Membranes

Question 1

3 simple states

Answer 1

micelle, liposome, bilayer sheet

Question 2

Functions of biological membranes

Answer 2

• cell boundary
• define / enclose compartments
• control movement of material in and out of cell
• allow response to external stimuli
• enable interactions between cells
• provide scaffold for biochemical activities
• first 5 are unique to plasma membrane

Question 3

which type of membranes don’t have a bilayer?

Answer 3

micelle

Question 4

What is a lipid bilayer

Answer 4

polar / hydrated biological membrane made up of phospholipids

• proteins are embedded in the lipid bilayer
• about equal in protein and lipid contents
• also includes cholesterol and associated sugar

Question 5

The plasma membrane

Answer 5

• separates the interior of the cell from the outside environment
• consists of a semipermeable lipid bilayer
• regulates the transport of materials entering and exiting the cell

Question 6

Fluid mosaic model

Answer 6

describes the structure of the plasma membrane

• fluid refers to individual lipid molecules move
• mosaic refers to diverse particles penetrate the lipid layer
• components are mobile and can interact

Question 7

amphipathic

Answer 7

having both hydrophobic and hydrophilic regions

- lipid bilayer is hydrated due to hydrophilic region

Question 8

Types of proteins found in the membrane

Answer 8

integral (transmembrane), peripheral, lipid anchored

Question 9

What makes biological membranes dynamic?

Answer 9

• lipids move easily, laterally, within leaflet
• lipid movement to other leaflet is slow
• membrane proteins diffuse within the bilayer

Question 10

integral membrane proteins

Answer 10

span the lipid bilayer via alphahelixes that form with hydrophobic amino acids

• permanently imbedded in the membrane
• functions include channel / transport molecules across the membrane, cell-cell communication, attachment

Question 11

Peripheral membrane protiens

Answer 11

associate with surfaces of the bilayer and with other integral protein anchors

• usually exist on the inside of the cell
• can attach via polar head group interactions or via protein-protein interactions with integral membrane proteins

Question 12

Myelin sheath

Answer 12

• the myelin sheath of a neuron consists of very low amounts of protein
• consists of layers of plasma membrane forming insulation around the nerve axon

Question 13

symmetry of biological membranes

Answer 13

Asymmetrical

• two leaflets have distinct composition
• outer leaflet usually contains glycolipids and glycoprotiens

Question 14

Membrane fluidity

Answer 14

determined by the nature of lipids in the membrane

• unsaturated lipids: increase fluidity
• saturated lipids: decrease fluidity

Question 15

How does temperature affect fluidity?

Answer 15

• warming increases fluidity: liquid crystal
  (fluid like consistency)
• cooling decreases fluidity: crystalline gel
  (gel like consistency)

Question 16

How does membrane fluidity affect cell function?

Answer 16

membrane fluidity must be maintained

- changes in temperature and lipid composition can result in the desaturation of the lipids or exchange of lipid chains

Question 17

unsaturated vs saturated fatty acid chains

Answer 17

Unsaturated: have one or more double bonds that introduce kinks in the phospholipid, reducing tightness of packing
Saturated: lacks double bonds, resulting in phospholipids with straight structure that favours tight packing

Question 18

How does cholesterol effect the membrane?

Answer 18

Cholesterol modulates membrane fluidity

• amphipathic structure of cholesterol allows it to pack tightly with phospholipids, alters flexibility
• add to liquid crystal membrane: fluidity decreases
• add to crystalline gel membrane: fluidity increases
• important for maintaining homeostasis of membranes

Question 19

True or false: cells with different functions have membranes with different protein compositions?

Answer 19

True

Question 20

Transmembrane domain

Answer 20

a hydrophobic (uncharged) peptide sequence that spans the PM

• permanently insert proteins into the PM
• alpha-helical
• foster protein-protein interactions

Question 21

movement of different substances across the membrane

Answer 21

small, uncharged molecules - move across the membrane easily. Ex: O2, CO2, NO2, H2O
large, charged, polar molecules - cannot easily cross lipid bilayers

Question 22

4 basic mechanisms for moving molecules across membranes

Answer 22

1. simple diffusion
2. diffusion through a channel
3. carrier-mediated diffusion
4. active transport

Question 23

Which basic mechanism requires an input of energy?

Answer 23

Active transport - often uses chemical energy in the form of ATP

Question 24

Passive transport

Answer 24

Does not require an input of energy

• consists of simple diffusion, diffusion through a channel and carrier mediated diffusion
• moves molecules from high concentration to low concentration

Question 25

simple diffusion

Answer 25

- very small uncharged molecules - flow down the concentration gradient - molecules like CO2 and O2 - H2O travels via osmosis

Question 26

diffusion through a channel

Answer 26

- ion channels are integral membrane proteins that form an aqueous pore - selective and only allow certain ions to pass - move down the concentration gradient Voltage gated channels: responds to changes across the membrane ligand gated channels: responds to binding of a specific molecule which causes conformational change in structure

Question 27

What is the target of toxins?

Answer 27

Ion channels - Tetrodotoxin is an Na+ channel blocker (potent neurotoxin) - Curare is an acetylcholine competitive antagonist of the acetylcholine receptor

Question 28

Aquaporins

Answer 28

specific water channels | - water moves through aquaporins channels in a single file line

Question 29

Facilitated diffusion

Answer 29

- compound binds specifically to integral membrane protein (facilitated transporter) - change in transporter conformation allows the compound to be released on the other side of the membrane

Question 30

Transportation of glucose across the membrane

Answer 30

Glucose fits like a lock and key in the transporter that induces a conformational change in the transporter. The glucose is released on the other side of the membrane and the protein is ready to receive the next glucose

Question 31

Na+ - glucose symporter

Answer 31

- sodium concentration outside is high because of Na K ATPase - 2 sodium and 1 glucose start on outside of the cell - follows facilitated diffusion - Energy is provided by a chemical gradient of high Na+ concentration outside of the cell

Question 32

Active transport

Answer 32

compound to be transported binds specifically to integral membrane protein (active transporter) - moves AGAINST concentration gradient - Requires an input of energy

Question 33

Sodium Potassium pump

Answer 33

- most common example of active transport - The S-P pump moves sodium from inside the cell to outside the cell - involves 4 proteins - For the hydrolysis of 1 ATP molecule we are transporting The 3 sodiums out and 2 potassiums in

Question 34

What property of integral membrane proteins allows them to remain imbedded in the lipid bilayer?

Answer 34

They have a transmembrane domain made of hydrophobic amino acids

Question 35

Roles of integral membrane proteins

Answer 35

- transporter - receptor - enzyme - anchor