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Chapter 5: Membranes
Terms in this set (95)
Similar in structure to a fat, but have only two fatty acids attached to the glycerol backbone, with the third space linked to a phosphorylated molecule; contains a polar hydrophilic "head" end (phosphate group) and a nonpolar hydrophobic "tail" end (fatty acids). Form the foundation of a cell's membranes. Include primarily glycerol phospholipids and sphingolipids.
fluid mosaic model
Proposal that the globular proteins are inserted into the lipid bilayer, with their nonpolar segments in contact with the nonpolar interior of the bilayer and their polar portions protruding out from the membrane surface. A mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond.
two categories of membrane proteins
integral: embedded in the membrane
peripheral: associated with the surface of the membrane
Four components of cell membranes
1. Phospholipid bilayer
2. Transmembrane proteins
3. Interior protein network
4. cell-surface markers
Bilayers are all composed of phospholipids that provide flexible matrix and imposes barrier to permeability.
Animal cell membranes also contain cholesterol.
Provide transport and communication across the membrane. Not in a fixed position -- They can move about.
Interior protein network
Intracellular proteins that reinforce the membrane's shape
different cell types exhibit different varieties of glycoproteins and glycolipids on their surfaces that act as cell identity markers
Protein molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.
Lipid molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.
How do lipids exist in the bilayer?
Was originally though that due to fluidity, the membrane was uniform with lipids and proteins freely diffusing in membrane plane.
Now, evidence shows that membrane is not homogenous and contains microdomains with distinct lipid and protein composition. Lipids can exist in either a disordered or an ordered phase within a bilayer.
lipid microdomains heavily enriched in cholesterol and sphingolipids that interact together and form ordered structure.
"dynamic nanometer-sized, sterol and sphingolipid-enriched protein assemblies"
examining cell membranes with electron microscopy
1. tissue is embedded in a hard epoxy matrix, cut with a microtome, and placed on a grid with beam of electrons directed through grid with TEM. Can be stained.
The membranes foundation
the field defining the number and biological function of lipids
how many lipids are identified in cells?
Three classes of lipids
1. Glycerol phospholipids
3. Sterols such as cholesterol
Classical phospholipid bilayer consists of...
combo of glycerol phospholipids and sphingolipids
glycerol phospholipids are BLANK with their head groups having both negative and positive charge, or BLANK with a primarily negative charge
phospholipid fatty acid tails are either BLANK or BLANK
saturated or cis-unsaturated
sphingolipids usually contain...
saturated hydrocarbon chains
why do phospholipids spontaneously form bilayers?
Because of their amphipathic structure: polar hydrophilic head, nonpolar hydrophobic tail
what happens when phospholipid molecules are placed in water?
The polar water molecules repel the nonpolar tails of phospholipids while seeking partners for hydrogen bonding. Nonpolar phospholipid tails clump together away from water. Form spontaneously into bilayers. Nonpolar interior of bilayer impedes passage of any water-soluble substances through the bilayer.
key biological property of lipid bilayer
nonpolar interior impeding passage of ay water-soluble substances through the bilayer
why is a lipid bilayer stable?
water's affinity for hydrogen bonding never stops
what holds the lipid bilayer together?
hydrogen bonding of water molecules
the degree of membrane fluidity...
changes with the composition of membrane itself depending on their fatty acid composition. Glycerol phospholipids that are saturated or mono cis-unsaturated make the membrane less fluid as they pack well. Sphingolipids are usually unsaturated and also make the membrane less fluid.
membrane fluidity and temperature
increased temperature, increased fluidity. Decreased temperature, decreased fluidity.
where are lipids synthesized?
In the endoplasmic reticulum.
