Transport across membranes Flashcards
3 main methods of transportation across the plasma membrane
Diffusion, osmosis and active transport
Solubility definition
The simplest path through a membrane. Gases dissolve in the membrane- partition and then repartition once they leave the membrane.
What methods of transportation rely on solubility?
diffusion and osmosis
Diffusion definition
the passive net movement of particles down a concentration gradient, until equilibrium is reached.
Osmosis definition
The passive net diffusion of water molecules from a region go high water potential to a region of low water potential.
Different osmolarities of solutions explained
Hypotonic- water potential within the cell is lower
hypertonic- water potential within the cell is higher
isotonic- water potential on both sides of the semi permeable membrane is equal
Tonicity definition
A measure of the effective osmotic pressure gradient when the two solutions are separated by a semipermeable membrane.
What molecules can diffuse through membrane + why?
Gases, such as oxygen and nitrogen can diffuse as they are lipid soluble. Can dissolve in membrane and diffuse passively via Brownian motion.
Water molecules; permeability is increased due to the existence of aquaporins
Fick’s Law of Diffusion function
Net diffusion flux = Dx x Area x concentration difference/distance
Each variable
Net diffusion flux- how fast the molecules will diffuse
Dx- diffusion coefficient
area- surface area of membrane
concentration difference
distance- how far the molecules must diffuse
What affects the diffusion coefficient + how?
Molecular weight- the greater the weight the more slowly diffusion takes place
temperature- the higher the temperature the faster the diffusion
Electrodiffusion explained
Direction of diffusion depends on the concentration gradient of the ions and the electrical gradient, provided by the potential difference.
E.g positive ions will diffuse towards a negative electrode, even if there is no chemical gradient.
Where does transport take place?
Pores, protein channels, carriers and pumps
3 different types of channels and what they transport
Voltage gated- sodium and potassium ions
ligand gated- acetylcholine
stretch mediated- sodium
Protein channel structure + definition
Transmembrane intrinsic proteins that form a continuous hydrophilic pore that extends across the lipid bilayer. Interact with the solute to be transported very weakly via their selectivity pore. When open, the pores allow specific solutes to pass through.
Have a negatively charged amino acid residue at the mouth of the opening to attract cations.
Selectivity filter explained + example
Bacterial K+ channel
Bacterial K+ channel structure was determined by X-ray crystallography. Polypeptide chain forms the pore helix with a protruding loop that forms a selectivity filter. Potassium ions shed their water coat and can then fit through the filter.
The inside of the pore is lined with carbonyl oxygen atoms which are spaced so that potassium can interact with them. They are too far away for a smaller sodium ion so they cannot pass.
Rate of transport across channel
10^8 particles per second
What movement occurs in channels?
Passive diffusion
How do channels discriminate between solutes? v carriers
Channels have a selectivity filter that recognises size and electric charge whereas carriers have a specific binding site for the solute.
Are channels always open- why?
Some are always open- potassium leak channels and aquaporins.
Some are not, contain gates which block the entrance through the pore. Oscillate between open and closed state
Voltage gated channels structure
Sodium and potassium channels formed of polymers of several polypeptide chains. Pore is formed by alpha subunits.
Fourth transmembrane segment acts as a voltage sensor
Portion of the protein complex also acts as a pore cap.
Voltage gated channels explained
Transport cations at a rate that is close to free diffusion. Voltage activation of sodium channels allows the cation to enter the cell and generate the action potential.
Ligand gated channel structure
5 oligomeric complexes with four alpha helix transmembrane segments each
Ligand gated channel function
Binding of the chemical intermediate to its receptor causes a conformational change which opens the channel - inotropic effect.
Stretch mediated channels explained
Membrane becomes deformed when pressure is added, mechanically changing the shapes of the channels, opening the pore
Aquaporin structure related to function
Six transmembrane alpha helices with the nitrogen and carbon termini being intracellular. Alpha helices are arranged symmetrically to form a pore so the water molecules can pass through.
contain a narrow pore which allows the water molecules to transverse in a single line, attracted to the oxygens on the carbonyl groups lining one side of the pore
Other side of the pore lined with hydrophobic amino acids.
Pore is too large to allow hydrated ions to enter. Energy cost of dehydrating is too much, as the then dehydrated ions cannot interact with the hydrophobic wall.
Contain two asparagines that bind to the oxygen atom of the central water molecule in the line of water molecules moving through, prevents making and breaking sequence of H30+ preventing H+ ions from moving in.
Another example of diffusion through a pore
Movement through gap junctions- passage of ions that allow a current to flow through the cardiomyocytes of the heart- forming an electrical syncytium.
Active transport definition
The active movement of particles against the electroconcentration gradient.
What mediates active transport + how?
