Prokaryotic Cells and Viruses

The Edexcel A-Level Biology specification (9BI0) requires a thorough understanding of prokaryotic cell structure and how it differs from eukaryotic cells. You must also understand the structure of viruses and the debate surrounding whether they can be classified as living organisms. This lesson covers all the key content for Topic 2.

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What Is a Prokaryotic Cell?

Prokaryotic cells are cells that lack a true nucleus — their genetic material is not enclosed within a nuclear envelope. The term "prokaryote" comes from the Greek pro (before) and karyon (kernel/nucleus). Prokaryotes include the domains Bacteria and Archaea.

Prokaryotic cells are typically much smaller than eukaryotic cells, usually in the range of 0.2–5.0 $\mu m$μm in diameter, compared to 10–100 $\mu m$μm for most eukaryotic cells.

Exam Tip: The specification requires you to understand the structural features of prokaryotic cells and be able to compare them with eukaryotic cells. A comparison table is almost always the best way to present this in an exam answer.

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Structure of a Prokaryotic Cell

Cell Wall

• All bacteria have a cell wall made of peptidoglycan (also called murein)
• Peptidoglycan consists of polysaccharide chains cross-linked by short peptide bridges
• Provides structural support and prevents the cell from bursting due to osmotic lysis
• Gram staining distinguishes two types of bacterial cell wall:

Feature	Gram-positive bacteria	Gram-negative bacteria
Cell wall thickness	Thick peptidoglycan layer (20–80 nm)	Thin peptidoglycan layer (5–10 nm)
Outer membrane	Absent	Present (contains lipopolysaccharides)
Gram stain result	Purple/violet	Pink/red
Examples	Staphylococcus aureus	Escherichia coli

Cell Surface Membrane (Plasma Membrane)

• A phospholipid bilayer with embedded proteins
• Controls the entry and exit of substances
• Site of some metabolic reactions (e.g. respiration in bacteria, as they lack mitochondria)

Cytoplasm

• Contains 70S ribosomes (smaller than the 80S ribosomes of eukaryotes)
• Contains all the enzymes and molecules required for metabolism
• No membrane-bound organelles are present

Genetic Material

• A single, circular chromosome of double-stranded DNA located in the nucleoid region (not membrane-bound)
• DNA is not associated with histone proteins (it is described as "naked" DNA)
• Many bacteria also contain small, circular, extra-chromosomal DNA molecules called plasmids

Plasmids

Plasmids are small, circular DNA molecules that replicate independently of the main chromosome. They typically carry genes that confer a selective advantage, such as:

• Antibiotic resistance genes
• Genes encoding toxins
• Genes for metabolic functions (e.g. the ability to degrade unusual substances)

Plasmids can be transferred between bacteria via conjugation (horizontal gene transfer), which is a major factor in the spread of antibiotic resistance.

Exam Tip: Plasmids are also essential tools in genetic engineering and biotechnology. You may be asked to explain how plasmids are used as vectors to introduce foreign genes into bacterial cells.

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Additional Prokaryotic Structures

Not all bacteria possess the following structures, but they are commonly found:

Capsule (Slime Layer)

• A thick, gelatinous layer outside the cell wall
• Protects the bacterium from desiccation, phagocytosis by white blood cells and chemical damage
• Helps bacteria adhere to surfaces and form biofilms
• Capsulated bacteria are often more pathogenic (disease-causing)

Flagella (singular: flagellum)

• Long, whip-like structures used for locomotion
• Composed of the protein flagellin
• Rotate like a propeller, powered by a proton motive force
• Allow bacteria to move towards favourable conditions (chemotaxis)

Pili (singular: pilus)

• Short, hair-like projections on the cell surface
• Used for attachment to surfaces and to other cells
• Sex pili are longer and are involved in conjugation — the transfer of plasmid DNA between bacteria

Mesosomes

• Infoldings of the cell surface membrane
• Were historically thought to be involved in respiration and cell division
• Now widely considered to be artefacts of the preparation process for electron microscopy
• However, the cell membrane itself does contain respiratory enzymes in prokaryotes

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Comparison: Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	0.2–5.0 $\mu m$μm	10–100 $\mu m$μm
Nucleus	No true nucleus (nucleoid region)	True nucleus with nuclear envelope
DNA	Circular, naked (no histones)	Linear, associated with histones
Chromosomes	Single chromosome + plasmids	Multiple chromosomes
Ribosomes	70S	80S
Membrane-bound organelles	Absent	Present (e.g. mitochondria, ER, Golgi)
Cell wall	Peptidoglycan	Cellulose (plants), chitin (fungi), absent (animals)
Reproduction	Binary fission	Mitosis / meiosis
Flagella	Simple (flagellin)	Complex (9+2 microtubule arrangement)

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Viruses

Viruses are non-cellular, obligate intracellular parasites. They are much smaller than bacteria, typically 20–300 nm in diameter, and can only be seen using an electron microscope.

