A vacuole is a membrane bound, multifunctional
organelle found in the cells of plants (including algae and fungi) and some
protists and bacteria. Vacuoles are acidic in nature and share some basic
properties with lysosomes that are predominantly found in plant cells.
Depending on the type of plant, there are different types of vacuoles with
specific properties that are crucial to their functions.
* Unlike lysosomes in animals, there is only one
of a few vacuoles in individual plant cells.
* Vacuoles take up 80-90 percent of the entire
plant cell volume
Vacuoles are complex organelles and much about
their biogenesis remains unknown. However, studies suggest that vacuoles found
in the root tips originate from vesicles that arise from Golgi bodies.
process involves the fusion of these vesicles to produce prevacuoles, which are
vacuole precursors. It is the fusion of these prevacuoles that ultimately
results in the formation of a vacuole.
Types and Functions of Vacuoles
Lytic vacuoles share similar properties with
lysosomes found in animals. As such, they contain different types of hydrolytic
enzymes responsible for the degradation of such molecules as nucleic acids,
proteins and polysaccharides.
Researchers have suggested that these particular
organelles either originate from the trans-Golgi network or the dilation of a
part of the smooth endoplasmic reticulum.
These types of vacuoles are also referred to as lytic
compartments and are characterized by the optimum pH of 5. Research studies
have found lytic vacuoles to contain the following types of hydrolytic and
Hydrolases - Essentially,
hydrolases are different types of hydrolytic enzymes that use water to break up
chemical bonds. This allows them to divide larger molecules into smaller ones.
Esterases - This includes
hydrolase enzymes that specifically serve to break down esters (compounds made
up of an acid and alkyl group) in to acids and alcohol group.
Nucleases - Enzymes
responsible for breaking down the bonds (phosphodiester bonds) to produce
Peroxidases - Peroxidases
include enzymes that typically break down hydrogen peroxide, removing it from
chloroplast and cytosol among others in plants.
There are different processes through which
cells eliminate unwanted/old material, unwanted cytoplasm or the entire cell.
In plant cells, this includes:
In plants, autophagy is an important process
that helps in the elimination of unwanted material from the cells. Here,
various materials in the cytoplasm that are no longer required by the cell are
enclosed within a vesicle refered to as the autophagosome and transported to
the vacuole where they are degraded.
The invagination of the double membrane of
the autophagosome makes it possible for this vesicle to enclose and hold
cytoplasmic material/components to be delivered to the vacuole. This process is
also involved in recycling of material.
By breaking down various cell
components, they are reduced to their basic components that can then be used by
the cell. For instance, the breakdown of proteins produces peptides that can
later be transported through the endoplasmic reticulum and Golgi apparatus to
Autophagy in cells occurs in response to
different conditions within the cells or in response to factors affecting the
body in general. For instance, such stressful conditions as starvation result
in the degradation of various components of the cell such a proteins and even
lipids in order to produce energy.
While it was previously believed that autophagy
non-selectively eliminates various components in the cell, recent studies have
shown that this mechanism can and does selectively eliminate given components
such as proteins in specific conditions or in response to given stressful
conditions in yeast cells.
Cell Defence and Cell Death
Vacuoles play a crucial role in the defense and
death of cells.
Although the process is yet to be fully
understood, vacuoles play an important role in immunity of the cell by
releasing various enzymes (hydrolytic enzymes) and antimicrobes that destroy
the invading pathogen. However, the mechanism has also been associated with
programmed cell death (PCD).
In reaction to invading organisms in the cell, an
enzyme refered to as vacuolar processing enzyme triggers the disruption of the
vacuolar membrane, causing the vacuole to collapse and release hydrolytic
enzymes and other antimicrobes. This not only results in the destruction of the
invaders, but also the cell itself.
On the other hand, the fusion of the central
vacuole with the plasma membrane in the presence of proteasome can cause the
vacuole to release antibacterial protease as well as other vacuole components
that can cause the death of the cell.
