the entire specification (including all the specification points in bold) and there is absolutely no guarantee that topics tested in paper 1 may not reappear in some form in paper 2.

So the proviso is this:  the only way to be fully prepared for paper 2 is to revise the entire specification so that you are prepared for whatever the examiners might throw at you.

But having said that, it seems sensible to focus your revision for paper 2 onto topic areas that were not examined in paper 1.  If I were in your position, these are the topic areas for which I would be doing most of my revision in the coming weeks:

  • Respiration 2.33 – 2.37
  • Gas exchange in Plants 2.38-2.43
  • Transport in Plants 2.49 – 2.56
  • Transport in Humans 2.57 – 2.66
  • Kidney 2.68 – 2.76
  • Reproduction 3.1 – 3.12
  • Food Chains and Energy Flow 4.4 – 4.7
  • Nitrogen and Water Cycles 4.8, 4.10
  • Human Influences on Environment 4.11 – 4.17
  • Food Production (including fish farming) 5.1 – 5.9
  • Selective Breeding and Genetic Modification 5.10 – 5.16

The bad news is that this list above still forms a large proportion of the extensive EdExcel IGCSE Biology specification but the good news is that there are PMGBiology blog posts on all the above.  So please use the search function on my homepage to find material to help you revise.

Practice papers and mark schemes are available online (and for my students on the school Firefly page)

Keep working hard – you are almost there and the summer to come will be long and restful….

Final advice before iGCSE Biology paper 1 on Tuesday

Well we are nearly there….. 24 hours to go until the main IGCSE Biology exam (worth 120 of the total 180 marks).  I hope you are all excited and looking forward with optimism to being able to show the examiner how much you understand from the extensive specification.  What should you be doing in this final 24 hours?

The most important thing is that you all get a good night’s sleep tonight.  Please do not stay up late cramming -it does not work! The evidence base for this is completely clear: you will perform better tomorrow with a normal night’s sleep tonight.  So please stop work an hour before you intend to go to bed, relax for an hour watching TV or socialising and then go to bed……

This afternoon you should want to look over a paper or two to familiarise yourself with the kinds of questions and the mark schemes.  I would have a final look again at the summer 2015 and summer 2016 paper 1B scripts.  Look at your answers and the kind of ways you lost marks.   In particular focus on the longer answer questions (for 4,5 or 6 marks) and look at the mark schemes.  Often there are marks available for saying obvious things but only if the correct vocabulary is used correctly.

Remember the PMG list of banned words:

Amount (oh no, please don’t ever write this in an exam – think – do you mean mass/volume/concentration?)

Level (do not talk about the level of something, you always mean “concentration” and concentration is a noun that actually means something!)

Substance – what substance are you talking about?  oxygen? glucose?

Gases – in questions on gas exchange or transport, please do not write about “gases” – say which gases you mean!

Nutrients – not really a banned word but one that needs very very careful use…..  A nutrient is a food molecule, for example glucose, an amino acid or a lipid. Nutrients like these are transported dissolved in blood plasma in mammals but if asked about it, don’t use the word but actually state which molecule you are talking about.  None of these nutrients are absorbed into the roots of a plant.  Plants absorb mineral ions (nitrate, phosphate, magnesium, potassium etc) through their roots into root hair cells by active transport.

Look over your revision notes a few final times to familiarise yourself with the key words.  Focus your final revision on the key areas that you know will definitely come up:  there will be a genetics question, there will be an experimental design question, the chances are that fish farming and fermenters will be there as usual……..

If you have worked hard for several weeks (and I know many of you have) you have little to fear in the exam from a lack of knowledge.  The thing to fear is losing marks due to rushing, due to not reading the question and due to not giving yourself time to think.  None of you will be rushed for time I promise, so please

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  • read every word in every question
  • give yourself time to think before answering – even easy marks can be lost by rushing!
  • plan longer answer questions to make sure you cover all the key points using the correct jargon – think before you write anything, “what are the key terms in this topic?” – and then make sure you use them correctly in your answer

Good luck!  By the end of tomorrow you will have completed two thirds of your GCSE exams in Biology and that should be a happy thought……