Download Phospholipid Bi-Layer - Pre AP Biology: 1(A)

Lipid Bilayer
History of Lipid Bi-Layer
• Researchers noted lipid soluble molecules entered cells
more rapidly than water soluble molecules, suggesting
lipids are component of plasma membrane.
• Later analysis showed it was phospholipids formed into a
bilayer, with proteins embedded in outer membrane.
(RBC soaked in acetone)
• Plasma membrane is phospholipid bilayer in which
protein molecules are partially or wholly embedded.
Embedded proteins are scattered throughout membrane
in irregular patterns, varies among membranes.
• Proteins have the ability to move throughout exterior of
membrane, along with phospholipids. They do not flipflop. This supports the fluid mosaic model.
Plasma Membrane
• Membrane structure has two components, lipids
and proteins.
• Lipids are arranged into a bilayer.
• Most plasma membranes are phospholipids,
which spontaneously arrange themselves into a
bilayer.
• Nonpolar tails (Fatty acid tails) are hydrophobic
and are directed inward; polar heads (phosphate
head) are hydrophilic and are directed outward
to face extracellular and intracellular fluids.
Membrane Fluidity
• At body temperature, membrane has consistency of olive
oil.
• In each monolayer, the hydrocarbon tails wiggle, and
entire phospholipid molecule can move sideways at a
rate of about 2 micrometers/second.
• Phospholipid molecules rarely flip-flop from one layer to
the other.
• Fluidity of the phospholipid bilayer allows cells to be
pliable.
• Some proteins are held in place by cytoskeletal
filaments; most drift in fluid bilayer.
Membrane Structures
• Glycolipids: have a structure similar to phospholipids
except the hydrophilic head is a variety of sugar, they are
protective and assist in various functions.
• Cholesterol: lipid found in animal plasma membranes;
reduces the permeability of membrane.
• Glycoproteins: have an attached carbohydrate chain of
sugar that projects externally.
• The plasma membrane is asymmetrical; glycolipids and
proteins occur only on outside and cytoskeletal filaments
attach to proteins only on the inside surface.
Membrane Proteins
• Plasma membrane and organelle membranes have
unique proteins; RBC plasma membrane contains 50+
types of proteins.
• Membrane proteins determine most of the membranes
functions.
• Channel Proteins: allow a particular molecule to cross
membrane freely.
• Carrier Proteins: selectively interact with a specific
molecule so it can cross the plasma membrane
• Receptor Proteins: shaped so a specific molecule can
bind to it. (Hormones)
• Enzymatic Proteins: catalyze specific metabolic
reactions.
Phospholipid Bilayer
Cell to Cell Recognition
• Carbohydrate chains of glycolipids and glycoproteins
identify cell; diversity of the chain is enormous.
• Chains vary by number of sugars(15 to several hundred)
• Chains vary in branching.
• Sequence of sugars in chains vary.
• Glycolipids and glycoproteins vary from species to
species, from individual to individual of same species,
and even from cell to cell of same ind.
• Immune system rejection of transplanted tissues is due
to recognition of unique glycolipids and glycoproteins.
Blood types are due to unique glycoproteins on the
membranes of RBC.
Membrane Permeability
• The plasma membrane is differentially permeable, only certain
molecules can pass through freely.
• A permeable membrane allows all molecules to pass through; an
impermeable membrane allows no molecules to pass through; a
semipermeable membrane allows some molecules to pass through.
• Small non-charged lipid molecules (alcohol, oxygen) pass through
the membrane freely.
• Small polar molecules (carbon dioxide, water) easily pass following
their concentration gradient.
• Macromolecules cannot freely cross a plasma membrane.
• Ions and charged molecules have difficulty crossing the hydorphobic
phase of the bilayer.
Passive and Active Transport
• Both passive and active mechanisms move
molecules across membrane.
• Passive transport moves molecules across
membrane without expenditure of energy by cell;
includes diffusion and facilitated transport.
• Active Transport uses energy (ATP) to move
molecules across a plasma membrane; includes
active transport, exocytosis, endocytosis, and
pinocytosis.
Diffusion and Osmosis
• In diffusion, molecules move from higher to
lower concentration (ex. down their gradient)
• Lipid soluble molecules (Alcohol) diffuse.
• Gases readily diffuse through lipid bilayer.
• Osmosis is the diffusion of water across a
differentially permeable membrane.
• Tonicity is strength of a solution in relationship to
osmosis; determines movement of water into or
out of cells.
Phospholipid Bi-Layer
Receptor Mediated Endocytosis
Endocytosis
Gap Junction
Gap Junctions
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Gap junctions are intercellular channels some 1.5–2 nm in diameter. These permit the
free passage between the cells of ions and small molecules (up to a molecular weight
of about 1000 daltons).
They are constructed from 4 (sometimes 6) copies of one of a family of a
transmembrane proteins called connexins.
Because ions can flow through them, gap junctions permit changes in membrane
potential to pass from cell to cell.
Examples:
The action potential in heart (cardiac) muscle flows from cell to cell through the heart
providing the rhythmic contraction of the heartbeat.
At some synapses in the brain, gap junctions permit the arrival of an action potential
at the synaptic terminals to be transmitted across to the postsynaptic cell without the
delay needed for release of a neurotransmitter.
As the time of birth approaches, gap junctions between the smooth muscle cells of
the uterus enable coordinated, powerful contractions to begin.
Several inherited disorders of humans such as
certain congenital heart defects and
certain cases of congenital deafness
have been found to be caused by mutant genes encoding connexins.
Desmosomes
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Desmosomes
Desmosomes are localized patches that hold two cells tightly together. They
are common in epithelia (e.g., the skin). Desmosomes are attached to
intermediate filaments of keratin in the cytoplasm.
Pemphigus is an autoimmune disease in which the patient has developed
antibodies against proteins (cadherins) in desmosomes. The loosening of
the adhesion between adjacent epithelial cells causes blistering.
Carcinomas are cancers of epithelia. However, the cells of carcinomas no
longer have desmosomes. This may account for their ability to metastasize.
Hemidesmosomes
These are similar to desmosomes but attach epithelial cells to the basal
lamina ("basement membrane" – View) instead of to each other.
Pemphigoid is an autoimmune disease in which the patient develops
antibodies against proteins (integrins) in hemidesmosomes. This, too,
causes severe blistering of epithelia.
Plasmodesmata
• Plasmodesmata
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• Although each plant cell is encased in a boxlike cell wall, it turns out
that communication between cells is just as easy, if not easier, than
between animal cells. Fine strands of cytoplasm, called
plasmodesmata, extend through pores in the cell wall connecting
the cytoplasm of each cell with that of its neighbors.
• Plasmodesmata provide an easy route for the movement of ions,
small molecules like sugars and amino acids, and even
macromolecules like RNA and proteins, between cells. The larger
molecules pass through with the aid of actin filaments.
• Plasmodesmata are sheathed by a plasma membrane that is simply
an extension of the plasma membrane of the adjoining cells. This
raises the intriguing question of whether a plant tissue is really made
up of separate cells or is, instead, a syncytium: a single,
multinucleated cell distributed throughout hundreds of tiny
compartments!
Plasmodesmata
Tight Junction
Adhesion Junction