Plant Cell Plastids: Types, Structure, and Functions

Posted on Nov 22, 2024 in Biology

Plastids are a group of organelles unique to plant cells, varied in shape and size, delimited by a double membrane, and containing circular DNA.

All plastids are derived from proplastids, undifferentiated structures present in dividing cells of plant roots and shoots. Depending on the needs of the differentiated cells, proplastids give rise to different types of mature plastids, which can transform from one type to another. All the plastids of a cell are called the plastidome.

There are several types of plastids, which differ in structure and function:

Chloroplasts

The most abundant, green because of chlorophyll manufactured and stored in a membrane called thylakoids. They are in all green parts of plants.

Chromoplasts

With various pigments such as carotenoids (yellow/orange), xanthophylls (yellow), lycopene (red), etc., which determine their color. They have no chlorophyll or thylakoids. They are found in roots like carrots, fruits such as tomatoes and peppers, or some petals.

Leucoplasts

Colorless, lacking pigment. Different plants and stored products are found in photosynthetic tissues, in embryonic cells, and meristematic regions of the plant exposed to light. We can distinguish:

  • Amyloplasts

    Store starch and can be spherical, oval, or elongated, usually showing a deposition in layers around a point. They are stained blue-black with iodine compounds.

  • Proteinoplasts

    Store proteins that can be crystals or filaments. They are common in phloem sieve elements.

  • Elaioplasts or Oleoplasts

    Store lipids and oils. They are common in the olive pulp and cotyledons of sunflower and peanut.

During ripening of some fruits, chromoplasts originate from chloroplasts. Throughout this process, carotenoid synthesis occurs, accompanied by the modification or disappearance of the thylakoid system and the decomposition of chlorophyll. This differentiation is irreversible. For example, on the top of carrot roots exposed to light, chloroplasts differentiate into chromoplasts, may lose pigment and developing thylakoids, and acquire a green color.

Chloroplasts

They are located in the cytoplasm of all photosynthetic plant cells and are responsible for carrying out photosynthesis. In the photosynthetic cells of leaves, in the presence of light, chloroplasts develop from proplastids. If plants are kept in the dark, proplastid development stops at an intermediate state called an etioplast, which does not contain chlorophyll and develops a semi-crystalline structure of tubular internal membranes. If plants that have grown in the dark are exposed to light, etioplasts continue their development into chloroplasts.

  • Chloroplast Outer Membrane

    A continuous membrane in contact with the hyaloplasm. It contains proteins called porins and is permeable to small molecules.

  • Chloroplast Inner Membrane

    A continuous membrane surrounded by the outer chloroplast membrane. It is impermeable to ions and metabolites, which may only enter the chloroplast via specific membrane transporters.

  • Intermembrane Space

    A small space between the outer and inner chloroplast membranes.

  • Stroma

    A space defined by the inner chloroplast membrane, which occupies most of the chloroplast and contains proteins, DNA, and ribosomes. It is the site of CO2 fixation and synthesis of carbohydrates, fatty acids, and some proteins. Within the stroma are distinguished:

    • Thylakoid Membrane

      Forming a network of closed, flattened disks called thylakoids whose membranes are oriented according to the axis of the chloroplast. There are two types of thylakoids:

      • Thylakoids of grana, flattened disks stacked one upon another, like coins. Each stack is referred to as grana.

      • Stroma thylakoids or lamellae. They are sacs that extend and connect the stroma thylakoids of different grana. The thylakoid membranes enclose a third compartment, the intratylakoid or intralamellar space, which has no communication with the intermembrane space. Thylakoid membranes are of fundamental importance in chloroplasts because they are where electron transport and ATP synthesis occur. They contain the chlorophylls and proteins necessary for the light phase of photosynthesis.

    • Circular Chloroplast DNA, similar to bacterial DNA, with several molecules, but larger and more complex than in mitochondria. The existence of chloroplast DNA confers a certain autonomy from the nucleus, but does not make them completely self-sufficient as they need many proteins encoded by nuclear DNA.

    • Chloroplast Ribosomes or plastoribosomes. They have a sedimentation coefficient of 70S, are smaller than those found in the cytoplasm, and are similar to bacterial ribosomes.

    • Starch Granules.

    • Soluble proteins responsible for the anabolic reactions of the dark phase of photosynthesis and the Calvin cycle, including those responsible for the conversion of CO2 into carbohydrates during photosynthesis.