⚡ Quick Revision: Photosynthesis Basics
🔹 The Process of Food Synthesis
- ✔ Definition: The process by which living plant cells containing chlorophyll produce food (glucose) using $CO_2$ and $H_2O$ in the presence of light.
- ✔ The Equation: $$6CO_2 + 12H_2O$$ $$\xrightarrow[\text{Chlorophyll}]{\text{Light Energy}}$$ $$C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow$$
- ✔ Importance: Provides food for all living beings and maintains the oxygen-carbon dioxide balance in the atmosphere.
🔹 Site of Photosynthesis: Chloroplast
- ✔ Grana: Piles of thylakoids containing chlorophyll; the site for Light Reactions.
- ✔ Stroma: The colorless ground substance; the site for Dark Reactions.
- ✔ Chlorophyll: The magnesium-containing green pigment that traps solar energy.
Photolysis: The splitting of water molecules into hydrogen ions and oxygen in the presence of light and grana.
Chloroplast with Chlorophyll. Remember: Chloroplast is the organelle (the "kitchen"), while Chlorophyll is the pigment (the "stove" that traps heat).
⚡ Quick Revision: Two Phases of Photosynthesis
🔹 Phase 1: Light-Dependent Reaction (Photochemical)
Occurs in the Grana of the chloroplast. It requires direct light energy.
- ✔ Activation: Chlorophyll absorbs photons and becomes "excited."
- ✔ Photolysis: Water splits into $H^+$, electrons ($e^-$), and Oxygen ($O_2$).
- ✔ Formation of ATP & NADPH: Solar energy is stored as chemical energy (Assimilatory Power).
🔹 Phase 2: Light-Independent Reaction (Biosynthetic)
Occurs in the Stroma. It does not require light but depends on the products of the light reaction.
- ✔ $CO_2$ Fixation: $CO_2$ is converted into glucose using ATP and NADPH.
- ✔ Polymerization: Several glucose molecules transform into Starch for storage.
Photophosphorylation: The process of converting ADP to ATP using light energy during the light-dependent phase.
Dark Reaction with "Night Reaction." Remember: The Dark reaction happens simultaneously with the Light reaction during the day; it just doesn't use light directly.
⚡ Quick Revision: Factors & Leaf Adaptations
🔹 External Factors
- ✔ Light Intensity: Rate increases with intensity but stabilizes at a point (Light Saturation).
- ✔ $CO_2$ Concentration: Increasing $CO_2$ up to 0.1% increases the rate; beyond this, it becomes inhibitory.
- ✔ Temperature: Optimum temperature is around 35°C. Very high temps denature enzymes.
🔹 Adaptations of Leaf for Photosynthesis
- ✔ Large Surface Area: For maximum light absorption.
- ✔ Leaf Mosaic: Arrangement of leaves to avoid shading each other.
- ✔ Extensive Vein System: Rapid transport to and from mesophyll cells.
Limiting Factor: A factor which is in shortest supply and directly affects the rate of a process (e.g., light at dawn/dusk).
Starch with Glucose. Remember: Glucose is the immediate product; Starch is the storage form. Test for "Destarching" a leaf involves keeping it in the dark for 24-48 hours.
⚡ Quick Revision: Photosynthesis Experiments
🔹 The Iodine Test for Starch
The standard procedure to check if photosynthesis has occurred.
- 1 Boil in Water: To kill cells and make the leaf permeable.
- 2 Boil in Methylated Spirit: To remove chlorophyll (leaf becomes pale white).
- 3 Add Iodine: Blue-black color indicates starch; brown color indicates no starch.
🔹 Essential Factors Experiments
-
1. Necessity of $CO_2$ (Moll's Half-leaf Experiment):
One half of a leaf is inserted into a flask containing KOH (Potassium Hydroxide). KOH absorbs $CO_2$. That part remains brown with iodine. -
2. Necessity of Sunlight:
Covering a part of a destarched leaf with black paper. The covered part does not turn blue-black. -
3. Necessity of Chlorophyll:
Using Variegated leaves (e.g., Coleus or Money plant). Only green parts turn blue-black.
Destarching: Keeping a plant in the dark for 24-48 hours so that all stored starch is used up or translocated to storage organs.
When testing for Oxygen production, use an aquatic plant like Hydrilla. The presence of gas bubbles in the funnel proves oxygen is evolved during photosynthesis.