Cell Structure and Division

Cells are the basic structural and functional units of all living organisms. A-Level Biology requires a thorough understanding of cell ultrastructure, the differences between prokaryotic and eukaryotic cells, microscopy techniques, the mechanisms of cell division, and the roles of stem cells and their relevance to cancer.

Key Definition: A cell is the smallest unit of life that can carry out all life processes. All living organisms are composed of one or more cells.

---

Eukaryotic Cell Ultrastructure

Eukaryotic cells possess a true nucleus enclosed by a double membrane (nuclear envelope) and a range of membrane-bound organelles. Each organelle is a compartment that provides a specialised internal environment for particular metabolic reactions.

Organelle	Structure	Function
Nucleus	Double membrane with nuclear pores; contains chromatin and nucleolus	Houses DNA; controls cell activities; nucleolus produces ribosomal RNA
Rough endoplasmic reticulum (RER)	Flattened membrane sacs studded with ribosomes	Synthesises and transports proteins (especially secretory proteins)
Smooth endoplasmic reticulum (SER)	Tubular membrane network without ribosomes	Synthesises lipids and steroid hormones; detoxification
Golgi apparatus	Stack of flattened membrane cisternae	Modifies, packages, and secretes proteins; produces lysosomes
Mitochondria	Double membrane; inner membrane folded into cristae; own circular DNA and 70S ribosomes	Site of aerobic respiration; produces ATP
Chloroplasts (plants only)	Double membrane; internal thylakoid membranes forming grana; own circular DNA and 70S ribosomes	Site of photosynthesis
Lysosomes	Single membrane containing hydrolytic enzymes	Intracellular digestion; autophagy; destruction of worn-out organelles
Ribosomes	Two subunits (80S in eukaryotes, 70S in prokaryotes)	Site of translation (protein synthesis)
Centrioles (animal cells)	Paired cylindrical structures of nine triplets of microtubules	Organise spindle fibres during cell division
Cell wall (plants, fungi)	Cellulose microfibrils (plants); chitin (fungi)	Provides structural support; prevents lysis
Vacuole (plants)	Large, membrane-bound (tonoplast), fluid-filled	Maintains turgor pressure; stores ions, sugars, and pigments

A diagram of a typical animal cell would show a roughly circular cell bounded by a cell surface membrane. Inside, the large nucleus sits centrally with its double membrane and visible nucleolus. Mitochondria appear as oval or sausage-shaped organelles with folded inner membranes (cristae). The rough ER is shown as flattened sacs near the nucleus with dots (ribosomes) on the surface, whilst the smooth ER has a more tubular appearance without ribosomes. The Golgi apparatus appears as a stack of curved sacs with vesicles budding off.

Exam Tip: When labelling an electron micrograph, ensure you can distinguish between the RER (studded with ribosomes, appearing as dark dots) and SER (smooth, tubular). Questions often ask you to identify organelles from electron micrographs — practise with real images.

---

Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic	Eukaryotic
Nucleus	No true nucleus — DNA free in the nucleoid region	Membrane-bound nucleus
DNA	Circular, naked (no histones), single molecule	Linear chromosomes with histones
Ribosomes	70S (smaller)	80S (larger); 70S in mitochondria/chloroplasts
Organelles	No membrane-bound organelles	Extensive membrane-bound organelles
Cell wall	Peptidoglycan (murein)	Cellulose (plants) or chitin (fungi); absent in animals
Size	Typically 1–5 µm	Typically 10–100 µm
Reproduction	Binary fission	Mitosis and meiosis

Prokaryotes may also possess plasmids (small circular DNA molecules, often carrying antibiotic resistance genes), flagella (for locomotion, powered by a rotary motor), pili (for attachment and conjugation — the transfer of plasmid DNA between cells), a capsule (slime layer for protection against phagocytosis and desiccation), and mesosomes (infoldings of the cell membrane, though their existence is debated).

Binary Fission

Key Definition: Binary fission is the process by which a prokaryotic cell divides into two genetically identical daughter cells.

