Lecture 9: Membrane Transport

What are the four different mechanisms that are used to move solutes across a membrane?

• passive transport, active transport

• simple diffusion and facilitated diffusion

What’s the difference between passive and active transport?

passive:

• down a concentration gradient

• no energy is expended

• transport proteins may/may not be needed

active:

• against a concentration gradient

• requires input of energy (often ATP)

• transport proteins “pumps” are required

What does simple diffusion require?

membrane permeability and favourable gradients conditions

What is permeability determined by?

properties of the solute such as:

Molecular size: Small molecules penetrate the lipid bilayer more rapidly than larger ones.

Partition coefficient (measure of polarity): The greater the lipid solubility, the faster the molecule penetrates the membrane.

Charge: Membranes are impermeable to ions.

What two gradients does the tendency of a molecule to move between two compartments depend on?

• concentration gradient (increase entropy)

• electric potential gradient (charged molecules want to move towards the compartment with the net opposite charge)

Respectively, what is the thermodynamically favourable transport direction for molecules with no net charge and charged molecule (ions)?

concentration gradient; electrochemical gradient

What is the electrochemical gradient?

combined effect of the concentration gradient and the electric potential gradient across the membrane

What is simple diffusion only possible for?

• gases

• nonpolar molecules

• small polar molecules (water, glycerol, or ethanol)

What is passive/simple diffusion? Describe an example?

unassisted movement of a molecule across a membrane down its concentration gradient at a rate proportional to the gradient and the permeability of the membrane.

• oxygen inhaled by the lungs and taken up by erythrocytes

Compared to capillaries of body tissues and lungs, what is the relationship between O2, CO2 and hemoglobin?

In the capillaries of body tissues:

low [O2] and high [CO2] relative to erythrocytes

• O2 is released from hemoglobin and diffuses outwards to meet tissue needs

In the capillaries of lungs:

high [O2] and low [CO2] relative to erythrocytes

• O2 diffuses inward and binds hemoglobin

What are membranes permeability to water?

semi-permeable

Thermodynamically, simple diffusion is an _____ process, requiring __ input of energy.

exergonic; no

What is the relationship of the net rate of transport for a substance to the concentration difference?

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

What is the relationship of simple diffusion between inward flux of solute and the concentration gradient of the solute?

linear

What are the two ways transport proteins can be classified as?

Carriers: transporters that alternate between two conformations

Channels: water-filled pore through which specific ions or small molecules can diffuse

What are the two main classes of facilitated diffusion transports?

carrier and channel proteins

• integral membrane proteins

• multiple transmembrane segments

How do channel proteins function?

form hydrophilic channels through the membrane that allow passage of solutes without a major conformational change

How do carrier proteins function?

bind one or more solute molecules, undergoes a conformational change that transfers the solutes to the other side

Channel proteins exhibit what type of diffusion via what channel?

facilitated diffusion via creating hydrophilic transmembrane channels

What are the three kinds of transmembrane channel proteins?

Ion channels: highly-specific channel that can conduct almost a million ions per second.

Porins: passage of a variety of hydrophilic solutes, determined by pore size; some antibiotic resistance has been linked to mutations in certain bacterial porins

Aquaporins: Water flows through at a rate of several billion per second. Amino acid residues discriminate against other ions of similar size

How do ion channels function?

• allow for the rapid passage of very specific ions (usually one kind of ion)

• are bidirectional, with flow determined by the electrochemical gradient

• different channels transport different ions (Na+, K+, Ca2+, Cl-)

• possess tiny pores (size filter), which are lined with hydrophilic amino acids

• most channels are gated

What does selectivity from the ion channels result from?

• from ion-specific associations (with amino acid side chains and the polar backbone)

• results from constriction in the center to serve as a size filter 16

Most ion channels are gated. What triggers the channel to open?

a specific stimulus, but do not need to undergo conformational changes with each ion it passes

What is voltage-gated?

open and close in response to changes in membrane potential

What is ligand-gated?

triggered by the binding of a specific substances to the channel protein?

What is mechanosensitive-gated?

respond to mechanical forces that act on the membrane

Between prokaryotic and eukaryotic ion channels, which features more complexity and what do these features do?

eukaryotic ion channels have additional features/complexity, and these features provide additional layers of regulation

How do porins function?

• allow for the rapid passage of various solutes (low specificity)

• found on the outer membranes of mitochondria, chloroplasts and bacteria

• close cylindrical β-barrel with a water-filled pore at its center

• on the β-barrel, polar side chains line the inside and nonpolar side chains point into the membrane

• upper size limit of the solute molecules is determined by the pore size

• mutations in bacterial porins have been associated with antibiotic resistance

Water diffuses slowly across membranes by what?

simple diffusion

What are aquaporins and its function?

water flows (in single file) through at a rate of several billion per second.

• Specialized cells in your kidneys have a high density of aquaporin to facilitate the re- absorption of water

What is the alternating conformation model?

when the membrane protein can adopt two conformational states

How does the alternating conformation model work?

1. The solute-binding site is open to one side of the membrane

2. Solute binding causes the protein to change to the other conformation

3. The second conformation has the solute-binding site open to the other side of the membrane

4. The solute is then released

Transport can go in either direction, based on the concentration gradient

Facilitated diffusion shares similarities with enzymatic activity (kinetics) in carrier proteins. What are these similarities?

