Solute transport

Question 1

Solutes

Answer 1

Describes anything that is dissolved in aqueous solution. This can include hydrophobic molecules, small and large uncharged polar molecules, and ions

Question 2

How are hydrophobic molecules transported across the membrane?

Answer 2

Small and nonpolar molecules diffuse readily across the membrane. This includes oxygen, carbon dioxide, nitrogen, and steroid hormones

Question 3

How are small, uncharged polar molecules transported across the membrane?

Answer 3

Small uncharged molecules will diffuse across the membrane, but much more slowly. This includes water, urea, and glycerol

Question 4

How are large, uncharged polar molecules transported across the membrane?

Answer 4

These molecules will diffuse, but very slowly. This includes glucose and sucrose

Question 5

How are ions transported across the membrane?

Answer 5

No matter the size, ions will not freely diffuse due to their charge and degree of hydration. They require help to cross the membrane

Question 6

Membrane transport proteins

Answer 6

Create hydrophilic regions through which polar molecules can travel. This includes ions, sugars, amino acids, nucleotides, metabolites. Membrane transport proteins include transporters and channels

Question 7

Membrane transporters function

Answer 7

Protein complexes that bind a solute and undergo conformational changes to allow transfer across the membrane. Each has multiple solute binding sites, and solutes need to bind to all of the sites (saturate). The transporter only undergoes conformational changes under saturation. They have functions in both passive and active transport

Question 8

Channels

Answer 8

Water filled pores, interact with solutes more weakly

Question 9

Types of membrane transport (2)

Answer 9

1. Passive transport (facilitated diffusion)
2. Active transport

Question 10

Passive transport (facilitated diffusion)

Answer 10

Movement of solutes down their concentration gradient, which does not require energy. If the solute is charged, the concentration and electrical gradient drive transport (electrochemical gradient)

Question 11

Membrane potential

Answer 11

The electrochemical gradient of the cell- usually negative on the inside of the cell and positive on the outside. It favors the entry of positive ions

Question 12

Active transport

Answer 12

Transport against the concentration gradient, which is facilitated by transporters or pumps. It requires energy, which is provided by ATP hydrolysis or an ion gradient

Question 13

Passive transport via a transporter

Answer 13

Occurs when the solute is moving down its concentration gradient. The solute needs to bind to all of the transporter’s binding sites (saturation). Once the solute is bound, the transporter undergoes a conformational change, which lets the solute into the cell. In this case, transport is driven passively by the concentration gradient

Question 14

3 types of active transporters

Answer 14

1. Coupled transporters
2. ATP driven pumps
3. Light driven pumps

Question 15

Coupled transporters

Answer 15

Occurs when one solute needs to move down its concentration gradient and one needs to move against its concentration gradient, which requires energy. In this case, the energy comes from the solute being transported down its concentration gradient. As it moves down the concentration gradient, it releases energy, and the energy is harnessed for the active transport phase. The solute providing energy is usually an ion, like sodium or hydrogen. In prokaryotic cells, hydrogen is usually a co-transported ion moving down its gradient

Question 16

ATP driven pumps

Answer 16

Work through ATP hydrolysis

Question 17

Light driven pumps

Answer 17

Active in photosynthetic cells

Question 18

Uniport

Answer 18

A transporter that carries only one type of solute

Question 19

Symport

Answer 19

A coupled transport where both solutes are moving in the same direction across the membrane

Question 20

Antiport

Answer 20

A coupled transport where the solutes are moving in opposite directions across the membrane

Question 21

Lactose permease symporter

Answer 21

Bacterial transporter which transports lactose across the E. coli membrane against its concentration gradient. It is made of 12 loosely packed alpha helices. The helices slide and tilt, exposing a binding site on either side of the membrane. Both lactose and a hydrogen ion bind. Once the binding sites are saturated, a conformational change can occur, and lactose and the H+ are transported across the membrane together

Question 22

Lactose permease symporter mechanism

Answer 22

The function of the transporter is based on a specific amino acid interaction. Arginine 144 (R144) bonds to glutamate 126 (E126), which allows E269 to be free to accept a proton. Therefore, solute loading is favored on the extracellular side. The binding of the proton increases affinity for lactose, and binding saturation occurs when lactose binds. This is an example of cooperative binding. Different amino acid interactions occur after the conformational change. The R144-E126 bond breaks, and R144 bonds with E269. This bond favors solute unloading on the cytosolic side, and the proton is displaced.

