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
What is an action potential?
An action potential propagates along the length of an axon. The changes in the Na+ channels and the consequent flow of Na+ into the axon alters the membrane potential and gives rise to the traveling action potential
3 steps to voltage gated Na+ channels
- open in response to depolarization( more positive inside cell)
- inactivate
-closed
Shortly after Na+ channels open, depolarization triggers the opening of voltage-gated K+ channels, allowing K+ to flow out of the axon, and returning the membrane to its resting potential.
Depolarization of neuron’s plasma membrane
- how action potential starts
- depolarizing stimulus is sufficient to open voltage-gated Na+ channels in the membrane and thereby trigger an action potential
- If there had been no amplification by voltage-gated ion channels in the plasma membrane, the membrane potential would simply have relaxed back to the resting value after the initial depolarizing stimulus (need certain amount to meet stimulus)
- without voltage gated Na+ channels, no action potentials
Describe Imaging technique that tracks glucose metabolism reveals activation of different brain regions.
In this noninvasive scan, a radioactive tracer is used to measure the metabolic activity in different parts of the brain. The areas with the highest activity are colored red. The subject in the scanner is either resting quietly, looking at a picture (seeing), counting backward from 100 by 7s (thinking), or listening to a story (hearing). Much of the energy consumed by the brain as it engages in such tasks is used to power the Na+ pumps that restore the membrane potential in neurons involved in vigorous electrical signaling
Why does the action potential only travel in one direction down the axon?
- away from the site of depolarization
- This is because Na+ channel inactivation in the aftermath of an action potential prevents the advancing front of depolarization from spreading backward
Inactive Na+ channel
When the membrane is at rest and highly polarized, positively charged amino acids in the voltage sensors of the channel (blue bars) are oriented so they remain exposed to the negative charges on the cytosolic side of the membrane; this orientation, stabilized by the interacting charges, keeps the channel in its closed conformation.
Active Na+ channel
When the membrane becomes depolarized, the voltage sensors twist so that the sensors’ positive charges remain exposed to the negative charges, which are now concentrated on the extracellular surface of the membrane; this realignment of the sensors alters the channel’s conformation so that the channel has a high probability of opening. However, in a depolarized membrane, the inactivated conformation of the channel is even more stable than the open conformation, and so, after a brief period, the channel becomes temporarily inactivated and cannot open.
Synapse
- where neurons connect to next target cell
Neurotransmitters carry the signal across the synaptic cleft that separates the presynaptic and postsynaptic cells. The neurotransmitter in the presynaptic terminal is contained within synaptic vesicles, which release neurotransmitters into the synaptic cleft. Note that both the presynaptic and postsynaptic membranes are thickened and highly specialized at the synapse.
Describe how an electrical signal is converted into a secreted chemical signal at a nerve terminal
When an action potential reaches a nerve terminal, it opens voltage-gated Ca2+ channels in the plasma membrane, allowing Ca2+ to flow into the terminal. The increased Ca2+ in the nerve terminal stimulates the synaptic vesicles to fuse with the plasma membrane, releasing their neurotransmitter into the synaptic cleft—a process called exocytosis
Describe how a chemical signal is converted into an electrical signal
- by postsynaptic transmitter-gated ion channels at a synapse.
- The released neurotransmitter binds to and opens the transmitter-gated ion channels in the plasma membrane of the postsynaptic cell. The resulting ion flows alter the membrane potential of the postsynaptic cell, thereby converting the chemical signal back into an electrical one
Provide a summary of Ion Channels and Nerve cell signaling
-Nerve Cell Signaling is Initiated by Transmitter-gated Ion Channels on Dendrites or Cell Body
-Action Potentials Are Mediated by Voltage-gated Na+ Channels on Axon
-Voltage-gated Ca2+ Channels in Nerve Terminals Convert an Electrical Signal into a Chemical Signal
-Transmitter-gated Ion Channels in the Postsynaptic Membrane Convert the Chemical Signal Back into an Electrical Signal
-Neurotransmitters Can Be Excitatory or Inhibitory
Optogenetics
- exploiting properties of ion channels for research
Describe how light-gated ion channels can control the activity of specific neurons in a living animal.
In this experiment, the gene encoding channelrhodopsin was introduced into a subset of neurons in the mouse hypothalamus.
- When the neurons are exposed to blue light using a tiny fiber-optic cable implanted into the animal’s brain, channelrhodopsin opens, depolarizing and stimulating the channel-containing neurons.
- When the light is switched on, the mouse immediately becomes aggressive; when the light is switched off, its behavior immediately returns to normal
