Photosynthesis Early Experiments & Factors affecting - Biology Class 11 - NEET

Photosynthesis is a physico-chemical process by which green plants synthesize organic compounds using light energy. All living forms depend on sunlight for energy through photosynthesis. It serves two critical functions: it is the primary source of all food on earth and responsible for release of oxygen into the atmosphere. Green plants are called autotrophs because they synthesize their own food, while organisms depending on plants for food are heterotrophs.

1. Early Experiments in Photosynthesis

A series of simple yet elegant experiments gradually revealed the process and requirements of photosynthesis. These historical experiments form the foundation of our understanding.

1.1 Joseph Priestley's Experiment (1770)

Priestly's Experiment

• Setup: Used a bell jar with a candle, a mouse, and a mint plant in different combinations.
• Observation 1: A candle burning in a closed bell jar gets extinguished. A mouse suffocates in a closed space.
• Observation 2: When a mint plant was placed in the bell jar, the mouse stayed alive and the candle continued to burn.
• Conclusion: Plants restore to the air whatever breathing animals and burning candles remove. Breathing animals and burning candles damage the air.
• Discovery: Priestley discovered oxygen in 1774.

1.2 Jan Ingenhousz's Experiment (1730-1799)

Ingenhousz's Experiment Setup

• Setup: Used similar setup as Priestley but placed it once in dark and once in sunlight.
• Key Experiment: Used an aquatic plant and observed bubble formation.
• Observation: In bright sunlight, small bubbles formed around the green parts of the plant. In the dark, no bubbles formed.
• Identification: Later identified these bubbles as oxygen.
• Conclusion: Sunlight is essential for the plant process that purifies air. Only the green part of plants can release oxygen.

1.3 Julius von Sachs's Discovery (1854)

• Evidence Provided: Production of glucose when plants grow.
• Storage Form: Glucose is usually stored as starch.
• Localization: The green substance (chlorophyll) is located in special bodies called chloroplasts within plant cells.
• Site of Synthesis: Glucose is made in the green parts of plants and stored as starch.

1.4 T.W. Engelmann's Experiment (1843-1909)

• Setup: Used a prism to split light into spectral components. Illuminated green alga Cladophora placed in a suspension of aerobic bacteria.
• Purpose of Bacteria: Used to detect sites of O₂ evolution.
• Observation: Bacteria accumulated mainly in the region of blue and red light of the split spectrum.
• Conclusion: Described the first action spectrum of photosynthesis. It resembles the absorption spectra of chlorophyll a and b.
• Significance: Showed which wavelengths of light are most effective for photosynthesis.

1.5 Cornelius van Niel's Contribution (1897-1985)

• Background: A microbiologist who studied purple and green bacteria.
• Key Finding: Photosynthesis is essentially a light-dependent reaction where hydrogen from a suitable oxidizable compound reduces CO₂ to carbohydrates.
• General Equation: 2H₂A + CO₂ → 2A + [CH₂O] + H₂O (in presence of light)
• In Green Plants: H₂O is the hydrogen donor and is oxidized to O₂.
• In Purple and Green Sulphur Bacteria: H₂S is the hydrogen donor; oxidation product is sulphur or sulphate, not O₂.
• Critical Inference: The O₂ evolved by green plants comes from H₂O, not from CO₂.
• Proof: Later proved using radioisotopic techniques.

1.6 Overall Photosynthesis Equation

By the middle of the nineteenth century, key features were known: plants use light energy to make carbohydrates from CO₂ and water.

• Empirical Equation: CO₂ + H₂O → [CH₂O] + O₂ (in presence of light), where [CH₂O] represents a carbohydrate.
• Correct Overall Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in presence of light)
• Note: Twelve molecules of water are used as substrate because during the process, 6 molecules of water are used and 6 molecules are produced.
• Source of O₂: The O₂ released is from water, proved using radioisotope techniques.
• Important: This is not a single reaction but a multistep process.

2. Factors Affecting Photosynthesis

Understanding factors that affect photosynthesis is necessary to determine the yield of plants including crop plants. Several factors, both internal and external, influence the rate of photosynthesis.

2.1 Classification of Factors

2.1.1 Internal (Plant) Factors

• Number of leaves: More leaves provide greater surface area for photosynthesis.
• Size of leaves: Larger leaves capture more light.
• Age of leaves: Young mature leaves photosynthesize most efficiently.
• Orientation of leaves: Affects light interception.
• Mesophyll cells: Number and arrangement affect CO₂ diffusion.
• Chloroplasts: Number and size determine photosynthetic capacity.
• Internal CO₂ concentration: Depends on stomatal opening and respiration rate.
• Amount of chlorophyll: More chlorophyll captures more light.
• Genetic predisposition: Determines inherent photosynthetic capacity.
• Growth of plant: Affects overall photosynthetic machinery.

2.1.2 External Factors

• Availability of sunlight: Provides energy for photosynthesis.
• Temperature: Affects enzyme activity in dark reactions.
• CO₂ concentration: Raw material for photosynthesis.
• Water: Raw material and maintains turgidity.

2.2 Blackman's Law of Limiting Factors (1905)

When several factors affect any biochemical process, the rate is determined by the factor available at sub-optimal levels.

