Overview: Lab Cell membrane The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Bi-lipid layer and proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept : Cellular membranes are fluid mosaics of lipids and proteins The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

Transport Chapter 5 Homeostasis Homeostasis – cell maintains internal equilibrium by adjusting its physiological processes

Movement across a membrane The movement of a substance across a membrane is active transport if it requires energy from the cell. The diffusion of a substance across a biological membrane is passive transport if it requires no energy from the cell to make it happen

Concept : Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction At dynamic equilibrium, as many molecules cross one way as cross in the other direction Animation: Membrane Selectivity Animation: Membrane Selectivity Animation: Diffusion Animation: Diffusion Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

concentration gradient, the difference in concentration of a substance from one area to another Substances diffuse down their concentration gradient high to low No work must be done to move substances down the concentration gradient no energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Molecules of dye Fig. 7-11a Membrane (cross section) WATER Net diffusion (a) Diffusion of one solute Equilibrium

(b) Diffusion of two solutes Fig. 7-11b Net diffusion Equilibrium

Occurs when molecules of one substance is spread evenly through out another substance. just so you really get it …. cells never reach a state of equilibrium. Instead, they achieve a "steady state". The chemical composition of the cell is "dynamic"; it is constantly in a state of flux. At dynamic equilibrium, as many molecules cross one way as cross in the other direction Chemical processes that reach equilibrium cease. If the chemical processes within the cell ceased the cell would die.

Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration I like to say high water conc. to low water conc. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Lower concentration of solute (sugar) Fig H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis

Brownian Motion The random movement of microscopic particles suspended in a liquid or gas, caused by collisions with molecules of the surrounding medium.

Water Balance of Cells Without Walls Tonicity is the ability of a solution to cause a cell to gain or lose water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water Balance of Cells with Walls Turgor – pressure in Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cytolysis breakdown of cells by the destruction of the animal cells outer membrane. If the cell membrane cannot regulate excessive influx of water Ex – RBC which is cytolysis ?

Video: Plasmolysis Video: Plasmolysis Video: Turgid Elodea Video: Turgid Elodea Animation: Osmosis Animation: Osmosis lethal effect called Plasmolysis when plant cells lose water; eventually, the membrane pulls away from the wall, Occurs in a hypertonic environment, Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Hypotonic solution (a ) Animal cell (b ) Plant cell H2OH2O Lysed H2OH2O Turgid (normal) H2OH2O H2OH2O H2OH2O H2OH2O Normal Isotonic solution Flaccid H2OH2O H2OH2O Shriveled Plasmolyzed Hypertonic solution

Polar molecules do not cross the membrane easily small – 02, C02,ethanol can pass large – sugars can’t pass ions – H+, NA + can’t pass So these large compounds and ions use …….

carrier molecules - are transport proteins, called bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves 1. carrier - facilitated diffusion 2. carrier- NA + K - pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed up the passive movement of molecules across the plasma membrane Facilitated diffusion is still passive because the solute moves down its concentration gradient – Ex- move glucose and amino acids Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The sodium-potassium pump is one type of active transport system 3 Na + out, 2 K +in = maintains a – cytoplasm Active transport allows cells to maintain concentration gradients that differ from their surroundings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Channel proteins include – Gated channels - Ion channels that open or close in response to a stimulus – also facilitated diffusion, passive because the solute moves down its concentration gradient

Fig EXTRACELLULAR FLUID Channel protein (E) A gated channel protein Solute CYTOPLASM Solute Carrier protein (D 1,2) A carrier protein

2 EXTRACELLULAR FLUID [Na + ] high [K + ] low [Na + ] low [K + ] high Na + CYTOPLASM ATP ADP P Na + P 3 K+K+ K+K+ 6 K+K+ K+K+ 5 4 K+K+ K+K+ P P 1 Fig

Fig Passive transport Diffusion Facilitated diffusion Active transport ATP

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins Two types of endocytosis: – Pinocytosis (“cellular drinking”) – Phagocytosis (“cellular eating”) Animation: Exocytosis and Endocytosis Introduction Animation: Exocytosis and Endocytosis Introduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Animation: Pinocytosis Animation: Pinocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20b PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM)

In phagocytosis a cell engulfs a particle in a vacuole cell eating The vacuole fuses with a lysosome to digest the particle Animation: Phagocytosis Animation: Phagocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-20a PHAGOCYTOSIS CYTOPLASM EXTRACELLULAR FLUID Pseudopodium “Food” or other particle Food vacuole Food vacuole Bacterium An amoeba engulfing a bacterium via phagocytosis (TEM) Pseudopodium of amoeba 1 µm

Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Animation: Exocytosis Animation: Exocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-UN3 Environment: 0.01 M sucrose 0.01 M glucose 0.01 M fructose “Cell” 0.03 M sucrose 0.02 M glucose

Fig. 7-UN4