Water Potential Review

Transcription

1 Water Potential Review Which way will the water flow? Ψ s = -7 Ψ s = -4

2 Chapter 7 Review Membrane Structure and Function

3 Selective Permeability The plasma membrane exhibits selective permeability It allows some substances to cross it more easily than others Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

4 Cellular membranes are fluid mosaics of lipids and proteins Phospholipids (fluid part) Are the most abundant lipid in the plasma membrane Are amphipathic, containing both hydrophobic and hydrophilic regions Proteins (mosaic part) can be Transmembrane (integral) Peripheral Fluid Mosaic Model

5 Lipids Phospholipid structure Consists of a hydrophilic head and hydrophobic tails CH 2 + N(CH 3 ) 3 Choline CH 2 O O P O Phosphate O CH 2 O CH O CH 2 Glycerol C O C O Fatty acids Hydrophilic head Hydrophobic tails Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

6 Lipids The structure of phospholipids Results in a bilayer arrangement found in cell membranes Hydrophilic head WATER WATER Hydrophobic tail Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

7 Lipids Saturated fatty acids Have the maximum number of hydrogen atoms possible Have no double bonds Are typically solid at room temperature Stearic acid Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

8 Lipids Unsaturated fatty acids Have one or more double bonds Are typically liquid at room temperature Oleic acid Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings cis double bond causes bending

9 How fluid? The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails Membrane fluidity Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

10 How fluid? The steroid cholesterol Has different effects on membrane fluidity at different temperatures Cholesterol within the animal cell membrane Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cholesterol

11 Thinking Question Would you expect an amoeba that lives in a cold climate to have a higher or lower amount of saturated hydrocarbon chains in its phospholipids compared to a species that lives in a warm climate?

12 Thinking Question Would you expect an amoeba that lives in a cold climate to have a higher or lower amount of saturated hydrocarbon chains in its phospholipids compared to a species that lives in a warm climate? Hint: saturated hydrocarbons are MORE fluid at room temperature.

13 Membrane fluidity Thinking Question Would you expect an amoeba that lives in a cold climate to have a higher or lower amount of saturated hydrocarbon chains in its phospholipids compared to a species that lives in a warm climate? Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails

14 Fluid-Mosaic Model Membrane proteins (mosaic part) are dispersed and individually inserted into the phospholipid bilayer Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

15 Membrane Carbohydrates Cell-cell recognition Is a cell s ability to distinguish one type of neighboring cell from another Short carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cells These carbohydrates mediate cell-cell recognition

16 Crossing the Membrane Cells need to control the exchange of material with their environment Hydrophobic molecules (non-polar) Are lipid soluble and can pass through the membrane rapidly Hydrophilic molecules (polar, ionic) Do not cross the membrane rapidly Need help of transport proteins

17 Passive vs. Active Transport Passive transport is the movement of a substance across a membrane with no energy investment Involves the process of simple diffusion Or facilitated diffusion Active transport requires an energy input

18 Passive Transport Simple Diffusion Is the tendency for molecules to move from areas of high concentration to areas of low concentration Small, hydrophobic molecules can move across the membrane this way Molecules of dye Membrane (cross section) Net diffusion Net diffusion Equilibrium Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

19 Facilitated Diffusion In facilitated diffusion Transport proteins speed the movement of molecules across the plasma membrane Does NOT require energy input (no ATP)

20 Facilitated Diffusion Transport proteins Provide corridors that allow a specific molecule or ion to cross the membrane EXTRACELLULAR FLUID Channel protein (a) Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings Solute CYTOPLASM

21 Special Case of Facilitated Diffusion Osmosis Is the diffusion of water across a selectively permeable (semipermeable) membrane Lower Higher concentration concentration of solute = Higher of solute = Lower concentration of concentration of water water Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings s Water moves from an area of higher water concentration to an area of lower water concentration

22 Tonicity Tonicity Is the ability of a solution to cause a cell to gain or lose water If a solution is isotonic The concentration of solutes is the same as it is inside the cell There will be no net movement of water If a solution is hypertonic The concentration of solutes is greater than it is inside the cell The cell will lose water If a solution is hypotonic The concentration of solutes is less than it is inside the cell The cell will gain water

23 Tonicity Diagram 5% NaCl 5% NaCl 10% NaCl 20% NaCl Isotonic Hypotonic Hypertonic Hypotonic Hypertonic Water

24 Osmosis in Cells Water balance in cells without walls Such as animal or protist Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled/Crenation Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

25 Osmosis in Cells Water balance in cells with walls Such as plant cells Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O Turgid (normal) Flaccid Plasmolyzed Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

26 Thinking Question Imagine you are in the hospital recovering from an operation and require intravenous fluids. Why should you be concerned about the concentration (tonicity) of the solution in the IV bags?

27 Active Transport Active transport Moves substances against their concentration gradient from low to high Requires energy, usually in the form of ATP

28 Review: Passive and Active Transport Compared Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. ATP Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

29 Movement of Macromolecules Transport of large molecules across the plasma membrane occurs by exocytosis and endocytosis In exocytosis Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane

30 Endocytosis Three types of endocytosis EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 µm Pseudopodium of amoeba Food or other particle Food vacuole Bacterium Food vacuole Plasma membrane PINOCYTOSIS 0.5 µm Vesicle Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

31 RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Ligand Coated pit Coat protein Plasma membrane Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0.25 µm

32 Key Points of Chapter 7 Cellular membranes are fluid mosaics of lipids and proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance across a membrane with no energy investment Active transport uses energy to move solutes against their gradients Bulk transport across the plasma membrane occurs by exocytosis and endocytosis