2bCell structure

Every living organism is built from cells. For 4BI1 you need to recognise eight structures and know which cells contain them.

The trap examiners set

The cell membrane and the cell wall are not the same thing. A plant cell has both: the membrane sits just inside the wall. An animal cell has only the membrane.

Write "cell wall" when you mean "cell membrane" and you lose the mark, even if the rest of the answer is right.

Drawing cells in the exam

• Use a sharp pencil and draw clean, continuous lines — no sketchy or hairy lines
• Do not shade or colour in
• Label lines must be straight, drawn with a ruler, and must touch the structure they name
• Labels go outside the diagram, never inside it

Knowing the names is half the marks. The other half is knowing what each structure does.

Nucleus

Contains the chromosomes, which are made of DNA. The DNA carries the genetic instructions that control the cell's activities, including which proteins the cell makes.

Cytoplasm

A jelly-like substance where most of the cell's chemical reactions happen. Organelles are suspended in it.

Cell membrane

Partially permeable — it controls which substances enter and leave the cell. Small molecules such as oxygen and water pass through; larger ones cannot.

"Partially permeable" is the term Edexcel wants. "Semi-permeable" and "selectively permeable" are older terms — use the spec's wording.

Cell wall

Made of cellulose. It is fully permeable (anything can pass through it) and gives the plant cell strength and support, stopping it bursting when it takes in water.

Mitochondria

The site of aerobic respiration, which releases energy from glucose for the cell to use.

Cells that need a lot of energy — muscle cells, sperm cells — contain many mitochondria. That link is a very common exam question.

Ribosomes

The site of protein synthesis. Ribosomes assemble amino acids into proteins.

Chloroplasts

Contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Found in the green parts of a plant — which is why root cells have none.

Permanent vacuole

Filled with cell sap (a solution of sugars and salts). When full it pushes outwards on the cell wall, making the cell turgid and helping support the plant.

The pattern behind the marks

Structure and function are linked. If a question describes a cell, ask what job it does — then the organelles follow:

• Lots of mitochondria → needs lots of energy
• Lots of ribosomes → makes lots of protein
• Lots of chloroplasts → photosynthesises a lot

This objective is worded as similarities and differences — so learn it both ways round. Questions asking only for similarities are common, and listing differences instead scores nothing.

Similarities — found in both

• Nucleus
• Cytoplasm
• Cell membrane
• Mitochondria
• Ribosomes

Differences

Two things students get wrong

"Plant cells have chloroplasts." Only cells in the green parts do. Root hair cells are plant cells with no chloroplasts — there is no light underground. Examiners test this often.

"Animal cells have no vacuoles." They can have small, temporary ones. The reliable difference is that a plant's vacuole is large, permanent and central.

Why plant cells keep their shape

The cellulose cell wall is rigid, and the full vacuole presses outwards against it. Together these keep the cell turgid and hold the plant upright. An animal cell, with no wall, would burst if it took in that much water.

Every cell in your body came from one fertilised egg, and every one of them carries the same genes. A muscle cell and a nerve cell hold an identical set of instructions. What makes them different is not which genes they have — it is which of those genes they use.

Differentiation is the process by which an unspecialised cell becomes a specialised cell, suited to one particular job. It happens because different genes are switched on and off in different cells.

Why a multicellular organism needs it

Structure follows function

Three examples worth being able to quote. The mark is always in the link between the structure and the job:

• Red blood cell — no nucleus, so there is more room for haemoglobin; a biconcave disc, which increases surface area for absorbing oxygen.
• Nerve cell — very long, so it can carry an electrical impulse a great distance; branched endings, so it can connect to many other cells.
• Root hair cell — a long, thin extension into the soil, which greatly increases the surface area for absorbing water and mineral ions.

Points that gain the marks

• Differentiation is not cell division. Mitosis makes more cells; differentiation makes them different. A question about a ball of identical cells becoming an embryo needs both — the cells divide, and then they differentiate.
• All the cells still have the same genes. Say that different genes are switched on or expressed. Do not say the cell loses the genes it does not use.
• Always connect the structure to the job. "A red blood cell has no nucleus" earns little on its own. "It has no nucleus, so there is more space for haemoglobin to carry oxygen" is the mark.
• Most animal cells cannot go back. Once differentiated they stay that way, which is why stem cells matter — see the next note.

Examiner tip. If a question asks you to explain the importance of differentiation, it wants the consequence, not the definition. Saying what differentiation is only describes it. The marks are for what it allows: cells specialise, they form tissues and organs, and the work is shared out between them.

A stem cell is an unspecialised cell that can keep dividing and can differentiate into other types of cell. That is the whole reason they are useful in medicine: they can be turned into the cell type a patient has lost.

The advantages

• They can replace cells the patient can no longer make. Insulin-producing cells for type 1 diabetes, or nerve cells after spinal injury, are the two examples questions use most.
• They can cure the condition rather than treat the symptoms. A diabetic who makes their own insulin again no longer needs injections.
• Bone marrow transplants already work. This is not a future technology — adult stem cells are used to treat leukaemia now, which makes it a strong example to quote.
• Stem cells divide rapidly, so large numbers can be grown from a small sample.
• Tissue could be grown to order, reducing waiting lists for donor organs.
• If the stem cells come from the patient themselves, the new cells are genetically identical and carry the same antigens, so there is no rejection and no lifelong immunosuppressant drugs.

The disadvantages

• Obtaining embryonic stem cells destroys the embryo. Many people believe an embryo is already a human life, so this is the objection the question is really about.
• Transplanted cells from a donor may be rejected by the patient's immune system.
• Because stem cells divide rapidly, there is a risk they carry on dividing uncontrollably and form a tumour.
• A virus present in the cultured cells could be passed to the patient.
• The work is slow and expensive, and most of it is still research rather than routine treatment.

Points that gain the marks

• "Discuss" means both sides and then a judgement. An answer that is only advantages, however many, cannot reach full marks on a discuss question.
• Name the source. Most objections apply to embryonic stem cells specifically. Many people who object to those accept adult stem cells from bone marrow, and saying so is often the judgement the question wants.
• Say what the stem cells would become. "They can treat diabetes" is vague; "they can be made to differentiate into insulin-producing cells in the pancreas" is the mark.
• Do not write that stem cells "cure everything". Give a named condition — diabetes, paralysis, leukaemia.

Examiner tip. Ethical questions still score on biology. Marks come from knowing what a stem cell can do, where it is taken from and what could go wrong — not from how strongly the answer argues. Write the science, then give your view in one sentence at the end.