Bio Lecture 03/11: Photosynthesis

Photosynthesis Overview

• Energy Source: Sunlight energy is captured via photosynthesis.

• Earth's System: Earth is not a closed system; energy from sunlight enters and is transformed.

Importance of Photosynthesis

• Sugars Production: Photosynthetic organisms produce sugars consumed by other organisms.

• Life Dependency: All life on Earth is interconnected through photosynthesis.

• Basic Reaction:

  • Reaction Equation: 6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

Light Energy and Wavelengths

• Energy Levels: Light varies in energy:

  • Short wavelengths = high frequency = high energy.

  • Long wavelengths = low frequency = low energy.

• Visible Light: Represents a small segment of the spectrum of light.

Light Interaction with Objects

• Reflection & Absorption:

  • Light can be reflected (bounced back), absorbed (captured), or transmitted (passed through).

• Color Perception: The color of an object is due to the wavelengths it reflects; others may be absorbed or transmitted.

Leaf Structure and Function

• Stomata: Openings that facilitate gas exchange (CO₂ in, O₂ out).

• Mesophyll Layer: Middle layer where photosynthesis occurs; contains chloroplasts.

• Epidermis & Veins: Outer layer provides protection; veins supply water and nutrients, transporting sugars.

Photosynthetic Cells

• Chloroplasts: Sites of photosynthesis within mesophyll cells, featuring:

  • Outer and inner membranes

  • Granum, lumen, stroma, and thylakoids structure.

Photosynthetic Reactions

• Types of Reactions:

  • Light-dependent Reactions: Occur in thylakoids capturing light energy.

  • Light-independent Reactions: Occur in stroma, utilizing ATP and NADPH to produce glucose.

Light-Dependent Reactions

• Pigments:

  • Chlorophyll captures violet, red, and blue wavelengths, reflecting green.

  • Accessory pigments absorb additional wavelengths, transferring energy to chlorophyll.

• Energy Dynamics:

  • Electrons from chlorophyll become excited upon light absorption.

  • Excited electrons may return to a lower energy state, releasing heat.

• Photosystems: Grouped in thylakoids, consisting of:

  • Photosystems I and II as light-harvesting complexes.

  • Reaction centers consist of chlorophyll that collect energy and pass electrons down the chain.

Electron Transport Chain (ETC)

• Light Energy Capture:

  • Electrons travel through the ETC, releasing energy which is used to pump H⁺ ions across thylakoid membrane.

• ATP Production: ATP synthase uses the resulting H⁺ gradient to synthesize ATP.

• Water Splitting:

  • Necessary to replenish electrons by splitting water: 2H₂O → 4H⁺ + O₂ + 4e⁻.

Light-Independent Reactions (Calvin Cycle)

• C3 Cycle: Carbon fixation process, requiring CO₂ to bond with RuBP, producing PGA, which is reduced to G3P.

  • Two G3P molecules are utilized to synthesize glucose.

• C4 Pathway:

  • Separates carbon fixation from the Calvin cycle to minimize photorespiration by converting CO₂ into a 4-carbon compound (PEP).

• CAM Pathway:

  • Involves stomata opening at night to allow CO₂ influx while keeping them closed during the day, storing CO₂ in vacuoles until it's needed for the Calvin cycle.