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.