Prokaryotes, Viruses and Cell Comparisons · Notes

1.Lesson overview

Syllabus focus

Cambridge IAL syllabus reference

1.2 Cells as the basic units of living organisms

Edexcel IAL syllabus reference

3.5 Prokaryotic cell structure
6.5 Bacteria and viruses

AQA IAL syllabus reference

1.2 Cells and cell structure
2.10 Cells divide by binary fission and mitosis

AP Biology syllabus reference

2.1 Cell Structure and Function
2.2 Cell Size
2.10 Origins of Cell Compartmentalization

By the end of this lesson you should be able to

1
Identify the core structures of a bacterial prokaryotic cell.
2
Relate bacterial-cell structures to their functions.
3
Compare prokaryotic and eukaryotic cells using structural evidence.
4
Describe why viruses are non-cellular.
5
Compare the genome type and envelope of named viruses, including Ebola virus, TMV, HIV and lambda phage.
6
Explain why viruses depend on host cells for reproduction.
7
Interpret scale and visible features in micrographs without overclaiming.

A bacterium is a complete living cell, even though it lacks a nucleus and membrane-bound organelles. A virus is a different kind of biological entity: a nucleic-acid genome packaged for delivery into a host cell. The distinction affects classification, microscopy, disease treatment and experimental interpretation. The central question is not which particle is smaller; it is whether it has the machinery needed to maintain itself and reproduce. Bacteria have membranes, ribosomes and metabolism. Viruses have information and structural components but rely on a host cell to supply the rest.

How this chapter fits together

We will map a bacterial cell, compare its organisation with eukaryotes, then examine virus architecture and replication. The final sections use scale and evidence to decide what a diagram or micrograph can legitimately show. Keep the same three tests throughout: identify the structure, connect it to a function, and decide whether the evidence is enough for the claim being made.

2.The bacterial cell plan

A typical bacterium is a unicellular prokaryote. Its DNA is not enclosed in a nucleus and it lacks mitochondria, chloroplasts, ER and Golgi bodies. It still has a cell surface membrane, cytoplasm, ribosomes and regulated pathways for obtaining energy and building cell material. The absence of organelles does not mean the cell lacks biochemical organisation. Enzymes may be concentrated in the cytoplasm or associated with the cell membrane, and the membrane can support electron transport and ATP formation. A bacterium maintains gradients, repairs damage and responds to its surroundings using a compact but complete cellular system.

Bacterial cells commonly range from about 1–5 µm in diameter, although bacteria vary widely in shape and size. Their small dimensions create a high surface-area-to-volume ratio and short diffusion distances. “Typical bacterium” is a model, not a claim that every species has the same envelope or surface structures. Size is useful evidence but not a diagnosis: a small eukaryotic cell and a large bacterium may overlap in apparent dimensions. Always combine size with the presence or absence of a nucleus, organelles, wall type and ribosome type.

Core structures

Cell surface membrane

A phospholipid bilayer controlling exchange and enclosing the cytoplasm.

Peptidoglycan cell wall

A strong mesh outside the membrane that prevents bursting and maintains shape.

Cytoplasm

An aqueous region containing enzymes, metabolites, DNA and ribosomes.

Circular DNA

The main chromosome in the nucleoid region, not surrounded by a nuclear envelope.

70S ribosomes

Sites of protein synthesis in the cytoplasm.

Capsule, pili or flagellum

Optional structures that can protect, attach, exchange DNA or provide movement.

