Transport Across Cell Membranes Pt. 1

Front 1

What is the job of membrane transport proteins?

Back 1

They are proteins that span the lipid bilayer, providing private passageways across the plasma membrane for select substances that otherwise wouldn't be able to cross the membrane.

Front 2

What is facilitated transport?

Back 2

A process that accelerates the passage of hydrophilic molecules across the cell membrane using specialized transport proteins.

Front 3

What protein-free, artificial lipid bilayers are impermeable to most water-soluble molecules? Why?

Back 3

Liposomes. This is because hydrophilic molecules can't diffuse passed the fatty acid part of the lipid bilayer.

Front 4

What are lipid bilayers typically impermeable to?

Back 4

Most ions and most uncharged polar molecules.

Front 5

True or false: in general, the larger the molecule and the more hydrophilic or polar it is, the more rapidly it will diffuse across the membrane.

Back 5

False. The smaller and the more hydrophobic or nonpolar the molecule is, the more rapidly it will diffuse across the membrane.

Front 6

What two general types of diffusion does selective transport facilitate?

Back 6

The passive diffusion of specific molecules or ions across the membrane.

The active pumping of specific substances either out of or into the cell.

Front 7

What does the combined action of different membrane transport proteins allow?

Back 7

It allows for the creation of very specific interior conditions of membrane-enclosed compartments such as the cell or an organelle.

Front 8

What is the rate at which each if the following crosses a protein-free artificial lipid bilayer by simple diffusion?

1. Small, nonpolar molecules

2. Small uncharged polar molecules

3. Larger uncharged polar molecules

4. Ions

Back 8

1. These dissolve readily in lipid bilayers and rapidly diffuse across them.

2. If they are small enough, they also readily diffuse, though less rapidly than small nonpolar molecules

3. Many organic molecules cells use are of this type, and they are often too large and polar to pass through efficiently without transport proteins.

4. Ions are charged and have a strong attraction to water, and therefore cannot simply diffuse through the lipid bilayer.

Front 9

Why are cells able to maintain internal ion concentrations that are very different from concentrations outside of the cell?

Back 9

Because cell membranes are impermeable to inorganic ions.

Front 10

What are the most important inorganic ions for cells?

Back 10

Na+ (Sodium), K+ (Potassium), Ca2+ (Calcium), Cl- (Chlorine), and H+ (hydrogen protons).

Front 11

What are the intracellular and extracellular concentrations of the following ions:

1. Na+, 2. K+, 3. Mg2+, 4. Ca2+, 5. H+, 6. Cl-

Back 11

1. 5-15mM inside. 145mM outside

2. 140mM inside. 5mM outside

3. 0.5mM inside. 1-2mM outside

4. 10^-4 inside. 1-2mM outside

5. 7x10^-5 (10^-7.2 or pH 7.2) inside. 4x10^-5 (10^-4 M or pH 7.4) outside

6. 5-15mM inside. 110mM outside.

Front 12

How do cells prevent themselves from being torn apart by electrical forces?

Back 12

The quantity if positive charge inside the cell must be balanced by an almost exactly equal quantity of negative charge, and the same is true for the charge in the surrounding fluid.

Ex: The high concentration of Na+ outside is balanced by the concentration of Cl- outside of the cell. The high concentration of K+ inside the cell is balanced by a variety of negatively charged organic and inorganic ions such as nucleic acids, proteins, and many cell metabolites.

Front 13

What is membrane potential?

Back 13

A voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other side.

Front 14

What is resting membrane potential?

Back 14

Occurs when a cell is stimulated and the exchange of anions and cations are precisely balanced. The voltage across the cell membrane will hold steady.

Front 15

Which is more negatively charged, the cell interior or exterior?

Back 15

The cell interior.

Front 16

What are the two classes of membrane transport proteins? Where are they present, and what do they do in general?

Back 16

1. Transporters

2. Channels

Present in all cell membranes and each provides a private portal across the membrane for a particular small, water-soluble organic molecules (sugar, amino acid) and small inorganic ions.

