MCB5 - Transport Across Cell Membranes

Question 1

Which types of molecules can pass freely through lipid bilayer and give examples.

Answer 1

Small uncharged or non polar molecules e.g oxygen, co2, nitrogen etc.

Question 2

Which type of molecule can not pass the lipid bilayer at all.

Answer 2

Charged molecules eg all ions.

Question 3

Why do cells need to transport materials across cell membrane.

Answer 3

Import nutrients, glucose and amino acids.

Export waste material.

Question 4

Why can hydrophilic molecules not cross a lipid bilayer membrane freely.

Answer 4

Interior of bilayer is hydrophobic meaning hydrophilic molecules cannot pass through easily

Question 5

What are the three ways in which transmembrane proteins transverse the entire bilayer.

Answer 5

Single alpha helix.
Multiple alpha helices.
Beta barrels.

Question 6

Discuss how a transmembrane protein can traverse the membrane in terms of the structure of the polypeptide chain.

Answer 6

Hydrophobic region of polypeptide chain sits in the hydrophobic interior of the bilayer.

Question 7

What are the two types of transmembrane proteins for transport.

Answer 7

Transporters and channels.

Question 8

What are membrane transport proteins.

Answer 8

Proteins existing within a membrane that aid transporting of various molecules.

Question 9

Discuss structure of channels as transport proteins.

Answer 9

Hydrophobic amino acid side chains form exterior of channel whilst hydrophilic side chains form interior pore allowing water soluble substances through.

Question 10

Discuss how transporter proteins work including their specificity.

Answer 10

Only allows molecules that can fit into the binding site through. Conformational changes occur which move the molecule across the membrane.

Question 11

Discuss how channels work as transporter proteins making reference to their specificity.

Answer 11

Specific to molecules depending on size and charge. Allows many molecules through at one time. Can have open or closed conformations based on different stimuli.

Question 12

How is the selectivity of ion channels maintained.

Answer 12

Internal structure as a specific size and charge which must be complementary to the molecule being transported.

Question 13

Discuss simple diffusion.

Answer 13

Passive process. Small uncharged and non-polar molecules. Down a concentration gradient,

Question 14

Discuss channel mediated diffusion.

Answer 14

Passive process. Transported down concentration gradient. Commonly for ions moving through channel during open conformation.

Question 15

Discuss transporter mediated transport of molecules.

Answer 15

Can be active or passive. Passive - down concentration gradient. Active - against concentration gradient and requires energy.

Question 16

Define pumps involved in transport across cell membranes. J

Answer 16

Transporter proteins that carry out active transport so require energy.

Question 17

Discuss membrane potential

Answer 17

Voltage difference across a membrane as a result of varying ion concentrations on either side of membrane.

Question 18

Which actions are abundant on either side of the cell membrane.

Answer 18

Na+ abundant on extracellular side.

K+ abundant on intracellular side.

Question 19

What is the normal membrane potential and how is it created.

Answer 19

Usually a negative membrane potential. More Na+ on exterior than K+ in interior resulting in negative membrane potential of inside compared to outside.

Question 20

Which amino balances Na+ on extracellular side.

Answer 20

Cl-

Question 21

Which negative anion balances K+ on intracellular side.

Answer 21

Organic and inorganic anions.

Question 22

Define an electrochemical gradient.

Answer 22

Net force driving a charged molecule across a cell membrane depending on both the concentration gradient and the membrane potential

Question 23

Why does electrochemical gradient affect only charged molecules

Answer 23

Membrane potential has no effect on uncharged molecules

Question 24

Discuss the role of electrochemical gradient in the movement of Na+ ions.

Answer 24

Na+ abundant on outside so can move in, along its concentration gradient. Membrane potential is negative so pulls Na+ into cell. Overall large movement of Na+ into cell.

Question 25

Discuss the role of electrochemical gradient in the movement of K+ ions.

Answer 25

K+ abundant in cell interior so moves down its concentration gradient to the extracellular space. Membrane potential opposes this as K+ is moving from negative to positive even though it’s positive therefore only small amount of K+ moves to the extracellular side.

Question 26

Give three examples of membrane transporter proteins in a cell.

Answer 26

Plasma membrane transporters. Mitochondrial transporters. Lysosomal transporters

Question 27

Discuss one way in which lysosomal transporters aid the processes carried out by lysosomes.

