Molecular Movement across cell membranes

Question 1

What is the lipid bilayer ?

Answer 1

Lipid bilayer cell membrane is a selectively permeable barrier to movement of substances.

Question 2

What is the role of intrinsic proteins ?

Answer 2

Intrinsic proteins in the membrane provide pathways for some substances to cross the membrane.

Question 3

Uniport

Answer 3

Moving 1 type of molecule in one direction

Question 4

Co- transport

Answer 4

Moving different types of molecules in different directions

Question 5

Symport

Answer 5

Different molecules being moved in the same direction

Question 6

Antiport

Answer 6

Different molecules being moved in different directions

Question 7

Diffusion

Answer 7

Occurs down a concentration gradient

Through lipid bilayer or involves a protein ‘channel’ or ‘carrier’

No additional energy required

Question 8

Active transport

Answer 8

Occurs against a concentration gradient

Involves a protein ‘carrier’

Requires energy (ATP)

Question 9

Describe passive transport

Answer 9

Particles eventually reach an equilibrium
Particles move along the concentration gradient

Net diffusion is equal to outside - inside concentration

Question 10

What does the rate of diffusion in a cell depend on ?

Answer 10

Thickness and viscosity of membrane and size, shape, polarity and solubility in membrane of substrate.

Question 11

Delta S

Answer 11

Concentration gradient across a membrane

Question 12

Partition coefficient (K)

Answer 12

Gives a measure of how well a substance dissolves in lipid or aqueous phase.

X/X
e.g. Oil / Water

Question 13

Emulsion

Answer 13

An emulsion is a mixture of two or more liquids that are normally immiscible owing to liquid-liquid phase separation.

Question 14

What does the rate of transport through lipid phase of membrane depend on ?

Answer 14

Polarity

Less Polar —-> Increase in diffusion rate

Except methanol which is HIGHLY polar, but diffuses quickly.

Question 15

What transports water ?

Answer 15

Intrinsic protein channels called aquaporins
These were discovered in 1992 and explained the anomaly, as to why water moves more quickly, than in diffusion.

Question 16

Describe aquaporins

Answer 16

They form tetramers in the membrane - each monomer acts as a water channel.

Each channel is approximately 2.8A at its narrowest point allowing continuous passage of one water molecule at a time.

1 water molecule wide, they are too narrow to permit any of the hydrated ions to pass through.

Question 17

How many molecules of water can pass through an aquaporin per second ?

Answer 17

Up to 3x10^9 molecules

Question 18

Describe the membrane arrangement of aquaporins

Answer 18

Secondary structure of aquaporins contains 6 α-helices connected by 3 extracellular and 2 intracellular loops.

All 6 α-helices exist in a closely associated tertiary monomer structure. Water passes through the transmembrane pore formed through the centre of the three-dimensional barrel

AQP monomers homotetermerize and create a five-pore quaternary structure.

Function of the central pore, formed by the space between all four monomers, remains largely unknown.

Question 19

Describe passive transport of diffusion through aqueous membrane channels

Answer 19

Substance stays in aqueous solution and passes through hydrophilic channels.

Usually highly specific, rates can be very high

Few pores needed to give a big difference in permeability.

Question 20

How many aquaporins are found ?

Answer 20

13 aquaporins found in different cells/tissues of the human body that differ in number/time/demand thus controlling differential water permeability/flow/function.

Question 21

Where are aquaporins most commonly found ?

Answer 21

Red Blood cells
Kidneys

Question 22

Types of Aquaporins in the kidney

Answer 22

AQP1,2,3,4,6,8,11

Question 23

Aquaporins with a low water permeability

Answer 23

AQP :

0
6
9
10

Question 24

ADH

Answer 24

Antidiuretic hormone
Regulates AQP-2 in the kidney

Question 25

Function of AQP-2

Answer 25

ADH regulates AQP-2 in the kidney by stimulating movement of AQP-2 to the luminal side of the renal cell membrane in late distal tubules, collecting tubules and collecting duct This increases water absorption There are other AQPs on the basolateral membranes, probably not regulated by ADH

Question 26

Describe some facts about protein channels

Answer 26

Many are highly specific May be open/closed by a gate Gated channels can be voltage/ligand gated Channels are either open/closed Channels are open for less than to a few ms

Question 27

Voltage gated channels

Answer 27

Potential difference inside/outside cell causes a conformational change.

