The source discusses cellular membrane transport, explaining the different mechanisms by which substances move into and out of cells across the selectively permeable plasma membrane. It distinguishes between passive mechanisms, which don't require cellular energy (ATP), such as simple diffusion (like the spread of a scent or the movement of oxygen and CO2) and osmosis (the movement of water), highlighting factors that influence diffusion rate and providing examples like the effects of tonicity on red blood cells. It also covers facilitated diffusion, a passive process aided by carrier proteins, which can be subject to transport maximums. Finally, the source describes active transport, which requires cellular energy, including primary active transport (using ATP directly, like sodium-potassium pumps) and secondary active transport (using ATP indirectly through the movement of another substance, like sodium-glucose co-transport). It also briefly mentions vesicular transport for larger molecules.