Cell Bio Exam 2 - Membrane Transport

Describe the selective permeability of the cell membrane

cell cannot prevent things crossing that have high solubility in fat; trying to protect itself

What are the two modes of transport? How do they use energy?

Passive transport - solute moves in favorable fashion, does not require energy; active transport - solute moves in non-favorable fashion, requires energy so requires a protein

What are the two types of passive transport?

Simple, diffusion and facilitated diffusion

What controls the rate and direction of simple diffusion?

The concentration gradient across the membrane; small polar molecules can pass through because of micro channels created by cholesterols ring structures; small nonpolar molecules cannot be restricted

What happens in facilitated diffusion?

A membrane protein facilitates the movement across the membrane; binds to the solute, undergoes conformation shift that brings the solute across

What are features of carrier mediated facilitated diffusion?

Solute must be moving down the concentration gradient (favorable concentration gradient); the carrier physically combines with the solute; carriers are specific for what they transport; carriers saturate at high solute concentration - have set numbers of carrier proteins, if necessary can upregulate to make more; rate of transport reflects number of carriers; carriers can be regulated (inhibitor)

What is the ping-pong model? Give an example of one of these proteins.

The protein alternates between high affinity state for binding and low affinity state - switches conformation; passive glucose transporter (GLUT1) - has amphipathic helices that may be involved in the conformation shift

How is information about transporter proteins discovered?

Cannot watch in real time – infer by removing parts of protein by deleting genes

What are features of the channels in facilitated diffusion?

R group chemistry of the amino acids around the channel is important; selectivity filter determine size and charge of solute that can pass; channels are regulated

What are the different kinds of gating stimuli? Which cell type is each found in?

Voltage gated – neurons and muscle cells; ligand gated (extracellular or intracellular ligand) – neurons and muscle cells; mechanically gated – muscle cells, bone cells, epithelial cells, special sensory cells, (skin, bone, responding to tension, heart, responding to stretch and pressure.)

How do voltage gated channels work? What are the three conformations?

No ion movement – gate is closed; membrane depolarized, ion movement – gate is open; no ion movement – gate is inactivated, blocked but gate not closed

What are examples of voltage gated channels?

Potassium channel –four smaller proteins that associate together; sodium channel and calcium channel – one large protein with four homologous domains

What does the potassium channel look like?

Each of the four protein units has an anti-parallel beta sheet that lines the channel; helix number four is the voltage sensor – highly conserved among all voltage, gated channels; change in voltage induces conformation shift in voltage sensor - leads to change in protein shape, and gates channel; at the N terminal, 19 amino acids form a globular domain – ball and chain, used for inactivation; in potassium, there are four balls and chains – in sodium and calcium only one – only one is needed to include the channel

How do chemically gated channels work? What does it mean that they are promiscuous? How does this apply with neurotransmitters?

Made up of 5 transmembrane spanning proteins – two identical; the R groups flanking the channel have a negative charge; when acetylcholine binds to the two identical subunits, it induces a conformation shift - Gates channel - must have both binding sites occupied; promiscuous, meaning they permit small monovalent cations through; synapse environments have an enzyme to degrade neurotransmitters so the receptor becomes inactive as soon as one site is cleared

What the two types of active transport?

Direct, indirect

What happens in direct transport?

The transporter binds and hydrolyzes ATP – transporter is a carrier, but also an enzyme with ATP as a substrate; uniport

What happens in indirect transport?

energy is used to establish an ion gradient; but the energy expenditure occurs at a time prior to transport; the ion and solute are co-transported – ion moves down concentration gradient, solute moves against concentration gradient; symport and antiport; most often the ion is sodium; the ion gradient works like potential energy so solute can be transported against a gradient; usually maintains electrical balance, ions moving in opposite directions will have the same charge

What are the types of direct active pumps?

P type and V type

How does a P type active pump work?

P type – pump protein is phosphorylated during transport (autophosphorylation – phosphate group is covalently attached to an amino acid within pump protein) – typically ion transporters

what are the structural features of P type pumps?

enzyme function happens inside the cell in the cytosol because that is where ATP is present; target site for phosphorylation – see slide 51

Describe how a P type pump works, using H+ ATPase as an example.

Hydrogen ion is transported against its gradient – transporter is in a high affinity state for binding hydrogen ions, but won't move it until the protein is phosphorated; protein is phosphorated, ATP becomes ADP; after hydrogen ion moves through proton is dephosphorylated by water

What are some examples of P type transporters?

H+ ATPase - in plasma, membrane of plant cells, fungi; H+/K+ ATPase – in plasma membrane of cells that line the stomach – pumps hydrogen from inside cells out into stomach lumen, pumps potassium from stomach lumen into cells, so stomach is acidic – maintains electron gradient; Ca+ ATPase - in sarcoplasmic reticulum membrane, endoplasmic reticulum membrane, in plasma membrane of many cells - removes calcium from the cytoplasm continually – so concentration of calcium is low intracellularly, extra is stored in ER – clears calcium out of muscle cells so they can relax; Na+/K+ ATPase – in plasma membrane of all animal cells, vital for animal cell, almost 1/3 of total energy requirement of a cell is used for this pump

How does the Na+/K+ ATPase work?

Establishes gradient for sodium and potassium; transports three sodium ions out and two potassium ions into the cell for every phosphate bond broken (every ATP used); both are moved against their concentration gradient; electrogenic pump - polarizes the membrane so cells respond to an electric current – so humans have a nervous system – creates a voltage difference because unequal movement of charged ions (3 positive out, 2 positive in); also volume regulation – affects osmotic gradient across membrane, because unequal movement of solute across membrane, more solute moving out than in – so drives water out of the cell, necessary because water is produced in the formation of macromolecules, don't want the cell to lyse; thermal regulation – constant activity, so high ATP consumption - generates heat – constantly working, never stops; establishes ion gradient across the membrane – can't move ions unless you have a transporter, creating a gradient because want to exploit it – gradient is necessary for excitation of cell by electrical impulses, also for some membrane transporters

what does V type mean for direct pumps? What is one kind of V-type pump?

Vesicle type because usually on internal membrane; V1-ATPase

How does V1-ATPase work?

Related to the F1 ATP synthesis in the in mitochondrial membrane – does the opposite, uses ATP to move hydrogen ions continually against the concentration gradient from the cytoplasm where ATP is into the interior of the intracellular compartment - results in acidification of the interior of the organelle

Give an example of indirect transport. How does this pump work?

Na+ driven glucose transporter (SGLT) – in intestine and kidney; transitions from occluded, empty to outward open to occluded occupied to inward open back to occluded, empty; moves glucose and sodium into the cell

Describe glucose transport in the intestine.

Indirect active glucose transport (SGLT) - sodium driven against the concentration gradient - from contents of gut to inside cell; passive glucose transport (Glut1) down the concentration gradient - from inside cell to blood vessels