Biochemistry: Hexose Sugars Overview

Photosynthesis Overview

• Photosynthesis is the process by which green plants synthesize organic matter using light.
• The general equation: 6CO2 + 6H2O → C6H12O6 + 6O2.
• Light energy is converted into chemical energy, stored as carbohydrates, with oxygen as a byproduct.
• CO2 and H2O are the primary raw materials; it is an anabolic process.

Historical Contributions to Photosynthesis

• Van Helmont (1648): Suggested tree growth primarily resulted from water.
• Stephan Hales (1727): Identified air as a nutrition source for plants and emphasized the importance of sunlight.
• Priestley (1772): Proposed that plants could restore oxygen in CO2-laden air.
• Ingenhousz (1779): Discovered that only green parts of plants can purify air with sunlight.
• Jean Senebier (1783): Noted that air-purifying activities depend on CO2 presence.
• Nicolas Theodore de Saussure (1804): Concluded that CO2 and water are essential raw materials.
• Meyer (1845): Recognized light's role as the energy source in photosynthesis.
• Julius Sachs (1862): Demonstrated that photosynthesis occurs in chloroplasts, producing organic matter.

Importance of Photosynthesis

• Directly provides food for humans and indirectly via animal products (meat, milk).
• Maintains atmospheric oxygen equilibrium.
• Supplies energy resources (e.g., coal, oil, wood).

Types of Photosynthetic Pigments

• Chlorophylls: Main pigments for light absorption, insoluble in water.
• Carotenoids: Includes carotenes and xanthophylls, also insoluble in water.
• Phycobilins: Water-soluble pigments.
• Different pigments absorb various wavelengths of light and exhibit unique absorption peaks.

Light Energy Absorption

• The sun is the primary energy source for photosynthesis.
• Plants receive about 40% of total solar energy; the rest is absorbed or scattered.
• Only a small portion of light is utilized by pigments, predominantly in the 400-700 nm range (Photosynthetically Active Radiation - PAR).
• Chloroplasts appear green due to weak absorption of light in the green spectrum.

Mechanisms of Photosynthesis

• Involves two main reactions:
  • Primary light reaction (Hill's reaction).
  • Dark reaction (Calvin cycle, carbon fixation).

Light Reaction (Hill's Reaction)

• Occurs in the presence of lipids within chloroplasts.
• Utilizes labeled carbon dioxide (^14CO2) to trace carbon assimilation pathways.
• Results in the formation of products necessary for the Calvin cycle.

Calvin Cycle (C3 Pathway)

• Steps include:
  • Carboxylation: CO2 combines with ribulose 1,5-bisphosphate (RuBP) to form a 6-carbon compound, which then splits into 3-phosphoglyceric acid (3PGA).
  • Reduction: 3PGA is converted to 3-phosphoglyceraldehyde (G3P).
  • Formation of Hexose Sugar: Some G3P molecules convert into fructose 1,6-bisphosphate and then into hexose sugars like glucose and sucrose.
  • Regeneration of RuBP: Excess G3P is used to regenerate RuBP for the continuation of the cycle.

Summary Points

• The Calvin cycle highlights the photosynthetic carbon reduction (PCR) pathway, playing a crucial role in synthesizing carbohydrates in plants.
• The cycle is also known as the Reductive Pentose Phosphate (RPP) cycle.

Additional Notes on Sugars

• Monosaccharides: Individual sugars (e.g., glucose, fructose) typically consist of 5 or 6 carbon atoms.
• Disaccharides: Composed of two monosaccharides linked by a glycosidic bond, with examples including sucrose, lactose, and maltose.

Questions for Review

• Name four scientists who contributed to the study of photosynthesis.
• Identify four necessary conditions for photosynthesis.
• What is the main source of energy in photosynthesis?
• Explain the two types of reactions involved in the mechanism of photosynthesis.

This quiz covers the essential characteristics of hexose sugars, focusing on glucose, fructose, galactose, and mannose. It delves into their structural classifications as aldoses and ketoses, highlighting their significance in animal tissues. Test your knowledge on monosaccharides and their properties.