OCR GCSE Biology

Eukaryotic and prokaryotic cells

Cells are the basic units of living organisms, and each contains smaller structures called sub-cellular structures that carry out particular jobs. Plants, animals and fungi are built from eukaryotic cells, which keep their genetic material inside a nucleus. Bacteria are prokaryotic cells, which have no nucleus. A cell is a three-dimensional object, so a slice through it on a slide can look quite different from the whole cell.

Sub-cellular structures

The receptor molecules in the cell membrane detect chemical signals such as hormones, and because the membrane is selective, the cell can hold on to the substances it needs and keep others out.

Bacterial cells

A bacterial cell is much smaller than a plant or animal cell. Its genetic material is not in a nucleus but lies free in the cytoplasm, mostly as one circular chromosome. It may also carry small rings of extra DNA called plasmids, which carry a few genes and can be passed between bacteria. Bacterial cells have a cell membrane, cytoplasm, ribosomes and a cell wall, but no mitochondria or chloroplasts.

Comparing plant and animal cells

Plant and animal cells share a nucleus, cytoplasm, membrane, mitochondria and ribosomes. Plant cells also have a cell wall, a permanent vacuole and, in the parts that are exposed to light, chloroplasts. Animal cells have none of these three, and they often store food as small grains of glycogen instead.

Light microscopes

A light microscope shines light through a thin specimen. The specimen is mounted on a slide and usually covered with a thin square of glass called a cover slip. It sits on the stage, and lenses at the eyepiece and on the objective turret magnify the image. A lamp or mirror below the stage supplies the light.

Most cells are transparent, so a specimen is often coloured with a stain. A stain either makes a colourless specimen visible or picks out particular structures. Methylene blue stains the nucleus of cheek cells, and iodine solution makes the nuclei of onion epidermis cells stand out.

Preparing a slide

1. Put a drop of water or stain on a clean slide.
2. Place a thin piece of specimen on the drop, for example a layer of the inner surface of an onion scale leaf peeled off with forceps, or cells taken from the inside of the cheek with a clean cotton bud.
3. Lower a cover slip onto the specimen at an angle, using a mounted needle, so that air bubbles are not trapped beneath it.
4. Put the slide on the stage and start with the lowest-power objective lens.
5. Turn the coarse focus knob until the image is roughly in focus, then use the fine focus knob to sharpen it. Move to a higher-power lens only when the specimen is in focus at low power.

A scientific drawing uses a sharp pencil and clear continuous lines with no shading. It is large, labelled with straight lines that do not cross, and given a title and the magnification used.

Magnification

Magnification says how many times larger an image is than the real object.

Both measurements must be in the same unit before dividing. The equation can be rearranged to find any of the three quantities.

Units

Cells are small, so they are measured in micrometres.

There are 1,000 µm in 1 mm and 1,000 nm in 1 µm. To change from millimetres to micrometres, multiply by 1,000.

Worked example

A drawing of a plant cell is 45 mm across, and the magnification is $\times 400$.

In standard form, $0.1125\ \text{mm} = 1.125 \times 10^{-1}\ \text{mm}$, which is $1.125 \times 10^{-4}\ \text{m}$.

If the same cell were viewed with an eyepiece of $\times 10$ and an objective of $\times 40$, the total magnification of the microscope is $10 \times 40 = 400$.

A sensible check is an estimate. A cell of about 100 µm across drawn at a width of 45 mm implies a magnification of roughly $45\,000 \div 100 = 450$, which agrees with the answer above.

Resolution and electron microscopes

Resolution is the smallest distance between two points at which they can still be seen as separate. Magnification only makes an image bigger. If the resolution is poor, a larger image is just a bigger blur.

A light microscope cannot resolve anything closer together than about 0.2 µm, so the smallest structures are beyond it. Electron microscopes use a beam of electrons instead of light. In a transmission electron microscope the beam passes through a very thin slice of specimen. Electrons have a much shorter wavelength than light, so the resolution is far higher, and the useful magnification is many thousands of times greater.

Electron microscopy allowed biologists to see:

• the internal folded structure of mitochondria and the membranes inside chloroplasts, which light microscopes show only as small blobs;
• ribosomes, which are too small to be seen with light;
• plasmids and the detail of the bacterial cell, and the layers of the cell membrane.

This changed how cells were understood, because a structure that had been seen as a blob could then be linked to how it worked. The specimen must be dead and very thin for an electron microscope, and the image is formed in black and white, but the extra detail was a large step in understanding sub-cellular structures.