Passive Mechanisms Move Some Small Molecules Across Membranes

Molecules can passively traverse the bilayer down electrochemical gradients by simple diffusion or by facilitated diffusion. This spontaneous movement toward equilibrium contrasts with active transport, which requires energy because it constitutes movement against an electrochemical gradient. Figure 41-8 provides a schematic representation of these mechanisms.

As described above, some solutes such as gases can enter the cell by diffusing down an electrochemical gradient across the membrane and do not require metabolic energy. The simple passive diffusion of a solute across the membrane is limited by the thermal agitation of that specific molecule, by the concentration gradient across the membrane, and by the solubility of that solute (the permeability coefficient, Figure 41-6) in the hydrophobic core of the membrane bilayer. Solubility is

Table 41-3. Transfer of material and information across membranes.

Cross-membrane movement of small molecules

Diffusion (passive and facilitated) Active transport Cross-membrane movement of large molecules

Endocytosis Exocytosis

Signal transmission across membranes

Cell surface receptors

1. Signal transduction (eg, glucagon ^ cAMP)

2. Signal internalization (coupled with endocytosis, eg, the LDL receptor)

Movement to intracellular receptors (steroid hormones; a form of diffusion) Intercellular contact and communication inversely proportionate to the number of hydrogen bonds that must be broken in order for a solute in the external aqueous phase to become incorporated in the hydrophobic bilayer. Electrolytes, poorly soluble in lipid, do not form hydrogen bonds with water, but they do acquire a shell of water from hydration by electrostatic interaction. The size of the shell is directly proportionate to the charge density of the electrolyte. Electrolytes with a large charge density have a larger shell of hydration and thus a slower diffusion rate. Na+, for example, has a higher charge density than K+. Hydrated Na+ is therefore larger than hydrated K+; hence, the latter tends to move more easily through the membrane.

The following factors affect net diffusion of a substance: (1) Its concentration gradient across the membrane. Solutes move from high to low concentration. (2) The electrical potential across the membrane. Solutes move toward the solution that has the opposite charge. The inside of the cell usually has a negative charge. (3) The permeability coefficient of the substance for the membrane. (4) The hydrostatic pressure gradient across the membrane. Increased pressure will increase the rate and force of the collision between the molecules and the membrane. (5) Temperature. Increased temperature will increase particle motion and thus increase the frequency of collisions between external particles and the membrane. In addition, a multitude of channels exist in membranes that route the entry of ions into cells.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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