Chapter 7 Review Membrane Structure and Function

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

 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  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

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

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

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

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

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

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?

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? FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails Membrane fluidity

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

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  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

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  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

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  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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Equilibrium (a)

Facilitated Diffusion  In facilitated diffusion  Transport proteins speed the movement of molecules across the plasma membrane  In facilitated diffusion  Transport proteins speed the movement of molecules across the plasma membrane

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

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

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  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

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmosis in Cells  Water balance in cells without walls  Such as animal or protist Hypotonic solution Isotonic solution Hypertonic solution H2OH2O H2OH2O H2OH2O H2OH2O Lysed NormalShriveled/Crenation

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmosis in Cells  Water balance in cells with walls  Such as plant cells H2OH2OH2OH2O H2OH2O H2OH2O Turgid (normal)Flaccid Plasmolyzed Hypotonic solution Isotonic solution Hypertonic solution

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

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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  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

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

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

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  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