Chapter 7Membrane Structure &

Function

I Can’s

Explain why membranes are selectively permeable

Describe the roles of phospholipids, proteins, & carbohydrates in membranes

Outline how water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution

Identify how electrochemical gradients are formed

7.1

The cell (plasma) membrane is selectively permeable Allows some things to cross easier than others

Made primarily of phospholipids & proteins Held together by weak interactions that cause the

membrane to be fluid The FLUID MOSAIC MODEL describes the the membrane

as fluid, with proteins embedded in or associated with the phospholipid bilayer

phospholipids

Provide a hydrophobic barrier that separates the cell from its liquid environment

Hydrophilic cannot easily enter the cell Hydrophobic can enter more easily

Proteins

2 types of proteins: 1) Integral

Completely embedded in the membrane Some are transmembrane proteins that span the

membrane completely

2) Peripheral Loosely bound to the surface of the membrane

Six major functions of membrane proteins: Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and

extracellular matrix (ECM)

Carbohydrates

Crucial for cell-cell recognition (for immune function)

Important for developing organisms (tissue differentiation)

Cell surface carbs vary from species to species and are the reason that blood transfusions must be type-specific

7.2

Membrane structure results in selective permeability

Nonpolar molecules (hydrocarbons, CO2, & O2) Hydrophobic & can dissolve in the phospholipid bilayer

& cross the membrane

Hydrophobic core of the membrane Impedes the passage of ions & polar molecules

(hydrophilic) Hydrophilic substances can avoid the lipid bilayer by

passing through transport proteins (transmembrane proteins)

Carrier proteins bind to molecules and change shape to shuttle them across the membrane

Movement of water Move through special transport proteins called aquaporins

by accelerating passage

7.3

Passive transport is diffusion of a substance across a membrane with NO ENERGY investment

Hydrocarbons, CO2, & O2 exhibit PT Passive diffusion

Travels from high concentration to a less concentration Flows down the concentration gradient Requires NO WORK Relies on thermal motion energy

Osmosis = diffusion of water across a selectively permeable membrane3 relationships:

1) Isotonic2) Hypertonic3) Hypotonic

Isotonic No net movement across the membrane Water crosses at the same rate in both

directions

Hypertonic solution Cell will lose water to its surroundings More solutes around the cell so water moves

to the higher concentration Cell loses water, shrivels, & dies

Hypotonic Water will enter faster than it leaves Fewer solutes in the water around the cell Movement of water follows the higher

concentration of solutes The cell will swell and possibly burst

Ions and polar molecules Cannot easily pass across the membrane Called facilitated diffusion

Utilizes transport proteins TP are specific for what they transport

How transport proteins work: 1) provide a hydrophilic channel that

molecules can pass through 2) bind loosely to the molecules and

carry them through the membrane

7.4

Active transport uses energy to move solutes against their gradients

Moved from less concentrated to higher concentrated (think of uphill movement)

Requires energy (usually ATP)

Sodium-Potassium Pump Pumps sodium out of the cell and

potassium into the cell Necessary for proper nerve transmission Major energy consumer in your body

Diffusion of ions

Membrane Potential The difference in electric charge across a membrane

that is expressed in voltage The inside carries a (-) charge Leads to an attraction with a Cation such as sodium This leads to 2 forces called the electrochemical

gradient: 1) a chemical force, which is the ion’s [ ] gradient 2) a voltage gradient, attracts + ions and repels - ions

Electrogenic pump A transport protein that generates

voltage across a membrane Na-K pump & Proton pump are examples

Cotransport An ATP pump that transports a specific solute

indirectly drives the active transport of other substances

The substance that was initially pumped across the membrane can do work as it moves back across the membrane by diffusion & will bring a second compound against its gradient

7.5

Bulk transport across the membrane occurs by exocytosis & endocytosis

Exocytosis Vesicles from the cell’s interior fuse with the cell

membrane Expels the contents of the vesicles

Endocytosis Cell forms new vesicles from the membrane

(reverse of exo) Allows the cells to take IN large molecules 3 types: 1) Phagocytosis 2) Pinocytosis 3) Receptor-mediated endocytosis

Phagocytosis – “cellular eating” Occurs when the cell wraps pseudopodia

around a solid particle and brings it into the cell

Pinocytosis – “Cellular drinking” Cell takes in small droplets of extracellular

fluid within small vesicles Not specific, because it takes in anything

Receptor-mediated endocytosis VERY SPECIFIC Certain substances (ligands) bind to specific

receptors on the cell’s surface (clusters) Causes a vesicle to form around the

substance and then pinch off into the cytoplasm