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1.4 Specialisation in Animal Cells

GCSE Biology · AQA · Chapter 1: Cell Structure and Transport

A free GCSE Biology lesson for AQA (8461) from OBA School: how sperm cells, nerve cells and muscle cells are adapted to do their jobs. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain what a specialised cell is and how cells become specialised
  • Describe how a sperm cell is adapted for fertilisation
  • Describe how a nerve cell is adapted to carry electrical impulses
  • Describe how a muscle cell is adapted to contract
  • Link the structure of a specialised cell to its function

Key words

  • Specialised cell: A cell with a structure that suits one particular job (its function).
  • Differentiation: The process by which a cell changes to become specialised. It gains the sub-cellular structures it needs.
  • Adaptation: A feature that helps a cell (or organism) do its job or survive.
  • Impulse: An electrical signal carried along a nerve cell.
  • Contract: Get shorter. Muscle cells contract to move parts of the body.

Why cells specialise

A simple single-celled organism does every job in one cell. A large animal is made of many cells, so different cells can share out the jobs. As an embryo grows, most of its cells differentiate: they switch on some genes, switch off others, and develop the sub-cellular structures they need. Most animal cells differentiate early in development and then stay that way.

Exam tip: When you explain an adaptation, always give the feature and then how it helps the cell: “has many mitochondria so that lots of energy is released by respiration”.

Sperm cells

A sperm cell must swim to an egg, get inside it and join its genetic material with the egg’s genetic material (fertilisation). A human sperm cell is about 55 µm long, most of it tail.

  • Long tail: whips from side to side to swim towards the egg.
  • Mid-piece full of mitochondria: releases the energy the tail needs.
  • Acrosome at the tip of the head: stores digestive enzymes that break down the outer layers of the egg.
  • Large nucleus: holds half the genetic information of a normal body cell (23 chromosomes in humans), so the fertilised egg gets a full set.
  • Streamlined head: helps it move easily through fluid.

Nerve cells

Nerve cells (neurones) carry electrical impulses around the body, for example from your toes to your spinal cord.

  • Long axon: carries impulses over long distances quickly.
  • Many dendrites: branched connections that pick up impulses from lots of other nerve cells.
  • Nerve endings (synapses): contain many mitochondria to release the energy needed to make chemical transmitters, which pass the impulse to the next cell.
  • Fatty (myelin) sheath around many axons: insulates the axon and speeds up the impulse.

Worked example: comparing sizes

A nerve cell body is 25 µm wide. Its axon is 0.8 m long. How many times longer is the axon than the cell body is wide?
Same units: 0.8 m = 800 mm = 800 000 µm
800 000 ÷ 25 = 32 000

Answer: 32 000 times (about 4 orders of magnitude)

Muscle cells and other examples

Muscle cells contract to move bones, pump blood or squeeze food along the gut.

  • Special proteins (protein fibres) that slide over each other to make the cell contract.
  • Many mitochondria: release the energy needed for contraction.
  • Store glycogen: a store of glucose that can be broken down quickly for respiration.
  • Red blood cell; Key adaptation No nucleus, leaving more room for haemoglobin to carry oxygen. Biconcave disc shape gives a large surface area.
  • Ciliated cell (airways); Key adaptation Tiny hairs (cilia) on the surface beat to sweep mucus and trapped dust away from the lungs.

Common mistakes

  • Saying mitochondria “make energy” or “produce energy”. They release energy by respiration.
  • Giving a feature without its function. “Has a tail” only scores when you add “to swim to the egg”.
  • Saying the acrosome contains “enzymes to digest the egg”. The enzymes break down the egg’s outer layers so the sperm can get in.
  • Saying a sperm nucleus has “half a nucleus” or “half the DNA of an egg”. It has half the genetic material of a normal body cell.

Revision questions

Try these, then check your answers below each one.

Question 1. How do muscle cells contract? (Choose from: Their nucleus divides, Protein fibres slide over each other, They absorb water by osmosis, Their cell wall shrinks.)

Answer: Protein fibres slide over each other

Working: Muscle cells contain protein fibres that slide over each other, making the cell shorter.

Question 2. True or false? The nucleus of a sperm cell contains half the genetic information of a normal body cell.

Answer: True

Working: True. Sperm and egg each carry half (23 chromosomes in humans), so the fertilised egg has a full set (46).

Question 3. What is the name of the process by which a cell becomes specialised?

Answer: differentiation

Working: Differentiation is the process by which cells become specialised.

Related lessons

Continue in the full course

This lesson is section 1.4 (AGCBH0104) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

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