B2.1 - Membrane and Membrane Transport

Question 1

Singer and Nicolson - Fluid Mosaic Model

Answer 1

• phospholipid bilayer model
  Features:
  1) Integral Membrane Proteins
  2) Phospholipids
  3) Cholesterol
  4) Glycolipids
  5) Glycoproteins

Question 2

Membrane Proteins

Answer 2

1) Integral
2) Peripheral

Question 3

Integral

Answer 3

1) Transmembrane - traverse both the membranes
2) Non transmembrane - traverses only one membrane

Question 4

Peripheral

Answer 4

Both sides are hydrophilic
- anchors cell membrane to the cytoskeleton

Question 5

Factors that affect fluidity

Answer 5

• saturation of hydrocarbon tail
• hydrocarbon tail length
• presence of cholesterol and proteins
• phospholipid molecules are constantly moving about by diffusion accounting for fluidity
• It’s sphere so there is often lateral movement but occasionally there is also movement along the vertical plane (flip-flopping).

Question 6

Lipid bilayers as barriers

Answer 6

Since any phospholipid molecule shows both hydrophobic and hydrophilic properties, they are said to be amphipathic.

Tails form a core of low permeability to large molecules and hydrophilic particles

• polar molecules cannot enter through barries but nonpolar molecules can

Question 7

Relationship between saturation and fluidity

Answer 7

more unsaturation = more fluidity
shorter length of hydrocarbon tail = more fluidity

(decreases both their freezing and melting point)

Question 8

Key phrases

Answer 8

favoring solidity
favoring fluidity
membrane raft
kinks
hydrophobic interaction

Question 9

Cholesterol

Answer 9

• increases stability of membrane, acts as a fluidity buffer
• steroid
• head group is hydroxyl group

In high temperatures, phospholipids become more mobile compromising the fluidity of the membrane. Cholesterol contrains movement of fatty acid tails and decreases fluidity

In low temperatures, the bilayer would usually become rigid. Cholesterol disrupts close package of non polar fatty acid chains so that fluidity increases.

Question 10

Transport mechanism

Answer 10

1) Passive (doesn’t require energy in the form of ATP)
2) Active (requires energy in the form of ATP)

Question 11

Passive transport

Answer 11

• Non spontaneous
  2 types of diffusion:
• passive diffusion
• facilitated diffusion (protein transporter mediated)

Takes place along a concentration gradient, which shows the difference in gradient between two areas. Steepness of gradient determines the rate at which particles diffuse.

Question 12

Active transport

Answer 12

1) Bulk (vesicle mediated)
- endocytosis [taking in]:
phagocytosis
pinocytosis
- exocytosis [releasing out]
2) Active

Question 13

Simple diffusion

Answer 13

the passive transport (passive net movement) of molecules from a region of high concentration to a region of lower concentration across a semi-permeable membrane until a dynamic equilibrium is reached.

Question 14

Factors affecting rate of diffusion

Answer 14

1) Concentration
2) Size and type of molecules
3) Diffusion pathway - the longer the pathway, the slower diffusion and volume of the cell.
4) Surface area
5) Moisture (based on their solubility)

Question 15

Facilitated diffusion

Answer 15

Diffusion using a helper protein

Faster and mediated by specialized cells. Increases the specificity of the process.

Some cells have aquaporins (channel proteins) that specifically allow water to pass through. Permeability of water is therefore increased

The transporter molecule must bind to the carrier protein

The carrier protein undergoes conformation change

Question 16

Transporters

Answer 16

Class of carrier protein

Question 17

Uniporter

Answer 17

Unidirectional, high to low conc

Question 18

Symporter

Answer 18

transports two different molecules along the conc grad (high to low)

Question 19

Anti-porter

Answer 19

transports two molecules, one against the gradient and one along the gradient

Question 20

Ion Channels

Answer 20

Rate of transport is higher in ion channels than transporters
- highest concentration is in neurons

Gated ion channel involves the rapid movement of sodium and potassium ions (signal) across the axon membrane.

Question 21

Ligand gated

Answer 21

Ligand can be a neurotransmitter which opens or closes the channel (binds to the protein channel)

Acetylcholine (present in neurotransmitter junction) binds to nicotinic acetylcholine receptor in the channel which opens and Na+ ions come inside cell, triggering a signal

Question 22

Mechanically Gated

Answer 22

Signals make pressure open or close the channel

Question 23

Voltage Gated

Answer 23

Voltage changes membrane potential which triggers open/close of channel.

