ICSE 7 Biology Photosynthesis Basic

Photosynthesis & Respiration

1.0 Autotrophic Nutrition: Photosynthesis

Definition: Photosynthesis is the process by which green plants (autotrophs) synthesize food (carbohydrates) from carbon dioxide and water using solar energy captured by chlorophyll.

Key Components:

• 🌿 Chlorophyll: A green pigment found in leaves that captures the sun’s energy.
• ☀️ Sun: The ultimate source of energy for all plants.
• 🔋 Starch: The form in which carbohydrates are stored in plants after photosynthesis.

The Photosynthesis Equation:
Carbon Dioxide + Water → (Sunlight/Chlorophyll) → Glucose + Oxygen

2.0 Stomata: The Respiratory Pores

Definition: Stomata are tiny pores found primarily on the underside (lower surface) of leaves that allow for gaseous exchange (entry of $CO_2$ and release of $O_2$).

Structure of Stomata:

• Guard Cells: Two bean-shaped cells surrounding the stomatal opening.
• Chloroplasts: Present within the guard cells.
• Cell Walls: Guard cells have a thin outer wall and a thick inner wall.

Regulation (Opening and Closing):

Phase	Mechanism	Result
Day (Sunlight)	Water moves into guard cells; they swell up (Turgid).	Inner walls move apart; Stoma Opens.
Night	Water moves out of guard cells; they shrink (Flaccid).	Inner walls come together; Stoma Closes.

3.0 Site of Photosynthesis: The Chloroplast

The Chloroplast is the cell organelle responsible for photosynthesis. It is divided into two distinct functional regions:

• 1. Grana (Light Reaction): Reactions dependent on sunlight. Water molecules break down into Hydrogen ions and Oxygen. Chemical energy is stored as ATP.
• 2. Stroma (Dark Reaction): Light-independent reactions. Uses chemical energy from the grana to convert Carbon Dioxide into Starch.

📅 Key Discovery: Julius Von Sachs (1854) discovered that chlorophyll is located in chloroplasts and that glucose is the first product of photosynthesis, stored as starch.

4.0 Factors Affecting Photosynthesis

The rate of photosynthesis is influenced by several environmental factors. If these factors are not optimal, the plant's ability to manufacture food is reduced.

• ☀️ Light: Photosynthesis occurs only up to a certain concentration of light. Above that level, chlorophyll is destroyed and the process is negatively affected.
• 🌡️ Temperature: Photosynthesis is enzyme-dependent. It does not take place at very high temperatures, and the rate is significantly lower at very low temperatures.
• ☁️ Carbon Dioxide (CO₂): Higher levels of CO₂ generally increase the rate. Low levels lead to a lower rate of photosynthesis.

5.0 Experiments Related to Photosynthesis

Note: To test for food (starch), we use Iodine Solution, which turns blue-black in the presence of starch.

🔬 5.1 Sunlight is Essential

Procedure:

1. Destarch a healthy green potted plant by keeping it in a dark room for 1-2 days.
2. Cover a portion of a leaf on both sides with black paper using paper clips.
3. Expose the plant to bright sunlight for a few hours.
4. Remove the leaf, decolourize it with alcohol, and test with iodine solution.

Observation: The portion covered with black paper does not turn blue-black.
Explanation: Only parts exposed to sunlight could photosynthesize and store starch.

🔬 5.2 Chlorophyll is Essential

Definition: A Variegated Leaf is a leaf that has both green and non-green areas (e.g., Croton or Money plant).

Procedure:

1. Destarch a variegated plant in a dark room for 2-3 days.
2. Place it in sunlight for 6 hours.
3. Pluck a leaf, trace its green areas on paper, decolourize with alcohol, and dip in iodine.

Observation: Only the green areas of the leaf turn blue-black.
Explanation: Non-green parts lack chlorophyll and cannot manufacture food.

🔬 5.3 CO₂ is Essential

Procedure:

1. Take two destarched plants (A and B).
2. Place a watch-glass containing Potassium Hydroxide (KOH) beside Plant A.
3. Cover both with separate bell jars and seal with Vaseline.
4. Expose to sunlight for 2-3 hours and test leaves for starch.

