Movement across membranes (passive channels)

Question 1

Passive mechanisms

Answer 1

• Passive movement of a substance across cell membranes relies on molecular concentrations does not require energy

-Only works for only very small uncharged molecules like O2 and CO2

Question 2

Water and passive mechanisms

Answer 2

• Water is also an uncharged molecule because of covalent bonds
• Water cannot pass through hydrophobic areas

Question 3

Crossing membranes: Channels

Answer 3

• Protein channels provide another form of passive transport
• This mode is particularly effective for small charged molecules (ions) like Na+, K+, Ca2+, Cl-
• Ions move down concentration gradient
• Channels are selective, only allowing a particular type of ions to pass
• Ion channels are formed by integral membrane proteins, typically multiple subunits that line an aqueous pore
• Channels are often gates, can be opened or closed based on responses to different stimuli

Question 4

Voltage gates channels

Answer 4

-Na+ and K+ channels
- Some channels can respond to changes in charges across the membrane

Question 5

Action potential

Answer 5

• Example of a Voltage gated channel
• The passage of electrical signals down a nerve axon
• Under polarized conditions (when they are quiet) neurons have little amount of Na+ inside of them

Question 6

Ligand gated channels

Answer 6

• E.g acetylcholine receptor
• The channel responds to the binding of specific molecules on it s surface (a ligand )
• Binding of a ligand produces conformational change in the structure of the receptor/channel

-(Lock and key)

Question 7

Tetrodoxin (TTX)

Answer 7

• Very potent neurotoxin
• Discovered in pufferfish (but found in others)
• TTX is an Na+ channel blocker
• It inhibits the firing of of action potentials bu binding to the voltage gated sodium channels in nerve cell membranes and blocking the passage of Na+ ions into the neurons
• This prevents the nervous system from carrying messages to muscles including the diaphragm (causing death via respiratory failure)

Question 8

Curare

Answer 8

• A mixture of organic compounds (found in different plants)
• Used as a paralyzing function
• Competitive antagonist of the nitotinic acetylcholine receptor (nAChR)

-It occupies the same position on the receptor as ACh with an equal or greater affinity, and elicits no response