Boron

Question 1

What happens when phospholipids are added to water at a fairly low concentration?

Answer 1

They form a monolayer on the water surface at the air-water interface with tails sticking up rather than interacting with water because it is energetically less costly.

Question 2

What determines the rate at which phospholipids diffuse within the leaflets in which they reside?

Answer 2

The rate of two-dimensional diffusion of phospholipids is extremely temperature dependent.

Question 3

When are the lipids said to be in sol state?

Answer 3

At high temperature, when the thermal energy is greater than the interaction energy that tends to hold adjacent lipids together, so lipids diffuse rapidly.

Question 4

When are the lipids said to be in gel state?

Answer 4

At low temperatures, interaction energies exceed thermal energies of most lipids, thus lipids diffuse slowly because they lack the energy to free themselves from their neighbors.

Question 5

What is the transition temperature?

Answer 5

The temperature at which the bilayer membrane converts from gel to sol state or vice versa.

Question 6

What are the four subgroups of glycerol-based phospholipids?

Answer 6

Phosphatidylethanolamines Phospatidylinositols Phosphatidylserine Phosphatidylcholine

Question 7

What are the three subgroups of sphingolipids?

Answer 7

Sphingomyelins Glycosphingolipids Gangliosides

Question 8

What are the two main lipid classes composing lipid bilayer?

Answer 8

Glycerol-based phospholipids Sphingolipids

Question 9

What does cholesterol do at modest concentration and why?

Answer 9

It decreases fluidity because its rigid steroid rings binds and partially immobilize fatty acid chains,

Question 10

What does cholesterol do at higher concentrations?

Answer 10

It can substantially disrupt the ability of phospholipids to interact among themselves, increasing fluidity and lowering transition temperature.

Question 11

How does water pass through pure phospholipid bilayer?

Answer 11

Through transient crack between the hydrophobic tails of phospholipids, so it does not have to surmount an enormous energetic barrier. The degree of permeability of water varies extremely with lipid composition.

Question 12

Describe the overall asymmetry of the plasma membrane in terms of phospholipid composition:

Answer 12

The leaflet that faces the cytoplasm contains phosphatidylethanolamine and phosphatidylserine whereas the outward facing leaflet is composed almost exclusively of phosphatidylcholine.

Question 13

What do the biochemical processes involved in cell function necessitate?

Answer 13

The maintainance of a precisely regulated intracellular environment.

Question 14

What are the foundamental properties of the plasma membrane?

Answer 14

1) Impermeable to large molecules (ensure retention within cytoplasm) 2) Selectively permeable to small molecules 3) Can accumulate substances against concentration gradient 4) Create a concentration gradient by using metabolic energy to draw substances uphill (Active transport) 5) Can rapidly modulate the permeability in response to metabolic stimuli.

Question 15

What can a simple pore do and cannot do?

Answer 15

A simple pore in the membrane can modulate the rate at which a gradient dissipates but cannot concentrate anything.

Question 16

What does the head groups of phospholipids determine?

Answer 16

The identity of the head group determines the name as well as many properties of the individual phospholipids.

Question 17

What does the physical characteristics of the bilayer depend on?

Answer 17

On the the chemical composition of its consituent phospholipid molecules.

Question 18

What determines the width of the bilayer?

Answer 18

The lenght of the fatty-acid chains.

Question 19

What does the nature of the head group determines physically?

Answer 19

It determines how densely packed adjacent phospholipid molecules are in each leaflet of the membrane.

Question 20

Why can detergent dissolve phospholipids?

Answer 20

Because they are also amphipatic and are water soluble at much higher concentration than are phospholipids.

Question 21

What are the four different classes of glycerol-based phospholipids?

Answer 21

1) Phospatydilethanolamine 2) Phosphatidylinositol 3) Phosphatidylserine 4) Phospatidylcholines

Question 22

What are the three different subgroups of sphingolipids (derived from sphingosines)?

Answer 22

Sphingomyelin Glycosphingolipids Gangliosides

Question 23

What does the presence of different types of lipids that interact to different degrees with phospholipids side chains results in?

Answer 23

This changes the strenght of interaction that prevent phospholipids from diffuse within the plane of the leaflet, consequently, the membrane has a different fluidity and a different transition temperature.

Question 24

What is the peculiarity of bilayers that contains different classes of lipids?

Answer 24

They do not undergo the gel-sol transition at a single, well-defined tempreature but rather they interconvert more gradually over a temperature range that is defined by the composition of the mixture. The membrane can become therefore divided into compositionally distinct zones.

