Week 8: Membrane Proteins and Transport Across Membranes Flashcards
Section 2 Week 2
What is the permeability of an artificial bilayer?
Impermeable to most water soluble molecules
Describe the permeability of the cell membrane
Membrane transport proteins transfer specific proteins via facilitated transport
What are the two types of movement that can happen across the lipid bilayer?
Permeable and impermeable movement
Describe permeable movement
Movement via simple diffusion through the lipid bilayer. Movement goes down the concentration gradient. There are more hydrophobic or non-polar molecules as they go through faster diffusion across lipid bilayer.
Describe impermeable movement
These require membrane proteins for transport due to how long it would take to move without it.
What molecules undergo permeable movement across the bilayer?
- Small, nonpolar molecules like O2, CO2, N2 and steroid hormones (some resistance but it really gets through)
- Small, uncharged polar molecules like H2O, ethanol, glycerol (more resistance, it takes longer to get through)
What molecules undergo impermeable movement
- Larger uncharged polar molecules like amino acids, glucose, nucleosides
- Ions like H+, Na+, K+, Ca2+, Cl-, Mg2+ and HCO3-
What is the function of transmembrane transport proteins?
They create a protein-lined path across the cell membrane and transport polar and charged molecules (such ass ions sugars, amino acids, nucleotides, various, cell metabolites)
What does the statement, “Each transport protein is selective”, mean?
It means that each transport protein transport a specific class of molecules. However this specificity varies and depends on the protien.
What are the two main classes of membrane transport proteins? Describe how they act during transporting.
- Channel: binds weakly to the transported molecule and does not change in conformation a lot during transportation.
- Transporter (aka carrier protien): binds strongly to the transported molecule and changes conformation significantly during transporation
Describe the selectivity and transport of a channel protein
Selectivity:
* size and electric charge of solute
Transport:
* transitent interactions as solute passes through
* no conformational changes for transport through an open channel
Describe the selectivity and transport of a transporter protein
Selectivity:
* solute fits into its binding site
Transport:
* specific binding of solute
* series of conformational changes for transport
What is the difference between passive and active transport?
Passive transport: molecules transported down the gradient, does not directly require energy
Active transport: molecules transported up the gradient, does directly require energy
Concentration gradient + Membrane potential = ?
Electrochemical gradient
True or False: In a cell, the concentration gradient is usually smaller than the membrane potential
FALSE
What is membrane potential?
The charge across the membrane (the charge on opposite sides)
How does the a negative charge within a cell affect the transport of positive ions into the cell?
The negative charge within a cell acts as a boost to get the positive ions into the cell.
How does a positive charge outside of a cell affect the transport of positive ions out of that cell?
The positive charge outside of the cell acts as a bit of resistance to the positive ions being transported out of the cell - however, for transport going down a gradient, it will happen despite the resistance
Channel proteins have a ____________ pore across the membrane
hydrophilic
What does it mean when channel proteins are selective?
It means that ion channels transport a specific ion, and therefore ion size and electric charge are taken into consideration
Which transport protein allows for faster passive transport of solutes?
Channel proteins
What kinds of interactions take place between a solute and the channel wall during selective passive transport?
Transient interactions
Where are ion channels found?
In animals, plants, and microorganisms
What are the two kinds of ion channels?
Non-gated: always open
Gated ion: some type of signal required for channel opening (specific ions transported)
What is a K+ Leak Channel?
It is a type of non-gated ion channel. It moves K+ out of cell. It plays a major role in generating resting membrane potential in plasma membrane of animal cells.
What are the four types of gated ion channels and what signals open them?
- Mechanically-gated: Signal - mechanical stress
- Ligand-gated (extracellular ligand): Signal - ligand (e.g. neurotransmitters)
- Ligand-gated (intracellular ligand): Signal - ligand (e.g. ion, nucleotide)
- Voltage-gated: Signal - change in volatge across membrane
A transporter binds a ________ solute and goes through a ____________ change
specific; conformational
A uniport transports how many solutes?
One
A uniport is involved with ____________ transport down its electrochemical gradient
passive
How does a uniport deal with a change in the electrochemical gradient?
The direction of transport for a uniport is reversible. This means that if the gradient changes, the protein will switch the drection of transport.
What is a GLUT Uniporter?
A GLUT Uniporter is a glucose transporter. It transports glucose down the concentration gradient and can work in either direction
Active transport transports ____________ the electrochemical gradient, this means it requires ___________
against; energy
What are the types of active transport?
- Gradient-driven pump: 1st solute down its gradient (energy); 2nd solute against its gradient
- ATP-driven pump (ATPases): ATP hydrolysis (energy); moves solute against its gradient
- Light-driven pump (bacteria): light energy; moves solute against its gradient
What are the two types of gradient-driven pumps?
- Symport: two solutes moved in the same
- Antiport: two solutes moved in opposite directions
In both symport and antiport gradient-driven pumps, where does the energy used to transport the 2nd solute against its electrochemical gradient come from?
