Plant Physiology: Photosynthesis

1. The Process and Importance of Photosynthesis

Photosynthesis is a fundamental biochemical process by which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy. This chemical energy is stored in organic compounds, primarily glucose, synthesized from carbon dioxide and water. Oxygen is released as a byproduct.

1.1. Overall Balanced Chemical Equation

The overall balanced chemical equation representing photosynthesis is:

6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Light Energy → C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)

This equation summarizes a complex series of reactions, highlighting the inputs (carbon dioxide, water, light energy) and outputs (glucose, oxygen).

1.2. Importance to Life in General

Photosynthesis is arguably the most important biological process on Earth due to its profound impact on all life forms:

• Primary Energy Source: It is the ultimate source of energy for almost all living organisms. Photosynthetic organisms (producers) form the base of most food chains, providing food for herbivores, which in turn are consumed by carnivores.
• Oxygen Production: Photosynthesis is responsible for maintaining the oxygen levels in the Earth's atmosphere. Oxygen is essential for aerobic respiration, the process by which most organisms release energy from food.
• Carbon Dioxide Regulation: It removes vast amounts of carbon dioxide from the atmosphere, helping to regulate the Earth's climate and mitigate the greenhouse effect.
• Formation of Fossil Fuels: Over geological time, the organic matter produced by ancient photosynthetic organisms has been converted into fossil fuels (coal, oil, natural gas), which are major energy sources for human civilization.

2. Site of Photosynthesis: The Chloroplast

Photosynthesis primarily occurs in specialized organelles called chloroplasts, found mainly in the mesophyll cells of plant leaves.

2.1. Internal Structure of Chloroplast

A chloroplast is a double-membraned organelle with a complex internal structure optimized for photosynthesis:

• Outer Membrane: Permeable to small molecules.
• Inner Membrane: Regulates the passage of materials into and out of the chloroplast.
• Stroma: The fluid-filled space enclosed by the inner membrane. It contains enzymes, ribosomes, and DNA. The dark reactions (biosynthetic phase) of photosynthesis occur here.
• Thylakoids: A system of interconnected flattened sacs or discs suspended within the stroma. The light reactions (photochemical phase) occur on the thylakoid membranes.
• Grana (singular: Granum): Stacks of thylakoids. Each granum is like a stack of coins.
• Stroma Lamellae (Intergranal Thylakoids): Unstacked thylakoids that connect adjacent grana.
• Chlorophyll: The green photosynthetic pigment, along with other accessory pigments, is embedded in the thylakoid membranes.

3. Phases of Photosynthesis

Photosynthesis is broadly divided into two main phases:

3.1. Light-Dependent Reactions (Photochemical Phase)

These reactions require light energy and occur on the thylakoid membranes of the chloroplast. They are called the photochemical phase because light energy is directly used to drive chemical reactions.

1. Activation of Chlorophyll: When light energy strikes the chlorophyll molecules in the photosystems (clusters of pigments and proteins) embedded in the thylakoid membranes, the chlorophyll molecules absorb this energy and become excited or 'activated'.
2. Photolysis of Water: The absorbed light energy is used to split water molecules (H₂O) into their components: protons (H+), electrons (e-), and oxygen gas (O₂). This process is called photolysis (photo = light, lysis = splitting). The oxygen released is the oxygen we breathe.
   • 2H₂O → 4H⁺ + 4e⁻ + O₂
3. Electron Transport and ATP Formation (Photophosphorylation): The excited electrons from chlorophyll are passed along an electron transport chain embedded in the thylakoid membrane. As electrons move down the chain, their energy is used to pump protons, creating a proton gradient across the thylakoid membrane. This gradient is then used by ATP synthase to produce ATP (adenosine triphosphate) from ADP and inorganic phosphate. This process of ATP synthesis using light energy is called photophosphorylation.
4. NADPH Formation: At the end of the electron transport chain, the electrons, along with the protons (H+) from water photolysis, are used to reduce NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH. NADPH is an energy-rich molecule that carries reducing power.

Summary of Light Reactions: Light energy is converted into chemical energy in the form of ATP and NADPH, and oxygen is released.