Question 23

Which ions are usually co-transported?

Answer 23

In animal cells, sodium is usually the co-transported ion. In bacterial cells, H+ is the co-transported ion

Question 24

Glucose/Na+ symporter

Answer 24

Sodium levels are high extracellularly in animals. Sodium moves down its concentration gradient to provide energy for the active transport of glucose. It binds to the transporter, which induces conformational change and increases affinity for glucose. Glucose is moving up its concentration gradient since glucose is present in higher amounts in the cell than it is in the gut. This is another example of cooperative binding. The transporter is important for all cells, but it is especially important for those in the lumen of the digestive tract to take up glucose from food digestion

Question 25

Cooperative binding

Answer 25

When in a co-transport situation, both solutes must be bound in order for the transporter to undergo a conformational switch

Question 26

Transcellular transport

Answer 26

All cells need a glucose/sodium symport, since they all need glucose. However, epithelial cells in the GI tract take up a lot of glucose and need more access. Epithelial cells in the GI tract also have to transport glucose to the underlying cells in the underlying tissue. Therefore, transcellular transport is achieved through non-uniform distribution of transporters. There are different transporters found in different membranes of the epithelial cells

Question 27

Epithelial cells terminology

Answer 27

Epithelial cells have a rectangular shape, and therefore there are different names for the different sides of the cell. The top part of the cell is called the apical membrane, the sides are called the lateral sides, and the bottom is called the basal membrane

Question 28

Differential diffusion of transporters in epithelial cells

Answer 28

Sodium/glucose symporters are only found in the apical membrane. These symporters are in contact with the lumen of the GI tract, where all of our digested food would be, and are responsible for uptake. Sodium independent passive glucose transporters are found in the lateral and basal membranes, which allows for release of glucose to the underlying cells. Additional transporters like Na/K ATPase maintain low sodium cellular concentration. They drive the Na/glucose symporter by maintaining the extracellular sodium concentration

Question 29

3 types of ATP driven pumps (ATPases)

Answer 29

1. P type pumps- ion transport
2. F type and V type proton pumps- ATP generation or pumping of protons
3. ABC transporter- small molecule transport

Question 30

P type pumps

Answer 30

These pumps autophosphorylate when they begin to undergo conformational changes. They P type pump family includes many ion pumps responsible for maintaining gradients of sodium, potassium, hydrogen, and calcium across membranes. These ATP driven transporters mediate primary active transport- movement of solutes against their concentration gradient. This sets up concentration gradients that will drive secondary active transport, or the transport of coupled transporters

Question 31

Calcium ATPase

Answer 31

A P-type pump that pumps calcium across membranes. Its function is best understood in the sarcoplasmic reticulum of muscle cells. The calcium ATPase is the membrane transporter that actively pumps calcium into the sarcoplasmic reticulum. It maintains a high gradient of calcium in the sarcoplasmic reticulum

Question 32

Sarcoplasmic reticulum

Answer 32

A specialized endoplasmic reticulum in muscle cells that serves to store large amounts of calcium. Calcium serves an important role in muscle cells. It needs to be released into the cytoplasm by calcium release channels in order for muscle contraction to occur. Calcium is released when action potentials depolarize the myocyte membrane

Question 33

Calcium ATPase structure

Answer 33

A multiprotein complex that consists of 10 transmembrane alpha helices. 3 of the alpha helices line the central channel that spans bilayer. It exists in 2 states- unphosphorylated and phosphorylated- the phosphate is obtained from ATP hydrolysis. The pump will only be phosphorylated when we have hydrolyzed ATP. In its unphosphorylated state, the pump acts as a binding site for calcium. Once it has been phosphorylated, a conformational change can occur and calcium is released.

