Chapter 7 Flashcards
what cell ability is fundamental to life? what makes this posible
. The ability of the cell to discriminate in its chemical exchanges is fundamental to life, and it is the plasma membrane and its component molecules that
make this selectivity possible.
how are proteins distributed in the membrane and what have researchers proposed calling something related to them
The proteins are not randomly distributed in the membrane,
however. Groups of proteins are often associated in long-lasting,
specialized patches, where they carry out common functions.
Researchers have found specific lipids in these patches as well
and have proposed naming them lipid rafts, but there is ongoing controversy about whether such structures exist in living
cells or are an artifact of biochemical techniques.
Like all models, the fluid mosaic model is
continually being refined as new
research reveals more about membrane structure.
how do some memb proetins seem to move and why maybe?
Some membrane
proteins seem to move in a highly directed manner, perhaps
driven along cytoskeletal fibers in the cell by motor proteins connected to the membrane proteins’ cytoplasmic regions.
However, many other membrane proteins seem to be held
immobile by their attachment to the cytoskeleton or to the
extracellular matrix
A membrane remains fluid as temperature decreases until
the phospholipids settle into a closely packed arrangement
and the membrane solidifies, much as bacon grease forms lard
when it cools
Therefore, extreme environments pose
a challenge for
life, resulting in evolutionary adaptations
that include differences in membrane lipid composition.
integral proteins
Integral proteins penetrate the hydrophobic interior of the
lipid bilayer. The majority are transmembrane proteins, which
span the membrane; other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions
of an integral protein consist of one or more stretches of nonpolar amino acids (see Figure 5.14), typically 20–30 amino
acids in length, usually coiled into α helices (Figure 7.6). The
hydrophilic parts of the molecule are exposed to the aqueous
solutions on either side of the membrane. Some proteins also
have one or more hydrophilic channels that allow passage
through the membrane of hydrophilic substances (even of
water itself; see Figure 7.1
peripheral
Peripheral proteins are not
embedded in the lipid bilayer at all; they are loosely bound
to the surface of the membrane, often to exposed parts of
integral proteins
Furthermore, a single membrane protein may itself
carry out
multiple functions. Thus, the membrane is not only
a structural mosaic, with many proteins embedded in the
membrane, but also a functional mosaic, carrying out a range of fxns
Cell-cell recognition d and fxn
, a cell’s ability to distinguish one type of
neighboring cell from another, is crucial to the functioning
of an organism. It is important, for example, in the sorting
of cells into tissues and organs in an animal embryo
how do cels recognize other cells
). Cells recognize other cells by binding to
molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
membrane carbs- how long, what are they bonded to and called consequentyl
ular surface of the plasma membrane (see Figure 7.7d).
Membrane carbohydrates are usually short, branched chains
of fewer than 15 sugar units. Some are covalently bonded to
lipids, forming molecules called glycolipids. (Recall that glyco
refers to carbohydrate.) However, most are covalently bonded
to proteins, which are thereby glycoproteins (see Figure 7.3).
The carbohydrates on the extracellular side of the plasma
membrane vary from species to species, among individuals of
the same species, and even from one cell type to another in
a single individual. T
A transport protein is specific for
the substance it translocates (moves), allowing only a certain substance (or a small
group of related substances) to cross the membrane. For
example, a specific carrier protein in the plasma membrane
of red blood cells transports glucose across the membrane
50,000 times faster than glucose can pass through on its own.
This “glucose transporter” is so selective that it even rejects
fructose, a structural isomer of glucose.
Thus, the selective
permeability of a membrane depends on
both the discriminating barrier of the lipid bilayer and the specific transport
proteins built into the membrane.
osmosis (sugar water fake barrier example, j explan it)
Two sugar solutions of different
concentrations are separated by a membrane that the solvent (water)
can pass through but the solute (sugar) cannot. Water molecules
move randomly and may cross in either direction, but overall, water
diffuses from the solution with less concentrated solute to that
with more concentrated solute. This passive transport of water,
or osmosis, makes the sugar concentrations on both sides more
nearly equal. (The concentrations are prevented from being exactly
equal due to the effect of water pressure on the higher side, which
is not discussed here, for simplicity.)
y. Note that each substance diffuses
down its own concentration gradient, unaffected by
the concentration gradients of other substances
Much of the traffic across cell membranes occurs by diffusion. One important example is
the uptake of
oxygen by a cell performing cellular respiration. Dissolved
oxygen diffuses into the cell across the plasma membrane.
As long as cellular respiration consumes the O2 as it enters,
diffusion into the cell will continue because the concentration gradient favors movement in that direction.
osmosis across an artificial barrier
Two sugar solutions of different
concentrations are separated by a membrane that the solvent (water)
can pass through but the solute (sugar) cannot. Water molecules
move randomly and may cross in either direction, but overall, water
diffuses from the solution with less concentrated solute to that
with more concentrated solute. This passive transport of water,
or osmosis, makes the sugar concentrations on both sides more
nearly equal. (The concentrations are prevented from being exactly
equal due to the effect of water pressure on the higher side, which
is not discussed here, for simplicity.)
. However, tight clustering of water molecules
around the hydrophilic solute molecules makes some of the
water unavailable to cross the membrane. As a result
e solution with a higher solute concentration has a lower free water
concentration. Water diffuses across the membrane from the
region of higher free water concentration (lower solute concentration) to that of lower free water concentration (higher
solute concentration) until the solute concentrations on both
sides of the membrane are more nearly equal. The diffusion of
free water across a selectively permeable membrane, whether
artificial or cellular, is called osmosis
why doesnt a paramecium cell burst when it has contractile vacuole
. The Paramecium cell doesn’t burst because it is also
equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it
enters by osmosis
t, the bacteria and
archaea that live in hypersaline (excessively salty) environments
(see Figure 27.1) have cellular mechanisms that
balance the
internal and external solute concentrations to ensure that water
does not move out of the cell
Plants that are not woody, such as most houseplants, depend
for mechanical support on
cells kept turgid by a surrounding
hypotonic solution. If a plant’s cells and surroundings are isotonic, there is no net tendency for water to enter and the cells
become flaccid (limp); the plant wilts.
However, a cell wall is of no advantage if the cell is
immersed in a hypertonic environment
In this case, a plant
cell, like an animal cell, will lose water to its surroundings
and shrink
This phenomenon is called facilitated diffusion
, many polar molecules and ions impeded by the lipid bilayer of the membrane
diffuse passively with the help of transport proteins that span the
membrane. T
