Page 16 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

11. The carbohydrates exist as short branched chain of sugars attached
to proteins (glycoproteins) or lipids (glycolipids) on the outer
surface of the membrane. They function as receptors for chemicals
like hormones, adhesion to neighbouring cells and for immune
responses.

12. Functions of cell membrane: 2
(a) The membrane protects the cell. Thus, any chemical or reaction
happening outside the cell will not harm it.
(b) It serves as a boundary between the cell and its environment.
Therefore, ions outside the cell cannot easily enter the cell.
(c) It regulates or controls the passage of substances in and out of
the cells. This happens especially through the protein channels,
which allow only specific polar molecules to move in or out.
(d) It acts as receptor sites in recognising external stimuli such as
hormone and antigen molecules. This also enables the cells to
recognise other cells and so to behave in an organised manner
during the formation of tissues in the embryo.
(e) Within the cells, membranes allow compartmentalisation and
division of labour especially within membrane-bound organelles
like nucleus, mitochondrion and chloroplast.
(f) Certain membranes can perform special functions such as light
reaction in the internal membranes of chloroplast and oxidative
phosphorylation in the inner membrane of mitochondria.
(g) It helps in cell mobility such as in the white blood cells where
the membrane can carry out amoeboid movement.

Cell wall

1. The cell wall is a carbohydrate layer of cellulose found outside the
plasma membrane of plant cells.

2. There are two types of cell wall, the primary cell wall and the
secondary cell wall.
(a) Primary cell wall
The primary cell wall is found in young cells and cells that
are not highly differentiated such as meristem, parenchyma
and collenchyma. The primary cell wall has the following
characteristics:
(i) It is a thin layer, found just outside the plasma membrane
of most plant cells. It is also found on the outer layer of
cells with secondary cell wall.
(ii) It consists of randomly arranged microfibrils of cellulose
in an amorphous matrix as in Figure 2.8.

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