MCB5 - Transport Across Cell Membranes Flashcards
Which types of molecules can pass freely through lipid bilayer and give examples.
Small uncharged or non polar molecules e.g oxygen, co2, nitrogen etc.
Which type of molecule can not pass the lipid bilayer at all.
Charged molecules eg all ions.
Why do cells need to transport materials across cell membrane.
Import nutrients, glucose and amino acids.
Export waste material.
Why can hydrophilic molecules not cross a lipid bilayer membrane freely.
Interior of bilayer is hydrophobic meaning hydrophilic molecules cannot pass through easily
What are the three ways in which transmembrane proteins transverse the entire bilayer.
Single alpha helix.
Multiple alpha helices.
Beta barrels.
Discuss how a transmembrane protein can traverse the membrane in terms of the structure of the polypeptide chain.
Hydrophobic region of polypeptide chain sits in the hydrophobic interior of the bilayer.
What are the two types of transmembrane proteins for transport.
Transporters and channels.
What are membrane transport proteins.
Proteins existing within a membrane that aid transporting of various molecules.
Discuss structure of channels as transport proteins.
Hydrophobic amino acid side chains form exterior of channel whilst hydrophilic side chains form interior pore allowing water soluble substances through.
Discuss how transporter proteins work including their specificity.
Only allows molecules that can fit into the binding site through. Conformational changes occur which move the molecule across the membrane.
Discuss how channels work as transporter proteins making reference to their specificity.
Specific to molecules depending on size and charge. Allows many molecules through at one time. Can have open or closed conformations based on different stimuli.
How is the selectivity of ion channels maintained.
Internal structure as a specific size and charge which must be complementary to the molecule being transported.
Discuss simple diffusion.
Passive process. Small uncharged and non-polar molecules. Down a concentration gradient,
Discuss channel mediated diffusion.
Passive process. Transported down concentration gradient. Commonly for ions moving through channel during open conformation.
Discuss transporter mediated transport of molecules.
Can be active or passive. Passive - down concentration gradient. Active - against concentration gradient and requires energy.
Define pumps involved in transport across cell membranes. J
Transporter proteins that carry out active transport so require energy.
Discuss membrane potential
Voltage difference across a membrane as a result of varying ion concentrations on either side of membrane.
Which actions are abundant on either side of the cell membrane.
Na+ abundant on extracellular side.
K+ abundant on intracellular side.
What is the normal membrane potential and how is it created.
Usually a negative membrane potential. More Na+ on exterior than K+ in interior resulting in negative membrane potential of inside compared to outside.
Which amino balances Na+ on extracellular side.
Cl-
Which negative anion balances K+ on intracellular side.
Organic and inorganic anions.
Define an electrochemical gradient.
Net force driving a charged molecule across a cell membrane depending on both the concentration gradient and the membrane potential
Why does electrochemical gradient affect only charged molecules
Membrane potential has no effect on uncharged molecules
Discuss the role of electrochemical gradient in the movement of Na+ ions.
Na+ abundant on outside so can move in, along its concentration gradient. Membrane potential is negative so pulls Na+ into cell. Overall large movement of Na+ into cell.
Discuss the role of electrochemical gradient in the movement of K+ ions.
K+ abundant in cell interior so moves down its concentration gradient to the extracellular space. Membrane potential opposes this as K+ is moving from negative to positive even though it’s positive therefore only small amount of K+ moves to the extracellular side.
Give three examples of membrane transporter proteins in a cell.
Plasma membrane transporters. Mitochondrial transporters. Lysosomal transporters
Discuss one way in which lysosomal transporters aid the processes carried out by lysosomes.
Allow the movement of H+ into the lysosome creating acidic conditions which help digestion of waste material.
Why are mitochondrial transporters vital for their role.
Imports pyruvate created in glycolysis from cytosol to the mitochondrion interior.
In what scenario does glucose Move into a cell.
Following a meal, glucose concentration in the blood is high meaning it will move into the cell.
Discuss process by which glucose moves into a cell.
Glucose binds to exposed binding site whilst transporter protein is in outward open state. Conformational changes occur, which pass through occluded state, forming the inward open state. Glucose released on intracellular side.
What is the occluded state.
State of glucose transporter protein where neither the extracellular or intracellular binding site is exposed.
Give scenario where glucose moves from inside cell to outside.
If blood glucose concentration is low, Liver produces glucose from glycogen. Glucose must then be transported out of hepatocytes.
What are the three types of pump transport proteins.
Coupled, ATP-driven and light-driven
Discuss mechanism of action for coupled pumps.
Couples downhill transport of one molecule to uphill transport of next molecule. Uses electrochemical gradient.
Discuss mechanism of action for ATP driven pumps.
Couples uphill transport of one molecule to ATP hydrolysis, which provides the energy.
Discuss mechanism of action of light driven pump and where they are found.
Uses energy from sunlight to drive uphill transport. Mainly found in bacteria,
Discuss how the sodium potassium pump works.
3 Na+ ions bind to sodium potassium pump on extracellular binding site. On inside, ATP is cleaved resulting in phosphorylation of pump protein. Conformational change occurs which releases Na+ on cytosolic side. 2 K+ ions bind to intracellular binding sites. Triggers release of Phosphate group which causes conformational change back to original, releasing K+ on extracellular side.
What are three types of transporter proteins.
Uniport,symport, antiport
Discuss mechanism of action for uniport transport proteins.
Movement of one type of solute from one side of membrane to other.
Discuss mechanism of action for antiport transporter proteins.
Carries two solutes in opposing directions. One solute moves downhill here as other moves uphill..
Discuss mechanism of action for symport transporter proteins.
Carries two solutes in the same direction, one downhill and one uphill.
Discuss mechanism of action for ion channels
Ion solvent shell is shed. Ions move in single file through the narrow pore in a channel transporter protein. Selectivity filter within ion channel ensures correct ion is moving through the channel based on charge and size. Once out of the pore, the aqueous solvent shell reforms around the ion.
What selectivity filter is present for K+ ions in K+ ion channels.
Polar carbonyl groups are found in the selectivity filter which interact with K+ and form transient bonds.which are unable to be formed with other ions based on their size or char.ge
Why can’t ions move through the ion channel with their solvent shell.
Would be too large.
What are the two main characteristics of ion channels.
Only open briefly but still allow many ions to pass through. Differ from transporters as they don’t have conformational changes per ion that passes through.
What are the four types of stimuli that can change the conformation of a channel transporter protein.
Voltage. Ligand binding - extracellular or intracellular. Mechanical stimulus.
Define osmolarity.
Total concentration of solute particles inside the cell.
Name of channel specifically for transport of water.
Aquaporins.
Discuss difference between osmolarity and concentration of solutes outside cell.
Osmolarity is generally higher than concentration of solutes outside cell meaning water is pulled into the cell by the osmotic gradient.
Why are aquaporin channels necessary.
Membrane is semi permeable to water as water is small and uncharged however movement occurs slowly so aquaporins increase efficiency of water movement into the cell.
Give example of cell that would require aquaporins
Epithelial cells of kidney for water reabsorption.
Which amino acids acts as a selectivity filter for aquaporins.
Asparagine. Allows water but prevents h+ moving through channel.
