Eukaryotic cells
Animal and plant cells are examples of eukaryotic cells. Eukaryotic cells all have a cell membrane, cytoplasm, and genetic material that is enclosed in a nucleus. The genetic material is a chemical called DNA and this forms structures called chromosomes that are contained within the nucleus. All animals (including human beings), plants, fungi, and protista are eukaryotes.
Prokaryotes
Bacteria are single-celled living organisms. They are examples of prokaryotes. At 0.2-20 um in length prokaryotic cells are 1-2 orders of magnitude smaller than eukaryotic cells. You could fit hundreds of thousands of bacteria on to the full stop at the end of this sentence, so you cannot see individual bacteria without a powerful microscope. When you culture bacteria on an agar plate, you grow many millions of bacteria. This enables you to see the bacterial colony with your naked eye. Bacteria have cytoplasm and a cell membrane surrounded by a cell wall, but the cell wall does not contain the cellulose you see in plant cells. In prokaryotic cells the genetic material is not enclosed in a nucleus. The bacterial chromosome is a single DNA loop found free in the cytoplasm. Prokaryotic cells may also contain extra small rings of DNA called plasmids. Plasmids code for very specific features such as antibiotic resistance.
cell membrane
The semipermeable membrane surrounding the cytoplasm of a cell.
slime capsule
Some bacteria have a protective slime capsule around the outside of the cell wall.
cell wall
A rigid layer that surrounds the cell membrane in bacteria, providing structural support.
plasmids
Extra small rings of DNA in prokaryotic cells that code for very specific features such as antibiotic resistance.
Orders of magnitude
Orders of magnitude are used to make approximate comparisons between numbers or objects. If one number is about 10 times bigger than another, it is an order of magnitude bigger. You show orders of magnitude using powers of 10. If one cell or organelle is 10 times bigger than another, it is an order of magnitude bigger or 10’. If it is approximately 100 times bigger it is two orders of magnitude bigger or 102.
Example of orders of magnitude
A small animal cell has a length of around 10 um. A large plant cell has a length of around 100 pm. 100 = 10. So, a large plant cell is an order of magnitude or 10’ bigger than a small animal cell.
Key points
• Eukaryotic cells all have a cell membrane, cytoplasm, and genetic material enclosed in a nucleus. • Prokaryotic cells consist of cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not in a distinct nucleus. It forms a single DNA loop. Prokaryotes may contain one or more extra small rings of DNA called plasmids. • Bacteria are all prokaryotes.
