Diffusion
Diffusion is the fundamental process by which substances move into and out of cells. It is a passive process, meaning it does not require energy from the cell's respiration.
To understand diffusion, you must grasp three key ideas:
- Random Movement: All molecules and ions are constantly moving randomly due to their kinetic energy.
- Concentration Gradient: This is the difference in concentration of a substance between two regions. A 'steep' gradient means there is a large difference (high concentration on one side, low on the other).
- Net Movement: While particles move randomly in all directions, more particles will naturally move from the area where they are crowded (high concentration) to the area where they are sparse (low concentration). This overall shift is called 'net movement'.
Note: The term 'net' is crucial. It acknowledges that while individual particles move randomly in all directions, the overall trend is from high to low concentration.
- Respiration releases chemical energy (ATP) used for active processes like active transport or muscle contraction.
- Diffusion requires no metabolic energy from the cell.
The energy for diffusion comes entirely from the kinetic energy of random movement of molecules and ions. This kinetic energy is inherent to the particles themselves; as long as the temperature is above absolute zero, the particles possess this energy.
| Factor | Effect on Rate of Diffusion |
|---|---|
| Surface Area | Increasing surface area increases the rate. A larger surface area allows more particles to cross the membrane simultaneously. |
| Temperature | Increasing temperature increases the rate. Higher temperatures give particles more kinetic energy, causing them to move faster and collide more frequently. |
| Concentration Gradient | Increasing the concentration gradient (making it steeper) increases the rate. A larger difference in concentration between the two regions drives a faster net movement. |
| Distance | Decreasing the distance decreases the time taken, effectively increasing the rate. Particles have less distance to travel to reach equilibrium. |
Diffusion is essential for the survival of cells because it allows the exchange of necessary substances without using energy.
1. Gases:
- Oxygen (O_2): Diffuses from areas of high concentration (e.g., alveoli in lungs or water around gills) into cells where it is used for respiration.
- Carbon Dioxide (CO_2): Diffuses out of cells (where it is produced by respiration) into the blood or surrounding medium to be removed.
2. Solutes:
- Glucose and Amino Acids: After digestion, these nutrients diffuse from the small intestine into the bloodstream.
- Mineral Ions: Roots absorb mineral ions from the soil solution by diffusion (when soil concentration is higher than root hair cell concentration).
A common practical investigation involves placing agar cubes (containing an indicator that changes color when acid enters) into hydrochloric acid solutions of different concentrations or temperatures.
Experimental Setup:
- Independent Variable: The factor you change (e.g., concentration of HCl, temperature, or size of the cube).
- Dependent Variable: The factor you measure (e.g., time taken for the cube to turn completely colorless).
- Control Variables: Factors kept constant to ensure a fair test (e.g., volume of acid, size of agar cubes if testing concentration).
If you increase the concentration of the acid, the time taken for the cube to turn colorless decreases. This means the rate of diffusion increases.
Why? Because a higher concentration creates a steeper concentration gradient between the outside of the cube and the inside.
The Correct Understanding: Diffusion is a passive process. The energy comes from the kinetic energy of the particles themselves, not from cellular respiration. Respiration provides ATP for active transport, which moves substances against the gradient and requires energy input.
The Correct Understanding: You must specify net movement from high to low concentration. While individual particles move randomly in all directions, the overall (net) flow is down the gradient.
When to use: When asked to describe how a factor (like temperature or surface area) affects diffusion.
Why examiners accept this: Examiners look for a clear link between the change in the factor and the physical reason for the change in rate. Simply stating 'it increases' is often insufficient; you must explain the mechanism.
Correct Usage Example:
- Question: Describe the effect of increasing temperature on the rate of diffusion.
- Answer: Increasing the temperature increases the kinetic energy of the particles. This causes them to move faster and collide more frequently, leading to a faster net movement down the concentration gradient.
When to use: When asked to define diffusion.
Why examiners accept this: The definition has specific keywords that are mandatory for full marks. Missing any of these key components will result in lost marks.
Correct Usage Example:
- Question: Define diffusion.
- Answer: Diffusion is 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.
- Net movement of particles (from high to low concentration). [1 mark]
- Down a concentration gradient / as a result of random movement. [1 mark]
- Increases the rate of diffusion. [1 mark]
- More particles can cross the membrane simultaneously / provides more space for exchange. [1 mark]
- The concentration of oxygen is higher in the blood/capillary than in the muscle cell. [1 mark]
- Therefore, there is a concentration gradient / net movement occurs down the gradient into the cell. [1 mark]