Biology Chapter 13: Photosynthesis

Overview of Photosynthesis

• Essential for life on Earth, plants synthesize food through photosynthesis, relying on light energy.
• All organisms, including animals, depend on plants for food and oxygen.

Importance of Photosynthesis

• Primary energy source for food production on Earth.
• Vital for oxygen release into the atmosphere.

Early Experiments in Photosynthesis

• Joseph Priestley demonstrated plants restore oxygen to the air using a mint plant in a closed environment with a candle.
• Jan Ingenhousz confirmed sunlight's role in photosynthesis, illustrating oxygen release during light exposure with aquatic plants.
• Julius von Sachs identified glucose production and storage as starch in chloroplasts.
• T.W. Engelmann used bacteria to find that oxygen production occurs mainly in blue and red light regions.

Photosynthesis Equation

• Overall reaction:
  • (6 CO_2 + 12 H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6 H_2O + 6 O_2)
• Oxygen released during photosynthesis originates from water, not carbon dioxide.

Site of Photosynthesis

• Occurs primarily in green leaves, particularly in mesophyll cells containing chloroplasts.
• Chloroplasts have internal structures like grana and stroma, where light-dependent reactions (light reactions) and Calvin cycle (dark reactions) occur.

Photosynthetic Pigments

• Four main pigments involved:
  • Chlorophyll a (blue-green),
  • Chlorophyll b (yellow-green),
  • Xanthophylls (yellow),
  • Carotenoids (yellow to orange).
• Chlorophyll a is the primary pigment for photosynthesis, absorbing light mainly in blue and red wavelengths.

Light Reactions

• Light reactions involve:
  • Light absorption, water splitting, oxygen release, and production of ATP and NADPH.
• Photosystems I (PS I) and II (PS II) contain light-harvesting complexes and have distinct reaction centers (P700 and P680).

Electron Transport in Light Reactions

• Excited electrons in PS II are transferred through an electron transport chain, reducing NADP+ to NADPH.
• The process is called the "Z scheme," characterized by the movement of electrons through photosystems and various acceptors.

Water Splitting

• Water splitting in PS II sustains a continuous supply of electrons necessary for the electron transport chain.

Summary of Photosynthesis

• Multistep process crucial for converting light energy into stored chemical energy, enabling the production of glucose and oxygen essential for life on Earth.### Electron Replacement in Photosynthesis
• Electrons lost from Photosystem II (PS II) are replenished by electrons derived from the splitting of water.
• Water is split into hydrogen ions (H+), oxygen (O2), and electrons during this process, contributing to the overall process of photosynthesis.
• This reaction can be summarized as: 2H₂O → 4H⁺ + O₂ + 4e⁻.
• The water-splitting complex is associated with PS II, located on the thylakoid membrane's inner side.

Photophosphorylation Processes

• Photophosphorylation refers to ATP generation from ADP and inorganic phosphate in the presence of light.
• Non-cyclic photophosphorylation occurs when both PS II and PS I function in sequence, producing ATP and NADPH + H+; they are linked through an electron transport chain.
• Cyclic photophosphorylation involves only PS I, where electrons are circulated within the system, synthesizing ATP but not NADPH + H+.

Chemiosmotic Hypothesis

• ATP synthesis in chloroplasts adheres to the chemiosmotic hypothesis, mirroring mechanisms in respiration.
• A proton gradient develops across the thylakoid membrane, primarily due to the accumulation of protons in the lumen.
• Protons result from water splitting, and as electrons traverse photosystems, they transport protons across the membrane.

ATP and NADPH Utilization

• ATP and NADPH produced during the light reactions drive the biosynthetic reactions that synthesize sugars in the stroma, independent of light.
• The Calvin cycle, named after Melvin Calvin, initiates CO₂ fixation using ribulose bisphosphate (RuBP) as the primary acceptor.

The Calvin Cycle Stages

• The Calvin cycle consists of three stages:
  • Carboxylation: CO₂ is fixed into an organic intermediate via RuBP carboxylase (RuBisCO), yielding two molecules of 3-phosphoglyceric acid (3-PGA).
  • Reduction: ATP and NADPH are used to synthesize carbohydrates from 3-PGA, producing glucose after six cycles.
  • Regeneration: RuBP is regenerated for continuous CO₂ fixation, requiring additional ATP.

