Exchange surfaces and surface area to volume ratio
Cell biology • Transport in cells
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Definition and calculation of surface area to volume ratio
Surface area (SA) is the total area over which exchange can occur. Volume (V) is the space that contains the organism’s cells and their demands for nutrients and oxygen. For simple shapes, SA and V compute from dimensions, then SA:V simplifies to a ratio. For a cube with side length s: SA = 6 × s² and V = s³. Example: a 1 cm cube has SA 6 cm² and V 1 cm³, so SA:V = 6:1; a 2 cm cube has SA 24 cm² and V 8 cm³, so SA:V = 3:1. A decreasing SA:V with increasing size reduces the rate at which substances reach internal cells by diffusion .
Why multicellular organisms need exchange surfaces and transport systems
SA:V falls as an organism grows: volume (the demand for resources) increases faster than surface area (the supply route). A lower SA:V limits diffusion per unit volume; cells deep inside larger organisms are too far from the outer surface for diffusion to supply oxygen and remove wastes quickly enough. Therefore specialised, highly folded exchange surfaces and circulatory transport systems maintain steep concentration gradients and move substances between exchange surfaces and internal cells .
Four main ways to increase effectiveness of an exchange surface
1) Large surface area: Folding, branching or fine divisions increase total area available for diffusion, so more particles cross per unit time. 2) Thin barrier: A short diffusion distance (often one cell thick) reduces the time for molecules to cross a membrane. 3) Efficient blood supply (in animals): Moving blood rapidly past the exchange surface maintains steep concentration gradients, sustaining diffusion. 4) Ventilation (in animals for gas exchange): Regular replacement of external medium (air or water) refreshes the supply of oxygen-rich medium and removes carbon dioxide, preserving concentration gradients. Each feature causes faster net exchange by increasing flux or maintaining gradients fileciteturn0file2turn0file5.
Limiting factors for exchange by diffusion
Distance (diffusion path length) limits rate: increased distance slows diffusion. Surface area limits rate: smaller area reduces flux. Concentration gradient strength affects net movement: a shallower gradient slows diffusion. Temperature affects particle kinetic energy and thus diffusion speed. These limiting factors set maximum sizes for organisms that depend solely on diffusion and drive the evolution of specialised exchange systems in larger organisms .
Key notes
Important points to keep in mind