Key classes of membrane protein
3. cell-surface receptors
4. cell-surface identity markers
5. cell-to-cell adhesion proteins
6. Attachments to the cytoskeleton
certain solutes are allowed to enter or leave cells through protein channels or carriers
enzymes attached to membrane help cells carry out chemical reactions
receptor proteins help detect chemical changes
cell-surface identity markers
identify cells to other cells
cell-to-cell adhesion proteins
glue cells together by temporary interactions or permanent bonds
attachments to cytoskeleton
surface proteins that interact with other cells are often anchored to the cytoskeleton by linking proteins
how are membrane proteins attached to the surface of the membrane?
special molecules that associate with phospholipids. They are modified lipids that have 1) nonpolar regions inserted into the internal portion of the lipid bilayer and 2) chemical bonding domains that link directly to proteins
transmembrane with nonpolar amino acids (water avoids these) that are held within interior of bilayer. Polar ends protrude from both sides of membrane.
primary difference between transmembrane proteins
number of times that the protein crosses the membrane
hydrophobic region of a transmembrane protein that anchors it in the membrane. Often composed of hydrophobic amino acids arranged in a-helices, but sometimes uses B-pleated sheets to form barrel-shaped pore.
the movement of substances in and out of a cell without the cell's having to expend energy
a difference between the concentration on the inside of the membrane and that on the outside
net movement of particles from place of high concentration to place of lower concentration
major barrier to crossing a biological membrane
the hydrophobic interior that repels polar molecules but not nonpolar molecules
what allows for membrane diffusion to be selective?
process of diffusion mediated by a membrane protein
channel protein (ion channel)
A transmembrane protein with a hydrophilic interior that provides an aqueous channel allowing diffusion of species that cannot cross the membrane. Usually allows passage of specific ions such as K+, Na+, or Ca2+ across the membrane.
a membrane protein that binds to a specific molecule that cannot cross the membrane and allows passage through the membrane
when channels and carriers are selective for one type of molecule and no others
ions interaction with polar vs nonpolar
react well with polar molecules (water) but are repelled by nonpolar molecules (interior of plasma membrane)
channel proteins that can be opened or closed in response to a stimulus that's either chemical or electrical.
3 conditions that determine the direction of net movement of ions
1. their relative concentrations on either side of the membrane
2. the voltage difference across the membrane and for the gated chanels
3. the state of the gate (open or closed)
the voltage difference as an electrical potential difference across the membrane
Changes form the basis for transmission of signals in the nervous system and some other tissues. Specific channels for particular ions (Ca2+, Na+, K+, Cl-)
T or F: ions play essential role in signaling by the nervous system
Relationship btw concentration and rate of transport
differs due to simple diffusion. As concentration increases, rate of transfer increases linearly. When a carrier protein is involved, concentration increase means more carriers are bound to transported molecule. When all carriers are occupied, rate of transport will be constant IE saturated
How do RBCs keep internal concentration of glucose low?
immediately add phosphate group to any entering glucose molecule, convert it to highly charged glucose phosphate that can no longer bind to glucose transporter.
Transporter protein is not a channel membrane protein. Binds to glucose molecule and flips its shape, and reverts to original shape after releasing the glucose.
diffusion of water across a selectively permeable membrane (membrane that permits the free passage of water but prevents or retards the passage of a solute); in the absence of differences in pressure or volume, the net movement of water is from the side containing a lower concentration of solute to the side containing a higher concentration
the medium in which one or more solutes is dissolved
substance dissolved in a solvent
How do water molecules interact with dissolved solutes?
By forming hydration shells around the charged solute molecules
Water forms hydrogen bonds with BLANK molecules to create a BLANK around them in solution.
charged or polar
the property of a solution that takes into account all dissolved solutes in the solution; if two solutions with different osmotic concentrations are separated by a water-permeable membrane, water will move from the solution with lower osmotic concentration to the solution with higher osmotic concentration
a solution with a higher concentration of solutes than the cell. A cell in a hypertonic solution tends to lose water by osmosis.
a solution with a lower concentration of solutes than the cell. A cell in a hypotonic solution tends to take in water by osmosis.
a solution having the same concentration of solutes as the cell. A cell in an isotonic solution takes in and loses the same amount of water.
compared to the extracellular fluid, the cytoplasm of the cell is
specialized channels for water to cross membranes easier than just by diffusion.
