Membrane Transport (Lec. 5) Flashcards
Distinguish molecules that can diffuse through a lipid bilayer from those that require transporters to cross a membrane.
Small nonpolar molecules are soluble in the lipid bilayer, and can therefore easily cross cell membranes. Small uncharged polar molecules (H2O) can also, but larger uncharged polar molecules (glucose) cannot. Charged molecules (ions) cannot cross regardless of size. Those that cannot freely diffuse across the cell membrane require transport proteins.
What do channel proteins do?
Allow the free passage of any molecule of the appropriate size
What do carrier proteins do?
Selectively bind and transport specific small molecules (glucose)
Describe the transport of small molecules by carrier proteins
Carrier proteins bind specific small molecules on one side of the membrane, then undergo conformational changes that allow the molecule to pass through the membrane and be released on the other side.
Contrast ion channels and carrier proteins
Channels are just open pores through the membrane, that allow anything to pass through as long as it’s the right size. Carrier proteins bind to specific molecules and control their passage through the membrane.
Summarize the role of ion channels in transmission of nerve impulses
Transport through these channels is extremely rapid, highly selective, and not permanently available, because the opening of the channels is regulated. This regulated opening and closing is responsible for the transmission of electric signals. Because ions are electrically charged, their transport results in an electric gradient. As nerve impulses travel along axons, the membrane depolarizes due to the opening and closing of voltage-gated channels. The membrane potential then increases drastically, then returns to resting level.
Describe the action of the sodium/potassium pump
Powered by ATP, it moves three sodium ions out of a cell and two potassium ions into a cell (both against their concentration gradients), which establishes the resting membrane potential.
Explain how ion gradients across the plasma membrane can drive active transport
The Na+ gradient established by Na+ - K+ pump provides a source of energy that is frequently used to power the active transport of sugars, amino acids, and ions in mammalian cells. The H+ gradients established by the H+ pumps of bacteria, yeast, and plant cells play similar roles.
Why do most cells have a negative membrane potential?
Uneven ion concentrations on either side of the membrane
Differentiate between the four different types of channels
Leak channels: always open
Voltage-gated channels: open/close in response to a change in membrane potential
Ligand-gated channels: open/close when bound by a ligand
Mechanically-gated channels: open/close in response to a physical force on the membrane
Describe the structure of voltage-gated ion channels
six transmembrane alpha-helices, with 5 and 6 separated by a pore loop (P-loop). Helix #4 has several positively charged amino acids, which are attracted to the normal intracellular negative charge, maintaining the channel closed. If the membrane depolarizes, helix 4 moves away from the cytosol, opening the channel pore
What is the purpose of the P-loop?
it’s the selectivity filter: the size of it is designed to just match the diameter of the dehydrated ion
What are the three types of facilitator carrier proteins?
Uniporters (one substance down a concentration gradient), symporters (an ion and an organic molecule in similar direction), and antiporters (an ion and an organic molecule in different direction)
What is the meaning of the 12-spanning structural class?
In uniporter facilitators, the first 6 and second 6 helices form separate domains, with the ligand binding site between the domains; this design makes it so that it can naturally ‘flicker’ or ‘vibrate’ between the inward-open and outward-open states
What are the two types of carrier pumps?
ABC pumps: specialty pumps that move mixed organics and ions; can function as importer or exporter; have two transmembrane (T) domains, and two ATP-binding (A) domains.
Ion pumps: use energy to create ion gradients; P-type pumps become phosphorylated in the process of creating cation gradients; V-type pumps concentrate H+ in vacuoles and vesicles; F-type pumps run ‘in reverse,’ producing ATP in response to a pre-existing H+ gradient (also called ATP synthases).
