Comparing Cells, and Stem Cells

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.4 — Similarities and differences between plant and animal cells
• 2.5 — The importance of cell differentiation in producing specialised cells (bold B statement — Paper 2 only)
• 2.6 — Advantages and disadvantages of using stem cells in medicine (bold B statement — Paper 2 only)

Specification section 2(c) Biological molecules — also taught in Carbohydrates, Proteins and Lipids, Enzymes: Catalysts With a Shape.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Every cell in your body carries the same complete set of instructions. A liver cell and a nerve cell have identical DNA. What makes them different is which parts of those instructions they use — and once a cell has committed to a job, it usually cannot go back.

Think of it like a huge cookbook

Every cell owns the same 20,000-recipe cookbook. A muscle cell keeps it open at the muscle-protein pages and glues the rest shut. A pancreas cell opens the insulin page. Nothing was deleted — the pages are just no longer used. Differentiation is choosing which pages to read, and specialising the whole kitchen around them: a cell that will spend its life contracting builds itself full of mitochondria and protein fibres. That is why a specialised cell is so much better at one job than a general one.

A stem cell is a cell that has not yet chosen. That is the whole idea, and the whole controversy. Give a stem cell the right signals and it can become a nerve cell, a blood cell, a muscle cell. Medicine wants that because it means damaged tissue could, in principle, be replaced rather than merely managed — a severed spinal cord repaired, insulin-producing cells rebuilt in a person with type 1 diabetes.

Why it is not simple

1. Embryonic stem cells can become any cell type, which makes them the most useful — but obtaining them destroys an embryo, and many people hold that this is destroying a potential human life.
2. Adult stem cells (for example from bone marrow) avoid that objection entirely, but can only become a limited range of cell types.
3. Cells from another person may be rejected by the patient's immune system.
4. Stem cells divide rapidly, and rapid uncontrolled division is what a tumour is — so there is a real risk of cancer.
5. The techniques are new, expensive, and long-term effects are still unknown.

Comparing the two cells properly — start from what they share

1. Both are eukaryotic, so both have a nucleus holding the DNA, cytoplasm where the reactions happen, and a cell membrane controlling what enters and leaves.
2. Both have mitochondria, because plants respire too — all day and all night, not only in the dark. This is the single most common error on the comparison.
3. Both have ribosomes, because every living cell has to build proteins.
4. The plant cell then adds three things: a cellulose CELL WALL for support, CHLOROPLASTS to trap light, and a large PERMANENT VACUOLE of cell sap that keeps it firm.
5. Two warnings on the additions. Chloroplasts are not in every plant cell — root cells have none, because nothing underground gets light. And the wall is fully permeable, so it is not what controls entry; the membrane still does that.

It is worth being clear about what differentiation costs, because the exam asks for advantages and the interesting part is the price. A specialised cell is dramatically better at one job — a red blood cell carries far more oxygen than a general cell could, because it has thrown out its nucleus to make room for haemoglobin. But it has thrown out its nucleus. It cannot divide, cannot repair itself, and lasts about four months. That trade is the reason a body needs stem cells at all: once cells have committed, something uncommitted has to be kept in reserve to replace them.

Think of it like a hospital that has to keep training people

A hospital full of specialists treats patients far better than a hospital of generalists. But a heart surgeon cannot cover a shift in radiology, and when one retires you cannot make another out of the remaining surgeons — you need someone who has not specialised yet. Stem cells are the medical students. Embryonic ones have chosen no subject at all and could become anything; adult ones, like a junior doctor already in a department, can still become several things but no longer anything. That is exactly why embryonic stem cells are the more useful and the more contested.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Plant and animal cells — both have

• Nucleus
• Cytoplasm
• Cell membrane
• Mitochondria
• Ribosomes

Only plant cells have

• Cell wall made of cellulose — support, prevents bursting
• Chloroplasts — photosynthesis (not in every plant cell; root cells have none)
• A permanent vacuole containing cell sap — keeps the cell turgid

Learn this definition · Differentiation

The process by which a cell becomes specialised for a particular function, developing a structure suited to that function.

Learn this definition · Stem cell

An undifferentiated cell that can divide to produce more cells of the same type, and which can differentiate into other types of cell.

Stem cells in medicine — advantages

• Could replace damaged or diseased tissue that the body cannot repair itself
• Potential treatments for paralysis, type 1 diabetes, Parkinson's disease and blindness
• Adult stem cells from the patient's own body would not be rejected
• Can be used to grow tissue for testing drugs, reducing animal testing

Stem cells in medicine — disadvantages and objections

• Obtaining embryonic stem cells destroys an embryo — an ethical objection many hold strongly
• Cells from a donor may be rejected by the immune system
• Risk of uncontrolled division, and therefore tumour formation
• Techniques are expensive and long-term effects are not yet known
• Risk of viral infection being transferred with donated cells

Not this: 'Specialised cells have different DNA from each other.'

This: All body cells of an organism contain the same DNA. They differ in which genes are switched on.

Plant cell vs animal cell — write this out, shared structures first

Embryonic vs adult stem cells

A. EMBRYONIC: can become ANY cell type, so the most useful — but obtaining them destroys an embryo.

B. ADULT (e.g. bone marrow): only a LIMITED range of types — but no embryo is destroyed, and the patient's own carry no rejection risk.

