CCEA GCSE Biology

Animal cells

An animal cell is bounded by a cell membrane, which controls what enters and leaves. Inside it the cytoplasm is the watery jelly where most chemical reactions of the cell take place. The nucleus, itself surrounded by a nuclear membrane, contains the chromosomes that carry the cell's genetic information and controls the activities of the cell. Mitochondria are the site of aerobic respiration, which releases the energy the cell uses, so cells that do a lot of work, such as muscle cells, have many of them.

Plant cells

A plant cell has all of those structures and three more that an animal cell lacks:

• a cell wall made of cellulose, which is outside the cell membrane, supports the cell and stops it bursting when water enters;
• a large permanent vacuole filled with cell sap, which keeps the cell firm and stores dissolved substances;
• chloroplasts, which contain chlorophyll and are where photosynthesis takes place. Only the green parts of a plant have them, so a root cell has none.

Bacterial cells

A bacterial cell is much smaller than a plant or animal cell and has a simpler structure. Its cell wall is not made of cellulose. It has no nucleus: its genetic material lies free in the cytoplasm, and as well as this main loop of DNA it often carries one or more small rings of DNA called plasmids. Plasmids can be passed from one bacterium to another, and they are the structures used in genetic engineering.

Using a light microscope

A light microscope lets you see cells by passing light through a thin specimen and magnifying the image with an eyepiece lens and an objective lens. The specimen has to be thin enough for light to pass through it, which is why a temporary slide is made from a very thin layer of tissue.

Making a temporary slide

1. Put a drop of water on a clean slide. For a plant cell, peel a thin layer of epidermis from the inside of an onion scale leaf with forceps and lay it flat in the drop. For an animal cell, gently scrape the inside of the cheek with a clean cotton bud and smear it on the slide.
2. Add a drop of stain: iodine solution for the onion, which makes the nucleus and cell wall easier to see, or methylene blue for cheek cells.
3. Lower a coverslip onto the specimen at an angle, using a mounted needle, so that air bubbles are not trapped beneath it.
4. Place the slide on the stage, start with the lowest-power objective lens and focus with the coarse focus, then use the fine focus and move to a higher power if needed.

Drawing what you see

A biological drawing is done in pencil with clear, continuous lines and no shading. It should fill a good part of the space, show only what is seen, and have label lines that are ruled and do not cross. The drawing is given a title and, where possible, the magnification used.

Light and electron microscopes

An electron microscope uses a beam of electrons in place of light. It has a much greater resolution, which means it can show two points that are very close together as separate. A light microscope cannot separate structures closer together than about 0.0002 mm.

Higher tier only: the greater resolution of the electron microscope has increased our understanding of cell structure, because it shows the fine detail inside cells, such as the internal structure of mitochondria and chloroplasts, and structures too small to see with light, such as ribosomes and the plasmids of bacteria. A light microscope magnifies but gives a blurred image beyond a certain point, so magnifying further shows no new detail.

Size and magnification

The sizes of cells are small, so units smaller than the millimetre are used. The SI units, each a thousand times smaller than the last, are:

So $1\ \text{mm} = 1000\ \mu\text{m}$. To convert millimetres to micrometres, multiply by 1000.

The size of a specimen can be estimated by comparing it with something of known size, such as the width of the field of view under a known objective, or measured with a ruler on a drawing or photograph.

Magnification links the size of an image to the size of the real object:

Both sizes must be in the same unit before dividing, and the equation can be rearranged to find either of the sizes.

Worked example

A drawing of a cheek cell is 54 mm across and the cell is 0.06 mm across in real life.

The magnification is $\times 900$. To find the real size of a cell whose image is 36 mm wide at $\times 400$:

Using a scale bar

Higher tier only: a scale bar is a line drawn on a micrograph that stands for a known real length. Measure the bar on the picture with a ruler and use it as the image size, with the length it stands for as the real size, to find the magnification. If a bar measures 25 mm and is labelled as 50 µm, then the magnification is $25\ \text{mm} \div 0.05\ \text{mm} = 500$. Once the magnification is known, the real size of any structure in the picture is its measured length divided by the magnification.