Chloroplasts: Definition, Structure, Functions

Chloroplasts are double membraned rod-like oval or spherical cell organelles in the cytoplasm of most plant cells that help in photosynthesis. It is a green plastid containing chlorophylls. Animal cells normally lack plastids(all forms of plastids including chloroplasts). Several unicellular protozoans like- Euglena, and Chrysamoeba contain chloroplasts.

In 1866, Haeckel has first used the term plastid for double membrane-bound cell organelles that contain pigments. However, Schimper(1883) coined the term chloroplasts for the green particles(plastids). He also classified the plastids of plants in 1885.

Although in the early 1700s, chloroplasts were characteristics by Nehemiah Grew and Antonie van Leeuwenhoek.

Definition of Chloroplasts

Chloroplasts are the elliptical, oval, spherical, star-shaped, ribbon-like lipoproteinous double membrane-bound organelles found in the cytoplasm of plant cells that contain chlorophylls and help in photosynthesis.

Structure of Chloroplasts

Chloroplasts are large cell organelles about 5 μm-10 μm long that are bounded by a double membrane called chloroplast envelope and also have their own genome (double helix, circular DNA).

Labelled diagram of chloroplast
Figure: Labelled diagram of chloroplast

In electron microscopic studies, each chloroplast contains three parts- chloroplast envelope, grana, and stroma.

Envelope or Limiting Membrane

The double lipoproteinous membrane that surrounds the chloroplast from the outside is called the chloroplast envelope or limiting membrane.

The membrane facing the cytoplasm is called the outer membrane. While the membrane facing the stroma or chloroplast matrix is called the inner membrane. Both the membranes are made up of protein, lipid, and protein (fluid mosaic in ultrastructure) and are about 40-60 Å in diameter.

Between the outer and inner membrane, a continuous fluid containing lumen or space is found, which is called intermembrane space or periplastidial space. The intermembrane space is about 25-75 Å in diameter.

The outer membrane also contains a special type of integrated protein called porin. Porin formed many channels in the outer membrane which make the membrane permeable to solutes of molecular weight as high as 10000 daltons. In contrast, the inner membrane of the envelope is relatively impermeable.

Stroma or Matrix

The opaque proteinaceous ground substances(colorless fluid) in the chamber(inner chamber) surrounded by the inner membrane of the chloroplast are called the stroma or matrix.

The ingredients of the stroma are approximately 50% of proteins and are soluble types. It also contains ribosomes (70S ribosomes or plastidoribosomes), circular naked DNA(plastidome), RNA(e.g. rRNA, tRNA, and mRNA), enzymes(photosynthetic enzymes), and some metallic ions (Mg2+, Fe2+, Mn2+).

Stroma and stroma lamellae
Figure: Stroma or chloroplast matrix

The enzymes present in the matrix of chloroplasts control the dark reaction of photosynthesis.

Grana

During the formation of the chloroplasts from the proplastids, the inner membrane folds to form lamellae in a parallel manner within the stroma. Later, the lamellae formed flattened elliptical closed membranous sacs called thylakoids.

The space inside a thylakoid is the lumen, and the outer surface of it is in contact with the stroma.

Thylakoids are arranged in orderly stacks called “grana,” which resembles a stack of coins. In each chloroplast, there are about 40-60 grana present in the matrix. The grana are about 0.3-2.7 μm in diameter and consist of as many as 2 to 100 thylakoids.

Grana with thylakoid in chloroplasts
Figure: Grana with thylakoids

In the grana, the single membrane of two thylakoids joins together to form a double unit-membrane called a granum lamella (in plural grana lamellae) or grana thylakoid. While the adjacent two grana are connected to each other by a flattened membranous cisterna called a stroma lamella(in plural stroma lamellae), or stroma thylakoid.

The thylakoid membrane and the membrane of the stroma lamellae contain a variety of photosynthetic pigments(e.g.-chlorophyll, carotene, xanthophyll). The photosynthetic pigments form mainly two pigment systems, such as pigment system-I or photosystem-I and pigment system-II or photosystem-II.

Also, there are various types of carrier proteins, and ATPase is present in the thylakoid membrane and the membrane of the stroma lamellae. On the inner side of the thylakoid membrane, many stalk-less quantasomes are present.

Quantasomes

Quantasomes are many stalkless chlorophyll-containing granules present inside the thylakoid membrane that play an important role in the light chemical reaction of photosynthesis.

In 1962, the term quantasome was first used by Park and Biggins and was called quantasome a physiological photosynthetic unit.

Quantasome
Figure: Quantasomes inside the thylakoid membrane

The molecular weight of each quantasome is about 1× 106 daltons and the dimension of the quantasome is 180 Å × 160 Å with a height of 100 Å.

Quantasomes help in ATP synthesis by absorbing photons from sunlight.

Functions of Chloroplasts

  • Photosynthesis: Chloroplasts are the photosynthetic organelles that function as the sites of photosynthesis. In the chloroplasts, light reaction(light-dependent) of photosynthesis occurs in the grana region, and dark reaction(light-independent) of photosynthesis occurs in the stroma region.
  • Protein synthesis: Ribosomes in the chloroplasts participate in protein synthesis.
  • Energy transduction: The chlorophyll pigments in the chloroplasts absorb solar energy and convert it into chemical energy.
  • Photosensitivity: In certain algae, the stigma or eyespots are present within the chloroplasts that operate as light receptors and can respond to light stimuli.
  • Storage of lipids: Lipids or fats are stored in the chloroplasts as plastogobuli.

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