3.Bacterial structures and their functions

Structure–function relationships

Structure	Function	Biological consequence
Cell surface membrane	Selective barrier; transport and some energy-generating reactions	Maintains the internal chemical environment
Peptidoglycan wall	Support and resistance to osmotic lysis	Cell retains shape in dilute surroundings
Capsule	Protection from desiccation and immune attack; adhesion	May help form a biofilm
Flagellum	Movement in liquid	Can propel the cell towards or away from stimuli
Pili or fimbriae	Attachment; some pili transfer DNA	Supports colonisation or gene exchange
Plasmid	Additional circular DNA carrying accessory genes	May carry antibiotic-resistance genes
Ribosome	Translation of mRNA	Protein synthesis occurs without a nucleus
Nucleoid	Region containing the main chromosome	DNA is present without a nuclear membrane

Some bacteria have infoldings of the cell surface membrane associated with respiration or photosynthesis. Do not call these mitochondria or chloroplasts: the decisive difference is the absence of membrane-bound eukaryotic organelles. The cell surface membrane can also contain transport proteins, receptors and enzymes. Because the membrane is the boundary and a reaction surface at the same time, damage to it can disrupt both homeostasis and energy generation.

4.Cellular and acellular agents differ in reproduction

Prokaryotic cells lack a membrane-bound nucleus but have DNA, ribosomes, membrane and metabolism. Viruses contain genetic material in a protein coat and depend on host-cell machinery for replication; they do not divide by binary fission. Some bacteria have plasmids and a cell wall, but those features vary and should not be assumed universally. Antibiotics target bacterial processes and generally do not directly stop a virus whose replication uses host machinery.

5.Prokaryotic and eukaryotic cells compared

Both cell types have a cell surface membrane, cytoplasm, DNA and ribosomes. The decisive contrast is organisation: eukaryotic DNA is held in a nucleus and the cytoplasm is compartmentalised by membrane-bound organelles; prokaryotic DNA lies in a nucleoid and the cytoplasm lacks those organelles. This difference changes how information and metabolism are arranged. In a eukaryote, transcription occurs in the nucleus and mRNA can be processed before it reaches a cytoplasmic ribosome. In a bacterium, transcription and translation can occur in the same cytoplasmic region, allowing a rapid response but providing less membrane compartmentalisation.

Structural comparison

Feature	Typical prokaryote	Typical eukaryote
Nucleus	Absent; DNA in nucleoid	Present; DNA enclosed by nuclear envelope
Main DNA	Usually one circular chromosome	Several linear chromosomes
Additional DNA	Plasmids may be present	Circular DNA in mitochondria and chloroplasts
Ribosomes	70S in cytoplasm	80S in cytoplasm; 70S in mitochondria and chloroplasts
Membrane-bound organelles	Absent	Present, including ER, Golgi and mitochondria
Cell wall	Peptidoglycan in bacteria	Cellulose in plants; absent in animals
Typical size	About 1–5 µm diameter	Often about 10–100 µm, with wide variation
Cell division	Binary fission	Mitosis or meiosis in the cell cycle

Comparison rule

Use matched statements: “prokaryotes have 70S ribosomes in the cytoplasm, whereas eukaryotic cytoplasm has 80S ribosomes.”

6.Binary fission

A bacterium reproduces asexually by binary fission. The circular chromosome is copied, the copies become associated with different regions of the growing cell, and a septum forms as membrane and wall grow inward. The result is two daughter cells that are usually genetically very similar. Binary fission is not mitosis: there is no nucleus whose chromosomes pass through prophase, metaphase, anaphase and telophase. Nevertheless, DNA replication, separation of the copies and accurate partitioning are still essential. The time between divisions depends on nutrients, temperature, pH, oxygen availability and the species.

Stages of binary fission

1
DNA replication
The circular chromosome is copied; a plasmid may also be copied.
2
Cell growth
The cell elongates and the chromosome copies move apart.
3
Septum formation
New membrane and peptidoglycan wall grow between the chromosome regions.
4
Separation
The septum closes and two daughter cells separate or remain attached.

Mutations and horizontal gene transfer can make daughter cells genetically different. In conjugation, a plasmid moves from one bacterial cell to another through direct cell-to-cell contact, often via a pilus; bacteriophages can also transfer bacterial genes between cells. Asexual reproduction therefore gives a strong expectation of similarity, not a guarantee that variation is impossible. This matters clinically because a resistance gene can spread through a population faster than a new mutation would arise independently in every cell. Selection then increases the frequency of cells that survive an antibiotic treatment.

