BIO 1140 - Cell Transport Across Plasma Membrane (Pt. 1)

Cell membranes contain specialized membrane transport proteins - Membrane transport proteins facilitate the passage of selected small water-soluble molecules - Liposomes are impermeable to most water-soluble molecules

The molecule’s size and solubility determine the diffusion rate - Simple diffusion

Na+, K+, Ca2+, Cl–, and H+ (protons) are among the most important inorganic ions for cells. - Na+ is most abundant outside the cell. - K+ is most abundant inside the cell.

Electrical charges inside and outside are generally balanced (Na+ by Cl- and K+ by organic molecules).

Small excesses of positive or negative charge on the two sides of the plasma membrane create a voltage difference

This voltage difference across the membrane is called the membrane potential - The activity of membrane transport proteins create and maintain the membrane potential

For a cell at rest, the exchange of anions and cations across the membrane is steady (resting membrane potential)

Membrane transport is mediated by two classes of membrane proteins • Transporters: transfer small organic molecules or inorganic ions • Channels: form tiny hydrophilic pores and allow substances to pass by diffusion • Most channels are ion channels

Molecules flow from a region of high concentration to a region of low concentration (downhill movement) - Downhill movement is passive and requires no energy(passive transport) - Passive transporters change conformation to mediate transport across the membrane

The uphill movement requires energy (active transport) - against a concentration / electrical gradient requires an input of energy - Only transporters can carry out active transport

Transmembrane proteins responsible for active transport are called active transporters or pumps

For an uncharged molecule movement direction is determined only by its concentration gradient

For a charged molecule, both concentration gradient and the membrane potential exert forces

The net driving force moving a charged solute is called electrochemical gradient - The electrochemical gradient is the sum of a force from the membrane potential and a force from the concentration gradient of the solute

Osmosis is the diffusion of water across membranes - Water molecules diffuse rapidly through aquaporin channels in the plasma membrane of some cells

Aquaporin channels responsible for water reabsorption in our kidneys - A nephron tissue

Glucose Transporter

• Uniporter: transports one substance in one direction • Symporter: transports two different substances in the same direction • Antiporter: transports two different substances in opposite directions

• ATP (used by primary active transporters) • Electrochemical gradient (used by secondary active transporters) • Light

ATP-driven Ca2+ transporters (pumps) keep the cytosolic Ca2+ concentration low - Ca2+ binds to a variety of proteins in the cell and alters their activities - An influx of Ca2+ into the cell serves as an intracellular signal - It triggers cell processes, such as muscle contraction, fertilization and nerve cell communication

ATP-driven Na+ transporters (pumps) uses energy supplied by ATP to expel Na+ and bring in K+ - The ATP-driven Na+ pump plays a pivotal role in the energy economy of animal cells - This pump accounts for 30% or more of their total ATP consumption - This pump is also called Na+-K+ ATPase or the Na+-K+ pump

ATP-driven Na+ transporter generates a steep concentration gradient of Na+ across the plasma membrane - With the Na+ pump, Na+ concentration remains about 10–30 times lower and K+ concentration about 10–30 times higher inside the cell - High Na+ concentration outside the cell represents a huge store of energy

Coupled transporters (pumps) are secondary active transporters - Coupled transporters mediate coupled transport using electrochemical gradients. - Symport & Antiport

Glucose–Na+ transporters use the electrochemical Na+ gradient to drive the active import of glucose