Membrane Properties & Transport

Question 1

What are the three classes of membrane proteins, and how are they distinguished?

Answer 1

1. Integral proteins
2. Peripheral proteins
3. Lipid-anchored proteins
   They are distinguished by the intimacy of their relationship to the lipid bilayer.

Question 2

Define integral proteins.

Answer 2

Integral proteins penetrate the lipid bilayer, acting as transmembrane proteins. This means they pass through the entire bilayer and have domains that protrude from both the extracellular and cytoplasmic sides of the membrane.

Question 3

Define peripheral proteins.

Answer 3

Peripheral proteins are located entirely outside of the lipid bilayer, on either the cytoplasmic or extracellular side, yet are associated with the surface of the membrane by noncovalent bonds.

Question 4

Define lipid-anchored proteins.

Answer 4

Lipid-anchored proteins are located outside the lipid bilayer, on either the extracellular or cytoplasmic surface, but are covalently linked to a lipid molecule that is situated within the bilayer.

Question 5

What are the three primary functions of integral proteins?

Answer 5

1. Receptors that bind specific substances at the membrane surface.
2. Channels or transporters involved in the movement of ions and solutes across the membrane.
   3.. Agents that transfer electrons during the process of photosynthesis and respiration.

Question 6

Describe the bonds integral proteins form with the cellular membrane.

Answer 6

Integral membrane proteins are amphipathic. The transmembrane domains of the proteins tend to have a hydrophobic character. Amino acid residues in transmembrane domains form van der Waals interactions with the fatty acyl chains of the bilayer, which seals the protein into the lipid wall of the membrane. This preserves the permeability barrier of the membrane, anchors the protein within the bilayer, and brings the protein into direct contact with surrounding lipid molecules.

Question 7

Describe the portion of integral membrane proteins that project either into the cytoplasm or into the extracellular space.

Answer 7

These nonembedded domains tend to have hydrophilic surfaces that interact with water-soluble substances at the edge of the membrane. In some proteins, the transmembrane domains are essentially devoid of water molecules, whereas others allow the aqueous solvent to penetrate deep into the protein’s membrane-embedded regions.

Question 8

What is the purpose of detergents, and how do they work?

Answer 8

Detergents can remove proteins from the cellular membrane or isolate proteins for study. Like membrane lipids, detergents are amphipathic, which means they can substitute for phospholipids in stabilizing integral proteins while rendering them soluble in aqueous solution. Once the proteins have been solubilized by the detergent, various analyses can be carried out to determine the protein’s amino acid composition, molecular mass, amino acid sequence, and so forth.

Question 9

What is homology modeling?

Answer 9

Integral membrane proteins are difficult to isolate and study. Therefore, when one is studied, scientists use a technique called homology modeling which allows them to learn about other members of that protein family. It essentially groups proteins together by characteristics and makes studying proteins easier for scientists.

Question 10

Define transmembrane domains.

Answer 10

Transmembrane domains are the segments of a protein embedded within the membrane. They consist of a string of about 20 predominantly nonpolar amino acids that span the core of the lipid bilayer as a helix.

Question 11

What is shown in a hydropathy plot?

Answer 11

A hydropathy plot is used to identify the transmembrane segments of an amino acid sequence of an integral membrane. This plot assigns each site along a protein a measure of the hydrophobicity of the amino acid at that site.

Question 12

What type of bonds link peripheral proteins to the membrane, and how are they broken?

Answer 12

These proteins are associated with the membrane by weak electrostatic bonds. These bonds can be broken by solubilizing them with high-concentration salt solutions that weaken the electrostatic bonds holding peripheral proteins to a membrane.

Question 13

What role do peripheral proteins on the internal surface of the cellular membrane accomplish?

Answer 13

They form a fibrous network that acts as a membrane skeleton. They provide mechanical support and function as an anchor for integral membrane proteins. Other specialized peripheral proteins function as enzymes or transmit transmembrane signals.

Question 14

Define GPI-anchored proteins.

