2 Learning Outcomesexplain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis;
3 Exchange across the plasma membrane The membrane provides an effective barrier against the movement of substances, however some exchange between the cell and the environment is essential.
4 Transport across membranes Materials can move across cell membranes:PassivelyDiffusion (simple or facilitated)OsmosisActivelyActive transportBulk transport
5 DiffusionNet movement of molecules or ions from a region of high concentration to a region of low concentrationOccurs along a concentration gradientResult = equilibrium (molecules or ions evenly spread out within a given space or volume)
6 Factors affecting the rate of diffusion Concentration gradientGreater the difference in concentration the greater the rate of diffusionTemperatureAt higher temperature kinetic energy particles increasesDiffusion is fasterSurface areaGreater the surface area, more particles can crossIncreases rate of diffusion
7 Factors affecting the rate of diffusion Nature of molecules or ionsLarge molecules diffuse slowerNon-polar molecules diffuse more easilyThe respiratory gases (CO2 and O2) are small enough to diffuse quickly through the membrane.Large, polar molecules (glucose and amino acids) and ions (Na+ and Cl-) cannot diffuse through the phospholipid bilayer
8 Facilitated Diffusion Protein molecules exist in membranes to facilitate diffusion.2 type of protein moleculeChannel proteintransmembrane protein that forms a tunnel through the bilayer.Carrier proteinschange shape to help molecules move into and out of cells.
10 Active TransportEnergy consuming transport of molecules or ions across a membrane against a concentration gradient, made possible by transferring energy from respiration.Energy makes the carrier proteins change shape, transferring ions across the membrane.
11 Examples of active transport Reabsorption in kidneysDigestion in gutHelps absorb glucose from our intestinesLoad sugars into phloemInorganic ion uptake in root hairsMagnesium ions are in short supply in the soil but are needed for photosynthesis
12 Bulk transportThis is the method of transporting large quantities of materials into cells (endocytosis) or out of cells (exocytosis)Endocytosis - Engulfing of material by cell membrane to form a endocytic vacuole.2 formsPhagocytosis the uptake of solid materialPinocytosis the uptake of liquidExocytosis - Process by which materials are removed from cells
13 Examples of bulk transport Hormones released into bloodstream from endocrine glandsWhite blood cells engulf invading microorganisms by phagocytosisIn plant cells materials to build the cell wall are carried outside in vesicles.
14 OSMOSIS Special type of diffusion involving water molecules Example: Two solutions are separated by a partially permeable membrane. Solute molecules are too large to pass through pores in the membrane, but water molecules are small enough.
15 What would happen if the membrane were not present?
16 Net movement of solute molecules from B to A by diffusion Net movement of water molecules from A to B by diffusionEquilibrium – concentrations of water molecules and solute molecules in A would equal that in B.
19 Solute molecules too large to pass through membrane Water molecules pass easily from A to BNet movement of water from A to B until equilibrium is reached, i.e. solution A has the same concentration of water molecules as solution B.The level of liquid A will fall and the level of liquid B will riseEquilibrium is brought about by the movement of water molecules alone.
20 Definition of osmosis Water potential Ψ Osmosis Tendency of water molecules to diffuse from one place to another.Measured in kPaPure water has a water potential of 0kPaOsmosisIs the net movement of water molecules from a region of high water potential to a region of low water potential (down a water potential gradient) across a partially permeable membrane.
21 Water potential Highest water potential 0kPa Pure water No solute Lower water potential-50kPaDilute solutionSmall amount of solute dissolvedVery low water potential-500kPaConcentrated solutionLarge amount of solute dissolvedDecreasing water potential
22 Some Important Terms Hypotonic Hypertonic Isotonic a region of higher water potential.Lower solute concentrationHypertoniclower water potentialHigher solute concentrationIsotonica region where there are equal water potentials on either side of a membrane.
23 Determining Water Potential in Potato tubers Salt Soluntion(mol-1)Starting Mass (g)Finishing mass (g)Change in mass (g)%age change in mass0.10.20.30.40.5
26 Important Terms Turgid Plasmolysed the term used to describe a plant cell where the protoplast exerts a pressure on the cell wall.Plasmolysedthe term used to describe a plant cell where the protoplast has shrunk away from the cell wall due to loss of water by osmosis.