B2.1 membranes and membrane transport

Question 1

Describe the phospholipid molecule

Answer 1

1. 3-carbon glycerol compound + 2 fatty acids + 1 highly polar organic alcohol
2. AMPHIPATHIC - has both hydrophobic and hydrophilic areas. Fatty acids are hydrophobic and the organic alcohol is hydrophilic

Question 2

How do amphipathic lipids form continuous sheet-like bilayers in water?

Answer 2

The hydrophobic and hydrophilic layers cause phospholipids to naturally align as a bilayer in the presence of water, where hydrophobic regions are attracted to one another and hydrophilic regions are attracted to water.

As the fatty acid tails to not attract each other very strongly, the bilayer formed is flexible (this variable shape allows for endocytosis)

Question 3

Describe the two major types of proteins in the membrane

Answer 3

Integral - embedded in one or both layers of the bilayer, has amphipathic character
(inside membrane)

Peripheral - attached to surface of bilayer, often attached to an integral protein
(on top/bottom of membrane)

Question 4

Compare active and passive transport

Answer 4

Passive - does not require ATP, moves down conc. gradient, continues until equilibrium

Active - requires ATP, moves against conc. gradient, no equilibrium reached

Question 5

Aquaporins - function?

Answer 5

• Cell membrane is impermeable to solute molecules, - polar molecules like water cannot pass through quickly, due to the hydrophobic properties of the fatty acid tails (in the middle of the membrane)
• channel proteins called aquaporins allow these water molecules to pass through down the conc. gradient.

Question 6

Compare carrier and channel proteins (integral proteins, facilitated diffusion)

Answer 6

Carrier proteins - changes shape in order to carry a specific ion. Can go both against and down conc. gradient. Can carry both water soluble and insoluble molecules.

Channel proteins - does not change shape. Can only go down conc gradient. Can only carry water soluble molecules.

Question 7

Describe the process of active transport with the sodium-potassium pump

Answer 7

1. A specific molecule binds to a specific binding site on the protein pump.
2. ATP binds to the pump and hydrolyses to become ADP.
3. A phosphate remains attached to pump and causes the pump to change shape.
4. The molecule is moved against conc. gradient and released.
5. The phosphate is released, protein pump returns to its original shape

Question 8

How is the membrane permeability selective?

Answer 8

Facilitated diffusion and active transport are selective processes, as only specific molecules are able to pass through.

Simple diffusion is not a selective process as any small or hydrophobic particle is able to pass through the phospholipid bilayer.

Question 9

Structure of glycolipids and glycoproteins

Answer 9

Glycolipids - phospholipids with carbohydrate chains attached to the head.

Glycoproteins - membrane proteins with carbohydrate chains attached

carbohydrate chains attached by a process called glycosylation.

Question 10

Function of glycolipids and glycoproteins

Answer 10

Cell to cell adhesion:
Some glycoproteins are cell adhesion molecules (CAM) and are responsible for direct attachment to neighbouring cells.
The glycoproteins and glycolipids form an EXTRACELLULAR MATRIX with the glycoproteins and glycolipids with the neighbouring cells, leading to stable cell-to-cell adhesion. The matrix provides structural support and plays and important role in the formation of tissues.

Cell recognition:
the carbohydrate chains on glycoproteins and glycolipids are specific in shape, allowing the immune system to recognise its own cells. If the immune system does not recognise the cell, the unfamiliar carbohydrate chains act as antigens, stimulating an immune response or the production of antibodies.

Receptors for horomones

Cell to cell communication (neurotransmitters bind to glycoproteins)

Question 11

Compare unsaturated fatty acids vs saturated fatty acids and how they relate to membrane fluidity.

Answer 11

Unsaturated fatty acids have double bonds, meaning that they have fewer attached hydrogen atoms. These double bonds within the fatty acid tails cause the molecule to become kinky, or less straight, and hence the fatty acid molecules do not bond together as tightly. Hence membrane fluidity increases, and the membrane has a lower melting point, allowing the membrane to survive low temperatures.

Saturated fatty acids do not have double bonds and hence are bonded to more hydrogen atoms. As there are no double bonds the fatty acid molecule is straighter in shape, allowing the phospholipid bilayer to be denser. This decreases membrane fluidity, making the membrane stronger and allowing the membrane to survive higher temperatures.

