Cells have evolved specialized transport mechanisms to facilitate the uptake of nutrients and removal of waste products in the form of ions, small compounds, and macromolecules. Although the mode of transportation is dependent on both physical (size) and chemical (solubility) characteristics of the nutrient/waste product, to enter or leave the cell all of these small molecules must cross the cell membrane. Likewise, nutrients and waste products must cross the membranes of subcellular organelles like the nucleus in eukaryotes. Transport mechanisms across these biomembranes can be divided into two classes based on the requirement for added energy: passive transport, which is energy-independent, and active transport, which is energy-dependent.
Energy-independent transport mechanisms:
To understand passive transport, it is important to remember the second law of thermodynamics (entropy), which states that objects will move from a state of order to a state of disorder. In other words, nutrients or waste compounds will naturally move from a state of high concentration to a state of low concentration. This movement is termed diffusion, and can be seen when a single drop of concentrated red food coloring is added to a glass of water. In short order the food coloring will have diffused into the water and turned it a uniform light red color. The diffusion of aqueous solutions across a semi-permeable membrane like the cell membrane is termed osmosis. For example, when a wilting plant is given water, it straightens and generally becomes firmer to the touch. Water moved from an area of high concentration outside each cell to an area of low concentration inside each cell. A final type of passive transport is filtration or dialysis. The pores that are used in biomembranes for passive transport are generally small, and therefore only allow small molecules or ions to diffuse across them. Larger compounds are filtered out by this gating process and must gain entry into the cell by active means.
Energy-dependent transport mechanisms:
Active transport requires the addition of energy, usually supplied by hydrolysis of adenosine triphosphate (ATP), to open large channels in biomembranes or to selectively transport molecules. Active transport may require the help of a carrier or shuttle molecule, and, because of the added energy, can operate against a concentration gradient to localize molecules or ions. Such active transport mechanisms are used everyday to acidify the stomach or to break down unwanted cellular components in organelles called lysosomes.
Learn more about the structure of biomembranes.