Cell structure

All organisms are built from cells, the basic unit of structure. Using deductive reasoning, cell theory predicts that a newly discovered organism will consist of one or more cells.

Practise making temporary mounts, staining, focusing with coarse and fine adjustment, and measuring with an eyepiece graticule. Magnification = image size / actual size. For example, a cell 8 µm wide drawn 40 mm wide has a magnification of 40 000 / 8 = ×5000.

Watch out: convert to the same units first. 1 mm = 1000 µm.

All cells have DNA as genetic material, cytoplasm composed mainly of water, where reactions take place, and a plasma membrane of lipids, which separates inside from outside.

A plasma membrane encloses a compartmentalized cytoplasm, with 80S ribosomes. The nucleus has a double membrane with pores, and chromosomes of DNA bound to histones. There is a cytoskeleton of microtubules and microfilaments.

Single-celled organisms carry out homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. In Paramecium, for example, contractile vacuoles expel water, food vacuoles digest food, and cilia provide movement.

Many nuclei: aseptate fungal hyphae and skeletal muscle. No nucleus: mature red blood cells and phloem sieve tube elements.

A prokaryote is small, with a nucleoid and a cell wall. A plant cell has a cell wall, chloroplasts and a large vacuole. An animal cell has a nucleus and mitochondria, but no cell wall. Practise finding each organelle in electron micrographs.

Use a sharp pencil and clear, continuous lines, without shading. Keep to the proportions of the micrograph. Annotate each structure with a function: for example, microvilli increase the surface area for absorption.