Structuring questions on speciation

The snowy Atlas Mountains of Morocco. As a cold environment unlike any other in Africa, these mountains support or supported many unique species trapped there when the last ice age ended.
The snowy Atlas Mountains of Morocco. As a cold environment unlike any other in Africa, these mountains support or supported many unique species or populations that were trapped there when the last ice age ended.

Speciation is how a species splits into two or more species.

What to say

1) Two populations of the same species are isolated geographically.
2) That means they can’t interbreed.
3) The two areas have different environments and different selection pressures.
4) Genetic differences develop between the two populations.
5) That could be due to selection or just random mutations.
6) Differences in allele frequency accumulate between populations.
7) Over time…
8) …the two populations are so different that they cannot mate and have fertile offspring.
9) That makes them different species.

What not to say

Don’t say genes, say alleles. Evolving new genes takes a long, long time-evolving different alleles, variant forms of genes you already have, happens much faster.

You should still emphasise that this process takes quite a long time, though.


This topic comes up in AQA A2 biology on unit 4, BIOL4. You can see a question on it here as question 8c, with mark scheme here.

An old problem

For many years after Darwin explained evolution, scientists had a problem: they couldn’t see how it could happen with sexual reproduction. If children are (basically) some sort of average of their parents, why don’t successful animals have their successful traits diluted down in their offspring until they disappear? Why don’t tiny subspecies with unusual, successful characteristics see those advantages wash away like a drop of coffee in a glass of water when they mate with other animals? Fleeming Jenkins, a Scottish engineer and creationist, explained this vividly just seven years after Darwin published Origin of Species-you can read his argument here, in the passages beginning “An illustration will bring this conception home.” and  “a very highly-favoured”. (Trigger warning: his choice of example is quite spectacularly racist.)

It wasn’t a question Darwin could give an answer to, and an answer didn’t emerge until the arrival of genetics in the early twentieth century explained what was going on. Dominant and recessive alleles explained the answer: people don’t have to be an average of their parents. Instead, they can completely take after one parent in some ways, and completely take after the other in other ones.


The picture of the Atlas Mountains was taken by Stephen Colebourne, and released under this license. All pictures used with thanks.


Structuring questions on antibiotic resistance

3060243118_cd0a6ec6eb_oThis is a classic A-level biology question topic: describing the steps of evolution that lets bacteria become resistant to antibiotics. You need to say:

1) Before bacteria are exposed to antibiotics, some may be resistant.
2) There is variation in the population, caused by things like mutation.
3) After the antibiotic course, the resistant bacteria survive.
4) They reproduce and have offspring.
5) The offspring have the same alleles, so they are also resistant.
6) There’s now a selection pressure in favour of antibiotic resistance.
7) Over many generations, antibiotic-resistant bacteria become more common in the population.

Of these, the easiest to forget are probably 1 and 5, but they’re important. Point 1 explains that there may be bacteria antibiotic-resistant for some random reason before the course starts, and point 5 emphasises that the offspring bacteria are resistant like their parents.

What not to say

Don’t say immune. Immune is a term for animals with immune systems, and it’s normally used about being protected from pathogens, not chemicals. Bacteria aren’t immune to antibiotics, they’re resistant to them.

Try not to say ‘same gene’ for step 5 in general evolution questions. Often it’s not that a whole new gene emerges. What emerges is just a new allele, a variant form of the old gene.

Example questions:
This comes up on the Edexcel biology A-level at unit 4, 6BI04/01. See an example here (it’s question 8b3) and the original mark scheme here.

It also comes up on the OCR biology AS-level at unit 2, F212. See an example question here (question 3c) and the mark scheme here.

Antibiotic resistance and you

Obviously, antibiotic resistance is a problem-it makes doing routine surgery riskier. And developing new antibiotics is hard-seriously, read that article, it’s well-informed and well-written. That’s partly because of what seems to be a lack of money in it compared to other drug types, partly because, well, a lot of smart people have tried their hand at it and failed. Given this, the NHS tries to make everyone who takes antibiotics finish the course, so that any partly-resistant bacteria get killed before they have time to evolve better resistance.


The picture of antibiotic pills-specifically amoxycillin, was taken by Flickr user Sheep purple, and released under this license. All pictures used with thanks.


8123803523_abb258b72e_h“I don’t think they make this decision out of a place of ignorance…It’s one they’ve thought about deeply. They’re reading all about this and making what they feel is the best-informed decision they can for their child.”

Tammy Murphy, superintendent of the Montecito Union School District in California, explaining her approach to vaccination. California has an incredible level of anti-vaccine paranoia, and 27.5% of the students in her care have parents who refuse to vaccinate them against a list of preventable diseases that includes hepatitis B, tetanus, HPV, polio, pneumonia, meningitis

Image taken by Damian Gadal, released under this license. All pictures used with thanks.