3.2. Light-Independent Reactions (Biosynthetic Phase / Calvin Cycle)

These reactions do not directly require light but depend on the ATP and NADPH produced during the light reactions. They occur in the stroma of the chloroplast and are known as the biosynthetic phase because complex organic molecules (sugars) are synthesized.

1. Carbon Fixation: Carbon dioxide (CO₂) from the atmosphere enters the stroma and is combined with an existing five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), by the enzyme RuBisCO. This forms an unstable six-carbon compound that immediately splits into two molecules of a three-carbon compound, 3-phosphoglycerate (PGA).
2. Reduction: The ATP and NADPH produced during the light reactions are used to convert PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This step involves the reduction of carbon compounds.
3. Regeneration of RuBP: Most of the G3P molecules are used to regenerate RuBP, allowing the cycle to continue. Some G3P molecules are used to synthesize glucose and other organic compounds (e.g., sucrose, starch, cellulose).

Summary of Dark Reactions: Carbon dioxide is fixed and converted into glucose using the energy (ATP) and reducing power (NADPH) generated during the light reactions.

4. Adaptations in Plants for Photosynthesis

Plants have evolved numerous adaptations to optimize their photosynthetic efficiency:

• Leaf Structure:
  • Large Surface Area: Broad, flat leaves maximize the absorption of sunlight.
  • Thinness: Allows for rapid diffusion of CO₂ into the leaf and O₂ out of the leaf.
  • Cuticle: A waxy layer that reduces water loss, but is transparent to light.
  • Epidermis: Transparent outer layer allowing light to pass through.
  • Palisade Mesophyll: Tightly packed, elongated cells rich in chloroplasts, located near the upper surface for maximum light absorption.
  • Spongy Mesophyll: Loosely packed cells with large air spaces for efficient gas exchange (CO₂ and O₂).
• Stomata: Pores on the leaf surface (mostly on the lower epidermis) that regulate the entry of CO₂ and the release of O₂ and water vapor. Their opening and closing are crucial for balancing CO₂ uptake and water loss.
  • Opening and Closing of Stomata (briefly, based on K+ ion exchange theory): In light, K+ ions are actively pumped into guard cells, lowering their water potential. Water enters by osmosis, making guard cells turgid and causing the stomata to open. In darkness or water stress, K+ ions move out, guard cells become flaccid, and stomata close.
• Chloroplasts: Abundant in photosynthetic cells, especially in the palisade layer, and can orient themselves to maximize light absorption.
• Vascular Bundles (Veins): Contain xylem (for water transport) and phloem (for sugar transport), ensuring efficient delivery of water and removal of synthesized sugars.
• Root System: Efficiently absorbs water and minerals necessary for photosynthesis.

5. Experiments to Show the Necessity of Factors for Photosynthesis

Before conducting most photosynthesis experiments, plants are often destarched. This involves keeping the plant in darkness for 24-48 hours, forcing it to use up any stored starch. This ensures that any starch detected after the experiment was newly formed during the experimental period.

Steps involved in the Starch Test (Iodine Test):

1. Boil the leaf in water: To kill the cells and break down cell membranes, making them permeable.
2. Boil the leaf in alcohol (ethanol): To remove chlorophyll. This is done in a water bath to prevent the alcohol from catching fire. Removing chlorophyll allows the color change of the iodine to be clearly visible.
3. Wash the leaf in cold water: To soften the leaf and remove any remaining alcohol.
4. Add iodine solution: Place the leaf on a white tile and add a few drops of iodine solution.
   • Observation: If starch is present, the leaf will turn blue-black. If no starch is present, it will remain yellowish-brown (the color of iodine).

5.1. Experiment to Show Necessity of Light

Procedure:

1. Destarch a potted plant.
2. Cover a part of one leaf on both sides with a black paper or aluminum foil clip, ensuring no light reaches the covered part.
3. Expose the plant to sunlight for several hours.
4. Perform the starch test on the covered and uncovered parts of the leaf.

Observation: Only the uncovered part of the leaf will turn blue-black, indicating starch formation. The covered part will remain yellowish-brown, showing that light is necessary for photosynthesis.

