Lecture 18

Question 1

Membranes

Answer 1

• membranes are involved in receiving information information, export and import of molecules, motility
• they are selective barriers
• external and internal membranes (like ER, Golgi, nucleus)

Question 2

membranes are _____

Answer 2

amphipathic –> one part hydrophilic, one part hydrophobic. Hydrophobic tails hide from water and hydrophilic heads are attracted

Question 3

Hydrophilic vs hydrophobic

Answer 3

• hydrophilic molecules (acetone) form hydrogen bonds with water molecules
• hydrophobic molecules cannot. water forms a cage like structure around it (energetically unflavoured)
• if there are multiple hydrophobic molecules they will aggregate together to minimize contact with water

Question 4

The lipid bilayer

Answer 4

• the lipids in the cell membrane are made out of phospholipids which have a phosphate-containing head and two hydrophobic tails

Question 5

How are bilayers self-sealing

Answer 5

• bilayers spontaneously rearrange to eliminate free edges because this would expose hydrophobic tails to water
• this forms a liposome

Question 6

In what ways can the lipid bilayer move?

Answer 6

• lipid bilayers have the ability to bend and adapt to changes in membrane structure
• lipids within the bilayer can move in lateral direction, flexion, rotation, and flip flop. NOT vertically

Question 7

What is fluidity

Answer 7

• the ease with which the lipid molecules can move within the plane of the bilayer

Question 8

What affects fluidity?

Answer 8

• length of carbon chain
• number of double bonds

Question 9

Chain length

Answer 9

• shorter chain lengths: less tail-tail interaction, increasing fluidity
• longer chain lengths: decreasing fluidity because more tightly packed

Question 10

Number of double bonds

Answer 10

• unsaturated chains form a kink and have double bonds which makes it more difficult to pack together, making the membrane more fluid
• saturated chains pack together very densely

Question 11

saturated vs unsaturated chain

Answer 11

• saturated chain: the carbons are fully saturated with hydrogens
• unsaturated chain: the carbons are not fully saturated with hydrogens

Question 12

Organisms like bacteria and yeast adapt to different
temperatures by adjusting the carbon chain length and
the unsaturation of double bond
To maintain membrane fluidity at colder temperatures
bacteria would:

Answer 12

Increase the number of double bonds in the carbon chains

Question 13

Food for thought

Answer 13

• many fats produced by plants are unsaturated: more fluid, exists as liquids at room temperature, melting point is lower than room temperature
• fats produced are saturated: solid at room temperature, melting point is higher than room temperature

Question 14

Cholesterol

Answer 14

• short, rigid lipid molecules that fill the space between neighbouring phospholipids caused by kinks in hydrophobic tail
• can stiffen membrane, making things more compact (decreasing fluidity)
• it will increase melting point

Question 15

membrane assembly

Answer 15

• new phospholipids are manufactured on the cytosolic side of the endoplasmic reticulum
• unless they are transferred to the other leaflet of the bilayer by a protein, they will remain on that leaflet (monolayer)

Question 16

What enzymes are used for the redistribution of phospholipids

Answer 16

• scramblases: randomly moves phospholipids from one monolayer to the other
• flippases: the Golgi membrane contains flippases which removes specific phospholipids from one side to the other

Question 17

Redistribution of phospholipids causes…

Answer 17

• asymmetric distribution

Question 18

Asymmetric distribution:

Answer 18

• phospholipids and glycolipids are distributed asymmetrically in the lipid bilayer of the eukaryotic plasma membrane
• for example: phophatidylserine is normally localized to the cytosolic face, but when facing the extracellular fluid it signals for apoptosis

Question 19

membrane orientation

Answer 19

• phospholipids and membrane inserted proteins transported in the lumen will be exported to the plasma membrane and be in contact with the extracellular fluid
• cytosolic facing protein remain facing the cytosol when transported
• example: attached sugar to membrane protein is synthesized in the lumen of the Golgi

Question 20

What are the membrane proteins

Answer 20

• transmembrane: extends through the bilayer
• monolayer associated a helix: amphipathic a helix is embedded in monolayer
• lipid linked: attached to the membrane through 1 or more covalently linked lipid groups
• protein attached: attached to the membrane indirectly through association with a membrane protein

Question 21

Which membrane proteins are integral and peripheral

Answer 21

Integral: transmembrane, monolayer associated a helix, lipid linked
Peripheral: protein attached

Question 22

detergents

Answer 22

• amphipathic molecules with only one hydrophobic tail

Question 23

integral membrane proteins

Answer 23

• require detergents to remove from the membrane

Question 24

peripheral membrane proteins

Answer 24

cambe removed by gentler procedures

Question 25

Peripheral membrane proteins

Answer 25

- can be removed by gentler procedures

Question 26

Why do detergents readily form micelles while phospholipids liposomes

Answer 26

detergents have a wedge shape and phospholipids are more cylindrical

Question 27

How do polypeptide chains usually cross the lipid bilayer

Answer 27

- with an a-helix

Question 28

single pass vs. multipass

Answer 28

- only crosses membrane once - cross membrane multiple times (can create channels with hydrophilic interiors)

Question 29

B-sheets cross the membrane as a

Answer 29

B-barrel

Question 30

animal cells don't have a cell wall like plant cells instead they have a ________

Answer 30

- cell cortex: --> meshwork of fibrous proteins located on the cytosolic side of the membrane - involves restricting diffusion of protein within the membrane - spectrin is a major component

Question 31

cells can confine proteins to specific regions known as ______________

Answer 31

membrane domains

Question 32

tight junctions

Answer 32

- prevent diffusion of proteins to other side of cell - ensures unidirectional transport of macromolecules

Question 33

Glycoproteins

Answer 33

- proteins with short chains of sugar

Question 34

proteoglycans

Answer 34

- proteins with long chains of sugar

Question 35

where are sugars located

Answer 35

sugars are located on the outside of the plasma membrane forming a carbohydrate layers known as glycocalyx

Question 36

glycocalyx

Answer 36

- protects against mechanical damage, makes the cell slimy - functions in cell-cell recognition and adhesion