Membrane Structure and Function

APBiology

Chapter7(Part)

Passive Transport

• Passivetransport- themovementofmaterialsacrossthecellmembranewithoutusingenergy

Passive Transport

–Diffusion:theprocessbywhichsoluteparticlesmovefromanareaofhighconcentrationtoanareaoflowerconcentration.– Solutionà Solute+solvent– Concentrationà amountofsubstanceinagivenvolume

Passive Transport

– Diffusion– Down concentration gradient

– àFrom higher concentration to lower concentration

– Affected by size (more you have, the longer it takes)

Simple Diffusion

• Suppose a substance is present in unequal concentrations on either side of a cell membrane.

Simple Diffusion

• If the substance can cross the cell membrane, its particles will tend to move toward the area where it is less concentrated until it is evenly distributed.

Simple Diffusion

• At that point, the concentration of the substance on both sides of the cell membrane is the same, and equilibrium is reached.

Simple Diffusion

• Even when equilibrium is reached, particles of a solution will continue to move across the membrane in both directions.

• Because almost equal numbers of particles move in each direction, there is no net change in the concentration on either side.

Passive Transport Summary

– Diffusion isthetendencyofmoleculesofanysubstancetospreadoutintheavailablespace.

– Diffusionisdrivenbykineticenergy(thermalmotionorheat)ofmolecules.

– Intheabsenceofotherforces,asubstancewilldiffusefromwhereitismoreconcentratedtowhereitislessconcentrated,downitsconcentrationgradient.

– Diffusionisaspontaneousprocessthatincreasesentropybycreatingarandomizedmixture.

Osmosis: An Example of Diffusion

– OSMOSIS: the movement of water through cell membranesàextremely important process

àmovement of water molecules from an area of higher concentration to an area of lower concentration

Osmosis Definitions

– Differencesintherelativeconcentrationofdissolvedmaterialsintwosolutionscanleadtothemovementofionsfromonetotheother.– Solutionwithhigherconcentrationofsolutesishypertonic relativetotheothersolution.

– Solutionwiththelowerconcentrationofsolutesishypotonic relativetotheothersolution.

– Solutionswithequalsoluteconcentrationsareisotonic.

Movement of Water

– Hypertonicsolutionwillhaverelativelylessfreewaterthanahypotonicsolution

– Thedirectionofosmosisisdeterminedonlybyadifferenceintotal soluteconcentration.– Thekinds ofsolutesinthesolutionsdonotmatter.

– Whentwosolutionsareisotonic,watermoleculesmoveatequalratesfromonetotheother,withnonetosmosis.