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ACTIVE TRANSPORT

Involves energy in the form of ATP being used to transport molecules or ions
across a membrane against a concentration gradient or an ionic gradient (this
happens a lot in nerve cells). Movement is unidirectional (i.e. in one
direction).

Exocytosis is the active transport of materials out of cells and Endocytosis
refers to the active transport of materials into cells.
o Exocytosis of materials in vacuoles involves the vacuole fusing with
the cell membrane and then opening up to the external environment to
expel the contents.

Active Transport may involve:
o Carrier molecules (proteins) in the membrane; or
o The formation or breakdown of membrane-bound ‘packages’ of
matter. These packages are membrane-bound vesicles formed by the
process of phagocytosis or pinocytosis.

Active transport of substances can be stopped (inhibited) by some poisons.
These are chemicals that can destroy the molecular structure of the protein
carrier molecules or block the ‘channel within the protein molecule that
provides the means of substances moving across the membrane. These poisons
are called ‘inhibitors’.
Phagocytosis (cell eating)

Occurs when a cell membrane surrounds and engulfs particulate matter e.g.
when a white blood cell engulfs a foreign body or bacterium (pathogen).
o Or when a unicellular organism, like an amoeba, engulfs its food.

In both cases the membrane fuses around the particle and, in doing so, forms a
vacuole which becomes incorporated into the cytoplasm.
o Lysosomes carrying digestive enzymes may fuse with the vacuole and
the released enzymes will digest the particles and the products of
digestion are absorbed into the surrounding cytoplasm. The vacuole
will then move toward the cell membrane, fuse with it and release any
indigestible matter outside the cell.
Pinocytosis (cell drinking)

Occurs when a cell membrane infolds to form a channel. The liquid material
enclosed in the channel is pinched off in a vacuole as the membrane fuses at
the surface.
 See the following http://www.biology4kids.com/files/cell2_activetran.html
Contractile Vacuole
Tony Molyneux
1
14/3/2012
A contractile vacuole is a sub-cellular structure (organelle) involved in
osmoregulation. It pumps excess water out of a cell and is found prominently in
freshwater protists. They are found in both plant and animal cells. It pumps the water
out from the cytoplasm.
In Paramecium, a common freshwater protist, 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. Other protists,
such as Amoeba, have contractile vacuoles 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.
The contractile vacuole stores extra water. If the cell has a need for water, the
contractile vacuole can release more water into the cell. But if water is in excess, the
contractile vacuole will remove it to maintain homeostasis. If you put fresh water
protists in marine environment, its contractile vacuole will disappear. In protists, it is
considered as an organelle for osmoregulation and excretion.
Protist Paramecium aurelia with contractile vacuoles.
http://en.wikipedia.org/wiki/Contractile_vacuole
Tony Molyneux
2
14/3/2012
Below is another example of a combination of Passive Transport of water (Osmosis)
and Active transport of water out of the cell is filling of the Contractile Vacuole by
osmosis and its emptying by the pumping action of the Contractile Vacuole, which
requires energy (ATP).
Contractile Vacuole (water expelling vesicle) in the Protist Amoeba proteus.
The enlarged image shows the flowing of the cytoplasm when the amoeba moves. At the front end several
pseudopods (foot-like extensions) are formed. In the centre of the cell the single nucleus is visible. Left of
it lies a water expelling vesicle called the Contractile Vacuole. Remains of prey are enclosed in food
vesicles. Tiny crystals make the cell look granular.
Nucleus
Pseudopods
Food vesicles
Contractile
vacuole
Amoeba proteus in closer detail
by Wim van Egmond, The Netherlands
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/artsep01/amoeba.html
Tony Molyneux
3
14/3/2012