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Chapter 7: Membrane Structure and Function
Terms in this set (58)
What are the 4 classes of macromolecules?
protein, lipids, carbs, nucleic acids
What is an amphipathic molecule?
a molecule that has a hydrophillic and hydrophobic region
In the 1960s, the Davson-Danielli model of membrane structure was widely accepted. Describe this model and then cite two lines of evidence that were inconsistent with it.
It was a sandwhich model: phospholipid bilayer btwn 2 layers of proteins
1. generalization that all membranes in cell were identical
2. Protein placement: if proteins surrounded the membrane they would be in aqueous surroundings
Who proposed the fluid mosaic model of membrane structure? When? Describe this model.
Proposed by S. J. Singer and G. Nicolson in 1972
Has fluid structure with "mosaic" of different proteins embedded in membrane
What is meant by membrane fluidity? Describe the movements seen in the fluid membrane.
phospholipids can separate (made of unsaturated fats), allow materials to pass in and out of them,
-phospholipids change place 10^7 times per second, proteins drift as well
How does decreasing temperature affect membrane fluidity?
decreases membrane fluidity generally,but membrane generally will remain fluid
How do phospholipids with unsaturated hydrocarbon chains affect membrane fluidity?
increase membrane fluidity
-not packed as closely together
How does cholesterol affect membrane fluidity?
-wedged btwn phospholipids in plasma membrane of animal cells
-at high temps (37 deg C) makes it less fluid by restricting phospholipid movement
-hinders close packing of phospholipids, lowers tem. required for membrane to solidify
penetrate hydrophobic core, transmembrane proteins go all the way through, others only extend partway into the core
-hydrophillic regions are exposed to aqueous solns
-hydrophobic regions have non-polar amino acids in alpha helixes
not embedded at all, are appendages loosely bound to the surgace, exposed to parts of integral proteins
-may provide a hydrophillic channel for particular solutes
-others shuttle a substance by changing shape, some hydrolyze ATP to actively pump substances
-enzyme with active site exposed to substances in adj. soln's
-sev. can be organized as a team that carries out sequential steps of a metabolic pathaway
-receptor may have site with shape for chem. messenger (hormone), causes change in shape that causes a change in the inside of the cell, usually binds to a cytoplasmic protein
serve as identification tags sp. identified by other cells (recognize glycoproteins)
membrane protein of adj. cells hook together in gap junctions/tight junctions
attachment to cytoskeleton and ECM
-microfilaments and other elements non-covalently bonded to membrane-bound proteins, maintains cell shape, stabilize location of certain proteins, can coordinate extracelluar and intracellular proteins
What are examples of membrane carbs being important in cell-cell recognition?
-sorting of cells into tissues and organs in an animal embryo
-rejection of foreign cells
membrane carbohydrates that are covalently bonded to carbohydrates (short, branched with less than 15 monomers)
On which side
of the vesicle membrane are the carbohydrates?
the inside, because the inside of vesicles becomes the outside, and the outside becomes the insides
membrane carbohydrates that are covalently bonded to proteins
What passes easily through the cell membrance?
nonpolar, hydrophobic molecules like hydrocarbons, carbon dioxide, oxygen
Which polar molecules pass through the cell membrane?
glucose and other monossaccharide sugars, but it crosses slowly
transport proteins that have hydrophilic channels that certain molecules/ions use as a membrane
transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane
Are transport proteins specific?
yes!! glucose passes through 50,000 faster through a transport protein, but only accepts glucose, even rejects fructose (structural isomer of glucose)
channel proteins that allows water molecules through the cell membrane, brings increase in amount of water molecules that can diffuse
-peter agre received the nobel prize in 2003 for this
-especially important in RBC
How does CO2 cross the membrane?
How does glucose cross the cell membrane?
regular diffusion, or with help of carrier proteins
How does H+ cross the cell membrane?
through carrier proteins
how does O2 cross the cell membrane?
How does H2O cross the cell membrane?
regular diffusion (slowly) or with help of aquaporins
the movement of molecules of any substance so they spread out evenly into the available space
region along which the density of a chemical substance decreases
cell does not have to expend energy to make it happen, occurs down the concentration gradient
diffusion of water across a selectively permeable membrane
there is no net movement of water across a plasma membrane
a solution that has more non-penetrating solutes, water always flows toward this solution
a solution that has less non-penetrating solutes, water always flows away from this solution
very firm, results when a plant cell is placed an hypotonic solution (normal for plant cell)
limp, occurs when a plant cell is placed in an isotonic solution
phenomenon that occurs when plant cells shrivel to death, occurs when they are placed in an hypertonic solution
What is facilitated diffusion? Is it active or passive? 2 examples
molecules and ions diffuse passively with the help of transport proteins over the cell membrane
the control of cells of water balance
Why don't plant cells burst when placed in a hypotonic solution?
because they have cell walls
What is active transport? -transport proteins involved?
-role of ATP?
when the cell must expend energy to move a solute across its concentration gradient
-transport proteins are carrier proteins
-ATP causes the membrane to change shape
part of facilitated diffusion, open or close in response to a stimulus (can be electrical or chemical)
What is membrane potential? Which side of the membrane is positive?
the voltage across a membrane, acts like a battery
the outside is positive, inside is negative because of unequal distribution of cations and anions on opp sides of the membrane
What are the 2 forces that drive the diffusion of ions across the membrane? What is the combination of these forces called?
1) A chemical force - the ion's concentration gradient
2) The electrical force - the effect of the membrane potential on the ion's movement
-favors passive transport of cations into the cell and anions out of the cell
-combination is called the electrochemical gradient
the combination of the chemical and electrical forces acting on an ion
a transport protein that generates voltage across a membrane
ex: sodium potassium pump = most common in animal cells
-store energy that can be tapped for cellular work (ATP sythesis)
main electrogenic pump of plants, fungi, bacteria, pumps H+ ions out into the extracellular solution
an ATP powered pump that transports a sp solute can indirectly drive the active transport of several other solutes
ex: sucrose H+ pump needs H+ ions to actively transport sucrose, so a proton pump pumps H+ ions outside the cell membrane
What is the role of cotransport in diarrhea? How does it fix dehydration?
Patients given solution containing a high conc of glucose and salt, solutes taken up by proteins on surface of intestinal cells, passed into blood
Increase in blood's solute conc. means water is drawn out from intestinal cells in bloostream, rehydrating patient
cell takes up biological molecules by forming new vesicles from the plasma membrane, pocket is formed in the membrane and it pinches inward
a type of endocytosis, "cellular eating", wrapped by a psudopodia in a food vacuole
a type of endocytosis, "cellular drinking", droplets of extracellular fluid taken up, non-specific, results in a vesicle
proteins with sp. receptor sites (clustered), bind to their ligands, vesicle is formed.
-recycled by same vesicle
a specific substance that binds to protein receptors in receptor mediated endocytosis
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