Question 12

What effect does cholesterol have on the membrane?

Answer 12

Cholesterol allows for fluidity and stability of the cell membrane, stabilising at higher temperatures and maintaining flexibility at lower temperatures.

At low temperatures (to prevent tighter packing to prevent freezing), cholesterol forces the phospholipids apart to prevent them from packing too closely. This allows components of the membrane to move, preventing freezing.

At high temperatures (to prevent the membrane from breaking apart), bonds between cholesterol and phospholipids prevent them from moving too far away from one another. This prevents the membrane to become too fluid, maintain its structural integrity.

Question 13

Why do plants have lower cholesterol than animals?

Answer 13

Plant cells have very little cholesterol in their fatty acid tails compared to animal cells. This is as cholesterol allows for fluidity and stability of the phospholipid bilayer. Plants do not need this as they have the cell wall.

Question 14

Explain the fluidity of the membrane

Answer 14

Fluidity is dependent on the ratio of unsaturated and saturaed fatty acids in the phosphlipid bilayer
- saturated = less fluid
- unsaturated = more fluid
cholesterol is a buffer at high and low temp

Question 15

Endocytosis example

Answer 15

Endocytosis: the process by which large particles enter the cell

a portion of the plasma membrane is pinched off to enclose macromolecules or particulates within a vesicle in the cell.

• the pinching off results in a change in the shape of the cell membrane
• the result is a vesicle that enters the cytoplasm of the cell
• the ends of the membrane are able to reattach because of the hydrophobic and hydrophilic properties of phospholipids and the presence of water.

Question 16

Exocytosis example

Answer 16

Exocytosis: the release of large particles from the cell.

1. Proteins produced by the ribosomes on the RER enters the lumen of the RER and are packaged into a vesicle.
2. The vesicle is transported to the cis end of the Golgi apparatus.
3. As the vesicle moves through the Golgi apparatus to the trans side it is modified.
4. the vesicle with the modified protein inside fuses with the plasma membrane, resulting in the secretion of the contents from the cell.

Question 17

Explain why enzymes stay within the vesicle during exocytosis

Answer 17

enzymes, being large protein molecules, have charged amino acids and are unable to pass through the phospholipid bilayer.

Question 18

neurotransmitter gated ion channel example

Answer 18

Nicotinic acetylcholine receptors.

When acetylcholine attaches to the channel, the channel is opened, allowing positive ions to flow through (Na, K, Ca)
- this causes membrane potential to change, generating a nerve impulse.

Question 19

voltage gated ion channel example

Answer 19

Sodium and potassium channels.

Voltage gated ion channels are opened by changes in membrane polarity.
1. Sodium channel opens first, Na molecules move from the outside of the neuron to the inside. The channel closes quickly
2. This depolarises the membrane
3. Potassium channels open more slowly, K ion move from the inside of the cell to the outside.
4. This repolarises the membrane, returning the membrane to its normal potential

Question 20

Example of exchange transporters (and active transport)

Answer 20

Sodium-potassium pump

Animals cells have a higher concentration of sodium in the extracellular environment than within the cells, vice versa for potassium.

To generate nerve impulses, neurotransmitter gated ion channels require a certain concentration of Na and K.

This concentration is maintained by the sodium-potassium pump. As it is active transport it requires energy.

Essentially sodium potassium pump maintains the membrane potential, or the electrical charge differential between the interior and exterior of the membrane.

Every cycle, 3 Na out, 2 K in.

Question 21

Example of INDIRECT active transport

Answer 21

Indirect active transport uses energy produced by the movement of a molecule down a conc. gradient to move another molecule against the conc. gradient.

Sodium-dependent glucose cotransporters
1. There are more Na ions outside than inside the cell
2. Na ions and glucose molecules bind to a specific transport protein on the cell membrane
3. Na ions pass through the carrier to the inside of the cel down the conc. gradient, the energy from this movement is captured by the carrier and used to move the Glucose molecule through the carrier against the conc. gradient.

Question 22

Purpose of CAMs

Answer 22

Cell adhesion molecules are involved in cell connections

Cell connections allow for coordinated behaviour and have impt structural functions.

Plant cells produce plasmodenta, tubes connecting cytoplasm of adjacent cells, allowing for exchange of water and small solutes.