Transport Across Cell Membranes Flashcards
What is the difference between passive and active transport? Examples?
Passive transport:
* Solutes move down their concentration gradient
- Without expenditure of energy
Active transport:
* Movement of solutes against its concentration gradient
- Requires input of energy
What is the difference between primary and secondary active transport? Examples?
Primary active transport - energy of ATP hydrolysis is used directly to drive transport of a solute against its gradient. (e.g., Na+ pump)
Secondary active transport - energy stored in electrochemical gradient of one solute is used to transport another solute (e.g., Na+-glucose symporter)
What is the purpose/function of a Na+-K+ pump? How does it work?
(1)
Keep cytosolic concentrations of Na+ low and K+ high
Contributes to polarized membrane potential (partially negative inside, partially positive outside)
(2)
Uses energy from ATP hydrolysis to transport Na+ out of cell as it carries K+ in
- Na+ binds to carrier
- ATPase causes transfer of phosphate (changes conformation)
- Na+ is released
- K+ on outside binds to pump
- Phosphatase (dephosphorylation) - converts pump back to Na+ confirmation -> spits out K+ and is able to accept more Na+
What is a neurotransmitter? How does it work?
Small signaling molecule secreted by a nerve cell at a synaptic to transmit information to a postsynaptic cell
(1) What is a resting potential? (2) What is a threshold potential? (don’t need exact #s here)
(1) voltage difference across plasma membrane when a cell is not stimulated (flow of positive and negative ions across plasma membrane is balanced)
(2) critical level of membrane depolarization that a neuron must reach to initiate an action potential
How is an action potential propagated?
Membrane becomes depolarized (membrane shifts to less negative value)
What are the roles of voltage-gated Na+, K+, and Ca2+ channels in neuronal signaling?
Voltage-gated Na+ channels:
- open in response to depolarization
- allow influx of positive charge
- further depolarizes membrane
- makes membrane potential even less negative
Voltage-gated K+ channels:
- return depolarized membrane to its resting potential
- K+ flows out of cell down their electrochemical gradient
Voltage-gated Ca2+ channels:
- allow Ca2+ to flow into nerve terminal
- Increase in Ca2+ concentration triggers fusion of synaptic vesicles with plasma membrane -> releases neurotransmitter into synaptic cleft
What are the roles of ligand-gated Na+ and Cl- channels in neuronal signaling?
Ligand-gated Na+ channels:
- allows influx of Na+
- activates postsynaptic cell (depolarizes membrane)
- promotes excitatory effect
Ligand-gated Cl- channels:
-allows influx of Cl- to enter cell
-inhibits postsynaptic cell (makes membrane harder to depolarize)
- promotes inhibitory effect
How is glucose transported across the gut epithelium?
Glucose is pumped into cell through apical domain by glucose-Na+ symporter (use the energy
of the Na+ gradient to actively import glucose, creating a high concentration of the sugar in the cytosol)
Glucose passes out of cell (down concentration gradient) by passive movement through glucose uniporter in basal and lateral domains
What is the difference between a transporter and a channel?
Transporter:
* Undergoes conformational changes to transfer small solutes
* Lock-and-key
* One-to-one
Channel:
* Does not need to undergo conformation changes
* Form pores that allow specific solutes to pass through
Compare the ions concentration inside and outside a cell.
Na+ = high concentration (extracellular); low concentration (intracellular)
K+ = low concentration (extracellular); high concentration (intracellular)
Ca2+ = high concentration (extracellular); low concentration (intracellular)
What are aquaporins?
Channels that facilitate transport of water across cell membranes (i.e., allows selective passage of water molecules)
Compare the three types of tonicity.
- Hypotonic - lower solute concentration compared to intracellular fluid
- Isotonic - same solute concentration as intracellular fluid
- Hypertonic - higher solute concentration compared to intracellular fluid
What are the three ways pumps can carry out active transport?
- Gradient-drive pumps - link uphill transport of one solute to downhill transport of another
- ATP-driven pumps - use energy released by ATP hydrolysis to drive uphill transport
- Light-driven pumps - use energy derived from sunlight to drive uphill transport
What are some examples of transmembrane pumps? Explain the function of each.
- Na+ pump - active export of Na+ and import of K+
- Ca2+ pump = active export of Ca2+
- H+ pump - active export of H+
Ca2+ pumps functions. Location
(1) Actively remove Ca2+ from cytosol
(2) Plasma membrane and endoplasmic reticulum
Compare uniport, symport, and anitport.
- Uniporter - facilitate passive movement of a single solute
- Symporter - pump moves a pair of solutes in same direction
- Antiporter - pump moves a pair of solutes in opposite direction
Provide and explain an example of an antiport.
Na+-H+ exchanger:
* Downhill influx of Na+ to pump H+ out of cell
?
- Mechanically-gated channel - channel is opened by application of physical force
- Ligand-gated channel - channel is opened by binding of a molecule (intracellular or extracellular ligand)
- Voltage-gated channel - opening is controlled by membrane potential
The probability of gate opening is controlled by what three factors?
- Mechanical stress
- Binding of a chemical ligand
- Change in voltage difference across membrane
What are the main parts of a neuron?
- Cell body - contains nucleus
- Axon - conducts electrical signals away from cell body toward its target cells
- Dendrites - radiate from cell body and receive signals from axons of other neurons
What would happen if gut epithelial cells only had one or the other glucose transporter?
Only symport - glucose would be taken up and never released for use by other cells
Only uniport - glucose would be released into gut lumen after fasting as freely as it is taken up after a meal
Epithelials cells have two different membrane domains. What keeps two types of transporters separate?
Tight junctions
Why does the action potential not go in reverse?
Refractory period (channel is in inactivated conformation/state)
Which direction through the channel do the K+ ions move?
Outside of cell
How is the resting concentration of Ca2+ in the nerve terminal restored post- transmission of the signal?
Ca2+ pump
What are two ways animal cells can increase pH in cytoplasm?
- Pump into lysosome
- Drag out protons with an influx of sodium
H2O associated with ions must be temporarily stripped off (Why?)
Water would disrupt and put restrictions on size
A lot of water - would transport all water
Throws off balance
What is membrane potential?
Difference in electrical charge on two sides of a membrane
What is depolarization?
Membrane potential shifts to a less negative value inside
Compare excitatory and inhibitory neurotransmitters. Provide examples.
Excitatory neurotransmitters:
- Open Na+ channels (depolarizes postsynaptic membrane)
- Ex: acetylcholine, glutamate
Inhibitory neurotransmitters:
- Open Cl- channels (makes it harder to depolarize postsynaptic membrane)
- Ex: GABA, glycine
What are synaptic vesicles?
Vesicles that store neurotransmitters
Explain the different conformations of voltage-gated Na+ channels.
Membrane is at rest and highly polarized = closed conformation: positive charged amino acids in voltage sensors are exposed to negative charges on cytosolic side of membrane (negative)
Membrane becomes depolarized = open conformation: voltage sensors twist –> sensors’ positive charges remain exposed to the negative charges, which are now concentrated on the extracellular surface of the membrane
What contributes to a polarized membrane potential (partially negative inside, partially positive outside)?
Na+-K+ pump
Low cytosolic concentration of Ca2+ is important for what processes?
- Cell signaling leading to muscle contractions
- Fertilization (prevent polyspermy)
- Nerve cell communication)
Besides ions, how are Ca2+ pumps different from Na+ pumps?
Na+ pump:
- antiporter
