2.9 Photosynthesis Notes

Introduction

Photosynthesis is a fundamental biological process that converts light energy into chemical energy, enabling plants, algae, and certain bacteria to produce their own food. This process is crucial for the sustenance of life on Earth as it forms the base of most food chains and is responsible for the oxygenation of the atmosphere. In this study note, we will delve into the details of photosynthesis, its impact on Earth's history, and the biochemical mechanisms involved.

What is Photosynthesis?

Photosynthesis is the process by which autotrophic organisms (producers) convert light energy into chemical energy stored in glucose. The general word equation for photosynthesis is:

Carbon dioxide + Water + Light energy → Glucose + Oxygen

The chemical equation is represented as:

$$ 6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2 $$

Inputs and Outputs

• Inputs: Sunlight, carbon dioxide ($CO_2$), and water ($H_2O$).
• Outputs: Glucose ($C_6H_{12}O_6$) and oxygen ($O_2$).

Transformations

• Light Energy to Chemical Energy: Light energy is captured by chlorophyll and other pigments in the chloroplasts and transformed into chemical energy stored in glucose.

The Impact of Photosynthesis on Earth

Historical Changes

• Early Atmosphere: The Earth's early atmosphere had no oxygen. The first life forms, which emerged around 4 billion years ago, were anaerobic.
• First Photosynthetic Organisms: About 3.5 billion years ago, photosynthetic prokaryotes began to release oxygen into the atmosphere.
• The Great Oxidation Event: Around 2.2 billion years ago, oxygen levels reached 2%, leading to significant changes in Earth's atmosphere and geology.

Geological and Atmospheric Changes

• Banded Iron Formations: Photosynthetic bacteria released oxygen into the oceans, oxidizing dissolved iron to form iron oxide, which precipitated and formed banded iron formations.
• Reduction of Greenhouse Gases: The increase in oxygen and decrease in methane and carbon dioxide led to climatic changes, including an Ice Age.
• Evolution of Multicellular Life: By 600 million years ago, oxygen levels supported the evolution of large multicellular organisms, including many photosynthetic plants.

Energy Requirements of Photosynthesis

Endothermic and Anabolic Nature

Photosynthesis is an endothermic reaction, meaning it requires an input of energy. It is also an anabolic reaction, where smaller molecules are built into larger ones.