5.2. Experiment to Show Necessity of Carbon Dioxide

Procedure:

1. Destarch two potted plants of similar size.
2. Place one plant under a bell jar with a watch glass containing potassium hydroxide (KOH) solution (which absorbs CO₂).
3. Place the second plant under another bell jar with a watch glass containing plain water (as a control).
4. Seal the bell jars to make them airtight.
5. Expose both setups to sunlight for several hours.
6. Perform the starch test on a leaf from each plant.

Observation: The leaf from the plant in the bell jar with KOH will not show starch (yellowish-brown), while the leaf from the control plant will turn blue-black. This demonstrates that carbon dioxide is necessary for photosynthesis.

5.3. Experiment to Show Necessity of Chlorophyll

Procedure:

1. Take a destarched potted plant with variegated leaves (e.g., Coleus or Croton, which have green and non-green/white patches).
2. Expose the plant to sunlight for several hours.
3. Draw an outline of the leaf, marking the green and non-green areas.
4. Perform the starch test on the leaf.

Observation: Only the green parts of the leaf will turn blue-black, while the non-green (white) parts will remain yellowish-brown. This proves that chlorophyll is essential for photosynthesis.

5.4. Experiment to Show Formation of Starch

This is implicitly shown by all the above experiments. If the necessary conditions (light, CO₂, chlorophyll) are met, starch is formed, as indicated by the positive iodine test (blue-black color).

5.5. Experiment to Show Release of Oxygen

Procedure:

1. Take an aquatic plant (e.g., Hydrilla or Elodea) and place it in a beaker containing pond water.
2. Invert a funnel over the plant and place a test tube filled with water over the stem of the funnel.
3. Place the entire setup in sunlight.

Observation: Bubbles will be seen rising from the plant and collecting in the inverted test tube. When a glowing splint is introduced into the test tube, it will rekindle, confirming the presence of oxygen. This demonstrates that oxygen is released during photosynthesis.

6. The Carbon Cycle

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Photosynthesis plays a central role in this cycle.

6.1. Diagrammatic Representation of the Carbon Cycle (Detailed Description)

Imagine carbon moving through different reservoirs on Earth:

1. Atmospheric Carbon Dioxide (CO₂): Carbon exists in the atmosphere primarily as carbon dioxide gas. This is the main source of carbon for photosynthesis.
2. Photosynthesis (Carbon Fixation): Plants and other photosynthetic organisms absorb atmospheric CO₂. Using light energy, they convert this inorganic carbon into organic compounds (glucose, starch, cellulose, etc.). This process removes carbon from the atmosphere and incorporates it into living matter.
3. Consumption (Food Chains): When herbivores eat plants, the carbon compounds are transferred to their bodies. Carnivores then eat herbivores, further transferring carbon up the food chain. Thus, carbon moves through the biotic components of the ecosystem.
4. Respiration: All living organisms (plants, animals, microbes) perform cellular respiration. During respiration, organic carbon compounds are broken down to release energy, and CO₂ is released back into the atmosphere (or water, if aquatic respiration).
5. Decomposition: When plants and animals die, decomposers (bacteria and fungi) break down their organic remains. During decomposition, carbon is released as CO₂ through microbial respiration, returning it to the atmosphere and soil.
6. Fossil Fuel Formation: Over millions of years, under specific geological conditions (high pressure and temperature), dead organic matter that did not fully decompose can be transformed into fossil fuels (coal, oil, natural gas). This process locks carbon away in the geosphere.
7. Combustion: The burning of fossil fuels (for energy, transportation, industry) releases large amounts of stored carbon back into the atmosphere as CO₂. Natural combustion (e.g., forest fires) also releases CO₂.
8. Oceanic Carbon: Oceans act as a major carbon sink. CO₂ from the atmosphere dissolves in ocean water. Marine organisms use this dissolved carbon to build shells and skeletons (e.g., calcium carbonate). When these organisms die, their remains can form sedimentary rocks (like limestone), storing carbon for long periods. Volcanic activity also releases CO₂.

In essence, the carbon cycle is a continuous loop: Carbon moves from the atmosphere to living organisms (photosynthesis), through food chains, back to the atmosphere (respiration, decomposition, combustion), and also cycles through oceans and geological reservoirs. Photosynthesis is the crucial step that brings atmospheric carbon into the biological world, making it available for all other life forms.