Diffusion

Diffusion — Movement Along a Gradient, for Free

Diffusion is the simplest way a substance moves inside a plant: molecules in constant, random motion spread out from a region where they are concentrated to a region where they are scarce, until the concentration is roughly equal everywhere. No metabolic energy is spent to make this happen — it is a purely physical process driven by the molecules' own kinetic energy.

Key features to remember:

• Passive — needs no ATP.
• Non-living process — it works just as well in a dead system as a living one, and happens in gases, liquids, and (more slowly) even solids.
• Slow over distance — fine for movement across a cell or a short air gap, hopeless for moving material from root to shoot.
• The only way gases move inside a plant — oxygen, carbon dioxide, and water vapour all diffuse through internal air spaces and out through stomatal pores. …