Question 34

Calcium ATPase mechanism

Answer 34

In its unphosphorylated state, the 2 alpha helices crossing the membrane are disrupted. They form a cavity that acts as a binding site for 2 calcium ions. The binding site is accessible from the cytosolic side, since calcium is being taken from the cytoplasm and loaded into the sarcoplasmic reticulum. ATP binds to the pump on the cytosolic side, and once ATP is phosphorylated, it transfers a phosphate to an aspartic acid on an adjacent domain. The phosphorylation is significant because it means that ATP has been hydrolyzed and is providing energy for this process. Once the phosphorylation has occurred, there is a drastic rearrangement/conformation change in the alpha helices. This conformational change disrupts the calcium binding site, which favors unloading of the calcium

Question 35

How does the conformational change work for calcium ATPase?

Answer 35

Two domains of the pump snap together, sort of like a clothespin. The two domains are driven together by ATP hydrolysis and phosphorylation, which drives out 2 calcium

Question 36

Na/K ATPase

Answer 36

A P-type pump that maintains a high extracellular concentration of sodium and a high intracellular concentration of potassium. It is an important pump for establishing concentration gradients that will be necessary for secondary active transport, like with the Na/glucose symporter. It also has a functioning in maintaining cytosolic pH. The Na/K ATPase uses ATP to drive out 3 sodium ions and drive in 2 potassium ions.

Question 37

Na/K ATPase mechanism

Answer 37

Phosphorylation causes a change in conformation, so the sodium ions are released out of the cell. This action also exposes binding sites for potassium in the pump. Binding of potassium triggers the release of the phosphate group and return to the pump’s original conformation. At this point, potassium is released inside the cell, and the cycle repeats

Question 38

Why is the Na/K ATPase so important?

Answer 38

It is an important pump for establishing concentration gradients that will be necessary for secondary active transport, like with the Na/glucose symporter. It also has a functioning in maintaining cytosolic pH. Additionally, without the pump, all sodium and chloride ions would leak into the cytoplasm. This would be problematic for all cells, but most cells have an extensive cytoskeleton and could resist it. However, this is problematic for RBCs, as they have no nucleus, no organelles, and a highly permeable membrane. If the pump were inactive, RBCs would swell and lyse

Question 39

V-type pumps

Answer 39

Turbine-like proteins, evolutionarily ancient. They pump protons into lysosomes, synaptic vesicles, and plant vacuoles to acidify the environment and therefore lower the pH

Question 40

F-type pumps

Answer 40

Turbine-like proteins that are structurally related to V-type pumps. Found in the membrane of bacteria, mitochondria, and chloroplasts. They work in reverse of V-type pumps, as they use the proton gradient to synthesize ATP. This includes ATP synthases

Question 41

How do F-type pumps synthesize ATP?

Answer 41

They use the proton gradient to generate ATP from ADP and phosphate. It is generated during the ETC or during photosynthesis

Question 42

V-type ATPase mechanism

Answer 42

The hydrolysis of ATP causes repeated conformational changes, moving the pump in a turbine-like motion. This allows protons to move from the cytoplasm into the interior of the lysosome

Question 43

ATP synthases

Answer 43

A type of F-type pump that uses energy from the proton gradient to generate ATP. The proton concentration is higher in the intermembrane space due to specialized proteins in the inner membrane that make up the ETC. Uses the energy of electrons to pump out H+, generating gradient

Question 44

Ion channels

Answer 44

Hydrophilic pores across the bilayer that are concerned specifically with inorganic ion transport. They select for only one type of ion. The pores are narrow, highly selective, and open and close rapidly. They go through less conformational changes than membrane transporters. Channels can pass up to 100 million ions per second, which is 10^5 times greater than transporters. They primarily transmit sodium, potassium, calcium, and chloride. Ion channels cannot be coupled to an energy source as they only carry out passive transport. Present in prokaryotes and eukaryotes

Question 45

Importance of ion channels

Answer 45

They are responsible for the excitability of neurons/the nervous system, as well as myocytes. Each channel is characterized by the ions (only one type) it conducts and the mechanism by which it is gated. The channels are highly selective, which prevents other unnecessary molecules from getting across the membrane.

