Cell Membrane and Cell Transport. Molecular Structure of the Cell Membrane.
Membrane Structure and Function Chs. 8 and 11. Cell Membrane – Introduction Separates the living...
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Transcript of Membrane Structure and Function Chs. 8 and 11. Cell Membrane – Introduction Separates the living...
Membrane Structure and Function
Chs. 8 and 11
Cell Membrane – Introduction Separates the living cell from its
nonliving surroundings 8 nm thick Controls traffic into and out of the cell
(selectively permeable) Composed of lipids (phospholipids) and
proteins, but include some carbohydrates
Cell Membrane – Introduction
Phospholipids and most other membrane constituents are amphipathic moleculesAmphipathic molecules have both
hydrophobic and hydrophilic regions Described by the fluid mosaic model
Membrane Model Development 1895 – Charles Overton; hypothesized
membranes made of lipidsObserved lipid-soluble substances
move across membrane easier than lipid-nonsoluble substances
1917 – Irving Langmuir; Dissolved phospholipids in benzene
and mixed with waterWhen benzene evaporated,
phospholipid film formed on water
1925 – E. Gorter and F. Grendel concluded membrane must be bilayer of phospholipidsPolar phosphorous head interacts
with polar water (hydrophilic)Nonpolar fatty
acid tails are sheltered from the water (hydrophobic)
Experiments showed real membranes attract water stronger than artificial ones
Hypothesis: proteins aid in water attraction
1935 – H. Davson and J. Danielli propose sandwich model: bilayer
between layers of proteins
Davson-Danielli model considered dominant, even after EM images
Two problems:Membranes differed in size,
composition, and stained appearanceMembrane proteins are amphipathic;
can’t be on surface only
Fluid Mosaic Model
1972 – S.J. Singer and G. Nicolson present revised model; hypothesize proteins are distributed throughout and among the bilayer
Membranes are fluid Membrane molecules are not held
together by bonds; they can slip/move past/around each other
Evidence: when human and mouse cells are fused together, membrane proteins don’t stay separated.
Most membrane molecules can move laterally; rarely do they flip-flop
Some proteins can’t move; bound to the cytoskeleton
Fluidity influenced by two factors:Temp: As temp decreases, lipids
pack closer together – become more solid
Saturation: unsaturated fatty acid tails make the membrane
more fluid
Cholesterol is wedged in the plasma membraneWarm temps: it restrains the movement
of phospholipids and reduces fluidityCool temps: it maintains fluidity by
preventing tight packing
Membranes are mosaics
Membranes are mosaics Membranes each have a unique
collections of proteins Membrane functions determined mostly
by proteins Two types of membrane proteins:
Peripheral proteins: not embedded in lipid bilayer
Integral proteins: penetrate the hydrophobic core of lipid bilayer, often completely spanning the membrane (transmembrane protein)
Membranes are mosaics Membranes have
distinctive inside and outside facesThe outer surface
has carbohydratesThis asymmetrical
orientation begins during synthesis of new membrane in the endoplasmic reticulum
Membranes are mosaics Membrane protein functions:
Cell-Cell Recognition Ability of a cell to distinguish one type of
neighboring cell from another The membrane plays the key role in
cell-cell recognition Cells recognize other cells from surface
molecules, often carbs, on membraneGlycolipidsGlycoproteins (more common)
Cell-Cell Recognition Carbs on external side of membrane
vary from species to species, individual to individual, and even from cell type to cell type within the same individual Variation marks each cell type as distinct The four human blood groups (A, B, AB,
and O) differ in the external carbohydrates on red blood cells
It is also the basis for rejection of foreign cells by the immune system
This attribute is important in cell sorting and organization as tissues and organs in development
Transport Membranes act as gatekeepers
(selectively permeable) Select based on size and charge
Small, uncharged atoms/molecules don’t have problems
Large and/or charged atoms/molecules do have problems
Proteins can help transport
Transport Proteins Each transport protein is specific as to
the substances that it will translocate Some act like a channel or tunnel
through the membrane Others bind to their specific molecules
and physically carry them across the membrane
Passive Transport No E required Requires gradient (separation of
