Comprehensive Notes on Photosynthesis in Higher Plants

Organizational Perspectives and the Study of Plant Physiology

• The description of the structure and variation of living organisms has historically been divided into two apparently irreconcilable perspectives on biology, based on the level of organization of life forms:
  • Organismic and Above: This perspective focuses on the organism as a whole and its interactions, leading to the development of ecology and related disciplines.
  • Cellular and Molecular: This perspective focuses on the internal components of life, leading to the development of physiology and biochemistry.
• Plant Physiology, specifically in flowering plants, bridges these perspectives. Processes such as photosynthesis, respiration, and plant growth and development are described in molecular terms but analyzed within the context of cellular activities and the organism as a whole.
• Physiological processes are also examined in relation to their environment throughout the study of this unit.

The Scientific Legacy of Melvin Calvin

• Melvin Calvin (1911–1997) was born in Minnesota in April 1911 and earned his Ph.D. in Chemistry from the University of Minnesota. He served as a Professor of Chemistry at the University of California, Berkeley.
• Scientific Contribution Post-World War II: Amidst the shock of the Hiroshima-Nagasaki bombings, Calvin and his co-workers sought to use radioactivity for beneficial purposes.
• Mapping Carbon Assimilation: Along with J.A. Bassham, Calvin used the radioactive isotope $C^{14}$ to label carbon dioxide. This allowed them to study the reactions in green plants that form sugar and other substances from raw materials like $CO_2$, water, and minerals.
• Energy Transformation: Calvin proposed that plants convert light energy into chemical energy by transferring an electron within an organized array of pigment molecules and other substances.
• Nobel Prize: For his work in mapping the pathway of carbon assimilation in photosynthesis, Calvin was awarded the Nobel Prize in 1961.
• Modern Applications: Calvin’s principles are currently used in research regarding renewable energy resources, basic solar energy research, and material science.

Photosynthesis: The Basis of Life on Earth

• Interdependence of Life: All animals, including human beings, depend on plants for food. Green plants are autotrophs because they synthesize the food they need, while organisms that depend on them are heterotrophs.
• Definition: Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds.
• Importance of Photosynthesis:
  1. It is the primary source of all food on Earth.
  2. It is responsible for the release of oxygen ($O_2$) into the atmosphere.
• Biological Foundation: Ultimately, all living forms on Earth depend on sunlight for energy.

Early Experiments and the Discovery of Photosynthetic Requirements

• Chlorophyll, Light, and $CO_2$: Simple experiments demonstrate that these three factors are essential for photosynthesis.
  • Starch Formation Analysis: Testing variegated leaves or leaves partially covered with black paper for starch shows that photosynthesis occurs only in green parts exposed to light.
  • The KOH Experiment: A leaf is partially enclosed in a test tube containing KOH-soaked cotton (which absorbs $CO_2$), while the other half is exposed to air. Testing for starch shows that only the part exposed to air (and thus $CO_2$) tests positive, proving $CO_2$ is required.
• Joseph Priestley (1733–1804): In 1770, he performed experiments with bell jars, candles, and mice. He observed that a candle in a closed space extinguishes and a mouse suffocates, concluding they "damage" the air. However, when a mint plant was added, the mouse survived and the candle burned. He hypothesized: "Plants restore to the air whatever breathing animals and burning candles remove."
• Jan Ingenhousz (1730–1799): Using Priestley's setup in both light and dark, he showed sunlight is essential for the process. He used an aquatic plant to show that small oxygen bubbles formed around green parts in bright sunlight but not in the dark.
• Julius von Sachs (1854): Provided evidence that plants produce glucose (stored as starch) as they grow. He identified that chlorophyll is located in "special bodies" (chloroplasts) within plant cells, and green parts are the sites of glucose production.
• T.W. Engelmann (1843–1909): Using a prism to split light, he illuminated a green alga (Cladophora) in a suspension of aerobic bacteria (used to detect oxygen evolution). Bacteria accumulated in the blue and red light regions, describing the first action spectrum of photosynthesis.
• Cornelius van Niel (1897–1985): A microbiologist who studied purple and green bacteria. He demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidizable compound reduces $CO_2$ to carbohydrates. The general formula is: $2H_2A + CO_2 \xrightarrow{\text{Light}} 2A + [CH_2O] + H_2O$.
  • In green plants, $H_2O$ is the hydrogen donor and is oxidized to $O_2$.
  • In purple/green sulfur bacteria, $H_2S$ is the donor, and the product is sulfur or sulfate, not oxygen.
  • Inference: The $O_2$ evolved by green plants comes from $H_2O$, not from $CO_2$. This was later confirmed by radioisotopic techniques.

