Photochemical reactions

       Photochemical reactions in photosynthesis are chemical reactions triggered by the absorption of light, where light energy is directly used to excite molecules and drive essential biological transformations. They are fundamental for the conversion of solar energy into chemical energy in green plants, algae, and certain bacteria.

Definition

        A photochemical reaction is defined as a chemical reaction initiated by the absorption of photons (light energy) by atoms or molecules. In photosynthesis, these reactions specifically refer to the initial steps where chlorophyll and other pigments absorb sunlight and convert it into chemical forms of energy.

Detailed Explanation

• Location: Photochemical reactions in photosynthesis take place in the thylakoid membranes of chloroplasts.

• Light Absorption: When plant pigments like chlorophyll absorb photons, their electrons are excited to higher energy states.

• Initiation of Chemical Changes: The energy from excited electrons drives a series of rapid, highly specific chemical reactions. These include water splitting (photolysis), oxygen evolution, and transfer of electrons through photosystems I and II.

• Products: The main outcomes of these photochemical reactions are the generation of ATP (energy currency) and NADPH (a reducing agent), which are then used in the subsequent “dark” reactions (Calvin cycle) to fix carbon dioxide into carbohydrates.

• Why “Photochemical”? These reactions only occur when photons are absorbed—they cannot proceed in darkness. The process is extremely fast and efficient to use the absorbed energy before it is lost via heat or fluorescence.

Photochemical Reaction Pathway in Photosynthesis

• Photosystem II: Absorbs light, leading to the photolysis of water and release of O₂, electrons, and protons.

• Electron Transport Chain: Excited electrons move through membrane proteins, generating a proton gradient for ATP synthesis.

• Photosystem I: Absorbs light, further energizing electrons that reduce NADP⁺ to NADPH.

• Energy Capture: The ATP and NADPH produced power the fixation of carbon during the Calvin cycle.

Importance

• Primary Step in Photosynthesis: Without the photochemical (“light”) reactions, plants could not capture solar energy or produce the carbohydrates and oxygen essential for life on Earth.

• Basis of Food Chains: Most ecosystems depend on this conversion of light to chemical energy as their foundation.

Key Points

• Photochemical reactions require light, are incredibly fast, and involve highly specific molecular events.

• They are essential for ATP and NADPH production—fuels for synthesizing sugars in plants.

• These light-driven reactions distinguish photosynthesis from pure chemical or thermal processes.

       Photochemical reactions in photosynthesis are the initial, light-dependent reactions where light energy is converted to chemical energy, enabling plants to drive the biosynthesis of essential organic compounds from inorganic sources.

Cell cycle

What is cell cycle?

The cell cycle is a series of ordered events that a cell goes through from the time it is formed until it divides into two daughter cells. It encompasses the cell's growth, DNA replication, preparation for division, and the actual division process.

The cell cycle is essential for cell growth, development, and reproduction. It ensures that genetic material is accurately duplicated and distributed to daughter cells. The cell cycle is divided primarily into two broad phases:

1. Interphase

• The longest phase, sometimes called the "resting phase" but actually very active.

• The cell grows and prepares for division.

• Interphase is subdivided into three stages:

  • G1 phase (Gap 1): The cell grows physically, produces RNA, proteins, organelles, and carries out normal metabolic activities. It prepares for DNA replication but does not replicate DNA yet.

  • S phase (Synthesis): DNA replication occurs, duplicating the cell's chromosomes. Each chromosome now consists of two sister chromatids.

  • G2 phase (Gap 2): The cell continues to grow and produces proteins and organelles necessary for mitosis. The cell also begins reorganizing its contents to prepare for division.

Cells that do not divide can enter a resting phase called G0, where they remain metabolically active but do not progress through the cell cycle unless stimulated.

2. M Phase (Mitotic Phase)

• The phase of actual cell division.

• It includes:

  • Mitosis: Division of the cell’s nucleus and its genetic content into two nuclei.

  • Cytokinesis: Division of the cytoplasm, creating two daughter cells.

Checkpoints in the Cell Cycle

The cell cycle has built-in checkpoints to ensure the process proceeds correctly:

• G1 checkpoint: Determines if the cell is ready for DNA synthesis.

• G2 checkpoint: Checks for DNA damage and completeness of DNA replication before mitosis.

• Metaphase checkpoint (during M phase): Ensures chromosomes are properly aligned before division proceeds.

Major events of the cell cycle

Phase	Description	Key Activities
G1 Phase	Cell growth and metabolism	RNA and protein synthesis; organelle production
S Phase	DNA replication	Duplication of chromosomes
G2 Phase	Preparation for mitosis	Synthesis of proteins, organelles; cell growth
M Phase	Cell division (mitosis + cytokinesis)	Nuclear division and cytoplasmic division
G0 Phase	Resting/non-dividing phase	Cells exit cycle temporarily or permanently

Cell cycle regulation read here

Cell division read here