Section 5: Membrane Transport

Question 1

What is less permeable with regards to charge, polarity, size?

Answer 1

More charged or polar and larger is less permeable

Question 2

Calcium, potassium, and sodium in and outside cells

Answer 2

• There’s more potassium inside cell than outside cell.
• There’s less calcium and sodium inside cell than outside.

Question 3

Channels

Answer 3

• Form a continuous hole in the membrane.
• Channels must be OPEN or CLOSED.
• There need to be gating mechanism (a switch).
• Channels support faster transport.

Question 4

Carriers/permeases

Answer 4

• Passive or active.
• Don’t form continuous hole in membrane are transporters.
• Proteins w two different conformations & both are same energy.
• Delta G of one state vs the other is the same.

Question 5

Passive Transport of charged solutes

Answer 5

• Transporters have two conformations open to diff sides.
• Switching between states doesnt depend on solute concentration
• Movement of solute simply depends on electrochemical gradient

Question 6

Electrochemical gradient

Answer 6

The net driving force that consists of concentration and electric potential difference.
Sum of concentration gradient and electrical charges (relative amount of ions on either side).

Question 7

Electric potential difference

Answer 7

• Movement of a charged molecule (by facilitated diffusion) follows the electrochemical gradient
• The electric potential difference across a membrane is called the membrane potential.

Question 8

Three mechanisms used by transporters for active transport

Answer 8

1. Coupled transporter: electrochemical gradient of other molecule.
2. ATP-driven pump: ATP hydrolysis drives unfavorable reaction.
   * Enzyme that can facilitate movement of things against concentration gradient.
   * But overall is favorable because it’s coupled with favorable hydrolysis of ATP.
3. Light-driven pump: Bacteriorhodopsin in section 3C.

Question 9

The Coupled-Transporter Pump Working Mechanism

Answer 9

• A coupled-transporter joins movement of one solute species (A) against its gradient with movement of another solute species (B) that diffuses along its gradient.
• Therefore: Species A is actively transported and Species B is passively diffused. Species B is the source of the electro- chemical potential energy.

Question 10

Symporter and Antiporter

Answer 10

Symporter: Transport of molecule A (unfavorable) coupled to transport of molecule B (favorable) IN THE SAME DIRECTION.

Antiporter: Transport of molecule A (unfavorable) coupled to a transport of molecule B (favorable) IN OPPOSITE DIRECTION.

Question 11

Plasma membrane sodium-glucose symporter

Answer 11

• The Na+-glucose symporter is found in the plasma membrane of epithelial cells in kidney and intestines.
• Function: recover glucose from extracellular before excretion.
• However, [Glucose]cytosol&raquo_space; [Glucose]extracellular
• To transport glucose against this concentration gradient, cells use the strong electrochemical gradient of Na+:
  [Na+]extracellular&raquo_space; [Na+]cytosol

Question 12

Plasma membrane sodium-glucose symporter steps

Answer 12

• Binding of Na+ causes a conformational change in transport protein.
• This conformational change leads to tighter binding of glucose.
• Once the transport protein “flips” its confirmation to face inward, the binding sites are of low affinity for Na+ and glucose, so these are both “dumped” in the cytosol.
• This makes sure that during switch from inward to outward facing, the binding sites for glucose and Na+ are empty, while in the “switch” from outward to inward, the sites are occupied.

Question 13

Asymmetric distribution of transporters in epithelial cells

Answer 13

It allows for transcellular transport of molecules.
* Na+/glucose transport occurs at apical side (active transport)
* Glucose permeation (by passive transport) on the basal side allows glucose to leave cells and go into deeper tissues
* Over time Na+ enters cell & builds up inside cell except that
* Na+/K+-pump: reestablishes Na+ gradient
* Na+/K+-pump occurs on the basal side to avoid loss of electrolytes (sodium) in the urine/feces.

Question 14

ATP-Driven Pumps (Transport ATPases)

Answer 14

Membrane transport enzymes that couple the energy released by ATP hydrolysis to drive transport of solutes against their electro-chemical gradient.
* Primary active transport is consuming ATP.
* Secondary active transport uses a gradient (doesnt directly consume ATP).