The Chemical Equation of Photosynthesis

• By the mid-19th century, the empirical equation was understood as: $CO_2 + H_2O \xrightarrow{\text{Light}} [CH_2O] + O_2$, where $[CH_2O]$ represents a carbohydrate (e.g., glucose).
• The correct, balanced equation representing the overall multi-step process is: $6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$, where $C_6H_{12}O_6$ is glucose.
• Twelve molecules of water are used as a substrate because the process involves the splitting of water to provide electrons and protons, with water also being a product of the reaction.

The Site of Photosynthesis: Chloroplast Structure and Organization

• Mesophyll Cells: Located in the leaves, these cells contain a large number of chloroplasts. Chloroplasts align along the cell walls to optimize light absorption—parallel to walls for moderate light and perpendicular for high intensity.
• Chloroplast Anatomy:
  • Membrane System: Consists of the outer membrane, inner membrane, grana (stacks of thylakoids), and stroma lamellae. This system is responsible for trapping light energy and synthesizing ATP and NADPH.
  • Stroma: The fluid matrix where enzymatic reactions synthesize sugar (the biosynthetic phase).
• Division of Labor:
  • Light Reactions (Photochemical): Directly light-driven reactions occurring in the membrane system.
  • Dark Reactions (Carbon Reactions): Occur in the stroma; they are not directly light-driven but depend on the products of light reactions (ATP and NADPH). They should not be assumed to happen only in the dark.

Photosynthetic Pigments

• Pigment Types: Leaf color is determined by four main pigments separated via paper chromatography:
  1. Chlorophyll a: Bright or blue-green; the chief pigment.
  2. Chlorophyll b: Yellow-green.
  3. Xanthophylls: Yellow.
  4. Carotenoids: Yellow to yellow-orange.
• Absorption vs. Action Spectrum:
  • Chlorophyll a shows maximum absorption in the blue and red regions of the visible spectrum ($VIBGYOR$).
  • The action spectrum (rate of photosynthesis) aligns closely with the absorption spectrum of chlorophyll a, though they do not overlap perfectly.
• Accessory Pigments: Chlorophyll b, xanthophylls, and carotenoids absorb light at different wavelengths and transfer the energy to chlorophyll a. They protect chlorophyll a from photo-oxidation and widen the spectrum of light used.

The Photochemical Phase: Light Reactions

• Components: Light absorption, water splitting, oxygen release, and formation of ATP and NADPH.
• Photosystems: Pigments are organized into Light Harvesting Complexes (LHC) or antennae within two systems:
  • Photosystem I (PS I): Reaction center chlorophyll a absorbs at $700\,nm$ ($P700$).
  • Photosystem II (PS II): Reaction center chlorophyll a absorbs at $680\,nm$ ($P680$).
• Electron Transport (The Z-scheme):
  1. Light hits PS II, exciting electrons.
  2. Electrons are picked up by an acceptor and passed to an Electron Transport System (cytochromes).
  3. This "downhill" movement on a redox potential scale reaches PS I.
  4. Simultaneously, light excites electrons in PS I, which move to another acceptor.
  5. These electrons reduce $NADP^+$ to $NADPH + H^+$.
• Splitting of Water: Associated with PS II on the inner side of the thylakoid membrane. Equation: $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$. Electrons replace those lost by PS II.

Photophosphorylation: Cyclic and Non-cyclic

• Phosphorylation: The process of synthesizing ATP from ADP and inorganic phosphate ($P_i$) in organelles.
• Non-cyclic Photophosphorylation: Occurs when PS II and PS I work in series. It produces both ATP and NADPH.
• Cyclic Photophosphorylation: Occurs when only PS I is functional or light beyond $680\,nm$ is available. Electons are cycled back through the ETS.
  • Location: Stroma lamellae (which lack PS II and NADP reductase enzyme).
  • Result: Only ATP is synthesized; no NADPH or $O_2$ is produced.

The Chemiosmotic Hypothesis of ATP Synthesis

• Mechanism: ATP synthesis is linked to a proton gradient across the thylakoid membrane.
• Proton Accumulation in the Lumen:
  1. Water splitting on the inner membrane side releases protons into the lumen.
  2. The primary electron acceptor on the outer side transfers electrons to an H-carrier, which pumps protons from the stroma into the lumen.
  3. NADP reductase on the stroma side removes protons from the stroma to reduce $NADP^+$ to $NADPH$.
• Result: High proton concentration (low pH) in the lumen and low proton concentration in the stroma.
• ATP Synthase (ATPase): Consists of two parts:
  • $CF_0$: Embedded in the membrane, forms a channel for facilitated diffusion of protons.
  • $CF_1$: Protrudes into the stroma.
• Synthesis: The breakdown of the gradient provides energy for a conformational change in $CF_1$, catalyzing ATP formation from $ADP$ and $P_i$.

