Lecture 4

Question 1

Why do molecules need to be transported across membranes?

Answer 1

Molecules need to be transported across the membranes because we need them for cell communication.

Ex. mRNA must pass the membrane to be translated in the cytoplasm.

Question 2

How can solutes cross membranes?

Answer 2

Solutes cross membranes by simple diffusion, facilitated diffusion, and active transport. the final one requires energy - ATP

Question 3

What can diffuse freely?

Answer 3

Small and uncharged molecules.

Ex. O2, CO2, N2, Ethanol, Glycerol

Question 4

What cannot diffuse freely?

Answer 4

Large polar molecules, charged ions.

Ex. Ca, Na, Cl, Glucose, Amino acids, H2O

Question 5

What is simple diffusion?

Answer 5

Simple diffusion does not involve any transport proteins or channels embedded int he membrane.
- Items move from high to low concentration.

Question 6

Example of simple diffusion?

Answer 6

In body tisuses: low oxygen and high co2 (plasma). Co2 diffuses into the RBC and o2 diffuses into the plasma.

In the lungs: high oxygen and low co2 (plasma). co2 will diffuse into the plasma, and o2 diffuses into the RBC.

Question 7

Facilitated diffusion?

Answer 7

Facilitated diffusion is a type of passive transport where molecules move down their concentration gradient across a cell membrane with the help of membrane proteins like channel proteins or carrier protein.

Question 8

Features of facilitated diffusion?

Answer 8

● It does not require energy (ATP)
● It only moves molecules from high to low concentrations
● Glucose is a good example of facilitated diffusion.

Question 9

What protein types mediate facilitated diffusion?

Answer 9

Channels and carriers are the two major protein types that mediate facilitated diffusion.

Question 10

Channels are…?

Answer 10

Channels are pores. The atom that they allow through their pore is in high concentration outside and low concentration inside.

Question 11

Carrier proteins undergo…?

Answer 11

Carrier proteins undergo a conformational change as the solute moves. The solute “binding” causes a conformational change that allows the carrier protein to release the molecule on the other side of the membrane.

Question 12

Features of carrier proteins?

Answer 12

They alternate between two conformational states.
- They are highly specific to their solute
- Glucose transporter is a good example. It binds to the outside of the cell, is carried into thr centre of the carrier potion which undergoes a conformational change and allows the release of glucose on the other side of the membrane (inside).

Question 13

Carrier proteins transport how many molecules?

Answer 13

Carrier proteins can transport either one or two molecules (uniport, symport, antiport).

Question 14

Symporters?

Answer 14

• Symporters transport two molecules in the same direction.

Question 15

Antiporters?

Answer 15

Antiporters move two molecules that are transported in opposite directions.

Question 16

Symporters and Antiporters?

Answer 16

Use the gradient of one molecule to transport the other. They can move items against their gradient using energy generated by moving a molecule alongside its gradient.

Question 17

LacY and GlpT example?

Answer 17

LacY transports lactose and hydrogen ions into the cell (sym), and GlpT transports phosphate and glycerol-3-phosphate in opposite directions (anti).

Question 18

how do channel proteins facilitate diffusion?

Answer 18

Channel proteins facilitate diffusion by forming hydrophilic transmembrane channels.

Question 19

Types of channels?

Answer 19

Ion channels, Porins, Aquaporins

Question 20

Ion channels?

Answer 20

Specific to certain ions

Question 21

Porins?

Answer 21

Not very specific, only it michochondria, chloroplasts and bacteria

Question 22

Aquaporins?

Answer 22

Specific to H2O (some of them can move glycerol)

Question 23

Features of ion channels?

Answer 23

Ion channels are transmembrane proteins that allow rapid passage of specific ions:

1. Typically gated to control when items can pass
2. Things that open the channel are:
   * voltage,
   * ligand,
   * temperature,
   * pressure
3. Tend to be kept closed

Question 24

Example of ion channel?

Answer 24

Potassium channels allow potassium ions to pass, but block smaller sodium ions.
- protein domains are connected to the channel and twist the four cahains of the channel to form the open or closed positions
- they change conformation when open
- amino acids sense if the correct ion is coming throigh

Question 25

Potassium channels have a filter for what?

Answer 25

Potassium channels have a filter for smaller ions: - potassium must shed its water molecules - Eight Oxygen atoms in the channel replace the water molecules - The ions move along oxygen sites - Sodium ions are too small to interact with the oxygen atoms and they cannot pass.

Question 26

Why is passing ions important?

Answer 26

This allows for communication between the cells and other cellular functions as well.

Question 27

Membrane potential?

Answer 27

The differene in the concentrations of ions on opposite sides of a membrane. Cell membranes are typically -70 to -40 mV.

Question 28

Depolarization?

Answer 28

Occurs when the interior voltage becomes less negative (-70 to -60)

Question 29

Hyperpolarization?

Answer 29

Occurs when the interior voltage becomes more negative (-70 to -80)

Question 30

Change in voltage is ______?

Answer 30

Cells read the change in voltage and use it as an intracellular signal.

Question 31

Therefore, ligand gated ion channels convert...?

Answer 31

Ligand gated ion channels convert a chemical signal into the cell (Neurotransmitters) - Acetylcholine conducts Na, K, and Ca, and is released at neuromuscular junctions to depolarize/hyperpolarize cells.

Question 32

Temperature gated ion channels?

Answer 32

TRPV1 is able to open due to a temperature change - heat pain can be transmitted from capsaicin molecules.

Question 33

Pressure gated ion channels?

Answer 33

PEIZO responds to mechanical force on the membrane. When closed it is bent, when open (due to mechanical force) they are straightened out.

Question 34

VGSCs transmit...?

Answer 34

VGSCs transmit signals in a wave through the nervous system. Voltage diff is high = closed Voltage diff is low = open, allow sodium through channel.

Question 35

Porins?

Answer 35

Transmembrane proteins that allow rapid passage of various solutes (they're large pores in the membranes). Restricted to: - Bacteria - Chloroplasts - Mitochondria

Question 36

ATP is.....?

Answer 36

ATP is the main energy storage molecule has negative charge. - ATP is made in the mitochondria - It cannot pass through membranes - ATP hydrolysis releases energy the cleaving the terminal bond.

Question 37

Function of voltage-dependent anion channel?

Answer 37

A voltage-dependent anion channel (VDAC) mediates exchange of negatively charged metabolites, most importantly ATP from mitochondria to the cytoplasm.

Question 38

Aquaporins?

Answer 38

Aquaporins are transmembrane channels that allow rapid passage of water. They are specifically made for water and nothing else can pass.