Molecule Transport

Two main categories of molecule transport exist in cells, active transport and passive transport (shown below). For small molecules such as oxygen, ethanol, and carbon dioxide, the molecules could easily cross the membrane via passive transport, in the form of simple diffusion through a concentration gradient. However, the means that cells use to transfer small molecules are not sufficient for transporting macromolecules, which include proteins, polynucleotides, and polysaccharides. To transport these macromolecules, cells rely on active transport. There are two basic means of active transport - by exocytosis and by endocytosis. Exocytosis involves sending macromolecules out of the cell, while the opposite applies to endocytosis.

Exocytosis

Macromolecules that are released don't simply drift towards the cell's membrane and squeeze their way out. They are actually packaged in a vesicle that separates them out from the rest of the cell. he vesicle fuses with its specific membrane structure and its contents are released without the vesicle, which is incorporated back into the cell's membrane.

Proteins, for example, that are to be secreted from the cell are made on the rough endoplasmic reticulum (ER). They are then transported to the Golgi complex by ER induced vesicles. The Golgi complex sorts and packages the proteins into vesicles that separate themselves off the Golgi complex and eventually fuse with the cellular membrane.

Some molecules are secreted continually from the cell, but others are selectively secreted. To control secretion, specific substances are stored in secretory vesicles, which are released when triggered by an extracellular signal. The signal, hormones being an example, binds to its specific cell surface receptor. Then the concentration of free Ca²⁺ is increased in the cell. The increased concentration of the Ca²⁺ triggers exocytosis, causing the secretory vesicles to fuse with the cellular membrane, releasing the substances outside the cell.

Endocytosis

There are two types of endocytosis. Pinocytosis involves ingesting small molecules and/or fluids surrounding the cell in a process known as fluid-phase endocytosis. Phagocytosis involves the ingestion of large molecules, such as microorganisms or cell debris using large vesicles, or vacuoles.

Eukaryotic cells are nearly continuously ingesting the surrounding fluids and molecules. In doing so they are also ingesting their own cellular membrane at a rapid rate. Macrophages, for example ingest 3% of its cellular membrane each minute, or 100% each half and hour! Obviously the membrane is being added by exocytosis at about the same rate it is being removed by endocytosis.

Microscope image of cell undergoing endocytosis. A. The cell membrane begins to sink
in. B. The membrane is attempting to envelop the material. C. The material has been
captured inside of a capsule of cell membrane. D. Endocytosis is complete. 

Most of the ingested molecules, which are surrounded by small vesicles called the primary vesicle, are fused with secondary lysosomes. The molecules that have been ingested are then deposited at the cytosol, where the cell uses them and most of the vesicle is reintegrated into the cellular membrane. In some cases the vesicle bypasses the secondary lysosome and goes directly to its target.

Often, a cell needs to selectively ingest a molecule. For this case it uses a special process, called receptor-mediated endocytosis. The molecules ingested bind to specific cell surface receptors and are internalized at a greater rate than fluid is by fluid-phase endocytosis. An example receptor-mediated endocytosis is the intake of low-density lipoproteins, or LDL. When a cell needs cholesterol to make more membranes it makes receptors for LDL and places them in the cellular membrane. The LDL then binds to the receptors and is rapidly taken in by the cell.

Some people contain defective genes for making the cell surface receptor for LDL. These people are unable to ingest LDL and thus have a higher concentration of LDL in their bloodstream, giving them a high cholesterol rate. These people are more likely to die from heart disease at an early age.

Phagocytosis

[Phagocytosis in action: A t-lymphocyte is attacking invading E. coli population. Copyright Daniel Kunkel]

White blood cells use phagocytosis to remove foreign particles from the blood stream. These cells will literally engulf foreign particles that are the same size as itself. There are two types of white blood cells which act as phagocytes: macrophages and polymorphic leucocytes. To be phagocytosed, particles need to bind to the receptors on the phagocyte. Then the particle is engulfed and absorbed. 

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