Chapter 7 - Membrane Structure and Function Flashcards
what are state ingredients of membranes?
lipids and proteins - carbohydrates also important
lipids are the most abundant in the plasma membrane
what does amphipathic mean? what demonstrates this characteristic?
phospholipids form membranes - they are an amphipathic molecules meaning it has both a hydrophilic region and hydrophobic region
membrane proteins are also amphipathic - embed themselves in phospholipid bilayer
which part of the phospholipid faces water? inside?
hydrophobic tail faces inwards while hydrophilic heads face water
what is the fluid mosaic model?
the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
where are lipids made?
where are membrane phospholipids made and the the assembly of the membrane take place?
smooth ER
membrane phospholipids and assembly of membrane + protein occurs in rough ER and golgi apparatus
phospholipid movement in the plasma membrane?
Most drift laterally (weak hydrophobic interaction)
Rarely does a lipid flip b/c it requires energy like ATP
proteins don’t usually move b/c larger but some can
which experiment showed that the plasma membrane isn’t static?
took two cells and labeled them with different colors and after an hour the proteins become equally distributed – this experiment shows that membrane isn’t static
phospholipid phase in relation to temperature
As temperature cools, membranes go from fluid to solid state b/c phospholipids pack together
what allows a membrane to stay fluid at lower temperatures?
membrane remains fluid to a lower temperature if it’s rich in phospholipids with unsaturated hydrocarbon tails - kinks in the tail b/c of double bond means they can’t pack closely together
different environments have different temperatures so you need a balance of sat/unsat to maintain fluidity
effects of cholesterol on the membrane
it’s a fluidity buffer
at high temperatures (body temp): cholesterol makes the membrane less fluid by restraining the phospholipid movement - gives structure to cell membrane
but at lower temperatures cholesterol also hinders the close packing of phospholipids and lowers the temperature required for the membrane to solidify
what determines most of the function of the plasma membrane?
the proteins
what are the two types membrane proteins?
1) integral proteins
2) peripheral proteins
what is an integral protein?
Integral protein is able to penetrate the hydrophobic interior of the lipid bilayer
the majority of integral proteins of transmembrane proteins - some only extend partway into the hydrophobic interior
what are trans-membrane proteins?
a type of integral proteins that span the membrane: a portion is inside and the other part is sticking outside
the hydrophobic regions of an integral protein consist of one or more stretches of non polar amino acids usually coiled into alpha helices
the hydrophilic regions are exposed to the aqueous solutions on either side of the membrane (extracellular and cytoplasmic side) - some proteins also have hydrophilic channels that allow passage through the membrane of hydrophilic substances
what are peripheral proteins?
peripheral proteins are not embedded in the lipid bilayer at all - they are appendages loosely bound to the surface of the membrane (often to exposed parts of integral proteins) or phospholipid head
how do membrane proteins attach?
on cytoplasmic side, some membrane proteins are held in place by attachment to the cytoskeleton
on extracellular side, they are attached to fibers of the ECM
what are the 6 major functions of membrane proteins?
1) Transport: if they’re too large or hydrophilic, proteins
help these molecules get across w/ channels
2) Enzyme activity: enzymes are embedded in the membrane which is useful because spatially you can keep chemical reactions close together
3) Signal transduction: may have a binding site with a specific shape that fits the shape of the chemical messenger (receptor) or the external messenger(signaling molecule) may cause protein shape to change to allow message to be relayed to the inside of the call
4) Cell-cell recognition: glycoproteins serve as ID tags
5) Intercellular joining: tight or gap junctions can form
6) Cytoskeleton and ECM attachment
how do cells recognize other cells?
cell recognize other cells by binding to molecules, that often contain carbohydrates, on the extracellular surface of the plasma membrane
membrane carbohydrates
usually short, branched, chains of fewer than 15 sugar units
some are covalently bonded to lipids forming glycolipids
most are covalently bonded to proteins which are glycoproteins
membrane sidedness and synthesis
o Distinct inside and outside faces
o The asymmetrical distribution of proteins, lipids, and associated carbohydrates is determined when the membrane is built by the ER and golgi
what are the components that make a cell’s membrane selectively permeable?
1) hydrophobic interior: polar molecules and ions can’t pass
2) specific transport proteins
what are transport proteins? what do they do?
transmembrane proteins that help hydrophilic substances pass through the lipid bilayer
channel proteins are a type of transport protein that have hydrophilic channels for certain molecules or atomic ions to use as a tunnel through the membrane
what are aquaporins?
channel proteins that facilitate the diffusion of water through the membrane
what are some types of transport proteins?
channel proteins
carrier proteins
what are carrier proteins
they are transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane
what is diffusion?
the movement of particles of any substance so that they speak out into the available space
spontaneous process, no work must be done to make it happen = no input of energy
what is the rule of diffusion?
in the absence of other forces, a substance will diffuse from where it is more concentrate to where it is less concentrate
any substance will diffuse down its OWN concentration gradient!
what is a concentration gradient?
the region along which the density of a chemical substance increases or decreases
concentration gradient represent potential energy and drive diffusion
what is passive transport?
the diffusion of a substance across a biological membrane
passive because the cell doesn’t have to input energy
solute will diffuse down its concentration gradient
(random but directional)
what is dynamic equilibrium?
