Presentation on theme: "Movement across Cell Membranes."— Presentation transcript:
1 Movement across Cell Membranes. WALTbe able to describe how substances move across cell membranes and identify the 5 ways in which substances do thisWILFTo be able to explaindiffusion as the passive movement of substances in the direction of a concentration gradientthe role of carrier and channel proteins in facilitated diffusionosmosis as a special case of diffusion across a partially permeablemembraneActive transport as the movement of molecules or ions through a membrane by carrier proteins against a concentration gradient,
2 Cell membranes are a barrier to most substances. This property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment.
3 This is compartmentalisation and is essential for life, as it enables reactions to take place that would otherwise be impossible.Eukaryotic cells can also compartmentalise materials inside organelles.Materials need to be able to enter and leave cells
4 There are five main methods by which substances can move across a cell membrane. 1. Lipid Diffusion2. Osmosis3. Passive Transport4. Active Transport5. Vesicles
5 Lipid Diffusion (or Simple Diffusion) A few substances can diffuse directly through the lipid bilayer part of the membrane.The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2.Lipid diffusion is a passive diffusion process, no energy is involved and substances can only move down their concentration gradient.Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.
6 Osmosis Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells the diffusion of water has its own name - osmosis.The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are.Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.
8 Dissolved substances attract a ‘cloud’ of polar water molecules around them. (Usually held by weak hydrogen bonds)The water molecules can no longer move freelyThe more concentrated the solution the more water molecules are ‘tied up’When a solution is separated from pure water or a more dilute solution by a membrane permeable to water, the free water molecules are still able to diffuse, moving across the membrane.
9 The lipid bilayer of the plasma membrane is theoretically impermeable to water because they are polarDue to the fluid properties of the membrane, and the fact that water is a very small molecule, as well as there being protein lined pores, there is unrestricted water movement across the membrane
10 Water PotentialThe water potential of a solution is the name given to the tendency of water molecules to enter or leave a solution by osmosis‘Water potential’ is really a measure of the free kinetic energy of the water molecules.The Greek letter psi (symbol ψ) is used to represent water potential
11 Solute potentialThis is the effect of the amount of dissolved solute present.It is shown by the symbol ψsThis used to be referred to as the Osmotic pressureA solution with a high solute potential would have a high inflow of water molecules to dilute it.
12 Pressure potentialPressure potential (or turgor pressure) is the mechanical pressure acting on a substanceIf a pressure greater than atmospheric pressure is applied to a solution then a pressure potential is created in the solutionPressure potential is represented by the symbol ψp
13 ExampleVisking tubing containing sucrose solution is lowered into pure waterUptake of water over time due to high solute potential. The water molecules in the pure water have a high water potential.Visking tubing becomes stretched by high internal pressureNet water uptake ceases due to high hydrostatic pressureThe pressure potential (ψp) offsets the solute potential (ψs)
15 100% pure water has ψ = 0, which is the highest possible water potential, so all solutions have ψ < 0, and you cannot get ψ > 0.Once a solute is dissolved in water, the water molecules become less mobile and are less likely to diffuse.The effect of dissolving a solute in water is to lower its water potential, making it more negative
16 QuestionsWhen a concentrated solution of glucose is separated from a dilute solution of glucose by a partially permeable membrane, which solution has:The higher water potential?Dilute glucose solutionA higher concentration of water molecules?Will show a net gain of water molecules?Concentrated glucose solution
17 Water potential is measured in units of pressure (Pa, or usually kPa) Because the highest water potential is pure water when ψ = 0 all other values are negative.When you write about water potential, try to write in terms of less negative and more negative, rather than higher and lower, and then you won’t get confused.If you have to use the words higher and lower, think about temperatures. A temperature of–10 °C is higher than one of –20 °C. In the same way, a water potential of –10 Pa is higher than a water potential of –20 Pa.
18 Cells and OsmosisThe concentration (or solute potential) of the solution that surrounds a cell will affect the state of the cell, due to osmosis.There are three possible concentrations of solution to consider:Isotonic solution a solution of equal ψs (or concentration) to a cellHypertonic solution a solution of higher ψs (or concentration) than a cellHypotonic solution a solution of lower ψs (or concentration) than a cell
20 ProblemsSimple animal cells (protozoans) in fresh water habitats are surrounded by a hypotonic solution and constantly need to expel water using contractile vacuoles to prevent swelling and lysis.
21 Cells in marine environments are surrounded by a hypertonic solution, and must actively pump ions into their cells to reduce their water potential and so reduce water loss by osmosis.
22 Young non-woody plants rely on cell turgor for their support, and without enough water they wilt. Plants take up water through their root hair cells by osmosis, and must actively pump ions into their cells to keep them hypertonic compared to the soil.This is particularly difficult for plants rooted in salt water.
23 Six cylinders of a standard size were cut from a single large potato Six cylinders of a standard size were cut from a single large potato. One cylinder was placed in distilled water and the others were placed in sucrose solutions of different concentrations. The length of each cylinder was measured every 5 minutes for the next 50 minutes. The graph shows the changes in length at each sucrose concentration.Explain whya) (i) the potato cylinder in distilled water increased in length;(ii) the potato cylinder in the 1.0 mol dm–3 sucrose solution showed no furtherdecrease in length after 40 minutes.(b) (i) Describe the difference in the rate of decrease in length during the first 10 minutes between the cylinder in the 0.4 mol dm–3 and the cylinder in the 0.8 mol dm–3 solution.(ii) Use your knowledge of water potential to explain this difference.
24 Explain why(i) the potato cylinder in distilled water increased in length;potato more negative water potential/hypertonic;(accept more concentrated)water enters by osmosis;cells extend/are turgid; 2 max(ii) the potato cylinder in the 1.0 mol dm–3 sucrose solution showed no furtherdecrease in length after 40 minutes.little/no water remaining in potato/fully plasmolysed/all waterhas moved out;cell wall prevents further shrinkage/sucrose solution moves in;or, water potentials are equal/equilibrium/isotonic;no net movement of water/no further osmosis; 2 marks(b) (i) Describe the difference in the rate of decrease in length during the first 10 minutes between the cylinder in the 0.4 mol dm–3 and the cylinder in the 0.8 mol dm–3 solution.faster rate (of decrease) in 0.8 mol dm-3 ; 1mark(ii) Use your knowledge of water potential to explain this difference.bigger water potential gradient/greater difference in waterpotentials (between potato and surrounding solution); 1mark
25 Points to note• Try to think of osmosis in terms of water potential rather than water or solute concentration.• Osmosis is the movement from a high water potential to a low water potential through a partially permeable membrane.• The water potential of pure water is 0. Since pure water has the highest water potential, all other values will be negative.