Cell Biology Membrane Transport and Cell Signaling

Membranes

act as a permeable shield; retain metabolic intermediates, release waste compounds

What kinds of molecules can diffuse across the membrane

size and polarity matter, but you have to ask if it’s charged

Free energy

a measure of the potential energy of a system; a function of enthalpy and entropy

Entropy

a measure of the degree of disorder or randomness in a system; the greater the spots in the room, the greater the entropy

enthalpy

total bond charges present in a chemical reaction

If delta G is -

energetically favorable

if delta G is +

not energetically favorable

Gibb’s Law

all system change in such a way that free energy is minimized

if delta G is -

the reaction will occur spontaneously

if delta G is +

reaction will only occur with an input of energy

if delta G is 0

the reaction is at equilibrium

simple diffusion

does not use any energy to move across the membrane; movement down the concentration gradient

will entropy increase or decrease in diffusion

increase in entropy due to more space

will delta G increase or decrease with diffusion

delta G will decrease due to the increased entropy

whether or not something diffuses is dependent on

it’s partition coefficient

if you are moving something from low concentration to high concentration…

the number of available spaces does down

if you are moving something form high to low concentration…

the number of available spaces goes up

partition coefficient

the measure of a substance’s solubility in lipid

in simple diffusion

rate is linear; partition coefficient is relevant

in simple diffusion, if there is a higher partition coefficient

you have a steeper slope

in simple diffusion, if you have a lower partition coefficient

you have a less steep slope

in simple diffusion

there is no theoretical maximum

what is the rate of simple diffusion dependent upon

concentration

what is the rate like in transporter-mediated diffusion

greater than simple diffusion

the partition coefficient in transporter-mediated diffusion

is irrelevant because it would be so low that you would need a transporter to move the molecule

the max rate in transporter-mediated diffusion

is due to the limited number of transporters

max rate

Vmax

transporter-mediated diffusion is

transporter specific

the rate of transporter mediated diffusion

is dependent on the affinity(Km) of the transporter for the transported molecule

as actual affinity increases,

Km decreases

what is the relationship between Km and affinity

reciprocal

what is the rate in transporter-mediated diffusion dependent on

concentration

for uriporters, we go

down the concentration gradient

the function of uniporters depends on

the number of transporters, the affinity of transporters for substrate, the concentration outside of the cell

in uniport function, what is the function of the partition coefficent(K)

K is irrelevant

what happens to the shape of the curve as affinity increases in simple diffusion

simple diffusion will be a straight line

what happens to the shape of the curve as affinity increases in transporter-mediated diffusion

the slope of the line can be affected by the partition coefficient

the higher the partition coefficient

the higher the curve

in simple diffusion, the higher rate of concentration means

the greater the rate of transport

in transporter-mediate diffusion, the plateau

is due to the limited number of transporters

active transport uses energy because

we are moving something against its concentration gradient, entropy is negative

Why do ions need to use an ion channel

ions are really smal, but they are charged

how do ion channels know what ions to move across the membrane

multipass transmembrane proteins have a selectivity loop

where are ion channels placed within the membrane

within small pockets known as vestibules

how specificity in ion channels determined

by the distance between oxygens within the vestibule

within the vestibule

oxygens are free to move around

how is gating within ion channels regulated

changed in membrane potential or ligand binding

K+ leak channels

are always open even in an unstimulated or resting state

what does it mean when the K+ leak channels are always open

this channel will always be moving ions form an area of high concentration to low concentration

what are the three main sources of energy in active transport

light, ATP-drive pump, coupled transporter

where is light used within active transport

a limited number of places

ATP-driven pump

primary active transport; will use ATP as a source of energy

within a p-type pump, are kinases used

no

within a p-type pump, what is being phosphorylated

aspartic acid

sarcoplasmi reticulum

when the ER contains a high amount of Ca2+

the p-type pump is composed of

ten alpha helices

in a p-type pump what will bind

calcium

within the p-type pump, how many calciums will the binding site accommodate

two

where does the ATP bind in the p-type pump

the nucleotide binding domain

in the p-type pump, where will the phosphate move

to the aspartic acid

what is the movement of the phosphate going to lead to

a conformational change

within the p-type pump ADP is

converted back to ATP, but the P still remains

within the p-type pump, calcium is..

