Download Cell Membrane PPT - Gulfport School District

A membrane’s structure and functions are
determined by its constituents: lipids, proteins,
and carbohydrates.
The general design of membranes is known as
the fluid mosaic model.
Phospholipids form a continuous bilayer which
is like a “lake” in which a variety of proteins
“float.”
The lipid molecules are usually phospholipids
with two regions:
• Hydrophilic regions—electrically charged
“heads” associate with water molecules
• Hydrophobic regions—nonpolar fatty acid
“tails” that do not dissolve in water
A bilayer is formed when the fatty acid “tails”
associate with each other and the polar
“heads” face the aqueous environment.
Membranes may differ in lipid composition; there
are many types of phospholipids.
Phospholipids may differ in:
• Fatty acid chain length
• Degree of saturation
• Kinds of polar (phosphate-containing)
groups present
Cholesterol is an important component of animal
cell membranes. (up to 25% of lipid content)
Hydroxyl groups interact with the polar heads of
phospholipids.
Cholesterol is important in modulating
membrane fluidity; other steroids function as
hormones.
The fatty acids make the membrane somewhat
fluid. This allows some molecules to move
laterally within the membrane.
Membrane fluidity is influenced by:
• Lipid composition—short, unsaturated
chains increase fluidity
• Temperature—fluidity decreases in colder
conditions
All biological membranes contain proteins; the
ratio of proteins to phospholipids varies.
Not all amino acid R groups are the same.
Peripheral membrane proteins lack
hydrophobic groups and are not embedded in
the bilayer.
Integral membrane proteins are at least partly
embedded in the phospholipid bilayer.
Anchored membrane proteins have hydrophobic
lipid components that anchor them in the
bilayer.
Proteins are asymmetrically distributed on the
inner and outer membrane surfaces.
Transmembrane proteins extend through the
bilayer; they may have domains with different
functions on the inner and outer sides of the
membrane.
Some membrane proteins can move within the
phosopholipid bilayer; others are restricted.
• Cell fusion experiments illustrate this
migration.
Proteins inside the cell can restrict movement of
membrane proteins, as can attachments to the
cytoskeleton.
Diverse carbohydrates are located on the outer
cell membrane and play a role in
communication.
• Glycolipid—carbohydrate covalently
bonded to a lipid
• Glycoprotein—one or more
oligosaccharides covalently bonded to a
protein
• Proteoglycan—protein with more and
longer carbohydrates bonded to it
Cells can adhere to one another through
interactions between cell surface
carbohydrates and proteins.
Membranes are constantly forming, transforming
into other types, fusing, and breaking down.
Though membranes appear similar, there are
major chemical differences among the
membranes of even a single cell.
Selective permeability: biological membranes
allow some substances, but not others, to
pass
Two processes of transport across
membranes:
1. Passive transport does not require
metabolic energy.
•
A substance moves down its
concentration gradient.
2. Active transport does require input of
metabolic energy.
•
A substance moves against its
concentration gradient.
Passive transport can occur by:
• Simple diffusion through the phospholipid
bilayer
• Facilitated diffusion through channel
proteins or aided by carrier proteins
Diffusion is the process of random movement
toward equilibrium; a net movement from
regions of greater concentration to regions of
lesser concentration.
Speed of diffusion depends on three factors:
• Diameter of the molecules—smaller
molecules diffuse faster.
• Temperature of the solution—higher
temperatures lead to faster diffusion.
• Concentration gradient—the greater the
concentration gradient, the faster a
substance will diffuse.
Cell cytoplasm is an aqueous solution, as is the
surrounding environment.
Diffusion of each solute depends only on its
own concentration.
A higher concentration inside the cell causes
the solute to diffuse out; higher concentration
outside causes the solute to diffuse in.
Some molecules cross the phospholipid bilayer
by simple diffusion:
• O2, CO2, and small, nonpolar, lipid-soluble
molecules.
