chapter 7 membrane structure & function. i can’s explain why membranes are selectively...
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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
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