The Biosynthetic Phase: The Calvin Cycle (C3 Pathway)

• Radioactive Tracing: Melvin Calvin found the first stable product of $CO_2$ fixation in some plants was a 3-carbon acid, 3-phosphoglyceric acid ($PGA$).
• The Cycle: Occurs in all photosynthetic plants (C3, C4, etc.) across three stages:
  1. Carboxylation: Fixation of $CO_2$ into RuBP (5-carbon sugar) using the enzyme RuBisCO ($RuBP\,carboxylase-oxygenase$). This forms two molecules of $PGA$.
  2. Reduction: A series of reactions using $2\,ATP$ and $2\,NADPH$ per $CO_2$ fixed to form glucose.
  3. Regeneration: RuBP is regenerated using $1\,ATP$ to ensure the cycle continues.
• Net Energy Requirement: To make one molecule of glucose ($6\times CO_2$), the cycle requires:
  • $6$ turns of the cycle.
  • $18\,ATP$.
  • $12\,NADPH$.

The C4 Pathway (Hatch and Slack Pathway)

• Adaptation: Found in dry tropical plants (e.g., maize, sorghum).
• Kranz Anatomy: Large "bundle sheath" cells surround vascular bundles, layered like a wreath. These cells have thick walls, many chloroplasts, and no intercellular spaces.
• Mechanism:
  1. Primary $CO_2$ acceptor is Phosphoenolpyruvate ($PEP$, 3-carbon) in mesophyll cells.
  2. Enzyme: PEP carboxylase ($PEPcase$); mesophyll cells lack RuBisCO.
  3. $CO_2$ forms Oxaloacetic acid ($OAA$, 4-carbon), then malic or aspartic acid.
  4. These acids move to bundle sheath cells, where they release $CO_2$ and form a 3-carbon molecule.
  5. The 3-carbon molecule returns to the mesophyll to regenerate $PEP$.
  6. The released $CO_2$ enters the standard Calvin Cycle in the bundle sheath cells (which contain RuBisCO but lack PEPcase).
• Advantages: No photorespiration, tolerance to high temperatures, high light intensity response, and greater biomass productivity.

Photorespiration: The Wasteful Process

• RuBisCO Action: The most abundant enzyme in the world. It has an active site for both $CO_2$ and $O_2$. Binding is competitive and depends on relative concentrations.
• C3 Plants: Some $O_2$ binds to RuBisCO. Instead of $2\,PGA$, it forms one molecule of phosphoglycerate and one molecule of phosphoglycolate (2-carbon).
  • Results: No synthesis of sugar, ATP, or NADPH. Instead, it utilizes ATP and releases $CO_2$.
• C4 Plants: Successfully avoid photorespiration by maintaining a high concentration of $CO_2$ at the RuBisCO site (via the breakdown of C4 acids in bundle sheath cells).

Factors Affecting Photosynthesis

• Blackman’s Law of Limiting Factors (1905): If a process is affected by more than one factor, the rate is determined by the factor nearest its minimal value.
• Internal Factors: Number, size, age, and orientation of leaves; mesophyll cells; chloroplasts; internal $CO_2$; and chlorophyll content.
• External Factors:
  1. Light: Linear relationship at low intensities. Saturation occurs at $10\%$ of full sunlight. Very high intensity can cause chlorophyll breakdown.
  2. Carbon Dioxide: Major limiting factor. Level is $0.03\%$–$0.04\%$. Increase to $0.05\%$ can increase rates. C4 plants saturate at $360\,\mu lL^{-1}$; C3 plants saturate beyond $450\,\mu lL^{-1}$.
  3. Temperature: Dark reactions are highly sensitive. C4 plants have higher temperature optima ($30\text{-}40^{\circ}C$ or above) than C3 plants ($20\text{-}25^{\circ}C$).
  4. Water: Affects the plant indirectly by closing stomata (reducing $CO_2$) and causing leaf wilting (reducing surface area and metabolic activity).

Questions & Discussion

• Why use 12 water molecules in the equation? Because 6 water molecules are produced as a byproduct, and 12 are needed to provide sufficient electrons/protons for the multi-step reduction of $CO_2$.
• Chloroplast Alignment: They align parallel to the walls to avoid excessive light damage during high intensity and perpendicular to maximize capture in low light.
• C3 vs. C4 Differentiation:
  • External: Hard to tell; depends on habitat (tropical vs temperate).
  • Internal: Kranz anatomy (bundle sheath cells) is the hallmark of C4.
• RuBisCO Efficiency: RuBisCO is more efficient in C4 plants because the specialized anatomy acts as a "CO2 pump," ensuring the enzyme works as a carboxylase rather than an oxygenase.
• Accessory Pigments: Plants lacking chlorophyll a but having chlorophyll b would not survive long because chlorophyll a is essential for the reaction center. Accessory pigments exist to widen the light absorption spectrum and protect against photo-oxidation.
• Chlorosis: Leaves in the dark turn yellow because chlorophyll is unstable and breaks down without light to trigger its synthesis, while xanthophylls/carotenoids may be more visible.