Lecture 5: Membrane Structure

Question 1

State 3 functions of the plasma membrane

Answer 1

1- Import and export of molecules.
2- receiving information.
3- expansion.

Question 2

nutrients pass _______ the plasma membrane and waste products pass _______ the plasma membrane.

Answer 2

inward and outward.

Question 3

The plasma membrane serves and prevents

Answer 3

• barrier that separates in the interior of the cell from the surrounding medium.
• prevents contents of the cell from escaping and mixing with the surrounding medium.

Question 4

Some proteins in the plasma membrane act as

Answer 4

sensors in the plasma membrane to enable cell to respond to changes in its environment.

Question 5

How does the plasma membrane expand?

Answer 5

when the cell grows or changes shape, the plasma membrane enlarges its area by adding new membrane while deforming without tearing.

Question 6

The membranes that surround the organelles of cells

Answer 6

separate one aqueous phase (cytosol) from another (interior of organelle).

Question 7

Internal membranes serve as

Answer 7

selective barriers.

Question 8

membrane-bound organelles (internal membranes) maintain

Answer 8

the characteristic differences in composition between these organelles.

Question 9

Name some membrane-bound organelles

Answer 9

• endoplasmic reticulum.
• peroxisomes.
• nucleus.
• lysosome.
• Golgi.
• transport vesicles.
• mitochondria.

Question 10

General structure of cell membranes

Answer 10

lipids and proteins.

Question 11

The lipid component of a cell membrane consists of

Answer 11

millions of lipid molecules arranged in two closely opposed sheets, forming a bilayer.

Question 12

Properties of a single lipid molecule

Answer 12

1- hydrophilic head.

2- one or two hydrophobic hydrocarbon tails.

Question 13

Define amphipathic molecule

Answer 13

molecules with both hydrophilic and hydrophobic properties.

Question 14

Name five types of lipids

Answer 14

1- fatty acids. 
2- phospholipids.
3-glycerol phospholipids.
4- non-glycerol phospholipids. 
5- cholesterol.

Question 15

A fatty acid consists of

Answer 15

a long hydrocarbon chain with a negatively charged carboxyl group.

Question 16

Types of fatty acids

Answer 16

• in saturated fatty acids all of the carbon atoms are bonded to a maximum number of hydrogen atoms.
• unsaturated fatty acids contain one or more double bonds between carbon atoms, resulting in one covalent bond with hydrogen.

Question 17

The most abundant lipid are

Answer 17

phospholipids.

Question 18

Phospholipids consist of

Answer 18

two fatty acids linked to a polar head group.

Question 19

In glycerol phospholipids

Answer 19

• the two fatty acids are bound to carbon atoms in glycerol.
• the third carbon atom is bound to a phosphate group, which is attached to another small polar molecule (radical).

Question 20

Types of small polar molecules (aminos) found in glycerol phospholipids

Answer 20

choline, serine, inositol, ethanol amine.

Question 21

The most common glycerol phospholipid in most cell membranes is

Answer 21

phosphatidylcholine.

Question 22

The net charge of small polar molecules found in phospholipids

Answer 22

• choline and ethanol amine are neutral.

- serine and inositol are negatively charged.

Question 23

What typer of lipid is sphingomyelin?

Answer 23

non-glycerol phospholipid.

Question 24

Define the structure of non-glycerol phospholipids

Answer 24

• contain two hydrocarbon chains linked to a polar head group formed from serine rather than glycerol.
• the third carbon atom of serine is bound to a phosphate group which in turn is attached to choline.

Question 25

Define the structure of glycolipids

Answer 25

-consist of two hydrocarbon chains linked to a polar head group formed from serine, which is attached to a polar carbohydrate molecule.

Question 26

Type of carbohydrate molecule in glycolipids

Answer 26

glucose or galactose.

Question 27

Define the structure of cholesterol

Answer 27

-consists of four hydrocarbon rings, one hydrophobic tail and a hydroxyl hydrophilic head.

Question 28

electrostatic bonds are between

Answer 28

charged atoms with polar polar groups and polar molecules (water).

Question 29

Why are hydrophobic molecules insoluble in water

Answer 29

because of their uncharged and non polar atoms, thus cannot form bonds with polar water.

Question 30

What happens when hydrophobic molecules are immersed in water

Answer 30

• a cage-like structure of water molecules forms around the tails, this requires energy.
• to minimize energy cost if tails cluster together.

Question 31

Name two conflicting forces in amphipathic molecules

Answer 31

1- the heads attract water.

2- the tails shun water and aggregate together.

Question 32

How are the conflicting forces in amphipathic molecules resolved

Answer 32

this is resolved by the formation of a phospholipid bilayer- most energetically favoured state.

Question 33

Name two types of synthetic lipid bilayers

Answer 33

1- liposomes, closed spherical vesicles.

2- flat bilayers, phospholipids applied to a small hole separating two aqueous compartment.

Question 34

Three types of phospholipid mobility in lipid bilayers

Answer 34

1- lateral diffusion: lipid molecules within a monolayer exchange places.
2- rotation: lipid molecules within a monolayer rotate around their axis.
3- “flip-flop”: lipid molecules rarely flip from one monolayer to the other.

Question 35

membrane fluidity depends on

Answer 35

a- length of hydrocarbon tails.
b- level of saturation of tails with respect to hydrogen.
c- presence of sterol cholesterol.

Question 36

In what do length of hydrocarbon tails affect membrane fluidity

Answer 36

• shorter chain increase fluidity because the distance between tails are larger (less packing).
• length of chain is inversely proportional to membrane fluidity.

Question 37

In what way do levels of saturation affect membrane fluidity

Answer 37

• unsaturated= more fluid, increase distance between neighbouring molecules.
• saturated= less fluid.

Question 38

In what way do cholesterol affect membrane fluidity

Answer 38

• the rigid hydrocarbon rings of cholesterol interact and partially immobilize the regions of fatty acid chains, therefor stiffening the bilayer and decreasing fluidity.

Question 39

Lipid bilayer is symmetric (T/F)

Answer 39

False, it is assymetric.

Question 40

The lipid composition of two leaflets of the lipid bilayers in many membranes

Answer 40

are different.

Question 41

Lipids present in the extracellular leaflet + explain assymetry

Answer 41

• glycolipids and sphingomyelin.
• the asymmetry is generated by both glycolipids and sphingomyelin are produced by enzymes exposed to the Golgi lumen and are not substrates for flippases.

Question 42

Phosphatidylcholine is mostly present in which leaflet

Answer 42

extracellular leaflet.

Question 43

Glycerol phospholipids mostly present in the cytosolic leaflet + explain asymmetry.

Answer 43

• phosphatidylethanolamine and phosphatidylserine.
• asymmetry generated by flippases, moving these glycerol phospholipids from the extracellular leaflet t the cytosolic leaflet.

Question 44

Type of lipid that distributes equally in each bilayer leaflet

Answer 44

• cholesterol, spontaneously shuttles (flip-flops) between the leaflets.
• does not require flippases.

Question 45

Specific flippases recognize different substrates (T/F)

Answer 45

True.