C4 Pathway Overview

• C4 plants exhibit a specialized adaptation to hot environments; their CO₂ fixation product is oxaloacetic acid (OAA).
• The C4 pathway includes distinct anatomical features known as Kranz anatomy, characterized by tightly packed bundle sheath cells containing chloroplasts.

Hatch and Slack Pathway

• The primary CO₂ acceptor in C4 plants is phosphoenol pyruvate (PEP), catalyzed by the enzyme PEP carboxylase.
• OAA formed in mesophyll cells can be converted into malic or aspartic acid, which is then transported to bundle sheath cells.
• In bundle sheath cells, these acids release CO₂, which is subsequently utilized in the Calvin cycle for sugar synthesis.

Conclusion of Photosynthesis Pathways

• Both C3 and C4 plants utilize the Calvin cycle for sugar biosynthesis; however, their initial mechanisms for CO₂ fixation differ.
• Understanding these pathways is crucial for leveraging plant efficiency in various environmental conditions.### C3 and C4 Plants Overview
• C4 plants conduct CO2 fixation primarily in bundle sheath cells, unlike C3 plants which fix CO2 in mesophyll cells.
• The enzyme RuBisCO, the most abundant enzyme globally, can bind both CO2 and O2, affecting the Calvin cycle's efficiency.

Photorespiration

• Occurs when RuBP, instead of binding with CO2, binds with O2, leading to the formation of phosphoglycerate and phosphoglycolate.
• Photorespiration consumes ATP and releases CO2, but does not contribute to sugar synthesis, rendering it a wasteful process.
• C4 plants avoid photorespiration by concentrating CO2 around RuBisCO, thus enhancing carbon fixation efficiency.

Advantages of C4 Plants

• Higher productivity and yields due to the prevention of photorespiration.
• Enhanced tolerance to elevated temperatures compared to C3 plants.

Comparison of C3 and C4 Pathways

• C3 plants: Calvin cycle directly in mesophyll; include RuBisCO; high photorespiration; lower temperature optimum.
• C4 plants: Initial CO2 fixation product is a 4-carbon compound (OAA); primary fixation occurs in both mesophyll and bundle sheath; lower photorespiration rates; adapted for high temperatures.

Factors Affecting Photosynthesis

• Internal Factors:

  • Leaf number, size, age, orientation, and internal factors such as chlorophyll content and mesophyll cell efficiency influence photosynthesis.

• External Factors:

  • Involve sunlight availability, ambient temperature, CO2 concentration, and water availability.
  • Blackman’s Law of Limiting Factors suggests that the rate is determined by the factor closest to its minimal value.

Light Intensity

• Affects CO2 fixation rates, particularly at lower intensities where a linear relationship exists.
• Light saturation occurs at approximately 10% of full sunlight; further increases may damage chlorophyll and lower photosynthesis rates.

Carbon Dioxide Concentration

• CO2 is a primary limiting factor for photosynthesis.
• Optimal CO2 concentration can increase fixation rates; C4 plants reach saturation at lower concentrations (360 µL/L) than C3 plants (450 µL/L).
• Elevated CO2 conditions can enhance productivity in C3 plants when managed in controlled environments.

Temperature

• C4 plants possess a higher temperature optimum for photosynthesis compared to C3 plants.
• Temperature adaptability is linked to the respective habitats of the plants; tropical plants can tolerate higher temperatures.

Water Stress

• Influences stomatal closure, reducing CO2 uptake, and adversely affecting photosynthesis efficiency.
• Water stress also leads to wilting and reduced surface area of leaves, limiting their metabolic activity and photosynthesis rate.

Summary of Photosynthesis Process

• Occurs mainly in the leaves, utilizing chloroplasts within mesophyll cells.
• Comprises light reactions (electron transfer, ATP synthesis, O2 release) and light-independent reactions (Calvin cycle).
• CO2 fixation linked to RuBisCO; process yields carbohydrates crucial for plant growth.

Explore the fascinating process of photosynthesis in higher plants with this quiz based on Chapter 13 of your biology textbook. Understand how plants synthesize their own food and the importance of this process for all life forms. Test your knowledge and discover key experiments related to photosynthesis.