CIE 1.2 Cell Structure

CIE 1.2 Cell Structure - 20 Multiple Choice Questions

AQA - Possible essays - as forecast by AI.....

How I suggested some the POSSIBLE 2025 AQA A-Level Biology Essay Titles

One of the most challenging aspects of A-Level Biology Paper 3 is preparing for the 25-mark synoptic essay. With so many potential topics across the full specification, students often feel overwhelmed. That’s why I’ve taken a systematic approach to identify four high-probability essay titles that could appear in the 2025 exam.

Here’s how I did it:

1. Analysing Past Essay Titles

I reviewed a complete set of past essay questions and their mark schemes, identifying which themes have come up repeatedly and which have been underused in recent cycles. This helped rule out repeats and spot patterns in the kinds of synoptic themes the exam board favours.

2. Cross-Referencing the AQA Specification

Using the official AQA Biology specification, I matched every past title to its relevant topic codes. I then looked for specification areas that:

• Are heavily weighted in content but haven't been examined recently

• Offer rich synoptic potential (e.g. enzymes, feedback, biological molecules)

• Align with the mark scheme’s focus on integration and application

3. Designing Original Titles

To avoid duplicating previous questions, I crafted entirely new titles that:

• Require a synoptic approach using at least four topics

• Encourage explanation, analysis, and application across biological scales

• Are rooted in specification content but phrased in fresh and exam-appropriate language

Overview: Gaining and Losing Atoms from the Human Body

Many topics in A level biology relate to how the human body gains and loses atoms. But have you ever stepped back and thought of how these processes all fit together? Having a wide overview can help make sense of individual topics.

Atoms in, atoms out

Your body is an atom-organising machine, with biochemical processes moving atoms between different molecules and locations. Your body takes new atoms in from the environment, and it releases atoms back into the environment.

A surprisingly large amount of mass is exchanged with the environment every day. About 98% of the atoms in your body are replaced each year - a startling thought. This article is focussed on the processes that result in the biggest mass changes.

Atoms gained and lost by the body usually exist as part of molecules.

• Taking atoms/molecules into the tissues from the environment = absorption

• Sending useful atoms/molecules out into the environment (or bloodstream) = secretion

• Sending waste atoms/molecules out into the environment = excretion

Most of the mass we exchange with the environment is in the form of water. Remember that water is a important substrate/product in many metabolic and catabolic biochemical reactions; it’s not just a solvent.

Keeping it under control

Our bodies must control what molecules can (and cannot) get in from (and out to) the environment. Most of our bodies are covered with skin for this reason - skin acts as a barrier between us and our environment. We have specialised organs that manage absorption: the lungs and the gut. These organs both create spaces where molecules from the outside environment are put for absorption to take place in a controlled way. We also have specialised organs that control what substances leave our bodies.

Quick check: remember that molecules don’t count as being inside our bodies until they have been absorbed into our tissues. Just breathing or swallowing something isn’t enough. For example, the gut is a tube that goes from mouth to anus. It goes through our bodies, but the space inside the gut (the lumen, where the food is digested) is not part of our bodies any more than the hole in a doughnut is part of the doughnut. Imagine passing a tiny pebble through the hole in a doughnut; the pebble doesn’t go into the doughnut, just through it; it’s the same with us. Similarly if you swallowed the pebble it would pass through your gut but not be absorbed through the gut wall into your body.

Absorption - gaining body mass

The most common atoms we gain from the molecules we absorb are: carbon (C), oxygen (O), hydrogen (H) and nitrogen (N). This makes sense as these are the atoms most frequently found in biological molecules. We get these from:

The gut

absorption from the gut varies a lot, especially depending on what you eat and drink. These are rough averages:

• About 7 litres of water per day (O, H) from fluids that were secreted higher up the gut

• About 2-3 litres of water per day (O, H) from food and drink

• Maybe something like 500 g total of other food molecules, comprised of:

  • carbohydrates (C, O, H)

  • fats (C, O, H)

  • proteins (C, O, H, N, S)

  • much smaller masses of other ions and molecules, with all the atoms we need to live

A lot of the water we ingest (eat/drink) comes from our food - you don’t need to drink litres of water every day. Any food that isn’t hard, dry and crunchy will contain water. Consider: a healthy diet includes a lot of plant and animal cells, and cells are about 70-80% water; a piece of steak contains a lot more water than protein, even after cooking.

The lungs

absorb gas from the air:

• Something like 750 g per day of oxygen, as the molecular gas O2 (O)

Just look at that daily body mass increase we get just from absorbing oxygen gas!! It’s more mass than carbohydrates, fats and proteins combined. And it’ll be even higher if you excercise. It’s kinda wild tbh. This oxygen is required for respiration and ends up in water molecules (not in CO2!).

How do we use this stuff?