Answer 27

Allow the movement of H+ into the lysosome creating acidic conditions which help digestion of waste material.

Question 28

Why are mitochondrial transporters vital for their role.

Answer 28

Imports pyruvate created in glycolysis from cytosol to the mitochondrion interior.

Question 29

In what scenario does glucose Move into a cell.

Answer 29

Following a meal, glucose concentration in the blood is high meaning it will move into the cell.

Question 30

Discuss process by which glucose moves into a cell.

Answer 30

Glucose binds to exposed binding site whilst transporter protein is in outward open state. Conformational changes occur, which pass through occluded state, forming the inward open state. Glucose released on intracellular side.

Question 31

What is the occluded state.

Answer 31

State of glucose transporter protein where neither the extracellular or intracellular binding site is exposed.

Question 32

Give scenario where glucose moves from inside cell to outside.

Answer 32

If blood glucose concentration is low, Liver produces glucose from glycogen. Glucose must then be transported out of hepatocytes.

Question 33

What are the three types of pump transport proteins.

Answer 33

Coupled, ATP-driven and light-driven

Question 34

Discuss mechanism of action for coupled pumps.

Answer 34

Couples downhill transport of one molecule to uphill transport of next molecule. Uses electrochemical gradient.

Question 35

Discuss mechanism of action for ATP driven pumps.

Answer 35

Couples uphill transport of one molecule to ATP hydrolysis, which provides the energy.

Question 36

Discuss mechanism of action of light driven pump and where they are found.

Answer 36

Uses energy from sunlight to drive uphill transport. Mainly found in bacteria,

Question 37

Discuss how the sodium potassium pump works.

Answer 37

3 Na+ ions bind to sodium potassium pump on extracellular binding site. On inside, ATP is cleaved resulting in phosphorylation of pump protein. Conformational change occurs which releases Na+ on cytosolic side. 2 K+ ions bind to intracellular binding sites. Triggers release of Phosphate group which causes conformational change back to original, releasing K+ on extracellular side.

Question 38

What are three types of transporter proteins.

Answer 38

Uniport,symport, antiport

Question 39

Discuss mechanism of action for uniport transport proteins.

Answer 39

Movement of one type of solute from one side of membrane to other.

Question 40

Discuss mechanism of action for antiport transporter proteins.

Answer 40

Carries two solutes in opposing directions. One solute moves downhill here as other moves uphill..

Question 41

Discuss mechanism of action for symport transporter proteins.

Answer 41

Carries two solutes in the same direction, one downhill and one uphill.

Question 42

Discuss mechanism of action for ion channels

Answer 42

Ion solvent shell is shed. Ions move in single file through the narrow pore in a channel transporter protein. Selectivity filter within ion channel ensures correct ion is moving through the channel based on charge and size. Once out of the pore, the aqueous solvent shell reforms around the ion.

Question 43

What selectivity filter is present for K+ ions in K+ ion channels.

Answer 43

Polar carbonyl groups are found in the selectivity filter which interact with K+ and form transient bonds.which are unable to be formed with other ions based on their size or char.ge

Question 44

Why can’t ions move through the ion channel with their solvent shell.

Answer 44

Would be too large.

Question 45

What are the two main characteristics of ion channels.

Answer 45

Only open briefly but still allow many ions to pass through. Differ from transporters as they don’t have conformational changes per ion that passes through.

Question 46

What are the four types of stimuli that can change the conformation of a channel transporter protein.

Answer 46

Voltage. Ligand binding - extracellular or intracellular. Mechanical stimulus.

Question 47

Define osmolarity.

Answer 47

Total concentration of solute particles inside the cell.

Question 48

Name of channel specifically for transport of water.

Answer 48

Aquaporins.

Question 49

Discuss difference between osmolarity and concentration of solutes outside cell.

Answer 49

Osmolarity is generally higher than concentration of solutes outside cell meaning water is pulled into the cell by the osmotic gradient.

Question 50

Why are aquaporin channels necessary.

Answer 50

Membrane is semi permeable to water as water is small and uncharged however movement occurs slowly so aquaporins increase efficiency of water movement into the cell.

Question 51

Give example of cell that would require aquaporins

Answer 51

Epithelial cells of kidney for water reabsorption.

Question 52

Which amino acids acts as a selectivity filter for aquaporins.

Answer 52

Asparagine. Allows water but prevents h+ moving through channel.