Transport Across Cell Membranes



The cell membrane, a lipid bilayer, uses a variety of transmembrane proteins to transport ions, water, and molecules either into or out of the cell. Some proteins, such as channel proteins and uniporters, carry out passive transport, facilitating diffusion down the electrochemical gradient. Pumps and cotransporters carry out active transport, using energy to move a substrate against its concentration gradient. The energy may be derived either from hydrolysis of adenosine triphosphate (ATP) or from the gradient of a cotransported substrate. Most transport proteins are highly selective for their substrate molecules. Transport proteins play important roles in the cell, both in cell maintenance and in responding to stimuli.

At A Glance

  • Channel proteins form hydrophilic pathways through which water-soluble molecules can move by passive transport down their concentration gradient.
  • Ion channels can selectively open and close to control the flow of ions.
  • Transmembrane pumps use ATP to drive active transport of substances against their concentration gradient.
  • Cell membrane transporters are highly selective and use concentration gradients to carry out either passive transport or active cotransport.
  • The sodium-potassium pump maintains a steep concentration gradient of Na+ across the cell membrane, with a greater concentration outside of the cell.
  • Calcium pumps are used in cell signaling and trigger muscle contraction in animal cells.
  • Proton pumps generate electrochemical proton gradients, which can be used to control pH levels or synthesize ATP.
  • Potassium leak channels are important in maintaining membrane potential.
  • Animal cells contain Na+/K+ pumps and Ca2+ pumps, while plant, fungal, and bacterial cells contain H+ pumps; all utilize transporters.
  • Voltage-gated Na+ and K+ ion channels enable a nerve impulse to travel from one end of a neuron to the other.