1. Overview
Diffusion is a fundamental physical process by which substances move into and out of cells. It is a passive process, meaning it requires no energy input from the cell’s metabolism, and it is essential for vital functions such as gas exchange and the absorption of nutrients.
Key Definitions
- Diffusion: The net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement.
- Concentration Gradient: The difference in the concentration of a substance between two areas. The steeper the gradient (the larger the difference), the faster the rate of diffusion.
- Net Movement: The overall direction of movement of a substance (even if individual particles move in all directions, the majority move from high to low concentration).
- Kinetic Energy: The energy possessed by particles due to their motion; this provides the "power" for diffusion.
Core Content
The Process of Diffusion
Diffusion occurs because all molecules and ions are in constant, random motion.
- Particles possess kinetic energy, causing them to bounce off each other and spread out.
- While individual particles move randomly in all directions, the net movement will always be from where there are many particles to where there are fewer.
- Eventually, particles reach equilibrium, where they are spread evenly. (Note: Particles still move at equilibrium, but there is no further net change).
Two boxes separated by a dashed line. The left side is crowded with circles (High Concentration); the right side has very few circles (Low Concentration). An arrow points from left to right labeled "Direction of Diffusion".
Diffusion in Living Organisms
Living cells are surrounded by a partially permeable cell membrane. Small molecules can diffuse directly through this membrane to enter or leave the cell.
Importance of Diffusion:
- Gas Exchange: Oxygen diffuses from the lungs into the blood, and Carbon Dioxide diffuses from the blood into the lungs. In plants, CO2 diffuses into leaves for photosynthesis.
- Absorption of Solutes: Dissolved food molecules (like glucose) diffuse from the small intestine into the bloodstream.
- Excretion: Waste products like urea diffuse out of cells into the blood for transport to the kidneys.
Factors Influencing the Rate of Diffusion
- Surface Area: A larger surface area (e.g., folded membranes like root hair cells or villi) provides more space for particles to pass through, increasing the rate.
- Temperature: Higher temperatures increase the kinetic energy of particles, making them move faster and thus diffuse faster.
- Concentration Gradient: A greater difference in concentration between two areas increases the "push" toward the lower concentration area.
- Distance: The shorter the distance particles have to travel (e.g., thin capillary walls), the faster diffusion occurs.
Extended Content (Extended Only)
There are no specific supplement objectives listed for topic 3.1 in the current syllabus requirements provided.
Key Equations
While there are no complex mathematical formulas for diffusion at this level, you must understand the Surface Area to Volume (SA:V) Ratio:
$$\text{Surface Area to Volume Ratio} = \frac{\text{Total Surface Area}}{\text{Total Volume}}$$
- Large organisms: Have a small SA:V ratio and need specialized exchange surfaces.
- Small organisms (e.g., bacteria): Have a large SA:V ratio, allowing diffusion to happen quickly enough to meet all their needs.
Common Mistakes to Avoid
- ❌ Wrong: Particles stop moving once they are evenly spread out.
- ✓ Right: Particles continue to move randomly, but there is no net movement in any one direction.
- ❌ Wrong: Diffusion requires energy from respiration (ATP).
- ✓ Right: Diffusion is a passive process; the energy comes from the intrinsic kinetic energy of the molecules.
- ❌ Wrong: Molecules move from low to high concentration.
- ✓ Right: Molecules move down the gradient, from high to low concentration.
Exam Tips
- Command Word - "Describe": If asked to describe diffusion, always mention "net movement," "high to low concentration," and "random movement."
- Command Word - "Explain": If asked to explain why a rate increased, link it to the factors (e.g., "The rate increased because higher temperature gave particles more kinetic energy").
- Contexts: Expect questions about the Alveoli (lungs) or Villi (intestines). In these cases, always mention that they are "one cell thick" to explain the factor of "short diffusion distance."
- Typical Values: Be prepared to compare rates. For example, if Surface Area doubles, the rate of diffusion generally increases significantly.