Passive and Active Transport for exam 1 PCB3134

What is passive transport? How do molecules move?

The movement of materials across the cell membrane without using cellular energy

• molecules move down their concentration gradient

What are the two kinds of passive transport? What movement do they involve?

simple diffusion and facilitated diffusion

• involve exergonic (negative deltaG) movement down the concentration gradient

What are the 4 important considerations to make when talking about transport?

1. solute properties (size and charge)

2. relative solute concentrations

3. availability of specific transmembrane proteins

4. availability of an appropriate energy source

How are each of the following molecules transported across the membrane (simple diffusion, facilitated diffusion, and active transport)?

• small nonpolar

• large nonpolar

• small polar

• large polar

• ionic

• small and large nonpolar > simple diffusion

• small polar > simple diffusion (slow) & facilitated diffusion (fast)

• large polar and ionic > facilitated diffusion and active transport

What is the movement of a molecule that has no net charge determined by?

its concentration gradient

What is the movement of a charged molecule (ion) determined by?

its electrochemical potential

What is electrochemical potential?

combination of concentration gradient and the charge gradient across a membrane (membrane potential)

• these aspects can work against each other

What is membrane potential? What is the symbol used for it?

charge gradient across the membrane (Vm)

What is an example of simple diffusion in erythrocytes?

Both O2 and CO2 pass the membrane through simple diffusion because they are small, nonpolar molecules

• O2 is taken up by cells in the lungs where the concentration is high and release it in the body tissues where O2 concentration is low

• CO2 is taken up by cells in body tissues where CO2 concentration is high and released in the lungs where CO2 concentration is low

What is osmosis? Why is it given a special name? When can it occur? How does it occur?

diffusion of water across a selectively permeable membrane

• given special name because it is a solvent moving across the membrane (not a solute)

• if two solutions of different solute concentrations are separated by a selectively permeable membrane that is permeable to water but not the solutes

• water is a small, polar molecule so it can move by simple diffusion but slowly so it is also aided by protein channels called aquaporins

Where does water move across the mem. through osmosis? What is osmolarity?

water will move towards the region of higher solute concentration (diluting it to equalize the concentrations across the membrane)

• relative concentrations of solutes between cytoplasm and extracellular solution

What does hypertonic, hypotonic, and isotonic mean? What do you need to think about when talking about tonicity?

important to think about which solution you are referring to and to know that solutions can only be hyper-, hypo-, and isotonic compare to another solution (not on its own)

• hypertonic > higher solute concentration

• hypotonic > lower solute concentration

• isotonic > same (equal) solute concentration

Plant cells must maintain internal pressure (turgor pressure) to force their plasma membrane against the cell wall. To do so, the interior of the cell must be _________________ to their environment.

hypertonic

What kind of process is simple diffusion thermodynamically?

exergonic, requiring no input of energy

What is the relationship between simple diffusion and concentration difference across a membrane kinetically?

net rate of transport for a substance is proportional to its concentration difference across the membrane

• these share a linear relationship with difference in concentration on the x-axis and rate of diffusion on the y-axis

What 3 proteins are used in facilitated diffusion?

• transport proteins

• carrier proteins

• channel proteins

What is the alternating conformation model?

states that a carrier protein is an allosteric protein and alternates between two conformational states

• one state > solute-binding site of the protein is accessible on one side of the membrane

• protein shifts to the alternate conformation, and the solute-binding site switches to the other side of the membrane which triggers release of solute

what are the 4 ways in which proteins relate to enzymes?

1. specificity (binding of substrate on a specific solute-binding site

2. intermediate forms (between carrier protein and solute)

3. conformational change (solute binds, carrier protein changes shape - affinity change, solute is released)

4. regulated (by external factors)

What is uniport transport? What is coupled transport?

Movement of a single substance in one direction

• movement of two substances with aid of single transmembrane protein

What are the two kinds of coupled transport?

1. symport > both molecules move in the same direction

2. antiport > molecules move in opposite directions

What is an example of uniport? Describe how it works.

Transport of glucose by GLUT1 carrier protein

• glucose binds to GLUT1 which has binding-site open to outside of the cell (T1 conformation) > GLUT1 shifts to its T2 conformation with binding site open to inside of cell > glucose is released to the interior or the cell which initiates conformational change from T2 back to T1 > GLUT1 is now ready for another cycle

How can glucose be kept in the cell? What are examples?

any chemical change done to glucose molecule inside the cell causes altered forms of glucose that do not count as far as transport is concerned in the glucose concentration gradient

• examples are phosphorylation and polymerization (liver polymerizes glucose into glycogen)

What is an example of antiport? Describe the process.

erythrocyte anion exchange

• anion exchange protein (chloride-bicarbonate exchanger) facilitates reciprocal exchange of Cl- and HCO3- ions only in a strict 1:1 ratio

• in lungs > Cl- is released and HCO3- is taken up by the cell

• in body tissue > Cl- is taken up and HCO3 is released by the cell

How to channel proteins facilitate diffusion? What are the 3 different kinds?

by forming hydrophilic transmembrane channels that allow specific solutes to cross the membrane directly

1. aquaporins

2. ion channels

3. porins

What are aquaporins? What kind of protein are these? What are some specific membranes that have these?

channel proteins that allow rapid movement of water across cell membranes

• tetramer (identical subunits) > each acts as own water channel so one aquaporin has 4 water channels

• erythrocytes, kidney cells, vacuolar membranes in plants

What are ion channels? What is an important characteristic?

tiny pores lined with hydrophilic atoms

• highly selective > so need different channel for every ion except porins

What are the functions of ion channels?

cellular communication (i.e. muscle contraction and electrical signaling of nerve cells)

• maintaining salt balance in cells and airways linking the lungs

What is a specific example of a ion channel involved in cystic fibrosis?

cystic fibrosis transmembrane conductance regulator (CFTR) which helps maintain proper Cl- concentration in lungs

• defects in the protein cause cystic fibrosis

• Cl- ion channel is faulty and does not allow movement of Cl- out of cells lining the airways into airway lumen, causing water to remain inside the cells as well > leads to thick, sticky mucus in the lumen which is associated with bacterial infections etc.

