Electrical Properties and Transport of Small Molecules

Question 1

What is the net charge within a cell at rest and what molecules contribute to it?

Answer 1

The net charge of cells is approximatelly neutral
whereas mostly ions (whether they are free or bound to proteins) contribute to its overall charge. Examples: Na+, K+, Ca2+, Mg2+, H+, Cl-, HCO3-, PO43-, proteins, nucleic acids or any phosphate compounds, carboxyl groups (-COOH an -C=O)

Question 2

What substances can readily pass the PM and what substances need transporters? Name examples for both

Answer 2

Small unpolar (CO2, O2, benzenes, partially water, urea). Sugars or ions need carriers.

Question 3

What two types of treansporter proteins exist?

Answer 3

• carrier proteins (switch between two states)

- channels (aquaeous pore for transport)

Question 4

What is the difference betwee passive and active transport?

Answer 4

Passive: along (down) the concentration gradient, no additional E needed. Simple difussion, channel transport, carrier mediated transport.
Active: against (up) the concentration gradient. E input requred. Carrier mediated transport.

Question 5

What is an electrochemical gradient? Which two aspect does it influence?

Answer 5

Concentration of substances (ions, molecules), their charge and the charge of membranes. Based on the membrane potential, it drives or retains the transport of specific substances.

Example:

• K+ (c) higher outside than inside, membrane outside more positive = high driving force
• K+ (c) higher outside than inside, membrane outside more negative = driving force is smaller compared to the previous example

Question 6

Name examples of an active transport.

Answer 6

• clasical or coupled carriers
• ATPase pumps
• light pumps

Question 7

What types of carriers do you know? Briefly describe how they work.

Answer 7

• uniport: binds a specific type of substrare and transports it, based on the carrier type and concentration gradient, the transport can be passive or active
• symport (coupled): capable of binding two types of substrates, binding of one increases the affinity to the other, has to be co-operative, utilizes a high concentration of one substrate (for example K+) to transport the other substrate too (for example glucose) even though it goes against the concentration gradient
• antiport (coupled): also capable of binding two types of substrates, two different substrates cannot be bound at the same time

Question 8

Describe the distribution of glucose, Na+ and K+ in the intestinal epithelial cells.

Answer 8

Glucose is transported agains the concentration gradient from the intestinal lumen into the epithelium via Na+ driven glucose symport (glucose is low in the epithel) and then using concentration gradient passively passed via glucose carrier protein. Na+ which has been transported into the ephitel is pumped out into the extracellular fluid by Na+-K+ pump (antiport) to maintain a stable concentrarion and drive the glucose transport.

Question 9

Describe the Donnan effect.

Answer 9

Cells contain many organic metabolites such as sugars, amino acids, nucleic acids and other molecules which are partially charged and attrack small molecules with an opposite charge (called counterparts). These counterparts, hence their structure, slowly leak intot the cell and contribute to overall osmolarity. If they would not be pumped out, the concentration inside would rise and result in a higher concentration of water outside the cell. Accordingly, water would travel inside the cell which would result in its rupture.

Question 10

How does the PM ATPase pump work? What does it maintain? What is a common inhibitor for such pump?

Answer 10

The ATPase pumps maintain osmotic balance. Upon ATP hydrolysis to ADP and Pi, the ubiquitous P-type pump pumps 3 Na+ out and 2 K+ into the cell against their electrochemical gradient. Such ATPases sequentially phosphorylate and dephosphorylate themselves during the pumping cycle.
Ouabain competes with K+ for the binding site

Question 11

Name and define classes of pumps.

Answer 11

• P type pumps: ATPase, they phosphorylate themselves, responsible for setting up and maintaining gradients of ions (Na+, K+, Ca2+, H+)
• F type pumps: ATP synthases, turbine like proteins with multiple different subunits, use H+ gradient, during electron transport steps of oxidative phosphorylation (inner mitochondrial membrane)
• V type pumps: ATPases, structure similar to F type pumps, but they normally pump H+ rather than synthesize ATP, pH maintenance in lysosomes, endosomes, vesicles
• ABC Transporters: pump small molecules across cell membranes

Question 12

Name examples of P-type pumps and define them.

Answer 12

P-type Ca 2+ ATPase:
Eucaryotic cells maintain low concentrations of Ca2+ in their cytosol. For signal transduction processes it is essential
that this steep concentration gradient is maintained.

SR:

1. Action potential initiated at neuromuscular junctions
2. Ca2+ released into cytosol through calcium release channels.
3. Muscle contraction.
4. Ca2+ pump brings calcium ions back into the SR

Na+/Ca2+ exchanger (antiporter) as alternative mechanism

P-type Na+K+ ATPase:

Establishes the Na+ gradient across the plasma membrane. All animal cells. Uses the energy obtained by ATP hydrolysis to pump Na+ out and K+ into the cell. Both ions go against the concentration gradient.

• 3 Na+ out
• 2 K+ in
• Inside: negative
• Outside: positive

Question 13

Define ABC transporters

Answer 13

Each member contains 2 highly conserved ATPase domains/ATP binding cassettes.
ATP binding: dimerization of ATP binding domains
ATP hydrolysis: dissociation of ATP binding domains
The resulting conformational changes transport small
molecules across the lipid bilayer

Transport inorganic ions, amino acids,
mono and polysaccharides, peptides, proteins

Used for export in eukaryotes.