1.1.3The Nature and Variety of Living Organisms
Common Features: Prokaryotic Organisms
5 min read 5 parts Lesson 4 of 5 Unit quiz
Section 1 of 5Features of Prokaryotes
Single-celled
- Prokaryotes are always single-celled, and bacteria are the example you need to know.
No nucleus
- Their DNA is not enclosed in a nucleus but lies free in the cytoplasm.
Circular chromosome
- The main genetic material is a single circular chromosome of DNA.
Small size
- Prokaryotic cells are much smaller than eukaryotic cells, and bacteria are visible only with a microscope.
Section 2 of 5Inside a Bacterial Cell
Inside a Bacterial Cell
Cell wall
- A cell wall surrounds the bacterium, protecting it and maintaining its shape.
Cell membrane
- The cell membrane lies inside the wall and controls what enters and leaves the cell.
Cytoplasm
- The cytoplasm is where the chemical reactions of the cell take place and where the DNA is found.
Plasmids
- Plasmids are small extra rings of DNA that carry additional genes, often separate from the main chromosome.
Missing structures
- Bacteria have no mitochondria and no other membrane-bound organelles.
Section 3 of 5Bacteria in Everyday Life
Shapes
- Bacteria come in different shapes, including rods, as in Lactobacillus bulgaricus, and spheres, as in Pneumococcus.
Useful bacteria
- Lactobacillus bulgaricus is used to turn milk into yoghurt.
Harmful bacteria
- Pneumococcus is a pathogen that causes pneumonia, an infection of the lungs.
Feeding
- Most bacteria feed on other living organisms or on dead organic matter, and those that feed on dead matter are decomposers.
Section 4 of 5Prokaryote and Eukaryote Cells Compared
1
Both types of cell have a cell membrane and cytoplasm, so these are shared features.
2
In a eukaryotic cell the genetic material is enclosed in a nucleus, whereas in a prokaryote it lies free in the cytoplasm.
3
Bacteria always have a cell wall, whereas only some eukaryotes, such as plants and fungi, have one.
4
Prokaryotes lack mitochondria and other organelles with membranes, which eukaryotic cells contain.
5
Prokaryotes are much smaller than eukaryotic cells and are never multicellular.
| Component | Eukaryote | Prokaryote |
|---|---|---|
| Nucleus | Present | Absent |
| Cell wall | Some types | Present |
| Mitochondria | Present | Absent |
| DNA | In nucleus | Circular, in cytoplasm |
Section 5 of 5Exam Checklist & Tips
1.1.3 – Key Points for the Exam
- Prokaryote A single-celled organism with no nucleus, such as a bacterium.
- Plasmid A small circular loop of DNA found in bacteria in addition to the main chromosome.
- Saprobiont An organism that feeds on dead organic matter, also called a decomposer.
- Pathogen An organism that causes disease.
- Circular chromosome The single ring of DNA that carries most of a bacterium's genes.
Common Mistakes
- Writing that bacteria have a nucleus; their DNA lies free in the cytoplasm.
- Saying all bacteria are harmful, when many are useful in food production and decomposition.
- Describing bacteria as plants or animals instead of a separate group of prokaryotes.
- Forgetting that bacterial cells do have a cell wall, unlike animal cells.