1. Carrier proteins are very specific for their target molecule (imparted by the precise fit between the solute + binding site)

2. Carrier proteins can have their activity regulated

3. Carrier proteins exhibit saturation kinetics

What are uniporters?

transports a single solute

What are symporters?

transports two solutes in the same direction across the membrane

What are antiporters?

transports two solutes in opposite direction across the membrane

What is glucose transporter GLUT1?

facilitated diffusion of glucose by a uniport carrier protein

What are the characteristics of the glucose transporter GLUT1?

• found on all mammalian plasma membranes

• human genome encodes 14 glucose transporters; only GLUT1 is ubiquitous

• process is reversible – can function in either direction depending on the concentration gradient

• glucose rapidly phosphorylated inside the cell. Keeps the intracellular concentration of glucose low and thus maintains a concentration gradient (also the 1st step in glucose metabolism)

What is chloride-bicarbonate exchanger?

facilitated diffusion via an antiport carrier protein AKA anion exchange protein

Characteristics of the chloride-bicarbonate exchanger?

facilitates the reciprocal exchange of chloride and bicarbonate in the opposite directions

• exchange is obligatory (1:1), transport will stop if either anion is missing

• critical role in waste CO2 production in metabolically active tissues

What influences a charged solutes transport?

concentration gradient and its electrical potential

Concentration gradient and electrical potential difference can be combined to calculate the net driving force called?

electrochemical potential or gradient

What are membrane proteins involved in active transport called?

pumps; their energy is required to move substances against their concentration gradients

With respect to the membrane, directed by the concentrations of the transported substances, what directionality does diffusion and active transport exhibit?

non-directional; unidirectional

What are some functions of active transport?

1. Uptake of essential nutrients

2. Removal of wastes (even if the concentration is higher outside the cell)

3. Creation of gradients and maintenance of non-equilibrium concentrations of ions

Active transport couples what processes?

endergonic transport to an exergonic process, usually ATP hydrolysis (energy can also come from absorbance of light, electron gradients...)

What is direct (primary) active transport?

solute accumulation is coupled directly to an exergonic chemical reaction (such as ATP hydrolysis) such as ATPases or ATPase pumps

• ATP hydrolysis drives the outward transport of protons, thereby establishing an electrochemical potential for protons across the membrane

What is indirect (secondary) active transport?

an endergonic (uphill-against a concentration gradient) is coupled to the exergonic (downhill-down a concentration gradient). This may have been pumped uphill by primary active transport

• Indirect: Simultaneous transport of two solutes. Exergonic inward movement of H+ provides the energy to move the transported solute against its concentration gradient or electrochemical potential

Difference between direct and indirect transport is based on what?

the source of energy

Characteristics of the 4 classes of ATPases?

• High selectivity

• Differ in structure, mechanism, localization and physiological roles

• All have one or more ATP binding sites on the cytosolic membrane leaflet

What are P-type ATPases and how many sub-families are there?

• are reversibly phosphorylated by ATP on a specific aspartic acid residue

• 5; which are found on the plasma membrane (p2-p5 are only in eukaryotes)

P4 transport lipids act as what?

flippase to help maintain membrane asymmetry

What is the function of 1deg active transport?

maintains intracellular sodium and potassium concentrations in animal cells which maintains osmotic balance and stabilizes cell volume

Most energy consumed by the brain is used to maintain ___, which maintains what?

Na+/K+; maintains the membrane potential required for the transmission of nerve impulses

What organelles do V-type ATPases pump protons (H+) into?

vacuoles, vesicles, lysosomes, endosomes, and the Golgi complex (why are these pumps only found in eukaryotes?)

What two multi-subunit components do V-type ATPases contain?

• an integral component embedded in the membrane

• a peripheral component that juts out from the membrane surface

F-type ATPases only transport protons (H+) in?

mitochondria, bacteria, and chloroplasts MBC

F-type ATPases can work in both directions transporting what flow?

• endergonic flow of H+ up their concentration gradient hydrolysis ATP

• exergonic flow of H+ down their concentration gradient is used to synthesize ATP

they can act in the reverse direction, which is why they are more accurately called ATP synthases

• not only can ATP be used as an energy source to generate ion gradients, but such gradients can be used as an energy source to synthesize ATP

What are ABC-type ATPases and what are two characteristics that all ABC-type ATPases family members have? ?

ATP binding cassette transporters

• comprise a very large family of transport proteins (~150 proteins found in all organisms)

• first discovered were importers, involved in the uptake of nutrients (amino acids, peptides, proteins, metal ions, lipids)

• a large number of clinically relevant exporters were identified in both bacteria and humans

They all have:

• Two nucleotide (ATP) binding domains

• Two transmembrane domains

What is the multi-drug resistant transporter and what do they pump out?

a sub-class of ABC-type ATPases

• ABC transporters are medically important because some of them pump antibiotics or drugs out of cells, rendering the cell resistant to the drug

• Some human tumors are resistant to drugs that normally inhibit growth of tumors; the resistant cells have high concentrations of an ABC transporter called MDR (multidrug resistance) transport protein

• The MDR protein of some bacteria renders them resistant to antibiotics by a similar mechanism

• MDR transport protein pumps hydrophobic drugs out of cells, reducing the cytoplasmic concentration and hence their effectiveness

What is indirect active transport (secondary active transport) powered by?

the potential energy stored in ionic gradients

What is the inward transport of molecules up their electrochemical gradients coupled to and driven by?

simultaneous inward movement of Na+ (animals) or H+ (plants, fungi, bacteria) down their gradients

What does indirect active transport ultimately rely on?

ATP, because the Na+ or H+ concentration gradient necessary for this process is generated by a ATPase pump.