facilitated diffusion

(noun)

The spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins.

Related Terms

  • membrane protein
  • active transport
  • passive diffusion

Examples of facilitated diffusion in the following topics:

  • Transport of Electrolytes across Cell Membranes

    • Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
    • Electrolyte ions may not be able to passively diffuse across a membrane, but may instead require special mechanisms to cross the semi-permeable membrane.
    • The mechanisms that transport ions across membranes are facilitated diffusion and active transport.
    • Facilitated diffusion of solutes occurs through protein-based channels.
    • Passive transport, such as diffusion, requires no energy as particles move along their gradient.
  • Facilitated transport

    • Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
    • Channel-mediated facilitated diffusion functions much like a bridge over a river that must raise and lower in order to allow boats to pass.
    • Unlike simple diffusion where materials pass through a membrane without the help of proteins, in facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins.
    • Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.
    • Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second.
  • Absorption in the Small Intestine

    • Absorption of nutrients occurs partially by diffusion through the wall of the small intestine.
    • Digested nutrients pass into the blood vessels in the wall of the intestine through a process of diffusion.
    • Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: iron is absorbed in the duodenum; vitamin B12 and bile salts are absorbed in the terminal ileum; water and lipids are absorbed by passive diffusion throughout the small intestine; sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport; and fructose is absorbed by facilitated diffusion.
  • The Role of Passive Transport

    • This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport.
    • In solutions containing more than one substance, each type of molecule diffuses according to its own concentration gradient, independent of the diffusion of other substances.
    • Many factors can affect the rate of diffusion, including, but not limited to, concentration gradient, size of the particles that are diffusing, and temperature of the system.
    • In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport.
    • Diffusion is a type of passive transport.
  • The Plasma Membrane and the Cytoplasm

    • During this type of transport, materials move by simple diffusion or by facilitated diffusion through the membrane, down their concentration gradient.
    • Water passes through the membrane in a diffusion process called osmosis.
    • Osmosis is the diffusion of water through a semi-permeable membrane down its concentration gradient.
    • Water molecules will diffuse out of the hypotonic solution and into the hypertonic solution (unless acted upon by hydrostatic forces).
    • Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient.
  • Bulk Flow: Filtration and Reabsorption

    • Capillary fluid movement occurs as a result of diffusion (colloid osmotic pressure), transcytosis, and filtration.
    • Bulk flow is one of three mechanisms that facilitate capillary exchange, along with diffusion and transcytosis.
  • Osmosis

    • The semipermeable membrane limits the diffusion of solutes in the water.
    • Not surprisingly, the aquaporin proteins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.
    • Osmosis is a special case of diffusion.
    • However, only the material capable of passing through the membrane will diffuse through it.
    • In this example, the solute cannot diffuse through the membrane, but the water can.
  • Skin, Gills, and Tracheal Systems

    • A dense network of capillaries lies just below the skin, facilitating gas exchange between the external environment and the circulatory system.
    • The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.
    • In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid.
    • As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration) .
    • Similarly, carbon dioxide molecules diffuse from the blood (high concentration) to water (low concentration).
  • Diffusion

    • Diffusion is the movement of particles from regions of high concentration towards regions of lower concentration.
    • Molecular diffusion, often called simply diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero.
    • However, diffusion can still occur in the absence of a concentration gradient.
    • The result of diffusion is a gradual mixing of material.
    • Discuss the process and results of diffusion, identifying factors that affect its rate
  • Diffusion

    • Diffusion is a passive process of transport.
    • Materials move within the cell's cytosol by diffusion, and certain materials move through the plasma membrane by diffusion.
    • Diffusion expends no energy.
    • Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly.
    • If the medium is less dense, diffusion increases.
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