What’s the point of transpiration?

5445351125_aec7188390_oWhat is transpiration?

Transpiration is the evaporation of water from plant leaves. Did that explain things? No? Let’s try again…


Plants work like great big drinking straws

Plants need to get water up from the roots, which are in the soil where the water is, to the top of the plant, where the leaves which do photosynthesis are. They do that by transpiration.

At the top of the plant, water evaporates out of the leaves, through holes called stomata on the underside. This sucks water up from further down the plant through tubes called the xylem which creates a pull that reaches all the way down to the roots.

The pull of transpiration is so strong that not all the water coming up the plant needs to go out of the leaves. About 5% of it-that’s one twentieth-stays behind. That’s what plants live on: a lot of the water goes out of the plant, but some can be diverted to the cells in the leaf, for them to do photosynthesis with.

What goes through the stomata

The xylem is a network of tiny tubes. This is a microscope image of them. Like the folded part of a straw, the xylem can bend.
The xylem is a network of tiny tubes. This is a microscope image of them. Like the folded part of a straw, the xylem can bend.

You can figure this out by thinking about what the plant does and how. Let’s recap the equation for photosynthesis:

Carbon dioxide + water glucose + oxygen

Photosynthesis makes oxygen and uses up carbon dioxide, powered by energy from sunlight. So in daylight oxygen diffuses out of the stomata and carbon dioxide diffuses in. Water diffuses out by transpiration.

At night, the plant can’t do photosynthesis, but it still needs energy. So it does respiration, just like people and animals. Oxygen diffuses into the stomata, and carbon dioxide and water diffuse out

(Historical note: people used to get a bit worried about this and said plants didn’t belong in rooms where people slept because they could suck all the oxygen out of the room. They don’t use anything like that much oxygen! You’ll notice that people manage to sleep with each other without sucking all the oxygen out of the room and killing each other very well.)

When the stomata open

The stomata are the little holes, and the guard cells are on either side of them.
The stomata are the little holes, and the guard cells are on either side of them.

Many stomata close at night. That’s so the plant doesn’t waste water through them at a time when it can’t do photosynthesis. Stomata are opened and closed by the guard cells, cells bordering the gap on either side.

Back down to the roots

The roots help to bring the water up into the leaves, and they do that by sucking water in by osmosis.

It’s not just water that the xylem carries: it’s also minerals from the soil that plants need to make things like proteins, enzymes, DNA and chlorophyll.

A picture of the roots, stained to show the different tissues.
A picture of the roots, stained to show the different tissues.

Plants want those minerals a lot, so they actively transport them in from the soil: they grab them out of the soil and pull them from low concentration in the soil to high concentration in the roots. That’s called active transport, because it uses energy to drag the minerals against their concentration gradient into the plant.

That also helps water get into the plant. Why? Because having more minerals in the roots lowers the concentration of water in them. Water diffuses in by osmosis, following the minerals as they’re loaded into the xylem.

(What is osmosis? It’s how water moves across a cell membrane. Water moves to the side with a lower concentration of water-that means a higher concentration of things dissolved in the water. In practice, that means water moves to the side with more sugar, salt, minerals-things like that.)


The photo of a leaf was made by Flickr user Nicola, and the picture of stomata was made by Andrea Scauri. Both were released under this license.
The picture of root tissue was made by Ishikawa Shihchuan, the picture of xylem tissue by A.J. Cann and the picture of a smoothie by Flickr user Nomadic Lass, all released under this license. All images used with thanks.

Getting a pure dry sample of a precipitate


This post is targeted at the Edexcel iGCSE, but should be useful for any board.

A lot of the chemistry GCSE syllabus on every course is pure facts and theory: balance this equation, describe a covalent bond. But some is more practical, and it’s aimed at seeing if you remember how you did experiments in class.

A good example of this is the question: how do you separate out a precipitate once you’ve formed it? Practising this answer is useful, because it often becomes the answer to questions with a lot of marks. In the January 2012 and June 2013 2C papers, this topic was part of five-mark questions.

So, to get a pure, dry sample of the precipitate, you:

1) Filter the precipitate out with filter paper.

2) Wash the solid (you have to specify this) with distilled/deionised/pure water

3) Dry the crystals. Specify a method: leave them to dry, gentle heating or rubbing with filter paper.

They won’t accept sieving for step 1, that’s because the powder is too fine-grained. You need to specify a method for 3: any of the ones listed are sensible.

