Lecture 16: Movement Across Membranes

Question 1

Transmembrane domain (TMD) (3) is a…

Answer 1

• alpha helical peptide sequence that is largely hydrophobic and spans the membrane - consists of AA with hydrophobic side chains
• permanently attaches the protein to the PM - hydrophobic FA tai;s interact with hydrophobic TMD
• can facilitate protein-protein interactions

Question 2

Example of a transmembrane protein is…

Answer 2

Glycoproteins A dimer

Question 3

Lipid bilayers allow many compounds or molecules to pass through them freely
True or False?

Answer 3

False, they do not allow many compounds or molecules to pass through them

Question 4

Which molecules are able to pass through membranes easily? (4)

Answer 4

Small, uncharged molecules (H2O, O2, CO2, NO)

Question 5

Large, polar, charged compounds ______ pass through membranes easily

Answer 5

Cannot
- ex. Of these molecules are ions, glucose

Question 6

Mechanisms for moving molecules across membranes (4)

Answer 6

1. Simple diffusion
2. Diffusion through a channel
3. Facilitated diffusion
4. Active Transport

Question 7

Passive transport mechanisms are (3)

Answer 7

Simple diffusion, diffusion through a channel, facilitated diffusion

Question 8

Active transport mechanisms are (1)

Answer 8

Active transport

Question 9

Non mediated transport mechanisms are (2)

Answer 9

Simple diffusion, diffusion through a channel

Question 10

Transporter mediated transport mechanisms are (2)

Answer 10

Facilitated diffusion, active transport

Question 11

Passive Transport (simple diffusion) moves from..

Answer 11

• Relies on molecular concentrations of substances across membranes
• Moves from high to low concentration, down the concentration gradient
• Doesn’t require energy

Question 12

Simple diffusion (passive mechanisms) works for only…

Answer 12

Very small and uncharged molecules

Question 13

Aquaporins are…?

Answer 13

Specific water channels; H2O moves through aquaporin in single file down concentration gradient in passive mechanisms

Question 14

Passive mechanisms: Channels

Answer 14

• channels (ion channels) are formed by integral membrane proteins - multiple subunits - that line aqueous pore
• mode is effective for small charged molecules
• ions move down concentration gradients
• channels are selective
• multiple ions at the same time go through

Question 15

Ion channels

Answer 15

• gated, can be open or closed
• turned on/off
• gate is on the low concentration side of membrane

Question 16

What are the two types of gated ion channels

Answer 16

1. Voltage gated
2. Ligand gated

Question 17

Voltage-gated channels (Na+ and K+)

Answer 17

respond to changes in charge across membrane due to ions changing charge as they move across

Question 18

Ligand-gated channels

Answer 18

Responds to binding of specific molecule on its surface - a ligand, this produces conformational change in the structure of the receptor/channel
- only responds to ONE type of ligand

Question 19

Tetrodotoxin (TTX) is a…

Answer 19

Na+ channel blocker

Question 20

Curare is a

Answer 20

Toxin that inhibits the receptors from working

Question 21

Passive mechanisms: Carriers, facilitated diffusion

Answer 21

Compound binds to integral protein called facilitating transporter
Change in transporter conformation allows compound to be released on other side of membrane

Question 22

Facilitation transport, steps:

Answer 22

1. Transporter ready to accept glucose
2. Glucose accepted
3. Intracellular side of transporter opens
4. Glucose is released

Question 23

In some cases, cells need to move substances from a

Answer 23

Low to high concentration gradient (this is not the norm)

Question 24

How can cells transport glucose from a low to a high concentration gradient?

Answer 24

Rely on chemical gradient of symporters and transport both molecules in the same direction

Question 25

Na+ glucose symporter

Answer 25

Glucose being transported against [] gradient (low to high), and Na+ with [] gradient

Question 26

Na Glucose symporter steps

Answer 26

1. Binding of 2 Na+ and 1 glucose to transporter
2. Causes conformational change in transporter
3. Transporter adopts inward-facing conformation
4. Dissociation of two Na+ molecules in cytosol, glucose gets pushed as well
5. Return to outward-facing conformation - no ATP required

Question 27

Antiporter

Answer 27

Two molecules being transported in opposite directions

Question 28

What happens during antiportation?

Answer 28

Concentration gradient of one molecule is used to transfer a second molecule in opposite direction

Question 29

Active transport

Answer 29

• compound binds to integral protein called active transporter
• change in conformation caused by hydrolysis of ATP molecule allows molecules release on other side of membrane
• using this mechanism, molecules move against [] gradient
• requires energy