Page 311 - Biology_F5

Biology for Advanced Level Secondary Schools

6.2 Gas exchange in plants woody stems, and roots of dicotyledonous
flowering plants. Like the stomata, it
Task 6.4 functions as a pore that provides a pathway
Search from the internet sources the for direct exchange of gases between
simulations or videos on gas exchange internal tissues and the atmosphere.
in plants including the mechanism
of stomatal opening and closing, and Stomatal opening and closing
FOR ONLINE READING ONLY
prepare short notes on what you have The opening and closing of stomata
observed. depends on changes in the turgor pressure
of the guard cells. When water flows into
Plants obtain oxygen and carbondioxide the guard cells by osmosis, their turgor
mainly through their leaves. They also increases and they expand. Due to the
use lenticels (raised pores found on stems relatively inelastic inner wall, the guard
of woody plants) for exchanging minute cells bend and draw away from each other,
amounts of gases. They require oxygen allowing the opening of the pore, hence
for respiration and carbon dioxide for the air passes into the leaf. If the guard
photosynthesis. These gases diffuse into cells lose water, the opposite happens and
the intercellular spaces of the leaf through the pore closes. The guard cells lower
stomatal pores, which are normally found their water potential in order to draw in
on the underside of the leaf and then water from the surrounding epidermal
gases diffuse into the cells that require cells by actively accumulating potassium
them (Figure 6.8). The lenticel is a porous ions. This process requires energy in the
tissue, consisting of cells with large form of ATP. The energy is supplied by
intercellular spaces in the periderm of the mitochondria that are found in the
secondary thickened organs; the bark of
guard cells.

Waxy cuticle

Upper epidermis

Palisade
mesophyll Air space

Vascular bundle
containing xylem and

Spongy phloem
mesophyll
O 2

Guard cell with CO Lower epidermis
chloroplasts Stoma 2

Figure 6.8: Internal structure of a leaf showing the position of stomata for gas exchange

Form Five Student’s Book
304