C2.1 Membrane Transport Flashcards
what is the structure + function of the cell membrane
Lipid bilayers basis of membrane and other amphipathic molecules
serves as a border between cell and environment controls passage of substances.
What do phospholipid molecules contain
Phosphate “head”, two hydrocarbon “tails”
Why does the membrane core have low permeability to hydrophilic particles, ions, polar molecules?
The core is formed through interactions between hydrophobic tails, hence why low permeability.
Why may substances be drawn back to aqueous solution outside when reaching the core?
Hydrophobic hydrocarbon core doesn’t repel solutes, they are more attracted to each other.
solutes more attractedd to water.
How does molecular size affect membrane permeability
larger the molecule, lower ermeability.
What is diffusion?
Spreading out of particles in liquids and gases, particles are in random motion.
Net movement from high to low concentration, down the concentration gradient.
Why does diffusion occur across membranes?
Can only happen as phospholipid bilayer is permeable to particles.
Non polar particles can diffuse through easily, e.g if higher concentration outside will passively diffuse inside.
Why can ions/polar molecules cannot easily diffuse through
polar molecules have partial charges, so diffuse at low rates, ions cannot easily diffuse through due to hydrophobic membrane core.
What are the two groups of membrane proteins?
Peripheral and Integral proteins.
proteins oriented in a way so that they can carry out function
What are integral proteins?
hydrophobic on surface, embedded in hydrocarbon chains in centre of membrane.
may fit in one of two phospholipid layers/extend across both (transmembrane, many are)
hydrophilic parts project through regions of phosphate heads.
what are peripheral proteins?
hydrophilic on surface, not embedded in membrane.
most attached to surface of integral proteins, attachment reversible.
some have hcarbon chain attached, inserted to membrane, acting as an anchor.
How does protein content of membrane vary?
variable as the function of membrane also varies.
More active a membrane, the higher its protein content.
Why does osmosis occur?
- due to differences in concentration of substances dissolved in water. passive
how does a solute dissolve in water?
forms intermolecular bonds.
Bonds restrict movement of water molecules.
Higher solute concentrations have lower concentration of water molecules that are free to move than lower solute concentration. Hence why net movement of low to high solute concentration.
Why can osmosis occur in all cells?
Water molecules small enough to pass through bilayer.
Some cells have aquaporin (Water channel, integral protein) greatly increasing membrane permeability to water, e.g kidneys, facilitates rapid movement of water.
What can help ions and polar molecules to pass through phospholipids?
Diffusion is possible with channel proteins, they are integral and transmembrane.
Protein with pore that connects cyto to aq solution outside.
diameter and chemical properties of its sides ensure only one type of particles passes through.
can pass either direction, no energy expended, high to low
facilitated diffusion
channel proteins required for diffusion to occur.
cells can select which substances diffuse in and out by types of channels synthesised
some closed/open, so permeability can be changed.
What are pump proteins?
carry out transport tasks, substance can be absorbed against concentration gradient/pump substances out
cells have many diff pumps, each transfer one specific type, allows cell to control content of cyto
how do pump proteins differ from channel proteins?
use energy, carry out active transport, diffusion passive.
can only move particles in one direction, pump proteins both
move against/down concentration gradient, whereas channel is down concentration gradient.
What are the different conformations that pump proteins can interconvert to
1) Transported particle enters pump from one side to reach central chamber/binding site.
2) Next conformation, ion/molecule passes out in opposite side of the membrane.
energy used to change from more stable to less stable conformation.
reverse change, no energy.
What is a semi-permeable membrane?
Membrane allows passage of certain small solutes. freely permeable to solvent.
what are properties of a cell membrane?
More selective and has variable permeability to water. Described as partially permeable, both semi and selectively permeable.
selectively permeable membrane?
allows passage of particular particles, facilitated diffusion+active transport allow for selective permeability, as they are specific.
What is the fluid mosaic model of membrane structure?
bilayer of phospholipids with proteins in variety of positions.
peripheral proteins, attached to inner/outer surface
integral proteins, embedded into bilayer
as phospholipid molecules are free to move laterally, proteins can also move.
how do membranes attain its specific required properties?
relative amounts of sat/unsat fatty acids regulated.
Must remain fluid, but be strong enough to avoid become perforated.
must be permeable but not too porous
ideal ratio depends on temp.
what is cholesterol?
makes up 20-40% of lipids in p membrane
steroid, most of molecule is hydrophobic.
attracted to centre
one end (-OH group) hydrophilic, attracted to phosphate heads, faces outwards
molecules positioned between phospholipids
intercalates (insert) between saturated.
helps maintain orderly arrangement of lipids, stabilises membrane at higher temps, maintaining impermeability to hydrophilic particles
ensures sat acids do not solidify at low temps.
Why must Na an K channels be specific?
both carry + charge but unable to fit in different channel. K channels do not allow Na+ ions, pore is 0,.3nm wide, ions in K channel are slightly smaller than 0.3nm
- however, when bonded to shell of water molecules, too large to pass through.
- to pass, bonds between ion and water break and bonds form between ion and amino acids in narrow part of pore.
after passing it can be associated with shell of water
- na ions too small to form bonds with narrowest part of pore, so cannot shed their shell.
what is the purpose of Na-Glucose cotransporter proteins?
transfer na+ and glucose together across p membrane into cell.
glucose, against concentration gradient, as na+ moves down
energy released by movement of na+ > energy needed to move glucose
e.g used in proximal tubule in the kidney, cells reabsorb glucose
how are cells in tissue linked by cell-to cell junctions?
junctions depend on cell adhesion molecules in plasma membranes of adjacent cells.
usually proteins with domains embedded in bilayer, and others portruding outwards.
junction formed by binding of CAMs in adjacent cells binding together their extracellular domains.
CAMs can be same/different, if different asymmetrical functioned produced.
