Transport across the plasma Transport across the plasma membranemembrane

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Diffusion and facilitated Diffusion and facilitated diffusiondiffusion

• DiffusionDiffusion – the net movement of molecules (or ions) from – the net movement of molecules (or ions) from a region of their higher concentration to a region of their a region of their higher concentration to a region of their lower concentration (molecules move down a lower concentration (molecules move down a concentration gradientconcentration gradient))

• Molecules tend to reach an equilibrium where evenly Molecules tend to reach an equilibrium where evenly spreadspread

• The rate at which substance diffuses across a membrane The rate at which substance diffuses across a membrane depends on:depends on:– The ‘steepness’ of the concentration gradient. The The ‘steepness’ of the concentration gradient. The

greater the difference in concentration, the greater the greater the difference in concentration, the greater the difference in the number of molecules passing in the 2 difference in the number of molecules passing in the 2 directions and hence the faster the net rate of diffusiondirections and hence the faster the net rate of diffusion

– Temperature. Diffusion takes place faster in high Temperature. Diffusion takes place faster in high temperatures due to higher kinetic energytemperatures due to higher kinetic energy

– The surface area. The greater the surface area, the The surface area. The greater the surface area, the more molecules or ions can cross it at any one moment more molecules or ions can cross it at any one moment (faster diffusion)(faster diffusion)

– The nature of the molecules or ions. Substances with The nature of the molecules or ions. Substances with large molecules tend to diffuse more slowly. Non-polar large molecules tend to diffuse more slowly. Non-polar molecules diffuse more easily through cell membranes molecules diffuse more easily through cell membranes as they are soluble in non-polar phospholipid tailsas they are soluble in non-polar phospholipid tails

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ALBIO9700/2006JK

• Facilitated diffusionFacilitated diffusion – diffusion of a – diffusion of a substance through protein channels in substance through protein channels in a cell membranea cell membrane

• The proteins provide hydrophilic areas The proteins provide hydrophilic areas that allow the molecules or ions to that allow the molecules or ions to pass through a membrane that would pass through a membrane that would otherwise be less permeable to themotherwise be less permeable to them

• Rate depends on how many Rate depends on how many appropriate channels there are in the appropriate channels there are in the membrane and on whether they are membrane and on whether they are open or not open or not

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ALBIO9700/2006JK

Active transportActive transport• The energy-consuming transport of The energy-consuming transport of

molecules or ions across a membrane molecules or ions across a membrane against a concentration gradient made against a concentration gradient made possible by transferring energy from possible by transferring energy from respirationrespiration

• Like facilitated diffusion, active transport is Like facilitated diffusion, active transport is achieved by special transport proteins, achieved by special transport proteins, each of which is specific for a particular each of which is specific for a particular type of molecule or iontype of molecule or ion

• However, it requires energy (molecule ATP) However, it requires energy (molecule ATP) because movement occurs up a because movement occurs up a concentration gradientconcentration gradient

• Energy used to make the transport protein Energy used to make the transport protein ((carrier proteincarrier protein) change its 3D shape, ) change its 3D shape, transferring the molecules or ions across transferring the molecules or ions across the membrane in the process the membrane in the process

ALBIO9700/2006JK

ALBIO9700/2006JK

Bulk transportBulk transport• Transport of large quantities of Transport of large quantities of

materials into cells (endocytosis) or materials into cells (endocytosis) or out of cells (exocytosis)out of cells (exocytosis)

• ExocytosisExocytosis – process by which – process by which materials are removed from cellsmaterials are removed from cells

• EndocytosisEndocytosis – engulfing of the – engulfing of the material by the plasma membrane to material by the plasma membrane to form a small sac or ‘endocytotic form a small sac or ‘endocytotic vacuole’vacuole’– Phagocytosis: bulk uptake of solid materialPhagocytosis: bulk uptake of solid material– Pinocytosis: bulk uptake of liquidPinocytosis: bulk uptake of liquid

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Osmosis Osmosis

• Solution = solute + solventSolution = solute + solvent• Partially permeable membranePartially permeable membrane – –

membrane which allows only certain membrane which allows only certain molecules throughmolecules through

• If solution B has higher concentration of If solution B has higher concentration of solute molecules than solution A – solution solute molecules than solution A – solution B is more concentrated than solution B is more concentrated than solution A/solution A is more dilute than solution BA/solution A is more dilute than solution B

