A2.2 Cell structureIB Biology SL: Revision notes

Cell theory states that all living organisms are made of one or more cells, and that the cell is the smallest unit of life. Because the theory is so well supported, deductive reasoning lets biologists predict that a newly discovered organism will consist of one or more cells before they have examined it.

All typical cells share: DNA as genetic material (instructions for making proteins, copied when the cell divides); cytoplasm composed mainly of water, where metabolism happens; and a plasma membrane made of lipids, which separates the inside from the surroundings and controls what enters and leaves.

Temporary mounts: a thin layer of cells in a drop of water or stain (e.g. methylene blue, iodine) under a cover slip. Focus with the coarse then the fine adjustment.

An eyepiece graticule is calibrated against a stage micrometer for each objective. Then:

• actual size = image size ÷ magnification
• magnification = image size ÷ actual size

Convert units first: 1 mm = 1000 µm, 1 µm = 1000 nm. A scale bar is a line on an image labelled with the actual length it represents; its length on the image = actual length × magnification. Measurement with instruments is quantitative observation.

Electron microscopes have far higher resolution (below 1 nm, against about 200 nm for light), revealing ultrastructure. Freeze fracture splits frozen membranes through the middle of the bilayer, showing transmembrane proteins. Cryogenic electron microscopy (cryo-EM) freezes samples very rapidly without stain and can reveal the 3D structure of proteins. In light microscopy, fluorescent stains make particular structures glow, and immunofluorescence uses antibodies carrying a fluorescent marker to show where a specific protein is. Light microscopes can still show living cells and colour.

Prokaryotes (e.g. Gram-positive Bacillus, Staphylococcus): cell wall, plasma membrane, cytoplasm, naked DNA in a loop (nucleoid) and 70S ribosomes. Prokaryote structure varies, but this is the type you need.

Eukaryotes: a plasma membrane enclosing a compartmentalised cytoplasm with 80S ribosomes; a nucleus with chromosomes of DNA bound to histones, inside a double membrane with pores; membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles and vacuoles including lysosomes); and a cytoskeleton of microtubules and microfilaments.

A unicellular organism carries out all the functions of life in one cell: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.

• Cell walls: plants (cellulose), fungi (chitin), animals none.
• Vacuoles: plants have a large permanent sap vacuole; animal vacuoles are small and usually temporary (e.g. food vacuoles); fungi often have large vacuoles.
• Chloroplasts and other plastids: plants only (e.g. amyloplasts store starch).
• Centrioles: animals; absent in most plants and fungi. Cilia and flagella occur in animal cells (e.g. sperm, airway epithelium) and some other eukaryotes, but not in flowering plant cells.

Some eukaryotic cells have unusual numbers of nuclei: aseptate fungal hyphae and skeletal muscle fibres have many; red blood cells and phloem sieve tube elements have none.

In micrographs, identify cells as prokaryote, plant or animal, and recognise the nucleoid, cell wall, nucleus, mitochondrion, chloroplast, sap vacuole, Golgi apparatus, rough and smooth ER, chromosomes, ribosomes, plasma membrane and microvilli. Annotated drawings must include the function of each labelled structure (e.g. rough ER: synthesis of proteins for secretion; Golgi: processing and packaging proteins into vesicles; microvilli: increase surface area for absorption).