Transport Function

GeneralBiologyOnline

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Ch. 8 Contents

General
---------Biology-----------------------------------------------------
------------------Online-------------------------------------- Ch. 8 Membrane Structure and Function

2. Membrane Function:
Transport

Figure below illustrates three ways molecules and ions can enter cells. Two of the ways require a protein to provide a transport channel through the lipid bylayer.

Passive Transport Simple Diffusion: small molecules such as oxygen, carbon dioxide and water can diffuse between the phospholipid molecules and through the lipid bilayer. In some cases larger fat soluble molecules can diffuse through the hydrophobic cell membrane. Click the Simple Diffusion link for a discussion of diffusion, concentration gradients and equilibrium.

Facilitated Diffusion: larger polar molecules such as sugars, amino acids and nutrients (N, P, K ions) cannot pass through the hydrophobic zone and must diffuse through a protein channel.

Active Transport

Active Transport uses ATP energy to pump molecules "uphill" against their concentration gradient. This is a defining characteristic of life and the reason we eat the fish and plants for energy instead of drinking the water they grow in. Cells have the ability to concentrate nutrients several thousand times their concentration in the environment. With this supply of nutrients, the cell can assemble the proteins and other macromolecules essential to living chemical pathways.

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