Although I’ve been talking about atoms, we nearly always absorb molecules. Even Oxygen is absorbed as the molecular gas O2.

And this is really important - having ready-made molecules is vital. Unlike plants, we can’t make glucose through photosynthesis, nor synthesise the amino group of amino acids from scratch*. So we need to absorb these molecules ready-made.

Some of these absorbed molecules are immediately useful (eg glucose for respiration, amino acids for protein synthesis), while many provide really useful starting places for biochemical processes that synthesise other useful molecules, often making use of the absorbed molecules’ ready-made carbon backbones.

Excretion - losing body mass

Excretion is the deliberate loss from the body of waste products from metabolism. It’s about getting rid of unneeded/dangerous waste. This is part of homeostasis.

We excrete the same sort of mix of atoms as we absorb, but in different molecular forms, which are no longer useful to us and/or which may be unhealthy for us to retain in our bodies. We can lose kilograms of mass this way, every day.

Again there are specialised organs/processes for this, the main ones being:

The lungs

• Excrete something like 730 g per day of carbon dioxide, a waste product of cellular respiration (C, O)

  • This comprises 530 grams of carbon atoms and 200 grams of oxygen atoms

  • This is the main way we lose carbon atoms from our bodies!

  • Important: these oxygen atoms are not the same ones that were absorbed by the lungs (those ones became water)
    
    

The liver

• Excretes somewhere around 900 g per day of bile fluids into the gut lumen.

  • Bile fluids are mostly water (H, O), which can be reabsorbed.

  • Contains a small mass (mg) of broken-down haem proteins (C, O, H, N), bile salts, and other substances that need to be excreted.

  • Also contains a very small mass of substances to aid digestion (but these are being secreted, not excreted).

• Note: the liver breaks down unwanted proteins to form urea … but it does not excrete urea! The urea it produces is released into the blood to be excreted by the kidney. (Fun question: why doesn’t the liver just excrete this urea directly into the gut?)

The kidney

• Excretes about 25 g per day of urea (C, O, H, N), which must be dissolved in water for excretion.

• Excretes excess water, in volumes that can vary from zero to litres per day (H, O).

• Excretes excess salts, averaging somewhere around 5 g per day.

The skin

• Excretes a highly variable volume of sweat, depending on conditions.

  • Sweat is almost entirely water (H, O) but contains a very small mass of excreted salts, minerals and other substances.

  • A small amount of urea is also excreted in sweat (C, O, H, N).

You might be surprised not to see faeces listed here. Some things are excreted into the gut and become part of our faeces, but only in small amounts. Most of the bulk of our faeces is undigested food and bacteria that live in our gut - things that were never inside our bodies in the first place, are not products of our metabolism, and so which don’t count as excretions (no matter what everyday language might suggest). In scientific language, the loss of faeces from the body is egestion (compare with ingestion for eating).

Warning: remember all these numbers will vary wildly between different people and under different conditions. For example: if you exercise then you will excrete more carbon dioxide, and if you eat a lot of protein you will excrete more urea. The numbers given here are just to give you a very rough idea of the amounts involved. This means you can’t directly compare them to the numbers above.

What about secretions?

While excretion is the release of waste, secretion is the release of useful molecules (into the blood or external environment). Secretions that are sent into the external environment (including into the gut lumen) may be lost if not reabsorbed.

• The gut receives about 6 litres per day of liquid secretions, but much of the water is reabsorbed

• On a normal day, you might secrete about half a litre of sweat

These secretions are largely water. They do contain other things, like digestive enzymes and salts, but the total mass is of these are small compared to the excretions described above and so they aren’t so important for the big-picture story we’re telling here.

Water gain and loss

Remember water is not just a solvent, it’s a common substrate and product of biochemical processes. And we don’t only get it from absorption through our gut.

• About 300 ml of water per day is produced as a product of metabolism (from respiration, as mentioned above).

It’s worth mentioning that we also lose quite a lot of water just because of the way our bodies work. But this is not considered excretion. Again all these figures vary a lot depending on the individual and their level of activity:

• About 300 grams of water per day is lost through unavoidable evaporation from the wet surfaces of our lungs

• About 300 grams of water per day is lost by evaporation from the skin (this is not sweat, it’s just evaporation)

• About 500 grams of water per day is lost because it is required to dissolve urea for excretion (this is a bit different from the kidney’s ability to also excrete excess water in addition to this.

Finally, be aware that some water that enters our gut is never absorbed at all. This is important to keep the gut contents nice and squidgy so they can be pushed along to the anus to be egested. About 200 grams of water per day is lost in this way

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* you might be confused by me saying this as people do talk about amino acid synthesis in humans. However, when we synthesise them we do so by using the amino group from an existing amino acid, usually glutamate. We can’t make the amino group ourselves, we can only swap it from molecules that have been synthesised by plants.

