Diffusion and osmosis

Diffusion is the movement of particles, like molecules or ions, from an area where they are in high concentration to an area where they are in low concentration. This happens because particles are in constant, random motion. The difference in concentration between two areas is called a concentration gradient, and particles naturally move 'down' this gradient until they are evenly spread out. This process does not require any energy from the cell, so it is called a passive process. For example, in your lungs, oxygen is in high concentration in the air you breathe in and in low concentration in your blood. Therefore, oxygen diffuses from the lungs into the blood.

The speed at which diffusion happens is not always the same. Several factors can change the rate of diffusion. A steeper concentration gradient (a bigger difference in concentration) makes diffusion faster. Higher temperatures give particles more kinetic energy, so they move and diffuse faster. A larger surface area provides more space for diffusion to occur, increasing the rate. Finally, a shorter diffusion distance means particles have less far to travel, so the rate is faster. Organisms have evolved features like the thin walls of alveoli in the lungs to maximize the rate of diffusion.

Osmosis is a special type of diffusion specifically involving water molecules. It is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane. A partially permeable membrane has tiny pores that allow small molecules like water to pass through, but not larger solute molecules like sugar or salt. Think of a dilute solution as having a high concentration of 'free' water molecules, while a concentrated solution has fewer 'free' water molecules because many are clustered around the solute particles. This difference in 'free' water concentration creates a water potential gradient, driving the movement of water.

Plant cells have a cell wall, which dramatically affects how they respond to osmosis. When a plant cell is placed in a dilute solution or pure water (high water potential), water enters the cell by osmosis. The cell swells, but the strong cellulose cell wall prevents it from bursting. The cell contents push against the cell wall, making the cell firm and rigid. This state is called turgid, and the pressure exerted on the cell wall is turgor pressure. This is what keeps plants upright and leaves firm. If a plant cell is in a concentrated solution (low water potential), it loses water by osmosis. The vacuole and cytoplasm shrink, and the cell membrane pulls away from the cell wall. The cell becomes flaccid and then plasmolysed. This is why plants wilt if they don't get enough water.

Animal cells, such as red blood cells, do not have a cell wall. This makes them very sensitive to changes in the water potential of their surroundings. If an animal cell is placed in a dilute solution or pure water (high water potential), water will rapidly enter the cell by osmosis. Because there is no cell wall to withstand the pressure, the cell will swell and eventually burst. This is called lysis. If an animal cell is placed in a solution with the same water potential as its cytoplasm (an isotonic solution), there is no net movement of water, and the cell remains its normal shape. If an animal cell is placed in a concentrated solution (low water potential), it will lose water by osmosis and shrink. This process is called crenation. This is why it's vital that fluids given to patients intravenously (in a drip) are isotonic to their blood plasma.

Sometimes a cell needs to move substances from a low concentration to a high concentration, which is the opposite of diffusion. This process is called active transport. Because it moves substances 'against' their concentration gradient, it requires energy. This energy is supplied by respiration in the form of a molecule called ATP. Active transport is carried out by special 'carrier proteins' embedded in the cell membrane. These proteins bind to a specific substance on one side of the membrane, change shape using energy from ATP, and release the substance on the other side. Key examples include the uptake of mineral ions from the soil by plant root hair cells, and the absorption of glucose from the gut into the blood.