Membranes

Question 1

What is membrane fluidity controlled by?

Answer 1

Fatty acid composition and cholesterol content

Question 2

What are the two states lipid bilayer can exist in?

Answer 2

An ordered, rigid state

Relatively disordered, fluid state

Question 3

When does the phase transition occur?

Answer 3

At Tm - melting temperature

Question 4

Where do you find cholesterol?

Answer 4

Inserted between phospholipids - OH group of cholesterol aligns with polar head groups of lipids

Question 5

What does cholesterol do to hydrocarbon chain? At high concentrations?

Answer 5

Rigid steroid ring partially immobilises hydrocarbon chain at point closest to head

At high concentrations, prevents hydrocarbon chains coming together blocks phase transition

Question 6

How do phospholipids form sealed compartments?

Answer 6

Spontaneously

Question 7

What do many integral membrane proteins have?

Answer 7

One or more domains embedded in lipid bilayer

Can have single or multiple transmembrane (TM) domains

Question 8

What are transmembrane domains composed of?

Answer 8

20-30 hydrophobic residues that form an alpha helix that spans the lipid bilayer

Question 9

How can transmembrane domains be predicted?

Answer 9

From primary sequences

-hydropathy plots can identify stretches of hydrophobic residues

Positive = energy required = hydrophobic

Question 10

What are the hydrophobic amino acids?

Answer 10

F - phenylalanine
A - alanine
M - methionine
I - isoleucine
L - leucine
Y - tyrosine
V - valine
W - tryptophan

Question 11

What can a transmembrane hydrophilic pore be formed by?

Answer 11

Alpha helixes

Question 12

What do detergents do?

Answer 12

Solubilise membrane proteins

Question 13

What is SDS?

Answer 13

Strong ionic detergent

Question 14

What is Triton X-100?

Answer 14

Mild ionic detergent

Question 16

What are peripheral membrane proteins?

Answer 16

Proteins indirectly associated with the membrane

More easily dissociated from membrane (eg salt wash)

Components of some signalling pathways are associated with cytoplasmic face of TM receptors

Question 17

What are features of a typical single pass transmembrane protein?

Answer 17

Most transmembrane proteins glycosylated (sugar residues added in ER lumen and Golgi)

Disulphide bonds exist between folded parts, to help stabilise folded structure

Both features present on non-cytosolic side

Question 18

What does freeze fracture electron microscopy do?

Answer 18

Visualise certain membranes

Question 19

What is an experiment that demonstrates the diffusion of proteins in the plasma membrane?

Answer 19

Rhodamine labelled membrane protein (mouse) cells + fluorescein labelled membrane (human)

Fuse the two together

Incubate at 37 degrees

Different proteins are different colours

Question 20

What does FRAP flurescence recovery after photo bleaching do?

Answer 20

A method to measure lateral mobility of proteins within membranes

Question 21

What does single particle tracking allow?

Answer 21

Tracking of individual protein molecules

Question 22

What are detergents used for?

Answer 22

Disrupt cells membranes to isolate proteins for study

Question 23

What are small hydrophobic molecules in a lipid bilayer?

Answer 23

O2 CO2 N2 Steroid hormones

Question 24

What are small uncharged polar molecules in a lipid bilayer?

Answer 24

H2O

Glycerol

Ethanol

Question 25

What are the large uncharged polar molecules in a lipid bilayer?

Answer 25

Amino acids Glucose Nucleotides

Question 26

What are the ions in a lipid bilayer?

Answer 26

H+ Na+ HCO3- K+ Ca2+ Cl- Mg2+

Question 27

What are carrier proteins?

Answer 27

Bind molecule to be transported. Move through membrane by series of conformational changes Only allow passage of molecules that fit binding site

Question 28

What a re channel proteins?

Answer 28

Form hydrophilic pores through membrane Discriminate on basis of size and charge

Question 29

What is passive transport?

Answer 29

All channels and most carriers Molecules move down concentration gradient

Question 30

What is active transport?

Answer 30

Movement of molecules against electrochemical concentration gradient Requires energy Transport by carrier proteins can be either or passive - channels or passive

Question 31

What is the hypothetical mechanism of carrier proteins?

Answer 31

1- specific molecule binds to carrier protein on one side of the membrane - binding site 2- binding triggers conformational change in carrier protein. Protein flips or shifts shape to the opposite side of the membrane 3- protein is release into that space. Binding site now has a low affinity so the molecule detaches 4- carrier protein returns to original shape

Question 32

What is an electrochemical gradient?

Answer 32

Net driving force, composed of concentration and voltage Affects ability of charged molecules to cross

Question 33

Explain the Na+ K+ pump?

Answer 33

1- three Na+ Ions bind inside the cell 2- pump is phosphorylated - phosphate from ATP is used, giving it energy 3- energy causes shape change, and 3 Na+ to be released to outside 4- 2 K+ ions bind to pump (from outside) 5- phosphate released - shape change 6- 2K+ released

Question 34

What does the Na+ / K+ pump do?

Answer 34

Maintains the electrochemical gradient essential for things such as nerve impulses and medulla contraction

Question 35

How can active transport be driven by ion gradients?

Answer 35

Free energy released as ions move down electrochemical gradient essential This free energy can be used to drive active transport of another molecule - coupled transporters (Usually Na+)

Question 36

What is uniport? How does it work?

Answer 36

Simple transport of soluble molecule from one side of the membrane to the other Binds to protein, conformational change, molecule released

Question 37

What is symport? How does it work?

Answer 37

Coupled transport of two types of solute (eg glucose and Na+ in kidney) Bind to carrier protein, one goes with gradient, conformational change, both molecules released on other side

Question 38

What is antiport? How does it work?

Answer 38

Two molecules bind on opposite sides membrane Conformational change Each molecule is released on other side that it started One molecule against conc gradient and one with

Question 39

What are the two types of glucose transporter in intestinal epithelial cells?

Answer 39

Apical Basal

Question 40

What is apical glucose transport?

Answer 40

Glucose / Na+ symport, allows uptake of glucose when conc in cell is already high

Question 41

What is basal glucose transport?

Answer 41

Passive glucose transporters, glucose released as required