More than 11 different kinds found in mammals: those that are specific for only water, and those that allow other small hydrophilic molecules (glycerol or urea) to cross the membrane as well
flow of water across cell membrane is
the force needed to stop osmotic flow. Higher the solute concentration, the higher the osmotic pressure.
what happens to a cell in a hypotonic solution (cell's cytoplasm is hypertonic relative to extracellular fluid)?
water diffuses into the cell from extracellular fluid, causing the cell to swell. Pressure of cytoplasm pushing out against cell membrane increases. Amount of water that enters depends on diff. in solute concentration between cell and extracellular fluid.
animal cells must maintain osmotic balance as they only have plasma membranes. Prokaryotes, fungi, plants, protists have strong cell walls that can withstand high internal pressures without bursting.
strategies for maintaining osmotic balance
extrusion, isosmotic regulation, turgor
single-celled eukaryotes use contractile vacuoles with small pore that opens to outside of cell to remove water
marine organisms can adjust internal concentration to match that of surrounding saltwater. No net flow of water due to being isosmotic.
plant cells have hydrostatic pressure (turgor pressure) that results from them being hypertonic to their environment and containing high concentration of solutes in their central vacuoles. Allows plants to maintain shape without wilting when they lack enough water.
passive transport processes that move materials down concentration gradients
diffusion, facilitated diffusion, osmosis
when cells move substances up their concentration gradients which requires expenditure of energy in form of ATP
actively transport single type of molecule
transport two molecules in same direction
transport two molecules in opposite directions
active transport enables cells to...
take up additional molecules of a substance that is already present in its cytoplasm in concentrations higher than in the extracellular fluid
move substances out of its cytoplasm and into the extracellular fluid, despite higher external conditions
More than one-third of all of the energy expended by an animal cell that is not actively dividing is used in active transport of...
Sodium (Na+) and Potassium (K+) ions
concentration of Na+ and K+ ions in animals relative to surroundings
low internal concentration of Na+ and high internal concentration of K+. Animals maintain these concentration differences by actively pumping Na+ out of the cell and K+ into the cell.
Transmembrane channels engaged in the active (ATP-driven) transport of Na+, exchanging them for K+, where both ions are being moved against their respective concentration gradients; maintains the resting membrane potential of neurons and other cells
In every sodium potassium pump cycle, where do Na+ ions and K+ ions go? How many?
3 Na+ leave the cell and two K+ enter. These are rapid changes and each carrier can transport up to 300 Na+ per second.
Amount of pumps in each cell varies.
The movement of a substance against its electrochemical gradient (from lower to higher concentration, or from opposite charge to like charge) using the energy provided by the simultaneous movement of a different chemical down its electrochemical gradient.
How does the active glucose transporter work?
uses the Na+ gradient produced by the Na+/K+ pump as a source of energy to power the movement of glucose into the cell. Both glucose and Na+ bind to the transport protein, which allows Na+ to pass into the cell down its concentration gradient, capturing the energy and using it to move glucose into the cell. Both molecules are moving in the same direction across the membrane, so the transporter is a symporter.
The process by which large particles and macromolecules are transported through plasma membranes. Inc. exocytosis and endocytosis
the uptake of material into cells by inclusion within an invagination of the plasma membrane; phagocytosis, pinocytosis, receptor-mediated endocytosis. All require energy expenditure.
the uptake of solid material
the uptake of dissolved material. Common in animal cells.
process by which specific macromolecules are transported into eukaryotic cells at clathrin-coated pits, after binding to specific cell-surface receptors
low-density lipoprotein. Taken up by receptormediated endocytosis. Bring cholesterol into the cell where it can be incorporated into membranes.
a type of bulk transport out of cells in which a vacuole fuses with the plasma membrane, discharging the vacuole's contents to the outside
why exocytosis is important for plants
exports materials needed to construct the cell wall through the plasma membrane
why exocytosis is important for animals
provides mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances
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