What you need to know
- 3.1Common features of cells.
- 3.5(i–ii)prokaryoticDescribes cells with no nucleus and no membrane-bound organelles, such as bacteria. Their DNA is free in the cytoplasm. ultrastructureThe detailed structure of a cell as seen with an electron microscope. and functions of the specified structures.
By the end, you should be able to…
- Identify chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere., plasmids, ribosomeA small organelle made of rRNA and protein, where translation takes place. and surface structures.
- Compare equivalent features in prokaryotic and eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. cells.
- Avoid assuming every bacterium has every optional structure.
Build your understanding
A cell without a membrane-bound nucleus
Prokaryotes are cellular organisms whose DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. is not enclosed in a membrane-bound nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores.. They have a cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave., cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. and ribosomeA small organelle made of rRNA and protein, where translation takes place., but lack the membrane-bound organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome. typical of eukaryotes. Absence of mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. does not mean an absence of respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP.; relevant reactions can occur at the cell membrane and in the cytoplasm.
A cell without a membrane-bound nucleus (continued)
The specification’s model is a typical bacterial cell. Its main chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. is usually a circular DNA molecule in a nucleoid region. It is not inside a nuclear envelopeThe double membrane around the nucleus. Its pores let mRNA leave.. Bacterial ribosomes are 70S, smaller than the 80S ribosomes of the eukaryoticDescribes cells with a nucleus and membrane-bound organelles, such as animal and plant cells. cytoplasm. S describes sedimentation behaviour, not a length in nanometres.
Chromosome and plasmids
The main chromosome carries many geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. needed for cell function. Plasmids are smaller DNA molecules separate from the main chromosome that can replicate independently. They may carry useful additional genes, including some antibiotic-resistance genes, but not every bacterial cell has a plasmid.
A plasmid is not a membrane-bound organelle or a tiny nucleus. Having a plasmid does not establish that a bacterium causes disease, and resistance cannot be inferred merely from seeing a circular DNA diagram.
Walls, capsules and attachment
A typical bacterial cell wall contains peptidoglycan and helps maintain shape and resist bursting when water enters. The cell-surface membrane beneath it regulates exchange. Do not call the wall celluloseA structural polysaccharide of β-glucose. Straight chains are linked by hydrogen bonds into microfibrils that strengthen plant cell walls.; cellulose is associated with plant cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting..
Some bacteria have a capsule outside the wall that can help adhesion and protect the cell, for example against desiccation or host defences in certain contexts. Pili are short protein projections that can help attachment; specialised pili are involved in DNA transfer between cells. These functions should not be assigned to every projection in every speciesA group of organisms that can interbreed to produce fertile offspring..
Movement and comparison
Some bacteria have flagella whose rotation helps move the cell through liquid. A bacterial flagellum differs structurally from a eukaryotic flagellum; do not assume that similar functions imply identical ultrastructureThe detailed structure of a cell as seen with an electron microscope..
Compare by feature: location of DNA, chromosome form, membrane-bound organelles and ribosome type. Both cell types have a membrane and cytoplasm, and use genetic information and ribosomes. Prokaryotes are generally smaller, but size alone is not a secure classification test.
NOT: 70S is not a length in nanometres, and a plasmid is not a nucleus.
- No membrane-bound organelles
- Respiration can occur at the membrane and in the cytoplasm
- Capsule, flagellum and pili are not universal
- Peptidoglycan is typical of bacterial walls, not of Archaea as a rule to assume here
Two traps this diagram blocks
- Do not draw a nuclear envelope around the bacterial chromosome.
- Do not treat a plasmid as evidence that the cell causes disease.
From the diagram alone
Question
Give three structural differences between a typical bacterium and a eukaryotic cell. [3]
Show a model answer
The bacterium has no nuclear envelope, its ribosomes are 70S rather than 80S, and it has no mitochondria. It also has a cell wall outside the membrane, which an animal cell does not.
| Structure | Function or significance |
|---|---|
| Circular chromosome | Main genetic information in nucleoid region |
| Plasmid | Additional independently replicating DNA |
| 70S ribosomes | Polypeptide synthesis |
| Cell wall | Shape and resistance to osmotic bursting |
| Capsule, when present | Protection and adhesion |
| Pili, when present | Attachment; specialised forms help DNA transfer |
| Flagellum, when present | Movement |
Make your reasoning visible
Say “no membrane-bound nucleus”
Prokaryotes still have DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code., so “no nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores., therefore no genetic information” is wrong.