Through transmembrane protein carriers which bind the solute that they transfer and undergo a conformational change allowing the solute to pass.
What happens to the transport as a result?
Slower, more sensitive to temperature and can also become saturated at high concentrations of solute, therefore there is a Vmax and Km.
Different types of carrier + definition
Uniport- transport one molecule in one direction- use ATP
symport- transport one molecule using the free energy gained from the transport of another down its concentration gradient
antiport- transport two molecules, one in each direction. using the free energy gained from the transport of one down its concentration gradient.
How do transporters pass the solute?
Solute binds to specific binding site which opens the channel. Solute then becomes occluded- an intermediate state where the solute is inaccessible from either side of the membrane then becomes released on the other side. Constantly switch between two different conformations
Transporter shared structural features
Built from 10 or more alpha helices. Solute and ion binding sites are foundmidway through the membrane. Binding site only ever accessible from one side. Transporters built from inverted repeats- similar packing of alpha helices in both halves of the membrane- therefore pseduosymmetric.
Two forms of active transport explained
Primary- using energy supplied from the hydrolysis of ATP
secondary- uses the electrochemical gradient created by the operation of a primary transport system.
Three types of ATP-driven pumps
P-type, ABC and V
Explain P-type
ATPase P contains a subunit with an ATP-binding site and becomes transiently phosphorylated during the transport process. Undergo conformational changes during the phosphorylation/dephosphorylation. Contain a beta subunit to help transport a second ion.
Example of P type carrier
Na/K- ATPase
NA/K- ATPase structure and function
Complex formed of two major subunits, both of which are integral plasma membrane proteins. Alpha unit is multiples and transverse membrane 10 times. beta is a glycoprotein linked to an oligosaccharide chain on its outer side. Transverse membrane only once.
Has 3 main domains- acting domain, nucleotide binding domain and phosphorylation domain.
Pump 3 sodium out and 3 potassium in
primary function of P type carriers
Set up and maintain gradients of sodium, potassium, hydrogen and calcium ions across cell membranes
Primary function of ABC transporters
Primarily pump small endogenous and exogenous molecules across cell membranes
How does Multidrug resistance protein transporter work?
Uses energy of ATP hydrolysis to extrude hydrophobic compounds from the cell and also functions as a chloride channel.
Two secondary active transport channels
sodium dependent glucose transporter and sodium/calcium exchanger
Na+- dependent glucose transporter name, location
SGLT- Symporter mediates the entry of glucose into cells and can be found n the intestinal mucosa and renal tubules
SGLT structure and function
glycoprotein formed of 14 transmembrane helices forming a channel with glucose and sodium binding sites.
Binding of solutes causes a conformational change, allowing the molecules to diffuse in, down sodium’s electroconcentration gradient
2 sodiums bind for every glucose
Sodium/calcium exchanger location and system
Distal tubule cells of kidney, myocardial and arterial smooth muscle cells, antiporter
Sodium/calcium exchanger- structure and function
nine transmembrane domains.
Introduces three sodium ions into the cell for every one calcium expelled into the extracellular space in an electrogenic process.
sodium flows down its gradident into the plasma membrane, which indirectly provides free energy for the calcium removal
Facilitated diffusion definition
Exhibits specificity and satiability. Passive diffusion of solutes across a membrane down the electrochemical gradient through carriers.
Important facilitated diffusion channel + what it transports and its location
GLUT uniporter, which ensure the supply of glucose and other hexoses to cells.
What isomers do GLUT recognise?
D-steroeisomers
GLUT 1 function and structure
Expressed in virtually all cells and carries basal glucose transport. Has a high affinity Km 1.4 mM which allows the maximal uptake of glucose even if levels are low.
Dimer
How are intracellular calcium levels controlled?
Extrusion of calcium via calcium 2+ ATPase and secondary sodium, calcium exchange and SERCA within the endoplasmic reticulum.
Calcium liberated from stores, such as calsequestrin and calmodulin to increase intracellular concentration
What is normal intracellular pH?
Maintained around neutral
How is this maintained?
Sodium driven hydrogen exchanger, antiporter that directs H+ out of cell using inward sodium gradient - secondary active transport.
Pumps hydrogen out using H+ ATPase.
What mediates vesicle formation?
Proteins that polymerise in a mesh attached to cytosolic side of membrane and direct vesicle formation.
Three types of protein coat
Clathrin, COP I and COP II.
How do the proteins mediate vesicle formation?
Cathrin covers plasma membrane vesicles that give the vesicles their shape- polygonal mesh
Adapter proteins between clathrin and the membrane recognise the contents of the vesicle and provides specificity for the transport of vesicles via different pathways.
COP proteins form a dense diffuse layer
V snare protein on vesicle binds to receptor so can release its contents.
