Active transport moves substances from a more dilute solution to a more concentrated solution, which is against a concentration gradient. Diffusion and osmosis do not need energy, but active transport does. The energy comes from respiration.
Plants use active transport to absorb mineral ions into root hair cells from the very dilute solution in the soil. The concentration of ions is higher inside the root hair cell, so ions cannot diffuse in. Plants need these ions for healthy growth.
Active transport also absorbs sugar molecules from the gut into the blood. When there is a lower concentration of sugar in the gut than in the blood, diffusion cannot move it, so active transport does. The sugar is then used for cell respiration.
The three processes differ. Diffusion moves particles down a concentration gradient and needs no energy. Osmosis is the diffusion of water only, through a partially permeable membrane, and needs no energy. Active transport moves substances against a concentration gradient and needs energy from respiration.