Front 17

How do channels discriminate between solutes? What about transporters?

Back 17

1. Channels mainly discriminate by size and electric charge. When the channel is open, any ion molecule or (sometimes) polar organic molecules small enough with the appropriate charge can pass through.

2. Transporters will only transfer molecules or ions that fit into specific binding sites in the protein. They are highly specific, just like enzymes and their substrates.

Front 18

True or false: channels can exist in either an open or closed conformation, and opening/closing is controlled by an external stimulus or by conditions within the cell.

Back 18

True.

Front 19

True or false: transporters don't require a series of conformational changes to transport small solutes across the lipid bilayer. They also transfer much faster than channels.

Back 19

False. They do require a series of conformational changes, and they transfer much slower than channels.

Front 20

In many cases, what does the direction of transport depend on?

Back 20

The relative concentrations of the solute on either side of the membrane.

Front 21

What is passive transport?

Back 21

Occurs spontaneously when solutes travel from a region of high concentration to a region of low concentration, provided a pathway exists, until equilibrium is reached. There is no expenditure of energy by the transport protein.

All channels and many transporters act as conduits for passive transport.

Front 22

What is active transport?

Back 22

The movement of a solute against its concentration (ie. From low to high). This type of transport can only be carried out by special transporters called pumps. They require some type of energy to power the process. (Can be ATP hydrolysis, a transmembrane ion gradient, or sunlight).

Front 23

What two things influence the passive transport of charged solutes?

Back 23

The concentration gradient and membrane potential.

Front 24

Why does the membrane potential tend to pull positively charged solutes into the cell and drive negative ones out?

Back 24

Because the cytosolic side of the plasma membrane is usually at a negative potential relative to the extracellular side.

Front 25

What is a solute's electrochemical gradient?

Back 25

The net force driving a solute across a cell membrane due to that solute's concentration gradient and the cell's membrane potential. Determines which direction each solute will passively transport across the cell membrane (in or out).

Front 26

What ion, for example, do the membrane potential and concentration gradient work in the same direction?

Back 26

Na+. There is more of it outside than inside the cell, and it is positive and therefore more attracted to the negative interior voltage of the cell.

Passive transport out of the cell.

Front 27

What ion, for example, does the membrane potential act against the concentration gradient?

Back 27

K+. There is more K+ in the cell than outside of it, but K+ wants to stay in the cell interior because it is attracted to the negative voltage.

Active transport out of the cell.

Front 28

What is osmosis?

Back 28

The movement of water across cell membranes down its concentration gradient from an area of low solute concentration to high solute concentration

Front 29

What is an aquaporin channel?

Back 29

Found in the plasma membrane of some cells, it allows selective passage of water molecules.

Front 30

How do different cells deal with osmotic swelling, which if unchecked would cause the cell to burst?

Back 30

1. Protozoans like amoebas use contractile vacuoles. The vacuole first accumulates solutes, which cause water to flow in by osmosis. Then, the amoeba uses the vacuole to eject the water.

2. Plant cells have tough cell walls that prevent swelling.

3. Animal cells have a gel-like cytoplasm that resists osmotic swelling. They also reduce the intracellular solute concentration by pumping out ions.

Front 31

In what direction do passive transporters move solutes? How? Give an example.

Back 31

They move solutes along their electrochemical gradient. Passive transporters do this by transitioning randomly between 3 conformational states, which are completely reversible and don't depend on whether the solute-binding site is occupied.

This means if solute concentration outside cell is high, solutes will bind to the transporter in the outward-open conformation more often.

Ex: the glucose transporter, which moves uncharged glucose down its concentration gradient.

Front 32

In general, how does the Na+ Pump in animal cells work?

Back 32

It uses ATP hydrolysis energy to expel Na+ and bring in K+ against their electrochemical gradients. This keeps cytosolic concentrations of Na+ low and of K+ high.

Front 33

What are the steps for the Na+ pump transport process?

