Page 30 - Biology_F5

Cytology

Vacuoles cell (autolysis). Plant vacuoles
Vacuoles are membrane bounded sacs, can function in the same way as in
which are found within the cytoplasm lysosomes of animal cells as they both
of a cell. The vacuoles of plant cells are participate in an automatic cell death
bounded by single membranes called by autolysis. This process occurs when
tonoplast. These vacuoles are formed the tonoplast ruptures and releases its
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when vesicles released by the endoplasmic contents (enzymes) into the cytoplasm
reticulum and Golgi apparatus merge which digests the entire cell.
together. Young plants consist of a number (d) They play vital role in primary growth,
of smaller vacuoles, which during growth since the pressure exerted by fusion
and development, fuse to form a large of small vacuoles leads to elongation
central permanent vacuole. Therefore, in of the cells. Hence, this increases the
a mature plant cell, cytoplasm is displaced length of a plant organ.
and seen as a thin layer near the periphery (e) They contain anthocyanins, which
of the cell surrounding the large central provide colour to flowers, fruits,
vacuole. The central positioning of the and buds. These pigments facilitate
vacuole pushes the nucleus more or less pollination and seed dispersal.
to the periphery or near to the cell surface
membrane. At this stage, the vacuole is Ribosomes
filled with cell sap, which contains water, Ribosomes are tiny organelles found in
phenol, mineral salts, alkaloids, pigments, the matrix of mitochondria, chloroplasts,
sugars, and proteins. Animal cell lack and cytoplasm of the cells. They occur in
vacuoles, but if present they are small both, prokaryotic and eukaryotic cells. In
and temporary, associated with storage of prokaryotic cells they are found freely in
food and other materials to be secreted, the cytoplasm, whereas in the eukaryotic
transported or removed. An example cells they are either attached to the outside
of vacuoles is food vacuole or vesicle of the endoplasmic reticulum (or nuclear
(phagocytotic or pinocytotic vesicles). envelope) or they occur freely in the
cytoplasm. Structurally, each ribosome is
Functions of vacuoles made up of two sub-units, a small sub-unit
Vacuoles in plant cells play the following and a large sub-unit (Figure 1.13). Basing
roles: on their size and sedimentation coefficient,
(a) They support herbaceous plants and there are two types of ribosomes. These
herbaceous parts of woody plants by are:
providing an osmotic system which (a) 70s ribosomes: These are relatively
creates a pressure potential. smaller, with sedimentation coefficient
(b) They act as temporary stores for of 70s and molecular weight of 2.7x10 6
different substances, such as food, Daltons. They occur in prokaryotic
enzymes, and waste materials. cells, such as in blue-green algae and
bacteria.They also occur in eukaryotic
(c) They have hydrolytic enzymes which cells organelles, such as mitochondria
destroy dead cells or the entire plant and chloroplasts.

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