What are porins? Where are they found? What are they made of?

• large pores on outer membranes of bacteria, mitochondria, and chloroplasts

• single polypeptide with multipass transmembrane segments that cross the membrane as beta barrels

How do porins compare to other ion channels?

larger and less specific than ion channels

The graph of the rate of facilitated diffusion versus substrate concentration is not linear like the graph of simple diffusion versus substrate concentration. Why?

In facilitated diffusion there is a limited amount of active protein, so that at higher substrate concentrations, the protein becomes saturated.

What are the characteristics of active transport?

requires energy

• used to move solutes up a concentration gradient, away from equilibrium

• has intrinsic directionality

What are the versions of active transport? Explain them.

direct > hydrolysis of ATP is directly involved

• membrane protein has ATPase as a part of it

indirect > hydrolysis of ATP is not directly involved, does not have ATPase as a part of the membrane protein

• energy from another molecule moving down its cg can be used a the energy source for indirect active transport

What is an example of transport that releases energy?

any molecule moving down their concentration gradient

What are the four types of ATPases? How do they differ? What is special about one of them?

P-type, V-type, F-type, and ABC-type

• differ in structure, mechanism, location, and roles

F-type includes ATP synthases and can pump H+ reversibly to actually make ATP

What do V-type ATPases do? Where do they function?

pump protons into organelles such as vacuoles, vesicles, lysosomes, endosomes, and the Golgi complex

• only in internal membrane bound organelles (not plasma membrane)

What is the ratio of K+ ions inside versus outside? Na+ ions?

30:1

0.08:1 (or outside to inside ratio is 12:1)

What is the purpose of having the [K+] gradient be 2.5 times as strong as the [Na+] gradient?

To overcome of the membrane potential (Vm) of the cell

What happens every turn of the sodium potassium pump? What does it require? What is it primarily responsible for?

exergonic hydrolysis of ATP to drive the transport of both ions (needs to use metabolic energy)

• 2 K+ ions in, 3 Na+ ions out

• the asymmetric distribution of ions across the plasma membrane of animal cells

What is the structure of the Na+/K+ pump? What are the purposes of its components?

alpha, beta, and gamma subunits

• beta & gamma are primarily structural

• alpha has binding site for sodium, changes shape, and has binding sites for potassium, also has ATP binding site

What kind of protein is the Na+/K+ pump? What does this mean specifically for this pump?

allosteric protein

• has two conformational states (E1 & E2)

E1 is open to inside and has high affinity for Na+ ions

E2 is open to outside and has high affinity for K+ ions

Explain the steps for one cycle of the Na+/K+ pump.

Na+ from inside the cell binds to E1 triggering phosphorylation of the alpha subunit from ATP (ADP is released)

• phosphorylation of the alpha subunit by ATP hydrolysis triggers conformational change to E2 which expels the Na+ to the outside of the cell

• K+ on the outside of the cell binds to E2 triggering dephosphorylation of alpha subunit

• dephosphorylation of alpha subunit causes conformational change to E1 and expels K+ into the cell

• ATP binds to the alpha subunit and the pump is ready for a new cycle

Which of the following is true regarding the E2 conformation (open to the outside of the cell) of the Na+/K+ pump?

It binds K+ strongly and Na+ weakly

What are the energetics of transport?

every transport event in the cell is an energy transaction

• for uncharged solutes, the only variable is the concentration gradient

• for charged solutes, both cg and electrical potential are relevant

What is the formula for calculating deltaG for the transport of uncharged molecules? What do the variables represent? When is deltaG positive? Negative?

*deltaGinward = the energy associated with the solute moving

• into the cell

• R = gas constant

• T = temp

• I = natural log

• [X]in = solute concentration inside the cell

• [X]out = solute concentration outside the cell

• z = charge of solute

• Vm = membrane potential

ln of value great than 1 (not decimal) > positive deltaG > movement in is unfavorable if solute concentration is higher inside the cell

ln of value less than (decimal) 1 > negative deltaG > movement in is favorable if solute concentration is lower inside the cell

For charged solutes, we calculate the effect of electrical potential on ΔG separately from the effect of concentration gradient. We use zFVm to calculate this effect.

How would membrane potential (negative for most cells) affect ΔG for the import of calcium ions?

it would make ΔG more negative

What is the formula for calculating the deltG value of transport for charged molecules? What do the variables represent?

*deltaGinward

What is the typical membrane potential for cells? What does this mean for the transport of ions across the membrane and the associated energy?

Vm is negative

• positive ion will give a negative deltaG value and therefore be pulled in

• negative ion will give a positive deltaG value and therefore be pushed out

Consider a nerve cell with [Cl-] inside = 50 mM, in a solution with [Cl-] = 100 mM and membrane potential of -60 mV. If RTln([S]in/[S]out = -410 cal/mol and zFVm = +1384 cal/mol, what will the direction of transfer be?

deltaG = -410 + 1384 = 974 cal/mole

• since positive deltaG for transport in, Cl- will move out of cell

since positive deltaG for transport in, Cl- will move out of cell