Cell differentiation and the adaptations of specialised cells, the use of a light microscope, and calculating magnification, image size and real size, including the use of scale bars and unit conversion.

What this dot point is asking

WJEC wants you to explain that cells become specialised by differentiation, describe how named specialised cells are adapted to their function, and carry out magnification calculations using the equation, including converting between millimetres and micrometres and reading scale bars.

Cell differentiation

Most cells in the body start the same and then differentiate. Once specialised, animal cells usually cannot change into a different type. Specialised cells let an organism carry out many jobs efficiently, because each cell type is shaped for its role.

Examples of specialised cells

Each specialised cell links a feature to a function. Learn at least these examples.

• Red blood cell: a biconcave disc shape and no nucleus, giving more room and surface area for haemoglobin, which carries oxygen. The shape also means oxygen can diffuse in and out quickly.
• Root hair cell: a long, thin extension gives a large surface area for absorbing water and mineral ions from the soil. It has many mitochondria for the active transport of minerals.
• Ciliated epithelial cell: has tiny hairs called cilia that beat to move mucus, for example up the trachea away from the lungs.
• Sperm cell: has a tail (flagellum) to swim, many mitochondria to release energy for movement, and enzymes in the head to break into the egg.
• Egg cell: large and packed with nutrients to feed the developing embryo, with a membrane that changes after fertilisation to stop more sperm entering.
• Palisade mesophyll cell: packed with chloroplasts and near the top of the leaf to absorb the most light for photosynthesis.

Using a light microscope

A light microscope shines light through a thin specimen and uses lenses to magnify it. The total magnification is the eyepiece magnification multiplied by the objective lens magnification. For example, a x10 eyepiece with a x40 objective gives x400.

Two ideas are often confused:

• Magnification is how many times bigger the image is than the real object.
• Resolution is the ability to tell two close points apart as separate. A light microscope has lower resolution than an electron microscope, which is why electron microscopes show more detail.

The magnification equation

The key equation is:

You can rearrange it two ways:

To use it reliably, get every length into the same unit first. The common conversions are:

• $1\ \text{mm} = 1000\ \text{micrometres}$
• $1\ \text{micrometre} = 1000\ \text{nanometres}$

So to change millimetres to micrometres, multiply by 1000. To change micrometres back to millimetres, divide by 1000.

Reading a scale bar

A scale bar is a line on a micrograph labelled with the real length it represents, for example a 10 micrometre bar. To find magnification: measure the bar with a ruler (the image size), then divide by the real size the bar stands for. To find the real size of a structure: measure it, then scale it using the bar. Always keep units the same before dividing.