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Lipid bilayers exhibit differential ___ to certain molecules.



Which molecules are most permeable to a lipid bilayer?

Hydrophobic molecules (O2, N2, CO2, steroid hormones)


Which molecules are moderately permeable to a lipid bilayer?

Small uncharged polar molecules (water, urea, glycerol) and large uncharged polar molecules (glucose and sucrose)


Which molecules are least permeable to a lipid bilayer?

Ions (H+, Na+, HCO3-, K+, Ca2+, Cl-, Mg2+)


What three methods can be utilized to cross the lipid bilayer?

1. Simple diffusion - no energy required
2. Passive transport - with concentration gradient, no energy required
3. Active transport - against concentration gradient, requires energy


15-30% of all membrane proteins are ___ proteins.



Passive transport can occur via ___ proteins or ___ proteins.

Channel; transport


What is the electrochemical gradient?

The combinatorial gradient generated by both the concentration (chemical) and electrical gradients across a lipid bilayer.


Channel proteins allow for ___ of a molecule ___ its electrochemical gradient through an ___ pore.

Diffusion; down; aqueous


What are ion channels?

Regulated, conditional facilitators of diffusion (passive transport)


What are the three types of ion channels?

Voltage-gated, ligand-gated (extracellular and intracellular), and mechanically gated


Transporter proteins utilize ___ changes to transport solutes.



True or false - transporter proteins are only utilized in passive transport.

False - can be utilized in both active and pasive transport


What are the three major types of active transport mechanisms?

1. Coupled transporter
2. ATP-driven pumps
3. Light-driven pumps


What are the two types of coupled transporters?

Symport (2 molecules move in the same direction) and Antiport (2 molecules move in opposite directions)


Describe the difference between primary and secondary active transport.

Primary: uses the energy of ATP hydrolysis to drive cargo against its electrochemical gradient
Secondary: uses energy stored in an electrochemical gradient (usually Na+) to drive cargo against its gradient.


___ transport can be driven by a sodium gradient. Explain.

Glucose; sodium flows down its electrochemical gradient into the cytosol of the cell. Glucose is found at high concentrations in the cell and low concentrations outside the cell; however, it utilizes the energy of the sodium gradient through a secondary active transport process to move glucose against its gradient (into the cell).


What are the 4 classes of ATP-driven pumps and what do they do?

1. P-type: phosphorylate themselves during transport; transport ions.
2. F-type: ATP synthases, make ATP from gradient
3. V-type: hydrolyze ATP, pump hydrogen ions
4. ABC pumps: pumps molecules rather than ions while hydrolyzing ATP (can still pump ions)


What establishes the sodium gradient?

Sodium/potassium pump


An average animal cell devotes ___ of its energy to the sodium/potassium pump.



Describe the cycle of the sodium/potassium pump.

Initially, there is a high Na+ concentration outside the cell and a high K+ concentration inside the cell. The goal is to move 3 Na+ out (against its gradient) and 2 K+ in (against its gradient).

1. Na+ binds to the pump. ATP hydrolysis occurs. Phosphorylation of cytoplasmic residues occurs.
2. This induces conformation changes that transfer Na+ out of the cell.
3. K+ binds on the extracellular face of the pump. Dephosphorylation occurs.
3. K+ is transferred to the inside of the cell due to dephosphorylation induced conformation changes.


What are ABC transports?

ATP Binding Cassetes; transport all things "non-ionic," though some also transport ions


Name three diseases in which ABC transporters are important.

1. Multi-drug resistance (MDR)
2. CF
3. Malaria


What does MDR protein do?

Pumps numerous drugs out of the cytosol; its expression is increased in many cancers, making them drug resistant.


What happens in CF?

The CFTR (transmembrane conductance regulator protein) is not functioning correctly. The Cl- channel that removes Cl- from the cell is not working. Cl- ions are trapped the cell. This attracts more cations. There are more ions in the cytoplasm, which induces water to enter the cell via osmosis. This induces thick pathological mucus.


What is plasmodium falciparum?

A malaria agent that uses ABC transporter to pump chloroquine, an anti-malarial drug, out of disease-causing protists.


What is cystinuria?

Genetic defect in an amino acid transporter responsible for removing cystine (Cys dimer) from urine and intestine; leads to development of cystine stones in bladder and kidneys


Describe the mathematical characteristics of transport proteins.

vmax = maximum rate of transport; occurs when all transporter binding sites are occupied.
Km = concentration of 1/2 maximal transport; reflects affinity of transporter for solute


What allows the transcellular transport of molecules and is important in nutrient absorption?

Spatial regulation of specific transporters


What are the general concentrations of ions in a typical cell (intracellular, extracellular)?

Na+: (low, high)
K+: (high, low)
Mg2+: (low, high)
Ca2+: (low, high)
Cl-: (low, high)