Lecture 8: Membrane Transport, Part I

Question 1

Different molecules diffuse through the membrane at different rates due to

Answer 1

lipid bilayer having selective permeability

Question 2

What allows the lipid bilayer to have selective permeability?

Answer 2

Thin layer of hydrophobic material in aqueous environment

Question 3

What molecules rapidly diffuse across the membrane?

Answer 3

Small, nonpolar molecules
Ex. O2, CO2, N2, steroid hormones

Question 4

What molecules diffuse across the membrane?

Answer 4

Small, uncharged polar molecules
Ex. H20, Urea, glycerol, NH3

Question 5

What molecules rarely diffuse across the membrane?

Answer 5

Large uncharged polar molecules
Ex. Glucose, sucrose

Question 6

What molecules cannot cross the membrane?

Answer 6

Charged Ions
Ex. H+, Na+ HCO3-, K+, Ca2+, Cl-, Mg2+

Question 7

Collection of isolated membrane lipids (which contain proteins) thrown into water is example of

Answer 7

hydrophobic interactions as membrane lipids will form sphere around proteins –> causes proteins to be locked in and are very large, polar molecules (often have charges) with no way to escape

Question 8

What component of lipid bilayer prevents passage of hydrophillic, water soluble molecules?

Answer 8

Hydrophobic interior of lipid bilayer of hydrophobic tails

Question 9

In living cells, hydrophilic molecules typically pass through membrane using

Answer 9

transmembrane proteins

Question 10

What are the 2 Types of Transmembrane Proteins cells use to import or export molecules?

Answer 10

1. Transporters
2. Ion Channels

Question 11

Similarities between Transporters and Ion Channels

Answer 11

Both multipass proteins to form hydrophilic pores
Arrange alpha helices for hydrophobic outside with nonpolar amino acids and hydrophilic on the inside with polar amino acids

Question 12

Transporters

Answer 12

Transmembrane protein that undergo conformational changes for each specific molecule, solute or ion
Conformation change takes time so lower rates of transport
Can do passive and active transport

Question 13

Ion Channels

Answer 13

Transmembrane protein that for hydrophilic pores which can open or close to transport ions and water
After channels open, no need for conformation change so much higher rates of transport
Only does passive transport

Question 14

Transporter and Channels regulate

Answer 14

ion concentrations within the cell

Question 15

Sodium (Na+) concentration in mammalian cell

Answer 15

High concentration outside cell
Low concentration inside cell

Question 16

Potassium (K+) concentration in mammalian cell

Answer 16

High concentration inside cell
Low concentration outside cell

Question 17

Direction of transport for solutes often depends on

Answer 17

concentration between inside and outside of cell

Question 18

Simple Diffusion

Answer 18

Describes ability of molecule to travel through lipid bilayer without transmembrane protein (unassisted)
Hydrophobic molecules diffuse very quickly
Large polar charged molecules diffuse very slowly or not at all –> rate of simple diffusion is too slow to meet the cell’s needs

Question 19

Passive transport/ Facilitated transport (diffusion)

Answer 19

Solutes spontaneously travel from high to low concentration using transmembrane protein (assisted)
No energy is required due to concentration gradient
Done by all channels and some transporters

Question 20

How is water passive transported?

Answer 20

Water is passively transported by channel proteins called aquaporins to form hydrophilic pore with alpha helices for water to passively pass through

Question 21

Which transmembrane proteins use passive transport?

Answer 21

All channels
Some transporters

Question 22

Active transport

Answer 22

Solute travels from low to high concentration
Energy is required and is carried out by transporters

Question 23

What is common source of active transport that proteins use?

Answer 23

ATP as transmembrane proteins hydrolyze ATP which releases energy and couple reaction as energy pushes against gradient

Question 24

How does cell know to use passive or active transport for uncharged molecules?

Answer 24

Ex. glucose
Dependent on concentration gradient

Question 25

How does cell know how to use passive or active transport and rate for ion transport?

Answer 25

Dependent on electrochemical gradient

Question 26

Most cells have slight ______ on inside of cell membrane

Answer 26

negative charge

Question 27

What is the source of why cells have slight negative charge on inside of cell membrane?

Answer 27

DNA from phosphate backbone covered with negative charges

Question 28

If high concentration of positive ions on outside of cell …

Answer 28

Concentration and electrical gradient are in same direction
Positive wants to naturally go to negative interior
Rapid transport with higher electrochemical gradient
Ex. Sodium gradient

Question 28

Electrochemical gradient

Answer 28

Charged molecules (ions) have 2 gradients: electrical and concentration

Question 29

If high concentration of positive ions on inside of cell …

Answer 29

Concentration and electrical gradient are in different directions
Concentration gradient pointing outwards from cell and electrical gradient point inward due to positive charge of potassium ions
Lower gradient
Ex. Potassium gradient

Question 30

What differentiates each membrane are from

Answer 30

membrane proteins

Question 31

Membrane transporters are highly

Answer 31

selective and typically transfer one type of molecule

Question 32

Each cell membrane has unique set of _______ that import/ export specific molecules

Answer 32

transporter proteins

Question 33

Plasma membrane membrane transporters:

Answer 33

transporters for importing sugars, amino acids, and nucleotides
Pumps (type of transporter protein) maintain ion concentration within cell

Question 34

Mitochondria membrane membrane transporters

Answer 34

transporters for importing pyruvate to generate ATP and transporters to export ATP so it can be used by cell for chemical reactions

Question 35

Lysosome membrane membrane transporters

Answer 35

Transporters for importing H+ that acidifies the lysosome interior to digest molecules

Question 36

Does transporters have ability to do passive and/or active transport?

