Membranes & Transport

Question 1

In 1925, Gorter and Grendel extracted lipids from erythrocytes and spread them as a monolayer. The area of the monolayer was twice the area of the cells. What was their conclusion?

Answer 1

The cell membrane must be two molecules thick - it is a bilayer

Question 2

In 1925, Gorter and Grendel extracted lipids from erythrocytes and spread them as a monolayer. The area of the monolayer was twice the area of the cells, and they concluded the membrane must be a bilayer. Why were erythrocytes used?

Answer 2

Because they do not contain a nucleus or any membrane-bound organelles

Question 3

What model is the most accurate, current representation of the lipid membrane?

Answer 3

The Fluid Mosaic Model - 1972, Singer & Nicholson

Question 4

Why is the lipid membrane described by the fluid mosaic model?

Answer 4

It is a flexible structure, embedded with proteins

Question 5

Where are peripheral proteins located on a lipid membrane?

Answer 5

Peripheral proteins are adsorbed onto the surface of the lipid bilayer by electrostatic forces

Question 6

Where are intrinsic proteins located on a lipid membrane?

Answer 6

Intrinsic proteins can either penetrate the bilayer partially or fully

Question 7

What are the three main functions of the lipid membrane?

Answer 7

1. Compartmentalises the cell and the organelles within it
2. Selectively permeable barrier
3. A solvent for integral membrane proteins

Question 8

Peripheral proteins can be removed easily from the bilayer using mild techniques, such as?

Answer 8

By changing the pH or the ionic strength of the medium

Question 9

How can intrinsic proteins be removed from a lipid membrane?

Answer 9

Through the use of detergents or solvents

Question 10

The cytoskeleton does not form a part of the membrane, however it does play a role in what?

Answer 10

The cytoskeleton plays a role in forming membrane compartments in addition to moving things to and from the membrane

Question 11

How is membrane fluidity controlled?

Answer 11

By a degree of saturation and cholesterol content

Question 12

Is lipid composition in the bilayer membrane asymmetric or symmetric?

Answer 12

Asymmetrically

Question 13

Lipids that are electrically neutral at physiological pH are found in the outer half of the membrane. What are they?

Answer 13

Phosphatidylcholine (PC) and sphingomyelin (SP)

Question 14

What two phospholipids tend to be found on the inner half of the membrane?

Answer 14

Phosphatidylethanolamine (PE) and Phosphatidylserine (PS)

Question 15

What regulates membrane fluidity by altering the length of the fatty acid hydrocarbon chain and the number of double bonds they contain?

Answer 15

Bacteria

Question 16

By shortening the fatty acid hydrocarbon chain, and adding double covalent bonds to the chain, how is the melting temperature affected?

Answer 16

It decreases

Question 17

What are the two functions of cholesterol?

Answer 17

1. Reduces phospholipid movement
2. Stops membranes solidifying by disrupting the regular packing of phospholipids

Question 18

What are the similarities between simple/passive and facilitated diffusion?

Answer 18

1. Both move down a concentration gradient
2. Neither require energy

Question 19

How does the rate of diffusion differ between simple and facilitated diffusion?

Answer 19

The rate of diffusion increases linearly with the concentration difference in simple diffusion, but in facilitated diffusion, the transport systems are saturable and a limiting rate is approached when all carrier proteins are busy

Question 20

What type of molecules move across the membrane by simple diffusion?

Answer 20

Hydrophobic molecules and very small, uncharged polar molecules

Question 21

What type of molecules move across the membrane by facilitated diffusion?

Answer 21

Hydrophilic substances

Question 22

Which requires carrier proteins - simple or facilitated diffusion?

Answer 22

Facilitated diffusion

Question 23

What type of carrier protein does water require to be able to diffuse across a membrane?

Answer 23

An aquaporin

Question 24

What is the type of transport in which molecules move against a concentration gradient, requiring ATP?

Answer 24

Facilitated active transport

Question 25

What are the two types of facilitated active transport?

Answer 25

Primary and secondary

Question 26

How is an electrochemical gradient formed?

Answer 26

When the solute being transported across the membrane carries an electrical charge, an electrochemical gradient is formed

Question 27

What is an electrogenic pump?

Answer 27

A transport protein that generates voltage across a membrane

Question 28

A major electrogenic pump in animal cells pumps 3 sodium ions out and 2 potassium ions into a cell for each ATP hydrolysed. What is the pump called?

Answer 28

Sodium-potassium ATPase

Question 29

What is the main electrogenic pump found in plants, fungi, and bacteria called?

Answer 29

A proton pump

Question 30

What is the definition of co-transport?

Answer 30

Active transport driven by a concentration gradient

Question 31

What are the three general types of transport systems in a lipid membrane?

Answer 31

1. Symport
2. Antiport
3. Uniport

Question 32

What does a uniport do?

Answer 32

Allows the passage of a single molecule across a membrane - can be either active or passive

Question 33

What does a symport do?

Answer 33

Allows the passage of two molecules cross a membrane in the same direction

Question 34

What does an antiport do?

Answer 34

Allows the passage of one molecule in one direction across a membrane, and another molecule in the opposite direction

Question 35

What are the three types of endocytosis?

Answer 35

1. Receptor-mediated endocytosis
2. Pinocytosis
3. Phagocytosis

Question 36

Is electrogenic transport dependent upon ATP, or not at all?

Answer 36

It does require ATP