Apologies for a few days without many Biology posts on the blog. Three day trip up to St Andrews to play the beautiful Old Course. I hit a shot off the road on the road hole yesterday so today the aim is to get into a few pot bunkers…..
The Carbon cycle should really be much simpler to understand than the Nitrogen cycle I posted about yesterday. This is because the processes involved in moving carbon atoms from one compartment to the next in an ecosystem are more straightforward. There are four processes mentioned in the specification and you need to make sure you understand each.
If you want to draw a carbon cycle from scratch to check you understand it, follow the procedure below.
1) Draw the following boxes showing where carbon atoms are found in an ecosystem – CO2 in air, carbon compounds in plants, carbon compounds in animals, fossil fuels and limestone, detritus in soil
2) Draw arrows linking the boxes with the following labels: photosynthesis, respiration, feeding, combustion, death and decay, death and no decay
That’s about as complicated as it gets.
Warning: Do not under any circumstances draw an arrow from the detritus in the soil directly to plants. Plants do not absorb any carbon containing molecules from the soil into their roots. Honestly, please believe me they don’t however much you want them to…. It would make the cycle more straightforward but they don’t – sorry. The only carbon-containing molecule plants absorb from their environment is CO2 and that as you all know is absorbed from the air in leaves in the process of photosynthesis.
Zondle quiz on carbon cycle to follow in due course: keep working hard! (I am jealous of those of you in Portugal at the moment although weather has been fine today in Northants!)
Here is my paragraph about each of the four sets of bacteria involved in the Nitrogen Cycle. 
1) Decomposers (Putrefying Bacteria) These aerobic bacteria live in the soil. When an organism dies decomposers digest the proteins and DNA that are found in the cells of the organism and produce ammonium ions (NH4+) as a waste product. Decomposers will also break down the molecules in animal faeces and will decompose the urea in animal urine into ammonium ions.
2) Nitrifying Bacteria These are also aerobic bacteria that live in the soil. They get their energy by converting ammonium ions into nitrates (via an intermediate ion called a nitrite). Nitrifying bacteria are essential for the Nitrogen cycle because the nitrates they form are the ions that plants will absorb through their roots. Nitrates will be used by the plant to make amino acids, proteins and DNA and these can pass up food chains.
3) Nitrogen-fixing Bacteria The picture now gets a little more complicated. Nitrogen-fixing bacteria can convert atmospheric nitrogen found in air spaces in the soil into ammonium ions (see diagram above). These bacteria live in the soil and respire aerobically like all the bacteria mentioned so far. Some nitrogen-fixing bacteria have formed a symbiotic relationship with a family of plants that includes peas, beans and clover. These plants are called leguminous plants and have small nodules in their roots that contain the nitrogen-fixing bacteria. The relationship is a mutualistic one as both parties benefit. The plant benefits as these bacteria produce nitrate ions that the plant can absorb and use to make proteins and the bacteria benefit as they are protected from soil predators and have a stable environment in which to live.
4) Denitrifying bacteria
These are anaerobic bacteria that thrive in conditions where there is little oxygen in the soil. This often happens when the soil becomes water-logged so all the air spaces are flooded. Denitrifying bacteria are “bad news” for Nitrogen cycle as they get their energy by taking nitrates from the soil and converting them into nitrogen gas. This obviously reduces the nitrate available to the plants with roots in the soil and this is one of the reasons farmers like to keep soil well aerated for their crops.
I will post a quiz on Nitrogen cycle on Zondle later this evening and so if you want to test your understanding of this potentially tricky topic, I would suggest you have a go at my quiz. There are also plenty of past paper questions on Nitrogen Cycle in the red question booklet.
I will make a blog post on each of the three “cycles” you need to know about for iGCSE. By far the most complicated is the Nitrogen cycle, so we might as well start there….
The first bit of understanding you need to is be clear the difference between how energy moves through an ecosystem and how matter (i.e. atoms) are exchanged between organisms and their environment. Energy in the ecosystems moves in a linear flow: there is no recycling of energy. The energy comes in at one end (in the producers through the process of photosynthesis) and is ultimately all lost as heat to the environment through the process of respiration. There is no possible way energy can be recycled. The “circle of life” that students like so much from Disney certainly does not apply here…. People find this idea very difficult to appreciate. All the time students will tell me that the energy in dead plants and animals goes into the soil and is then absorbed through the roots of plants: “it’s the circle of life sir” they earnestly tell me. And in the words of the late, great Amy Winehouse, I say “NO,NO,NO”…
Matter on the other hand is recycled through the ecosystem. The individual atoms that make up your body (H,O,C,N,S etc.etc,) have all been in other organisms and indeed will be again in the future. You took them in through your food and use these atoms to build the molecules that make up your cells. But ultimately all these atoms will leave your body either through metabolic processes or when you die and are decomposed. You could draw up a cycle for any of the atoms that are found in living things but your specification only requires you to understand two. How are carbon atoms cycled – the Carbon cycle – and how are nitrogen atoms cycled – the Nitrogen cycle. (You will also look at the Water cycle as well…..)
