Page 146 - Biology Form Two

Biology for Secondary Schools

Lenticels During respiration, plants use oxygen
Epidermis
gas and produce carbon dioxide gas.
This causes the concentration of
Loosely
packed cork carbon dioxide in the root cells to be
cells
higher than in the soil. Carbon dioxide
FOR ONLINE READING ONLY
Parenchyma diffuses out from the root cells to the
cells soil through the epidermis.
Figure 6.10: Structure of the lenticels
Importance of gas exchange in plants
The loose arrangement of the cells (i) It enables plants to obtain carbon
facilitates the movement of gases dioxide gas, which is one of
between them. The cells have a thin the necessary raw materials for
layer of moisture so that gases diffuse photosynthesis.
in and out in solution form.
(ii) It enables plants to obtain oxygen
The concentration of oxygen is higher in gas, which is necessary for the

the intercellular air spaces than in the cork production of energy. Energy is
cells. Oxygen, therefore, diffuses into the produced during respiration.
cork cells surrounding the lenticels. The (iii) It enables plants to remove excess
cork cells use oxygen for respiration carbon dioxide gas during the
and produce carbon dioxide. Thus, the night. If not removed, the carbon
concentration of carbon dioxide in the dioxide gas would harm the plant.
cork cells becomes higher than in the

intercellular air spaces. Carbon dioxide, Exercise 6.2
therefore, diffuses out through the cork 1. Explain clearly how each of
cells into the intercellular air spaces and the following parts of the leaf
then out through the lenticels. facilitates gas exchange:

(a) Lamina
Gas exchange through the roots
Gas exchange occurs through (b) Lower epidermis
breathing roots. Plants with breathing (c) Spongy mesophyll
roots have a very thin epidermal layer
which enables the root to carry out 2. Describe the magnified side view
gas exchange. Oxygen is at a higher of a leaf.

concentration in the soil than in the root 3. Describe the effect of opening and
cells. Therefore, oxygen diffuses into closing of the stomata on the rate
the root cells through the epidermis.

140 Student’s Book - Form Two

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