Question 46

How are ions different from aqueous pores?

Answer 46

Ion channels are more selective due to narrowing of the pore (selectivity filter). Permeating ions make contact with the walls of the channel, and only ions of the appropriate size and charge can pass

Question 47

Selectivity filter

Answer 47

The narrowest part of the ion channel pore which selects for the appropriate ion. It limits the rate of ion passage in order to assess the exact size of the ion trying to pass through. Ions shed their associated water and pass single file through the filter.

Question 48

Ion channel conformational changes

Answer 48

Ion channels can be open or closed. This is basically the extent of their conformational changes

Question 49

Types of ion channels (4)

Answer 49

1. Voltage gated
2. Ligand gated
3. Mechanically gated
4. K+ leak channels

Question 50

Gating

Answer 50

Ion channels open briefly in response to a specific stimulus, then close. The stimulus depends on the type of channel

Question 51

Voltage gated channels

Answer 51

Opens due to change in voltage across the membrane (membrane potential)

Question 52

Ligand gated channels

Answer 52

Opens due to ligand binding, like when a neurotransmitter binds to its receptor

Question 53

Mechanically gated channels

Answer 53

Opens due to mechanical stress, like movement or stretching of the membrane

Question 54

K+ leak channels

Answer 54

Makes membrane permeable to K+, so it can leak across the membrane. This maintains membrane potential across all cell membranes

Question 55

Membrane potential

Answer 55

Difference in electrical charge on 2 sides of membrane. The extracellular environment is slightly more positive and the intracellular environment is slightly more negative. Because of all the transporters and channels in the membrane, the charge is rarely equal on both sides

Question 56

How is membrane potential maintained?

Answer 56

Potassium passively flows out of the cell through potassium leak channels. When potassium leaves, negative charge builds up intracellularly. This imbalance of charges is the membrane potential. Many different channels and membrane pumps also help to contribute to the membrane potential, and the Na/K pump is also important.

Question 57

Bacterial K+ channel

Answer 57

This is the most understood ion channel and can be used to study how ion channels work. It is made of 4 transmembrane subunits (quaternary structure). It has negative amino acids concentrated on the cytosolic side, which attract cations (like potassium) and repel anions. The subunits are 2 alpha helices which are tilter outward. There is a chain connecting the helices- this chain is a short helix (pore helix) and a loop. The loop protrudes into the pore, and it acts as a selectivity filter

Question 58

Bacterial K+ channel subunits

Answer 58

There are 4 subunits. Each one is made of 2 alpha helices, a pore helix, and a protruding loop that enters the ion channel and creates the narrow selectivity filter. The pore helix also has a negative charge that is oriented toward the interior of the ion channel in order to attract cations toward the selectivity filter

Question 59

Bacterial K+ channel mechanism

Answer 59

Hydrated cations are transported into the vestibule and can get about halfway across the membrane, which is the widest portion (vestibule). Once they get to the selectivity filter, the incorrect ions are excluded. Dehydrated K+ breaks its interactions with water enters the selectivity filter and interacts transiently with the carbonyl oxygens extending from the protruding loop. This interaction helps to identify that it is the correct ion, as selectivity filter dimensions only accommodate interactions between potassium and oxygen

Question 60

How does the bacterial K+ channel selectivity filter work?

Answer 60

The vestibule is the wider portion of the channel and can accommodate any hydrated cation. In the selectivity filter, however, the oxygens are placed precisely to only accommodate dehydrated K+. While K+ can bind with the oxygens (spacing), other ions like Na+ cannot. This ensures that the correct ion is being transported across the membrane

Question 61

Gating of the potassium channel

Answer 61

The inner helices tilt in on themselves, constricting the pore opening on the cytosolic side. A small opening still remains after tilting, but the bulky hydrophobic side chains interact, causing steric hindrance and closing the pore. Many other channels are built and gated similarly. Gating involves tilting, rotating, or bending of inner helices