The document discusses various processes that occur across cell membranes, including passive and active transport. The cell membrane is selectively permeable and allows certain particles to pass through protein gates. Passive transport involves diffusion and osmosis, moving particles from high to low concentration without energy. Active transport moves particles from low to high concentration and requires energy. Larger particles are transported within vesicles via endocytosis moving into the cell, and exocytosis moving out. Specialized cells are needed for organisms to have different tissues and organs.