Diffusion is the net movement of particles from a higher concentration to a lower concentration. It needs no energy from the cell. Oxygen diffuses into cells and carbon dioxide diffuses out. Diffusion is faster with a steeper concentration gradient, a higher temperature and a larger surface area.
Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Active transport moves substances against a concentration gradient, from low to high, using energy from respiration; root hair cells take in mineral ions this way. Large organisms have a small surface area to volume ratio, so they need exchange surfaces like lungs and villi, and transport systems.
Worked example
A potato piece in salty water loses mass. Why?
- The salt solution is more concentrated than the potato cells.
- Water moves out of the cells by osmosis.
Answer:The potato loses water, so its mass falls.
Key words
- diffusion
- net movement from high to low concentration
- osmosis
- water diffusing across a partially permeable membrane
- active transport
- movement against a gradient using energy
- surface area to volume ratio
- surface ÷ volume; smaller in big organisms
New questions every time.
NGSS-referenced: HS-LS1-2