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Membrane transport

By Levi Clancy for Student Reader on

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Passive TransportPolar Molecules
Less than 3 carbons
No net charge
No energy required for transport. Cross fast with no channels or carriers.
Passive or ActiveBiological monomers
5-6 carbon sugars
Sometimes energy is required to transport it across the membrane. Slow, too slow for metabolic processes, need channel or carrier-mediated mechanism usually taken via receptor-meidated endocytosis.
Active TransportIonsEnergy is required to transport ions across the membrane. Nope, need receptor-mediated endocytosi then cross vesicle membrane after alteration of macromolecule, needs energy.

Transport proteins

UniporterA uniporter accepts one solute and whisks it across the membrane.
SymporterA symporter accepts two solutes (a system called cotransport) and whisks them both to the other side of the membrane.
AniporterAn aniporter accepts two solutes, which is cotransport, but unlike a symporter each solute is on opposite sides of the membrane, and swaps them.

The structure of the transport proteins all share a common structure. Composed of 12 alpha-helices which wind back and forth in the membrane to form a nice channel structure. This is conserved. Translocases transport proteins.

Transport mechanisms

Simple transportConcentration gradients occurs whether charged or not. Electrostatic gradients (potential fields) drive net movememnts of charged particles such as ions in a fluid. Movement of materials across membrane is determined by properties of lipid bilayer unless movement occurs through a channel or by means of a carrier molecule. Very small molecules with no net charge such as water, O2, and CO2, cross lipid bilayers easily regardless of polarity. No carriers or channels required. Moderate-sized nonpolars readily, but more trouble. Large nonpolar molecules like fatty acids or triglycerides may primarily move through blood or tissue fluid complexed with a lipoprotein and may be taken up by cells through a receptor-mediated endocytosis mechanism recognizing protein component of lipoprotein and takes up lipid along with protein.
Group translocationCompounds are chemically modified as they transport. An example is the phosphotransferase system (PTS), in which solutes passed across the membrane are phosphorylated (including sugars glucose, mannose and fructose). Proteins alternately phosphorylate and dephosphorylate the sugar until the membrane-spanning Enzyme IIc receives a phosphate and phosphorylates sugar in the actual transport event. PEP is the first protein to get phosphorylated, following by EnzI, Hpr, EnzIIa, EnzIIb and finally EnzIIc; EnzIIc phosphorylates the solute and transports it into the cytoplasm. This phosphorylation prepared glucose for immediate entry into a central metabolic pathway.
ABC systemThe substrate binds to the periplasmic-binding protein, which brings it to the membrane-spanning transporter, which brings it into the cell through the ATP-hydrolyzing protein. Substrate-binding protein, membrane transporter and ATP-hydrolizing protein. Periplasmic binding proteins and the ABC system. There are periplasmic-binding, membrane-spanning and ATP-hydrolyzing (kinases) proteins. Thse make up the ABC system.
Passive transport
  1. Concentration gradients drive a net force and each component in any fluid will diffuse from regions of higher concentrations toward regions of lower concentrations.

  2. Electrostatic gradients cause charged particles to be attracted to opposite charges and repelled by similar charges.

Simple diffusion