Photosynthesis

Definition of Photosynthesis

• Photosynthesis: process where light energy is used to create carbohydrate molecules from carbon dioxide and water.

Location of Photosynthesis

• Takes place in chloroplasts, the green structures within plant cells.

  • Plant cells are packed with chloroplasts for active photosynthesis, contrary to schematic diagrams showing few per cell.

Chlorophyll and Light Absorption

• Chloroplasts contain chlorophyll, a pigment that absorbs specific light wavelengths.

• Chlorophyll absorbs:

  • Red, blue, and violet light.

• Reflection of light results in the perception of green color in plants.

Absorption Spectrum

• Chlorophyll shows two peaks in absorption spectrum:

  • Violet and blue wavelengths (absorbed).

  • Red wavelengths (absorbed).

• Middle wavelengths (green) are reflected, making plants appear green.

Other Pigments

• Carotenoids:

  • Reflect yellow, orange, and red, contributing to fall colors by absorbing shorter wavelengths.

• Phycocyanins:

  • Found in cyanobacteria, reflect blue wavelengths and absorb longer wavelengths.

Photosynthesis Process

Overview

• Consists of two main steps:

  1. Light-dependent reactions.

  2. Light-independent reactions (Calvin cycle).

Light-dependent Reactions

• Only occur in the presence of light.

• Key processes include:

  • ATP synthesis (energy carrier molecule):

    • ATP formed by adding a phosphate group to ADP.

  • Generation of NADPH (energy carrier).

  • Splitting of water (H₂O) releasing oxygen (O₂) as a byproduct.

Light-independent Reactions (Calvin Cycle)

• Can occur in light or dark.

• ATP and NADPH generated from light-dependent reactions are used to:

  • Convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆).

Energy Flow in Photosynthesis

• Energy transformation:

  • Sunlight ➜ Light-dependent reactions ➜ ATP and NADPH ➜ Light-independent reactions ➜ Glucose synthesis.

• Energy Levels:

  • Carbon dioxide and water have less energy than glucose.

  • Energy from sunlight is captured in chemical bonds of glucose.

Requirements and Products of Photosynthesis

• Requirements:

  • Sunlight, water, carbon dioxide.

• Products:

  • Glucose (C₆H₁₂O₆) is the main product.

  • Oxygen (O₂) is a byproduct.

Importance of Glucose in Plants

1. Source of Carbon:

   • Used to synthesize other organic molecules (proteins, nucleic acids, lipids, cellulose, etc.).

2. Energy Source:

   • Glucose can be broken down to extract energy, converting it into ATP for cellular processes.

Summary of Photosynthesis

• Essential process for energy conversion in plants.

• Key outcomes: glucose for energy and structural molecules, oxygen as an incidental byproduct.