Back 33

1. Na+ binds to the Na+ pump.

2. Phosphorylation of ATP of the cytosolic face of the pump occurs.

3. This phosphorylation triggers a conformational change, allowing Na+ to be ejected outside of the cell.

4. K+ binds to the pump, which is now open to the outside of the cell.

5. The pump dephosphorylates itself.

6. Pump returns to original conformation, and K+ is ejected into cytosol.

Front 34

How is each type of cell membrane able to carry out its unique functions?

Back 34

Each cell membrane has its own set of transporters.

Front 35

In what direction do pumps actively transport a solute? How?

Back 35

Against the solute's electrochemical gradient. Pumps carry out this process in 3 main ways:

1. Gradient-driven pump: link the uphill transport of one solute across the membrane to the downhill transport of another.

2. ATP-driven pump: hydrolize ATP to drive uphill transport. (Ex: the Na+/K+ ATPase pump)

3. Light-driven pump: found mainly in bacterial cells. Use energy from sun to drive uphill transport.

Front 36

How does the steep concentration gradient of Na+ across the plasma membrane provide energy for other processes?

Back 36

It is like water behind a dam. The Na+ concentration gradient wants Na+ to re-enter the cell. Like the water behind the dam, it has potential energy that can be used to drive active processes in a cell, including the active transport of other molecules across the plasma membrane.

Front 37

What does the Ca+ pump do? Where is it located? How does it work?

Back 37

Like the Na+ pump, it keeps the cytosolic Ca2+ concentration low. However Ca2+ is much less plentiful than Na+. These pumps are found in the sarcoplasmic reticulum of muscle cells.

1. Muscle cell is stimulated, causing Ca2+ to flood into the cytosol from the SR.

2. Ca2+ stimulates the cell to contract.

3. To recover, Ca2+ must be pumped back into the SR.

4. Pump uses ATP to phosphorylate itself, inducing a series of conformational changes similar to those of the Na+ pump.

5. When the pump opens to the SR lumen, the Ca2+ binding sites are eliminated, ejecting 2 Ca2+ ions into the SR.

Front 38

How do gradient-driven pumps exploit solute gradients to mediate active transport?

Back 38

They drive coupled pumps, meaning the transport of one solute helps facilitate the transport of another solute. They can act as symports (transport solutes in the same direction) or antiports (transport solutes in opposite directions).

Front 39

What is a uniport?

Back 39

They are gradient-driven transporters that only facilitate the movement of a solute down its concentration gradient passively. This doesn't require additional energy, so they are not considered pumps.

Front 40

How does the glucose-Na+ symport work?

Back 40

It uses the steep Na+ gradient to actively transport glucose into the cell. When open to the cytosol, the energy of the Na+ gradient carries a glucose molecule against its gradient into the pump. Both are then ejected into the cell cytosol.

The pump can only open or close when it is occluded-occupied by both or occluded-empty. Additionally, sometimes the pump will open back outward instead of inward. This happens because the conformational transformations are reversible.

Front 41

Besides the glucose-Na+ symport, what do gut epithelial cells need to properly transport glucose out of the small intestine, into the bloodstream, and then into the liver?

Back 41

1. The Na+/K+ ATPase pump to establish the Na+ and K+ gradients. Located on the basolateral domain of intestine epithelial cell.

2. A leaky K+ channel to trick the Na+/K+ ATPase into continuously pumping to create the steep gradient. Also located on the basolateral domain.

3. The glucose-Na+ symporter (obvs, yo).

4. A passive glucose uniport transporter on the opposite side of the cell, separated from the symporter in it's own domain by a tight junction.

The family of these receptors is called "gluts."

Front 42

What drives the transport of solutes in plants, fungi, and bacteria? How?

Back 42

Electrochemical H+ gradients, which function a lot like Animal Na+ electrochemical gradients.

An H+ ATPase pump use ATP hydrolysis to pump H+ out of the cell. An H+ driven symport uses that established gradient to bring solutes into the cell.

Front 43

What other organelles use the H+ gradient?

Back 43

In animals, lysosomes. In plants and fungi, vacuoles. They help to keep the internal environment of the organelles acidic.

Front 44

Study this table.

Back 44