B. Transport proteins—these will expend ATP (cellular energy) in order to transport molecules into or out of the cell. This is referred to as “active transport” because it requires energy in order to allow for the transport. C. Recognition proteins—these are cells that allow others to recognize the cell as being a self-cell and distinguish the cell as being of a certain type. They have short
polysaccharide (or sugar) chains extending from the surface of the cell. D. Adhesion proteins—these are proteins that attach to neighboring cells. They provide anchors for the internal filaments and tubules within the cells to provide stability and structure for the cell. E. Receptor proteins—these are located in and on the membrane and allow trigger molecules such as hormones to act on the interior of the cells. These are basically
binding proteins. F. Electron transfer proteins—these are involved from one molecule to another during various chemical reactions in the cell membrane.
TRANSPORT ACROSS THE CELL MEMBRANE There is passive and active transport across the cell membrane. Passive transport does not require the use of energy. Simple diffusion is the net movement of substances from an area of higher concentration to an area of lower concentration. There is random and constant movement of molecules that pass through the cell membrane to an area of lower concentration (also referred to as going down a gradient). Facilitated diffusion is the diffusion of solutes through the aid of proteins in the membrane. The biggest example of this is the facilitated diffusion of water with the help of aquaporins. This does not require ATP energy input but requires the aquaporin proteins. Osmosis refers to the diffusion of water molecules across a semipermeable membrane. When water moves into a cell by osmosis, hydrostatic pressure, also called osmotic pressure, can build up in the cell. On
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