released

what happens to H+ in the p-type pump

2H+ binds

what is the function of H+ and H2O

they stability the empty Ca2+ binding sites

after the Ca2+ binding site is stabilized

P is released from its binding site

under conditions when you don’t have an affinity change

passive transport will be used

a lower concentration of K+ outside the cell and a higher concentration inside the cell is possible with the..

K+ leak channel because there is a mechanism that transports K+ back into the cell

a higher concentration of Ca2+ outside the cell and a lower concentration of Ca2+ inside the cell is possible because…

as Ca2+ moves with its concentration gradient inside the cell there is a mechanism that transports it back outside the cell

Sodium-Potassium ATPase

protein that will move sodium out of the cell and move potassium into cell

within the Sodium-Potassium ATPase

for every three sodium molecules, two molecules of potassium will be pumped inside

how is a gradient created that pulls water into the cell

cells contain a high concentration of solutes including negatively charged organic molecules that are confined inside the cell and their accompanying cations that are required for change balance

how is the gradient pulling water into the cell counteracted

by an opposite osmotic gradient due to high concentration of inorganic ions in the extracellular fluid

the Na+-K+pump helps maintain this balance by

pumping out Na+

coupled transporters

facilitate the transport of one molecule to the energetically favorable

Na+-Glucose symporter

an example of a coupled transporter

Within the Na+-Glucose Symporter

Na+ and glucose binding are cooperative, transition to the occluded state only occurs when both Na+ and glucose are bound

within the Na+-Glucose Symporter what does it mean to say that the binding sites for Na+ and glucose are “cooperative“

Na+ binding increases the affinity for glucose

within the Na+-Glucose Symporter stochastic fluctuations caused by thermal energy

drive the transporter randomly

Signal transduction

the process whereby a cell receives an extracellular signal and converts it to an intracellular signal that alters cell behavior

effector proteins

metabolic enzymes, transcription regulatory proteins, cytoskeletal proteins

How are effector proteins altered

A signaling molecule of some sort is going to engage a receptor on the plasma membrane, which will activate a series of events with intracellular signaling proteins

what is the first messenger molecules

the extracellular signaling molecule

What is the second messenger molecule

small intracellular molecules whose concentration changes in response to binding of an extracellular signal and functions in signal transduction

Contact-dependent signaling

a signaling molecule is directly attached to the target cell via a membrane bound signaling molecule on the signaling cell and a receptor on the target cell

paracrine system signaling

signaling molecules are released from a signaling cell, and the molecules find their way to the receptor adjacent cells

synaptic signaling

combination of paracrine and contact-dependent signaling; involves neurons and synapses

endocrine system

a signaling molecule is dumped into the bloodstream

the response to a specific signaling molecule

is cell type dependent

cross talk

intermediates in one transduction pathway can affect intermediates in another pathway

when can the receptor be inside the cell

if the signaling molecule can cross the cell membrane if the signaling molecule is small and hydrophobic

what moves the cell signal across the membrane

a carrier protein

why are carrier proteins needed

since the signal molecules tend to be non polar, the molecule will aggregate before it can get to the receptor

signaling molecules need to be

something that can alter protein function or gene expression and can be elevated under certain conditions

how many domains do transcription factors have

two; DNA binding domain and activation/repressor domain

what are the classes of surface receptors

ion-channel-coupled receptors, g-protein-coupled receptors, and enzyme-coupled receptors

an ion-channel-coupled receptor

is both an ion channel and a receptor for a signaling molecule

ion channel-coupled receptors interact with chemical signals by

transducing a chemical signal into an electrical signal by allowing an ion to move across the membrane

small monomeric g-proteins

direct effects on effector molecules and relay signals from cell surface receptors

large trimeric g-proteins

bind and are activated by cell surface receptors