The netmovement of particles (atoms, molecules or ions) from a region where they are of higher concentration to a region where they are of lower concentration; that is, down a concentrationgradient (until equilibrium is reached)
What is Net movement
Overallmovement of particles when the movement of particles in one direction is greater than the movement of particles in the opposite direction
Concentration gradient
Difference in concentration of particles in two regions
Equilibrium
When both regions being compared have the same concentration
Diffusion does not require a partiallypermeablemembrane to be present
Both the solvent and solutes can pass through a permeable membrane
Factors that affect the rate of diffusion
Concentrationgradient
Diffusiondistance
Surfacearea-to-volumeratio
The steeper the concentration gradient
The faster the rate of diffusion
The shorter the diffusion distance
The faster the rate of diffusion
The greater the surface-area-to-volume ratio
The faster the rate of diffusion
The rateof food and oxygen intake is slower as the cell grows larger
It is not beneficial for the cell to grow too big
Osmosis
The netmovement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
Water potential
A measure of the tendency of water to move from oneplace to another
A dilute solution has a higher water potential
A concentrated solution has a lower water potential
A water potential gradient is established when a partiallypermeablemembraneseparates two solutions of different water potentials
Concentrated solution
High concentration of solute, low water potential of solvent
Diluted solution
Low concentration of solute, high water potential of solvent
Water potential
A water potential gradient is established when a partially permeable membrane separates two solutions of different water potentials
Partially permeable membrane
Allows somesubstances to pass through it but not others
Factors affecting rate of osmosis
Water potential gradient
Distancethatwater moleculesneedtomove
Surface area-to-volumeratio
Factors affecting rate of diffusion
Concentrationgradient
Diffusiondistance (travelled by solute molecules)
Surfacearea-to-volumeratio
Cells as living osmotic systems
A living cell is enclosed by a partially permeable membrane
A plant cell behaves differently from an animal cell when placed in solutions of differing water potentials due to its cellwall
What happens to a plant cell in a solution with higher water potential
1. Cell sap has lower water potential than surrounding solution
2. Water enters by osmosis
3. Cell expands and becomes turgid
4. Cell wall prevents cell from bursting
What happens to a plant cell in a solution with lower water potential
1. Cell sap has higher water potential than surrounding solution
2. Water leaves by osmosis
3. Cell becomes flaccid
4. Cytoplasm shrinks away from cell wall and cell becomes plasmolysed
Turgor
Plays an important role in maintaining the shape of softtissues in plants
Keeps herbaceous (non-woody) plants firm and upright
Loss of turgidity causes the plant to wilt
Plasmolysis causes tissues to become limp or flaccid
When cells of a plant become flaccid, the plant wilts
What happens to a cell in a solution with same water potential
A cell immersed in a solution with the same water potential as its cytoplasm will not change its size or shape
Dead cells no longer have an intact partially permeable cell membrane, and therefore osmosiscan no longer occur
Reverse osmosis also uses pressure to move water molecules against the water potential gradient
Active transport
The process in which cellularenergy is used to move the particles of a substance across a membrane against its concentration gradient, i.e. from a region where the particles are of lower concentration to a region where they are of higher concentration
Differences between diffusion and active transport
Diffusion: Down a concentration gradient, energy from respiration not required, cell membrane not required
Active transport: Against a concentration gradient, energy from respiration required, cell membrane required
Active transport requires energy obtained through respiration, and thus occurs only in living cells
Dissolved mineral salts are taken up by root hair cells via active transport