Unit 5

Question 1

2 types of transport through membrane barriers

Answer 1

1.) Transport can be passive - does not require external energy, only a conc. gradient where the molecule travels down its conc. gradient
2.) Transport can be active - molecules are transported against their conc. gradient, requires energy

Question 2

Classes of transport proteins

Answer 2

1.) ATP-powered pumps, 1-1000 moleucles / sec
a.) Couples hydrolysis of ATP to the transport of a molecule against it’s conc. gradient
2.) Channel proteins, 10^7 - 10^8 molecules / sec
b.) Transport ions down their conc. gradient through a hydrophillic pore in membrane protein. They can exist in an open / closed conformation & when open, many ions can pass simultaneously
3.) Carrier proteins, 100 - 10000 molecules / sec\
c.) Bind water-soluble molecules on one side of the membrane & deliver them to the other side. Involves a conformational change in the protein. Only bind one or a few molecules at a time

Question 3

Uniports

Answer 3

• Work in passive diffusion & are selective for one type of molecule. They move molecules down conc. gradient

Question 4

Symport

Answer 4

• Moves 2 molecules in the same direction

Question 5

Antiport

Answer 5

• Moves 2 molecules in opposite directions

Question 6

Uniports v.s simple diffusion

Answer 6

1.) Uniports are faster
2.) Partition coefficient is irrelevant for uniporters
3.) Uniport transport limited by # of uniporters in membrane
4.) Uniport transport is specific

Question 7

Km

Answer 7

• Measure of the affinity of an enzyme for its substrate. The lower the Km the tighter the binding between the two

Question 8

4 types of pumps

Answer 8

• P-class: Heterotetramer with 2 subunits: α & β, β is phosphorylated during transport
• V-class: Multiple subunits, trasport only proteins against their gradient, acydifies lysosome and vacuole
• F-class: Multiple subunits related to V-class, only member that generates ATP, pumps only protons, found in inner membrane of mitochondria, thylakoid, bacterial plasma membrane
• ABC superfamily: More members than other classes, transport sugars, amino acids, phospholipids proteins, responsible for multidrug resistance

Question 9

ABC transporters

Answer 9

> 100 members, 2 domains: Transmembrane domain (6 α-helices per monomer, 12 in total) & a nucleotide binding domain
- CFTR protein structurally related but not a pump. ATP hydrolysis opens an ion channel

Question 10

2 properties of Ion channels that distinguish them from just holes in the membrane

Answer 10

1.) Selectively - Some ions pass through, others don’t. This depends on the diameter & shape of the ion, & on the charges that line the channel
2.) Gating - Channels are not continuously open. Rather, they open in response to a stimulus for a short period of time. Unlike transporters, a conformational change is not needed for each ion. More than 10^6 ions can pass through the channel per second

Question 11

3 types of gating

Answer 11

1.) Voltage-gated: Respond to changes in electrical potential across membrane. Found in all nerve cells, also found in muscle cells, egg cells & plants. Opening of the channel further changes the membrane potential, which can activate / inactivate other voltage-gated channels
2.) Ligand-gated: respond to the binding of a ligand, for example a neurotransmitter or inward-rectifying potassium channels that respond to the intracellular ligand PIP2
3.) Mechanically-gated: Respond to mechanical force. Auditory hair cells use these channels. Sound waves cause the channels to open, causing an ion flow that affects voltage-gated channels who transmit the signal to the brain