Voltage - changes in Na+ or K+ ions

-70mv = resting potential
Voltage below a cetrain value, gate stays closed. Voltage above a certain value, gate opens.

1) When there is a change in voltage, the K+ channel opens allowing K+ ions to diffuse, causing the voltage to come back to equilibrium/resting state.
2) The channel closes again quickly due to an extra globular protein subunit which is attached to a flexible chain of amino acids
3) the ball fits inside the open pore, closing the channel again.

Question 24

Channel vs Transporter

Answer 24

Channel:
- channel opening is caused by a stimuli other than the ion to be transported (neurotransmitter, voltage, pressure)
- have a faster rate of transport
- transport only ions
Transporter:
- requires the molecule to bind to the transporter
- slower rate of transport
- transport both ions and molecules

Question 25

Sodium Potassium Pump

Answer 25

Na+ on the inside of the cell (goes out) K+ on the outside of the cell (goes in) Sodium potassium Pump - Actively transports ions against their concentration gradient 1) ADP + P forms ATP 2) Phophorlyation of ATP cause a comformational change in the protein channel. 3) Opens up binding site for sodium ions in the interior of the cell (3 Na+ ions bind to binding sites) 4) The 3 Na+ binding triggers a conformation change and the channel faces the exterior, releasing the 3 Na+ 5) On the exterior, the two K+ ions bind, triggering a conformation change 6) The ATP gets dephophorylated to ADP + P triggering the release of the two K+ ion inside the cell.

Question 26

Favourable vs unfavourable movement

Answer 26

favourable - releases energy unfavourable - demands energy

Question 27

CAM (cell-adhesion-molecules)

Answer 27

proteins with one part embedded in the phospholipid bilayer and other parts projecting outwards into the extracellular environment - forms a complex

Question 28

Gap Junction

Answer 28

- protein channels, allows ions to rush in and rush out (nerve cells and heart cells) - intracellular channels physically connecting neighbouring cells for the movement of molecules

Question 29

Tight Junction

Answer 29

- interlocked tightly with cytoskeleton proteins so less can pass through - present in epithelial cells - form a tight seal between two neighbouring cells and act as occluding (preventing entry) junctions. - this barrier prevents the unregulated movement of molecules across the barrier.

Question 30

Desmosomes

Answer 30

- anchoring junction (between cell and basement membrane) - facilitate cell-cell adhesion between adjacent cells in tissues to ensure structural stability allowing cells to withstand mechanical stress

Question 31

Junction proteins

Answer 31

Cell-cell = cadherin cell-extracellular matrix = integrin

Question 32

Signalling Junction

Answer 32

relay signal between 2 cells

Question 33

Plasmodesmata

Answer 33

- plants have plasmodemata instead of gap junction enables transport of proteins, ions, and water (phloem), communication (transfer of chemical signals) - channel between cells of neighbouring plant cells.

Question 34

Bulk Transport

Answer 34

1) Endocytosis 2) Exocytosis

Question 35

Endocytosis

Answer 35

can involve the uptake of solid substances (phagocytosis) or liquid substances (pinocytosis)

Question 36

Receptor-mediated endocytosis

Answer 36

Is done majorly - iron/Fe3+ transport inside the cell - hormones - fats - neurotransmitters

Question 37

Phagocytosis

Answer 37

generally used by bacterial cells, phagocytosis by WBCs, absorption of nutrients in in amoeba/bacteria, lysosome

Question 38

Pinocytosis

Answer 38

water and mineral uptake by unicellular organisms from the environment

Question 39

Exocytosis

Answer 39

involves the release of a large amount of molecules out of a cell.

Question 40

Glycolipids

Answer 40

- Carbohydrates (sugar) linked to lipids (of the cell membrane) - Part of the plasma membrane of cells - carbohydrate part projects into the extracellular environment. - contributes to membrane stability since it forms H-bonds with water molecules

Question 41

Glycoproteins

Answer 41

- Proteins with carbohydrate as an additional component - Part of the plasma membrane of cells - protein part embedded in membrane, carbohydrate parts projects into extracellular environment - cell-cell recognition

Question 42

Glycocalyx

Answer 42

carbohydrate rich layer on the outer surface of the plasma membrane of animal cells made up of glycolipids and glycoproteins. - signifies a specific type of cell (self cell and non self cell)