Movement across Cell Membranes.

Movement across Cell Membranes. • WALT • be able to describe how substances move across cell membranes and identify the 5 ways in which substances do this • WILF • To be able to explain • diffusion as the passive movement of substances in the direction of a concentration gradient • the role of carrier and channel proteins in facilitated diffusion • osmosis as a special case of diffusion across a partially permeablemembrane • Active transport as the movement of molecules or ions through a membrane by carrier proteins against a concentration gradient,

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.

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

There are five main methods by which substances can move across a cell membrane. 1. Lipid Diffusion 2. Osmosis 3. Passive Transport 4. Active Transport 5. Vesicles

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.

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.

Osmosis

Dissolved substances attract a ‘cloud’ of polar water molecules around them. (Usually held by weak hydrogen bonds) • The water molecules can no longer move freely • The 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.

The lipid bilayer of the plasma membrane is theoretically impermeable to water because they are polar • Due 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

Water Potential • The 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

Solute potential • This is the effect of the amount of dissolved solute present. • It is shown by the symbol ψs • This used to be referred to as the Osmotic pressure • A solution with a high solute potential would have a high inflow of water molecules to dilute it.

Pressure potential • Pressure potential (or turgor pressure) is the mechanical pressure acting on a substance • If a pressure greater than atmospheric pressure is applied to a solution then a pressure potential is created in the solution • Pressure potential is represented by the symbol ψp

Example • Visking tubing containing sucrose solution is lowered into pure water • Uptake 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 pressure • Net water uptake ceases due to high hydrostatic pressure • The pressure potential (ψp) offsets the solute potential (ψs)

Water potential = solute potential + pressure potential Ψ = ψs + ψp

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

Questions • When 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 solution • A higher concentration of water molecules? • Dilute glucose solution • Will show a net gain of water molecules? • Concentrated glucose solution

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.

Cells and Osmosis • The 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 cell • Hypertonic solution  a solution of higher ψs (or concentration) than a cell • Hypotonic solution a solution of lower ψs (or concentration) than a cell

Problems • Simple 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.

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.

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.

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 why • a) (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 further • decrease 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.

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 further • decrease in length after 40 minutes. • little/no water remaining in potato/fully plasmolysed/all water • has 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 water • potentials (between potato and surrounding solution); 1mark

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.

CAMS • Osmosis and water potential

Movement across Cell Membranes.