General Structure of a Virus

All viruses share certain structural features:

Component	Description
Nucleic acid (genome)	Either DNA or RNA (never both); can be single-stranded (ss) or double-stranded (ds)
Capsid	A protein coat surrounding the nucleic acid, made up of repeating protein subunits called capsomeres
Envelope (some viruses)	A lipid bilayer derived from the host cell membrane; contains viral glycoproteins
Attachment proteins	Surface proteins that enable the virus to bind to specific receptors on the host cell

Examples of Viruses

Virus	Nucleic acid	Envelope	Shape	Host
HIV (Human Immunodeficiency Virus)	ssRNA (retrovirus)	Yes	Spherical	Human T-helper cells
Influenza	ssRNA	Yes	Spherical	Respiratory epithelial cells
Tobacco Mosaic Virus (TMV)	ssRNA	No	Rod-shaped / helical	Tobacco plant cells
Bacteriophage (T4 phage)	dsDNA	No	Complex (head + tail)	E. coli bacteria
SARS-CoV-2	ssRNA	Yes	Spherical with spike proteins	Human respiratory cells

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HIV Structure in Detail

The Edexcel specification specifically requires knowledge of HIV structure:

• Envelope: A lipid bilayer derived from the host cell, studded with glycoprotein spikes (gp120 and gp41)
• Capsid: An inner protein coat (p24 protein)
• Genome: Two copies of single-stranded RNA (it is a diploid retrovirus)
• Reverse transcriptase: An enzyme packaged inside the capsid that converts viral RNA into DNA inside the host cell
• Integrase: Helps incorporate the viral DNA into the host genome

Exam Tip: HIV is a retrovirus because it carries the enzyme reverse transcriptase, which catalyses the synthesis of DNA from an RNA template. This is the reverse of the normal flow of genetic information (DNA → RNA → protein), hence the name.

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Are Viruses Alive?

This is a classic discussion point in biology. Viruses display some characteristics of living organisms but lack others:

Characteristic of life	Viruses
Contain genetic material	Yes (DNA or RNA)
Reproduce	Only inside a host cell (cannot reproduce independently)
Evolve / mutate	Yes (through mutation and natural selection)
Metabolism	No — viruses have no enzymes for metabolic reactions (except a few specific enzymes like reverse transcriptase)
Homeostasis	No
Growth	No — viruses do not grow
Response to stimuli	No
Cellular structure	No — viruses are acellular

The general scientific consensus is that viruses are not considered living organisms because they cannot carry out metabolic processes independently and must hijack host cell machinery to replicate.

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Viral Replication: The Lytic Cycle

The basic steps of viral replication (using a bacteriophage as an example):

1. Attachment — the virus binds to specific receptors on the host cell surface
2. Penetration (injection) — the viral nucleic acid is injected into the host cell
3. Replication — the host cell machinery is hijacked to replicate viral nucleic acid and synthesise viral proteins
4. Assembly — new virus particles are assembled from the replicated components
5. Lysis (release) — the host cell bursts (lyses), releasing hundreds of new virus particles to infect other cells

Some viruses (e.g. HIV) can also follow the lysogenic cycle, where the viral DNA integrates into the host genome and is replicated along with the host DNA during cell division, remaining dormant until triggered to enter the lytic cycle.

The following diagram summarises the stages of the lytic cycle:

graph TD
    A["Attachment<br/>Virus binds to host"] --> B["Penetration<br/>Nucleic acid injected"]
    B --> C["Replication<br/>Viral DNA/RNA copied"]
    C --> D["Assembly<br/>New virions formed"]
    D --> E["Lysis<br/>Cell bursts, viruses released"]
    E -->|"Infect new cells"| A

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Binary Fission

Binary fission is the method of asexual reproduction used by prokaryotic cells. It is simpler and faster than mitosis:

1. The circular DNA molecule replicates
2. The cell elongates and the two DNA copies move to opposite ends (poles) of the cell
3. The cell membrane and cell wall pinch inwards (invaginate) at the centre
4. The cell divides into two identical daughter cells

Under optimal conditions, some bacteria (e.g. E. coli) can divide every 20 minutes, leading to exponential population growth.

Exam Tip: Binary fission is not mitosis. Prokaryotes do not undergo mitosis because they lack a nucleus and the complex chromosome organisation of eukaryotes. Make sure you use the correct terminology.

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Summary

• Prokaryotic cells are smaller and simpler than eukaryotic cells, lacking membrane-bound organelles and a true nucleus
• Bacterial cell walls are made of peptidoglycan, and Gram staining distinguishes two types
• Plasmids carry additional genes and can be transferred between bacteria, contributing to antibiotic resistance
• Viruses are acellular, obligate intracellular parasites containing either DNA or RNA within a protein capsid
• Viruses are not considered living because they lack independent metabolism and cannot reproduce outside a host cell
• Binary fission is the rapid, asexual method of reproduction in prokaryotes

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A-Level Deep Dive: Prokaryotic Cells and Viruses

