Plastids and Vacuoles: Exploring Plant Cell Organelles
Plastids
Plastids are organelles found exclusively in plant cells. They vary in shape and size and are enclosed by a double membrane containing circular DNA. All plastids originate from undifferentiated proplastids present in the dividing cells of plant roots and shoots.
Based on the needs of differentiated cells, proplastids develop into various types of mature plastids, which can transform from one type to another. The collection of plastids within a cell is called the plastidoma.
Types of Plastids
Several types of plastids exist, each with distinct structures and functions:
Chloroplasts
The most abundant plastids, chloroplasts are green due to the chlorophyll they produce and store within internal membranes called thylakoids. They are present in all green parts of plants.
Chromoplasts
Chromoplasts contain various pigments like carotenoids (yellow/orange), xanthophylls (yellow), and lycopene (red), giving them their characteristic colors. They lack chlorophyll and thylakoids and are found in structures like carrot roots, tomato fruits, peppers, and some petals.
Leucoplasts
These colorless plastids lack pigments and store various products in different plant tissues. They are found in non-photosynthetic tissues, embryonic cells, and meristematic regions of plants not exposed to light. Leucoplasts include:
Amyloplasts
Amyloplasts store starch and can be spherical, oval, or elongated. They typically exhibit layered starch deposition around a central point and stain blue-black with iodine compounds.
Proteinoplasts
Proteinoplasts store proteins, which can be in the form of crystals or filaments. They are commonly found in phloem sieve elements.
Elaioplasts/Oleoplasts
Elaioplasts or oleoplasts store lipids and oils. They are common in the pulp of olives, sunflower cotyledons, and peanuts.
During the ripening of some fruits, chromoplasts develop from chloroplasts. This process involves carotenoid synthesis, modification or disappearance of the thylakoid system, and chlorophyll breakdown. This differentiation is irreversible. For example, in exposed carrot roots, chromoplasts cannot revert to chloroplasts by losing pigments and developing thylakoids to regain a green color.
Vacuoles
Vacuoles are organelles surrounded by a single membrane called the tonoplast. They are particularly abundant in plant cells.
Young plant cells have numerous small vacuoles. As cells grow and differentiate, these vacuoles enlarge and merge, often forming a large central vacuole that can occupy most of the cytoplasm. When this happens, the nucleus is pushed to the cell’s periphery. The entire vacuolar system of a cell is called the vacuoma.
Vacuoles contain water and various organic and inorganic compounds, storing large quantities for different purposes:
- Waste products: Vacuoles store toxic waste products that would harm the cell if they were free in the cytoplasm, such as tannins, which have a defensive role by inhibiting fungal and microbial growth in case of injury.
- Reserve substances: Vacuoles store sugars, starch, and proteins.
- Mineral salts: Vacuoles accumulate mineral salts, which can form crystals like calcium oxalate, resulting from intracellular calcium accumulation.
- Hallucinogens: Vacuoles store hallucinogenic compounds like alkaloids, which plants use to attract beneficial animals or deter herbivores, such as morphine.
- Soluble pigments: Vacuoles store soluble pigments like anthocyanins, which are red, purple, or blue and give a distinctive color to many plant organs, including petals, fruits (grapes, plums, cherries), roots (sugar beet), and autumn leaves.
- Large amounts of water: This water accumulation promotes plant cell growth and maintains their turgidity. The vacuole membrane is permeable to water but impermeable to small molecules that accumulate inside, making it hypertonic compared to the cytosol. This causes water to enter the vacuole by osmosis, increasing cell turgor.
Inclusions
Inclusions are temporary storage sites for reserve materials or waste products resulting from cellular metabolic activity within the cytoplasm. In plant cells, lipid inclusions are typically clusters or droplets of essential oils, balsams, resins (amber and incense), or latex deposits. They also accumulate pigments like carotenoids.
The primary functions of inclusions are attracting insects through scents and flavors and protecting plants from microorganisms. In animal cells, inclusions can be lipid droplets and glycogen granules, both serving as energy reserves. Deposits of pigments like melanin can also be found.
Glyoxysomes
Glyoxysomes are membrane-bound vesicles with an amorphous matrix that originate from the endoplasmic reticulum. They are found only in plant cells and filamentous fungi and are particularly abundant in oil seeds.
Glyoxysomes contain enzymes necessary for the glyoxylate cycle, a modified Krebs cycle that converts seed lipids into carbohydrates.
Peroxisomes
Peroxisomes are small vesicles enclosed by a single membrane, abundant in cells of organs like the liver and kidneys. They contain enzymes involved in various biochemical pathways, synthesized on free ribosomes and incorporated into peroxisomes as complete polypeptide chains.
These enzymes include peroxidases and catalase. Peroxidases oxidize certain substrates to generate energy and produce hydrogen peroxide (H2O2), which is highly toxic to cells. Catalase then decomposes hydrogen peroxide into water and oxygen.
Serial Endosymbiosis Theory
The serial endosymbiosis theory, formulated in the early 20th century and popularized by Lynn Margulis in 1967, attempts to explain the origin of mitochondria and chloroplasts through a series of symbiotic events. The theory, with some modifications from Margulis’s original version, can be summarized as follows:
The first eobiont was likely an anaerobic and heterotrophic prokaryotic cell.
Through an unknown mechanism, this early cell developed a system of internal membranes that enclosed DNA within an inner core, separating it from the rest of the cytoplasm by a double membrane (becoming eukaryotic). It also gained the ability to obtain materials through phagocytosis.
Once phagocytosis evolved, this primitive eukaryotic cell could undergo a series of symbiotic events that led to the emergence of mitochondria and chloroplasts.
First Symbiotic Event: Origin of Mitochondria
The primitive eukaryotic cell engulfed aerobic bacteria, which, instead of being digested, remained in the cytoplasm and reproduced freely, eventually becoming mitochondria.
Second Symbiotic Event: Origin of Chloroplasts
In a similar process, chloroplasts originated from cyanobacteria (oxygenic photosynthesis) ingested and retained by a eukaryotic cell that already possessed mitochondria.
Third Symbiotic Event: Origin of Chloroplasts in Chromista
Chromista chloroplasts, with up to four membranes, resulted from subsequent endosymbiotic events involving cells that already had chloroplasts.
This theory, now widely accepted by the scientific community, is supported by evidence such as the presence of circular DNA resembling bacterial DNA, ribosomes similar to those of bacteria (70S) inside mitochondria and chloroplasts, and the double (or triple or quadruple) membranes surrounding these organelles.