Introduction to Immunity (What is an Antigen? What is an Antibody?)

Immunity is a complex topic. This article overviews the basic concepts and vocabulary to give you a foundation before you start looking at the details. When you are studying, break this topic up into chunks so that you’re not overwhelmed by the amount of information.

Also: expect to find this topic confusing! Feeling confused is part of learning and shouldn’t be thought of as a bad thing, or some sort of failure. It’s just a step toward understanding. If you’re not confused, that’s more worrying because it means you’ve probably made incorrect assumptions.

What is Immunity

Our bodies are wet and warm, and full of useful molecules. This makes them an attractive environment for bacteria, fungi, viruses and other organisms to live. Our bodies host many such organisms that benefit us, some of which are absolutely required for normal functioning (eg the bacteria that break down food in our gut).

But there are other organisms that would like to live and reproduce in and on our bodies that can harm us by their presence. These are disease-causing organisms, and they are called pathogens. Remember these are tiny things without brains and they have no concept of our bodies as an entire organism. They just want to grow and reproduce, with no mind to consider they are damaging their environment.

Immunity is the body’s ability to identify and defend itself against pathogens.

What Defenses do we have?

The first line of defense is to stop invaders gaining entry to our bodies. For example, our skin protects us by making it harder for pathogens to get into our tissue fluids (wounds are more likely to become infected than unbroken skin). Another example: incoming food is passed through the highly acided stomach environment, which kills many organisms before they reach the intestines. But these defenses don’t count as part of the immune response.

If a pathogen does get into our bodies (especially into our blood, intracellular fluid, or cells), then the body - we hope! - will spot, kill, and remove the pathogen. This is the immune response.

The body has cells that are specialised for detroying and removing pathogens. However, it would be pretty difficult for any one cell to be able to accurately recognise every possible different threat, especially as pathogens are constantly evolving to try to get past our defenses. This is why the body uses antibodies.

What is an Antibody?

Antibodies are small protein molecules that are carried around by the blood and tissue fluids. They act like little sticky labels - they stick to anything weird/suspicious and label it as being “not part of my body”.

Antibodies aren’t huge; they only bind to one part of the pathogen, rather than grabbing the entire thing. Usually they bind to a large protein on the surface of the pathogen. The antibody’s physical presence might interfere with the functioning of the pathogen, but more importantly the antibody labels it up to other parts of our immune system as being something that needs to be removed.

But remember: it’s not possible to make one molecule that binds to everything suspicious. For this reason, there are many different antibodies in your body. These vary in a small part of the structure at the end of their arms. Depending on any one antibody’s particular structure, it will be able to bind to different things.

(Recap: do you remember how protein enzymes have a binding site that is very specific to their substrate? Antibodies, which are also proteins, have 3D structures that allow them to be similarly specific in their binding.)

What is an Antigen?

Antibodies with different tertiary structures are able to bind different antigens

Any molecule that an antibody binds to is called an antigen. It’s called that because it generates an immune response.

It’s important to understand that an antigen isn’t a particular type of molecule. Antigens can be viral coat proteins, polysaccharides, lipids, or … just about anything really. If an antibody binds to it, it’s called an antigen - that’s it.

• Anything a child plays with is called a “toy”

• Anything more than 100 years old is called an “antique”

• Anything that gets bound by antibodies is called an “antigen”

Not every part of a pathogen will act as an antigen. But you hope that at least some parts of it will, otherwise your body won’t know it’s there. Luckily, pathogens tend to come covered in loads of interesting proteins which very often do serve as antigens.

If the pathogen does carry an antigen, and if that antigen gets bound by an antibody (labelling it as “not part of my body”), then the immune system will destroy it and remove it from the body.

Where do Antibodies come from?

During an infection, antibodies are released by B-lymphocytes (a type of white blood cell). These cells have the ability to produce and release antibodies in huge numbers. They synthesise the antibodies using a normal protein synthesis process.

But … B-lymphocytes are not all the same. Remember how the immune system uses many different antibodies, with slightly different structures in that variable section? Each individual B-lymphocyte cell can only make ONE type of antibody, with one particular structure. So there are different B-lympocytes for every different antibody made in the body. Which is amazing as that means there are a LOT of different B-lymphocytes.

This is all possible because the B-lymphocytes have slightly different DNA coding just for that variable section of the antibody. This is more than a bit mindblowing because it means that B-lymphocytes are not genetically identical to each other!!! Exactly how this happens is complicated so I’m not going to get into that right now. But the fact they’re all different, and make different antibodies, is crucial to the whole way immunity works.

(There are also T-lymphocytes, which have many similarities to B-lymphocytes but play a different role. We’ll get to them later. )

How do we get the right Antibodies for our needs?