Protein storage vacuoles
Protein storage vacuoles can be found in the
storage tissues where they accumulate proteins. Seeds are good examples of tissues
where reserve proteins are stored. All proteins to be stored are first synthesized
in the rough endoplasmic reticulum and then transported to the protein storage
In some plants, this process involves the transport of proteins
through autophagy and protein bodies (PBs). On the other hand, they may be
released from the Golgi apparatus (having been synthesized in the ER) as
prevacuoles before arriving at the vacuole for storage.
For proteins to be
successfully transported from the Golgi apparatus to the vacuole, protein
targeting is essential. Here, peptide targeting sequence target given receptors
on the vacuole, which allows for proteins to be successfully transported and
Depending on the type of plant, storage tissues
(seeds, etc) will contain many, densely packed protein storage vacuoles.
Moreover, depending on the plant, there may be one or different types of proteins
Gas vacuoles are composed of hollow cylindrical
gas vesicles. They are typically found in bacteria and have a permeable
membrane that allows air to pass through. This membrane also serves to bind the
vesicles. These vesicles can either inflate (fill with air) or deflate allowing
the bacteria such as cyanobacteria
to float or remain at a given desired depth in water.
Contractile vacuoles are membrane bound
organelles that are typically found among members of kingdom protista (algae,
amoebas, and ciliates etc). In these cells, the contractile vacuole is particularly
important given that it helps in osmoregulation (regulation of osmotic
Although the entre mechanism is yet to be understood, researchers
suggest that the contractile vacuole system (contractive vacuole complex)
functions through the activities of two compartments that are bound by two differentiated
The two membranes have different properties that make it possible
for the vacuole to carry out osmoregulation. Here, the first membrane is
divided into many vesicles and tubules and contains numerous
proton-translocating V-ATPase enzymes. These components are responsible for
producing an electrochemical gradient of protons and fuse with the membrane of
the second compartment.
The second compartment
expands into the reservoir for fluid storage and can fuse with the cell
membrane of the cell. However, it lacks the V-ATPase holoenzymes. As such, it
undergoes contraction periodically which enables the vacuole to expel fluids.
Together with other solutes, this system works like a pump that pumps out
excess water from time to time to prevent the cell from swelling and getting ruptured.
The sap vacuole is also commonly refered to as
the central vacuole of a cell. It is the large, central organelles that occupy
most part of the cell volume. This organelle contains the fluid known as the
cell sap, which consists of such contents as water, sugars, minerals and amino
acids among others. As a plant and cells mature, provacules from the Golgi
complex fuse to form the sap vacuole at the center of the cell.
Contents of the cell sap are transported to the
vacuole from the cytoplasm in the cell.
Some of the other functions include:
Cell growth - Vacuoles are
important in plant cells given that they help maintain turgidity of the cell.
Enlargement of the vacuole consequently results in growth/increased size of the
cell. This ultimately contributes to tissue rigidity.
Storage - Apart from
proteins, vacuoles are storage compartments for metabolites, organic acids and
sugars among others
Pigment deposition -
Vacuoles are a site where pigments are deposited allowing for such vegetable
colors as red, blue, scarlet etc.
A light microscope can be used to view and study
the structure of a vacuole. Although the vacuole does not stain as other
organelles of the cell (because the vacuole does not contain much stainable
constituents) experiments have shown that it is possible to stain this
organelle as the sap inside the vacuole take in and accumulate colored dyes.
Most of the dyes used are basic in nature. However, the pH level of the dye
also depends on the type of the vacuole being stained.
Some of the dyes that have been used for the
purposes of staining vacuoles include:
Here, we will focus on staining using Neutral
red solution (pH less than 6.0).
A part of a plant (roots,
Neutral red stain (7.2 in
0.08M phosphate buffer)
A compound light microscope
Glass slides and cover
Obtain a thin skin of the plant using a pair of
forceps and stain the sample with 0.01 percent of neutral red
Rinse the sample with the phosphate buffer
Mount for observation
Through this procedure, the stain will only
stain the vacuole of live plant cells without staining any other organelle.
When viewed under the microscope, students will view the vacuoles appear deep
red in color.
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