Passive Movement Across Membranes
Biology: IB Diploma Higher Level, topic 6.1.1
Passive Movement
Passive movement is the net movement of particles from an area of high concentration to an area of lower concentration without the direct input of energy.
Diffusion
- Diffusion describes the passive movement of particles in fluids (liquids and gases).
- Molecules move randomly but tend to move into any space available until it is evenly distributed.
- So, the net movement of particles will be towards the area of lower concentration or down a concentration gradient.
- Substances such as CO2, O2 and urea can move in and out of cells across cell membranes via diffusion.
E.g. Perfume
- When you spray perfume, the smell diffuses across the room from the area of high concentration (where you just sprayed it) to the area of low concentration (the other side of the room).
Filtration
- A variation of diffusion is the process of filtration.
- In filtration, material moves according to its concentration gradient through a membrane. Sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly.
- This occurs in the kidney, where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules.
- The rate of diffusion in this instance is almost totally dependent on pressure.
Test yourself on Passive Movement Across Membranes
What do we call the net movement of particles from an area of high concentration to an area of lower concentration?
In which state of substance does diffusion NOT happen?
Which of the following substances can diffuse across cell membranes?