Specialised Cells and Stem Cells (Triple)

Explained

Specialisation, and the cells that have not chosen yet

Differentiation is the development of an unspecialised cell into a specialised one, with a structure suited to a particular job.

It matters because a multicellular organism works by division of labour. One cell cannot be good at carrying oxygen, transmitting impulses and absorbing water all at once, so different cells take on different roles and the organism as a whole does far more than any single cell could.

Named specialised cells

Each of these is a structure attached to a purpose, and questions always want both halves.

• Red blood cell: no nucleus, so there is more room for haemoglobin, so more oxygen is carried. Biconcave, so the surface area is larger for diffusion.
• Sperm cell: a tail so it can swim to the egg, many mitochondria to release the energy for swimming, and enzymes in the head to digest through the egg membrane.
• Egg cell: large food store to supply the embryo, and a membrane that changes after fertilisation so no further sperm can enter.
• Ciliated epithelial cell: cilia that beat to sweep mucus along the airways.
• Root hair cell: a long extension giving a large surface area for absorbing water and mineral ions, and many mitochondria for the active transport of those ions.
• Palisade mesophyll cell: many chloroplasts, packed at the top of the leaf where the light is strongest.
• Neurone: very long, so it can carry impulses over a distance, with branched ends to connect to other cells.

Write these as feature, then function, joined by so or because. No nucleus, so more room for haemoglobin, so more oxygen carried. The joining word is where the mark sits.

Stem cells

A stem cell is unspecialised. It can divide repeatedly and can differentiate into other cell types.

Embryonic stem cells come from early embryos and can become almost any type of cell, which makes them the most useful and the most contentious.

Adult stem cells are found in tissues such as bone marrow. They can divide and differentiate, but usually into a more limited range of types, so bone marrow stem cells produce blood cells rather than anything at all.

Plant stem cells are found in meristems, at the tips of roots and shoots, and they remain able to produce any plant tissue throughout the plant's life. That is why a cutting can grow into a whole new plant.

Medical uses, and the objections

The potential is to replace cells that have been damaged or lost: nerve cells after a spinal injury, insulin producing cells in diabetes, or blood cells in leukaemia, which is already routine using bone marrow transplants.

The objections divide into two kinds, and a good evaluation separates them.

Practical risks: the transplanted cells may be rejected by the immune system, the procedure carries a risk of infection, and cells that divide without control could form a tumour.

Ethical concerns: obtaining embryonic stem cells destroys the embryo, and people disagree about the moral status of an embryo. Some argue that using embryos created for fertility treatment and otherwise discarded is acceptable; others do not.

Using a patient's own adult stem cells avoids both the rejection problem and the ethical objection, which is why so much research aims at that route.

Answering an evaluation question

Give a benefit with its mechanism, give a risk or objection, and then answer the question that was asked rather than concluding that opinions differ.

A question asking whether embryonic stem cells should be used wants both sides, but it also wants a position with a reason attached, and the reason is what is being marked.