7.Scale and bacterial micrographs

Scale is essential when distinguishing bacteria from eukaryotic cells or viruses. Bacteria are usually measured in micrometres; many viruses are measured in nanometres. Apparent size on a screen is not evidence of actual size, so use the scale bar. If a scale bar is absent, identify the limitation rather than inventing a dimension. A stain or false colour can improve contrast, but neither proves that the visible object is alive or that the image is a complete three-dimensional cell.

Evidence and justified claims

Visible evidence	Reasonable inference	Overclaim
Small repeated cells with a wall and no visible nucleus	Consistent with bacterial cells	Every cell is the same species
A dense internal region without a nuclear boundary	Consistent with a nucleoid	Exact genes or metabolic pathways are visible
A labelled scale bar	Cell dimensions can be estimated	The specimen is alive
A filament-like surface projection	A flagellum or pilus may be present	The cell must be motile

Electron microscopy may reveal structures that light microscopy cannot resolve, but fixation can distort shape and cannot show whether a cell was metabolically active at the moment of fixation. Structural evidence and functional evidence are not interchangeable. For example, seeing many ribosomes supports a capacity for protein synthesis, but it does not measure the current rate of translation. A functional conclusion needs evidence such as incorporation of labelled amino acids, product formation or a controlled physiological response.

8.Envelope variation and biofilms

Bacteria vary in wall structure, shape and surface layers. Cocci are roughly spherical, bacilli are rod-shaped and spirilla are helical, but these labels describe form rather than complete identity. Cells may form chains, clusters or biofilms embedded in extracellular material. A capsule or extracellular polymeric matrix can reduce desiccation, improve attachment and limit penetration of immune molecules or disinfectants. Within a biofilm, cells may experience different oxygen and nutrient conditions from cells in the surrounding liquid, so the same species can show different growth behaviour in different locations.

The peptidoglycan wall is a mesh of sugars cross-linked by short peptides. It gives strength while remaining porous to many small molecules. In Gram-negative bacteria, an additional outer membrane contains lipopolysaccharide; in Gram-positive bacteria, the peptidoglycan layer is much thicker. These differences affect staining and sensitivity to some antibiotics. A wall is not the same as a capsule: the wall is a structural envelope closely associated with the membrane, whereas a capsule is a looser outer layer. Both can affect how a cell interacts with its environment, but they have different compositions and functions.

Do not equate shape with identity

Reliable identification normally combines staining, culture or biochemical tests, molecular evidence and ecological context.

9.Virus structure

A virus is not a prokaryotic or eukaryotic cell. It consists of a nucleic-acid core—DNA or RNA—surrounded by a protein capsid. Some viruses also have a phospholipid envelope derived from a host-cell membrane and containing viral proteins. Viruses have no cytoplasm, ribosomes or independent energy metabolism. Viral genomes vary in whether they are DNA or RNA, single-stranded or double-stranded, and linear, circular or segmented. Those differences change how the genome is copied and expressed, but they do not turn a virion into a cell.

Capsid

The protein coat surrounding a viral genome. It protects the genome and often contributes to attachment to a host cell.

Envelope

A phospholipid membrane surrounding some viruses. Viral surface proteins in the envelope bind host receptors; the envelope can be disrupted by detergents.

Virus components

Component	Role	Important limit
DNA or RNA genome	Carries information for viral replication and products	Normally cannot be translated without host machinery
Capsid	Protects the genome and helps deliver it into a host	Does not make the virus a cell
Envelope, if present	Supports entry by membrane fusion or related mechanisms	Derived from host membrane and vulnerable to solvents
Attachment proteins	Recognise specific host receptors	Help determine host range and tissue tropism

Comparing named viruses shows how genome type and envelope presence vary independently of each other, and of virus size. Ebola virus and HIV are both enveloped RNA viruses, but Ebola causes rapid, severe haemorrhagic disease while HIV establishes a long-term infection of immune cells; TMV (tobacco mosaic virus) is a non-enveloped RNA virus with a simple rod-shaped capsid that infects plant cells; and lambda phage (λ phage) is a non-enveloped DNA virus that infects bacteria and can adopt either a lytic or a lysogenic (latent) lifestyle.