Question 25

How do water molecules transverse the membrane?

Answer 25

Through transient cracks between hydrophobic tails of the phospholipids without having to surmount an enormous energetic barrier.

Question 26

What does the phosphatidylinositol composition of the cytoplasmic face of an organelle membrane do?

Answer 26

It defines the identity of that organelle and govern its trafficking and targeting properties.

Question 27

What is the peculiarity of the phospholipids that are characteristics of animals cells?

Answer 27

They have one saturated and one unsaturated fatty acid residue, therefore they are less likely to partition into sol-like or gel-like lipid domains.

Question 28

How can peripherally associated membrane proteins be removed from the membrane?

Answer 28

By mild treatments that disrupt ionic bonds (very high salt concentration) or hydrogen bonds (very low salt concentration).

Question 29

What are the three ways by which an integral membrane protein can eb associated with the membrane?

Answer 29

1) Span the bilayer one or more time (transmembrane) 2) Embedded without crossing it 3) Lipid-anchored proteins attached to the PM by a covalent bond that links them to either a lipid component or a fatty acid derivative that interacalates in the membrane.

Question 30

What are membrane spanning segments?

Answer 30

Short stretches of amino acids that cross the membrane once, composed mainly of nonpolar amino acids, in concert with polar, uncharged amino acids.

Question 31

What defines the membrane topology of a multi-spanning transmembrane protein?

Answer 31

The pattern with which the transmembrane protein weaves across the lipid bilayer.

Question 32

Describe glycophospholipid-linked proteins:

Answer 32

Proteins linked to a special type of glycosilated phospholipid molecule on the outer surface of the membrane.

Question 33

What does the conformation of the helix with 3.6 amino acids per turn guarantee?

Answer 33

That the polar atoms in the backbone are maximally hydrogen bonded to one another so that they do not require the solvent to contribute hydrogen-bond partners.

Question 34

What can proteins achieve through assembly into multimeric complexes?

Answer 34

Increase in stability and increase in variety and complexity of functions they perform.

Question 35

What do attachments of adhesion molecules to the extracellular environment regulate intracellularly?

Answer 35

Regulate shape, growth and differentiation.

Question 36

Describe integrins:

Answer 36

Cell-matrix adhesion molecules that link cells to components of the extracellular matrix (eg. fibronectin, laminin) at adhesion plaques. These linkages produce confromational changes to the laminin molecules that are trnamsitted to their cytoplasmic tails.

Question 37

Descrbie cell-cell adhesion molecules:

Answer 37

Attach cells to each other

Question 38

What are the two classes of cell-cell adhesion molecules?

Answer 38

Cadherins (Ca2+ dependent adhesion molecule) N-CAMs (Ca2+ independent neural cell adhesion molecules)

Question 39

Describe cadherins:

Answer 39

Glycoproteins with one membrane-spanning segment n a large extracellular domains that binds Ca2+.

Question 40

Describe N-CAMs:

Answer 40

Members of the immunoglobulin superfamily.

Question 41

What do cell-cell adhesion molecules regulate cytoplasmically?

Answer 41

1)Organize cytoplasm 2)Control gene expression in response to intercellular contacts.

Question 42

What transmembrane proteins classes assist ions and other membrane-impermeable substances to cross the bilayer?

Answer 42

• Pores -Channels -Carriers -Pumps

Question 43

Describe the function of channels and pores:

Answer 43

They serve as a conduit that allow water, specific ions or even very large proteins to flow passively throught the bilayer.

Question 44

Describe the functions of carriers:

Answer 44

Can either facilitate the transport of specific molecules or couple the transport of a molecule to that of another solute.

Question 45

Describe the functions of pumps:

Answer 45

Use the energy that is release through the hydrolisis of ATP to drive the transport of substances against their concentration gradients.

Question 46

Describe amphipathic alpha-helices:

Answer 46

Hydrophobic amino acids alternate with hyrophilic residues that are spaced at regular intervals (approx. 3-4 amino acids) in the sequence and thus are the hydrophilic and hydrophobcic amino acids are aligned along a single edge of the helix. The resultant helix has disinct hydrophilic and hydrophobic surfaces.

Question 47

What are the two forms of association between peripheral membrane proteins and the membrane?

Answer 47

1) Via ionic interactions with phospholipid head groups (Salt bridges) 2) Direct binding of peripheral membrane proteins to the extracellular or cytoplasmic surfaces of integral membrane proteins.