A2.2 Cell structure

Cell theory

Cell theory has three linked claims: all living things are made of one or more cells, the cell is the smallest unit that can carry out all functions of life, and new cells come only from the division of existing cells. It was built inductively from many observations, which means a single unusual case does not destroy it but does test its limits (see atypical cells below).

Microscopy and magnification

Light microscopes use visible light and lenses; they can show living, stained or coloured cells but are limited by resolution to roughly 0.2 μm. Electron microscopes use beams of electrons with a much shorter wavelength, so resolution is far better and internal membranes, ribosomes and organelle detail become visible, but specimens must be dead and prepared in a vacuum.

Magnification links three quantities:

\[\text{magnification} = \frac{\text{image size}}{\text{actual size}}\]

Always convert to the same unit first: 1 mm = 1000 μm and 1 μm = 1000 nm. Newer techniques widen what we can see: fluorescent stains and immunofluorescence tag specific molecules, freeze-fracture splits membranes to show embedded proteins, and cryogenic electron microscopy reveals the shapes of individual proteins.

Prokaryotic versus eukaryotic cells

Both types share a plasma membrane, cytoplasm, ribosomes and DNA as genetic material.

Plant, animal and fungal cells

Plant cells have a cellulose wall, a large permanent vacuole and chloroplasts in green tissues; they store starch. Animal cells lack a wall, have small temporary vacuoles, often have centrioles and store glycogen. Fungal cells have chitin walls, store glycogen and lack chloroplasts. Compartments let incompatible reactions run side by side and let enzymes and substrates be concentrated.

Atypical cells

Some cells stretch the "standard cell" idea: skeletal muscle fibres are huge and contain many nuclei; aseptate fungal hyphae have no cross walls; red blood cells lose their nucleus; phloem sieve tube elements lack a nucleus and rely on companion cells. These show how structure matches function.

Endosymbiosis (HL)

flowchart TD
  A["Large anaerobic host cell"] --> B["Engulfs aerobic bacterium by endocytosis"]
  B --> C["Bacterium is not digested"]
  C --> D["Both benefit: host gains ATP"]
  D --> E["Becomes mitochondrion over many generations"]
  E --> F["Later, some cells engulf photosynthetic bacteria"]
  F --> G["These become chloroplasts"]

Evidence: mitochondria and chloroplasts have their own circular DNA, 70S ribosomes, a double membrane and divide by a fission-like process, all features of bacteria. Eukaryotes probably arose from a common ancestor that already had these traits.

Multicellularity and differentiation (HL)

Multicellularity evolved many times. It allows larger size and division of labour. All cells in an organism contain the same genome, but during development different genes are switched on, so cells differentiate into specialised types with distinct shapes and functions.

Common mistakes

• Forgetting to convert units before using the magnification formula.
• Saying prokaryotes have "no DNA" or "no ribosomes". They have both, just no nucleus.
• Confusing resolution (ability to tell two points apart) with magnification (how much bigger the image is).
• Stating that plant cells have chloroplasts in every cell; root cells do not.

Exam tips

• Calculate: show the formula, substitute with units, give a sensible number of significant figures and the "×" symbol for magnification.
• Compare and contrast: include at least one similarity, and write differences as paired statements.
• Outline endosymbiosis as a short sequence, then give two pieces of evidence.
• For micrograph questions, identify organelles by shape: double membrane with folds for mitochondria, stacks of thylakoids for chloroplasts.