Passive diffusion

Definition

Passive diffusion is the process by which molecules move across a biological membrane from an area of higher concentration to an area of lower concentration without the use of energy. This movement occurs spontaneously due to the inherent kinetic energy of molecules, and it plays a crucial role in cellular processes, allowing for the exchange of substances essential for life.

5 Must Know Facts For Your Next Test

1. Passive diffusion does not require ATP or any other form of energy; it relies solely on the natural motion of molecules.
2. Small nonpolar molecules, such as oxygen and carbon dioxide, easily diffuse through cell membranes, while larger or polar molecules often require other methods of transport.
3. The rate of passive diffusion is influenced by factors like temperature, size of the molecule, and the permeability of the membrane.
4. Equilibrium is reached when the concentration of molecules is equal on both sides of the membrane, at which point net movement stops but individual molecules continue to move.
5. In living organisms, passive diffusion is essential for processes like gas exchange in the lungs and nutrient uptake in cells.

Review Questions

• How does passive diffusion differ from active transport in terms of energy requirements and directionality of movement?
  • Passive diffusion differs from active transport in that it does not require energy to move substances across a membrane. While passive diffusion moves molecules from areas of high concentration to low concentration, active transport moves substances against their concentration gradient, requiring ATP or another energy source. This fundamental difference influences how cells regulate their internal environment and manage the uptake and release of various substances.
• What factors influence the rate of passive diffusion across a selectively permeable membrane, and how do these factors interrelate?
  • The rate of passive diffusion is influenced by factors such as concentration gradient, temperature, molecular size, and membrane permeability. A steeper concentration gradient increases the rate as there is a greater difference driving the movement. Higher temperatures can enhance molecular movement, further accelerating diffusion. Additionally, smaller molecules tend to diffuse more quickly than larger ones. All these factors work together to determine how efficiently substances can cross cell membranes through passive diffusion.
• Evaluate the significance of passive diffusion in biological systems and its impact on cellular homeostasis.
  • Passive diffusion is crucial for maintaining cellular homeostasis by allowing essential substances like oxygen and nutrients to enter cells while facilitating the removal of waste products. This process ensures that cells can sustain life functions efficiently without expending energy for transport. Disruptions in passive diffusion can lead to imbalances that affect cell function and overall organism health. Understanding this process highlights its role in physiological processes such as respiration and nutrient absorption, showcasing how life depends on these fundamental mechanisms.

Related terms

Concentration gradient:

The difference in concentration of a substance between two areas, which drives the movement of molecules during passive diffusion.

Selectively permeable membrane: A type of biological membrane that allows certain substances to pass while blocking others, facilitating passive diffusion for specific molecules.

Osmosis: The specific case of passive diffusion involving the movement of water molecules across a selectively permeable membrane.