A2.2 Cell structureIB Biology HL: Revision notes

Cell theory: all organisms consist of one or more cells, the basic unit of life, so by deductive reasoning a newly discovered organism is predicted to be cellular. All cells have DNA, cytoplasm (mainly water) and a lipid plasma membrane.

Microscopy: make temporary mounts, stain, focus with coarse then fine adjustment, and measure with an eyepiece graticule calibrated against a stage micrometer. Magnification = image size ÷ actual size; a scale bar length on an image = actual length × magnification. Electron microscopy gives far higher resolution; freeze fracture shows the inside of membranes; cryo-EM reveals protein structure; fluorescent stains and immunofluorescence locate specific molecules in light microscopy.

Prokaryote (Gram-positive eubacteria such as Bacillus): cell wall, plasma membrane, cytoplasm, naked looped DNA, 70S ribosomes.

Eukaryote: compartmentalised cytoplasm with 80S ribosomes; nucleus with DNA bound to histones, in a double membrane with pores; mitochondria, rough and smooth ER, Golgi apparatus, vesicles, vacuoles and lysosomes; a cytoskeleton of microtubules and microfilaments.

Plants: cellulose wall, large sap vacuole, chloroplasts and other plastids, no centrioles. Fungi: chitin wall, no plastids. Animals: no wall, small temporary vacuoles, centrioles, some cells with cilia or flagella.

A unicellular organism performs all eight functions of life: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.

Atypical numbers of nuclei: aseptate fungal hyphae and skeletal muscle fibres are multinucleate; red blood cells and phloem sieve tube elements have no nucleus. In micrographs, identify cells as prokaryote, plant or animal, and annotate drawings of organelles with their functions.

Evidence suggests all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. Mitochondria then evolved by endosymbiosis: an aerobic prokaryote was engulfed but not digested and became a permanent part of the cell. In some eukaryotes (algae and plants), chloroplasts subsequently arose in the same way from a photosynthetic prokaryote.

Evidence: mitochondria and chloroplasts have 70S ribosomes, naked circular DNA and the ability to replicate by division; they also have a double membrane and are about the size of bacteria. Antibiotics that block 70S ribosomes inhibit protein synthesis inside them.

Cell differentiation is the process by which cells in a multicellular organism develop into specialised tissues. Every cell has the same genome; the basis of differentiation is different patterns of gene expression, often triggered by changes in the cell's environment (e.g. signals from neighbouring cells or position in the embryo). In Volvox, expression of the gene regA in somatic cells makes them swimmers that cannot reproduce.

Multicellularity has evolved repeatedly, in separate lineages. Many fungi and eukaryotic algae, and all plants and animals, are multicellular. Advantages: larger body size (e.g. harder for predators to eat, able to exploit new habitats) and cell specialisation, allowing a division of labour between tissues. Volvocine algae show stages from single cells (Chlamydomonas) to colonies of similar cells (Gonium) to organisms with specialised cells (Volvox).