Answer 14

These are specific types of peripheral membrane proteins containing a certain type of linkage bound by a small, complex oligosaccharide in the outer leaflet of the lipid bilayer.

Question 15

Define the transition temperature.

Answer 15

The transition temperature is the temperature at which the lipid bilayer transitions from a liquid crystalline phase to a frozen crystalline gel (when temperature falls), which restricts the movement of the phospholipid fatty acid chains.

Question 16

How and when does the remodeling of membranes occur?

Answer 16

If the temperature of the cell is lowered, the cell responds in two ways (using enzymes to accomplish these actions):
1. It desaturates single bonds in fatty acyl chains to form double bonds.
2. It reshuffles the chains between different phospholipid molecules to produce ones that contain two unsaturated fatty acids, which greatly lowers the melting temperature of the bilayer.

Question 17

Define desaturases.

Answer 17

Enzymes that desaturate single bonds to form double bonds.

Question 18

Define phospholipases.

Answer 18

Enzymes that split the fatty acid from the glycerol backbone.

Question 19

Define acyltransferases.

Answer 19

Enzymes that transfer fatty acids between phospholipids.

Question 20

Define lipid rafts.

Answer 20

Lipid rafts are patches of cholesterol and sphingolipids. They are microdomains within the cellular membrane that are more gelated and highly ordered than surrounding regions. Because of their distinctive physical properties, these microdomains float within the fluid of the environment.

Question 21

Why don’t phospholipid molecules commonly move from one leaflet to the other, and what can facilitate an exception to this?

Answer 21

To flip across the membrane, the hydrophilic head group of the lipid must pass through the internal hydrophobic sheet of the membrane, which is thermodynamically unfavorable. However, cells contain enzymes that actively move certain phospholipids from one leaflet to the other. These enzymes play a role in establishing lipid asymmetry and may also reverse the slow rate of passive transmembrane movement.

Question 22

Define cell fusion.

Answer 22

Cell fusion is a technique whereby two different types of cells, or cells from two different species, can be fused to produce one cell with a common cytoplasm and a single, continuous plasma membrane.

Question 23

What does FRAP stand for, and what does it help scientists study?

Answer 23

Fluorescence recovery after photobleaching. It allows researchers to follow the movements of molecules in the membranes of living cells using a light microscope.

Question 24

What two findings did early studies using FRAP reveal?

Answer 24

1. Membrane proteins moved much more slowly in a plasma membrane than they would in a pure lipid bilayer.
2. A significant fraction of membrane proteins (30 to 70 percent) were not free to diffuse back into the irradiated circle (a part of FRAP’s technique).

Question 25

What restricts the movement of a large number of proteins in the cellular membrane?

Answer 25

Many proteins are tethered to the membrane’s skeleton. In addition, the movement of a transmembrane protein through the bilayer is slowed by extracellular materials that can entangle the external portion of the protein molecule.

Question 26

What does scramblase accomplish in the plasma membrane?

Answer 26

It can move phospholipids over the lipid bilayer in both directions. Scramblase enzymes are rare because they interrupt the asymmetry of the bilayer, so they are only present in particular cells at particular times.

Question 27

What does flippase accomplish in the plasma membrane?

Answer 27

Flippase flips phospholipids from the extracellular leaflet to the cytosolic leaflet. It is an ATPase since it uses the energy of ATP to move phospholipids.

Question 28

What does floppase accomplish in the plasma membrane?

Answer 28

It flips phospholipids from the cytosolic leaflet to the extracellular leaflet. It is an ATPase since it uses the energy of ATP to move phospholipids.

Question 29

Within what temperature range does the cellular membrane alter its functions, and which functions does it alter?

Answer 29

From 0-4 degrees C, cells slow or halt functions dependent on membrane fluidity (such as movement, trafficking, secretion, etc).

Question 30

What three strategies do cells use to regulate membrane fluidity?

Answer 30

1. Alter length of lipid tails
2. Alter saturation of lipid tails
3. Alter cholesterol content (in animal cells).

Question 31

Describe the steps of FRAP.