There are vast numbers of different bacteria and fungi and viruses that might harm us, and they are constantly evolving. So we need a immune system that can react to unexpected threats. Something that can recognise anything strange, rather than just a check-list of well-known pathogens.

Important: we DO NOT respond to new pathogens by creating new antibody variations specially designed to bind to them!

This crucial fact is often missed by students.

At first this might seem unintuitive, especially as many news reports suggest otherwise. But think about it. How could we? How would the body even know something was a pathogen if an existing antibody hadn’t already labelled it as such? And even if it did magically know it was a pathogen, how could a lymphocyte know what DNA sequence it would need to be able to synthesise a protein sequence that folded to a structure that was able to bind to it?! And then create that sequence??!

This leads to another mindblowing fact: we ALREADY have B-lymphoctyes that can make antibodies for pathogens we have never encountered. Including for pathogens that haven’t even evolved yet.

Antibodies are not designed to fit specific antigens.

Instead, we create a huge random jumble of antibody variations with a huge variation of different binding sites. Some might be useless, and many will never be required, but there are so many that there’s a really good chance at least one of their binding sites will just happen to be a good fit to some part of a new pathogen.

The thing we do do in response to pathogens is make very large numbers of the particular antibodies we need.

What happens when an Antigen arrives in the body?

The B-lymphocytes don’t do it all alone. Every B-lymphocyte has a genetically-identical T-lymphocyte. For simplicity I’m going to call B-lymphocytes “B-cells”, and T-lymphocytes “T-cells”.

T lymphocyte (T-cell)

T-cells can make molecules with binding sites that match those of their matching B-cells’ antibodies. But T-cells’ don’t make and release antibodies - instead they make similar molecules that are anchored in the cell membrane, sticking their antigen-binding-sites out into the tissue fluids.

These proteins are, massively confusingly, called antibody receptors. You must remember that antibody receptors are receptors that have structures similar to antibodies, not receptors for antibodies. It’d be better if they were called antibody-like receptors, but unfortunately they’re not.

The T-cells expose their antibody receptors to the tissue fluids.

If a pathogen passes by, and if it has an antigen that happens to fit that receptor, then the T-cell can bind to it. This then sets off a cascade of signalling that results in replication of the corresponding B-cells, and those B-cells releasing huge amounts of that particular antibody (if there’s one molecule of that antigen about, there are likely many more to be found).

In summary:

• T-cells are the look-outs/scouts (using antibody receptors)

• B-cells are the antibody factories (releasing antibodies)

(Side note: B-cells have antigen receptors too, but they still need the T-cells to help them develop into full-speed-ahead antibody factories. It’s all a lot more complicated than I’m explaining here, but you don’t need to know every detail for A-level biology.)

How do Antibodies know that an Antigen is “not part of my body”?

They don’t - antibodies are just small protein molecules, they don’t know anything. Any individual antibody will just swoosh around in our tissue fluids and maybe bind something if it can. It doesn’t know what it’s binding.

So, how do we avoid sticking antibodies onto our own body? (N.b. this does happen in auto-immune diseases, causing huge problems.)

B-cells only release antibodies if their corresponding T-cell has caught an antigen (the T-cell signals to them). But B-cells with antibodies that would attack our own bodies never receive such signals. Because their corresponding T-cells are killed before they get the chance.

T-cells mature in the thymus, an organ between the tops of the lungs. Before release into the body, each newly-matured T-cell is tested to make sure it is functional and to check that it doesn’t bind molecules naturally found in our own bodies. If a T-cell fails either test, it is destroyed.

Autoimmune diseases (eg Type 1 diabetes) happen when this system fails and the immune system attacks the body’s own cells.

Immune Memory (ish)

The second time we encounter a pathogen, our immune systems respond much more rapidly, so we don’t tend to get so ill.

This ‘memory’ is possible due to special B-cells, known as memory cells. These cells live longer than usual, so they persist in the body. So we have more B-cells for previously-encountered antigens than for unknown ones. Immunity is a numbers game, and it takes time to replicate immune cells. The more B-cells you start with, the more quickly you can mount an immune response.

So, this ‘memory’ is simply that that we have more B-cells for antigens we’ve previously encountered. Not really a memory, more a case of being better-stocked for more-likely eventualities.

Vaccination: Antigen vs Pathogen

Vaccination is where we are deliberately exposed to an antigen before we meet the pathogen that carries it in real life, so that we develop B memory cells for that antigen and are ready to mount a rapid immune response.

Because our immune system only needs us to have encounted the antigen and not the whole pathogen, vaccination can be carried out very safely. A single antigen is isolated from the pathogen and copies of this are injected. No live pathogen is involved, so there is no risk of infection.

(In the past live or damaged viruses were used for vaccination, but this is now rare.)