Photosynthesis and the Calvin Cycle

Introduction to Photosynthesis

  • Photosynthesis is a crucial biological process in which organisms convert light energy into chemical energy in the form of glucose.

  • The green color of plants is attributed to a pigment named chlorophyll, which plays a significant role in absorbing light for this process.

  • Essential inputs for photosynthesis include light, carbon dioxide (CO2), and water (H2O), leading to the outputs of oxygen (O_2) and glucose.

Organelles Involved in Photosynthesis

  • Chloroplasts:

    • Organelles found in plant and algal cells that facilitate photosynthesis.

    • Chloroplasts contain chlorophyll and are primarily located in the mesophyll tissue of leaves.

    • Water is absorbed through roots and transported to leaves; carbon dioxide enters and oxygen exits through stomata.

  • Mitochondria:

    • Organelles involved in cellular respiration which utilizes organic molecules to produce ATP.

Types of Organisms

  • Heterotrophs:

    • Organisms that rely on consuming external food sources for energy, including fungi, animals, and most bacteria.

  • Autotrophs:

    • Organisms capable of producing organic compounds from inorganic sources through photosynthesis (e.g., plants).

  • Photoautotrophs: A subgroup of autotrophs that specifically use light as an energy source to synthesize organic molecules.

Photosynthesis and Cellular Respiration

  • There is a vital energy cycle between photosynthesis and cellular respiration:

    • Photosynthesis converts light energy into chemical energy, producing glucose and oxygen from CO2 and H2O.

    • Cellular respiration breaks down glucose back into CO2 and H2O, releasing energy stored in ATP.

The Structure of Chloroplasts

  • Chloroplast Components:

    • Outer membrane

    • Inner membrane

    • Intermembrane space

    • Thylakoids: Stack into structures called granum; site of light-dependent reactions.

    • Stroma: The fluid-filled space where the Calvin cycle occurs.

Light Energy and Photosynthesis

  • Light is a form of electromagnetic radiation essential for life.

  • The electromagnetic spectrum includes wavelengths ranging from gamma rays to radio waves, with visible light (380 to 740 nm) being the part that drives photosynthesis.

Pigments in Photosynthesis

  • Pigments absorb light energy, which can lead to various reactions:

    • Light could pass through, reflect, or be absorbed by an object.

    • Chlorophyll absorbs primarily blue light and reflects green light, explaining why leaves appear green.

The Process of Photosynthesis

  • Photosynthesis occurs in two main stages:

    1. Light Reactions (Hill Reaction): Occurs in the thylakoid membrane and captures light energy to produce ATP and NADPH.

    2. Calvin Cycle: Takes place in the stroma, utilizing ATP and NADPH to convert CO_2 into glucose.

Light-Dependent Reactions

  • Light energy is converted to chemical energy in the form of ATP and NADPH.

  • Occur in the thylakoid membranes, involving two main photosystems:

    • Photosystem II (PSII): Absorbs light, exciting electrons. Water molecules are split (photolysis) to replace these electrons, releasing oxygen (O_2) and protons (H^+) into the thylakoid lumen:

    • 2H2O \to O2 + 4H^+ + 4e^-

    • Electrons from PSII are passed through an electron transport chain, releasing energy that is used to pump protons from the stroma into the thylakoid lumen, building a proton gradient.

    • Photosystem I (PSI): Absorbs light, re-energizing the electrons. These electrons are then transferred to NADP+ reductase, which uses them to reduce NADP^+ to NADPH.

  • The proton gradient across the thylakoid membrane (higher concentration in the lumen) drives H^+ back into the stroma through ATP synthase. This process, called chemiosmosis, synthesizes ATP from ADP and inorganic phosphate (P_i).

Calvin Cycle

  • The Calvin cycle, which consists of three main stages occurring in the stroma:

    1. Carbon Fixation: An enzyme, RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), catalyzes the attachment of carbon dioxide (CO_2) from the atmosphere to an existing five-carbon sugar, Ribulose-1,5-bisphosphate (RuBP). This forms an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

    • RuBP + CO_2 \to \text{unstable 6C intermediate} \to 2 \times 3-PGA

    1. Reduction: Each molecule of 3-PGA receives an additional phosphate group from ATP (generated in the light reactions), becoming 1,3-bisphosphoglycerate. This compound is then reduced by electrons donated from NADPH (also from the light reactions), losing its phosphate group to become Glyceraldehyde-3-phosphate (G3P). This G3P is a three-carbon sugar.

    • 3-PGA + ATP + NADPH \to G3P + ADP + P_i + NADP^+

    1. Regeneration: For every three molecules of CO_2 fixed, six molecules of G3P are produced. Five of these six G3P molecules are used to regenerate three molecules of RuBP, a process that requires energy from ATP. The remaining one G3P molecule exits the cycle and is used to synthesize glucose and other organic compounds. This regeneration step ensures the cycle can continue.

Reaction Summary

  • Overall photosynthesis equation:

    • 6CO2 + 6H2O + \text{light} \to C6H{12}O6 + 6O2

  • Both stages (light reactions and Calvin cycle) are essential for the complete process of photosynthesis, where ATP and NADPH produced in light reactions drive the Calvin cycle to synthesize glucose.

Applications of Photosynthesis

  • The process has ecological and agricultural significance, contributing to the base of the food web and serving as a framework for understanding energy transfer in ecosystems.