Answer 36

Both passive and active transport

Question 37

Does ion channels have ability to do passive and/or active transport?

Answer 37

Only passive transport

Question 38

In passive transport, _____ gradient determines direction of movement

Answer 38

electrochemical and concentration

Question 39

Glucose Transporters

Answer 39

glucose is large, polar molecule so can not simply diffuse through lipid bilayer
glucose is uncharged so only concentration gradient dictates passive or active transport

Question 39

After a meal, passive transport in glucose transporters

Answer 39

high amount of glucose in blood stream
liver cells are in charge of brining glucose in and liver can store glucose in form of glycogen but other cells need to import glucose to harvest energy and will need to take glucose out of blood stream using passive transport
glucose transporters transport glucose from extracellular space to cytosol (high to low concentration)

Question 40

Fasting/ Hunger pains, passive transport in glucose transporters

Answer 40

low amount of glucose in blood stream
glucose transporter transports glucose from cytosol to extracellular space (high to low concentration)

Question 41

Gradient driven pump

Answer 41

transport solutes against gradient by linking one solute that goes down the gradient to another going against the gradient

Question 42

In active transport, membrane transporters linked to

Answer 42

source of energy to move against gradient

Question 43

What are the two sources to power active transport?

Answer 43

ATP
Gradients

Question 44

How is sodium an example of gradient driven pump?

Answer 44

Sodium has high electrochemical gradient and as traveling down, proteins able to harness energy from gradient to power transport of a different molecule

Question 45

ATP driven pump

Answer 45

use energy from ATP to transport solutes against gradient

Question 46

Light driven pump

Answer 46

use energy from sunlight to transport solutes against gradient (bacteriorhodopsin)

Question 47

Na+-K+ pump (Na+-K+ ATPase): Sodium Potassium Pump

Answer 47

ATP that sodium potassium pump uses accounts for 30% of food humans eat
ATP driven pump that transports Na+ outside of cell and brings K+ inside cell (against gradient)

Question 48

ATPase

Answer 48

Hydrolyzes ATP into ADP + Pi

Question 49

What powers the Na+-K+ pump?

Answer 49

Hydrolysis reaction of ATPase

Question 50

The number of molecules going in and out are not equal:

Answer 50

321NOKIA
3 Sodium (Na+) outside
2 Potassium inside (K+)
Use of ATP
Pushing more positive out, pulling less positive in

Question 51

Reasons why Na+-K+ Pump is important

Answer 51

1. maintain ion concentration differences for sodium and potassium
2. negative charge inside cell is maintained

Question 52

Steps to Na+-K+ pump

Answer 52

1. At resting state, Na+-K+ pump is opened pointing to inside of the cell. Cytoplasmic Na+ binds to Na+-K+ pump and keeps conformation until all 3 Na+ binding sites are occupied
2. Na+ binding stimulates phosphorylation by ATP where ATPase domain hydrolyzes ATP by attaching phosphorylation group to its protein
3. Phosphorylation and negative charge causes protein to change conformation for pump to open to outside of the cell and expel Na+ to the outside, binding sites for Na+ are no longer active
4. Extracellular K+ binds to protein as potassium binding sites are active and will change conformation once K+ binding sites are occupied
5. Binding K+ to protein induces conformational change which releases phosphate group which restores to protein’s original conformation
6. K+ is released to inside and Na+ sites are reactive for cycle to repeat

Question 53

Glucose Na+ symport pump

Answer 53

transports glucose into intestinal cells (against gradient) using high electrochemical gradient of Sodium (active transport)

Question 54

Glucose symport

Answer 54

pull glucose into the cell
restricted to apical domain (facing gut lumen) by tight junctions

Question 55

Glucose uniport

Answer 55

passive transport, glucose goes down concentration gradient

Question 56

Some cells have ______ which limit movement of proteins

Answer 56

tight junctions

Question 57

Coupled transport means

Answer 57

By using solute to travel down gradient for energy for another solute to travel against gradient
Active transport
Ex. gradient driven pump

Question 58

Intake of glucose by intestinal cells uses glucose Na+ symport pump

Answer 58

• Gut lumen found inside of small intestine
• Try to bring low glucose concentration from gut lumen to high glucose concentration in extracellular fluid (against gradient need active transport)
• After pulling glucose against its concentration gradient, now want to go from high to low concentration but not right back out so have to restrict protein fluidity
• Symport pump restricted on apical side that is facing gut lumen
• Uniport pump on basal side to release glucose from high to low concentration into blood stream (with concentration gradient, passive transport)
• glucose can go to connective tissue where it has access to blood vessels