Bio1151
Chapter
7
Membrane Structure and Function
The plasma membrane is a "fluid mosaic" made of
with a mosaic of
embedded
in it, and exhibits
permeability.
The fluid mosaic model. The plasma membrane is a fluid structure with a mosaic of proteins embedded in or attached to a bilayer of phospholipids In animal cells, glycoproteins such as collagen comprise the Extracellular Matrix ( ECM ).
Membranes contain amphipathic
, containing both hydrophobic and hydrophilic
regions,
and form a
, with
embedded
proteins.
Phospholipid bilayer. Phospholipid molecules form a bilayer with the hydrophilic heads exposed to the aqueous environments on both side of the membrane, and hydrophobic tails on the inside, away from water.
The fluid mosaic model for membranes. Proteins also have hydrophilic and hydrophobic regions and are embedded in the bilayer to provided various functions.
(nonpolar)
molecules
and small molecules like O
2
can pass through the membrane by diffusion;
molecules (except
water
) and large molecules like
glucose
cannot.
Lipids (also called fats) are constructed from a single glycerol molecule and usually 3 fatty acids, and may also be called "triglycerides". The fatty acid tails are nonpolar hydrocarbon chains that are hydrophobic.
Polar water molecules pass through the plasma membrane via channel proteins called aquaporins.
(passive transport) is the tendency for molecules to
spread
out evenly down their
gradient.
The diffusion of solutes across a membrane. The dye diffuses down a concentration gradient from where it is more concentrated to where it is less concentrated, leading to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.
is the
diffusion
of
across a
membrane, and is affected by the concentration gradient of dissolved substances, or
.
Osmosis. Two sugar solutions of different concentrations are separated by a semipermeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross through the pores in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute.
In
environments where solute concentration is the
as it is inside the cell, there is no net water movement.
In
environments where solute concentration is
than it is inside the cell, water tends to move
the cell.
In
environments where solute concentration is
than it is inside the cell, water tends to move
the
cell.
Organisms without cell
must have adaptations for
to maintain water
balance
in
hypotonic
and
hypertonic
environments.
Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.
The contractile vacuole of the freshwater protist Paramecium is an evolutionary adaptation for osmoregulation that offsets osmosis in a hypotonic environment by bailing water out of the cell.
1) Secretory cells actively transport salt from the blood into the secretory tubules by countercurrent exchange. The 1) secretory tubules make up the 1) salt glands, which empty via a duct into the nostrils.
Plant cells fare best in
environments.
Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.
diffusion is the movement of molecules across the membrane facilitated by
proteins,
the concentration
gradient.
proteins
provide corridors that allow a specific molecule or ion to cross the membrane.
Facilitated diffusion: channel proteins. A channel protein has a channel through which water molecules or a specific solute can pass.
proteins
undergo a change in shape that translocates the solute-binding site across the membrane.
Facilitated diffusion: carrier proteins. A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.
transport, such as
sodium-potassium
, is the movement of molecules
their concentration gradient using energy, usually in the form of
.
The sodium-potassium pump moves 3 sodium ions out of the cell for every 2 potassium ions pumped in. The active transport moves the ions against their concentration gradient and is powered by ATP.
Review.
Large proteins cross the membrane by
transport mechanisms.
In
. The cell takes in macromolecules by forming new
from the plasma membrane via invagination.
In
(cell
eating
), large particles are engulfed by
into a vacuole, and are
digested
in the
.
Phagocytosis A cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.
In
(cell
drinking
), small droplets of fluid are "gulped" into
.
Pinocytosis The cell gulps droplets of extracellular fluid, together with molecules dissolved in the droplet, into tiny vesicles. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.
In
,
transport
migrate to the plasma membrane, fuse with it, and release their contents.
Exocytosis. Many secretory cells use exocytosis to export their products such as hormones.