Lecture 16 - Movement Across Membranes

Question 1

The transmembrane domain (TMD):

Answer 1

• α helical peptide sequence that is largely hydrophobic (uncharged) and spans the membrane; consists of amino acids
  with hydrophobic side chains
• Is permanently attaches the protein to the PM.
• Hydrophobic fatty acid tails interacts with hydrophobic TMD
• Can facilitate protein-protein interactions
  α helices

Question 2

what is a dimer

Answer 2

multiple protein subunits that make a ring with a pore in the middle where things move. Forms a protein channel

Question 3

heterodimer

Answer 3

dimer made up of multiple different proteins forming the ring

Question 4

homodimer

Answer 4

the same proteins form the dimer (ring structure)

Question 5

octodimer

Answer 5

8 proteins form ring

Question 6

Lipid bilayers do not allow many _______ ___ _______ to pass through them freely

Answer 6

compounds or
molecules

Question 7

______,_______ molecules cross membranes relatively easily (H2O, O2, CO2, NO)

Answer 7

Small, uncharged

Question 8

_____/_____/_____ compounds cannot easily
cross lipid bilayers

Answer 8

Large/polar/charged

Question 9

Specific mechanisms are required for the
controlled

Answer 9

transport of many substances across
membranes

Question 10

Four basic mechanisms for moving molecules across membranes

Answer 10

1 Simple diffusion

2 Diffusion through a channel

3 Facilitated diffusion

4 Active transport

Question 11

which mechanism for moving molecules across membranes requires ATP

Answer 11

Active transport

Question 12

Passive movement of substances across cell membranes relies on

Answer 12

• molecular concentrations of the substance across the membrane
• Moves from a HIGH to a low concentration; down the concentration gradient; “downhill”
• DOES NOT require energy

Question 13

Simple Diffusion works only for

Answer 13

very small and
uncharged molecules like H2O, O2 and CO2

Question 14

Simple Diffusion moves

Answer 14

Down a concentration gradient—flow is downhill;
does NOT require energy

Question 15

what are AQUAPORINS

Answer 15

specific water
channels; H2O moves through aquaporin channels in “single
file” down the concentration gradient

Question 16

Channels (a.k.a. ion channels) are formed by

Answer 16

integral membrane proteins—typically
multiple subunits—that line an aqueous
pore

Question 17

channels are particularly effective for

Answer 17

small, charged molecules (ions) like Na+, K+, Ca2+, Cl-

Question 18

how do ions move

Answer 18

down concentration gradients —
flow is downhill

Question 19

Channels are selective. What does this mean?

Answer 19

allowing only particular types of ions to pass

Question 20

Ion channels are often _____ and can be _____ or ______

Answer 20

gated, opened, closed

Question 21

ion channels can be turned ____/____ in response to
different _______/_______
* e.g., changes in charge across the membrane;
neurotransmitters

Answer 21

ON/OFF, signals/stimuli

Question 22

Two types of Gated Ion Channels

Answer 22

1) Voltage-gated channels (e.g., Na+ and K+ channels)

2) Ligand-gated channels (e.g., acetylcholine receptor)

Question 23

Voltage-gated channels can respond to

Answer 23

changes in charge across membrane eg action potentials in neurons

Question 24

Under non-depolarized conditions (when they are quiet), neurons have

Answer 24

low [Na+] inside. When the membrane is depolarized, the gate will open to allow Na+ to pass

Question 25

Ligand-gated channels respond to

Answer 25

binding of specific molecule on its surface — a ligand

Question 26

Binding of a ligand produce

Answer 26

conformational change in the structure of the receptor/channel

Question 27

Only a ligand adapted to the ______ _____ can produce an effect.

Answer 27

binding site

Question 28

Tetrodotoxin (TTX) is a very potent

Answer 28

neurotoxin

Question 29

Tetrodotoxin (TTX) was discovered in the

Answer 29

pufferfish. (also found in several other aquatic animals like blue-ringed octopuses and moon snails)

Question 30

TTX is a Na+….

Answer 30

channel blocker

Question 31

TTX inhibits the firing of action potentials in neurons by

Answer 31

binding to the voltage-gated sodium channels in nerve cell membranes and blocking the passage of Na+ ions into the neuron.

Question 32

TTX prevents the nervous system from carrying messages to

Answer 32

muscles, including
the diaphragm

Question 33

TTX intoxication consequently causes death via

Answer 33

respiratory
failure.

Question 34

Curare is a mixture of organic compounds found in

Answer 34

different plants originating from Central and South America like members of the Strychnos species

Question 35

Curare was cleverly used as a _______ _______
and _______ _______ before modern techniques

Answer 35

paralyzing poison, hunting tool

Question 36

Curare is a _______ ______ of the nicotinic acetylcholine receptor (nAChR)

Answer 36

competitive antagonist

Question 37

It occupies the ______ ______ on the receptor as ACh with an equal or ______ affinity, and elicits
no response. It is an example of a ____________ muscle relaxant.

Answer 37

same position, greater, non-depolarizing

Question 38

In facilitated diffusion, a compound binds specifically to integral
membrane protein called a…..

Answer 38

facilitative transporter

Question 39

in facilitated diffusion: Change in transporter _______ allows compound to be released on other side of membrane

Answer 39

conformation

Question 40

in facilitated diffusion: the compound moves ______ a concentration
gradient. no energy requirement

Answer 40

down

Question 41

Most animal cells import glucose from the
blood into cells ____ a _______ ______ via a ________ _________

Answer 41

down, concentration gradient, facilitative transporter

Question 42

glucose transporter steps / mechanisms

Answer 42

1 Transporter ready to accept glucose molecule

2 Glucose is accepted by transporter

3 Intracellular side of
transporter opens

4 Glucose is released and cycle repeats steps

Question 43

Symporter: Under certain circumstances, cells need to move substances from a _______ ______ to ______ ________. Thus, AGAINST the concentration gradient

Answer 43

lower concentration, higher concentrations

Question 44

during reabsorption of glucose in kidney cells after blood filtration to prevent its loss through the urine, cells cannot rely on

Answer 44

the concentration gradient of glucose as kidney cells would stop reabsorbing glucose when an extracellular and
intracellular equilibrium is reached

Question 45

during reabsorption of glucose in kidney cells after blood filtration to prevent its loss through the urine, cells must rely on

Answer 45

the chemical gradient of a second molecule that
would not reach extracellular and intracellular equilibrium.

BOTH molecules are transported in the SAME direction.

Question 46

The Na+-Glucose Symporter steps / mechanisms

Answer 46

[1] Simultaneous binding of 2 Na+ and 1 glucose to the transporter with outward-facing binding sites.

[2] This cause a conformational change in the transporter (occluded conformation).

[3] Eventually the transporter adopts an inward-facing conformation that allows

[4] the dissociation of the two Na+ molecules in the cytosol. As a result, the glucose molecule gets pushed-in as well.

[5] Return to the outward-facing conformation to repeat the cycle.

Question 47

Antiporter: The concentration gradient of one molecule is used to

Answer 47

transfer a second
molecule in opposite directions.

Question 48

the sodium-proton exchanger
(Na+/H+ exchanger) in the nephron of the
kidney is an example of what method

Answer 48

Antiporter

Question 49

how does the sodium-proton exchanger
(Na+/H+ exchanger) work?

Answer 49

This antiporter transport Na+ into the
cell and protons (H+) out of the cell. This
carrier is specifically responsible to maintain
pH and sodium levels in specific kidney cells

Question 50

in Active transport: Compound binds specifically to integral
membrane protein called an

Answer 50

ACTIVE
TRANSPORTER

Question 51

in active transport: Change in the conformation of the
transporter is caused by

Answer 51

the hydrolysis of an
ATP molecule —- allows molecules to be released
on other side of the membrane

Question 52

active transport allows molecules to move against

Answer 52

a concentration gradient

Question 53

Active transport requires input of _______ in the form of ______ _____

Answer 53

ENERGY, ATP
molecules

Question 54

The Na+/K+ ATPase maintains cellular [Na+] and [K+] using

Answer 54

ATP