Osmoregulation

1. OSMOREGULATION:
The fresh water Protozoa animals will face
a problem with excess of wafer that enters
into the body by Osmosis. The process by
which water balance in the body is
maintained is called Osmoregulation. In
protozoans animal’s osmoregulation is
carried on by contractile vacuoles.

2. • In marine protozoans and parasitic protozoans
contractile vacuole is absent. In Amoeba
osmoregulation is carried out by contractile
vacuoles. The contractile vacuole will absorb
excess of water in the body Then it reaches the
surface and ruptures. This sends out the excess
water. Now-a-days "Duncan Wigg, EC. Bovee, T L
Jahn' in-1967 worked on the contractile vacuole
of Amoeba and came to the conclusion this
contractile vacuole has no contraction capacity. It
will rupture only to send out the excess water. So
they called the contractile vacuole as Water
pulsating vesicle'.
•

3. • In Paramecium two contractile vacuoles are
present. They are surrounded by 6 to 10
radiating canals. The two contractile vacuoles
will work alternately and perform
Osmoregulation.
• Maintenance of constant internal
environment is called homeostasis. This word
is coined by W.B Cannon, in Protozoans
homeostasis or Osmoregulation is carried on
by contractile vacuoles.

4. • Amoeba makes use of contractile vacuoles to
collect excretory wastes, such as ammonia,
from the intracellular fluid
by diffusion and active transport. As osmotic
action pushes water from the environment
into the cytoplasm, the vacuole moves to the
surface and pumps the contents into the
environment.

6. contractile vacuole (CV)
• A contractile vacuole (CV) is a sub-cellular structure (organelle)
involved in osmoregulation. It is found predominantly
in protists and in unicellular algae. It was previously known
as pulsatile or pulsating vacuole.
• The contractile vacuole is a specialized type of vacuole that
regulates the quantity of water inside a cell.
In freshwater environments,
the concentration of solutes is hypotonic, lesser outside than inside
the cell. Under these conditions, osmosis causes water to
accumulate in the cell from the external environment. The
contractile vacuole acts as part of a protective mechanism that
prevents the cell from absorbing too much water and
possibly lysing (rupturing) through excessive internal pressure.

7. • The contractile vacuole, as its name suggests, expels water out of
the cell by contracting. The growth (water gathering) and
contraction (water expulsion) of the contractile vacuole are
periodical. One cycle takes several seconds, depending on the
species and the environment's osmolarity. The stage in which water
flows into the CV is called diastole. The contraction of the
contractile vacuole and the expulsion of water out of the cell is
called systole.
• Water always flows first from outside the cell into the cytoplasm,
and is only then moved from the cytoplasm into the contractile
vacuole for expulsion. Species that possess a contractile vacuole
typically always use the organelle, even at very hypertonic (high
concentration of solutes) environments, since the cell tends to
adjust its cytoplasm to become even more hyperosmotic than the
environment. The amount of water expelled from the cell and the
rate of contraction are related to the osmolarity of the
environment. In hyperosmotic environments, less water will be
expelled and the contraction cycle will be longer.

8. • contractile vacuoles belong to the
protists Paramecium, Amoeba, Dictyostelium and Trypa
nosoma, and to a lesser extent the green
alga Chlamydomonas. Not all species that possess a
contractile vacuole are freshwater organisms;
some marine, soil microorganisms and parasites also
have a contractile vacuole. The contractile vacuole is
predominant in species that do not have a cell wall, but
there are exceptions (notably Chlamydomonas) which
do possess a cell wall. Through evolution, the
contractile vacuole has typically been lost
in multicellular organisms, but it still exists in the
unicellular stage of several multicellular fungi, as well
as in several types of cells
in sponges (amoebocytes, pinacocytes,
and choanocytes)

9. • The number of contractile vacuoles per cell varies,
depending on the species. Amoeba have
one, Dictyostelium discoideum, Paramecium
aurelia and Chlamydomonas reinhardtii have two, and
giant amoeba, such as Chaos carolinensis, have many.
The number of contractile vacuoles in each species is
mostly constant and is therefore used for species
characterization in systematics. The contractile vacuole
has several structures attached to it in most cells, such
as membrane folds, tubules, water tracts and
small vesicles. These structures have been termed
the spongiome; the contractile vacuole together with
the spongiome is sometimes called the "contractile
vacuole complex" (CVC). The spongiome serves several
functions in water transport into the contractile
vacuole and in localization and docking of the
contractile vacuole within the cell.

10. • Paramecium and Amoeba possess large contractile
vacuoles (average diameter of 13 and 45 µm,
respectively), which are relatively comfortable to
isolate, manipulate and assay. The smallest known
contractile vacuoles belong to Chlamydomonas, with a
diameter of 1.5 µm. In Paramecium, which has one of
the most complex contractile vacuoles, the vacuole is
surrounded by several canals, which absorb water by
osmosis from the cytoplasm. After the canals fill with
water, the water is pumped into the vacuole. When the
vacuole is full, it expels the water through a pore in the
cytoplasm which can be opened and closed.[2] Other
protists, such as Amoeba, have CVs that move to the
surface of the cell when full and undergo exocytosis.
In Amoeba contractile vacuoles collect excretory waste,
such as ammonia, from the intracellular fluid by
both diffusion and active transport.

11. Water flow into the CV
• The way in which water enters the CV had been a mystery for many years,
but several discoveries since the 1990s have improved understanding of
this issue. Water could theoretically cross the CV membrane by osmosis,
but only if the inside of the CV is hyperosmotic (higher solute
concentration) to the cytoplasm. The discovery of proton pumps in the CV
membrane[3] and the direct measurement of ion concentrations inside the
CV using microelectrodes[4] led to the following model: the pumping of
protons either into or out of the CV causes different ions to enter the CV.
For example, some proton pumps work as cation exchangers, whereby a
proton is pumped out of the CV and a cation is pumped at the same time
into the CV. In other cases, protons pumped into the CV drag anions with
them (carbonate, for example), to balance the pH. This ion flux into the CV
causes an increase in CV osmolarity and as a result water enters the CV by
osmosis. Water has been shown in at least some species to enter the CV
through aquaporins.[5]

12. • Acidocalcisomes have been implied to work
alongside the contractile vacuole in
responding to osmotic stress. They were
detected in the vicinity of the vacuole
in Trypanosoma cruzi and were shown to fuse
with the vacuole when the cells were exposed
to osmotic stress. Presumably the
acidocalcisomes empty their ion contents into
the contractile vacuole, thereby increasing the
vacuole's osmolarity