components of cell membrane Quiz on functions of components of cell membrane

Crossing membranes (passive processes) pages 22-27 Starting activity: What do you remember about diffusion? Define it... What affects its rate? What state can it occur in?

Diffusion The net movement of molecules or ions from a region of their high concentration to a region of their lower concentration, down a concentration gradient. What substances have to diffuse into and out of living cells? Metabolism Cross membranes

The rate of diffusion Temperature Concentration gradient Stirring/moving Surface area Distance/thickness Size of molecule

Diffusion Lipid soluble, small, uncharged molecules can diffuse across the phospholipid bilayer down a concentration gradient – random process relying on kinetic energy of particles Substances e.g. CO2, O2, H2O, steroid hormones (lipid soluble)

Larger or charged molecules Facilitated diffusion

Facilitated diffusion Large or polar molecules can’t pass easily through the bilayer These molecules move through gaps in channel proteins Transport depends on a concentration gradient (facilitated = made easy) Substances e.g. glucose, sodium ions, amino acids

Summary table Substance moved by Examples Simple diffusion Facilitated diffusion using channel proteins Facilitated diffusion using carrier proteins Compare and contrast facilitated diffusion and simple diffusion

Osmosis – a special case The net movement of water molecules from a region of higher water potential to a region of lower water potential (down a water potential gradient) across a partially permeable membrane. Water potential = a measure of the concentration of water molecules which are able to diffuse.

Key definitions Solvent – a liquid that dissolves solids Solute – a solid that dissolves in a liquid Solution – a liquid containing dissolved solids

Water Potential Water potential (Ψ = greek letter psi) of a solution is the tendency of water to diffuse out of it (unit is one of pressure) i.e. how frequently water molecules are hitting the membrane – kPa, kilopascals Pure water has a water potential of 0 Adding a solute to pure water decreases the concentration of water molecules in a given space - the chance of water molecules leaving the solution is less likely as the water molecules are attracted to the solute molecules and move less freely Solutions have a negative water potential 17g sucrose in 1dm3 water Ψ = -130kPa

Osmosis – a special case of diffusion Although water molecules are polar they are small enough to pass through the membrane Osmosis is…..the diffusion of water from a high water potential to a low water potential (down its water potential gradient) across a partially permeable membrane

Understanding water potential and osmosis Units = Greek letter psi ψ Measured in kPa. Pressure created by water molecules Pure water has a ψ 0 kPa. As the water potential decreases value for ψ becomes more negative. The more solute dissolved, the lower the water potential of the solution. Diagram 4 from books page 27

Osmosis in animal and plant cells crenated haemolysed Incipient plasmolysis plasmolysed

Potato Cylinders Investigation Results Examine the potato cylinders and measure their mass (g) Copy and complete this table. How could we display this on a graph? Sucrose Concentration (M) 0.0 0.1 0.2 0.4 0.6 0.8 1.0 Initial mass (g) Final mass (g) Mass change (+/- g) Percentage mass change

Discussion  Why do you calculate the percentage change in mass rather than the actual change in mass Estimate the concentration of the potato from the graph. 3. From your knowledge of osmosis in plant cells, what do you suppose has happened to the cells of the potato tissue (a) in 1M sucrose, (b) in water?  How would these changes in the cells account for any changes in size of the potato cylinder?  What do you think limits the change in mass of the potato cylinder which was immersed in water? How could the results of this experiment be used to explain the extension growth of plant shoots and roots ? 7. How could the experiment be modified to find out the osmotic concentration of cell sap ?  

Red onion practical

Crossing membranes (active processes) Diffusion alone will not meet the needs of a cell. Speed Concentration gradient (glucose, magnesium ions) Ensuring a one way flow Moving large amounts

Active transport The movement of molecules or ions across membranes, which uses ATP to drive protein ‘pumps’ within the membrane. Eg Na⁺ in nerve axons Some carrier proteins act as pumps Complementary shape to molecule they carry The molecule can fit on just one side ensuring unidirectionality of movement

Active transport requires energy: It acquires energy from respiration so cells that carry out a lot of A.T. will need lots of mitochondria. What materials are required for aerobic respiration to occur? Root cells actively transport ions e.g. NO₃⁻, K⁺ into roots. This process stops when soil is waterlogged such as in peat bogs. Can you suggest why?

Moving large amounts – bulk transport Pinocytosis (small molecules / liquids) Phagocytosis (large solids) Animation

Movement across membranes

Summary of movement across membranes: Passive processes Diffusion Facilitated diffusion Osmosis Active processes Active transport Endocytosis and exocytosis

Tasks to try: Explain why active transport allows substances to be accumulated in an area, whereas facilitated diffusion doesn’t. Answer Q’s 10, 11 &12 Homework: Due next lesson Finish Q’s above and Q9 from booklet