Observation: Plant B shows higher starch; Plant A shows almost none.
Explanation: KOH absorbs all CO₂ in Jar A, preventing photosynthesis.

🔬 Photosynthesis in a Laboratory (O₂ Release)

Method: Place an aquatic plant (Hydrilla) in a beaker filled with water. Cover it with a transparent funnel and invert a test tube over it.
Result: After some time in sunlight, air bubbles (Oxygen) emerge and collect in the test tube. This proves that plants release oxygen during photosynthesis.

6.0 Cellular Respiration

Definition: Cellular respiration is the process in which food (glucose) is broken down in the cells with the help of oxygen to release energy.

While breathing is the physical act of taking in air, respiration is the chemical process that happens inside every cell to power the body.

7.0 Aerobic and Anaerobic Respiration

7.1 Aerobic Respiration

Definition: The process of breakdown of food (glucose) in the presence of oxygen.
It takes place in all organisms and leads to the production of carbon dioxide, water, and a high amount of energy.

Glucose + Oxygen → Carbon Dioxide + Water + Energy

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7.2 Anaerobic Respiration

Definition: The process of breakdown of food (glucose) in the absence of oxygen.
It occurs in organisms such as Yeast and leads to the production of alcohol (ethanol) and carbon dioxide.

Glucose → Alcohol + Carbon Dioxide + Energy

[Image showing a comparison diagram between aerobic and anaerobic respiration processes]

8.0 Key Differences

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen	Occurs in presence of O₂	Occurs in absence of O₂
Energy Yield	High (38 ATP)	Low (2 ATP)
Location	Cytoplasm & Mitochondria	Only in Cytoplasm
End Products	CO₂ and Water	Alcohol or Lactic Acid

9.0 Respiration in Specific Conditions

⚡ 9.1 Muscle Cramps

During heavy exercise, the body experiences a temporary deficiency of oxygen. Muscle cells switch to anaerobic respiration to provide extra energy.
Result: This leads to the production and accumulation of Lactic Acid, which causes muscle pain and cramps.

🍞 9.2 Fermentation

Definition: The anaerobic conversion of sugar to carbon dioxide and alcohol by yeast is called Fermentation.
This process is used widely in the production of wine and beer.

[Image showing lactic acid fermentation in human muscle cells during exercise]

10.0 How Plants Breathe

Just like animals, plants respire 24/7. They do not have a single respiratory organ; instead, different parts of the plant exchange gases through specific structures.

• 🍃 Stomata: Primary pores on the surface of leaves for oxygen intake and $CO_2$ release.
• 🌳 Lenticels: Small openings or "scars" on the bark of woody trees and older roots. They consist of loosely arranged cells for gaseous exchange.
• 🌱 Root Hairs: In younger roots, oxygen is taken in from soil air spaces by the process of diffusion.
• 🌊 Pneumatophores: Specialized "breathing roots" found in mangroves or saline swamps. They emerge out of the soil to breathe through lenticels.

11.0 Proving CO₂ Production in Respiration

The Lime Water Test: Lime water turns milky in the presence of carbon dioxide.

Activity with Yeast:

1. Mix yeast with a sugar solution in a closed beaker.
2. Pass the gas produced through a tube into another beaker containing lime water.
Result: The lime water turns milky, indicating that yeast releases $CO_2$ during respiration.

12.0 Photosynthesis vs. Respiration

Feature	Photosynthesis	Respiration
Occurrence	Only in cells with chlorophyll	In all living cells
Time	Only in presence of light	Continuously (Day and Night)
Raw Materials	$CO_2$ and Water	Glucose and Oxygen
Energy Status	Manufactures food (Stores energy)	Breaks down food (Releases energy)
Gas Released	Oxygen ($O_2$)	Carbon Dioxide ($CO_2$)

⚠️ Critical Concept: Waterlogging

Too much water in soil fills up the air spaces, replacing oxygen. Without oxygen, roots cannot respire, leading to the death of the plant. This is why farmers ensure fields are not over-watered.

BASIC NOTES COMPLETED

Definitions, Processes, and Experiments successfully covered.