• Osmosis involves Osmosis involves netnet movement of water movement of water molecules only molecules only

ALBIO9700/2006JK

ALBIO9700/2006JK

• Water potential and solute potentialWater potential and solute potential– Water potentialWater potential: the tendency of water : the tendency of water

molecules to move from one place to another (psi, molecules to move from one place to another (psi, ψψ))

– Water always moves from a region of higher water Water always moves from a region of higher water potential to a region of lower water potential potential to a region of lower water potential (down concentration gradient)(down concentration gradient)

– Osmosis: the movement of water molecules Osmosis: the movement of water molecules from a region of higher water potential to a from a region of higher water potential to a region of lower water potential through a region of lower water potential through a partially permeable membranepartially permeable membrane

– Pure water has highest water potential (solutes Pure water has highest water potential (solutes lower the water potential) lower the water potential)

– Solute potentialSolute potential: the amount that the solute : the amount that the solute molecules lower the water potential of a solution. molecules lower the water potential of a solution. Always –ve (Always –ve (ψψss). ).

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• Osmosis in animalsOsmosis in animals– If the water potential of the solution surrounding If the water potential of the solution surrounding

the cell is too high, the cell swells and burststhe cell is too high, the cell swells and bursts– If it is too low, the cell shrinksIf it is too low, the cell shrinks

– In animal cells In animal cells ψψ = = ψψss (water potential is equal to (water potential is equal to solute potential)solute potential)

• Pressure potentialPressure potential– The greater the pressure applied, the greater the The greater the pressure applied, the greater the

tendency for water molecules to be forced backtendency for water molecules to be forced back– Increasing the pressure increases the water Increasing the pressure increases the water

potentialpotential– Pressure potentialPressure potential: contribution made by : contribution made by

pressure to water potential (pressure to water potential (ψψpp))

– The pressure potential makes the water potential The pressure potential makes the water potential less negative and is therefore positive less negative and is therefore positive

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ALBIO9700/2006JK

• Osmosis in plant cellsOsmosis in plant cells– Different from animal cells because of rigid Different from animal cells because of rigid

and strong cell walland strong cell wall– Cell wall prevents the cell from bursting Cell wall prevents the cell from bursting

(lysis)(lysis)– When fully inflated with water: When fully inflated with water: turgidturgid– Water potential is a combination of solute Water potential is a combination of solute

potential and pressure potential (potential and pressure potential (ψψ = = ψψss + + ψψpp))

– When protoplast shrinks and pulls away When protoplast shrinks and pulls away from the cell wall: from the cell wall: plasmolysisplasmolysis

– Incipient plasmolysisIncipient plasmolysis: the point at which : the point at which pressure potential has just reached zero pressure potential has just reached zero and plasmolysis is about to occur and plasmolysis is about to occur

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ALBIO9700/2006JK

Exchange surfacesExchange surfaces

• Gaseous exchange Gaseous exchange in mammalian lungsin mammalian lungs– Gaseous exchange Gaseous exchange

surfacesurface: where : where oxygen from the oxygen from the external environment external environment can diffuse into the can diffuse into the body, and carbon body, and carbon dioxide can diffuse dioxide can diffuse outout

– In humans: In humans: alveolialveoli in in the lungsthe lungs

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• Uptake of mineral ions in a plant rootUptake of mineral ions in a plant root– Specialised exchange surface: Specialised exchange surface: root hairsroot hairs (very (very

thin extensions of the cells that make up the thin extensions of the cells that make up the outer layer or outer layer or epidermis epidermis of a root)of a root)

– Root hairs make contact with thin layer of water Root hairs make contact with thin layer of water coating each soil particle and absorbs it by coating each soil particle and absorbs it by osmosisosmosis

– Lower concentration of solutes in the water in the Lower concentration of solutes in the water in the soil than there is inside the root hair cellsoil than there is inside the root hair cell

– Water potential higher outside the root hair and Water potential higher outside the root hair and water moves passively down the water potential water moves passively down the water potential gradient into the cellsgradient into the cells

– Mineral ions also absorbed by facilitated diffusion Mineral ions also absorbed by facilitated diffusion if concentration is higher outside root hairif concentration is higher outside root hair

– By active transport (carrier proteins and energy) By active transport (carrier proteins and energy) when concentration is relatively low in soilwhen concentration is relatively low in soil

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ALBIO9700/2006JK