2.3 Cell Size

The highly complex organization of living systems requires constant input of energy and the exchange of macromolecules. Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment. Surface area-to-volume ratios affect the ability of a biological system to obtain necessary resources, eliminate waste products, acquire or dissipate thermal energy, and otherwise exchange chemicals and energy within the environment. The surface area of the plasma membrane must be large enough to adequately exchange materials-- these limitations can restrict cell size and shape. Smaller cells typically have a higher surface area-to-volume ratio and more efficient exchange of materials with the environment. As cells increase in volume, the relative surface area decreases and the demand for internal resources increases. More complex cellular structures (e.g., membrane folds) are necessary to adequately exchange materials with the environment. As organisms increase in size, their surface area-to-volume ratio decreases, affecting properties like rate of heat exchange with the environment. Explain how specialized structures and strategies are used for the efficient exchange of molecules to the environment. Organisms have evolved highly efficient strategies to obtain nutrients and eliminate wastes. Cells and organisms use specialized exchange surfaces to obtain and release molecules from or into the surrounding environment.

surface area-to-volume ratio

• essential for exchanges between cell and environment (resource uptake, waste elimination, dissipating heat, other chemical/energy exchanges)

• decreases as volume increases, evolutionary advantageous to have smaller cells, limits set by cellular metabolism

• maximized through folding in brain tissue and membrane of mitochondria, endoplasmic reticulum, and chloroplast

surface area-to-volume ratio beyond a cellular level

• smaller organisms are more efficient with heat exchange

• evolved strategies to increase surface area: root hair in plants, villi/microvilli in animal intestines

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