Membrane Proteins Flashcards
What is facilitated transport?
movement of small water soluble molecules across the cell membrane membrane channels and transporters
what does diffusion rate depend on? How do small non polar molecules, polar molecules and ions cross?
Size and solubility properties of the molecule:
- small non polar molecules will diffuse via concentration gradient
- uncharged polar molecules will diffuse if small or will be transported if large via concentration gradient
- ions will not cross the bilayer freely and will move based on electrochemical gradient
What is membrane potential?
Voltage across the cell membrane ranging between -20 and -200 mV
What does the membrane potential favour and oppose?
FAVOURS entry of positively charged ions
OPPOSES entry of negatively charged ions or exit of positively charged ions
What is a membrane channel?
A transmembrane protein that select ions based on size and electric charge that exist in an open or closed state
what is a membrane transporter?
Transmembrane protein that transports molecules or ions that fit into specific binding sites and change shape to move substances across the membrane
What is passive transport?
Moving solutes down their concentration gradient without the use of energy (e.g. all channels, many transporters)
What is active transport?
Moving a solute against its concentration gradient with the use of ATP, light, or ion gradient (e.g. specific transporters called pumps)
What is the difference between transport of uncharged solutes and charged solutes?
Uncharged solutes will want to flow based on concentration gradient
Charged solutes will want to flow based on electrochemical gradient
what are aquaporins?
channel proteins that move water across the cell membrane via osmosis
How do protozoan, plants and animal cells avoid osmotic swelling?
Protozoan: eliminate excess water via contractile vacuole
Plant: Rigid cell wall
Animals: transmembrane pumps expel solutes shifting osmotic gradient out of cell
What are passive transporters?
Transmembrane proteins that move a solute along its electrochemical gradient (glucose transporter)
How does the glucose transporter work?
Glucose will bind to a specific binding site on the glucose transporter and will cause a conformational change in the transporter that will move glucose to the other side of the membrane. The direction is based on the concentration gradient.
What are three types of pumps and examples?
ATP driven pump: uses ATP hydrolysis to move solutes to a higher concentration (Na+/K+ pump)
Gradient driven pump: energy from downhill transport of one solute is used for the uphill transport of another
Light-driven pump: uses energy from sunlight to move solutes uphill
What does the Na+/K+ pump do?
Uses energy of ATP hydrolysis to pump 3 Na+ out and 2K+ in simultaneously to keep cytosolic Na+ low and K+ high
What are the steps of the Na+/K+ pump?
- Na+ binds
- Binding causes pump to phosphorylate
- Conformational change due to phosphorylation and ejection of Na+ outside the cell
- New shape allows K+ to bind
- Binding dephosphorylates
- Dephosphorylation triggers conformational change back to original shape and ejects K+
How does the Ca2+ work?
- Ca2+ binds to site on pump
- Binding causes phosphorylation
- Phosphorylation changes shape and changes shape of protein
- Change of shape ejects Ca2+ into the sarcoplasmic reticulum
- Ejection of Ca2+ results in dephosphorylation
What are the three types of gradient driven pumps?
Symport: pump that moves solute 1 downhill and solute 2 uphill in the same direction
Antiport: pump that moves solute 1 uphill and solute 2 down hill in opposite directioons
Uniport: transporter that moves one type oof solute downhill
What is the Glucose-Na+ symport?
Symport pump that brings glucose against its concentration gradient into the cell by bringing Na+ down its electrochemical gradient.
How does the Glucose-Na+ symport work?
- Na+ binds to empty protein
- Na+ binding allows glucose to bind
- Both solutes occupying membrane allows it to eject both into cytosol
How do gut epithelial cells take up glucose?
- At apical side, glucose-Na+ symport actively takes up glucose against concentration gradient
- At basal domain, glucose is taken down its concentration gradient via uniport via facilitated diffusion
How is resting membrane potential established?
Na+/K+ keeps cytosolic Na+ low and K+ high
How do Na+ ions stay outside the cell membrane if K+ channels are large enough for them to pass?
Narrow regions in the ion channel force the ions to touch the channel wall too allow them to pass. Na+ are to small to interact with the K+ channel walls.
What is a patch clamp recording?
Type of mechanism that monitors ion flow through a single channel in a small isolated patch of membrane. Currents appear and disappear during opening and closing of channels.
What are types of ion channels?
Voltage gated: controlled by membrane potential
Ligand gated: controlled by binding of ligand
Mechanically gated: controlled by mechanical force applied to the channel
How do auditory hair cells work?
Sound vibrations tilt stereocilia in a hair cell, which stretches the mechanically gated channels open allowing positively charged ions to enter from the surrounding fluid and cause an action potential.