• Statement: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value.
• It is the factor which directly affects the process if its quantity is changed.
• Example: Despite presence of a green leaf, optimal light and CO₂, the plant may not photosynthesize if temperature is very low. Temperature becomes the limiting factor.
• Application: At any point, several factors interact simultaneously, but usually one factor is the major cause limiting the rate.

2.3 Light as a Limiting Factor

Three aspects of light must be distinguished: light quality, light intensity, and duration of exposure.

2.3.1 Light Intensity

• At Low Light Intensities: There is a linear relationship between incident light and CO₂ fixation rates.
• At Higher Light Intensities: The rate does not show further increase as other factors become limiting (reaches a plateau).
• Light Saturation Point: Occurs at 10% of full sunlight.
• In Nature: Except for plants in shade or dense forests, light is rarely a limiting factor.
• Beyond Saturation: Increase in incident light beyond a point causes breakdown of chlorophyll and a decrease in photosynthesis.

Graph of light intensity on the rate of photosynthesis

2.4 Carbon Dioxide Concentration

CO₂ is the major limiting factor for photosynthesis.

2.4.1 Atmospheric CO₂ Levels

• Normal Atmospheric Concentration: Between 0.03% and 0.04% (very low).
• Optimal Increase: Increase up to 0.05% can cause an increase in CO₂ fixation rates.
• Beyond 0.05%: Levels can become damaging over longer periods.

2.4.2 Differential Response of C₃ and C₄ Plants

• At Low Light Conditions: Neither C₃ nor C₄ plants respond to high CO₂ conditions.
• At High Light Intensities: Both C₃ and C₄ plants show increase in rates of photosynthesis.
• C₄ Plants Saturation: Show saturation at about 360 µL L⁻¹.
• C₃ Plants Saturation: Respond to increased CO₂ concentration; saturation seen only beyond 450 µL L⁻¹.
• Current Limitation: Current availability of CO₂ levels is limiting to C₃ plants.

2.4.3 Agricultural Application

• Greenhouse Crops: C₃ plants like tomatoes and bell pepper respond to higher CO₂ concentration.
• CO₂ Enriched Atmosphere: These crops are allowed to grow in CO₂ enriched atmosphere.
• Result: Shows increased rates of photosynthesis leading to higher productivity and yields.

2.5 Temperature

2.5.1 Effect on Photosynthetic Reactions

• Dark Reactions: Being enzymatic, they are temperature controlled (highly sensitive).
• Light Reactions: Also temperature sensitive but affected to a much lesser extent.

2.5.2 Differential Response of C₃ and C₄ Plants

• C₄ Plants: Respond to higher temperatures and show higher rate of photosynthesis.
• C₃ Plants: Have a much lower temperature optimum.

2.5.3 Habitat Adaptation

• Tropical Plants: Have a higher temperature optimum.
• Temperate Climate Plants: Have a lower temperature optimum.
• Reason: Temperature optimum for photosynthesis depends on the habitat that plants are adapted to.

2.6 Water

Though water is one of the reactants in the light reaction, its effect as a factor is more through its effect on the plant rather than directly on photosynthesis.

2.6.1 Indirect Effects of Water Stress

• Stomatal Closure: Water stress causes stomata to close, hence reducing CO₂ availability.
• Wilting: Water stress makes leaves wilt, thus reducing surface area of leaves.
• Metabolic Activity: Wilting also reduces the metabolic activity of leaves.
• Overall Impact: Water affects photosynthesis indirectly by affecting stomatal opening, leaf area, and metabolism.

3. Early Simple Experiments (School Level)

Simple experiments demonstrate the basic requirements of photosynthesis: chlorophyll, light, and CO₂.

3.1 Starch Test Experiment (Variegated Leaf)

• Setup: A variegated leaf or a leaf partially covered with black paper is exposed to light.
• Test: Leaves tested for presence of starch.
• Observation: Photosynthesis occurred only in the green parts of the leaves in the presence of light.
• Conclusion: Both chlorophyll (green pigment) and light are required for photosynthesis.

3.2 CO₂ Requirement Experiment

• Setup: Part of a leaf is enclosed in a test tube containing KOH soaked cotton (which absorbs CO₂). The other half is exposed to air.
• Treatment: Setup placed in light for some time.
• Test: Test for presence of starch in both parts of the leaf.
• Observation: The exposed part tested positive for starch. The portion in the tube tested negative.
• Conclusion: CO₂ is required for photosynthesis.
• Explanation: KOH absorbed CO₂ from the air in the test tube. Without CO₂, that part of the leaf could not photosynthesize and produce starch.

Summary

• Photosynthesis is the light-driven synthesis of organic compounds from CO₂ and H₂O with the release of O₂; it occurs primarily in chloroplasts of green tissues.
• Classic experiments by Priestley and Ingenhousz established the role of plants in restoring air and the necessity of light and green tissues; later work by Sachs, Engelmann and van Niel clarified carbohydrate formation, action spectra and the source of oxygen (water).
• The rate of photosynthesis is controlled by multiple factors; according to Blackman’s Law of Limiting Factors, the most limiting factor at any time controls the rate.
• Understanding these factors has practical applications such as CO₂ enrichment in greenhouses and improving crop productivity by optimising light, water and nutrient availability.