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The concentration gradient is the graduated change in particle concentration between two areas. Diffusion is the net movement of particles from high to low concentration along this gradient. Although particles move randomly in both directions, the net directional movement follows the gradient until equilibrium is reached, where particle density becomes uniform throughout.

In hypotonic solutions, water moves into cells causing bloating. Isotonic solutions have equal solute concentrations inside and outside, producing no shape changes. Hypertonic solutions have greater external solute concentration, causing water to leave cells and the cell to shrivel. These effects occur because water moves toward areas of greater solute concentration.

Cell membranes are semipermeable because they allow certain molecules to pass through while blocking others. This selective permeability enables cells to regulate and maintain different solute concentrations inside versus outside their membranes. The phospholipid bilayer and embedded proteins control which substances can cross the membrane.

Osmosis is the movement of water across a semipermeable membrane from areas of low solute concentration to high solute concentration. Unlike diffusion, which involves any particles moving down a concentration gradient, osmosis specifically refers to water movement. Both processes involve random particle motion but result in net directional movement.

Plant cell walls are rigid and only permeable to small molecules. When water enters plant cells, the membrane pushes against the wall, creating hydrostatic or turgor pressure that limits water entry rate and amount. Animal cells lack this wall, so they can swell more freely in hypotonic solutions without this pressure constraint.

Diffusion is a major limiting factor to cell size because the surface area-to-volume ratio decreases as cells grow larger. Larger cells have less membrane area relative to their volume, reducing the ability to transport sufficient nutrients throughout the cell. Unicellular organisms remain small to maximize diffusion efficiency, while multicellular organisms use many small cells to increase total surface area.

Human lungs contain many small alveoli that increase surface area for efficient gas diffusion between air and bloodstream. This extra surface area accelerates oxygen and carbon dioxide exchange. Many physiological processes, including breathing and digestion, depend on diffusion across membranes to transport essential materials throughout the body. Understanding these principles connects to the physiology of the circulatory system.