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The plasma membrane separates the cytoplasm from its surroundings. Its phospholipid bilayer has hydrophilic heads facing water and hydrophobic tails facing inward.
Selective permeability: a membrane allows some substances to cross more readily than others.
Small nonpolar molecules can cross the bilayer. Ions and many polar molecules need transport proteins. Passive transport follows a concentration gradient; active transport requires energy.
For a linear concentration gradient, the magnitude of diffusive flux is J = D(c₁ − c₂) / L.
Assume c₁ > c₂. D is the diffusion coefficient and L is the diffusion distance. A larger concentration difference increases flux; a greater diffusion distance decreases it. This is a simple steady-state approximation.
Water (H₂O) crosses a selectively permeable membrane during osmosis, moving toward the side with greater solute concentration.
For this ideal example, the solute cannot cross. Aquaporins provide channels for water. A cell in a hypotonic solution gains water; in a hypertonic solution it loses water. Compare both sides before predicting the direction.
A membrane allows some substances to cross more readily than others.
Diffusive flux for a linear gradient. D is the diffusion coefficient; L is the diffusion distance (c₁ > c₂).
Water. During osmosis, water crosses a selectively permeable membrane toward the side with greater solute concentration.
A membrane allows some substances to cross more readily than others.
Diffusive flux for a linear gradient. D is the diffusion coefficient; L is the diffusion distance (c₁ > c₂).
Water. During osmosis, water crosses a selectively permeable membrane toward the side with greater solute concentration.