It comes from the free energy from moving the 1st solute down its electrochemical gradient
Describe the Na+-glucose symporter
The Na+-glucose symporter transports Na+ down its gradient and provides energy to move glucose against its gradient
When do conformational changes occur in Na+-glucose symporters?
After both sites occupied by Na+ and glucose and after both sites are emptied when both Na+ and glucose dissociate
What kind of transporter protein is a Na+-H+ exchanger?
It is a gradient-driven antiport pump
What is the role of the transporation of Na+ in a Na+-H+ exchanger?
It provides energy to move H+ against its gradient
What pH is needed in the cytosol to regulate optimal enzyme function?
~7.2
Why is excess H+ in the cytosol an issue?
Excess H+ causes the pH in the cytosol to decrease (pH ~5)
How does the Na+-H+ exchanger solve the problem when there is excess H+ in the cytosol?
It maintains cytsosolic pH by increasing transporter activity and transporting H+ out by transporting Na+ in
How is the Na+ electrochemical gradient maintained in animal cells?
The Na+-K+ pump
What are the kind of ATP-driven pumps?
- P-type pump (N+-K+ pump H+ pump)
- V-type proton pump
- ABC transporter
How do P-type pumps use ATP?
Through phosphorylation during the pumping cycle
How many Na+ and K+ are moved against their electrochemical gradients?
3 Na+ out, 2 K+ in
Describe the steps of the pumping cycle of the Na+-K+ pump
- three Na+ binds
- Pump phosphorylates itself (Pees itself), hydrolyzing ATP
- phosphorylation triggers conformation change and Na+ is ejected
- two K+binds
- pump dephosphorylates itself
- pump returns to orginal conformation and K+ is ejected
What is the H+ pump?
- is a type of V-type pump
- generates H+ electrochemical gradient which is used for H+-driven symport/antiport
- uses ATP to pump H+ into organelles to acidify the lumen
- its in lysosome and plant vacuole
What the ABC transporter?
It’s a type of ATP-driven pump which uses 2 ATP to pump small molecules across cell membrane (i.e. this is how toxins are removed from cells)
What is F-Type ATP synthase?
- it is structurally related to V-type proton pump but has an opposite mode of action
- it uses H+ gradient to drive the synthesis of ATP
- in mitochondria, chloroplasts and bacteria
What are examples of transport proteins regulating critical cellular processes?
- Transcellular transport of glucose by transporters
- Generation of membrane potentials
Transporters work together to transfer ____________ from the ____________ to the _______________
glucose; intestine; bloodstream
What kind of transport is used to transport glucose from the gut lumen into the epithelial cell?
Active transport using an Na+-driven glucose symport
What are tight junctions between epithelial cells used for when regarding glucose?
They make sure glucose can’t go between the cells to make sure they go through the cells
What type of transport is used to transport glucose from the epithelial cell into extracellular fluid (the bloodstream)
Passive transport using a passive glucose uniport
What maintains the Na+ concentration between the extracellular fluid and within the epithelial cell?
An Na+ pump
What does the apical domain of the plasma membrane face?
The lumen
What does the lateral domain of plasma membrane face?
The neighbouring cells
What does the basal domain face?
The basal side (where extracellular fluid is)
Basal + lateral = ?
Basolateral domain
What are tight junctions between epithelial cells used for when regarding transport proteins?
Keeps transport proteins on the right side even if they moved around
Membrane potential = ?
difference in electrical charge on two sides of membrane
True or False: There’s positive and negative on both sides
True! It’s just a matter which one they have more of
How does the K+ Leak channel contribute to the generation of membrane potential in animal cells?
It plays a major role in membrane potential as it controls the outward flow of K+
How does the Na+-K+ pump contribute to the generation of membrane potential in animal cells?
- It contributes to 10% of membrane potential.
- It maintains the Na+ gradient with low cystolic and the K+ gradient with high cystolic.
- With 3 Na+ ions pumped out and 2 K+ ions pumped in, there is a net 1 positive ion pumped out
Equilibrium = ?
Resting membrane potential
What does the membrane potential range from in animal cells?
Vary from -20mV to -200mV
When referring to mebrane potential, what is it from the perspective of?
Always from the perspective of the inside of the cell (i.e. ig it’s more negative (like -200mV) that’s the inside of the cell)
What is the net result of the generation of membrane potential in animal cells?
- A bit more positive on the outside (Na+ (from Na+-K+ pump), K+ (from K+ leak channel))
- A bit more negative on the inside (Cl- and fixed ions)
What generates the membrane potential in plant cells?
Plasma membrane P-type pump: H+ pump generates H+ electrochemical gradient which causes the membrane potential that varies from -120 mV to -160 mV
How are plasma membrane p-type pumps used in animal cells?
- Used by gradient-driven pumps to carry out active transport (e.g. H+ driven symport)
- Electrical signaling
- Regulate pH
