Chapter 12 Flashcards
Plants do not have a Na+-K+ pump. So how do plants concentrate K+ inside the cell and do the work of transporting amino acids and sugars into cells?
Plants, like fungi and many prokaryotes, use H+ pumps; hydrolysis of ATP is coupled to moving protons out of the cytosol leaving negative charge inside and excess positive charge at the outside surface of the membrane.
The proton electrochemical gradient can then be used to do the work of otherwise unfavorable uptake of metabolites and other ions, such as K+. Similar to animals, K+ is concentrated in the cytosol so that flickering of K+-leak channels allows some K+ to leak back out of the cell and helps set the resting membrane potential.
State how passive transport is different from active transport.
Passive transport requires no input of energy. The molecule being transported moves from the side of higher concentration to the side of lower concentration. Passive transport can be simple diffusion if the molecule diffuses across the lipid bilayer without a protein, or can be facilitated diffusion if the molecule crosses the membrane by way of a protein, which in general is called a transporter or ion channel.
Active transport requires an input of energy. A protein, in general called a pump, couples the hydrolysis of ATP to moving either sodium or protons across the membrane, which then results in an electrochemcial gradient of the sodium or protons. Active transport that utilizes ATP is called primary active transport.
Secondary active transport involves a protein that in general is called a coupled pump. The coupled pump couples the favorable flow of sodium or protons down their electrochemical gradient to moving a second molecule across the membrane against its gradient.
Describe how a cell uses its membranes for energy conversion and conservation.
A cell’s membrane can ‘hold’ an electrochemical gradient and a cell can use the gradient to drive ATP synthesis, thereby converting the gradient energy into chemical bond energy. Many different proteins can use ATP to do work, so the energy is conserved.
A cell can do the reverse that is use hydrolysis of ATP to create an electrochemical gradient, converting chemical bond energy into energy of the electrochemical gradient. Many different coupled pumps can use the electrochemical gradient to do the work of secondary active transport, so energy is conserved.
In animal cells, how do the Na+-K+ pumps and K+-leak channels set the resting membrane potential?
Na+-K+ pumps are used to concentrate K+ inside the animal cell, which is in turn used to set the resting plasma membrane potential: K+-leak channels flicker open/close allowing K+ to flow out leaving negative charge behind at the cytosolic surface and excess positive charge at the outside membrane surface. The difference in charge is the membrane potential in units of millivolts.
Correct or Incorrect
The plasma membrane is highly impermeable to all charged molecules.
Correct, the nonpolar interior of the membrane makes it energetically unfavorable for charged molecules to leave their hydrated states and enter the interior where there are no attractive surfaces, just nonpolar surfaces.
Correct or Incorrect
Channels must first bind to solute molecules before they can select those that they allow to pass.
Incorrect, channel proteins do NOT bind solute. Channel proteins do have a pore of a certain size and polarity that permits only an ion of the right size and polarity to pass through the central pore.
Correct or Incorrect
Transporters allow solutes to cross a membrane at much faster rates than do channels.
Incorrect. Because transporters have to bind solute and change conformation, it takes longer for the solute to cross the membrane. In contrast, once a channel protein opens its pore, solute (ions) can pass straight through.
Correct or Incorrect
The plasma membrane of many animal cells contains open K+ channels, yet the K+ concentration in the cytosol is much higher than outside the cell.
Correct. The Na+ K+ pump moves K+ into the cell. However, there are open K+- channels, so some K+ does diffuse back out, but the channel proteins prevent anion from diffusing with the K+. Only a small amount of K+ has to diffuse out before a voltage is generated - positive outside and negative inside. The electric field prevents more K+ from flowing out.
List the following compounds in order of increasing lipid bilayer permeability: RNA, Ca2+, glucose, ethanol, N2, water.
N2, water ~ ethanol, glucose, Ca2+ ~ RNA
Consider an animal cell using a symport in the plasma membrane to take up amino acids.
What is the most likely ion whose electrochemical gradient drives the import?
Na+. Animal cells have Na+K+ pumps at their plasma membrane. The pumps hydrolyze ATP, which causes conformational changes needed to move 3 Na+ out and 2 K+ in. The result is an electrochemical gradient of Na+ that favors flow of Na+ back into the cell: much more Na+ at the outside surface of the membrane than the inside surface, and attraction to the excess negative charge at the inside surface.
Consider an animal cell using a symport in the plasma membrane to take up amino acids.
Is ATP consumed in the process? If so, how?
Not by the symport, which uses the favorable flow of Na+ into the cell as the needed energy for the unfavorable uptake of amino acids. But ATP is needed by the Na+ K+ pump to generate the Na+ electrochemical gradient.
Why do Na+-K+ pumps concentrate K+ inside animal cells?
Na+-K+ pumps are needed to concentrate K+ inside animal cells which is in turn used to set the resting plasma membrane potential: K+-leak channels flicker open/close allowing K+ to flow out leaving negative charge behind at the cytosolic surface of the membrane.
Why do Na+-K+ pumps prevent NA+ from accumulating inside animal cells?
Na+-K+-pumps are also needed to prevent Na+ from accumulating in the cell, which otherwise would result in cell swelling and rupture as too much water moves in by osmosis.
Why do Na+-K+ pumps convert ATP energy into the free-energy available in a NA+ electrochemical gradient?
Na+-K+-pumps are also needed to convert ATP energy into the free-energy available in a Na+ electrochemical gradient , which is then used to drive uptake of a variety of metabolites (sugars, amino acids . . . .) by co-transport with Na+ down its gradient. Co- transport is called secondary active transport and the transport proteins are either symports or antiports.