Answer 31

1. Fluorescently label membrane protein in a living cell
2. Photobleach patch with laser beam: irreversibly destroys fluorophore
3. Monitor recovery of fluorescence (Does the fluorescence recover or not?)
4. Recovery rate is a measure of lateral diffusion of labeled protein.

Question 32

What diffusion barriers can restrict the mobility of membrane proteins?

Answer 32

1. Proteins can be anchored to the internal protein (cytoskeleton).
2. Anchored to external proteins (extracellular matrix)
3. Bound to transmembrane protein in another cell.

Question 33

How do proteins confine other proteins to certain sections of the cellular membrane, and what is each bounded section called?

Answer 33

Proteins can form tight compartments using a junction protein tightly bonded to another cell. This confines certain proteins to a specific section (called the apical membrane domain) of the cellular membrane. The part of the membrane NOT confined is called the basolateral membrane domain.

Question 34

Define the partition coefficient.

Answer 34

The partition coefficient is the energy required to move from hydrophilic to hydrophobic. It describes the energy required to move particles across the plasma membrane.

Question 35

Describe the characteristics of membrane transport protein channels.

Answer 35

They form a continuous, hydrophilic, aqueous pore in the membrane allowing molecules to pass through membrane and down gradient. The pore is usually small and selective by charge and size (determined by the lining of amino acids that make up the interior of the pore). The pore opening is regulated by a ligand, membrane potential, or other condition. Usually consists of multiple grouped amphipathic a-helices. Polar or charged amino acids form lining of the hydrophilic channel. Only allows materials to flow from higher concentration to lower concentration.

Question 36

Describe the characteristics of membrane transport protein carriers.

Answer 36

They do not form an open channel, but bind to a cargo and move it through the entry way in such a way so that a continuous pore is never formed. Conformation changes in carrier protein transfer molecules across membrane. Can carry molecules against their concentration gradient (which means this mode of transport requires a direct or indirect input of energy in a coupled reaction).

Question 37

Why is the partition coefficient irrelevant in relation to facilitated transport?

Answer 37

Since the ion or polar molecule transported never enters the hydrophobic core of the lipid bilayer, there is never an energy expenditure from moving through a thermodynamically unfavorable environment.

Question 38

What determines the maximum transport rate of facilitated transport?

Answer 38

The steepness of the concentration gradient and the maximum rate of transport via the channel or the carrier.

Question 39

What is the main distinction between passive and active transport?

Answer 39

Active transport requires the input of energy. In passive transport, small non-polar molecules cross the membrane by simple diffusion, down gradient. Transport up gradient is active and requires energy.

Question 40

Rank the ATP-powered pump, ion channel, and transporter in terms of transport speed.

Answer 40

1. Ion channels are the fastest in ions/second
2. Transporters are the next fastest in molecules/second.
3. ATP-powered pumps are the least fast in ions/second.

Question 41

Distinguish between uniporters, symporters, and antiporters.

Answer 41

Uniporter: Transports a single molecule down its concentration gradient.
Symporter: Transports two molecules in the same direction.
Antiporter: Transports two molecules in opposite directions.

Question 42

Briefly describe the ATP-powered ion pump.

Answer 42

This pump uses the energy of ATP hydrolysis to pump Na+ and K+ ions across the plasma membrane. The hydrolysis of one ATP powers the movement of 3 Na+ and 2 K+ ions. This is the primary energy-consuming process in all cells, costing about 25% of all ATP consumed.

Question 43

What is the point of the ATP-powered ion pump?

Answer 43

It segregates ions both intracellularly and extracellularly and generates charge separation across the membrane. It also maintains the osmotic equilibrium of animal cells, ensuring that water stays outside of the cell when it wants to rush in to achieve equilibrium (due to the high concentration of ions within the cell).

Question 44

Describe the function of cotransporters.

Answer 44

Cotransporters use energy stored in the gradients of Na+ and H+ to power the uphill movement of another molecule or ion. The classic example is glucose import from the intestinal lumen into cells. It first makes the energy gradient of sodium, then uses it to power the import of glucose.