state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids

Cells as the Basic Units of Living Organisms

Learning Objective

State that all viruses are non‑cellular structures with a nucleic‑acid core (DNA or RNA) surrounded by a protein capsid, and that some viruses possess an outer phospholipid envelope derived from the host cell membrane. Explain why viruses are not considered cells, the basic structure of a typical cell, and how to identify key organelles using a light microscope.

1. Microscopy Skills (Syllabus 1.1)

  • Preparing a temporary slide – place a thin fragment of onion epidermis on a clean slide, add a drop of distilled water, cover with a cover‑slip, and examine immediately.

  • Magnification calculation – Total magnification = ocular lens magnification × objective lens magnification.


    Example: 10 × ocular × 40 × objective = 400 × total magnification.

  • Resolution – the smallest distance that can be distinguished; for a light microscope it is ≈ 0.2 µm.

  • Eyepiece graticule – use the calibrated scale in the eyepiece to measure cell dimensions; convert to real size using the total magnification.

  • Identifying organelles in photomicrographs – students should be able to recognise nuclei, vacuoles, chloroplasts, and cell walls in light‑microscope images and, where provided, recognise membrane‑bound organelles in electron‑microscope pictures.

2. Cell Organelles & Their Functions (Syllabus 1.2)

OrganelleFunctionPresent in
NucleusStores DNA; controls gene expression and cell divisionEukaryotic cells
RibosomeSite of protein synthesisBoth prokaryotic and eukaryotic cells
Rough endoplasmic reticulum (RER)Synthesises membrane‑bound and secretory proteinsEukaryotic cells
Smooth endoplasmic reticulum (SER)Lipid synthesis; detoxificationEukaryotic cells
Golgi apparatusModifies, sorts and packages proteins for secretionEukaryotic cells
MitochondrionSite of aerobic respiration (ATP production)Eukaryotic cells
ChloroplastPhotosynthesis (light‑dependent reactions and Calvin cycle)Plant cells and some algae
Cell wallProvides shape and protection; prevents osmotic lysisPlant cells, fungi, most bacteria
Plasma membraneSelective barrier; regulates transportAll cells

2.1 Prokaryotic vs. Eukaryotic Cells

Prokaryotes lack a true nucleus and membrane‑bound organelles; their DNA is located in a nucleoid region. Eukaryotes possess a membrane‑bound nucleus and a variety of specialised organelles (see table above).

2.2 Cell‑Surface Membrane Composition (required for later topics)

The plasma membrane is a fluid mosaic of a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and carbohydrate‑containing molecules (glycolipids and glycoproteins) that contribute to cell recognition and signalling.

3. Viruses – Definition and General Structure (Syllabus 1.3)

  • Viruses are non‑cellular infectious particles; they consist of a nucleic‑acid genome surrounded by a protein capsid.

  • They lack cytoplasm, ribosomes and metabolic pathways, so they cannot grow or reproduce outside a host cell.

  • Because viruses do not possess the cellular machinery described above, they are not regarded as the “basic unit of life” despite their biological importance.

3.1 Structural Components

ComponentDescriptionPresent in
Nucleic‑acid coreDNA or RNA; may be single‑ or double‑stranded; some RNA viruses have segmented genomesAll viruses
CapsidProtein shell made of repeating subunits called capsomeres; determines overall shape (icosahedral, helical or complex)All viruses
Envelope (optional)Phospholipid bilayer derived from the host cell membrane; contains viral glycoprotein spikes that mediate attachment to new host cellsOnly enveloped viruses (e.g., Influenza virus, HIV)

3.2 Optional Extension – Advanced Virus Details (A‑Level enrichment)

For students wishing to explore further, the following topics are useful for deeper understanding (AO2):

  • Genome segmentation (e.g., influenza virus has eight RNA segments).

  • Complex capsid architectures (e.g., bacteriophage T4).

  • Detailed replication strategies for each genome type.

3.3 Genome Types

  • DNA viruses – double‑stranded DNA (dsDNA) or single‑stranded DNA (ssDNA).

  • RNA viruses

    • Double‑stranded RNA (dsRNA)

    • Single‑stranded positive‑sense (+) RNA (acts directly as mRNA)

    • Single‑stranded negative‑sense (–) RNA (must be transcribed to +RNA)

    • Some have segmented genomes (e.g., influenza).

3.4 Viral Replication Cycle (Key for AO1)

  1. Attachment – viral spikes bind to specific receptors on the host cell membrane.

  2. Entry

    • Enveloped viruses: fusion of envelope with host membrane or endocytosis.

    • Non‑enveloped viruses: direct penetration or endocytosis.

  3. Uncoating – release of the genome into the cytoplasm.

  4. Replication & transcription – host or viral enzymes synthesise viral nucleic acid and mRNA (strategy depends on genome type).

  5. Translation – host ribosomes produce viral proteins.

  6. Assembly – new capsids are formed around replicated genomes.

  7. Release

    • Lysis of the host cell (typical of non‑enveloped viruses).

    • Budding through the host membrane, acquiring an envelope (typical of enveloped viruses).

4. Practical Skills Linked to Cells & Viruses (Syllabus 1.4)

Preparing a temporary slide of onion epidermis

  1. Cut a small piece (≈ 5 mm) of onion skin.

  2. Place it on a clean microscope slide with a drop of distilled water.

  3. Gently lower a cover‑slip, avoiding air bubbles.

  4. Observe at low power (10 ×) to locate cells, then switch to 40 × objective for detail.

  5. Use the eyepiece graticule to measure cell dimensions; calculate the actual size using total magnification.

Drawing cells to scale

  • Select a convenient magnification (e.g., 400 ×).

  • Measure the cell with the graticule (e.g., 50 divisions = 10 µm at 400 ×).

  • On graph paper, let 1 mm represent 1 µm and draw the cell accordingly.

Practice problem – calculating real cell size

If a cell measures 30 graticule divisions at 400 × magnification and the eyepiece scale is 0.1 mm per division, what is the actual length of the cell in micrometres?

Solution: 30 divisions × 0.1 mm = 3 mm on the eyepiece.

Real size = 3 mm ÷ 400 = 0.0075 mm = 7.5 µm.

5. Comparison: Cells vs. Viruses

FeatureCellVirus
MembranePlasma membrane (phospholipid bilayer) with proteins, cholesterol, glycolipids/glycoproteinsMay have an envelope (phospholipid bilayer) or none
Genetic materialDNA (usually double‑stranded) in nucleus (eukaryotes) or nucleoid (prokaryotes)DNA or RNA; single‑ or double‑stranded; may be segmented
MetabolismSelf‑contained metabolic pathwaysNone; entirely dependent on host metabolism
ReproductionCell division (mitosis, meiosis, binary fission)Assembly inside a host cell after genome replication
SizeTypically 1–100 µm20–300 nm

6. Link to Later Topics

Understanding viral structure and replication underpins:

  • Topic 10 – Infectious Diseases: mechanisms of pathogenicity, transmission, control measures.

  • Topic 11 – Immunology: antigen–antibody interactions, vaccine design, innate and adaptive immunity.

7. Suggested Diagram

Two schematic drawings: (a) a non‑enveloped virus showing nucleic‑acid core and capsid; (b) an enveloped virus showing nucleic‑acid core, capsid, surrounding phospholipid envelope with embedded glycoprotein spikes.