Cell Structure
Structures and Functions in Living Organisms · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Cell structure and organelle function appear on nearly every paper; one of the highest-frequency topics.
Bacterial cells (prokaryotes)
Bacteria are very different from animal and plant cells. They are prokaryotic, which means they have no and no membrane-bound at all. The main features of a bacterial cell are:
- A made of peptidoglycan (not cellulose; this is different from a plant cell wall)
- A just inside the wall
- 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
- (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
- membrane
- (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).
Features of bacterial (prokaryotic) cells
Naming the features of a bacterial cell comes up, so you need to know bacteria have no nucleus — their DNA is a single circular chromosome plus plasmids — alongside a cell wall, membrane, cytoplasm and ribosomes, but no mitochondria or chloroplasts. Write "circular chromosome", not "nucleus", and name plasmids as a separate point.