Spec mapping

The Edexcel 9BI0 specification places prokaryotic ultrastructure and viral biology within Topic 2: Membranes, Proteins, DNA and Gene Expression and the wider Cells, Viruses & Reproduction strand. The relevant statements require candidates to: describe the ultrastructure of a prokaryotic cell (cell wall of peptidoglycan, plasma membrane, cytoplasm, 70S ribosomes, circular DNA in a nucleoid, plasmids, capsule, flagella, pili and mesosomes); compare prokaryotic with eukaryotic ultrastructure quantitatively in $\mu m$μm and $nm$nm; describe the structure of viruses with reference to a non-enveloped example (e.g. tobacco mosaic virus, $\lambda$λ-phage) and an enveloped example (e.g. HIV); and evaluate whether viruses should be classified as living. Synoptic threads run into Topic 4 (Microbiology, Disease and the Immune System) — selective antibiotic toxicity exploits the 70S/80S ribosome distinction — and Topic 6 (Modern Genetics) — bacterial plasmids serve as vectors for recombinant DNA technology and as the molecular substrate for horizontal gene transfer (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

A student examines a transmission electron micrograph of an unknown microorganism. The cell is approximately $2\,\mu m$2μm long, has no visible membrane-bound organelles, and shows ribosomes that sediment at $70\,S$70S in centrifugation. A separate image shows a distinct particle $80\,nm$80nm across with a protein capsid, an inner core of single-stranded RNA and a phospholipid envelope studded with glycoprotein spikes.

(a) Identify each of the two structures, justifying your conclusions with reference to the features given. (4)

(b) The scale bar on the bacterial image reads $1\,\mu m$1μm and prints as $25\,mm$25mm. Calculate the magnification of the printed image, expressing your answer in standard form. (2)

(c) Explain one reason why the second structure cannot reproduce on its own. (2)

Solution with mark scheme:

(a) Step 1 — interpret the bacterial clues. Cell size $\approx 2\,\mu m$≈2μm, absence of membrane-bound organelles and 70S ribosomes together identify a prokaryotic (bacterial) cell.

M1 (AO2.1) — correct identification of the prokaryotic cell from the size range and lack of membrane-bound organelles. A common pitfall is to write "small eukaryote" because the candidate has missed that 70S ribosomes are diagnostic.

A1 (AO1.2) — explicit justification that 70S ribosomes (smaller than the eukaryotic 80S) and the lack of a true nucleus are diagnostic features.

Step 2 — interpret the viral clues. A particle $80\,nm$80nm across with a capsid, ssRNA genome and phospholipid envelope bearing glycoprotein spikes identifies an enveloped RNA virus (e.g. an influenza-like or HIV-like particle).

M1 (AO2.1) — correct identification as a virus (acellular, sub-100 nm, capsid + nucleic acid).

A1 (AO1.2) — justification linking the envelope and glycoprotein spikes to host-derived membrane and receptor-binding function. A bare "virus" with no justification scores M1 only.

(b) Step 1 — convert units consistently. $1\,\mu m = 1 \times 10^{-3}\,mm$1μm=1×10−3mm. Printed length $= 25\,mm$=25mm.

M1 (AO2.1) — correct conversion of $\mu m$μm to $mm$mm before dividing.

Step 2 — apply $\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$magnification=actual sizeimage size​.

$M = \dfrac{25\,mm}{1 \times 10^{-3}\,mm} = 2.5 \times 10^{4}$M=1×10−3mm25mm​=2.5×104

A1 (AO1.1b) — magnification $= 2.5 \times 10^{4}$=2.5×104 (or $\times 25\,000$×25000). Many candidates lose marks here by dividing the raw numbers ($25 \div 1$25÷1) and reporting $\times 25$×25, three orders of magnitude wrong.

(c) M1 (AO1.2) — viruses lack ribosomes / metabolic enzymes / a plasma membrane bounding cytoplasm, so they cannot synthesise proteins, generate ATP or replicate their genome unaided.

A1 (AO2.1) — therefore the virus is an obligate intracellular parasite that must hijack a living host cell's ribosomes (70S in bacteria, 80S in eukaryotes) and nucleotide pools to replicate.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Bacteria, viruses and animal cells differ in their structure and in their capacity for independent reproduction. Using examples, evaluate the claim that viruses should be classified as living organisms.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	States that the conventional criteria for life include cellular organisation, metabolism, response to stimuli, growth, reproduction, homeostasis and heredity (MRS GREN or equivalent).
2	AO1.2	States that viruses are acellular, lack ribosomes and a plasma membrane bounding cytoplasm, and have no independent metabolism.
3	AO2.1	Applies the criteria: viruses do possess heritable nucleic acid (DNA or RNA), undergo evolution, and can self-assemble within a host.
4	AO2.1	Applies the criteria: viruses cannot reproduce, generate ATP or maintain homeostasis outside a host — they are obligate intracellular parasites.
5	AO3.1a	Evaluates: by strict criteria viruses fail the test for life; by an inclusive evolutionary criterion they are biological entities subject to selection.
6	AO3.2a	Reaches a justified conclusion (either side accepted if reasoned), e.g. "viruses are best classified as biological replicators rather than living organisms, because their dependence on host machinery means the cell, not the virion, is the unit of metabolism".

Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. This is a classic Section B "evaluate" question — Edexcel rewards candidates who weigh evidence on both sides and reach a defensible conclusion, not those who merely list features.

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Synoptic links