Page 125 - General and Inorganic Chemistry for Advanced Secondary Schools Students Book Form Five and Six

Biology for Secondary Schools

The light reaction or light dependent the dark reaction stage, hydrogen
reaction stage ions from light reaction stage and
This stage takes place in specialised carbon dioxide from the atmosphere
structures of the chloroplast called combine to form glucose that is the
grana. The grana (singular is granum) simplest carbohydrate. Glucose is
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contain chlorophyll that absorbs light later converted into starch. Starch
energy from the sun. The light dependent is insoluble and acts as a temporary
reaction changes light energy into storage of excess carbohydrates. It can
chemical energy. The formed energy is easily be converted into glucose when
stored in a chemical compound called
ATP (Adenosine Triphosphate). This a plant needs energy. Glucose may
energy is used in the dark reaction also be converted into cellulose (the
stage of photosynthesis. structural material of plants). Lipids
and proteins are also formed from
Light energy causes photolysis, a glucose through complex processes.
chemical process whereby water
molecules (H O) are split into hydrogen Activity 6.3: An experiment to
2
–
+
ions (H ) and hydroxyl ions (OH ). This determine the presence of starch in a
can be represented using the following leaf
equation.
Materials: A leaf, absolute ethanol,
+
lightenergy
H O → H + OH − iodine solution, source of heat, test
2
The hydroxyl ions undergo further tube, beaker, dropper, forceps, water,
reactions to produce water and white tile, cotton wool, notebook, and
oxygen. Some oxygen is released pencil
into the atmosphere. The rest of the
oxygen is used for respiration in the Procedure
plant. Hydrogen ions are used in the 1. Take a leaf from a plant that has
dark reaction stage to synthesise food been in the sunlight for at least 6
together with carbon dioxide from the hours.
air.
2. Put the leaf in boiling water for
The dark reaction or light 2-3 minutes, as shown in Figure
independent reaction stage 6.3. This stops further chemical
This stage takes place in the stroma reactions in the leaf because
in the absence of light. The stroma is boiling kills the leaf cells.
a colourless matrix of fine material
found in the chloroplast. During

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