Question 15

Three types of ATP-Driven Pumps

Answer 15

1. P-type pump
2. ABC Transporter
3. ​​V-type H+ pumps

Question 16

ABC transportters

Answer 16

• Uses ATP hydrolysis to pump small molecules across membrane.
• Does not get phosphorylated.

Question 17

V-type and F-type H+ pumps

Answer 17

F-type H+ pumps:
* These are multi-subunit, turbine-like complexes found in bacteria, mitochondria and chloroplasts.
* use H+ gradients to synthesize ATP (not active transport).
V-type H+ pumps:
* related to F-type but use ATP hydrolysis to pump H+ against electro-concentration gradients.
* acidifies organelles (falls into category).

Question 18

The P-type Na+-K+ ATPase Pump STEPS

Answer 18

1: Inward-facing conformation: TWO K+ dissociate (on inside), which allows binding of THREE Na+ (on inside).

2: ATP binds and gets hydrolyzed to ADP+Pi, which causes a conformation change in the protein (to the outward-facing conformation), the three Na+ now “flipped” to outside

3: Outward-facing conformation: The three Na+ dissociate (on the outside), which allows binding of 2 K+ (on the outside)

4: Pi (inorganic phosphate) removed from pump, which causes conformational change (to in conformation), 2 K+ now on inside.

Question 19

Osmosis, hypertonic, and hypotonic

Answer 19

Movement of water across a semipermeable membrane due to a difference in water concentration.
Hypertonic: More solutes outside cell than inside.
Water moves out of cell and cell shrinks.
Hypotonic: Less solutes outside cell than inside.
Water move in; create outward pressure & cell swells.

Question 20

Why is inside the cell a high osmolarity? Solution

Answer 20

Macromolecules are charged & require counterions to balance charge, causes a lot of solutes inside (hypoosmolarity).
P-type sodium-potassium pump controls osmolarity
* The Na+ electrochemical gradient leaves Cl- & balances the intracellular & extracellular solute (balances water movement).
Cell doesn’t let chloride out unless it is faced w hyperosmolarity.

Question 21

Ion channels

Answer 21

• Ion channels form a narrow aqueous pore
• Passage is highly selective for a single ion type
• Selection accomplished by a selectivity filter found in pore
• Aqueous pore can be closed or open: gated
• Movement is from high to low electrochemical gradient.
• Rate of ion flow can be 105x faster than known transporter.

Question 22

Gating mechanisms of ion channels

Answer 22

• Voltage-gated: responds to changes in membrane potential.
• Change in distribution of + or - charges across membrane.
• Ligand-gated: binds to ion channel causing it to open.
• Mechanically-gated: gated by pushing and pulling forces.

Question 23

Functions of channels

Answer 23

• Regulate propagation of electrical signals in neurons
• Regulate neuron-to-neuron communication
• Muscle contraction and senses (hearing, touch)
• Epithelial function (intestine, respiratory tract, etc)
  *Leaf-closing response
  *Even single cell organisms like reversing direction of Paramecium

Question 24

How Cells Generate Membrane Potential

Answer 24

1. Sodium/potassium pump (active transport).
2. Passive transport of potassium (K+ leak).

Question 25

How Cells Generate Membrane Potential IN DEPTH

Answer 25

*Animal cells typically have a negative plasma membrane potential of - 20 to -120 mV. *Due to a K+ leak from cytosol (high K+) to outside (low K+). *Leak is allowed by K+ leak channels in plasma membrane: K+ moves with its chemical gradient by P-type Na+-K+ Antiporter. *As K+ leaks, the cell becomes increasingly negative, creating an electrical field. *This electrical field eventually prevents further net movement of K+ despite there still being high [K+] in the cytosol relative to outside of the cell. *The net movement is a balance between electrical and chemical concentration gradients. *The resting membrane potential is the equilibrium condition when there is no net flow of ions.

Question 26

How a K+ Channel select K+ and exclude Na+

Answer 26

* Negative charges on cytosolic side allows +ve ions to pass. * Vestibule is a hydrating area (H2O bound ions). * Pore helix also selects for +ve ions. Selectivity filter allows only K+ to pass * Carbonyl oxygen has partial negative: displaces interactions of K+ with water, which allows K+ to pass. But K+ is larger than Na+, so why does Na+ not also pass? * The 4 carbonyl oxygens in the pore can substitute 4 water molecules that interact with K+ but not with Na+. * Selectivity of K over Na+ is based on complete dehydration of K+.

