Block 1 Unit 1 Plasma Membrane 2

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Dr. Smas's second lecture (126-152), including topics such as diffusion and active transport. These are her objective questions as well as some questions on mechanisms of transport and some additional vocab.

simple diffusion

Net transport of molecules from an area of high concentration to low concentration. Ex. Gases passing through the plasma membrane.

facilitated diffusion

Spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane transport proteins.

active transport

This type of transport requires the use of energy.

passive transport

This type of transport does not require energy.

primary active transport

This type of active transport uses chemical energy, such as ATP

secondary active transport

This type of active transport uses an electrochemical gradient.

1) The rate of facilitated diffusion is much higher than passive diffusion, 2) Transport is specific, and 3)Transport occurs via a limited number of carrier proteins.

Three main features that distinguish facilitated diffusion from simple diffusion?

1) Extracellular insulin binds to the plasma membrane receptor. 2) Signal transduction causes GLUT4-containing vesicles to be translocated. 3) GLUT4-vesicles fuse with membrane; increases GLUT4 at cell surface. 4) Glucose transported through GLUT4 (facilitated diffusion).

What is the mechanism of action of GLUT4 in insulin responsive glucose uptake?

Glut4

The major insulin-responsive glucose transporter. Found in muscle and fat cells. An example of facilitated diffusion.

symporter

In this type of secondary transport, both molecules come into the cell or both molecules go out of the cell (At least one molecule goes down its gradient).

antiporter

In this type of secondary transport, one molecule goes out and another comes into the cell.

Energy from ATP is used to establish and maintain an electrochemical gradient (lots of Na+ gets pumped out of the cell). This gradient is used to drive "downhill" flow of a second type of transport (glucose and sodium both come into the cell).

How does the Na+/K+ ATPase primary active transporter function to drive secondary transport of glucose?

Cardiotonic steroid drugs such as Digitalis, which contains ouabain and digitoxigenin and is used to treat congestive heart failure, inhibit dephosphorylation of the E2-P form. This causes the pump to be locked in a non-functional state. The decreased pump action increases the [Na+] in cardiac muscle cells. This leads to increased [Ca+2] in the cell via the action of a Na+-Ca+2 transporter. The calcium mediated signal acts to increase contraction strength of the heart muscle.

Explain the mechanism of action of cardiotonic steroid drugs on the Na+/K+ ATPase.

The selectivity filter is too small for a hydrated ion to pass any farther. The ion must shed its water shell and the shedding must be thermodynamically stable. Passage of K+ through the voltage gated K+ channel is thermodynamically favored whereas passage of Na+ is not thermodynamically favored.

What is the structural basis of the ion selectivity of the voltage-gated K+ channel?

CFTR normally pumps Cl- out of the cell. In cystic fibrosis, a deletion of 3 bp produces a mutant CTFR (deltaF508CTFR) which doesn't reach the cell surface. Cl- accumulate in the cell since they can't get out, causing water and Na+ to flow into the cell. The extracellular environment is thus left with a thick, dehydrated mucus that causes defects in respiratory tract cilia and infection. *About 20-30% of CF cases are caused by other CTFR mutations.

How are ABC-type ATP-powered pumps involved in cystic fibrosis?

MDR1 uses ATP energy to export drugs from the cytosol. MDR1, which is normally expressed in the liver to remove toxins, is overexpressed in tumor cells and thus drugs fail to exert their benefits.

How are ABC-type ATP-powered pumps involved in multi-drug resistance?

Higher concentrations in ICF: K+, Mg+2; Higher concentrations in the ECF: Na+, Ca+2, Cl-, HCO3-, glucose.

Consider the following: Na+, K+, Ca+2, Mg+2, Cl-, HCO3-,glucose. Due to selective permeability, which have higher concentrations inside the cell (ICF)? Which have higher concentrations outside the cell (ECF)?

The structure of ion channels facilitates rapid passage of ions (10^7-10^8 molecules/sec). Comparitively, Carrier Mediated Facilitative Diffusion is 10^2-10^4 molecules/second and ATP-Powered Pumps are 1-10^3 molecules/sec.

