Size and scale of cells to systems

Organisation • Principles of organisation

Flashcards

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Give a quick method for comparing SA:V between two shapes.

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Calculate surface area and volume for each shape, then divide surface area by volume to obtain SA:V for comparison; smaller objects tend to have higher SA:V .

Key concepts

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Units and orders of magnitude

Cell lengths commonly use micrometres (µm). One micrometre equals 1 × 10−3 millimetres and 1 × 10−6 metres, and one thousand nanometres make one micrometre . Sub-cellular structures may require nanometre (nm) precision while whole tissues and organs use millimetres or metres for practical measurement.

Typical sizes and scale relationships

Bacterial cells range roughly from 1 µm to 10 µm in length, so between 100 and 1000 bacterial cells fit in 1 mm in a straight line . Most animal and plant cells measure a few to several tens of micrometres. Tissues form from many similar cells, so tissue dimensions reach millimetres and centimetres; organs reach centimetres to metres depending on organism size. The organism’s organisation therefore spans many orders of magnitude from nm to m.

Magnification and calculating actual size

Magnification is the ratio of image size to actual size and is unitless: magnification = image size ÷ actual size. Rearrangement gives actual size = image size ÷ magnification and image size = actual size × magnification . Example calculation methods appear in worked micrograph questions where image measurements convert between mm and µm before applying the formula .

Surface area to volume ratio (SA:V) as a limiting factor

Increasing size reduces the surface area relative to volume, so SA:V decreases as cells or organisms enlarge. Lower SA:V causes slower exchange of heat and dissolved substances per unit volume, which limits cell size and affects organismal form. Organisms adapt by remaining small, increasing surface division (folds, villi, alveoli) or developing transport systems to overcome SA:V limits .

Diffusion limits and requirement for specialised systems

Diffusion operates effectively only across small distances because diffusion time increases with distance; therefore large multicellular organisms require exchange surfaces and circulatory transport to move substances quickly between organs and cells. Small organisms or tissues with short diffusion distances avoid complex transport systems; insects use tracheal tubes, while larger animals use lungs, gills or blood vessels to maintain rapid exchange .

Practical measurement and representation

Microscope images require careful unit conversion and accurate measurement of image size before applying magnification formulae. Scale bars provide direct reference for measuring features in micrographs; consistent units are essential to avoid calculation errors . Simple models and cube experiments demonstrate SA:V changes and provide evidence for why cells remain small .

Key notes

Important points to keep in mind

Use consistent units; convert mm ↔ µm ↔ nm before calculations .

Magnification is unitless: always record units for image and actual size and cancel them appropriately .

SA:V decreases as size increases; lower SA:V reduces exchange efficiency per unit volume .

Diffusion is only fast over short distances; long internal distances require transport systems .

Small cells maintain high SA:V and efficient exchange without complex transport.

Specialised exchange surfaces increase area (folding, villi, alveoli) rather than increasing cell size .

Bacterial cells are much smaller than typical eukaryotic cells (≈1–10 µm) .

Electron microscopes image nm-scale structures; light microscopes suit µm-scale cells .

Always show working: convert units, measure image, then apply magnification formula .

Cube or model experiments provide evidence for SA:V effects on diffusion and heat loss .