Four named viruses compared

Virus	Genome	Envelope?	Host and typical lifestyle
Ebola virus	RNA	Enveloped	Human/primate cells; lytic, rapid severe disease
HIV	RNA	Enveloped	Human T-helper cells; can integrate and persist (latency) before productive replication
Tobacco mosaic virus (TMV)	RNA	Non-enveloped	Plant cells; simple rod-shaped capsid
Lambda phage (λ phage)	DNA	Non-enveloped	Bacterial cells; lytic or lysogenic (latent)

Lytic and latent (lysogenic) infection

In a lytic infection, a virus immediately replicates and is released, usually destroying the host cell. In a latent (lysogenic) infection, the viral genome persists in the host cell — sometimes integrated into the host chromosome, as with HIV and lambda phage — without immediately producing new particles, until a later signal triggers productive replication.

10.Why viruses depend on host cells

A virus cannot reproduce by binary fission or mitosis because it has no ribosomes, cytoplasm or complete metabolic machinery. It must enter a suitable host cell and use host nucleotides, amino acids, ATP and ribosomes, together with viral enzymes where necessary. The viral genome redirects host processes to make new genomes and proteins. Dependence is specific rather than universal: a virus may infect one species or cell type but fail to replicate in another because receptors, intracellular conditions or antiviral defences are unsuitable. This is why host range and tissue tropism are biological properties, not simply geographical observations.

A simplified lytic cycle

1
Attachment
Viral proteins bind a receptor on a susceptible host cell.
2
Entry and uncoating
The genome or nucleocapsid enters and the genome becomes accessible.
3
Genome expression
Host and viral enzymes produce viral proteins from viral information.
4
Genome replication
New copies of viral nucleic acid are made using host resources.
5
Assembly
Capsid proteins and genomes assemble into new virions.
6
Release
Particles leave by lysis or budding; lysis damages the host cell.

Some viruses can remain latent or lysogenic, with the viral genome persisting in the cell without immediate lysis. The exact cycle varies, but dependence on living-cell machinery remains the defining principle. During latency, the host cell may divide and copy the viral genome along with its own DNA, or the viral genome may remain as a separate molecule. A later signal can activate productive replication. Do not treat “not currently causing lysis” as evidence that the virus is inactive or harmless.

11.Host range and tissue tropism

Infection requires more than contact. A virion must reach a host cell, bind a compatible receptor, enter, express its genome and avoid or overcome host defences. A change in an attachment protein can alter host range or tissue tropism, while a change in a host receptor can reduce susceptibility. The same logic explains why a virus may infect one tissue efficiently but not another in the same organism. Receptor distribution, temperature, proteases needed for entry and intracellular defence systems all contribute to whether replication succeeds.

Enveloped viruses often enter by fusion with a host membrane or by endocytosis followed by fusion. Non-enveloped viruses may enter by endocytosis or membrane disruption. Symptoms can reflect direct cell damage, immune responses or both. Because an envelope is lipid-based, detergents can disrupt it, but the effect of a cleaning method depends on contact time, concentration and whether all surfaces are reached. A control intervention should therefore be evaluated by mechanism and real-world exposure, not by the label “antiviral” alone.

Virus and bacterium in disease control

Feature	Virus	Bacterium
Cellular?	No; acellular particle	Yes; prokaryotic cell
Independent ribosomes?	No	Yes; 70S ribosomes
Treatment targets	Viral entry, enzymes, release or host response	Wall synthesis, bacterial ribosomes, DNA or metabolism
Reproduce outside host?	No	Many grow independently with nutrients and suitable conditions
Control examples	Vaccination, hygiene, antivirals and isolation where appropriate	Hygiene, vaccination for some species, antibiotics when indicated

12.Bacteriophages

A bacteriophage is a virus that infects bacteria. A common phage has a head containing nucleic acid, a protein tail and attachment fibres. The fibres recognise receptors on the bacterial surface and the tail helps deliver the genome through the cell envelope. The head protects the genome outside the host, while the tail provides an organised route for delivery. The structure is therefore adapted to the barrier it must cross, but the phage still lacks the ribosomes and energy metabolism needed to produce its own components.

