Chap 12 Membrane strcuture and function

Question 1

What is a lipid bilayer?

Answer 1

The favoured structure for most phospholipids and glu=ycolipids in aqueous media. Composed of teo lipid sheets. The hydrophobic effect is a driving force for the formation of lipid bilayers.

Question 2

Name some of the features common to all membranes

Answer 2

1. Membranes are sheet-like structures, two molecules thich
2. Membranes are composed of lipids and proteins
3. Membrane lipids are amphipathic molecules
4. Proteins, unique to each membrane, mediate the transfer of molecules or information.
5. Membranes are non covalent assemblies
6. The membranes are asymmetric
7. Membranes are fluid
8. Membranes are electrically polarized, inside being negative

Question 3

WHat conditions are required for a small molecules to spontaneously pass through a membrane?

Answer 3

First, the molecule must be lipophilic, and second, the concentration of the molecule must be greater on one side of the membrane than the other.

Question 4

Integral membrane protein

Answer 4

Interacts tightly with the membrane interior

Question 5

peripheral membrane protein

Answer 5

Interacts with the border of a membrane

Question 6

Channel

Answer 6

Allows rapid movement of molecules down a gradient across a membrane

Question 7

Passive transport

Answer 7

Facilitated diffusion

Question 8

Active transport

Answer 8

Movement against a concentration gradient

Question 9

Na+ K+ ATPase

Answer 9

Inhibited by digitalis

Question 10

Secondary transporter

Answer 10

Uses the energy of one gradient to create another

Question 11

Antiporter

Answer 11

Molecules moving in opposite directions

Question 12

Symporter

Answer 12

Molecules moving in the same direction

Question 13

IOn channel

Answer 13

Can be voltage-gated or ligand-gated

Question 14

Differenciate between simple and facilitated diffusion

Answer 14

Simple diffusion- the molecule can diffuse down its concentration gradient through the membrane.
Facilitated diffusion- the molecule is not lipophilic and cannot directly diffuse through the membrane. A channel or carrier is required to facilitate movement down the gradient

Question 15

Differentiate between passive and active transport

Answer 15

In passive transport (facilitated diffusion), a substance moves down its concentration gradient through a channel or transporter.
In active transport, a concentration gradient is generated at the expense of another source of energy, ATP.

Question 16

Suggest how a loss of fluidity or too much fluidity in the membrane might affect membrane-protein function

Answer 16

Most proteins require conformational changes to work, so if the membrane was too rigid, the required structural conformations would not be attained. If the membrane were too fluid, the interactions with the environment that the protein needs to maintain structure would be disrupted.

Question 17

WHat is the relation between the sodium potassium pump and the strength of a heart contraction. Identify the relevant primary and secondary active-transport components.

Answer 17

The heart contraction is initiated by the release of calcium from its stores and terminated by the removal of calcium from the cytoplasm. The removal is done by a sodium-calcium antiporter, which removes calcium out of the cell, against its concentraation gradient by allowing sodium to flow in, down its conc gradient. The sodium generated is established by the Na K ATPase. Cardiotonic steroids function by inhibiting the Na K ATPase, which inhibits the sodium-calcium antiporter. As a result, calcium, the signal for contraction, remains in the heart cell longer allowing for a more robust heartbeat.

Question 18

Suggest a biochemical basis for the lethal action of ouabain

Answer 18

Ouabain, like digitalis, inhibits the Na K ATPase, which is crucial to maintaining the sodium gradient that renders nervous and muscle cells electrically excitable. Inhibition of the enzyme shuts down cardiac and respiratory function.

Question 19

What are two fundamental properties of all ion channels?

Answer 19

Selectivity and the rapid transport of ions

Question 20

Differentiate between ligand-gated and voltage-gated channels

Answer 20

Ligand-gated channels open in response to the binding of a molecule by the channel, whereas voltage-gated channels open in response to changes in membrane potential,

Question 21

True or false;
The sodium-glucose linked transporter does not depend on the hydrolysis of ATP

Answer 21

False. Although the cotransporter does not directly depend on ATP, the formation of the Na+ gradient that powers glucose uptake depends on ATP hydrolysis.

Question 22

List two forms of energy that can power active transport

Answer 22

ATP hydrolysis and the movement of one molecule down its concentration gradient coupled with the movement of another molecule up its concentration gradient

Question 23

Differentiate between peripheral and integral proteins

Answer 23

Peripheral proteins are attached to the phospholipid head groups of membrane lipids or the exposed portions of integral membrane proteins. Integral proteins are embedded in the membrane.

Question 24

Lipid bilayers are able to fill in holes immediately. What is the energetic basis of this self-sealing?

Answer 24

The hydrophobic effect. If there is a hole, the hydrophobic tails of the phosphilipids will come together, freeing any associated water.

Question 25

What is the energetic basis of all biological membranes being asymmetric?

Answer 25

For both sides of the membrane to become identical, the hydrophilic parts of the lipids, proteins, and carbohydrates would have to pass through the hydrophobic interior of the membrane. Such movement is energetically unfavorable.

Question 26

Why is an alpha helix especially suitable for a transmembrane-protein segment

Answer 26

All of the amide hydrigen atoms and carbonyl oxygen atoms are stabilized in the hydrophoboc environment by intrachain hydrogen bonds. If the R groups are hydrphobic, they will interact with the hydrophobic interior of the membrane, further stabilizing the helix.