outline key structural features of a prokaryotic cell as found in a typical bacterium, including: unicellular, generally 1–5 µm diameter, peptidoglycan cell walls, circular DNA, 70S ribosomes, absence of organelles surrounded by double membranes

Cells as the Basic Units of Living Organisms (Cambridge 9700 – Topic 1)

1. The Microscope in Cell Studies (Syllabus 1.1)

1.1 Key Practical Concepts

• Wet‑mount (temporary) preparations – a drop of water, a cover‑slip and the specimen (e.g., pond water) are used to observe living cells.
• Dry‑mount (permanent) preparations – heat‑fixed or chemically fixed specimens on a slide; useful for staining.
• Magnification – total magnification = ocular lens power × objective lens power.

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

• Resolution limit of light microscopy – ≈0.2 µm; structures smaller than this (e.g., ribosomes, viruses) require electron microscopy.
• Scale‑bar calculations –

  
  Actual size (µm) = (length on image ÷ length of scale bar) × (scale‑bar length in µm).

  
  Example: a bacterium measures 2 cm on a printed photo; the scale bar is 1 cm = 10 µm.

  
  Actual size = (2 cm ÷ 1 cm) × 10 µm = 20 µm.

1.2 Safety & Handling of Light Microscopes

• Never look directly at the light source.
• Use the lowest‑power objective first to locate the specimen, then switch to higher‑power objectives.
• Oil‑immersion (100×) objectives require a drop of immersion oil and must be cleaned with lens tissue after use.
• Never force an objective into the slide; if resistance is felt, raise the stage and re‑align.

1.3 Electron Microscopy (Brief Overview)

• Scanning Electron Microscope (SEM) – produces a 3‑D surface image; useful for studying cell shape and surface structures (e.g., bacterial pili, flagella).
• Transmission Electron Microscope (TEM) – passes electrons through an ultra‑thin section; reveals internal structures such as ribosomes, nucleoid, and membrane layers.
• Resolution down to 0.1 nm, far beyond light microscopy, allowing direct visualisation of 70S ribosomes (≈20 nm).

1.4 Practice Question (LO 1‑5)

In a photomicrograph the scale bar is 5 mm and represents 10 µm. A rod‑shaped bacterium measures 12 mm on the image. What is its actual length?

Solution: Actual length = (12 mm ÷ 5 mm) × 10 µm = 2.4 × 10 µm = 24 µm.

2. Cells as the Basic Units of Living Organisms (Syllabus 1.2)

2.1 General Features of All Cells

• Plasma membrane – phospholipid bilayer with embedded proteins; described by the fluid‑mosaic model. Functions include selective permeability, transport, signalling and cell‑cell recognition.
• Size range – most prokaryotes 1–5 µm in diameter; typical eukaryotic cells 10–100 µm. The surface‑area‑to‑volume (SA:V) ratio decreases as size increases, influencing rates of diffusion and metabolic demand.

2.2 Typical Prokaryotic Cell (Bacterium)

• Organisation – unicellular; each bacterium functions as an independent unit.
• Size – usually 1 µm to 5 µm in diameter (rod‑shaped, cocci, spirilla, etc.).
• Cell wall – rigid peptidoglycan (murein) layer; provides shape and protects against osmotic lysis. Gram‑positive bacteria have a thick layer; Gram‑negative bacteria have a thin layer plus an outer membrane (not covered here as “typical”).
• Plasma membrane – single phospholipid bilayer directly beneath the cell wall; only membrane‑bound structure in a typical bacterium.
• Genetic material – single, circular DNA chromosome located in the nucleoid region (no true nucleus). Small extrachromosomal DNA molecules (plasmids) may also be present.
• Ribosomes – 70S ribosomes (30S small subunit + 50S large subunit) for protein synthesis; visible only with TEM.
• Inclusion bodies – non‑membranous storage granules (e.g., glycogen, polyphosphate, sulfur).
• Absence of double‑membrane organelles – no mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, or nucleus.

2.3 Typical Eukaryotic Cell – Plant vs. Animal

2.3.1 Common Eukaryotic Organelles (present in both plant and animal cells)

2.3.2 Plant‑Specific Organelles & Structures

2.3.3 Animal‑Specific Organelles & Structures

2.4 Prokaryote vs. Eukaryote – Quick Comparison (LO 1‑7)

3. Viruses – Non‑Cellular Infectious Particles (Syllabus 1.3)

3.1 Why Viruses Are Not Cells (LO 1‑8)

• They lack a self‑sustaining metabolic system – no respiration, no protein synthesis without a host.
• They have no plasma membrane or internal membrane‑bound organelles.
• Genetic material is either DNA or RNA, but never both, and is not packaged with histones.

3.2 Basic Structural Components

• Nucleic‑acid core – DNA or RNA, single‑ or double‑stranded.
• Capsid – protein shell formed from repeating subunits (capsomers); protects the genome.
• Envelope (optional) – lipid bilayer derived from the host cell membrane; contains viral glycoproteins that mediate attachment to new host cells.

3.3 Relevance to Later Topics

Understanding virus structure underpins later study of:

• Infectious disease and immunity (how the immune system recognises capsid proteins).
• Biotechnology – use of viral vectors for gene cloning and therapy.

3.4 Suggested Diagram (for revision)