Transporting Materials and Waste Elimination
The issue in transporting materials around the cell is somewhat problematic. Since the
inside of the cell is friendly to water soluble molecules and not compatible with hydrophobic
molecules.
Inside the cell, molecules that are destined for the cell membrane maybe hydrophilic
and would merge to within the aqueous cytoplasm of the cell. As a bioengineer you need to
protect the cell from the certain enzymes produced by the cell. Such enzyme might digest
essential cell structures.
However the Eukaryotic cell has develop mechanisms in transporting molecules to their
point of action.
One method is through the use of an organelle called vacuole. Earlier in our discussion
on compartmentalization of the cell, we know that vacuole are small compartments (sac like)
which is surrounded by a hydrophobic membrane. This vacuole can transport materials
through and out of the cell in a mode which is isolated from the rest of the cell interior.
This type of mechanism is known as the vesicle mediated transport. There are two
types of vesicle mediated transport namely the Endocytosis which involves three mechanisms,
the phagocytosis, pinocytosis and the receptor mediated Endocytosis, and the Exocytosis.
In Endocytosis, the cell engulfs some of its extracellular fluid (ECF) including material
dissolved or suspended in it. The materials to be internalized is surrounded by an area of
plasma membrane, this portion of the plasma membrane is invaginated and pinched off
forming a membrane-bounded vesicle called an endosome.
The term “endocytosis” was coined by Christian deDuve in 1963 to include both the
ingestion of large particles (such as bacteria) and the uptake of fluids or macromolecules in
small vesicles. The former of these activities is known as phagocytosis (cell eating) and the
latter as pinocytosis (cell drinking).
Figure 8.17
Three types of
Endocytosis in animal
cell. (a) Phagocytosis
(b) Pinocytosis
(c) Receptor-mediated
endocytosis
During phagocytosis, cells engulf large particles such as bacteria, cell debris, or even
intact cells, allowing the binding of the particle to receptors on the surface of the phagocytic
cell. This union between the particles and the receptors then triggers the extension of
pseudopodia—an actin-based movement of the cell surface.
The pseudopodia eventually surround the particle and their membranes fuse to form a
large intracellular vesicle (>0.25 μm in diameter) called a phagosome. The phagosomes then
fuse with lysosomes, producing phagolysosomes in which the ingested material is digested by
the action of lysosomal acid hydrolases. During maturation of the phagolysosome, some of the
internalized membrane proteins are recycled to the plasma membrane.
Figure 2
Phagocytosis. Binding of a bacterium
to the cell surface stimulates the
extension of a pseudopodium, which
eventually engulfs the bacterium.
Fusion of the pseudopodium
membranes then results in formation
of a large intracellular vesicle (a
phagosome).
The ingestion of large particles by phagocytosis plays distinct roles in different kinds of
cells (Many amoebas use phagocytosis to capture food particles, such as bacteria or other
protozoans. In multicellular animals, the major roles of phagocytosis are to provide a defense
against invading microorganisms and to eliminate aged or damaged cells from the body.
In mammals, phagocytosis is the function of primarily two types of white blood cells,
macrophages and neutrophils, which are frequently referred to as “professional phagocytes.”
Both macrophages and neutrophils play critical roles in the body's defense systems by
eliminating microorganisms from infected tissues. In addition, macrophages eliminate aged or
dead cells from tissues throughout the body. A striking example of the scope of this activity is
provided by the macrophages of the human spleen and liver, which are responsible for the
disposal of more than 1011 aged blood cells on a daily basis.
Phagocytic cells, like macrophages and neutrophils, are an early line of defense against
invading bacteria. However, some bacteria have evolved mechanisms to avoid destruction even
after they have been engulfed by phagocytes.
Two examples:


Salmonella enterica is a bacterium that causes food poisoning in humans. Once
engulfed by phagocytosis, it secretes a protein that prevents the fusion of its phagosome
with a lysosome.
Mycobacteria (e.g., the tubercle bacillus that causes tuberculosis) use a different trick.
o When the phagosome is first pinched off from the plasma membrane, it is coated
with a protein called "TACO" (for tryptophan-aspartate-containing coat protein).
o This must be removed before the phagosome can fuse with a lysosome.
o Mycobacteria taken into a phagosome are able, in some way, to keep the TACO
coat from being removed.
o Thus there is no fusion with lysosomes and the mycobacteria can continue to
live in this protected intracellular location.
In the process of pinocytosis the plasma membrane forms an invagination. Whatever
substance is found within the area of invagination is brought into the cell. In general this
material will be dissolved in water and thus this process is also referred to as "cellular
drinking" to indicate that liquids and material dissolved in liquids are ingested by the cell. This
is opposed to the ingestion of large particulate material like bacteria or other cells or cell
debris.
The best-characterized form of this process is receptor-mediated endocytosis, which
provides a mechanism for the selective uptake of specific macromolecules. The macromolecules
to be internalized first bind to specific cell surface receptors.
These receptors are concentrated in specialized regions of the plasma membrane, called
clathrin-coated pits. These pits bud from the membrane to form small clathrin-coated
vesicles containing the receptors and their bound macromolecules (ligands). The clathrincoated vesicles then fuse with early endosomes, in which their contents are sorted for transport
to lysosomes or recycling to the plasma membrane.
Clathrin-coated vesicle formation
(A) Extracellular macromolecules (ligands) bind to cell surface receptors
that are concentrated in clathrin-coated pits. These pits bud from the
plasma membrane to form intracellular clathrin-coated vesicles. (B, M. M.
Perry, 1979. J. Cell Science 39: 266.)
In Exocytosis is a cellular process where cells eject waste products or chemical
transmitters (such as hormones) from the interior of the cell. Exocytosis is similar in function
to endocytosis but working in the opposite direction.
There are five steps to exocytosis:
Vesicle Trafficking
In this first step, the vesicle containing the waste product or chemical transmitter is
transported through the cytoplasm towards the part of the cell from which it will be eliminated.
Vesicle Tethering
As the vesicle approaches the cell membrane, it is secured and pulled towards the part
of the cell from which it will be eliminated.
Vesicle Docking
In this step, the vesicle comes in contact with the cell membrane, where it begins to
chemical and physically merge with the proteins in the cell membrane.
Vesicle Priming
In those cells where chemical transmitters are being released, this step involves the
chemical preparations for the last step of exocytosis.
Vesicle Fusion
In this last step, the proteins forming the walls of the vesicle merge with the cell
membrane and breach, pushing the vesicle contents (waste products or chemical transmitters)
out of the cell. This step is the primary mechanism for the increase in size of the cell’s plasma
membrane.