Movement across cell membranes is characterized as passive or active, and is dependent upon the nature of the phospholipid bilayer, ion channels, and protein pumps , which permit simple diffusion, facilitated diffusion, and active transport respectively.
The cell membrane is more permeable to non-polar, hydrophobic molecules than to polar, hydrophilic molecules by virtue of the hydrophobic interior of the amphipathic lipids of the bilayer. As a result, some non-polar molecules such as H2O and CO2 are able to diffuse directly across the cell membrane down a concentration gradient. This osmotic, chemical gradient limits both the rate of diffusion and the maximum concentration of the diffusing molecule in the cytosol, or the extracellular fluid (ESF) in the case of waste products.
Small polar molecules, typically simple ions like H+, K+, Na+, or Cl- are sufficiently small for facilitated diffusion through transmembrane protein pores called ion channels. The rate of facilitated diffusion is determined both by the number of ion channels and the concentration gradient. image - ion channel proteins : animation - facilitated diffusionActive transport. Protein pumps utilize energy to move molecules or ions against concentration gradients. image - carrier proteins : animation - carrier proteins : animation - Na glucose symport : animation - Na K ATPase
In addition, though not strictly a form of transmembrane transport, molecules can be moved through membranes. Endocytosis involves movement of macromolecules from the exterior into the lumen of endomembranous organelles. This is achieved by phagocytosis, pinocytosis, or receptor-mediated endocytosis. animation - phagocytosis. Exocytosis is the reverse of endocytosis. Vesicles containing the substances for release are transported to the plasma membrane and fuse with it, effecting extrusion of vesicular contents. animation - exocytosis
The soluble NSF receptor (SNARE proteins) are the best characterized among many factors involved in exocytosis.
Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows transport along the microtubular conveyor belt and exocytosis; and, Interpretation: Inner Life of the Cell Џ
Google endocytosis / Google exocytosis / Google phagocytosis / Google pinocytosis / Google receptor-mediated endocytosis / Virtual Cell Textbook - Cell Biology :