Phages make the distinction between genetic information and cellular machinery clear. The phage genome carries instructions, but the bacterium supplies ribosomes, ATP and many raw materials. In a lytic cycle, newly assembled phages are released when the bacterium is broken open. Phages can be useful experimental tools because a plaque in a bacterial culture reveals successful infection and release. However, a clear plaque shows a population-level outcome; it does not by itself show which stage of the cycle limited growth.

A useful contrast

A bacterium is small but cellular: it has a membrane, cytoplasm, ribosomes and metabolism. A phage may be smaller, but size is not why it is non-cellular.

13.Worked example: identify an unknown particle

Question

An unknown particle contains RNA surrounded by a protein coat. It has no cytoplasm or ribosomes and produces many new particles only after entering a host cell. Is it a prokaryote, a eukaryote or a virus? Explain two pieces of evidence.

Numbered solution

1
It is a virus.
2
The RNA core and protein capsid match the basic structure of a virus.
3
It is non-cellular because it has no cytoplasm or ribosomes and depends on a host cell for reproduction.

Marking note

“It causes disease” is not sufficient because both bacteria and viruses can cause disease. Use structural and reproductive evidence. A high-scoring response identifies the missing cellular features and explains why host dependence follows from that absence, rather than treating “virus” as a label to memorise.

14.Worked example: compare a bacterium with an animal cell

Question

Give three structural differences between a typical bacterium and a typical animal cell, and explain one consequence of one difference.

Model answer

1
A bacterium has no nucleus; an animal cell has a nucleus enclosed by a nuclear envelope.
2
A bacterium has 70S ribosomes in its cytoplasm; an animal cell has 80S cytoplasmic ribosomes.
3
A bacterium has a peptidoglycan wall; an animal cell has no cell wall.
4
The bacterial wall helps retain shape and resist osmotic bursting while the membrane controls selective transport.

Marking note

Use matched pairs. Do not say that bacteria have no DNA or animal cells have no membrane. If the question asks for a consequence, connect the structural difference to a biological process: a missing nuclear envelope changes where transcription occurs; a peptidoglycan wall changes resistance to osmotic pressure; and the absence of bacterial ribosomes makes some antibiotic targets unavailable in viruses.

15.Extended worked case: apply and evaluate

Problem

A microbe is observed reproducing only inside host cells and has a protein capsid but no ribosomes. Classify it and justify.

Reasoned solution

1
A capsid with genetic material and no ribosomes is consistent with a virus.
2
Dependence on host cells for reproduction supports that classification.
3
A bacterium would be a cellular organism with ribosomes and its own binary-fission machinery.

Check or limitation

This classification does not tell whether the virus has DNA or RNA, or whether it has a lipid envelope.

16.Exam tips and common misconceptions

Exam tips

Use “nucleoid region” rather than “bacterial nucleus”.
Name peptidoglycan when the question asks for the bacterial wall.
Remember that plasmids are small circular DNA molecules, not organelles.
For viruses, include nucleic-acid core and protein capsid; mention an envelope only when present.
Use the scale bar, visible structure and microscope type before identifying a micrograph.

Common misconceptions

Viruses are tiny bacteria. Viruses are non-cellular and have no ribosomes or independent metabolism.
Prokaryotes have no internal organisation. They have a membrane, wall, nucleoid, ribosomes and sometimes specialised surface structures.
All viruses have an envelope. Some are non-enveloped and consist of genome and capsid.
Antibiotics kill viruses. Antibiotic targets such as peptidoglycan synthesis and bacterial ribosomes are absent from viruses.
A virus reproduces by cell division. It is assembled from newly made components inside a host cell.

