Substances cross cell membranes either by passive transport or by active transport. The difference is where the energy comes from and the direction of movement.Passive transport does not use energy from ATP. The energy comes from the random kinetic energy of the particles themselves. Substances move down a concentration gradient, from where they are more concentrated to where they are less concentrated, until equilibrium is reached. There are three types:
• diffusion: particles pass through the phospholipid bilayer;
• facilitated diffusion: particles pass through channel or carrier proteins;
• osmosis: the movement of water across a partially permeable membrane.Active transport uses energy from ATP, made by respiration, to move substances against a concentration gradient, from a lower concentration to a higher one. It uses specific carrier proteins that act as pumps. For example, root hair cells take up nitrate ions from the soil, where their concentration is lower than in the cell.
Bulk transport, endocytosis and exocytosis, also needs ATP, because the membrane must be moved to form or fuse vesicles.A useful test: if a substance keeps entering a cell when its concentration inside is already higher, or if uptake stops when respiration is blocked by an inhibitor such as cyanide, the process must be active.Summary: passive = no ATP, down the gradient, stops at equilibrium; active = ATP needed, against the gradient, carrier proteins, can build up high concentrations.
• diffusion: particles pass through the phospholipid bilayer;
• facilitated diffusion: particles pass through channel or carrier proteins;
• osmosis: the movement of water across a partially permeable membrane.Active transport uses energy from ATP, made by respiration, to move substances against a concentration gradient, from a lower concentration to a higher one. It uses specific carrier proteins that act as pumps. For example, root hair cells take up nitrate ions from the soil, where their concentration is lower than in the cell.
Bulk transport, endocytosis and exocytosis, also needs ATP, because the membrane must be moved to form or fuse vesicles.A useful test: if a substance keeps entering a cell when its concentration inside is already higher, or if uptake stops when respiration is blocked by an inhibitor such as cyanide, the process must be active.Summary: passive = no ATP, down the gradient, stops at equilibrium; active = ATP needed, against the gradient, carrier proteins, can build up high concentrations.