Describe how specialised cells are adapted to their function, explain how microscope technology has improved our understanding of cells, and use the magnification equation with the correct units.

What this dot point is asking

Edexcel statements 1.2 to 1.6 want you to describe how specialised cells are adapted to their function, explain how improvements in microscope technology (especially the electron microscope) revealed sub-cellular detail, understand size and scale, and use the magnification equation. Statement 1.6 is the core practical: investigating specimens with a microscope, including magnification calculations and labelled scientific drawings.

Specialised cells

After a cell differentiates it gains features that suit a particular job. Edexcel expects named examples:

• Sperm cell: a long tail (flagellum) to swim to the egg, many mitochondria to release the energy for swimming, and enzymes in the head (acrosome) to digest through the egg membrane.
• Egg cell (ovum): a large store of nutrients in the cytoplasm for the developing embryo, and a membrane that changes after fertilisation to stop other sperm entering.
• Red blood cell: a biconcave disc shape and no nucleus, giving more room and surface area for haemoglobin to carry oxygen.
• Root hair cell: a long thin extension giving a large surface area for absorbing water and mineral ions, with many mitochondria for active transport.
• Nerve cell (neurone): very long to carry impulses over distances, with branched ends to connect to other cells.
• Ciliated epithelial cell: tiny moving hairs (cilia) on its surface that sweep mucus and trapped particles along, for example up the airways.

Microscopy: light and electron microscopes

A light microscope uses light and glass lenses. It magnifies up to about $\times 2000$ and can show cells, nuclei and chloroplasts, but its resolution (the smallest distance between two points that can still be seen as separate) is limited by the wavelength of light. It is cheap, portable and can view living material.

An electron microscope uses a beam of electrons instead of light. Because electrons have a much shorter wavelength, it gives far higher magnification (up to about $\times 2\,000\,000$) and much higher resolution, revealing tiny structures such as ribosomes and the internal detail of mitochondria. Improvements in this technology are why scientists now understand sub-cellular structures so well. The trade-off is that electron microscopes are large, expensive and cannot show living cells.

Magnification and units

You can rearrange the triangle to find any value: real size $=$ image size $\div$ magnification, and image size $=$ real size $\times$ magnification. The single biggest source of lost marks is not converting units, so always put both lengths into the same unit before dividing.

Try this

Q1. State one advantage and one disadvantage of an electron microscope compared with a light microscope. [2 marks]

• Cue. Advantage: much higher magnification and resolution, so finer detail is visible. Disadvantage: expensive, large, and cannot view living cells.

Q2. An image is $45\ mm$ wide at a magnification of $\times 900$. Calculate the real width in micrometres. [2 marks]

• Cue. Real size $= 45 \div 900 = 0.05\ mm = 50\ \mu m$.