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.
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.
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
in. B. The membrane is attempting to envelop the material. C. The material
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 in action: A t-lymphocyte is attacking invading E. coli population.]
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.