Answering a stem-cell evaluation question — the four moves that score

1. State which kind of stem cell the question is about — embryonic or adult. Most of the marks turn on this.
2. Give the benefit in terms of what it could REPLACE: tissue the body cannot repair itself, and name a condition (paralysis, type 1 diabetes, Parkinson's, blindness).
3. Give the objection that fits the kind you named: the embryo for embryonic, the limited range for adult, rejection for anyone else's cells.
4. Give the risk that applies to BOTH and is not solved by choosing between them: stem cells divide rapidly, and uncontrolled rapid division is a tumour.

Model answer [4 marks]

Explain why a red blood cell is better at carrying oxygen than an unspecialised cell, and state the cost of that specialisation. [4]

During differentiation the red blood cell loses its nucleus, which leaves more room inside for haemoglobin, so it carries far more oxygen than a general cell could. Its biconcave shape also gives a large surface area for oxygen to cross. The cost is that without a nucleus it has no instructions to work from: it cannot divide, cannot repair itself, and lives only about four months, so it has to be continually replaced from stem cells in the bone marrow.

Model answer [3 marks]

A liver cell and a nerve cell contain identical DNA. Explain how they come to be so different. [3]

Every body cell of an organism contains the same complete set of genes. During differentiation only some of those genes are switched on in a particular cell, and the rest are not used. The cell then builds the structures the active genes code for — so the liver cell and the nerve cell develop different structures suited to different functions, from the same instructions.

Mark-losing trap. Plants have mitochondria. Plants respire day and night — 'plants photosynthesise instead of respiring' is wrong and costs marks all course.

Mark-losing trap. Not every plant cell has chloroplasts. Root cells have none: no light reaches them.

Mark-losing trap. Specialised cells do NOT have different DNA. They have the same genes with different ones switched on.

Mark-losing trap. Say WHICH kind of stem cell you mean. The embryo objection applies only to embryonic ones; the limited range applies only to adult ones.

Mark-losing trap. The tumour risk survives every fix. Using the patient's own cells removes rejection and the embryo objection, and leaves rapid uncontrolled division exactly where it was.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Name one structure found in a plant cell that is never found in an animal cell.
2. Grade 7 · Explain [2 marks] — Explain why a root hair cell contains no chloroplasts even though it is a plant cell.
3. Grade 8 · Explain [3 marks] — Explain how cell differentiation makes a multicellular organism more efficient than one made of identical general-purpose cells.
4. Grade 9 · Evaluate [4 marks] — Doctors propose treating a patient's spinal injury with stem cells taken from a donated embryo. Which response best evaluates this proposal?
5. 9+ · Suggest [4 marks] — Researchers reprogram a patient's own skin cells into stem cells, then grow them into insulin-producing cells. Which is the strongest statement of what this achieves and what still needs testing?

The people behind this science

Three ways into the same idea — the one who said a plant and an animal are built the same way, and the one who showed the instructions are a molecule you can photograph, and the one who drew a body becoming specialised. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Theodor Schwann — the one who said a plant and an animal are built the same way

The first half of this page compares a plant cell with an animal cell — and that comparison only makes sense because Schwann established, in 1839, that they are the same kind of thing at all. Before him, animals and plants were assumed to be built on different plans. He also asked the question the second half answers and could not answer it himself: if every cell is one elementary unit, why is a nerve cell nothing like a bone cell?

• “What did Schleiden tell you at dinner that changed everything?”
• “How can cartilage, muscle, nerve and bone all be built the same way?”
• “Why did it matter that animals turned out to be built like plants?”
• “Where did you think a brand new cell came from?”
• “If every cell is the same elementary unit, why are they so unlike one another?”

Rosalind Franklin — the one who showed the instructions are a molecule you can photograph

This lesson's key idea is that a liver cell and a nerve cell hold the SAME instructions and differ only in which are used. Franklin's X-ray work made those instructions a physical object — a structure you could photograph and measure, rather than an abstraction.

• “What did your X-ray photographs actually show about the shape of DNA?”
• “If every cell has the same DNA, why is a nerve cell nothing like a skin cell?”
• “How can you learn the structure of something far too small to see?”
• “Why does the shape of a molecule decide what it can do?”
• “How certain can you be of a structure you have only ever seen indirectly?”

Leonardo da Vinci — the one who drew a body becoming specialised

Differentiation is a body becoming specialised from something unspecialised. Leonardo's studies of the fetus in the womb are among the first careful records of that process, drawn from dissection at a time when nobody could say how it happened.

• “What did you see when you studied a body still developing?”
• “How does something so simple become a heart, a hand and an eye?”
• “Why did you draw growth in stages rather than as a single picture?”
• “What did you believe was guiding the body as it formed?”

Carry on through the course

• Previous lesson: Levels of Organisation
• Next lesson: Carbohydrates, Proteins and Lipids
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• Diffusion, Osmosis and Active Transport — The three ways anything gets into or out of a cell — which need energy, which do not, and what makes each one fast or slow.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification