Animal and plant cell structures and their functions, examples of specialised cells and their adaptations, the levels of organisation from cell to organism, and using a light microscope including magnification calculations.

What this dot point is asking

CCEA Double Award wants you to know the parts of animal and plant cells and what each part does, how some cells are specialised for a particular job, how cells are organised into tissues, organs and systems, and how to use a light microscope including the magnification calculation. This is shared with the separate Biology course, but for Double Award you are not asked for the extra cell-biology depth.

Animal and plant cell structures

A plant cell has three extra structures: a cell wall made of cellulose for support and shape, chloroplasts containing chlorophyll for photosynthesis, and a large permanent vacuole full of cell sap that keeps the cell turgid.

It helps to remember that every structure in the list maps to a job. If a question asks why a cell can carry out lots of respiration, point to many mitochondria. If it asks how the cell keeps its shape, point to the cell wall in a plant cell or the cell membrane controlling water balance in an animal cell.

Specialised cells

Cells become specialised so they can do a particular job well. CCEA expects named examples:

• A red blood cell has no nucleus and a biconcave shape, giving more room and a large surface area to carry oxygen as oxyhaemoglobin.
• A root hair cell has a long thin extension that increases the surface area for absorbing water and minerals from the soil.
• A nerve cell (neurone) is very long and thin to carry electrical impulses quickly over a distance.
• A sperm cell has a tail (flagellum) to swim to the egg and many mitochondria to release the energy for swimming.

The pattern in every example is the same: an adaptation, then the reason it helps. Markers want both halves, so write "long and thin so that impulses travel quickly", not just "long and thin".

Levels of organisation

A good worked example is the digestive system: muscle and gland cells form tissues, those tissues build the stomach (an organ), the stomach joins the intestines and pancreas to form the digestive system, and that system is one of several that make up the human organism.

Using a light microscope and magnification

A light microscope uses lenses to magnify a thin specimen on a slide. To prepare a slide you place the specimen on the slide, add a drop of water and a stain (such as iodine for plant cells), then lower a coverslip slowly to avoid air bubbles. You focus first on low power, then switch to high power.

The same triangle works in reverse. If you know the real size and the magnification, multiply them to get the image size; if you know the image size and the magnification, divide to get the real size. Practise rearranging it until you do not have to think.

Examples in context

Example 1. Why red blood cells lose their nucleus. A mature red blood cell has no nucleus. This frees up space inside the cell so it can be packed with more haemoglobin, the protein that carries oxygen. The biconcave disc shape also gives a larger surface area for oxygen to diffuse in and out. This is a clear case of structure fitting function: every adaptation makes the cell better at carrying oxygen, even though losing the nucleus means the cell cannot repair itself and only lasts about 120 days.

Example 2. Estimating cell size with a microscope. A student views onion cells along the diameter of the field of view, which is 2 mm wide. They count eight cells lying end to end across the diameter. The mean length of one cell is 2 mm divided by 8, which is 0.25 mm, or 250 micrometres. This method, estimating size from the field of view, is a standard CCEA practical skill and shows why you must always know the real width of your field of view.

Try this

Q1. Name the part of a cell that controls its activities and contains the DNA. [1 mark]

• Cue. The nucleus.

Q2. A cell is 0.1 mm wide and its image is 25 mm wide. Calculate the magnification. [2 marks]

• Cue. Magnification equals 25 divided by 0.1, which is x250.