Question 28

Ligand gated channels

Answer 28

Binding of a chemical ligand (e.g. acetyl choline) causes a conformational change.

Question 29

Sodium gated voltage channels

Answer 29

They are lined with negatively charged amino acids that pull the sodium ion away from its water shell. The smaller un-hydrated sodium ion can then diffuse through the channel. Un-hydrated potassium ions are too large. K+ uses another channel

Question 30

Describe the selectivity of the potassium channel

Answer 30

Carbonyl oxygens in the selectivity channel strip water molecules from the potassium molecule (NOT THE SODIUM MOLECULE). So only potassium ions can permeate.

Question 31

Similarity between facilitated diffusion and active transport

Answer 31

In both cases, a substance binds onto a specific carrier resulting in a conformational change which transports the substance.

Question 32

Saturate meaning

Answer 32

Limited binding sites and it takes time for transport to occur.

Question 33

Vmax

Answer 33

The limit of the carrier protein to operate.

Question 34

Primary active transport

Answer 34

Energy is directly from an energy source (e.g. ATP)

Question 35

Secondary active transport

Answer 35

Indirect use of energy Able to move 'uphill' without itself breaking down ATP Energy which is stored as a concentration difference resulting form a "secondary" process.

Question 36

Examples of primary active transport

Answer 36

Sodium Potassium pump Ca2+ ATPase transporter H+ ATPase transporter

Question 37

Ca2+ ATPase transporter

Answer 37

Involved in muscle contraction Present on the cell membrane and the sarcoplasmic reticulum in muscle fibres. Maintains a low systolic Ca2+ concentration

Question 38

H+ ATPase transporter

Answer 38

Cells in stomach Found in parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH) Concentrates H+ ions up to 1 million fold

Question 39

Sodium Potassium pump energy source

Answer 39

Na/K ATPase Obtains energy directly from breakdown of ATP Transports both sodium and potassium

Question 40

What determines the direction of the Na K ATPase pump ?

Answer 40

Na, K, ATP, ADP concentrations

Question 41

What can drive the Na K ATPase pump in reverse ?

Answer 41

ATP being made from ADP

Question 42

In electrically active cells, how much ATP is used to pump K+ in and Na+ out of cells ?

Answer 42

60-70% of the cells energy

Question 43

Importance of sodium potassium pump

Answer 43

Controls cell volume Contributes to electrical potential across the membrane : 3Na out and 2K in - a net loss of ions per pump action Can be used to drive secondary transport

Question 44

Coupling of primary and secondary active transport

Answer 44

Primary (direct) active transport 1. H+ is moved from a low to high concentration using energy from ATP Secondary (indirect) active transport 2. H+ at high concentration outside H+ passes through channel - doesn't need energy due to the electrochemical gradient. Process is EXERGONIC 3. Energy from 2 is used to transport S against its concentration gradient.

Question 45

Co transport examples

Answer 45

Sodium and Glucose Sodium and Amino Acids Sodium and Calcium (anti port)

Question 46

Describe secondary active transport with Na+ symporter

Answer 46

Na+ across the membrane is high outside and low inside the cell (maintained using ATP Na/K pump) Na+ creates electrochemical energy due to pressure to diffuse through the membrane. Na+ and a second molecule bind to the symporter and both are transported into the cell using Na+ electrochemical energy. (e.g. glucose, amino acids and 2x HCO3-)

Question 47

Symporter

Answer 47

Transport substance in the same direction as a 'driver' ion. e.g. Na+ Involves the use of an electrochemical gradient (usually from sodium) Protein co-transporters are classified as symporters or anti-porters.

Question 48

Na+ Glucose symporter

Answer 48

When both molecules engage conformational changes to allow transport.

Question 49

Antiport

Answer 49

Transports substance in the opposite direction of a 'driver' ion. Uses Na+ electrochemical energy to transport calcium or hydrogen ions out of the cell while Na+ enters. Transport is in the opposite direction to the primary ion (e.g. Na+)