Chapter 12: Transport across cell membranes Flashcards
Name the two main classes of membrane proteins which mediated transfer of molecules across lipid bilayers
transporters and channels (proteins!)
Transporters
- moving parts transport molecules
- undergo series of conformational changes to transfer small solutes across the lipid bilayer
- very selective for solute they bind, and transfer them at much slower rate than channels
Channels
- hydrophilic pore allowing passive transmembrane movement
- forms a pore across the bilayer through which specific inorganic ions (or in some cases polar organic molecules) can diffus
Simple diffusion: solute movement
- rate at which solute crosses protein-free, artificial lipid bilayer by simple diffusion varies
- depends on size and solubility
- small, nonpolar molecules mostly pass through
- the chances of permeability through bilayer decreases as molecules become larger, uncharged
- many organic molecules that are cell nurtients are too large and polar to pass through lipid bilayer without membrane transport proteins
Why is flow of ions across membranes necessary?
- necessary for cellular processes
- mito electron transport
- electrical properties of membranes and action potentials by neurons
Ion channels
- involved in setting up membrane potential, electrical excitability of cells
- transport inorganic cells Na+, K+, Ca2+, Cl-
- can exist in either opened or closed formation
- transport only in open formation
- opening and closing of channel often controlled by external stimulus or conditions within the cell
How do solutes cross membranes?
passive or active transport
Passive transport
- move down the concentration gradient, requires NO energy (high to low conc.)
- all channels and many transporters allow molecules to cross membrane only passively
-concentration gradient drives passive transport and determines direction - simple diffusion across lipid bilayer
Active transport
- move up/against concentration gradient, requires energy input (low to high conc.)
- energy either ATP hydrolysis or ion gradient
- always mediated by transporters, pump molecules against concentration gradient r electrochemical gradient
Channels vs transporters in active and passive transport
- only ion channels used in passive transport
- transporters used in passive and active transport
Name an example of a transporter
- Na+ pump
- its is located in animal cells and uses energy supplied by ATP to expel Na+ and bring in K+
- Na+/K+ ATPase sets up Na+ and K+ gradient
K+
- typically 10-30 times higher inside cells than outside
Na+
- 10-30 times higher outside cells than inside
Name components of electrochemical gradient
- force from concentration gradient of solute and force from membrane potential
- concentration gradient and membrane potential work together to increase driving force for movement of solute
- magnitude greater when gradients work together in same direction
Na+/K+ electrochemical gradient example
- for both, the membrane potential acts against the concentration gradient, decreasing the electrochemical driving force
Importance of Na+K+ATPase
- establishes Na+ gradient across plasma membrane
- K 10-30 times higher inside cells
- Na 10-30 times higher outside cell
- Na+ pump uses energy of ATP hydrolysis to pump out Na+ and keep K+ in (keeps cytosolic concentrations of Na+ low and K+ high
Na gradietn required to transport nutrients into cells and plays crucial role in regulating cytosolic pH
Name the two types of glucose transport
the glucose transporter proteins (GLUTs) that transport glucose through facilitative diffusion (a form of passive transport), and sodium-dependent glucose transporters (SGLTs) that use an energy-coupled mechanism (active transport)
What K+ processes play major parts in the resting membrane potential?
- the K+ concentration gradient and K+ leak channels
- when the K+ leak channels are closed, the membrane potential is zero
- K+ will leave when channel is open; assuming other channels unavailable, K+ will leave but negative ions unable to leave (this causes membrane potential to be created, driving K+ back into cell)
- at equilibrium, K+ concentration gradients balance the membrane potential; there’s no net change in K+ across membrane
Membrane potential
negatively charged start in cells, mostly due to ion channels
What influences passive transport of charged solutes?
- concentration gradient and membrane potential
- called electrochemical gradient!
What controls gated channels?
- a change in the voltage difference across the membrane,
- the binding of a chemical ligand to the extracellular face of a channel
- ligand binding to the intracellular face of a channel, or - mechanical stress.
Example of voltage gated channel
In the case of the voltage-gated channels, positively charged amino acids (white plus signs) in the channel’s voltage sensor domains become attracted to negative charges on the extracellular surface of the depolarized plasma membrane, pulling the channel into its open conformation.
Example of mechanical gated ion channel
The leaves snap shut in less than half a second when an insect brushes against them. The response is triggered by touching any two of the three trigger hairs in succession in the center of each leaf. This mechanical stimulation opens ion channels in the plasma membrane and thereby sets off an electrical signal, which leads to a rapid change in turgor pressure that closes the leaf.
3 steps of ion channel involvement in neurotransmission
Ligand gated ion channels
voltage Na+ gated channels
voltage gated Ca++ channels