Things to understand about the Nitrogen Cycle:
1) Which molecules in living things contain nitrogen atoms?
Well the answer is fairly simple. Proteins are polymers of amino acids. Amino acids all contain an -NH2 group (amine group) and so Nitrogen is found in proteins. The bases in DNA (Adenine, Cytosine, Guanine and Thymine) are described as nitrogenous bases and so they contain nitrogen too.
2) Where are nitrogen atoms found in the ecosystem other than in the molecules of living organisms?
This is more complicated. Nitrogen gas makes up 78% of the atmosphere so clearly there is a lot of nitrogen in the air. In the soil, there will be urea from the urine of animals and urea contains nitrogen. There are also a range of ions found in the soil that contain nitrogen: the two most important are ammonium NH4+ and nitrate, NO3-. (I don’t know how to do subscript and superscript in WordPress and so you will have to excuse the rather ugly molecular formulae)
3) How do nitrogen atoms move from the abiotic (non-living) parts of the ecosystem and into the organisms?
There is only one way nitrogen atoms can move from the abiotic environment and into the organisms in an ecosystem. This is via plants that can absorb nitrate ions from the soil in their roots. This is a slight simplification but it will do at the moment. Look at the diagram above and find the arrow that shows assimilation of nitrates from the soil into plants.
4) How many different kinds of soil bacteria are involved in cycling nitrogen?
You need to know about four different kinds of bacterial that live in the soil that play a role in recycling nitrogen. Use the diagram above and your notes to describe the role each of these organisms play in the cycling of nitrogen atoms in the ecosystem.
I am off to have my supper: another post about these bacteria will appear here later tonight or tomorrow. Please don’t read it until you have tried to write down a paragraph on each of the four types of bacteria in the bullet point list above.
Today is one of the few days where I can pinpoint exactly where I was and what I was doing 25 years ago. The Cobblers were playing Sheffield United at the old County ground on April 15th 1989. It was a beautiful sunny day, there was a big crowd and a good match. We lost 2-1 as I recall but they were chasing promotion and had a fearsome forward in Brian Deane. At half time there was a stadium announcement to say that rioting had broken out at the FA cup semifinal at Hillsborough and that match had been abandoned.
When I got home I remember watching the TV scenes of devastation and chaos. 25 years later, there has still been no justice for the 96 people who like me went to watch a football match that afternoon but unlike me, never returned. The South Yorkshire Police have still not been properly held to account for their lies, repeated year after year about what actually happened that day. The media reporting and political reaction from the Conservative government was atrocious, shameful and contemptible. My thoughts today are with the families of the 96 who continue to fight against officialdom for justice for their loved ones. A terrible, terrible day for this country.
For any young people reading this post, April 15th 1989 was one of those days where when you look back, you realise that your world was changed. Any naive trust that I might have had as a youngster that the police were always honest and that you could believe things that were reported in the newspapers was lost that day. My great sadness is that nothing in the 25 years since has convinced me that anything has changed for the better.
I have now taught two terms of this new post-16 Biology qualification to two year 12 groups. It is very early to be making judgements on the quality of the course but there is one thing about this course that is over-whelming: it is MASSIVE…. I don’t know if I am losing track of the pace of my post-16 teaching but I cannot ever remember in 20 years having to rush through so many topics.
The content is pretty standard and comparable to A level I would say, but why is there so much of it? It irritates me beyond belief to feel that I have to rush in the classroom just to cover the basics of the syllabus. If the aim is to promote independent learning among students then the sheer volume of the specification has the absolute opposite effect. My understanding of “independent learning” is not just to let the students do it in their own time with no input/help/feedback from the teacher. I want students to learn independently and to take initiative for their learning but I also want to be able to help/guide/assess their learning and there simply isn’t time to do this properly.
So please Cambridge, when you come to review this specification for the next draft, please can you remove about 25% of the content? There is nothing “rigorous” about a massive specification: it rewards a rather dull and limited approach to learning and removes the time needed for a less didactic teaching style. I want our post-16 biologists to finish their course with a deep understanding of the beauty of the natural world, having had the time to make interesting links between subject areas and with a skill-set as learners that will set them up for the next stage of their lives. I remain to be convinced that this course is going to manage that.
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…….
A great reminder of how difficult it must be for a tree to move water up xylem vessels to such a great height. Plant biology is always way more complex and confusing than you would think.
PMG Biology 