2. The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. A membrane is a fluid structure with a mosaic, which is a pattern consisting of numerous small pieces or elements fitted together, of various proteins embedded in it when viewed from the top. Phospholipids can move laterally a small amount and can flex their tails. Membrane proteins also move side to side or laterally making the membrane fluid.
Transport proteins allow passage of hydrophilic substances across the membrane. Passive transport is the diffusion of a substance across a membrane with no energy investment. In passive transport, substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. CO2, H2O, and O2 easily diffuse across plasma membranes. Diffusion of water is known as osmosis.
Diffusion is the tendency for molecules of any substance to spread out evenly into the available space. They move from high to low concentration. Facilitated diffusion is a type of Passive Transport aided by proteins. In facilitated diffusion, transport proteins speed the movement of molecules across the plasma membrane. Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane. Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane.
Active transport uses energy to move solutes against their concentration gradients. It requires energy, which is usually in the form of ATP. In active transport, some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Cotransport occurs when active transport of a specific solute indirectly drives the active transport of another solute. This involves transport by a membrane protein and is driven by a concentration gradient. Examples of active transport are exocytosis and endocytosis. In exocytosis, transport vesicles migrate to the plasma membrane, fuse with it, and release their contents. In endocytosis, the cell takes in macromolecules by forming new vesicles from the plasma membrane.
That is how the plasma membrane plays a role in active transport and passive transport.