5.1 Structure of Cell Membranes

Question 1

Not only do membranes physically separate cells from their external environment, they also:

Answer 1

define spaces within many cells that allow them to carry out their diverse functions

Question 2

what are the main component of cell membranes?

Answer 2

lipids because they have properties that allow them to form a barrier in an aqueous environment
also has proteins and carbohydrates embedded or associated

Question 3

what is the major type of lipid found in cell membranes?

Answer 3

phospholipids (made up of a glycerol backbone attached to a phosphate-hydrophilic group and two fatty acids-hydrophobic)

Question 4

amphipathic

Answer 4

having both hydrophilic and hydrophobic regions

Question 5

how do amphipathic molecules arrange themselves in aqueous environment? (in water)

Answer 5

they arrange themselves so hydrophilic side faces outwards to interact with water and non polar groups face away from water

Question 6

polar molecules like water tend to….

Answer 6

exclude nonpolar molecules or nonpolar groups of molecules (useful for healing cell membranes)

Question 7

what is the shape of the structure determined by?

Answer 7

the bulkiness of the head group relative to the hydrophobic tails

Question 8

micelle

Answer 8

a spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed

Question 9

bilayer (formed by lipids with less bulky head gouts and two hydrophobic tails)

Answer 9

a two-layered structure of the cell membrane with hydrophilic “heads” pointing outward toward the aqueous environment and hydrophobic “tails” oriented inward, away from water

Question 10

the bilayer structure forms spontaneously and without the action of an enzyme as long as:

Answer 10

the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell

Question 11

the pH of the environment is important for cell membranes/phospholipids because :

Answer 11

it ensure that the head groups are in their ionized (charged) form and thus hydrophilic

Question 12

liposome

Answer 12

an enclosed spherical bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water

Question 13

lipids freely associate with one another because of:

Answer 13

extensive van der Waals forces between their fatty acid tails (very weak forces, can easily be broken and re-formed)

Question 14

fluid

Answer 14

describes lipids that are able to move in the plane of the cell membrane

Question 15

the degree of membrane fluidity depends on:

Answer 15

which types of lipid make up the membrane

Question 16

in a single layer of the lipid bilayer, the strength of van der Waals interactions between the lipids’ tails depends on:

Answer 16

the length of the fatty acid tails and the presence of double bonds between neighbouring carbon atoms

Question 17

the longer the fatty acid tails, the more surface is available for van der Waals interactions resulting in:

Answer 17

tighter packing and reduced mobility

Question 18

saturated fatty acid tails have no double bonds which has what result on mobility?

Answer 18

reduced mobility because of tighter packing

Question 19

unsaturated fatty acid tails have double bonds and kinks resulting in:

Answer 19

enhanced lipid mobility due to reduced tightness of packing

Question 20

cholesterol

Answer 20

an amphipathic lipid that is a major component of animal cell membranes

Question 21

the amphipathic structure of cholesterol allows it to:

Answer 21

pack tightly with phospholipids: hydrophilic hydroxyl group interacts with polar head of phospholipids, ring structure participates in van der Waals interactions with the fatty acid chains

Question 22

cholesterol increases or decreases membrane fluidity depending on:

Answer 22

temperature

Question 23

at temperatures typically found in a cell, cholesterol:

Answer 23

decreases membrane fluidity because of interactions between rings of cholesterol and phospholipid fatty acid tail (van der Waals forces)

Question 24

at low temperatures, cholesterol:

Answer 24

increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids

Question 25

cholesterol helps maintain a consistent state of membrane fluidity by:

Answer 25

preventing dramatic transitions from a fluid to solid state.

Question 26

lipid raft

Answer 26

lipids assembled in a defined patch of the cell membrane

Question 27

membranes are not always a uniform fluid bilayer, but instead can contain regions with:

Answer 27

discrete components

Question 28

what is a lipid flip-flop?

Answer 28

the spontaneous transfer of a lipid between layers of the bilayer (VERY RARE)

Question 29

there is little exchange of components between the two layers of the membrane which results in:

Answer 29

the two layers to differ in composition; different types of lipids are present primarily in one layer or the other

Question 30

transporters

Answer 30

Membrane proteins that move ions or other molecules across the cell membrane.

Question 31

receptor

Answer 31

A molecule on cell membranes that detects critical features of the environment. Receptors detecting signals that easily cross the cell membrane are sometimes found in the cytoplasm.

Question 32

enzyme

Answer 32

A protein that functions as a catalyst to accelerate the rate of a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell.

Question 33

anchor

Answer 33

A membrane protein that attaches to other proteins and helps to maintain cell structure and shape.

Question 34

membrane proteins can be classified into two groups depending on:

Answer 34

how they associate with the membrane

Question 35

what are the two groups of proteins?

Answer 35

integral membrane proteins and peripheral membrane proteins

Question 36

integral membrane protein

Answer 36

A protein that is permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself.

Question 37

peripheral membrane protein

Answer 37

A protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions. easily separated from the lipid membrane

Question 38

transmembrane proteins

Answer 38

Proteins that span the entire lipid bilayer; most integral membrane proteins are transmembrane proteins

Question 39

transmembrane proteins are composed of three regions:

Answer 39

two hydrophilic regions, one protruding from each face of the membrane, and a connecting hydrophobic region that spans the membrane.

Question 40

structure of transmembrane proteins allows for:

Answer 40

separate functions and capabilities of each end of the protein

Question 41

Peripheral membrane proteins may be associated with either the internal or external side of the membrane, they interact with either:

Answer 41

the polar heads of lipids or with integral membrane proteins by weak noncovalent interactions such as hydrogen bonds.

Question 42

how can one prove protein mobility experimentally

Answer 42

fluorescence recovery after photobleaching, or FRAP

Question 43

fluid mosaic model

Answer 43

A model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins.