Membrane Transport I, Passive Transport

Question 1

diffusion

Answer 1

spontaneous movement of solute from regions of high to low concentrations

Question 2

what law does diffusion follow?

Answer 2

2nd law of thermodynamics

Question 3

when could diffusion be working against the cell?

Answer 3

active transport

Question 4

active transport

Answer 4

molecules & ions are moved against their concentration gradient, requires E (usually ATP)

Question 5

what are PMs permeable to?

Answer 5

o Water

o A few small & uncharged molecules (O2 & CO2) gases

Question 6

what are PMs not permeable to?

Answer 6

Ions such as
K+, Na+, Ca2+ (cations)
Cl-, HCO3 (anions)
Ions are small enough to get through but are charged, therefore, interact with hydrophobic interior & polar heads

Small hydrophilic molecules like glucose
Carried through transporters or move through channels

Macromolecules like PROs & RNA

Question 7

How does the cell store E within membranes?

Answer 7

By accumulating molecules within membranes, E can be stored & later released by permitting some molecules to move down the gradient

Question 8

osmosis

Answer 8

diffusion of a molecule through the cell membrane, net movement of water (or solvent) across a selectively permeable membrane

Question 9

how does osmosis work?

Answer 9

Driven by a diff in solute concentrations on either side of the membrane

Low solute concentration to high solute concentration

Question 10

when does osmosis occur?

Answer 10

when membranes are permeable to water but not to dissolved ions & smaller polar organic solutes

Question 11

what 2 factors change when osmosis occurs?

Answer 11

Volume change (until solute concentration is equalized) water creates an expansion but the PM prevents this which leads to pressure in the cell

Pressure change

Question 12

what determines the direction of water movement?

Answer 12

the envr the cell is situated

Question 13

Hypertonic & what occurs to the cell?

Answer 13

high salt medium
By osmosis, water will flow into high salt region
Cell will shrink in size

Question 14

Hypotonic & what occurs to the cell?

Answer 14

Lower salt medium, high water envr
o Water will flow by osmosis into the cell
o Cell will swell, increases to the point where it bursts or cell will work to prevent this

Question 15

what occurs to a cell after a hypertonic or hypotonic state is reached?

Answer 15

Cells usually return to normal b/c salts re-enter or leave after a period of time (through channels)

Cells will pump ions in or out to compensate for the change in water flow

Undesirable

Question 16

what organisms have a defense against a bursting cell? & what is it?

Answer 16

plants, fungi & bacteria

cell wall strong enough to fight pressure

if placed in a hypotonic solution, the cell won’t burst

Question 17

cell wall

Answer 17

specialized & rigid extracellular matrix located on the outside of the PM

Porous & doesn’t provide a barrier to the diffusion of small molecules

Question 18

isotonic & how is cell size affected?

Answer 18

salt concentration is the same in & out of the cell

Cells remain the same size

Question 19

what happens if the cell is placed in a hypotonic envr?

Answer 19

cell will swell & burst

Question 20

what happens if the cell is placed in a hypertonic envr?

Answer 20

cell will shrivel

Question 21

turgor pressure

Answer 21

pressure of water that the membranes feel due to the pressure of water against them

Question 22

what maintains movement through a plant?

Answer 22

plants tend to be hypertonic to their envr therefore, they keep salts in their cells & water is drawn inwards

transpiration keeps water moving through the plant

turgor pressure occurs

Question 23

how do cells without a wall deal with osmosis?

Answer 23

pump water back into the contractile vacuole

ions are pumped into the vacuole through active transport causing water to move into the vacuole

exocytosis occurs

ions are pumped back into the cell via active transport

Question 24

How are electric gradients established across membranes?

Answer 24

When a membrane is selectively permeable to ions

Net positive charge on one side of the membrane & net negative on the other 

Occurs only if channels are present

Solutes move down their concentration gradient

Question 25

what is an axon’s resting potential?

