Cell structure — AQA GCSE Combined Science: Trilogy

Revision notes

6 short notes, in the order of the specification. Each one in short:

1. Eukaryotes and prokaryotes

   Eukaryotic cells, such as plant and animal cells, have a cell membrane, cytoplasm and genetic material enclosed in a nucleus. Prokaryotic cells, such as bacteria, are much smaller. Their genetic material is not enclosed in a nucleus: it is a single DNA loop, and there may be one or more small rings of DNA called plasmids.

2. Animal and plant cells

   Most animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells have these too, and often also have chloroplasts, which absorb light for photosynthesis, and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

3. Required practical 1: using a light microscope

   Practical

   Required practical 1 uses a light microscope to observe, draw and label plant and animal cells. Mount thin tissue on a slide with water or a stain and a cover slip, focus on the lowest-power objective with the coarse focusing knob, then sharpen with the fine knob. Total magnification is the eyepiece lens magnification multiplied by the objective lens magnification.

4. Cell specialisation

   A specialised cell has structures that suit it to a particular function. A sperm cell has a tail and many mitochondria to release energy for swimming. A nerve cell is long, with branched connections, to carry impulses. A root hair cell has a long hair-like projection, giving a large surface area to absorb water and mineral ions.

5. Cell differentiation

   Differentiation is the process by which a cell acquires different sub-cellular structures so that it can carry out a certain function, becoming a specialised cell. Most types of animal cell differentiate at an early stage, but many types of plant cell keep the ability to differentiate throughout life. In mature animals, cell division is mainly for repair and replacement.

6. Microscopy

   An electron microscope uses electrons instead of light, so it has much higher magnification and resolving power than a light microscope. This lets biologists see much finer detail, such as ribosomes and the internal structure of mitochondria and chloroplasts. Magnification is calculated with magnification = size of image ÷ size of real object, using the same units for both.