Cell Transport
The movement of materials into and out of cells influences the internal concentrations of the molecules. So, you can see that what goes in or out will have a profound effect on the ability of cells to function properly. The ability of cells to move materials in and out efficiently can determine whether they will live or die. How do cells control these movements? They do it by both passive and active transport. We will examine these processes in this section. Before we begin, however, I want to stress the importance of the plasma membrane that surrounds cells. To get in or out of a cell, material must pass through this membrane. It is the membrane that is the gatekeeper of cells.
The purpose of cell transport is to maintain homeostasis. The different kinds of cell transport are divided into two categories: those that require energy and those that do not. Passive transport does not require energy. There are three kinds of passive transport. In diffusion substances move from high concentrations to low concentrations. In facilitated diffusion substances move from high concentrations to low concentrations via carrier proteins. Finally, in osmosis water moves from high concentrations (of water) to low concentrations. It is Passive transport is the movement of a substance across a cell membrane without the input of the cell's energy.
Simple Diffusion
Simple Diffusion involves the movement of atoms across the cytolemma from a region of higher concentration to a region of lower concentration. Atoms move across the cell membrane by by going between the lipid molecules that make up the cell membrane. Small atoms diffuse the easiest across the membrane. No outside chemical energy is needed for simple diffusion.
Facilitated Diffusion
Diffusion is facilitated by cell membrane proteins that provide a way for atoms or molecules to more easily diffuse across the membrane.
Osmosis
Osmosis is the simple diffusion of water molecules across a semipermeable membrane. It occurs when the concentration of solutes in the solution on the two sides of a semipermeable membrane are different moves from a solution with a higher water concentration to a solution with lower water concentration.
Active transport requires energy and usually moves substances from low concentrations to high concentrations against the concentration gradient. In endocytosis, a form of active transport, the cell engulfs material. In exocytosis, the cell expells material. Exocytosis
In exocytosis wastes and cell products are packaged by Golgi apparatus in sacs called Golgi vesicles. Gogi vesicles fuse with the cell membrane and the materials in the vesicles are secreted out of the cell.
Endocytosis
The cell membrane surrounds desirable macromolecules outside the cell. The cell pinches off a saclike portion of its outer membrane to form a tiny new vesicle . the vesicle moves into the cell where it releases its contents into the cytoplasm.
PinocytosisIn Pinocytosis the cell membrane encloses a droplet of fluid and its solutes and brings the droplet into the cell.
PhagocytosisIn Phagocytosis the cell engulfs a food particle . The vesicle containing food then fuses with a lysosome carrying digestive enzymes.
