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43 terms

Chapter 7: Membrane

Selective Permeability
A property of biological membranes that allows some substances to cross more easily than others.
A molecule that has both a hydrophilic region and a hydrophobic region.
fluid mosaic model
the membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer (bilayer) of phospholipids
integral proteins
Typically transmembrane proteins with hydrophobic regions that completely span the hydrophobic interior of the membrane.
peripheral proteins
are not embedded in the lipid bilayer at all. Instead, the are loosely bound to the surface of the protein, often connected to integral proteins
Functions of membrane proteins
Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoplasm and extracellular matrix
crucial in cell-cell recognition (which is necessary for proper immune function) and in developing organisms (for tissue differentiation)
provide a hydrophobic barrier that separates the cell from its liquid environment. (hydrophilic molecules cannot easily enter the cell easily, but hydrophobic can enter much more easily)
What are membranes made of?
phospholipids and proteins held together by weak interactions that cause the membrane to be luid
transport proteins
what hydrophilic substances pass through, span the membrane
(water channel proteins that facilitate the amount of diffusion)A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).
passive diffusion
a substance travels through from where it is more concentrated to where it is less concentrated
concentration gradient
the path molecules travel when an imbalance between separated molecule concentrations exists
the diffusion of water across a selectively permeable membrane (water moves from an area of high to lower free water concentration)
the ability of a solution to cause a cell to gain or lose water (depends in part on its concentration of solutes that cannot cross the membrane, non-penetrating solutes)
no net movement of water across the membrane, water flows across the membrane but at the same rate. (normal for animal cells but flaccid for plant cells)
higher, cell will lose water to its environment, shrivel, and probably die (lysed in animal cells, normal (turgid) in plant cells
lower, water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled ballon
the control of water balance
plant cells, the relatively inelastic wall will expand only so much before it exerts a back pressure on the cell that opposes further water uptake
plant cells, no net tendency for water to enter, surroundings are isotonic
causes the plant to wilt and can lead to plant death, plant cell shrivels (plasma membrane pulls away from the wall)
facilitated diffusion
when many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrance
two types of transport proteins
channel and carrier proteins
channel proteins
provide corridors that allow a specific molecule or ion to cross the membrane, (aquaporins, facilitate massive amounts of diffusion that occur in plant cells and in animal calls such as red blood cells. ion channels (gated channels), open or close in response to a stimulus)
carrier proteins
(ex. glucose transporter) undergo a subtle change in shape that somehow translocates the solute- binding site across the membrance
Two ways transport proteins work
1. provide a hydrophilic channel through which the molecules in question can pass
2. bind loosely to the molecules in question and cary them through the membrane
active transport
substances are moved against their concentration gradient (less concentrated to more concentrated), requires energy in the form of ATP
Example of active transport
sodium- potassium pump, pumps sodium out of the cell and potassium into the cell, necessary for proper nerve transmission and is a major energy consumer in your body
membrane potential
difference in electric charge across the membrane is expressed in voltage, acts like a battery, an energy source that affects the traffic of all charged substances across the membrane
Two forces driving diffusion of ions across membrane
1. chemical force, ions concentration gradient
2. voltage gradient, attracts a positively charged ions and repels negatively charged ions
electrochemical gradient
combination of forces acting on an ion
electrogenic pump
a transport protein that generates voltage across the membrane (ex. sodium- potassium pump and proton pump)
proton pump
main electrogenic pump for plants, fungi, and bacteria
an ATP pump that transports a specific solute indirectly drives the active transport of other substances (substance that was initially pumped across the membrane can do work as it moves back across the membrane by diffusion and brings with it a second compound)
vesicles from the cell's interior fuse with the cell membrane, expelling their contents, many secretory cells use exocytosis to export products
the cell forms new vesicles from the plasma membrane; reverse of exocytosis, and this process allows the cell to take in macromolecules
3 types of endocytosis
Phagocytosis, pinocytosis, and receptor- mediated endocytosis
"cellular eating" occurs when the cell wraps pseudopodia around a solid particle and brings it into the cell
"cellular drinking" the cell takes in small droplets of extracellular fluid within small vesicles
Receptor- mediated endocytosis
very specific process, substances (ligands) bind to specific receptos on the cell's surface (receptors usually coated pits), and this is the causes a vesicle to form sround the substance and then to pinch off into cytoplasm
any molecule that binds specifically to a receptor site of another molecule
stabilizes and keeps membrane from moving a lot, in between phospholipids