concentrations) Movement from areas of Hi to Low
(down, along, or with) concentrationsMovement continues even after
equilibrium is reached Rate of diffusion depends on size and
charge of molecules (interaction with the membrane)
Passive Transport Simple Diffusion: movement of
molecules from Hi to Low concentrations
Passive Transport Each substance diffuses down its own
concentration gradient, independent of the concentration gradients of other substances
Passive Transport Osmosis: diffusion of water across a
semi-permeable membrane Osmosis continues until the solutions
are isotonic When two solutions are isotonic, water
molecules move at equal rates from one to the other, with no net osmosis
Passive Transport A solution with a higher concentration of
solutes is hypertonic A solution with a lower concentration of
solutes is hypotonic These are comparative terms
Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water
Solutions with equal solute concentrations of solute are isotonic
Passive Transport
Passive Transport Paramecia have contractile vacuoles to
expel excess water
Passive Transport Facilitated diffusion: diffusion using
“helper” moleculesThose atoms and molecules that
were too big or charged can still move down their concentration gradient (hi to low)
Passive Transport: Facilitated diffusion
Some proteins (channel) act like corridors
Allow for fast, bulk flowEx: aquiporins
Passive Transport: Facilitated diffusion
Some channel proteins (gated channels) open or close depending on the presence or absence of a physical or chemical stimulus The chemical stimulus is usually different
from the transported molecule Ex: when neurotransmitters bind to specific
gated channels on the receiving neuron, these channels open
This allows sodium ions into a nerve cell When the neurotransmitters are not present,
the channels are closed
Passive Transport: Facilitated diffusion
Some proteins change shape to physically translocate the molecules
These shape changes could be triggered by the binding and release of the transported molecule
Transport proteins are much like enzymes They may have specific binding sites
for the soluteTransport proteins can become
saturated when they are translocating passengers as fast as they can
Transport proteins can be inhibited by molecules that resemble the normal “substrate”
When these bind to the transport proteins, they outcompete the normal substrate for transport
Active Transport
Requires E (ATP) Movement of molecules against or up
their concentration gradientsLow to Hi
Performed by receptor proteins
Active Transport
The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membraneTypically, an animal cell has higher
concentrations of K+ and lower concentrations of Na+ inside the cell
The sodium-potassium pump uses the E of one ATP to pump three Na+ out and two K+ in
Ions keep separate charges across a membrane
Membrane potential: voltage difference across the membrane
Electrochemical gradientGradient due to concentrations of
ionsGradient due to membrane potential
electrogenic pumps generate voltage gradient
Ions keep separate charges across a membrane
In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell
Proton pumps in the cristae of mitochondria and the thylaloids of chloroplasts, concentrate H+ behind membranes
These electrogenic pumps store energy that can be accessed for cellular work.
Cotransport A single ATP-powered pump that
transports one solute can indirectly drive the active transport of several other solutes through cotransport via a different protein
As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient
Plants commonly use the gradient of H+ that is generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cellThe high concentration of H+ on one
side of the membrane, created by the proton pump, leads to the facilitated
diffusion of protons back, but only if another molecule, like sucrose, travels with the H+
Endo- vs. Exocytosis
Both move large molecules into/out of the cell
Both use vesicles Reverse processes of each other
Endocytosis
A small area of the plasma membrane sinks inward to form a pocket
The pocket deepens, pinches in, and forms a vesicle containing the material that had been outside the cell
Endocytosis
Two types: Phagocytosis:
cell eating Pinocytosis:
cell drinking
Receptor mediated endocytosis
Receptor mediated Endocytosis Triggered when extracellular
substances bind to special receptors, ligands, on membrane surface, especially near coated pits