Cell Membranes

Question 1

Name 6 organelles in a eukaryotic cell

Answer 1

Endoplasmic Reticulum 
Nucleus
Lysosome
Peroxisome
Goglgi apparatus
Plasma membrane
Mitochrondrion

Question 2

Can you see biological membranes under light microscopes?

Answer 2

Nope

Question 3

What are cell membranes composed of?

Answer 3

Primarily proteins and lipids
Outer and inner monolayer.
Singer Nicholson model

Question 4

What are the 3 major classes of lipid?

Answer 4

Phospholipid
Sphingolipid
Cholesterol

Question 5

Describe the structure of a phospholipid

Answer 5

Glycerol backbone, CHOH x3
Phosphate on the sn-3 position
Two fatty acid chains on the sn-1 and sn-2 positions
Headgroup attached to phosphate.

Question 6

Structally which 2 lipids are similar?

Answer 6

Phosphoglycerides and sphingolipids

Question 7

In water, membrane lipids for what?

Answer 7

In water membrane lipids form vesicles

Question 8

Lipids spontaneously form what in an aqueous environment?

And why?

Answer 8

Lipids spontaneously form bilayers in an aqueous environment

System tries to minimize the free energy (DG) of the system

Hydrophilic/charged sites try to make favorable interactions with the aqueous environment.

Hydrophobic regions try to interact with other hydrophobic sites, minimizing entropic cost of placing lipids in water (hydrophobic effect)

Question 9

Describe the liquid crystalline phase

Answer 9

Membranes contain lipids with acyl chains with cis-double bonds.

These bonds prevent close packing of chains so have a bilayer with mobile acyl chains

At different temperatures the lipids move around in the plane of the bilayer

Question 10

What are the 2 dymamics in lipid membranes and explain them.

Answer 10

Lateral diffusion- rapid movement of molecules

Flip-Flop- energetically unfavourable due to long time scale

preserves membrane asymmetry and can be enhanced by membrane fusion or flippases

Question 11

At low temperature what happens to the liquid crystalline lipid?

Answer 11

At low temperatures the liquid crystalline lipid turns into a gel phase in which the acyl chains are effectively frozen

Question 12

Why is the liquid crystalline phase is essential

Answer 12

The liquid crystalline phase is essential for the function of the membrane proteins, which require a fluid environment in which to operate

Molecules need to be able to move within the membrane to carry substrates between enzymes and mediate signalling events

Question 13

Name the 4 major protein membrane proteins

Answer 13

Channels
Transporters
Receptors
Structural proteins

Question 14

To cross the hydro-phobic (non-polar) core of the bilayer membrane proteins typically adopt what conformation?

Answer 14

To cross the hydro-phobic (non-polar) core of the bilayer membrane proteins typically adopt an alpha-helical conformation

Question 15

What anchors the membrane proteins?

Describe the shape of the transmembrane domains

Answer 15

Transmembrane helices

Hydrophobic protein sequence with R groups projecting out to face the lipids

Question 16

How many amphipathic helices are needed to generate a polar route through the membrane?

Answer 16

To generate a polar route through the membrane you need at least 4 amphipathic helices

Question 17

What dictates what can pass through a pore?

Answer 17

Charges at entry to the pore dictate the charge of the ions that can pass

Question 18

What can channels not do?

Answer 18

Move ions or any solute against their electrochemical gradient, i.e., they cannot pump

They are unsuitable for facilitating the diffusion of molecules such as glucose and amino acids across the plasma membrane.- as K and Na can travel through these as well

Question 19

What can control passage of ions through a channel?

Answer 19

A gate

Question 20

What dictates the direction of ion flow?

Is this active or passive?

Answer 20

Direction of ion flow is dictated by the electrochemical gradient of the ion (passive process)

Question 21

What are the two types of transporters?

Answer 21

Passive ( down con. gradient)

Active (against con. gradient)

Question 22

Where does energy for active transporter come from?

Answer 22

Electrochemical gradient

Energy rich substrates (e.g. ATP)

Question 23

Why is it difficult to find out structure of membrane proteins?

Answer 23

Membrane proteins do not readily form 3-D crystals
too large for liquid state NMR
Little channels needed to perform so not large sample size

Question 24

How was the early protein structures solved?

Answer 24

from naturally abundant proteins
(mitochondrial/chloroplast, bacterio-/rhodopsin,
bacterial proteins etc.)

Question 25

What are the 3 main techniques used for structural analysis?

Answer 25

X-ray crystallography
Difficult to prepare crystals

Electron Microscopy (2D electron diffraction)
Low to medium resolution
Requires formation of 2D crystals

NMR Spectroscopy
Solution state NMR techniques – structure in micellar systems
Solid state NMR techniques – structure in the bilayer

Question 26

Describe the hydropathy plot

Answer 26

Each amino acid residue is assigned a value corresponding to its hydrophobicity

The average length of a transmembrane helix is 20 amino acid residues

A computer algorithm calculates and plots the total hydrophobicity of residues 1-20, 2-21,3-22, etc to reveal potential transmembrane helices

Question 27

What is the less common structure of transmembrane proteins?

Answer 27

Beta-barrel

Question 28

Where are Beta-barrels usually found?

Answer 28

β-barrels form hydrophilic pores (porins) in the outer membranes of bacteria and mitochondria
Found in only a few specialised locations where it is important for the membranes to be relatively leaky to small polar molecules. So in mitochondria they allow substrates for ATP production to pass through the mitochondrial outer membrane. They are then transported using transporters across the inner membrane.
In bacterial outer membranes they allow nutrients like maltose or phosphate to cross the outer membrane Likewise they are then carried across the plasma membrane by transporters. Too leaky to be of use in general membranes.

Question 29

Name an example of a Beta porin

Answer 29

This porin allows sucrose to diffuse across the outer membrane of the bacterium S. typhimurium

The pore is made up of 16 β-strands