Page 91 - Biology Form Two

Biology for Secondary Schools

Structure and functions of root root hairs are always in search of
hairs water supply. If root hair cells have a
Root hairs are extensions of the higher concentration of mineral salts
epidermal cells of the root. Figure 4.12 than their surroundings, mineral salts
shows the structure of a root hair. are absorbed by active transport. Root
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hairs are very thin to provide a short

Epidermal cells Soil particles distance over which the absorbed water
and mineral salts have to travel to reach
Cell membrane
the cell.

Cytoplasm Cell vacuole As the main roots elongate, the older
Nucleus root hairs die and are replaced with
Cellulose cell wall new ones. This helps the plant to have

Figure 4.12: Structure of a root hair a constant supply of water and mineral
salts from the soil.
Root hairs are long and slender to
provide a large surface area for the Movement of water and dissolved

absorption of water and mineral salts mineral salts
from the soil. The large number of When water is absorbed by the root
root hairs increases the total surface hairs, it dilutes the contents of the
area of the roots for the absorption cell sap vacuole. This causes the cells
of water and mineral salts. Root of the cortex, which are adjacent to
hairs contain vacuoles filled with cell the epidermis to have less water than
sap. The root hair cell sap is usually the root hair cells, hence hypertonic.
hypertonic; that is, more concentrated Therefore, water moves from the root
than the surrounding water. Hence, hair cells to the cortex cells by osmosis.

water enters the root hair cells by It moves the same way from the cortex
osmosis. They have permeable cell into the cells of the endodermis, then
walls which are hydrophilic in nature. into the pericycle and then into the
Hydrophilic means water loving. Thus, xylem vessel (Figure 4.13).

Student’s Book - Form Two 85

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BIOLOGY FORM 2 NEW.indd 85