Cell structure
Light and electron microscopes allow us to see inside cells. Plant, animal and bacterial cells have smaller components each with a specific function.
Bacterial cells
Bacteria are all single-celled. The cells are all prokaryotic. This means they do not have a nucleus or any other structures which are surrounded by membranes. Larger bacterial cells may be visible using a light microscope, however an electron microscope would be needed to see the details of the cell organelles.
Plant and animal cells have some components in common with bacterial cells. These include the cytoplasm and cell membrane. Bacteria have other components that are unique:
| Structure | How it is related to its function |
|---|---|
| Chromosomal DNA | The DNA of bacterial cells is found loose in the cytoplasm. It is called chromosomal DNA and is not contained within a nucleus. |
| Plasmid DNA | Bacteria also have small, closed-circles of DNA called plasmids present in their cytoplasm. Unlike the chromosomal DNA, plasmid DNA can move from one bacterium to another giving variation. |
| Flagella | Bacteria can have one or more flagella (singular: flagellum). These can rotate or move in a whip-like motion to move the bacterium. |
| Cell wall | Plant and bacterial cell walls provide structure and protection. Only plant cell walls are made from cellulose. |
Bacteria are amongst the simplest of organisms. Their cells do not divide by mitosis. Instead they copy themselves by binary fission. The process is similar, but we use a different name for it because prokaryotic bacteria are very different from other eukaryotic plant and animal cells.
Eukaryotic and prokaryotic cells can be compared:
| Feature | Eukaryotic cell (plant and animal cell) | Prokaryotic cell (bacterial cell) |
|---|---|---|
| Size | Most are 5 μm – 100 μm | Most are 0.2 μm – 2.0 μm |
| Outer layers of cell | Cell membrane. Surrounded by cell wall in plants and fungi. | Cell membrane. Surrounded by cell wall. |
| Cell contents | Cytoplasm. Cell organelles include mitochondria, chloroplasts in plants and ribosomes. | Cytoplasm. Ribosomes present. There are no mitochondria or chloroplasts. |
| Genetic material | DNA in a nucleus. Plasmids are found in a few simple eukaryotic organisms. | DNA is a single molecule, found free in the cytoplasm. Additional DNA is found on one or more rings called plasmids. |
| Type of cell division | Mitosis | Binary fission |