general stucture of membranes is known as
fluid mosaic model
like a "lake" in which is variety of proteins "float
phospholipid bilayer
maintain a bilayer organization spontaneously helps membranes fuse during phagocytosis, vesicale formation, etc
lipids
phospholipids vary fatty acid chain length, degree of saturation, phosphate groups
membranes may be up to 25 percent cholesterol
membranes contain proteins, the number of proteins varies with cell function
some membrane proteins extend across the lipid bilayer ith hydrophobic and heydrophilic regions or domains
spand the bilayer, hydrophilic ends protrude on either side
integral membrane proteins
dont penetrate teh bilayer
peripheral membrane proteins
may have different domains on either side of the membrane
transmembrane proteins
the two sides of the membrane can have very different properties
some membrane proteins can move freely within teh bilayer, while some are anchored to a specific region
membranes have cabs on the outer surface that serve as recognition sites for other cells and moelcules
glycolipids and glycoproteins
cells arrange themselves in groups by cell recognition and cell adhesion
these processes can be studied in sponge cells the cells are easily separated and will come back together again
binding of cells is usually homotypic: the same molecule sticks out from both cells and forms a bond
some binding is heterotypic the cells have different proteins
help ensure directional movement of materials
tight junctions
like spot welds
desmosomes
allow communication
gap junctions
some substances can pass through but not others in membranes
selective permeability
no outside energy required- diffusion
passive transport
energy is required for passing through
active transport
teh process of random movement toward equilibrium
diffusion
particles continue to move, but there is no net change in distribution
equilibrium
new movement is directional until equilibrium is reached
diffusion is net movement from regions of greater concentration to regions of lesser concentration
diffusion trate depends on diameter of the moelcuesl or ion, temperature of the solution, electric charges, concentration gradient
diffusion works very well over short distances
membrane properties affect the diffusion of solutes
the membrane is pereable to solutes that move easily across it and impermeable to those that cant
small moelcules pass through the lipid bilayer
simple diffusion
lipid soluble moecules can diffuse across the membrane as can water
electrically charged an polar molecules cant pass through easily
the diffusion of water
osmosis
equal solute concentration( and equal water concentraion)
isotonic solution
higher solute concentraion
hypertonic
lower solute concentration
hypostonic
water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution
animal cells may burst when placed in a hypotonic solution
polar molecules can cross the membrane through channel proteins and carrier proteins
facilitated diffusion(passive)
have a central pore lined with polar amino acids
channel proteins
important channels proteins, most are gated can be closed or open to ion passage, gate opens when protein is stimulated to change its shape. stimulus can be a molecule or electrical charge resulting from many ions
ion channels
gradients can be a concentration gradient of ions, or an electrochemical gradient resulting from a charge imbalance across the membrane
membrane potential is a charge imbalance across a membrane
water may pass through the membrane by hydrating ions that pass through a channel
water also enters cells through special water channels called aquaporins
transport polar molecules such as glucose across membranes
carrier proteins
moves substances against a concentration gradient requires energy
active transport
requires direct participation of atp
primary active transport
energy comes from an ion concentration gradient that is established by primary active transport
secondary active transport
primary active transport, found in all animal cells, the pump is an tegral membrane glycoprotein. it is an antiport
sodium- potassium pump
energy can be "regained" by letting ions move across a membrane with the concentration gradient- secondary active transport, aids in uptake of amino acids and sugars, uses symports and antiports
macromolecules (proteins, polysaccharides, nucleic acids) are too large to cross the membrane, teh can be taken in or excreted by means of vesicles
processes that bring molecules and cells into a eukaryotic cell ( the plasma membrane folds in or invaginates around teh material, forming a vesicle)
endocytosis
molecules or entire cells are engulfed. some protists feed in this way. some whte blood cells engulf foreign substances. a food vacuole or a phagosome forms which fuses w a lysosome
phagocytosis
a vesicle forms to bring small dissolved substances or fluids into a cell. vesicles are much smaller than in phagocytosis. constant in endothelial cells
pinocytosis
highly specific, depends on receptor proteins integral membrane proteins - to bind to specific substances, sites are called coated pits- coated with other proteins such as clathrin
receptor mediated endocytosis
mammalian cells take in cholesterol by receptor- mediated endocytosis
lipids are packaged by the liver into lipoproteins- secrete to boodstream
material in vesicles is expelled from a cell. indigestible materials are expelled. other materials leave cells such as digestive enzymes and neurotransmitters
exocytosis
(keeping different materials separated)endoplasmic reticulum segregates newly-formed proteins
(electrically excitable membranes)teh plasma membrane of neurons conducts nerve impulses
membranes help transform energy:-inner mitochondrial membranes- energy from fuel molecules is transformed to atp-thylakoid membranes of chloroplasts transform light energy to chemical bonds
membrane proteins can organize chemical reactions
many cellular processes involve a series of enzyme-catalyzed reactions- all the moelcules must come together for these to occur. forms an assembly line of enzymes
membrane proteins process info and binding of a specific ligand can initiate, stop or change cell functions