Membrane
Membrane
A cell membrane or plasma membrane is a thin barrier that separates a cell from its surroundings. It also keeps the cell's cytoplasm and organelles on the inside. Membranes are selectively permeable, meaning that some things can pass through the membrane and some cannot.
All cells have a cell membrane that is a thin but double layer of molecules that surrounds it. This membrane acts as a barrier and helps protect the cell while controlling the movement of substances in and out of
the cell. The membrane also allows a cell to maintain a constant internal environment, despite changes in its external environment. It is able to do this because of the semipermeable nature of its layers that regulate the passage of all substances going through it. The membrane is able to keep things out that it does not want or need while allowing in what it must have. A membrane surrounds not only the entire cell, but each organelle or specialized structure inside the cell also has a membrane around it. For both the cell and its organelles, the membrane is a place of constant activity. Although this membrane is extremely thin, it is very strong and can heal itself if broken. Examined very closely, a membrane is like a mesh bag whose little, square holes are small and strong enough to hold a dozen oranges or five pounds of potatoes, but which will also let water flow out or in completely. The bag is semipermeable, since it keeps certain-size things in (oranges) but let things of another size (water molecules) pass through. Most cell membranes are permeable to oxygen and water but not to large organic molecules like proteins.
One way that substances move through a membrane is by a process called passive transport. Passive transport involves no use of energy on the part of the cell, since certain substances are able to move freely in or out of it. Diffusion and osmosis are forms of passive transport since the cell does not need to use any energy to move substances. In diffusion, molecules of a substance spread themselves out more evenly from an area of high concentration. Substances like carbon dioxide, salts, and oxygen move in and out of cells this way. In osmosis, water molecules move from an area where they are crowded and cross a membrane to where they are less crowded. This process stops of its own accord when the solutions on either side of the membrane are at equal strength.
Active transport is a different way that a cell moves molecules in or out through its membrane, and it involves the use of energy. Active transport occurs when a cell wants to bring in more of a substance than will enter via passive transport. This happens when a plant's nearly-full root cells want to store even more minerals, and must move them from an area where they are less crowded to one where they are more crowded (inside the cell). In order to pack in more molecules, the cell uses carrier molecules that literally carry the desired molecule to a membrane slot into which it fits and then forces it into the cell. This process requires that the cell uses its own energy. Cell membranes are much more than walls or barriers that hold a cell together and keep it separate from its environment, since membranes control the movement of substances into and out of a cell.