Diffusion Osmosis and Active Transport
This topic covers the essential mechanisms by which substances cross biological membranes, a fundamental process for cell survival, communication, and function. Understanding these passive (no energy) and active (energy required) transport methods is key to solving problems in physiology and cell biology.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Diffusion and osmosis are passive processes, meaning they do not require metabolic energy (ATP) as they are driven by concentration or water potential gradients.
- Active transport moves substances against their concentration gradient, from a low to a high concentration, which requires energy from cellular respiration (ATP).
- Osmosis is a specific type of diffusion for water molecules, moving across a partially permeable membrane from a region of higher water potential to one of lower water potential.
- A high solute concentration corresponds to a low (more negative) water potential. Pure water has the highest possible water potential (0 kPa).
- The rate of diffusion is increased by a steeper concentration gradient, a larger surface area, a shorter diffusion distance, and higher temperature.
- Examples in living systems include gas exchange in alveoli (diffusion) and glucose uptake in the small intestine (active transport). Non-living examples include a tea bag in hot water (diffusion).
Formulae
Rate of diffusion ∝ (Surface Area × Concentration Gradient) / Diffusion Distance Use this proportionality to reason about how changing factors will affect the rate of diffusion. For example, doubling the surface area will double the initial rate, assuming other factors are constant.
Definitions
- Diffusion
- The net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
- Osmosis
- The net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
- Active Transport
- The movement of molecules or ions across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy (ATP).
- Water Potential (Ψ)
- A measure of the tendency of water to move from one area to another. It is lowered by the presence of solutes. Water always moves down a water potential gradient.
Worked example
A piece of Visking tubing, which is permeable to water and monosaccharides but not to disaccharides, is filled with a solution containing 0.4 M sucrose and 0.2 M fructose. This bag is then placed into a beaker of 0.3 M fructose solution. What is the initial net movement of fructose and water?
- 1
Step 1:
Analyse fructose movement.
The concentration of fructose inside the bag is 0.2 M and outside is 0.3 M.
Fructose will move via diffusion down its concentration gradient.
Therefore, there is a net movement of fructose INTO the bag.
- 2
Step 2:
Analyse water movement (osmosis).
Water movement is determined by the total water potential, which is influenced by the total solute concentration.
The membrane is impermeable to sucrose.
- 3
Step 3:
Calculate the effective solute concentration gradient for water movement.
Inside the bag, the total solute concentration is 0.4 M sucrose + 0.2 M fructose = 0.6 M.
Outside the bag, the solute concentration is 0.3 M fructose.
The total solute concentration is higher inside the bag.
- 4
Step 4:
Determine the water potential gradient.
A higher total solute concentration means a lower (more negative) water potential.
Therefore, the water potential is lower inside the bag than outside.
- 5
Step 5:
Conclude the direction of water movement.
Water moves by osmosis from a region of higher water potential to lower water potential.
Therefore, there is a net movement of water INTO the bag.
Answer: Net movement of fructose is into the bag. Net movement of water is into the bag.
Common mistakes
- ×Mixing up water potential and solute concentration. Remember: high solute concentration = low water potential. Water moves from high potential (dilute solution) to low potential (concentrated solution).
- ×Forgetting membrane permeability is a constraint. A large concentration gradient is irrelevant if the membrane is impermeable to that specific substance. Always check what can and cannot pass through.
- ×Confusing the responses of plant and animal cells. Animal cells burst (lyse) in pure water, while plant cells become rigid (turgid) due to their strong cell wall. Both shrink in concentrated solutions.
- ×Ignoring non-permeable solutes when determining osmosis. The overall direction of water movement depends on the difference in total solute concentration on both sides of the membrane, but is most strongly driven by solutes that cannot cross it.
No-calculator tips
- ✓Focus on the direction of net movement, not the exact rate. Determine which side has the higher concentration or higher water potential to predict the overall flow.
- ✓Use proportional reasoning. If a concentration gradient is halved, the initial rate of diffusion will also be halved. Look for simple multiplicative factors.
- ✓In osmosis problems, quickly sum the concentrations of all solutes on each side of the membrane to find which side has the lower water potential overall. This tells you where the water will go.