Chloroplasts, Chromatin, and Photosynthesis: A Deep Dive

Posted on Feb 15, 2025 in Biology

Chloroplasts

Chloroplasts are characteristic cell organelles of plants, where photosynthesis occurs. Their shapes and sizes vary.

Structure

Chloroplasts have a double membrane enclosing the stroma. Within the stroma are stacks of discs called thylakoids; a stack is called a granum.

Photosynthetic reactions occur in two phases:

  1. Light-dependent reactions occur in the thylakoid membrane.
  2. Light-independent (dark) reactions occur in the stroma.

Membranes

  • Inner and Outer Membranes: Both are highly permeable to O2 and contain numerous membrane transporters.
  • Thylakoid Membrane: Contains photosynthetic pigments that absorb light energy.
Photosynthetic Pigments
  • Chlorophylls: Highly effective photoreceptors containing a tetrapyrrole core and an alcohol.
  • Carotenoids: Protective devices preventing the oxidation of photosynthetic proteins.

The thylakoid membrane possesses enzymatic systems to capture light energy, facilitate electron transport, and produce ATP.

Stroma

The stroma contains enzymes that convert CO2 into organic compounds. It also contains plastid DNA, RNA, enzymes for plastoribosomes, and components for replication, transcription, and translation. In addition to chloroplasts, there are chromoplasts and amyloplasts.

Functions

  1. Photosynthesis: Chloroplasts conduct photosynthesis, using light energy to convert CO2 into organic compounds, mainly carbohydrates. Oxygen is released from H2O.

Phases of Photosynthesis

  1. Light-Dependent Phase: Light energy is captured by photosynthetic pigments and converted into chemical energy as ATP and NADPH. Oxygen molecules are released.
  2. Light-Independent (Dark) Phase: ATP and NADPH are used as energy sources and reducing power, respectively, to convert atmospheric CO2 into carbohydrates.
  1. Biosynthesis of Fatty Acids
  2. Reduction of Nitrate to Nitrites

Chloroplast DNA also performs additional functions.

Chromatin

Chromatin is the fundamental substance of the nucleus, formed by DNA strands in different degrees of condensation. In interphase, it appears as a web-like structure. The DNA of a human cell, if stretched out, would be 2 meters long. Therefore, compaction is significant, especially considering the negative charge repulsion from phosphate groups.

Chromatin is a supramolecular complex of DNA and associated proteins:

  • Histones: Small, basic proteins with few repeating units around which the DNA double helix is wound.
  • Non-histones: More heterogeneous proteins.

In the eukaryotic nucleus, chromatin organization occurs at different levels:

  1. Nucleosome: The DNA double helix is wound around an octamer of histones. Spacer DNA is found between octamers.
  2. Chromatin Fiber: A more complex folding, where groups of six nucleosomes form around a central axis, achieving greater compaction and shortening. At this level, genes are accessible for transcription and replication.
  3. Higher-Level Packaging: Fibers form loops around a non-histone protein axis, creating structural domains.

The chromosome represents the highest level of packaging.