17.Language in context: Capsid

Capsid means The protein coat surrounding a viral genome. It protects the genome and often contributes to attachment to a host cell.

Use the term in The bacterial cell plan

The surrounding idea is: A typical bacterium is a unicellular prokaryote. Its DNA is not enclosed in a nucleus and it lacks mitochondria, chloroplasts, ER and Golgi bodies. It still has a cell surface membrane, cytoplasm, ribosomes and regulated pathways for obtaining energy and building cell material. The absence of organelles does not mean the cell lacks biochemical organisation. Enzymes may be concentrated in the cytoplasm or associated with the cell membrane, and the membrane can support electron transport and ATP formation. A bacterium maintains gradients, repairs damage and responds to its surroundings using a compact but complete…

When explaining “Identify the core structures of a bacterial prokaryotic cell.”, name this term precisely and then state the relationship, mechanism, calculation, or evidence that makes it relevant.

18.Method checkpoint: Bacterial structures and their functions

This lesson-specific route is useful when working with Bacterial structures and their functions. Keep each stage visible so that a reader can check the reasoning rather than only the final claim.

1
DNA replication
2
Cell growth
3
Septum formation
4
Separation

Why the order matters

The method is tied to this lesson’s aim: Relate bacterial-cell structures to their functions.. A skipped stage can change the interpretation or invalidate the conclusion.

19.Summary and self-check

Chapter summary

A bacterium is a unicellular prokaryote with a membrane, peptidoglycan wall, cytoplasm, circular DNA and 70S ribosomes.
Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotic cells are compartmentalised.
Binary fission involves chromosome replication, cell growth, septum formation and separation.
A virus is a non-cellular structure containing DNA or RNA in a protein capsid; some also have a phospholipid envelope.
Viruses depend on host cells because they lack ribosomes, cytoplasm and complete energy-generating metabolism.
Micrographs support structural claims only as far as their scale, preparation and resolution allow.

Can you now…

Label a bacterial cell and link each structure to its function.
Write a matched comparison between a prokaryotic and a eukaryotic cell.
Explain why a virus is not classified as a cell.
Describe how a virus makes and releases new particles.
Use a scale bar and visible evidence without claiming information the image cannot show.

20.Curriculum alignment and applied reasoning

Detailed-note focus

This extension turns the lesson into an exam-ready sequence: identify the evidence, apply the mechanism or calculation, then state a qualified conclusion. Core outcomes revisited here include: Identify the core structures of a bacterial prokaryotic cell.; Relate bacterial-cell structures to their functions.; Compare prokaryotic and eukaryotic cells using structural evidence..

Cross-course alignment

Where this lesson transfers

Course	Mapped focus in this lesson
Cambridge International A Level Biology 9700	1.2 Cells as the basic units of living organisms
Edexcel IAL Biology	3.5 Prokaryotic cell structure 6.5 Bacteria and viruses
AQA International A-level Biology	1.2 Cells and cell structure 2.10 Cells divide by binary fission and mitosis
AP Biology	2.1 Cell Structure and Function 2.2 Cell Size 2.10 Origins of Cell Compartmentalization

Applied analysis: Choosing a treatment target

Scenario: Explain why an antibiotic that inhibits bacterial 70S ribosomes is not expected to treat a viral infection, and identify one feature that distinguishes a bacterial cell from a eukaryotic cell.

Worked reasoning: Viruses lack ribosomes and reproduce only inside host cells, so a drug aimed at bacterial 70S ribosomes has no viral target. A bacterial cell has no membrane-bound nucleus; its circular DNA lies in a nucleoid, whereas eukaryotic DNA is enclosed in a nucleus.

Exam-quality communication: Avoid saying viruses are “cells”; identify the missing cellular machinery.

Precision audit

Name the observation, quantity, structure or variable before interpreting it.
Show the causal step or calculation route; do not jump from data to a conclusion.
State a limitation, condition or comparison whenever the evidence cannot justify an absolute claim.