Dynamic equilibrium: no gradient and molecules cross the barrier at the same rate
how does water diffuse?
water diffuses across the membrane from the region of higher free water concentration/low solute contraction to that of lower free water concentration/higher solute concentration until the solute concentration on both sides of the membrane are more equal
WATER MOVES TO DILUTE
what is osmosis?
diffusion of free water across a selectively permeable membrane
isotonic solution
if a cell without a cell wall (like an animal cell) is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane
water diffuses across membrane but at the same rate in both directions
hypertonic solution
if a cell is in a solution that is hypertonic to the cell, the water will move out of the cell, shrivel, and probably die
hypotonic solution
if a cell is in a solution that is hypotonic to the cell then water will enter the cell faster than it leaves and the cell will swell and burst
what is osmoregulation?
the control of solute concentrations and water balance in cells that don’t have rigid cell walls
Ex. The protest paramecium which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
what does turgid mean? what’s it in reference to?
plant cells that have cell walls
when a plant cell swells as water enter by osmosis, the cell wall will only expand so much before it exerts a back pressure on the cell that opposes further water uptake and will become turgid = firm
what is flaccid?
if a plant’s cells and their surrounding are isotonic and there’s no net tendency for water to enter/leave then the cells become flaccid = limp
what is plasmolysis?
if a plant cell is in a hypertonic environment the cell will shrivel and its plasma membrane will pull away from the cell wall at multiple places –> causes plant to wilt and can lead to plant death
bacteria and fungi do the same thing
what are ion channels?
facilitated diffusion
channel proteins that transport ions - electrochemical gradient
moves down electrochemical gradient - no energy
what are gated channels?
ion channels that open or close in response to a stimulus
facilitated diffusion - no energy required
what is active transport?
Proteins use energy to move solutes across a membrane against their concentration gradient
from low to high concentration of solute
ATP supplies energy for active transport
what kind of proteins participate in active transport?
carrier proteins, no channel proteins
this is because when channel proteins are open they allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradient
what is the sodium-potassium pump?
active transport
exchanges Na+ for K+ across the plasma membrane of the animal cell
- 3 Na+ ions move out of the cell for every 2 K+ ions that go in
- a net of 1 positive charge is transferred from inside to out
- the electrochemical gradient created helps cotransport other molecules (such as glucose and amino acids) into the cell and is used to conduct nerve impulses
- Potassium pump changes shape depending on what it’s bound to
- Outside of cell membrane becomes more positive with every pump
what are types of passive transport?
1) diffusion (hydrophobic molecules and small uncharged polar molecules diffuse through lipid bilayer)
2) facilitated diffusion ( hydrophilic substances diffuse through membrane with assistance of transport proteins: channel/carrier down concentration gradient)
what are the charges of the extracellular and cytoplasmic sides of the membrane?
cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of uneven distribution of anions/cations
what is the membrane potential?
the voltage across a membrane
what is the electrochemical gradient?
two forces drive the diffusion of ions across the membrane:
1) a chemical force (ion’s concentration gradient)
2) electrical force (effect of membrane potential on ion
movement)
This determines the transport of ions: important in the transmission of nerve impulses
what is cotransport?
a type of active diffusion - a transport protein (cotranporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient
ex. the carrier protein like sucrose-H+ cotransporter in a plant cell is able to use the diffusion of H+ down its electrochemical gradient into the cell to drive the uptake of sucrose
how do larger molecules such as proteins and polysaccharides cross the membrane?
bulk transport!
the cross in bulk, packaged in vesicles = requires energy
what is exocytosis?
the cell secretes certain molecules by the fusion of vesicles with the plasma membrane
transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
ex. insulin
what is endocytosis?
the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane
almost like reverse exocytosis
what are the three types of endocytosis?
1) phagocytosis
2) pinocytosis
3) receptor-mediated endocytosis (form of pinocytosis)
what is a ligand?
a term for any molecule that binds specifically to a receptor site on another molecule
ex. iron, LDL
what is phagocytosis?
cell engulfs a particle by extending pseudopodia around it and packaging it into food vacuole
macrophages use phagocytosis
what is pinocytosis?
smaller solutes are brought it through pinching of membrane – molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
what is receptor-mediated endocytosis?
binding of ligands to receptors triggers vesicle formation
