passive diffusion

Examples of passive 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.
  • Water passes through semi-permeable membranes by passive diffusion, moving along a concentration gradient and equalizing the concentration on either side of the membrane.
  • Electrolyte ions may not be able to passively diffuse across a membrane, but may instead require special mechanisms to cross the semi-permeable membrane.
  • All movement can be classified as passive or active.
  • Passive transport, such as diffusion, requires no energy as particles move along their gradient.

• The Role of Passive Transport

  • Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.
  • The most direct forms of membrane transport are passive.
  • In passive transport, substances move from an area of higher concentration to an area of lower concentration .
  • The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes.
  • Diffusion is a type of passive transport.

• Dalton's Law of Partial Pressure

  • In the lungs, the relative concentration of gasses determines the rate at which each gas will diffuse across the alveolar membranes.
  • These pressure differences explain why oxygen flows into the alveoli and why carbon dioxide flows out of the alveoli through passive diffusion (just as a similar process explains alveolar and arterial gas exchange).
  • While inhaled air is similar to atmospheric air due to Dalton's law, exhaled air will have relative concentrations that are in between atmospheric and alveolar air due to the passive diffusion of gasses during gas exchange.

• 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.

• Diffusion

  • Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration.
  • 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.

• The Respiratory System and Direct Diffusion

  • As animal size increases, diffusion distances increase and the ratio of surface area to volume drops.
  • Diffusion is a slow, passive transport process.
  • Their cells are kept moist so that gases diffuse quickly via direct diffusion.
  • Flatworms are small, literally flat worms, which ‘breathe' through diffusion across the outer membrane .
  • This flatworm's process of respiration works by diffusion across the outer membrane.

• Blood Flow in the Lungs

  • Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction.
  • The air, along with the diffused carbon dioxide, is then exhaled.

• Facilitated transport

  • Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.
  • Facilitated transport is a type of passive transport.
  • 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.
  • A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy.
  • Others are carrier proteins which bind with the substance and aid its diffusion through the membrane.

• 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.
  • Re-absorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream

• The Plasma Membrane and the Cytoplasm

  • Passive (non-energy requiring) transport is the movement of substances across the membrane without the expenditure of cellular energy.
  • 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.
  • In contrast to passive transport, active (energy-requiring) transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).