BIO146

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Chapter 7
Chapter 7

selective permeability - allowing some substances to cross it more easily than others; exhibited by the plasma 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)

7.1 Cellular membranes are fluid mosaics of lipids and proteins

amphipathic molecule — has both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region; phospholipids

→ a phospholipid bilayer can exist as a stable boundary between two aqueous compartments

→ phospholipid -- hydrophobic tail & hydrophilic head

→ most membrane proteins are amphipathic

fluid mosaic model -- the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

The Fluidity of Membrane

a membrane is held together mainly by hydrophobic interactions, which are much weaker than covalent bonds

→ most of the lipids and some proteins can shift about sideways - in the plane of the membrane

→ very rarely, a lipid may flip-flop across the membrane, switching from one phospholipid layer to the other

adjacent phospholipids switch positions about 10^7 times per second

→ a membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies (bacon grease) steroid cholesterol - wedged between phospholipid molecules in the plasma membranes of animal cells

→ resists changes in membrane fluidity that can be caused by changes in temperature

Membrane Proteins and Their Functions

integral proteins - penetrate the hydrophobic interior of the lipid bilayer

transmembrane proteins - span the membrane; the majority of membrane proteins

peripheral proteins - not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, often to exposed of integral proteins

HIV - Human immunodeficiency virus

AIDS - acquired immune deficiency syndrome

The Role of Membrane Carbohydrates in Cell-Cell Recognition

--> cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane

membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units

glycolipids - membrane carbohydrates that some are covalently bonded to lipids

glycoproteins - membrane carbohydrates that most are covalently bonded to proteins

7.2 Membrane structure results in selective permeability

selective permeability - it allows some substances to cross more easily than others; a membrane exhibits this

Transport proteins

transport proteins - hydrophilic substances can avoid contact with the lipid bilayer by passing through the transport proteins

→ channel transport proteins - function by having a hydrophilic channel that certain molecules or ions use as a tunnel through the membrane

→ aquaporins - facilitate the passage of water molecules through the membrane in certain cells

→ carrier transport proteins - hold on to their passengers and change shape in a way that shuttles them across the membrane

7.3 Passive transport is diffusion of a substance across a membrane with no energy investment

diffusion - the movement of particles of any substance so that they spread out into the available space; needs no energy

→ rule - spread from a higher concentrated area to a lower concentrated area

concentration gradient - the region along which the density of a chemical substance increases or decreases

down concentration gradient movement — passive transport, no energy required

against concentration gradient movement — energy required

→ much of the traffic across cell membranes occurs by diffusion

passive transport - the diffusion of a substance across a biological membrane

osmosis - the diffusion of free water across a selectively permeable membrane, whether artificial or cellular

Water Balance of Cells Without Cell Walls

tonicity - the ability of a surrounding solution to cause a cell to gain or lose water

→ the tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane relative to that inside the cell

isotonic - no net movement of water across the plasma membrane

hypertonic - the cell will lose water, shrivel and probably die

less solute & more water inside the cell and more solute & less water outside of the cell

hypotonic - water enters the cell faster than it leaves, the cell will swell and lyse

more solute & less water inside of the cell and less solute & more water outside of the cell

osmoregulation - the control of solute concentrations and water balance

turgid (firm) - the healthy state for most plant cells

flaccid - limp

plasmolysis - causes the plant to wilt and can lead to plant death

Facilitated Diffusion: Passive Transport Aided by Proteins

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ion channels - channel proteins that transport ions

gated channels - open or close in response to a stimulus

The Need for Energy in Active Transport

active transport - to pump a solute across a membrane against its gradient requires work, the cell needs energy

sodium-potassium pump - exchanges Na+ for K+ across the plasma membrane of animal cells

How Ion Pumps Maintain Membrane Potential

membrane potential - the voltage across a membrane, ranges from 50 to 200 mV

→ the inside charge of the cell is negative compared with the outside

→ the membrane potential favors the passive transport of cations into the cell and

anions out of the cell

→ electrochemical gradient - a combination of forces acting on an ion by chemical force and electrical force

electrogenic pump - a transport protein that generates a voltage across a membrane

proton pump - the main electrogenic pump of plants, fungi, and bacteria, which actively transports protons (hydrogen ions) out of the cell

Cotransport: Coupled Transport by a Membrane Protein

cotransport - occurs when active transport of a solute indirectly transport of other solutes

→ plants use H+/sucrose cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves

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Exocytosis

exocytosis - the cell secretes certain molecules by the fusion of vesicles with the plasma membrane

→ many secretory cells use exocytosis to export products

Endocytosis

endocytosis - the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

three types of endocytosis

phagocytosis (”cellular eating”) - the vacuole fuses with a lysosome to digest food

pinocytosis (”cellular drinking”)

Receptor-mediated endocytosis - binding of ligand (any molecule that binds specifically to a receptor site of another molecule) to receptors triggers vesicle formation

→ endocytosis and exocytosis also provide mechanisms for rejuvenating or remodeling the plasma membrane

Passive transport

high → low (no energy required)

simple diffusion

osmosis

facilitated diffusion (involves transport protein)

Active transport

low → high (energy required)

Sodium and potassium ion pump

exocytosis

endocytosis

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