Question 27

The Action Potential, depolarization, and what it's controlled by

Answer 27

* Short, local depolarization of the plasma membrane that migrates along the neuronal axon: this is triggered event. * Depolarization refers to a shift towards a more positive or neutral membrane potential * Action potential controlled by voltage-gated cation channels.

Question 28

Electrochemical gradients before, during, and just after action potential

Answer 28

* Before action potential: Lots of Na+ outside (little inside) Lots of K+ inside (little outside) * During action potential: Begins with Na+ rushing in (Na+ channels opening) *Just after action potential: K+ net transport out cell (delayed K+ channel opening).

Question 29

Membrane Depolarization

Answer 29

* A is a stimulating current that happens for a short time. * B is when the sodium channel senses this change and causes it to change membrane potential causing depolarization. * Diagram C is little piece of the protein that sticks into cytosol and moves out of way when channel open allowing sodium in. * Potassium channels open to let potassium rush out making cell negative again.

Question 30

Membrane Repolarization

Answer 30

Membrane polarity is reestablished by: i) Voltage-gated Na+ channel inactivation. ii) Delayed voltage-gated K+ channels to let K+ out and offset Na+ gain in the cell.

Question 31

The Myelin Sheath, two types of glial cells, and diseases caused by lack of it

Answer 31

Layer surrounding axons to protect them, made by glial cells. * Axons are tightly wrapped by the plasma membrane of another cell: the glial cell. * Prevents flow of sodium and potassium in and out of cell. * Leaves nodes of ranvier that aren’t covered by myelin sheath. * Deficiency in myelination causes Multiple Sclerosis. * Glial Cell Types: Schwann cells enclose peripheral nerves & oligodendrocytes enclose nerves of central nervous system.

Question 32

Nodes of Ranvier and saltatory conduction

Answer 32

* Myelin sheath interrupted by spaces called nodes of Ranvier. * Axon membrane is exposed and voltage-gated Na+ channels have access to Na+ in the extracellular medium * The depolarization induced at one node is sufficient to cause depolarization at another node further down the axon. * Action potential jumps: called saltatory conduction. * Increases the speed of transmission and reduces energy expenditure.

Question 33

The neuronal synapse

Answer 33

* Neurons form contact sites with other neurons called synapses. * Permits unidirectional chemical communication between cells. * Chemicals are neurotransmitters enriched in secretory vesicles. Signal goes from axon terminus to dendrite of next neuron.

Question 34

What controls neuron to neuron signaling? How?

Answer 34

Transmitter-Gated Channels. * Action potential along presynaptic neuron causes release of neurotransmitters into the synaptic space. * Neurotransmitters diffuse and bind to transmitter-gated channels on post- synaptic neuron. * Channels open to create a new action potential. * Neurotransmitters are destroyed or recycled to reset synapse.

Question 35

How Muscles are Commanded OVERVIEW

Answer 35

- The transmitter-gated channels control neuromuscular signaling. - Motor neurons get close to muscle cells to release neurotransmitters that tell muscle cell to contract. - The Acetylcholine Receptor: Transmitter-gated cation channel that contro muscle contraction. Channel normally closed but can bind to acetylcholine to open.

Question 36

Muscle Contraction Steps: Five ion channels involved

Answer 36

1. Action potential activates voltage-gated Ca+2 channels to open in presynaptic neuron (wave of depolarization). Ca+2 causes secretion of acetylcholine. 2. Acetylcholine binds and opens acetylcholine receptors, causing Na+ to enter and depolarize muscle cell. ​​3. Nearby voltage-gated Na+ channels open to further depolarize and spread action potential in muscle cell. 4. Plasma membrane voltage-gated Ca+2 channels open allowing Ca+2 to enter cells. 5. Sarcoplasmic reticulum voltage-gated Ca+2 channels open to release stored Ca+2 into cytosol. 6. Very large spike in cytosolic Ca+2 causes muscles to contract. Whole goal is to get calcium inside the cell.