Passive Diffusion

Definition

Passive diffusion is a type of transport mechanism in which molecules or ions move across a cell membrane from an area of higher concentration to an area of lower concentration, without the expenditure of cellular energy. This process is driven solely by the concentration gradient, as the molecules or ions move down their concentration gradient.

5 Must Know Facts For Your Next Test

1. Passive diffusion is a key mechanism for the absorption of nutrients, such as glucose and amino acids, during chemical digestion in the small intestine.
2. In the kidneys, passive diffusion plays a crucial role in the reabsorption of water, glucose, and other essential molecules from the filtrate in the renal tubules.
3. The rate of passive diffusion is directly proportional to the concentration gradient of the substance and the permeability of the cell membrane to that substance.
4. Passive diffusion is a spontaneous process that does not require the input of cellular energy, such as ATP, to drive the movement of molecules or ions.
5. The size, charge, and lipid solubility of the molecules or ions are important factors that determine the rate and efficiency of passive diffusion across the cell membrane.

Review Questions

• Explain how passive diffusion is involved in the chemical digestion and absorption of nutrients in the small intestine.
  • During chemical digestion in the small intestine, the breakdown of complex food molecules into simpler, absorbable nutrients creates a concentration gradient. This concentration gradient drives the passive diffusion of these nutrients, such as glucose and amino acids, across the intestinal epithelial cells and into the bloodstream. Passive diffusion allows these nutrients to move from the area of higher concentration in the intestinal lumen to the area of lower concentration in the blood, without the need for cellular energy expenditure.
• Describe the role of passive diffusion in the process of tubular reabsorption in the kidneys.
  • In the kidneys, the filtrate that is formed in the glomeruli of the nephrons contains a variety of essential molecules, including water, glucose, and other nutrients. During the process of tubular reabsorption, these molecules are selectively reabsorbed back into the bloodstream. Passive diffusion plays a crucial role in this process, as it allows the movement of water, glucose, and other small, uncharged molecules down their concentration gradients from the filtrate in the renal tubules to the surrounding capillaries. This passive diffusion process helps to maintain the appropriate concentrations of these essential substances in the body.
• Analyze how the physicochemical properties of molecules, such as size, charge, and lipid solubility, can influence the rate and efficiency of passive diffusion across cell membranes.
  • The physicochemical properties of molecules can significantly impact their ability to undergo passive diffusion across cell membranes. Smaller, uncharged molecules with high lipid solubility, such as oxygen and carbon dioxide, can readily diffuse through the lipid bilayer of the cell membrane, as they can easily dissolve and pass through the hydrophobic core. In contrast, larger, charged molecules, or those with low lipid solubility, may face greater resistance to passive diffusion and require the assistance of transport proteins, such as in the case of facilitated diffusion. The rate of passive diffusion is directly proportional to the concentration gradient of the substance and the permeability of the cell membrane to that substance, which is influenced by the physicochemical properties of the diffusing molecules.

Related terms

Concentration Gradient: The difference in the concentration of a substance between two areas, which drives the movement of that substance from the area of higher concentration to the area of lower concentration.

Osmosis: The movement of water molecules across a semi-permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

Facilitated Diffusion: A type of passive transport that involves the use of transport proteins to facilitate the movement of molecules or ions across the cell membrane down their concentration gradient.