Page 267 - Biology_F5

Biology for Advanced Level Secondary Schools

sunlight among other reasons, it gives the end
6CO + 6H O C H O + 6O
2 2 chlorophyll 6 12 6 2 product of photosynthesis as a hexose
sugar. However, the end product is a triose
Thus, the raw materials for photosynthesis
are carbon dioxide and water. Plants obtain sugar called 3-phosphoglyceraldehyde (3-
carbon dioxide from the atmosphere, which PGAL). Moreover, the equation shows that
FOR ONLINE READING ONLY
diffuses into the leaf through stomata. Water the source of oxygen is carbon dioxide.
is normally absorbed from the soil into the This is not correct because experiments
using the two oxygen isotopes, O and
18
plant body by the roots and transported via O , show that the oxygen given off during
16
xylem tissue to the leaves which are the photosynthesis comes from water, and not
photosynthetic organs of the plants. The light from carbon dioxide. Thus, more water
energy is trapped by chloroplasts, which enters the reaction so that some of it is
contain chlorophyll, the photosynthetic evolved as a by-product, as revealed in
pigment of the leaves or stems in some a more accurate equation below. In this
plant species. One of the functions of light equation, CH O is an empirical formula of
2
energy is photolysis of water that refers the carbohydrate formed by photosynthesis.
to splitting of a water molecule in the sunlight
presence of light to produce hydrogen, 6CO + 12H O chloroplast C H O +6O + 6H O
2
6
2
6
12
2
2
oxygen, and electrons. These products The site for photosynthesis
of photolysis of water are required in the Photosynthesis takes place in the green parts
first stages of photosynthesis. The oxygen of the plants such as leaf mesophyll and
released during photosynthesis comes from bundle sheath cells which have chloroplasts.
water molecules. Two molecules of water A chloroplast is a double membranous
produce one molecule of oxygen. Light is organelle found in the cytoplasm. Its matrix
also important in producing biochemical is called stroma, which contains DNA, free
energy in plants. This energy is required in ribosomes, membrane ribosomes, starch
fixing carbondioxide into phosphoglycerate grains, lipid droplets and photosynthetic
which is rapidly converted into sugars and enzymes. The stroma also contains stacks
other forms of organic molecules, such as of flattened sacs or membranes called
starch and protein for storage and cellular thylakoids, which contain photosynthetic
uses. pigments called chlorophyll and other
accessory pigments. Thylakoids are stacked
Ultimately, carbohydrates and other forms of together to form column-like structures
organic molecules synthesised by autotrophs called grana (singular granum).
are the major source of energy for all
heterotrophs. Therefore, they are regarded Structurally, chlorophyll is a long

as energy harvesters in all ecosystems. hydrocarbon chain with a hydrophobic (water
hating) tail, a complex ring of hydrophilic
The traditional equation of photosynthesis (water loving) head in the interior. Based on
highlighted above does not illustrate the the structure, different types of chlorophylls
real situation in photosynthesis because, can be discerned. (Figure 5.1)

Form Five Student’s Book
260