Under the right conditions, you can get DNA to precipitate out of a mixture of broken-up cells. It looks like a rather stringy mess, to be honest.
Under the right conditions, you can get DNA to precipitate out of a mixture of broken-up cells. It looks like a rather stringy mess, to be honest. (It’s the stuff on the sides that looks like spit.)

Why do you need to wash in step 2? Simple: the solution still has dissolved chemical in it. (In the silver chloride example, it’s dissolved sodium nitrate: the bits of the chemicals from the start that didn’t turn into a precipitate.) They’re in the water, so if the crystals have the original water left on them it’ll leave the unwanted sodium nitrate on your silver chloride crystals when the water evaporates off. You need to wash that water away with pure water.

Other things that might come up as a step 0 to those methods:

0) Naming the chemicals that you mix to form a precipitate. They need to be chemicals with one name of the product each in them, that are soluble in water (so they react and don’t mix up with the precipitate). For example, to make barium sulfate, you could use calcium sulfate (soluble) and barium chloride (also soluble). If you’re making a chloride or sulfate, you could also use dilute hydrochloric/sulfuric acid.

0) Giving a reaction. Same principle: the dissolved chemicals will be (aq) and the product will be (s). Example:

CaSO4 (aq) + BaCl2 (aq) → CaCl2 (aq) + BaSO4 (s)

Or an ionic equation, that excludes the spectator ions:

Ba2+ (aq) + SO42-(aq) →  BaSO4 (s)

(Can’t remember what an ionic equation is? It might not be on your course. It leaves off the ions that don’t do anything in the reaction and just stay floating around in the water.)


The picture of precipitates was taken by Richard Masoner and released under this license. The picture of DNA precipitating was taken by Steve Jurvetson (work web address here), and released under this license. All pictures used with thanks.

Edexcel iGCSE chemistry: the tests

Edexcel iGCSE Chemistry testsOn the iGCSE course, at various points along the syllabus, you’re meant to know tests for some important chemicals. Here’s a list of them, guides to doing some of them, and a look at past paper questions.

The guides

You can download simple guides to the tests as pdf files: simple, one-page summaries that put everything you need to know on a single page. Here’s one showing all the colours, and here’s one in black and white.

But before we start: precipitation

Many of these reactions create precipitates, and it’s probably worth covering what that means.

It means that when you mix two chemicals dissolved in water (that is, two solutions), a solid powder appears in the test tube, so the water looks cloudy.

How does precipitation happen?

Well, it’s down to the way chemicals dissolve in water. Let’s start with an ionic chemical, like sodium chloride (salt, formula NaCl). It’s made of sodium ions, Na+, and chloride ions, Cl.

Solid saltWhen dissolves in water, it splits into those ions. They are now free to move around in solution, separately from one another.


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The Blood Vessels (GCSE Biology, OCR and AQA A-level biology)

(When you learn these topics depends on what course you’re doing. Check your specification. Sorry, but there are too many GCSE specifications now to write separate articles for all of them!)

Blood vessels entering and leaving the heart 

Here’s a table of the biggest blood vessels in your body. This is written in order as blood goes from the body to the heart and back out again: Blood Vessels So four key blood vessels to learn: the vena cava, pulmonary artery, pulmonary vein, aorta. On many courses, you’re meant to know which atriums (atria) and ventricles they go to and from.

Other key blood vessels 

On Edexcel iGCSE biology and AQA AS-level biology, you’re meant to know a few more. Here’s a table of them: Blood Vessels 2That makes the renal artery and vein, hepatic artery and vein and the hepatic portal vein.

On AQA AS-level and OCR A2 biology, you’re also meant to know about the coronary arteries. These loop round from the aorta to the heart muscle itself, giving it oxygen and food to beat with.

What’s the hepatic portal vein for? Simple. Blood coming from the small intestine is carrying a real mixture of stuff absorbed from food in the gut. Lots of useful chemicals, some not-so-useful ones, even some toxic ones. The liver takes them and reprocesses them into new, more useful chemicals before they’re allowed to go to the rest of the body.

But if the hepatic portal vein is taking blood into the liver, what’s the hepatic artery for? That’s simple too.  The liver uses lots of energy to do its jobs, but the hepatic portal vein carries deoxygenated blood. That’s no good for aerobic respiration! To give the liver some energy, the hepatic artery carries oxygenated blood to it. So the liver gets everything it needs to do its job: food from the small intestine, oxygen from the heart.


This isn’t on the syllabus, but if you’re planning to do medicine you might want to know that there are actually two renal arteries and two renal veins (each kidney has its own renal artery and renal vein). There are also two venae cavae (Stupid Latin plurals! Why can’t it be vena cavas or venus cava or something?): one from the body above the heart and one from below the heart, and there are several separate hepatic veins from different parts of the liver.