Page 22 - Biology_F5

Cytology

(b) Presence of hydrophilic pores for condition by selectively allowing
passage of polar substances. substances to enter and exit the cell.
(c) The phospholipid bilayer facilitates
the passage of fat soluble substances.
(d) Presence of microvilli increases the Cell wall
surface area for absorption. Plant cells, fungal cells, and bacterial
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(e) Presence of proteins with specific cells are surrounded by fairly rigid, non-
shapes makes the membrane a living wall called a cell wall (Figure 1.8).
receptor site for chemical stimuli such The chemical composition of cell walls
as hormones. of these organisms differs. The plant
(f) Presence of glycolipid and glycoprotein cell wall is made up of cellulose while
enables the membrane to bind to the the fungal cell wall is made up of chitin.
membrane of a neighbouring cell, that The bacterial cell wall, is made up of
is, cell-to-cell recognition. peptidoglycan called murein.
(g) Presence of cholesterol disturbs
the close packing of phospholipids. A cell wall is a protective layer around
This keeps them more fluid and the plasma membrane. It also determines
maintains stability and flexibility of the shape of plant cells. Originally, the
the membrane over a wide range of cell wall is a product of cytoplasm.
temperature. The cytoplasmic organelles such as
the endoplasmic reticulum and Golgi
Exercise 1.3 apparatus play an important role in the
formation of the cell wall. The formation
1. Describe the structure of a plasma of a new cell wall usually takes place in a
membrane as proposed by Danielli- dividing cell immediately after the nuclear
Davson's model. division.
2. Describe the structure of a cell
membrane as envisaged by Jonathan Structure of the cell wall
Singer and Garth Nicolson. Plant cell wall, which is made up of
cellulose and strengthened by other
3. Name the chemical constituents of polysaccharides, is a protective layer
cell membranes and enumerate the around the cell surface membrane. The
role of each. primary cell wall in plants is composed
of cellulose, hemicelluloses and pectin.
4. How is the cell membrane adapted This primary wall is laid down during cell
to its functions? division in plants. The primary cell wall

5. The dynamic properties and functions thickens into secondary cell wall. During
of cell membranes are essential for secondary thickening, some cells undergo
maintaining the internal environment lignification (hardening) whereby lignin
is deposited in all cellulose layers. Lignin
of the cell. Explain how cell cements and anchors fibres together,
membranes maintain homeostatic

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