Membrane Structure and Composition

Question 1

Functions of Membranes

Answer 1

Define boundaries of a cell and organelles and act as permeability barriers.
Serve as sites for biological functions such as electron transport.
Membrane proteins carry out and regulate the transport of substances across the membrane.
Contain protein molecules that act as receptors to detect external signals.
Provide mechanisms for cell-to-cell contact, adhesion, and communication.

Question 2

Fluid Mosaic Model

Answer 2

Describes the fluid lipid bilayer with a mosaic of proteins attached to or embedded in the bilayer

Question 3

Three classes of membrane proteins:

Answer 3

Integral membrane proteins
Peripheral proteins
Lipid-anchored proteins

Question 4

Fluidity of Bilayer

Answer 4

Lipids in bilayer are in constant motion; proteins are able to move laterally through membrane (except those anchored to internal structures)

Question 5

Transmembrane Proteins

Answer 5

Contain hydrophobic segments which span the lipid bilayer; anchors protein to membrane

Question 6

Membrane Lipids

Answer 6

Non-homogenous; diversity in lipid content

Question 7

Classes of Membrane Lipids:

Answer 7

Phospholipids, glycolipids, sterols

Question 8

Phospholipids

Answer 8

Most abundant lipids in membrane; include phosphoglycerides and sphingolipids; variation in content among types of membranes; polar head and hydrophobic tail

Question 9

Location of Lipid Synthesis:

Answer 9

Smooth ER

Question 10

Glycolipids

Answer 10

Sugar/carbohydrates attached to lipids; cerebrosides and gangliosides prominent in neurons

Question 11

Sterols

Answer 11

Lipid molecules derived from cholesterol with characteristic four-ring steroid structure; contain a polar group to embed in membrane

Question 12

Thin-Layer Chromatography

Answer 12

Used to separate different kinds of lipids based on their relative polarities

Question 13

Membrane Asymmetry

Answer 13

Lipids are distributed unequally between the two monolayers; established during the synthesis of the membrane; most glycolipids are present in the exoplasmic layer

Question 14

Lipids are mobile within their monolayer

Answer 14

Fast lateral diffusion - can move as fast as several um per second; slow transverse diffusion (flip-flop)

Question 15

Lateral diffusion can be demonstrated by:

Answer 15

Fluorescence Recovery After Photobleaching (FRAP)

Question 16

Why fluidity is important for membrane function

Answer 16

Membrane permeability, mobility of membrane proteins, proper organization of membranes, cell movement; membranes function properly only in the fluid state

Question 17

Tm

Answer 17

Membrane fluidity changes with temperature; transition temperature measures the temperature at which a membrane changes phases from a ordered gel phase to liquid cystalline

Question 18

How can Tm be measured?

Answer 18

Differential scanning calorimetry - uptake of heat is measured as temperature is increased

Question 19

Affects of fatty acid saturation on Tm

Answer 19

Unsaturated fatty acids lowers Tm

Question 20

Affects of fatty acid chain length on Tm

Answer 20

Increased fatty acid chain length increases Tm

Question 21

Sterols influences on membrane fluidity

Answer 21

Cholesterol (and other sterols) affect membrane fluidity by stacking in between phospholipids.

Question 22

Lipid Rafts

Answer 22

Localized regions of membrane lipids (and proteins) involved in cell signaling (non-random)

Question 23

Freeze-fracture Experiment

Answer 23

Membranes are frozen and broken; images revealed evidence of the fluid-mosaic model

Question 24

Types of membrane proteins:

Answer 24

Integral, peripheral, and lipid-anchored

Question 25

Integral Membrane Proteins

Answer 25

Contain hydrophobic regions embedded in the interior; difficult to isolate; can cross the membrane once or multiple times (single-pass or multi-pass); most common structures are alpha helices; can form a hydrophilic core with multiple alpha helices; beta barrels can also form channels/porins

Question 26

Peripheral Membrane Proteins

Answer 26

Bound to the membrane via weak electrostatic forces and hydrogen bonds

Question 27

Lipid-anchored Membrane Proteins

Answer 27

Covalently bound to lipid molecules embedded in the bilayer; acylation and prenylation used for cytoplasmic proteins; GPI-modifications used for exoplasmic proteins

Question 28

Topology of Membrane Proteins

Answer 28

Membrane proteins have a specific orientation with respect to the membrane face; topology maintained during membrane budding and fusion; radioactive labeling can be used to determine topology of membrane proteins

Question 29

Glyoproteins

Answer 29

Membrane proteins with carbohydrate chains covalently linked to amino acid side chains - glycosylation; occurs in the ER and Golgi; in plasma membrane proteins, carbohydrates present on exoplasmic domains;

Question 30

Glycosylation of Proteins

Answer 30

N-linked glycosylation occurs co- and post-translationally in the rough ER and Golgi
O-linked glycosylation occurs strictly post-translationally and only in Golgi

Question 31

Mobility of Membrane Proteins

Answer 31

Some proteins move freely, whereas others are constrained because they are anchored or associated in complexes; studied using FRAP and cell membrane fusion experiments (fluorescent antibody tags)

Question 32

How do we study membrane proteins?

Answer 32

SDS-PAGE
X-ray Crystallography
Molecular Biology

Question 33

Isolation of Membrane Proteins

Answer 33

Integral Membrane Proteins - use of detergents to disrupt lipid bilayer
Peripheral and Lipid-anchored Membrane Proteins - changes in pH or ionic strength or chelating agents

Question 34

SDS - Polyacrylamide Gel Electrophoresis

Answer 34

Separates proteins by size

Question 35

Hydropathy Plot

Answer 35

Hydrophobicity Plot; number and location of transmembrane segments in a membrane protein can be predicted