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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:
- 1. transporters
- 2. enzymes
- 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.
- -is specific
- -is passive
- -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