Binary fission proceeds as follows:

1. The circular DNA molecule replicates. Both copies attach to the cell membrane.
2. The cell elongates, and the two DNA copies move to opposite ends (poles) of the cell.
3. The cell membrane invaginates (grows inwards) at the centre of the cell, forming a septum.
4. New cell wall material is laid down, dividing the cell into two identical daughter cells.
5. Under optimal conditions, binary fission can occur every 20 minutes in E. coli.

flowchart TD
    A["Circular DNA replicates"] --> B["Both DNA copies attach to cell membrane"]
    B --> C["Cell elongates; DNA copies move to opposite poles"]
    C --> D["Cell membrane invaginates, forming a septum"]
    D --> E["New cell wall material laid down"]
    E --> F["Two identical daughter cells produced"]

Binary fission does not involve a mitotic spindle or the condensation of chromosomes. Any variation in the daughter cells arises from mutation, not from the division process itself.

---

Viruses

Key Definition: A virus is an acellular, non-living obligate intracellular parasite that can only reproduce inside a host cell.

Viruses are not classified as living organisms because they:

• Do not carry out metabolic reactions.
• Cannot reproduce independently (they require host cell machinery).
• Do not grow or develop.
• Are not made of cells.

General Virus Structure

• A nucleic acid core — this may be DNA or RNA, single-stranded or double-stranded.
• A protein coat (capsid) made of repeating protein subunits (capsomeres) that surrounds and protects the nucleic acid.
• Some viruses have a lipid envelope derived from the host cell membrane (e.g., influenza, HIV).
• Some bacteriophages have a tail and tail fibres for attachment to bacterial cells.

A diagram of a typical bacteriophage would show an icosahedral (20-sided) head containing DNA, attached to a hollow tail sheath with a baseplate and tail fibres at the bottom. HIV, by contrast, would be shown as a sphere with an outer lipid envelope studded with glycoprotein spikes, surrounding a capsid that contains two copies of single-stranded RNA and reverse transcriptase.

---

Cell Fractionation and Ultracentrifugation

Key Definition: Cell fractionation is the process of separating the different organelles of a cell so that they can be studied individually.

Method

1. Homogenisation — cells are broken open (using a blender or homogeniser) in a cold, isotonic, buffered solution.
   • Cold — to reduce enzyme activity that could damage organelles.
   • Isotonic — to prevent organelles from shrinking (in hypertonic solution) or swelling and bursting (in hypotonic solution) by osmosis.
   • Buffered — to maintain a constant pH and prevent denaturation of enzymes within organelles.
2. Filtration — the homogenate is filtered through gauze to remove large cell debris (cell walls, connective tissue).
3. Ultracentrifugation — the filtrate is spun at progressively higher speeds in an ultracentrifuge:

flowchart TD
    A["Tissue sample"] --> B["Homogenisation
(cold, isotonic, buffered solution)"]
    B --> C["Filtration
(remove cell debris)"]
    C --> D["1st spin: low speed ~1,000g
Pellet: Nuclei"]
    D --> E["2nd spin: ~3,500g
Pellet: Mitochondria, Chloroplasts"]
    E --> F["3rd spin: ~20,000g
Pellet: Lysosomes, ER"]
    F --> G["4th spin: ~100,000g
Pellet: Ribosomes, Membrane fragments"]

Fraction	Speed / Duration	Organelles in Pellet
1st spin	Low speed (~1,000 g, 10 min)	Nuclei
2nd spin	Medium speed (~3,500 g, 10 min)	Mitochondria, chloroplasts
3rd spin	Higher speed (~20,000 g, 20 min)	Lysosomes, endoplasmic reticulum
4th spin	Very high speed (~100,000 g, 60 min)	Ribosomes, cell membrane fragments

After each spin, the pellet (containing the heaviest organelles) is removed and the supernatant is transferred to a new tube for the next spin at a higher speed.

Exam Tip: Questions on cell fractionation frequently ask why the solution must be cold, isotonic, and buffered. Make sure you give three separate reasons with explanations — one for each condition. This is a classic 3-mark question.

---

Microscopy

• Light microscopes — magnification up to ×1,500; resolution ~200 nm; can observe living specimens; uses visible light and glass lenses.
• Transmission electron microscopes (TEM) — magnification up to ×500,000; resolution ~0.5 nm; thin sections, produces 2D images; specimens must be dead and stained with heavy metals.
• Scanning electron microscopes (SEM) — lower magnification than TEM but provides detailed 3D surface images; useful for studying cell surface structures.

Magnification Formula