7. Describe briefly the structure and functions of two
organelles that are found in plant cell but not in animal cell.
Answer:
Plant has cell
wall and chloroplasts. Both of this organelle is not found in the animal cell.
Cell wall is a rigid external cell wall that protects the plant and is only one
cell thick. Cell wall is a non-living component located outside the plasma
membrane and it is fully permeable. The cell wall made up a bundle of cellulose
molecules known as microfibrils. Each microfibril composed about 3000 glucose
residues condensed together and are held together by a matrix of pectin and
hemicelluloses but the spaces between fibrils are not entirely filled with
matrix. This lead to water, air and dissolved materials to pass freely through
cell wall. The cell wall provides mechanical strength and skeletal support for
the plant. The strong materials in cell wall has fibre-matrix system, the
matrix provides stress to the fibres, which will have high ensile strength. The
matrix also improves resistance to compression and protects them from infects
of pathogen and possible chemical attack. The mature plant cell wall is made up
of many layers. The first portion of young plant is created by primary cell
wall. This layer is continues to grow. Fibrils in primary cell wall are
arranged loosely. This arrangement of fibrils makes primary cell wall more
elastic and allows for stretching during cell growth. The cells of soft tissues
of plants have only primary cell wall and intercellular lamellae. After grow
stops, cells become harder and more woody portions of the plant. This is due to
the secondary wall is formed. Actually, the secondary wall is inserted between
the plasma membrane and primary cell wall. The secondary cell wall is composed
of compact layers or lamellae. In addition to cellulose, secondary wall also
contains lignin, which is impermeable to water. As a result, leaving hard tube
formed by their wall to functions as mechanical support and for internal
transport. The cell wall also develops a coating of waxy cutin, the cuticle on
the exposed epidermal surfaces to reduce water loss and risk infection.
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