Answer 25

-70mV - slightly more negative on the inside than outside

cell acts as a capacitor

Question 26

capacitor & what is it’s role in cells?

Answer 26

stores E in an electric field b/w a pair of conductors

Separates charge enabling the storage of E by building up potential E

Question 27

what are 4 methods of transport across the membrane?

Answer 27

1. Diffusion through lipids
   Not ions
   Water or gases
2. Diffusion through an aqueous channel or pore
   Selective of ions
3. Facilitated diffusion: transmembrane PROs create a water-filled pore through which ions & some small hydrophilic molecules can pass by diffusion
   o Channels can be open or closed depending on the needs of the cell
   o Not an open channel, transporter of facilitated diffusion channel
4. Active transport: transmembrane PROs called transporters use ATP to force ions of small molecules through the membrane against their concentration gradient

Question 28

what do aquaporins cause?

Answer 28

In some cell types the movement of water is much greater than it would be if it flowed through the lipid bilayer itself

Question 29

role of aquaporins in the gut

Answer 29

several liters of water per day are moved into the lumen of the stomach, intestine & then are re-absorbed in the lower gut

Want to move water into the gut, food provides high amount of solutes in the intestine & then water wants to be released at the bowels

Question 30

how is the movement of water through pores regulated?

Answer 30

Cell will be signaled to increase the amount of water absorption now, vesicles lined with aquaporins will move from the interior to the exterior PM & place those aquaporins

Aquaporin movement: internal membranes external membrane (PM)

when is the cell is signaled to do the opposite, aquaporins will be removed. Endocytotic vesicles will be formed

Flow is regulated by the location of aquaporins, not the aquaporins themselves

Question 31

role of aquaporins in kidneys

Answer 31

Water is reabsorbed (less excretion) in response to the hormone vasopressin (antidiuretic hormone)
Presence of vasopressin promotes permeability & reabsorption
Aquaporins move from internal membranes to cell membrane

Vesicles lined with aquaporins, vesicles will move to the perimeter in response to antidurettic
Made in ER, to Golgi

Water flow was enhanced by placing aquaporins in the membrane –> water flows by diffusion
Stop the flow, remove aquaporins, vesicles will form endocytotic vesicles

Question 32

transport of glucose within the cell via a carrier PRO

Answer 32

Diffusional transporter, designed only to capture glucose

Glucose fits into transporter, causing it to change shape

Opens up & dumps glucose into the cell

Once glucose is released, shape will revert

Can bind to another glucose molecule

Will take glucose away from the perimeter or phosphorylate it to maintain the low gradient in the cell

Question 33

what are 2 types of channels for passive transport?

Answer 33

carriers –> ex: glucose transporter

channel –> membrane PROs form a highly selective pore

Question 34

how is passive transport regulated?

Answer 34

presence or absence (expression) of PRO on membrane

“gated” PROs
Voltage is regulated
Ligand-regulated (hormone, neurotransmitter)
Mechanical or stretch (smooth muscle, sensory nerves)

Question 35

what are the types of gated ion channels?

Answer 35

o Ligand gated
o Mechanically gated
o Voltage gated
o Light gated

Question 36

types of facilitated diffusion (3) & exs of each

Answer 36

Ligand gated: ligand needs to bind to a particular place on the PRO for the channel to open
Ligands can be extracellular or intracellular
Ligand isn’t the substance being transported
Ex: Ach binds to neurotransmitters opening up the channel admitting Na+ to enter & initiate nerve impulse or muscle contraction

Mechanically gated ion channels: mechanical shift causes pores to open
Ex: sound waves bend cilia like projections causing the opening of ion channels

Voltage gated ion channels: response to changes in charge across the PM
Ex: neuron impulse, reduction in voltage opens Na+ channels allowing an influx of Na+ into the cell
Acts as a wave as Ach hits neurons
Pump ions back out to prepare for another signal