Lecture 8: Membrane Transport, Part I Flashcards
Different molecules diffuse through the membrane at different rates due to
lipid bilayer having selective permeability
What allows the lipid bilayer to have selective permeability?
Thin layer of hydrophobic material in aqueous environment
What molecules rapidly diffuse across the membrane?
Small, nonpolar molecules
Ex. O2, CO2, N2, steroid hormones
What molecules diffuse across the membrane?
Small, uncharged polar molecules
Ex. H20, Urea, glycerol, NH3
What molecules rarely diffuse across the membrane?
Large uncharged polar molecules
Ex. Glucose, sucrose
What molecules cannot cross the membrane?
Charged Ions
Ex. H+, Na+ HCO3-, K+, Ca2+, Cl-, Mg2+
Collection of isolated membrane lipids (which contain proteins) thrown into water is example of
hydrophobic interactions as membrane lipids will form sphere around proteins –> causes proteins to be locked in and are very large, polar molecules (often have charges) with no way to escape
What component of lipid bilayer prevents passage of hydrophillic, water soluble molecules?
Hydrophobic interior of lipid bilayer of hydrophobic tails
In living cells, hydrophilic molecules typically pass through membrane using
transmembrane proteins
What are the 2 Types of Transmembrane Proteins cells use to import or export molecules?
- Transporters
- Ion Channels
Similarities between Transporters and Ion Channels
Both multipass proteins to form hydrophilic pores
Arrange alpha helices for hydrophobic outside with nonpolar amino acids and hydrophilic on the inside with polar amino acids
Transporters
Transmembrane protein that undergo conformational changes for each specific molecule, solute or ion
Conformation change takes time so lower rates of transport
Can do passive and active transport
Ion Channels
Transmembrane protein that for hydrophilic pores which can open or close to transport ions and water
After channels open, no need for conformation change so much higher rates of transport
Only does passive transport
Transporter and Channels regulate
ion concentrations within the cell
Sodium (Na+) concentration in mammalian cell
High concentration outside cell
Low concentration inside cell
Potassium (K+) concentration in mammalian cell
High concentration inside cell
Low concentration outside cell
Direction of transport for solutes often depends on
concentration between inside and outside of cell
Simple Diffusion
Describes ability of molecule to travel through lipid bilayer without transmembrane protein (unassisted)
Hydrophobic molecules diffuse very quickly
Large polar charged molecules diffuse very slowly or not at all –> rate of simple diffusion is too slow to meet the cell’s needs
Passive transport/ Facilitated transport (diffusion)
Solutes spontaneously travel from high to low concentration using transmembrane protein (assisted)
No energy is required due to concentration gradient
Done by all channels and some transporters
How is water passive transported?
Water is passively transported by channel proteins called aquaporins to form hydrophilic pore with alpha helices for water to passively pass through
Which transmembrane proteins use passive transport?
All channels
Some transporters
Active transport
Solute travels from low to high concentration
Energy is required and is carried out by transporters
What is common source of active transport that proteins use?
ATP as transmembrane proteins hydrolyze ATP which releases energy and couple reaction as energy pushes against gradient
How does cell know to use passive or active transport for uncharged molecules?
Ex. glucose
Dependent on concentration gradient
How does cell know how to use passive or active transport and rate for ion transport?
Dependent on electrochemical gradient
Most cells have slight ______ on inside of cell membrane
negative charge
What is the source of why cells have slight negative charge on inside of cell membrane?
DNA from phosphate backbone covered with negative charges
If high concentration of positive ions on outside of cell …
Concentration and electrical gradient are in same direction
Positive wants to naturally go to negative interior
Rapid transport with higher electrochemical gradient
Ex. Sodium gradient
Electrochemical gradient
Charged molecules (ions) have 2 gradients: electrical and concentration
If high concentration of positive ions on inside of cell …
Concentration and electrical gradient are in different directions
Concentration gradient pointing outwards from cell and electrical gradient point inward due to positive charge of potassium ions
Lower gradient
Ex. Potassium gradient
What differentiates each membrane are from
membrane proteins
Membrane transporters are highly
selective and typically transfer one type of molecule
Each cell membrane has unique set of _______ that import/ export specific molecules
transporter proteins
Plasma membrane membrane transporters:
transporters for importing sugars, amino acids, and nucleotides
Pumps (type of transporter protein) maintain ion concentration within cell
Mitochondria membrane membrane transporters
transporters for importing pyruvate to generate ATP and transporters to export ATP so it can be used by cell for chemical reactions
Lysosome membrane membrane transporters
Transporters for importing H+ that acidifies the lysosome interior to digest molecules
Does transporters have ability to do passive and/or active transport?