Considering the following: Carrier-Mediated Facilitative Diffusion, Ion Channels, and ATP-Powered Pumps. Which one is the fastest in terms of molecules/second?

False. The two types of facilitated diffusion are Carrier Mediated Facilitative Diffusion and Ion Channels. Ion Pumps by comparison are an example of active transport.

True or False: Two types of facilitated diffusion are 1) Carrier Mediated and 2) Ion Pumps

Both the Ion Concentration and Ion Charge Gradient determine ion channel flow.

What determines direction of Ion Channel flow?

K+ leak channels are always open and they function to maintain a negative membrane resting potential (cytoplasm is negative).

Most ion channels are gated (voltage-gated, extracellular ligand gated, intracellular ligand gated, mechnically gated). What ion channel is always open and what is its function?

Voltage-Gated Ion Channels

These help propagation of Nerve Impulse Action Potentials.

The restricted (selective) part of the K+ channel has four binding sites. Binding of the second ion creates electrostatic repulsion to push the first ion out, then the process repeats.

Use the Voltage-Gated K+ Channel to explain how the rates of transport through ion channels is so fast.

Ball and chain model can cause inactivation of voltage gated ion channel in milliseconds.

What is one model that explains rapid inactivation of ion channels?

The Nicotinic Acetylcholine Receptor

What is one example of a ligand-gated ion channel?

Bind of acetylcholine to the receptor opens the channel and causes a large flux of Na+ and K+ into the cell. This causes transient depolarization of the membrane and the action potential leads to muscle contraction.

What happens after acetylcholine binds the nicotinic acetylcholine receptors at the neuromuscular junction?

Before binding, the channel is closed due to the position of bulky hydrophobic leucine side chains. After 2 acetylcholine molecules bind, a conformational change causes the helical subunits to slide and rotate and open the channel. Small polar amino acids now line the channel.

Describe the structure/conformation of the acetylcholine receptor before and after binding (Hint: what types of amino acids are involved?).

Channelopathies

Epilepsy, diabetes mellitus, cystic fibrosis, dilated cardiomyopathy, cardiac arrhythmias, and myasthenia gravis are all examples of this.

Myasthenia Gravis

An autoimmune response that produces Ab against the nicotinic acetylcholine receptor is the cause of this disease, which weakens the skeletal muscles.

Na+/K+ ATPase

This important P-class primary active transporter generates an ion concentration gradient important for maintaing cell volume, driving transport of sugars and amino acids, and establishing and maintaining the electrochemical gradient in all the cells. It is key to overall plasma membrane function.

-60mV

The electrochemical gradient created by the Na+/K+ ATPase and the K+ leak channels maintain the membrane potential, which is this value at rest.

The protein beings in the E1 conformation. 3 cytoplasmic Na+ bind, which stimulates phosphorylation by ATP. Phosphorylation causes a conformational change (E2-P), which expels the Na+ to the outside, and causes 2 extracellular K+ to bind. K+ binding causes the release of the phosphate group, which causes a conformational change back to E1. K+ is released into the cell and the cycle repeats.

Describe the mechanism of action of Na+/K+ ATPase.

P-Class or P-Type

This family of active transporters has alpha and beta subunits (alpha unit contains ATP binding-region), uses the hydrolysis of ATP to provide energy to pump against the EC gradient, and requires a Mg+2 cofactor.

ATP-Binding Cassette

This family of active transporters has six alpha helices in each transmembrane domain, transports ion or small molecules, and couples ATP to solute movement. Examples are MDR1 and CFTR.

Ligand binds from cell interior, which leads to conformational change that increases affinity for ATP. Two ATP bind, and the ATP cassettes interact, which causes a second conformational changes. Ligand is released outside of cell. ATP hydrolysis resets transporter for another cycle.

What is the general model for ABC transporter action?

[Glucose] and [Na+] in the intestinal lumen are high, therefore they both go down the concentration gradient and enter the epithelial cells via the glucose-Na+ symporter. Na+ is kept low in the epithelial cells via a Na+/K+ pump between the epithelial cells and the blood. Glucose travels from the epithelial cell to the blood via Glut2.

How is glucose transported from the intestinal lumen into the blood?

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