Chapter 6: Plasma Membrane

Fluid mosaic model

model that describes the arrangement and movement of the molecules that make up a cell membrane

Phospholipids vary in

-fatty acid chain length (# of carbons)

-degree of unsaturation (# of double bonds)

-polar group present (can vary)

Saturated fatty acids

-tails are relatively straight

-packing of phospholipids is tight

-higher melting point (more heat for them to separate

Unsaturated fatty acids

-tails have kinks

-loose packing

-lower melting point

Palmitic (16:0

16 carbons, no double bonds

Cholesterol

in animal cells: it fills in the spaces left by kinks of unsaturated fatty acids; its rigidity tends to decrease membrane fluidity

-4 fused rings

-OH of cholesterol is attracted to polar molecules (ex. polar heads in phospholipids)

Lateral diffusion

phospholipids in the membrane are able to move sideways between other phospholipids

Flip-flop

phospholipids in a membrane can flip over but this takes a while and is less likely to occur because polar head must go through non polar region in order to flip

Factors that increase membrane fluidity

-increase in temp (more kinetic energy; cells collide)

-increase unsaturated fatty acids

-increase shorter fatty acids

-lower cholesterol (at moderate temp.)

What factors decrease membrane fluidity?

-temperature

-unsaturated fatty acids

-length of fatty acids

-cholesterol

Peripheral membrane proteins

-do not penetrate the bilayer at all

-lack exposed hydrophobic groups

*some are lipid-anchored

Integral membrane proteins

-penetrate the bilayer (partially or fully)

-transmembrane proteins cross the entire membrane

-hydrophobic regions interact w/hydrophobic core of membrane

-hydrophilic regions are exposed to aq env.

Lipid-anchored membrane proteins

-have fatty acids or other lipid groups covalently attached to them

-anchored to specific region of the cell

Why are oligosaccharides (carbs) attached to proteins and lipids?

-membranes also have carbs on outer surface that serve as recognition and/or signaling sites for other cells and molecules

-carbohydrates are added to proteins and lipids via glycosylation (adding glycoside to protein)

gylcolipids

lipids attached to carbohydrates

glycoprotein vs proteoglycan

glycoproteins: mostly proteins

proteoglycans: carb & protein, but mostly carbs

What are membranes constantly doing?

forming, transforming, fusing, and breaking down

-they are DYNAMIC

cell recognition and adhesion

cells arrange themselves in groups by cell recognition and cell adhesion

-process is: tissue specific and/or species specific

Types of cell junctions

tight junctions, desmosomes, gap junctions

Tight junctions

proteins form a quilted seal that limits movement of solution through space between cells (proteins act like thread in a quilt)

Desmosomes

link adjacent cells tightly but permit materials to move around them in the intercellular space

Gap junctions

connexin protein channels allow molecules to pass between adjacent

Integrins

-cell membranes also adhere to extracellular matrix

-integrin protein binds to matrix outside epithelial cells and to actin filaments (cytoskeleton) inside the cells

-binding is non covalent & reversible

How do integrins work

-integrin is recycled from the "back" of the cell by endocytosis

-as the cell moves forward, vesicles deliver integrin to the "front," where the integrin attaches to the extracellular matrix

Factors that affect transport

-direction of concentration gradient

-size

-polarity

how do membranes function as gatekeepers?

-they are selectively permeable

-some substances can pass through unaided, others cannot

-cell needs to get nutrients into cell and waste out of cell

Passive transport

happens spontaneously, no energy is required to get something across the cell membrane

-simple diffusion

-osmosis

-facilitated diffusion via channel or carrier proteins

Active transport

requires an input of energy to get something across a membrane

-transport of substances against, or up a concentration gradient (low to high)

-uses carrier proteins

-energy source is usually ATP but it is sometimes a separate concentration gradient

Diffusion

things wants to spread out

-random movement of molecules toward a state of equilibrium (same concentration everywhere)

-movement from high to low concentration

-rate of diffusion depends on size of cell, temperature, and concentration gradient

Simple diffusion

-small, nonpolar/hydrophobic molecules can move freely through cell membrane (ex. co2, o2)

-these molecules move down concentration gradient

-molecules randomly move around

Osmosis

the diffusion of water; passive process

-depends on the # of other solute particles present on either side of the membrane

-water will flow towards the area of higher solute concentration

Aquaporin

A membrane protein, specifically a transport protein, that facilitates the passage of water through channel proteins

-facilitated diffusion of water

Isotonic solution

equal solute concentration inside and outside of cell

Hypotonic solution

lower solute concentration on outside of cell than inside

-water moves into cell due to osmosis and the cells swell (can burst - hemolysis)

Hypertonic solution

higher solute concentration on outside of cell than inside

-water moves out of cell and cell shrinks

Facilitated diffusion

diffusion for ions and water-soluble (polar) molecules (ex. Na+, K+, Cl-, glucose, amino acids, nucleotides) that can move down their concentration gradient

-channel proteins or carrier proteins get molecules across

Facilitated diffusion: channel proteins

-central pore lined w/polar amino acids

-like a tunnel ions can flow through

-high specificity (to certain molecules)

-are often gated - channel only opens when a stimulus is present

-some are always open while others are gated (stimulus binds, changes shape, & gate opens)

Facilitated diffusion: carrier proteins

-still no energy required, but only one molecule crosses at a time

-transport of larger polar molecules like glucose and amino acids

-binding of a chemical stimulus causes a shape change

Why is energy used on active transport?

-living cells often need to take in scarce molecules

-some things need to be maintained at high or low concentrations in or out of cells

Uniporter

A carrier protein that transports a single molecule across the plasma membrane in one direction

Symporter

A carrier protein that transports two molecules across the plasma membrane in the same direction. For example, the Na+-glucose cotransporter in intestinal cells is a symporter.

Antiporter

A carrier protein that transports two molecules acrss the plasma membrane in opposite directions.

Primary active transport

direct hydrolysis of ATP provides energy for transport

-ex. sodium potassium pump (antiporter)

-3Na+ out for every 2K+ in

Secondary active transport

an already established concentration gradient allows a different molecule to be transported against its own concentration gradient (uses something that's diffusing)

Transport of large molecules

some molecules that a cell needs to take in or release are too large for carrier proteins (ex. nucleic acids, polysaccharides, proteins)

-endocytosis

-exocytosis

Phagocytosis

cell membrane engulfs large particles, cellular "eating"

-makes a phagosomes

-nonspecific

Pinocytosis

cell membrane brings in liquids and small particles, still with vesicle formation, "cellular drinking"

-dissolved solutes

-non specific

Receptor-mediated endocytosis

uptake of specific materials, recognized on cell surface by receptor proteins

-very specific, coated vesicle

-ex. LDL molecules full of cholesterol

Exocytosis

how cells release large molecules; secretory vesicles fuse w/plasma membrane to release molecules outside the cell

Other roles for membranes

-energy transformation

-organizing chemical rxns

-information processing