Both passive and active transport
Does ion channels have ability to do passive and/or active transport?
Only passive transport
In passive transport, _____ gradient determines direction of movement
electrochemical and concentration
Glucose Transporters
glucose is large, polar molecule so can not simply diffuse through lipid bilayer
glucose is uncharged so only concentration gradient dictates passive or active transport
After a meal, passive transport in glucose transporters
high amount of glucose in blood stream
liver cells are in charge of brining glucose in and liver can store glucose in form of glycogen but other cells need to import glucose to harvest energy and will need to take glucose out of blood stream using passive transport
glucose transporters transport glucose from extracellular space to cytosol (high to low concentration)
Fasting/ Hunger pains, passive transport in glucose transporters
low amount of glucose in blood stream
glucose transporter transports glucose from cytosol to extracellular space (high to low concentration)
Gradient driven pump
transport solutes against gradient by linking one solute that goes down the gradient to another going against the gradient
In active transport, membrane transporters linked to
source of energy to move against gradient
What are the two sources to power active transport?
ATP
Gradients
How is sodium an example of gradient driven pump?
Sodium has high electrochemical gradient and as traveling down, proteins able to harness energy from gradient to power transport of a different molecule
ATP driven pump
use energy from ATP to transport solutes against gradient
Light driven pump
use energy from sunlight to transport solutes against gradient (bacteriorhodopsin)
Na+-K+ pump (Na+-K+ ATPase): Sodium Potassium Pump
ATP that sodium potassium pump uses accounts for 30% of food humans eat
ATP driven pump that transports Na+ outside of cell and brings K+ inside cell (against gradient)
ATPase
Hydrolyzes ATP into ADP + Pi
What powers the Na+-K+ pump?
Hydrolysis reaction of ATPase
The number of molecules going in and out are not equal:
321NOKIA
3 Sodium (Na+) outside
2 Potassium inside (K+)
Use of ATP
Pushing more positive out, pulling less positive in
Reasons why Na+-K+ Pump is important
- maintain ion concentration differences for sodium and potassium
- negative charge inside cell is maintained
Steps to Na+-K+ pump
- At resting state, Na+-K+ pump is opened pointing to inside of the cell. Cytoplasmic Na+ binds to Na+-K+ pump and keeps conformation until all 3 Na+ binding sites are occupied
- Na+ binding stimulates phosphorylation by ATP where ATPase domain hydrolyzes ATP by attaching phosphorylation group to its protein
- Phosphorylation and negative charge causes protein to change conformation for pump to open to outside of the cell and expel Na+ to the outside, binding sites for Na+ are no longer active
- Extracellular K+ binds to protein as potassium binding sites are active and will change conformation once K+ binding sites are occupied
- Binding K+ to protein induces conformational change which releases phosphate group which restores to protein’s original conformation
- K+ is released to inside and Na+ sites are reactive for cycle to repeat
Glucose Na+ symport pump
transports glucose into intestinal cells (against gradient) using high electrochemical gradient of Sodium (active transport)
Glucose symport
pull glucose into the cell
restricted to apical domain (facing gut lumen) by tight junctions
Glucose uniport
passive transport, glucose goes down concentration gradient
Some cells have ______ which limit movement of proteins
tight junctions
Coupled transport means
By using solute to travel down gradient for energy for another solute to travel against gradient
Active transport
Ex. gradient driven pump
Intake of glucose by intestinal cells uses glucose Na+ symport pump
- Gut lumen found inside of small intestine
- Try to bring low glucose concentration from gut lumen to high glucose concentration in extracellular fluid (against gradient need active transport)
- After pulling glucose against its concentration gradient, now want to go from high to low concentration but not right back out so have to restrict protein fluidity
- Symport pump restricted on apical side that is facing gut lumen
- Uniport pump on basal side to release glucose from high to low concentration into blood stream (with concentration gradient, passive transport)
- glucose can go to connective tissue where it has access to blood vessels
