Cell Structure

Bacterial cells (prokaryotes)

Bacteria are very different from animal and plant cells. They are prokaryotic, which means they have no nucleus and no membrane-bound organelles at all. The main features of a bacterial cell are:

• A cell wall made of peptidoglycan (not cellulose; this is different from a plant cell wall)
• A cell membrane just inside the wall
• Cytoplasm filling the inside of the cell
• A single circular chromosome of DNA floating free in the cytoplasm
• Smaller circular pieces of DNA called plasmids, which can carry useful genes such as antibiotic resistance and which bacteria can swap between each other
• Ribosomes (smaller than the ribosomes in eukaryotic cells) for making proteins
• Some bacteria have a flagellum for swimming and pili for sticking to surfaces

Bacteria are also much smaller than eukaryotic cells. A typical bacterium is around 1 to 10 μm across, compared with 10 to 100 μm for an animal or plant cell.

Yeast cells: a useful comparison

Yeast is a single-celled fungus (so it is a eukaryote, not a prokaryote). Yeast cells are roughly oval-shaped and contain:

• A nucleus with chromosomes
• Cytoplasm
• Cell membrane
• Mitochondria (yeast respires both aerobically and anaerobically)
• Ribosomes
• A cell wall made of chitin (the same material as in multicellular fungi, different from both plant cellulose and bacterial peptidoglycan)

Yeast is important in two big industries: it makes carbon dioxide in bread baking (which makes the dough rise) and ethanol in brewing (anaerobic respiration of yeast on sugar produces alcohol).