what is concentration gradient in active transport, osmosis, phagocytosis and pinocytosis?

Question

what is concentration gradient?

Answer 1

Let's look at osmosis first: it is the diffusion of water across a semi-permeable membrane and does not require energy, which is why it is a form of passive transport. Phagocytosis and pinocytosis are both forms of active transport (requiring energy) and are both used to transport material "in bulk" across/against a concentration gradient. An example of phagocytosis would be a white blood cell engulfing a damaged cell to destroy it. An example of pinocytosis would be a cell membrane forming little "pockets" to engulf amounts of water.

A concentration gradient means there is more "stuff" (solute) dissolved in a solution - in Biology this is usually referring to water inside/outside a cell. An example would be a human blood cell in seawater: there is more solute outside the cell than there is inside. Back to osmosis, this would cause the water to diffuse across the cell to make the concentration reach equilibrium (inside and outside would be the same). As a result, the cell would shrivel.