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The fluid mosaic model of membrane structure contends that membranes consist of
-phospholipids arranged in a bilayer
-globular proteins inserted in the lipid bilayer
Cellular membranes have 4 components:
1. phospholipid bilayer
2. transmembrane proteins
3. interior protein network
4. cell surface markers
Membrane structure is visible
using an electron microscope.
Transmission electron microscopes (TEM)
can show the 2 layers of a membrane.
separate the layers and reveal membrane proteins.
Phospholipid structure consists of
– a 3-carbon polyalcohol acting as a backbone for the phospholipid
-2 fatty acids attached to the glycerol
-phosphate group attached to the glycerol
are nonpolar chains of carbon and hydrogen.
-Their nonpolar nature makes them
The phosphate group is
polar and hydrophilic ("water-loving").
The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer
-fatty acids are on the inside
-phosphate groups are on both surfaces of the bilayer
Phospholipid bilayers are fluid.
-hydrogen bonding of water holds the 2 layers together
-individual phospholipids and unanchored proteins can move through the membrane
-saturated fatty acids make the membrane less fluid than unsaturated fatty acids
-warm temperatures make the membrane more fluid than cold temperatures
Membrane proteins have various functions:
3. cell surface receptors
4. cell surface identity markers
5. cell-to-cell adhesion proteins
6. attachments to the cytoskeleton
Peripheral membrane proteins
-anchored to a phospholipid in one layer of the membrane
-possess nonpolar regions that are inserted in the lipid bilayer
-are free to move throughout one layer of the bilayer
Integral membrane proteins
-span the lipid bilayer (transmembrane proteins)
-nonpolar regions of the protein are embedded in the interior of the bilayer
-polar regions of the protein protrude from both sides of the bilayer
Integral proteins possess at least one transmembrane domain
-region of the protein containing hydrophobic amino acids
-spans the lipid bilayer
Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane.
-b sheets in the protein secondary structure form a cylinder called a b-barrel
-b-barrel interior is polar and allows water and small polar molecules to pass through the membrane
Passive transport is movement of molecules through the membrane in which
-no energy is required
-molecules move in response to a concentration gradient
is movement of molecules from high concentration to low concentration
integral membrane proteins allow the cell to be selective about what passes through the membrane.
have a polar interior allowing polar molecules to pass through.
bind to a specific molecule to facilitate its passage.
Channel proteins include:
-ion channels allow the passage of ions (charged atoms or molecules) which are associated with water
-gated channels are opened or closed in response to a stimulus
-the stimulus may be chemical or electrical
bind to the molecule that they transport across the membrane.
is movement of a molecule from high to low concentration with the help of a carrier protein.
-saturates when all carriers are occupied
In an aqueous solution
-water is the solvent
-dissolved substances are the solutes
is the movement of water from an area of high to low concentration of water
-movement of water toward an area of high solute concentration
When 2 solutions have different osmotic concentrations
-the hypertonic solution has a higher solute concentration
-the hypotonic solution has a lower solute concentration
Osmosis moves water through aquaporins toward
the hypertonic solution.
Organisms can maintain osmotic balance in different ways.
1. Some cells use extrusion in which water is ejected through contractile vacuoles.
2. Isosmotic regulation involves keeping cells isotonic with their environment.
3. Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid.
-requires energy – ATP is used directly or indirectly to fuel active transport
-moves substances from low to high concentration
-requires the use of carrier proteins
Carrier proteins used in active transport include:
-uniporters – move one molecule at a time
-symporters – move two molecules in the same direction
-antiporters – move two molecules in opposite directions
Sodium-potassium (Na+-K+) pump
-an active transport mechanism
-uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell
-ATP energy is used to change the conformation of the carrier protein
-the affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane
-uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule
-a symporter is used
-glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient
Bulk transport of substances is accomplished by
1. endocytosis – movement of substances into the cell
2. exocytosis – movement of materials out of the cell
occurs when the plasma membrane envelops food particles and liquids.
1. phagocytosis – the cell takes in particulate matter
2. pinocytosis – the cell takes in only fluid
3. receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor
occurs when material is discharged from the cell.
-vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell
-used in plants to export cell wall material
-used in animals to secrete hormones, neurotransmitters, digestive enzymes