Movement Through the Membrane Cell Membrane

Cell Membrane… One of the main functions of the cell membrane is to regulate what enters and leaves the cell. It also protects and supports the cell. The membrane is composed of a lipid bilayer, which gives the cell a tough, flexible structure that forms a strong barrier between the cell & its surroundings.

Cell membrane continued… It also contains proteins that serve as channels for transport of certain materials. Carbohydrates form chains and attach to the proteins in the surface. Many of these carbohydrates act as chemical identification cards. This is so cells recognize each other.

Diffusion Diffusion is basically the movement of a substance from an area of high concentration to an area of low concentration. The concentration is basically how much of a solute (mass) is in a solvent (volume). Diffusion causes many substances to move across a cell membrane, yet requires no energy to take place.

Diffusion Continued… Diffusion is referred to as a form of ‘passive transport’ because no energy is needed. Diffusion continues until an equilibrium is met. In other words, until there is the same amount of material on each side of the cell membrane.

Osmosis Cell membranes are described as selectively permeable, meaning they only allow certain things in/out of the cell. Osmosis occurs when water diffuses through a selectively permeable membrane. Osmosis works when there is a difference in concentration of water inside and out of the cell.

Osmosis Continued For example lets say there is a higher concentration of salt inside of a cell and more water on the outside. The water molecules outside the cell will move into the cell to make it equal on both sides of the membrane. As this occurs, the cell will swell as water rushes in. (hypotonic solution) If there is more water inside the cell and salt outside, the water rushes out, causing the cell to shrink. (hypertonic solution)

Osmosis Continued… In an isotonic solution, the cell is equal to its surroundings in terms of solute/solvent.

Facilitated Diffusion Not all molecules can move across cell membranes without help. Glucose in one of these molecules and it must be moved across through a protein channel. Although these molecules, along with other ions, sugars, and salts are ‘helped’ across the cell membrane, it is still considered diffusion because no energy is used by the cell.

Active Transport This process occurs when substances must be moved against the concentration gradient, or moving from an area of low concentration to area of high concentration. It can be illustrated as a pump moving sub- stances across the membrane, like the Na+ and K+ pumps, which are proteins that transport these materials. Endocytosis transoports materials inside the cell by taking material into the membrane and forming a pocket and pulling it inside. This process is called phagocytosis.

Active Transport Continued… Exocytosis occurs when a vacuole forms a pocket around material inside a cell and then fuses with the cell membrane, forcing the contents out of the cell. All forms of active transport require energy from the cell to work, hence the term ‘active’ transport.