Introduction to cells

Cell theory — three classical principles:

1. All living organisms are composed of cells.
2. Cells are the smallest units of life.
3. Cells arise from pre-existing cells (by division).

Exceptions / atypical cells. Cells that do not fit the typical model:

• Striated muscle fibres — multinucleate (many nuclei sharing one cytoplasm).
• Giant algae (e.g. Acetabularia) — a single cell up to 100 mm tall.
• Aseptate fungal hyphae — long cytoplasmic threads without internal cell walls.

These do not disprove cell theory; they highlight that "cell" can be unusually large or unusually shared.

Seven functions of life (all shown by unicellular organisms such as Paramecium):

• Metabolism — chemical reactions
• Reproduction — making copies
• Sensitivity (response) — detecting and responding to stimuli
• Homeostasis — maintaining a stable internal state
• Growth
• Respiration / nutrition (acquiring energy and materials)
• Excretion (removing waste)

Mnemonic: MR SHGRE — order varies between resources; the IB lists them as MR H GREN (Metabolism, Response, Homeostasis, Growth, Reproduction, Excretion, Nutrition).

Magnification.

$M = \frac{\text{image size (in the drawing or photo)}}{\text{actual size (real)}}.$

Rearranged: actual size $=$ image size $/ M$.

Worked example. A scale bar reads 10 μm but measures 50 mm on the page. Magnification $= 50000 \,\mu\text{m} / 10 \,\mu\text{m} = \times 5000$.

Units. $1\,\text{mm} = 1000\,\mu\text{m} = 10^6\,\text{nm}$. Always match units before dividing.

Surface area to volume ratio (SA:V). As a cube doubles in side length, surface area $\times 4$ but volume $\times 8$ — SA:V halves. The same logic applies to cells: larger cells have proportionally less membrane per unit cytoplasm, which limits the rate at which they can exchange materials (oxygen, glucose, waste) with their surroundings.

Consequence. Cells stay small. Multicellular organisms get large by making MORE cells, not bigger ones. Cells with high metabolic demand (e.g. liver, intestinal lining) are often flattened or have folded membranes (microvilli) to boost SA:V.

Emergent properties. A whole multicellular organism shows properties that arise from interactions among differentiated cells — for instance, consciousness emerges from neurons, but no single neuron is conscious. This is a core idea in the 2025+ Theme A (Unity and Diversity).

Differentiation. Most cells in a multicellular body contain the same genome but express different subsets of genes. Specialised cells (muscle, neuron, red blood cell) result from controlled differential gene expression during development.