Polar (hydrophilic) molecules do not pass
through—they are not soluble in the
hydrophobic interior of the membrane.
• Amino acids, sugars, ions, water
Osmosis is the diffusion of water across
membranes through special channels.
It depends on the concentration of water
molecules on either side of the membrane—
water moves down its concentration gradient.
The higher the total solute concentration, the
lower the concentration of water molecules.
Osmotic pressure: pressure that must be
applied to a solution to prevent flow of water
across a membrane by osmosis
Π = cRT
c = total solute concentration
R = the gas constant
T = absolute temperature
The higher concentration of a substance on
one side of a membrane represents stored
energy.
If a membrane allows water, but not solutes, to
pass through, the net movement of water
molecules will be toward the solution with the
higher solute concentration and the lower
concentration of water molecules.
When comparing two solutions separated by a
membrane:
• A hypertonic solution has a higher solute
concentration.
• Isotonic solutions have equal solute
concentrations.
• A hypotonic solution has a lower solute
concentration.
Concentration of solutes in the environment
determines the direction of osmosis in all
animal cells.
In other organisms, cell walls limit the volume
of water that can be taken up.
Turgor pressure is the internal pressure
against the cell wall—as it builds up, it
prevents more water from entering.
Facilitated diffusion:
Channel proteins are integral membrane
proteins that form channels across the
membrane through which some substances
can pass.
Substances can also bind to carrier proteins
to speed up diffusion.
Both processes operate in either direction.
Ion channels: channel proteins that allow
specific ions to pass through
Most are gated channels—they open when a
stimulus causes the protein to change shape.
• Ligand-gated—the stimulus is a ligand, a
chemical signal.
• Voltage-gated—the stimulus is a change in
electrical charge difference across the
membrane.
Water crosses membranes at a faster rate than
simple diffusion.
It may “hitchhike” with ions such as Na+ as
they pass through ion channels.
Aquaporins are channels that allow large
amounts of water to move along its
concentration gradient.
Carrier proteins in the membrane facilitate
diffusion by binding substances.
Glucose transporters are carrier proteins in
mammalian cells.
Glucose molecules bind to the carrier protein
and cause the protein to change shape—it
releases glucose on the other side of the
membrane.
Cells maintain an internal environment with a
different composition than the outside
environment.
This requires work—energy from ATP is
needed to move substances against their
concentration gradients (active transport).
Specific carrier proteins move substances in
only one direction, either into or out of the
cell.
The sodium–potassium (Na+–K+) pump is an
integral membrane protein that pumps Na+ out
of a cell and K+ in.
One molecule of ATP moves two K+ and three
Na+ ions.
Macromolecules are too large or too charged to
pass through biological membranes, so
instead they cross within vesicles.
To take up or to secrete macromolecules, cells
must use endocytosis and exocytosis.
Exocytosis moves materials out of the cell in
vesicles.
The vesicle membrane fuses with the cell
membrane and the contents are released into
the environment.
Exocytosis is important in the secretion of
substances made by cells such as digestive
enzymes and neurotransmitters.
Endocytosis brings macromolecules and
particles into eukaryotic cells.
The cell membrane invaginates, or folds
around the particle and forms a vesicle.
The vesicle then separates from the
membrane.
Endocytosis depends on receptors—proteins
that bind to specific molecules (ligands).
The receptors are integral membrane proteins
on the cell membrane.
The resulting vesicle includes both the receptor
and its ligand, plus other substances present
near the site of invagination.
Phagocytosis (“cellular eating”): a specialized
cell engulfs a large particle or another cell
• A food vesicle (phagosome) forms and
usually fuses with a lysosome, where the
contents are digested.
Pinocytosis (“cellular drinking”): vesicles are
smaller and bring in fluids and dissolved
substances
Receptor endocytosis brings specific large
molecules into a cell via specific receptors.
This allows cells to control internal processes
by controlling location and abundance of each
type of receptor on the cell membrane.
It also plays a role in cell signaling.