TRANSPORT ACROSS Plasma MEMBRANES ! Biological Sciencess !!

TRANSPORT ACROSS MEMBRANES

1. Movement of Small Molecules Across Membranes can involve simple diffusion or protein-mediated transport. Cell membranes are selectively permeable.

(a) Passive Diffusion. Lipophilic solutes cross the membrane freely by dissolving
in the lipid bilayer. This is passive diffusion. Examples: ethanol (alcohol,contains both polar and non-polar regions); also fatty acids, glycerol, steroids,etc. Also nonpolar gases like O=O (O2)

(b) Selective transport by protein carriers = “permeases”. Polar or ionic small solutes may be transported across membranes if specific protein carriers are in the membrane. Examples: sugars, amino acids, ions.

(c) Many substances cannot cross the membrane. Examples: large molecules
such as proteins, nucleic acids. Also small polar molecules or ions for which
there is no protein carrier.

2. Some protein transporters require energy; others do not. There are 2 possible situations:

(a) facilitated diffusion. Membrane has specific protein carrier, will bind to
molecule and bring it across cell membrane. No energy required. No preferential direction. If molecule is more concentrated outside than inside cell, net movement will be out of cell. Examples include-
• Glucose transporters- 5 different GLUT proteins and 2 types that cotransport
Na and glucose (these are used for secondary active transport)
• Water channels- 8 different types of aquaporins

(b) Active transport. Membrane has specific protein carrier, also a requirement
for energy (ATP or other form of energy). Will move solute against a concentration
gradient, so can concentrate material even if diffusion would favor opposite direction of flow.

Example: Na+- K+ ATPase in nerve cells. Pumps Na+ to outside, K+ in, maintains electrical potential against diffusion. When nerve cell “fires”,momentary gates open to let diffusion occur. Then pumps are turned back on to restore potential.

3. Active transport can involve ATP pumps, symport, or antiport

(a) ATP pumps.

• ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP
with the movement of ions across a membrane against a concentration
gradient.
• ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy
released is used to move one or more ions across the cell membrane.
• As much as 25% of a cell’s ATP reserves may be spent in such ion transport.

• Examples include:
1. The Na+-K+ ATPase pumps Na+ out of the cell while it pumps K+ in Because the pump moves three Na+ to the outside for every two K+ that are moved to the inside, it creates an overall charge separation known as polarization. This electrical potential is required for nervous system activity, and supplies energy needed for other types of transport such as symport and antiport.

2. Ca++ ATPases are responsible for keeping intracellular Ca++ at low levels, a necessary precondition for muscle contraction.

(b) Symport

• To transport some substances against a concentration gradient, cells use energy already stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to power membrane proteins called transporters.
• When the transported molecule and the co-transported ion move in the same direction, the process is known as symport.

• Example: transport of amino acids across the intestinal lining in the human gut.

(c) Antiport

Cell uses movement of an ion across a membrane and down its concentration
gradient to power the transport of a second substance “uphill” against its gradient.
• In this process, the two substances move across the membrane in opposite
directions.
• Example: transport of Ca2+ ions out of cardiac muscle cells. Muscle cells are triggered to contract by a rise in intracellular Ca2+ concentration, so it
is imperative that Ca2+ be removed from the cytoplasm so that the muscle can relax before contracting again. This antiport system is so effective that it can maintain the cellular concentration of Ca2+ at levels 10,000 times lower than the external concentration.

4. Cells Regulate Permeability by Adding & Removing Membrane Transport Proteins

(a) Insulin is transported by transporter GLUT4. Insulin causes glucose transport
molecules to be inserted into muscle and adipose tissue cells. Glucose is then
taken up into those tissues, lowering the blood concentration.

(b) Antidiuretic Hormone (ADH) is transported via aquaporin. ADH causes aquaporin 2 proteins to be added to the kidney collecting duct membranes. The result is water conservation.

(c) Aldosterone is transported by Na Pump. Aldosterone causes cells in the distal
tubules and collecting duct of the kidney to make more Na pump molecules. The final result is that the body retains more Na and secretes more K into the urine.

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