Plant Life

• Balanced Chemical Equation:

$$6CO_2 + 12H_2O \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \mathbf{C_6H_{12}O_6 + 6H_2O + 6O_2 \uparrow}$$

• Sources of Raw Materials:
1. Carbon Dioxide ($CO_2$): Absorbed directly from the ambient atmosphere by diffusion through open leaf stomata.
2. Water ($H_2O$): Absorbed from the soil by root hairs through osmosis and conducted upward to the leaves via the xylem vessels.
3. Sunlight: Absorbed from solar radiation by the green pigment chlorophyll situated in chloroplast thylakoid membranes.

• Keeping the plant in a completely dark room for 48 hours is called Destarching.
• In darkness, photosynthesis ceases completely ($P = 0$), but the living plant cells continue to respire continuously.
• Consequently, all pre-existing starch stored in the chloroplasts of the leaves is completely converted into soluble glucose and translocated away to roots or consumed for energy.
• This ensures that any starch detected by the iodine test at the end of the experiment was newly manufactured during the experimental period only, validating the test.

• Steps:
1. Boil in water for 2 minutes: Kills protoplasm and ruptures cell membranes to make them permeable to iodine.
2. Boil in alcohol (water bath): Dissolves out all green chlorophyll, turning the leaf pale white so color changes can be observed.
3. Rinse in warm water: Softens the brittle leaf.
4. Add dilute Iodine solution: If starch is present, the leaf stains deep blue-black.
• Why a Water Bath is Used: Alcohol (methylated spirit) is highly inflammable and volatile. If heated directly over a naked burner flame, alcohol vapors will catch fire and cause a dangerous laboratory explosion. The water bath provides gentle, flameless heating.

1. Take a destarched potted plant.
   2. Insert half of an attached leaf into a split cork fitted into a wide-mouthed conical flask containing Potassium Hydroxide solution ($KOH$), while the outer half remains exposed to open air.
   3. (KOH absorbs all carbon dioxide gas inside the flask, creating a $CO_2$-free environment for the inner half).
   4. Expose the entire apparatus to bright sunlight for 4 to 6 hours.
   5. Pluck the leaf and test it for starch with iodine solution.
   • Result: The outer half exposed to normal air turns deep blue-black (starch made), while the inner half deprived of $CO_2$ remains brownish-yellow (no starch).
   • Conclusion: Carbon dioxide is strictly necessary for photosynthesis.

1. Oxygen Requirement: Aerobic respiration occurs strictly in the presence of oxygen; Anaerobic respiration occurs in the complete absence of oxygen.
   2. Breakdown of Glucose: Aerobic causes complete oxidation into $CO_2$ and $H_2O$; Anaerobic causes incomplete breakdown into Ethyl Alcohol (Ethanol, $C_2H_5OH$) and $CO_2$.
   3. Energy Yield: Aerobic respiration yields $38\text{ ATP}$ molecules ($2830\text{ kJ}$) per glucose molecule; Anaerobic respiration releases only a meager $2\text{ ATP}$ molecules ($118\text{ kJ}$).
   4. Site: Aerobic occurs in cytoplasm and mitochondria; Anaerobic occurs entirely in the cytoplasm.

• During the night, in the absence of sunlight, photosynthesis stops completely ($P = 0$), so no oxygen is produced.
• However, the immense biomass of the large tree continues to undergo respiration, continuously absorbing oxygen and releasing large volumes of carbon dioxide ($CO_2$) into the air beneath its canopy.
• A person sleeping under the tree can experience oxygen deficiency, breathing distress, headache, and suffocation due to localized $CO_2$ accumulation.

• Definition: The Compensation Point is the specific light intensity at which the rate of Photosynthesis is strictly equal to the rate of Respiration ($P = R$).
• At this point, the volume of oxygen released by photosynthesis is exactly consumed by respiration, and the carbon dioxide released by respiration is exactly consumed by photosynthesis.
• There is zero net gas exchange between the plant and the environment.
• Time of Occurrence: It occurs twice daily—during Dawn (early morning) and Dusk (early evening) when ambient light intensity is low.

• Opening (Daytime): In light, guard cells photosynthesize, accumulating sugars and potassium ions ($K^+$). This draws water into the guard cells by endosmosis, making them turgid. Because their outer lateral cell walls are thin and elastic while their inner pore walls are thick and inelastic, the outer walls bulge outward, pulling the inner walls apart to open the stomatal pore.
• Closing (Nighttime): In darkness, photosynthesis stops. Water exits the guard cells by exosmosis, causing them to become flaccid. The elastic inner walls spring back to their straight resting positions, closing the stomatal pore to prevent water loss.