5.1 Structure of Cell Membranes

Question 1

What are cells defined by?

Answer 1

membranes

Question 2

What do membranes do?

Answer 2

physically separate cells from their external environment and define spaces within many cells that allow them to carry out their diverse functions

Question 3

What is the main component of cell membranes?

Answer 3

lipids, they have properties that allow them to form a barrier in an aqueous environment

Question 4

What are other components of membranes?

Answer 4

• proteins or embedded in or associated with the membrane

- carbohydrates are found attached to lipids (glycolipids), and proteins (glycoproteins)

Question 5

What do embedded proteins in membranes do?

Answer 5

perform important functions such as transporting molecules

Question 6

What are the major types of lipids found in cell membranes?

Answer 6

phospholipids

Question 7

What are most phospholipids made up of?

Answer 7

glycerol backbone attached to a phosphate group and two fatty acids

Question 8

Why are phospholipids amphipathic?

Answer 8

• head is hydrophilic/polar

- tail is hydrophobic/nonpolar

Question 9

What does amphipathic mean?

Answer 9

having both hydrophilic and hydrophobic regions

Question 10

How do amphipathic molecules behave in an aqueous environment?

Answer 10

spontaneously arrange themselves into various structures in which the polar head groups on the outside interact with water and the nonpolar tail groups come together on the inside away from water

results from tendency of polar molecules to exclude nonpolar molecules/groups

Question 11

What is the shape of the structure determined by?

Answer 11

bulkiness of head group relative to hydrophobic tails

Question 12

What are micelles?

Answer 12

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

Question 13

What is a bilayer?

Answer 13

two-layered structure of the cell membrane with hydrophilic heads pointing outwards toward the aqueous environment and hydrophobic tails oriented inward away from the water

Question 14

What structures do bilayers form? What does it explain

Answer 14

closed structure with an inner space because free edges would expose the hydrophobic chains to the aqueous environment

• explains why bilayers are effective cell membranes
• explains why membranes are self-healing

Question 15

How are membranes self-healing?

Answer 15

small tears in a membrane are rapidly sealed by the spontaneous rearrangement of the lipids surrounding the damaged region because of the tendency of water to exclude nonpolar molecules

Question 16

What are the consequences of the ability of phospholipids to form a bilayer when placed in water?

Answer 16

bilayer structure forms spontaneously, dependent only on the properties of the phospholipid and without the action of an enzyme, as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell

Question 17

Why is the pH important?

Answer 17

it ensures that the head groups are in their ionized form and suitably hydrophilic

Question 18

What are lipsomes?

Answer 18

an enclosed bilayer structure that surrounds a central space, spontaneously formed by phospholipids in environments with neutral pH like water

Question 19

What can liposomes do as they form?

Answer 19

may capture macromolecules present in solution

Question 20

What can liposomes do?

Answer 20

• form, break, and re-form in environments such as tidal flats
• can grow, incorporating more and more lipids from the environment

Question 21

Why do lipids freely associate with one another?

Answer 21

because of the extensive van de Waals forces between their fatty acid tails

Question 22

Describe van der Waals forces between fatty acid tails.

Answer 22

• weak
• easily broken and re-formed, so lipids molecules are able to move within the plane of the membrane, sometimes very rapidly

Question 23

Why is the membrane said to be fluid?

Answer 23

because membrane lipids are able to move in the plane of the membrane

Question 24

What does the degree of membrane fluidity depend on?

Answer 24

which types of lipids make up the membrane

Question 25

Describe van der Waals forces in a single layer of the lipid bilayer.

Answer 25

strength between the lipid tails depends on the length of the fatty acid tails and teh presence of double bonds between neighbouring carbon atoms

Question 26

What do longer fatty acid tails mean?

Answer 26

more surface is available to participate in van der Waals forces

the tighter packing that results tends to reduce lipid mobility

Question 27

Describe saturated fatty acid tails.

Answer 27

• no double bonds
• straight
• tightly packed, reducing mobility

Question 28

Describe unsaturated fatty acid tails.

Answer 28

• double bonds

- have kinks, reducing the tightness of packing and enhancing lipid mobility in the membrane

Question 29

What is cholesterol?

Answer 29

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

Question 30

Describe the amphipathic nature of cholesterol.

Answer 30

• hydroxyl (OH) group is hydrophilic

- hydrophobic region consists of four interconnected carbon rings with an attached hydrocarbon chain

Question 31

What does the amphipathic structure of cholesterol allow?

Answer 31

allows a cholesterol molecule to insert itself into the lipid bilayer so that its head group interacts with hydrophilic head group of phospholipids, while the ring structure participates in van der Waals interactions with the fatty acid chains

Question 32

How does cholesterol increase or decrease membrane fluidity depending on the temperature?

Answer 32

at typical temperature in a cell:
- decreases fluidity because interaction of rigid ring structure with the phospholipid fatty acid tails reduces mobility of the phospholipids

low temperature:
- increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids

Question 33

What does cholesterol do?

Answer 33

helps maintain a consistent state of membrane fluidity by preventing dramatic transitions from a fluid to a solid state

Question 34

What are transporter proteins?

Answer 34

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

Question 35

What are receptor proteins?

Answer 35

membrane proteins that allow the cell to receive signals from the environment

Question 36

What are enzymes?

Answer 36

proteins that catalyze chemical reactions

Question 37

What are anchors?

Answer 37

membrane proteins that attach to other proteins and help to maintain cell structure and shape

Question 38

What are the two groups of membrane proteins?

Answer 38

integral membrane proteins and peripheral membrane proteins

Question 39

What are integral membrane proteins?

Answer 39

proteins that are permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself

Question 40

What are peripheral membrane proteins?

Answer 40

proteins that are temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions

are easily separated from the membrane by simple experimental procedures that leave the structure of the membrane intact

Question 41

What are transmembrane proteins?

Answer 41

(most) integral membrane proteins that span the entire lipid bilayer

Question 42

What are the 3 regions of transmembrane proteins?

Answer 42

• two hydrophilic: one protruding from each face of the membrane
• one connecting hydrophobic that spans the membrane

Question 43

What does the structure of transmembrane proteins allow?

Answer 43

allows for separate functions and capabilities of each end of the protein

ie. hydrophilic region on the external side of a receptor can interact with signalling molecules, whereas the hydrophilic region of that receptor protein on the internal side of the membrane often interacts with other proteins in the cytoplasm of the cell to pass along the message

Question 44

What are peripheral membrane proteins associated with?

Answer 44

either the internal or external side of the membrane

Question 45

What do peripheral membrane proteins interact with?

Answer 45

either with the polar heads of lipids or with integral membrane proteins by weak noncovalent interactions such as H-bonds

Question 46

How are peripheral membrane proteins associated with the membrane?

Answer 46

transiently associated, and can play a role in transmitting info received from external signals, while others limit the ability of transmembrane proteins to move within the membrane and assist proteins in clustering lipid rafts

Question 47

What is the fluid mosaic model?

Answer 47

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