Use “some” for optional features
Capsules, plasmids, pili and flagella vary between cells and speciesA group of organisms that can interbreed to produce fertile offspring..
Compare matching categories
Pair DNA location with DNA location, or ribosomeA small organelle made of rRNA and protein, where translation takes place. type with ribosome type, rather than listing unrelated differences.
Avoid this misconception
The claim: Bacteria have no organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome., so they cannot synthesise proteins.
The correction: They have ribosomes, which are not membrane-bound and carry out protein synthesis.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Give two features shared by typical bacterial and eukaryotic cells.
Show suggested answer
A cell-surface membrane, cytoplasm, DNA and ribosomes are shared features; any two are suitable.
Practice question
Retrieval and explanation · suggested answer
Distinguish the roles of a bacterial cell wall and flagellum.
Show suggested answer
The wall supports shape and helps resist osmotic bursting; a flagellum provides movement in bacteria that possess it.
Practice question
Retrieval and explanation · suggested answer
Why is “all bacteria have plasmids carrying resistance genes” incorrect?
Show suggested answer
Not all bacteria have plasmids, and plasmids can carry different genes. Resistance is one possible function, not a universal property.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
An electron image shows a small cell but no nucleus. What additional observations would strengthen a prokaryotic interpretation?
Show a hint
Avoid relying on size or one missing feature alone.
Explore the reasoning
Evidence across suitable sections for no membrane-bound organelles, DNA in a nucleoid region and bacterial surface organisation would help. A single thin section might miss a eukaryotic nucleus.
Recall this lesson
Lesson recall questions
Question 1 of 5: State the shared structural unit of all living organisms.
Cells as a shared organisation
Practice question
Retrieval and explanation · suggested answer
State the shared structural unit of all living organisms.
Lesson: 3A 5: The Organisation of Cells
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Useful extra DNA
3A · Prokaryotic CellsNHGRI describes plasmids as DNA separate from the chromosome and notes their use in research and their potential to carry resistance genes. A plasmid is a useful extra…Specification 3.5(i)Read & use the scienceClose article
Why this matters
NHGRI describes plasmids as DNA separate from the chromosome and notes their use in research and their potential to carry resistance genes. A plasmid is a useful extra piece of bacterial DNA, not a second nucleus. Its genes can affect the phenotype even though a school micrograph cannot show those genes. Microbiologists therefore combine cell observations with culture tests and DNA analysis when investigating resistance. Not every plasmid carries an antibiotic-resistance gene, and possession does not guarantee expression. The core cell comparison remains simpler: prokaryotes lack membrane-bound organelles and usually have circular chromosomal DNA. The real case explains why listing a plasmid in a diagram has consequences.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.5(i) know the ultrastructure of prokaryotic cells, including cell wall, capsule, plasmid, flagellum, pili, ribosomes and circular DNA
Use the science
Why can two cells with similar visible structures respond differently to an antibiotic?
Explore a possible response
They may carry different genes or express them differently. Appearance alone does not reveal the full genetic basis of susceptibility.
Explore the source
- NHGRI: plasmidsNational Human Genome Research Institute
The mobile resistance gene
3A · Prokaryotic CellsThe CDC reported that the resistance gene mcr-1 can occur on a plasmid, a small DNA molecule separate from a bacterium’s chromosome. Plasmids can move between bacteria, potentially…Specification 3.5(i) · 3.5(ii)Read & use the scienceClose article
Why this matters
The CDC reported that the resistance gene mcr-1 can occur on a plasmid, a small DNA molecule separate from a bacterium’s chromosome. Plasmids can move between bacteria, potentially spreading resistance to colistin. That gives the labelled plasmid in a prokaryote diagram a real consequence. A laboratory would still have to test whether an isolate carries the gene and whether it is resistant; seeing a plasmid-like structure is not sufficient. The exam point remains the location and function of prokaryotic structures. This case invites students to connect extra-chromosomal DNA to an observable phenotype while recognising that the connection needs genetic and susceptibility evidence. A superbug headline can be accurate about a risk without proving that every carrier is untreatable.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.5(i) know the ultrastructure of prokaryotic cells, including cell wall, capsule, plasmid, flagellum, pili, ribosomes and circular DNA
- 3.5(ii) understand the function of the structures listed in (i)
Use the science
Why could two bacteria of the same species differ in colistin susceptibility?
Explore a possible response
One may carry and express a resistance gene on a plasmid while the other does not. A susceptibility test and genetic evidence would check this.
Explore the source
- Federal scientists find the MCR-1 resistance geneUS Centers for Disease Control and Prevention