Cells and organisation · GCSE Biology
Bacterial cells
Label a bacterial cell for GCSE Biology: chromosomal DNA loop, plasmids, flagella, cell wall, cytoplasm and membrane, and why bacteria lack a nucleus and mitochondria.
Bacterium: prokaryotic. DNA loop plus plasmids, no nucleus, no mitochondria, no chloroplasts. Wall is not cellulose. Flagellum for movement if shown.
The important bits
What you need to know
- 1
Bacteria are prokaryotic unicellular organisms, typically 1–5 µm long — smaller than most animal cells (~20 µm) but larger than viruses.
- 2
Chromosomal DNA is a single circular loop in the cytoplasm, not enclosed in a nucleus. Do not label this loop as a nucleus.
- 3
Plasmids are extra small rings of DNA. They can replicate independently and often carry genes for antibiotic resistance or toxins. They are the vectors used to insert the human insulin gene in genetic engineering.
- 4
The cytoplasm contains enzymes for metabolism and smaller ribosomes for protein synthesis. There are no membrane-bound organelles: no mitochondria, no chloroplasts, no vacuole of the plant type.
- 5
A cell membrane controls entry and exit. Outside it, a cell wall provides shape and support; it is not made of cellulose (unlike plants). Some species have a slime capsule that reduces drying and helps evade phagocytosis.
- 6
A flagellum (plural flagella) allows motility in liquid. Not all bacteria have one; only label it if the diagram shows a tail-like structure.
- 7
Bacteria reproduce asexually by binary fission: DNA is copied, then the cell splits into two genetically identical cells (clones), unless plasmids or mutations add variation.
- 8
Some bacteria are pathogens (Salmonella, gonorrhoea). Others are useful: Lactobacillus in yoghurt, decomposers in the carbon cycle, and genetically engineered E. coli producing insulin.
Quotations worth analysing
Short evidence. Real method.
“Bacterial cells have cytoplasm, a cell membrane, a cell wall, chromosomal DNA and plasmids.”
This is the minimum label set. Add flagellum or capsule only if the diagram or the extra-mark demand includes them.
“Plasmids are small rings of DNA.”
Use “ring” or “loop”, not “chromosome”. Chromosomal DNA is the large loop; plasmids are the extras.
“Bacteria have no nucleus and no mitochondria.”
Pair this with “DNA is a loop in the cytoplasm” and “respiration occurs in the cytoplasm”.
Go deeper
How do I label a bacterium without inventing a nucleus?
Put the large coiled loop in the cytoplasm and call it chromosomal DNA. Put any small separate rings as plasmids. The outer thick line is the cell wall; the inner thin line is the cell membrane. Cytoplasm fills the interior. Ribosomes may be shown as dots; they synthesise proteins. A long whip is a flagellum for movement. There is no nuclear membrane, no mitochondria, no chloroplasts. If the question asks for differences from an animal cell, lead with no nucleus, DNA as a loop, plasmids, cell wall, and much smaller size. If it asks for differences from a plant cell, stress that the bacterial wall is not cellulose and there are no chloroplasts or permanent vacuole. Size is a useful check: 2 µm is bacterial; 20 µm is likely animal.
Go deeper
Why do plasmids matter beyond the diagram?
Plasmids carry genes that can move between bacteria, including antibiotic-resistance genes. That is a natural-selection story: a random mutation or a received plasmid lets a bacterium survive penicillin; it reproduces by binary fission; the resistance allele becomes more common. In genetic engineering, restriction enzymes cut a plasmid and a human gene (for example insulin) at matching sites; ligase seals the gene into the plasmid; the plasmid is inserted into bacteria; those cells are cultured and produce the protein. So the same small ring appears in cell biology, infection, and inheritance papers. In the exam, “plasmid is a nucleus” loses both topics. Write “small ring of DNA that can carry extra genes”.
Go deeper
How do bacteria reproduce, and is that mitosis?
Binary fission is not mitosis. Mitosis is a eukaryotic process with a nucleus, visible chromosomes lining up, and two identical nuclei. Bacteria have no nucleus: the DNA loop is replicated, the cell grows, and it splits into two. Offspring are clones unless mutation or plasmid transfer adds new DNA. Conditions that speed binary fission are warmth, moisture, nutrients and oxygen for aerobes — the same conditions that speed decay. That is why food is refrigerated and why compost heaps are kept aerated. A growth curve (lag, exponential, stationary, death) appears on some Triple papers: exponential means the population doubles at a regular interval when resources are unlimited. Do not write that bacteria “do mitosis in their nucleus”.
See the idea in action
A cell is 3 µm long with a wall, membrane, cytoplasm, one large DNA loop, three small DNA rings and a flagellum. It is a bacterium (prokaryotic). The rings are plasmids. It has no nucleus, so it is not an animal or plant cell. Protein synthesis occurs at ribosomes in the cytoplasm. If this cell carries a plasmid with a resistance gene, antibiotics may fail to kill it, and binary fission will copy that plasmid into daughter cells.
Exam technique
Turn knowledge into marks
Learn a standard labelled sketch: wall, membrane, cytoplasm, chromosomal DNA, plasmid, flagellum. In prose, say prokaryotic, no nucleus, DNA loop. Never transfer plant cellulose or mitochondria onto the bacterium.
Common mistakes
Do not give these marks away
- 01
Labelling the DNA loop as a nucleus, or adding mitochondria and chloroplasts to bacteria.
- 02
Saying the bacterial cell wall is cellulose.
- 03
Calling binary fission mitosis, or claiming plasmids are found in human body cells.
Which statement about a typical bacterial cell is correct?
AIt has a nucleus and mitochondria
BIts chromosomal DNA is a loop in the cytoplasm and it may have plasmids
CIt has chloroplasts containing cellulose
DIt is eukaryotic and about 20 µm across
Show the answer
Its chromosomal DNA is a loop in the cytoplasm and it may have plasmids. Bacteria are prokaryotic: no nucleus, no mitochondria, DNA as a loop plus optional plasmids. They are typically a few micrometres long, not 20 µm.
Quick questions
If this is the bit you searched
Do bacterial cells have a nucleus?
No. They are prokaryotic. Chromosomal DNA is a single loop in the cytoplasm, and they may also have plasmids.
What is the function of a flagellum in bacteria?
Movement. The flagellum rotates or whips so the cell can swim in liquid. Not all bacteria have flagella.
Why are plasmids important in genetic engineering?
They are small rings of DNA that can be cut and have a useful gene inserted, then placed back into bacteria to make a protein such as insulin.
How do bacteria reproduce?
By binary fission: DNA is copied and the cell splits into two genetically identical cells, unless mutation or plasmid transfer adds variation.