Chapter 6 - Cell Membranes Flashcards
Fluid mosaic model
General structure of biological membranes; the phospholipid bilayer of the membrane consists of transmembrane proteins that are noncovalently embedded in the bilayer; glycosilations or carbohydrates are attached to the transmembrane proteins on the outer membrane
Peripheral membrane proteins
Proteins that contain polar or charged regions that interact with integral membrane proteins or with the polar heads of phospholipids in the bilayer; do not penetrate the bilayer
Integral membrane proteins
Proteins that are held in the membrane by the distribution of the hydrophilic and hydrophobic side chains on its amino acids
Phospholipid movements
Lateral, rotation, flexion (leg kicking), flip-flop (flipping from one mono-layer to the other mono-layer)
Cholesterol
Animal cell membranes may be up to 25% cholesterol, which is important for membrane integrity
Membrane fluidity
Depends on temperature, cholesterol content and fatty acid chain composition; a membrane with shorter-chain fatty acids, unsaturated fatty acids or less cholesterol is more fluid; fluidity decreases at reduced temperatures
Flippase, floppase, scramblase
ATP-dependent transbilayer lipid translocators; flippase transfers phospholipids towards the inward monolayer and floppase transfers phospholipids toward the outward monolayer; scramblase flip flops cholesterol
Freeze fracturing
Technique using electron microscopy that reveals proteins embedded in the phospholipid bilayer
Transmembrane proteins
Proteins that extend all the way through the phospholipid bilayer; ; the domains on the inner and outer sides of the membrane can have specific functions
Membrane carbohydrates (glycosilations)
Branched oligosaccharides covalently bonded to lipids or proteins aka glycolipid, glycoprotein; function in cell recognition, adhesion and as identifiers and always located on the outer part of the membrane
Diffusion
Passive transport; random movement of molecules toward a state of equilibrium; kinetic energy in the environment moves the molecules to equally diffuse in the environment; at equilibrium, particles continue to move, but no net change in distribution
Rate of diffusion
Depends on the diameter of the molecules, temperature and concentration gradient; smaller molecules, higher temperatures and greater solute concentration diffuse faster
Osmosis
Movement of water across the membrane; direction depends on the relative concentrations of water molecules on each side of the membrane
Hypertonic solution
Higher solute concentration relative to the other side of the membrane or relative to another solution; cells crenate (shrivel) in hypertonic solution; plant cells plasmolyze (shrink)
Isotonic solution
Equal solute concentrations; plant cells are flaccid in isotonic solution
Hypotonic solution
Lower solute concentrations; plant cells become turgid or swollen; red blood cells lyse in hypotonic solutions
Aquaporin
Channel protein that provides a hydrophilic channel to allow H2O to pass through; composed of 6 transmembrane alpha-helices which form two hemi-pores that fold up into one complete pore
NPA motif
Asparagine-proline-alanine motifs bind together to regulate movement of molecules through the aquaporin
Aquaporin function
Water molecules travel single file through AQP-1 channel; 8 oxygen atoms aligned within the channel to serve as water binding sites; selectivity is based on physical diameter of channel and the hydrophobic/hydrophilic properties of amino acids within channel
Facilitated diffusion
Diffusion of polar molecules through transmembrane proteins (channel or carrier proteins)
Channel proteins
Integral membrane proteins that form a channel across the membrane that allows certain substances to pass
Ion channels
Channel proteins with hydrophilic pores that allows a specific ion to move through its center; gated channel opens when protein is stimulated to change shape by a chemical signal (ligand) or an electrical charge difference (voltage gated channels)
Carrier Proteins
Membrane proteins that bind some substances and speed their diffusion through the bilayer; forms 2 different conformations that either opens the carrier protein to the inside or outside of the plasma membrane; once the target molecule binds to the carrier protein, the protein undergoes a conformation change, releasing the molecule to the other side of the bilayer
Active Transport
Transport of molecules that moves against the concentration gradient that requires the input of chemical energy; active transport across the cellular membrane requires a transport protein (carrier or channel protein)
Symporter
Coupled active transport protein that moves 2 substances in the same direction
Antiporter
Coupled active transport protein that moves 2 substances in opposite directions across a membrane; ie. Sodium Potassium pump
Uniporter
Active transport protein that moves a single substance in one direction across a membrane
Sodium Potassium Pump
Active transport antiport system that utilizes a carrier protein to transport Na+ and K+ against their concentration gradient; pumps Na+ and K+ in opposite directions (antiport) and increases the concentration of Na+ outside the cell and increases the concentration of K+ inside the cell; binds to three molecules of Na+ inside the cell where ATP donates a phosphate group to initiate the conformation change; the carrier protein then releases the three Na+ molecules out of the cell and binds to two K+ molecules, triggering the release of the phosphate group and conformation of the carrier protein, releasing the K+ molecules into the cell
Primary Active Transport
Transport that requires the direct hydrolysis of ATP to move substances against a concentration gradient; ie. Sodium potassium pump
Secondary Active Transport
Transport that uses energy supplied by an ion concentration gradient established by primary, ATP-driven active transport ; no direct ATP expenditure; ie. glucose transporter requires the binding of Na+ from the sodium potassium pump (primary active transport) and glucose for the carrier protein to change conformation and allow both substances through the membrane (symporter)
Electrochemical Gradient
A voltage difference across a membrane due to separation of opposite charged ions; composed of two forces acting to drive diffusion of ions across the membrane; electrogenic pumps such as proton pumps actively transport ions across the membrane
Membrane potential
For a cell’s membrane potential, the reference point is the outside of the cell; in most resting neurons, the potential difference across the membrane is about -30 to -90 mv with the inside of the cell more negative than the outside
Exocytosis
Mechanism to transport molecules out of the cell; process by which materials packaged in vesicles are secreted from a cell when a vesicle membrane fuses with the plasma membrane
Endocytosis
A group of processes that bring small molecules, macromolecules, large particles into the eukaryotic cell; there are three types: phagocytosis, pinocytosis, and receptor-mediate endocytosis
Phagocytosis
“Cellular eating”; part of the plasma membrane engulfs large particles or even entire cells; the food vacuole or phagosome usually fuses with a vacuole or lysosome where the contents are be digested
Pinocytosis
“Cellular drinking”; smaller vesicles bring fluids and dissolved substances into the cell; relatively nonspecific with what it brings into the cell
Receptor-mediated endocytosis
Molecules at the cell surface recognize and trigger the uptake of very specific materials; efficient method of taking up substances that may exist in low concentrations in the cell’s environment
Clathrin coated pits
Receptor proteins are located at outer membrane regions over called clathrin coat pits that are located on the cytoplasmic side; made of a triskelion that looks like a cage that serves to strengthen and stabilize the vesicle; the receptor binds to the specific molecule and both are engulfed into the vesicle, covered in the clathrin coated pits on the outer membrane of the vesicle; the receptors release the desired molecules and the receptors form a new vesicle, budding off to be recycled back to the plasma membrane
