WJEC GCSE Biology

Animal and plant cells

The cell is the basic unit of living things, and most of what a cell does is carried out by parts inside it. Animal and plant cells share most of these parts, and plant cells have three extra ones.

An animal cell therefore has a membrane, cytoplasm, nucleus and mitochondria. A plant cell has all of those as well as a cell wall, chloroplasts and a permanent vacuole. Root cells have no chloroplasts because no light reaches them.

Using a light microscope

A light microscope magnifies a thin specimen by passing light through it, and lets you see the nucleus, cytoplasm, cell membrane and, in plants, the cell wall and vacuole. Mitochondria are too small to show detail with a school microscope.

The total magnification is the eyepiece lens magnification multiplied by the objective lens magnification, so a $\times 10$ eyepiece with a $\times 40$ objective gives $\times 400$.

The image size, actual size and magnification are linked by

The two sizes must be in the same units before dividing. For example, if a cell is drawn 45 mm across and the magnification is $\times 300$, its actual width is $45 \div 300 = 0.15$ mm, which is $150\ \mu\text{m}$ (1 mm is 1,000 $\mu$m). Rearranged, the actual size is image size divided by magnification and the image size is actual size multiplied by magnification.

Specified practical: examining animal and plant cells

The usual specimens are a thin layer of onion epidermis (a plant) and cheek cells scraped gently from inside the mouth (an animal).

1. Place a drop of water on a clean slide. For the onion, peel a small piece of the thin layer from the inside of a scale leaf and lay it flat in the drop. For the cheek cells, rub a clean cotton bud or sterile spatula inside the cheek and smear it on the slide.
2. Add a drop of a stain, which makes colourless structures easier to see. Iodine solution is often used for plant cells and methylene blue for cheek cells.
3. Lower a coverslip onto the specimen at an angle with a needle, so that air bubbles are not trapped.
4. Clip the slide to the stage, start with the lowest-power objective lens and focus with the coarse focus, then change to a higher power and focus with the fine focus only.
5. Draw what you see.

Safety matters because the stains can irritate skin and stain clothes, and a cheek swab slide should be disposed of in disinfectant afterwards rather than kept.

A good scientific drawing is done in sharp pencil with clear, continuous lines and no shading. It is large enough to show detail, it draws only what is seen (not every cell in the field of view, just a few clearly outlined ones), and its labels are written in straight lines that do not cross, each touching the part it names with a ruled line. A title and the magnification are added.

The microscope itself changed how cells were understood. Light microscopes first showed that living things are made of cells, and later improvements in lenses and in electron microscopes revealed smaller structures, so the understanding of cell structure grew as the instrument improved.

Specialised cells

In a multicellular organism, cells change during development so that each type is suited to its job. This change is called differentiation, and the result is a specialised cell. A specialised cell has the structures that help its function, often in greater numbers.

Levels of organisation

Similar specialised cells are found together. A tissue is a group of similar cells with a similar function, such as muscle tissue. An organ is made of several tissues working together to perform specific functions, and the stomach is an example, made of muscle, glandular and epithelial tissues. Organs are arranged into organ systems, such as the digestive system, which work together to make up the whole organism.