Page 25 - Ranger SPM 2022 Biology

Biology SPM Chapter 2 Leaf Structure and Function
Mechanism of Photosynthesis
1. There are two main stages in photosynthesis, namely light-dependent reactions,
and light-independent reactions (Figure 2.9).
2. Light-dependent reactions:
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• The light-dependent reactions occur in thylakoid.
• During the light-dependent reactions, photosynthetic pigments on the surface
of thylakoid absorb light energy.
• Light energy is used to excite electrons in the chlorophyll pigments to a higher
level.
• The excited electrons from the chlorophyll will pass a series of electron carriers.
Energy produced from the electrons are used to generate energy in the form of
ATP.
• These electrons will finally be accepted by the final electron carrier, namely
NADP .
+
• Simultaneously, light energy will also be used to breakdown water molecule to
hydrogen ion (H ) and hydroxyl ion (OH ) by photolysis of water.
+
-
• NADP combines with hydrogen ion to from NADPH. Hydroxyl ion loses an electron
+
to form oxygen and water.
Form 5
• The products of light-dependent reactions are ATP, NADPH, and oxygen. ATP
and NADPH then enters light-independent reactions, whereas oxygen is released
to the atmosphere.
3. Light-independent reactions:
• The light-independent reactions occur in stroma.
• During the light-independent reactions, carbon dioxide binds to the 5-carbon
organic compound to form a 6-carbon organic compound.
• NADPH and ATP produced from the light-dependent reactions reduce the
6-carbon organic compound to a glucose monomer.
• Glucose monomer is then condensed to form starch molecule which is kept in
stroma as granules.
4. The overall reaction of photosynthesis can be represented in the following equation:

Light energy
6H O + 6CO C H O + 6O
2
6
12
2
2
6
Water Carbon dioxide Chlorophyll Glucose Oxygen
Quiz

Photosynthesis What is the ratio of carbon dioxide
to water used in photosynthesis?
VIDEO

220

F5 Chapter 2.indd 220 3/29/22 4:35 PM