BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

V. CYTOLOGY. THE CONCEPT OF THE CELL

Structure of the Plant Cell

Plastids

A group of double-membrane Organelles found exclusively in plant Cells. The following types of Plastids exist: proplastids, METABOLISM/14.html">Chloroplasts, chromoplasts, amyloplasts, and etioplasts. All plant cells contain plastids of a single type.

Plastids of all types are surrounded by a double-membrane envelope, beneath which lies the stroma, or matrix. In addition, plastids contain a system of internal membranes (vesicles, thylakoids, frets), nucleoids in the form of DNA, and Ribosomes.

Proplastids are small (1–1.5 µm), colorless or pale-green undifferentiated plastids found in the meristematic cells of SHOOT and ROOT apices. They are spherical or ellipsoidal in shape. The Structure of proplastids is poorly developed. The inner membrane of the envelope folds inward in places, projecting into the homogeneous stroma. One or two isolated vesicles and thylakoids may be present at the sites of invagination.

Chloroplasts. In higher plants, chloroplasts are located in the leaf mesophyll cells, guard Cells of the epidermis, and other cells of green Tissues. The cells of the palisade and spongy parenchyma in leaf mesophyll contain 20–40 chloroplasts each.

In higher plants, chloroplasts are green and shaped like biconvex, plano-convex, or concavo-convex lenses with a circular or ellipsoidal outline. Their size ranges from 3 to 10 µm.

The Internal Structure of all chloroplasts is represented by pairs of parallel membranes that join at their ends to form disk-shaped sacs called grana thylakoids. Thylakoids (in groups of 5–20) stack on top of each other to form grana. A chloroplast contains 64 to 40–60 grana. The cavities of thylakoids from the same granum and adjacent grana are connected by tubules called frets (stroma thylakoids), the walls of which also have a membranous structure. Thus, the thylakoid system represents a continuous network of interconnected cavities bounded from the stroma by the thylakoid membrane system.

The Development of a chloroplast from a proplastid can occur directly (in the light) or indirectly—via an etioplast (in the dark).

In the light, the inner membrane of the double-membrane proplastid envelope folds into the stroma at several points. These invaginations then pinch off and transform into flat, round vesicles—thylakoids. Subsequently, the thylakoids unite to form grana.

In the second case, the membrane tubules of the prolamellar body in etioplasts (which developed from proplastids in the dark) are reorganized into the thylakoid system only upon exposure to light.

Chloroplasts can also multiply by division from pre-existing chloroplasts, as occurs in Algae (Chlorella, Chlamydomonas, etc.).

The thylakoid membranes of chloroplasts contain fat-soluble pigments (chlorophyll a, chlorophyll b, and carotenoids) as well as an Electron Transport Chain that drives the light reactions of Photosynthesis.

The chloroplast stroma contains plastoglobules, starch grains, ribosomes, and nucleoids.

Plastoglobules are lipid-rich droplets located adjacent to the thylakoids. The number of plastoglobules increases as chloroplasts age.

Starch grains in chloroplasts (1–5 in number) are positioned near the thylakoids. They form in chloroplasts in the light and disappear within 12–24 hours in the dark.

Ribosomes are found in the chloroplast either freely or adsorbed onto The surface of thylakoid membranes. They are smaller than cytoplasmic ribosomes.

Chloroplast DNA has a Primary Structure length of approximately 40–46 µm and is capable of encoding The Biosynthesis of about 120 Proteins with a Molecular Weight of 40,000 (about 300 amino acid residues). Chloroplast DNA self-replicates via a Replication mechanism. Consequently, chloroplasts possess a Protein Biosynthesis apparatus; nevertheless, many chloroplast proteins are encoded by nuclear genes, synthesized on cytoplasmic ribosomes, and subsequently imported into the chloroplasts. Chloroplasts are also the site of biosynthesis for chlorophylls, carotenoids, Fatty acids, Glycolipids, and other substances.

The function of chloroplasts is photosynthesis—the primary synthesis of organic carbon from inorganic carbon utilizing light energy.

Etioplasts are plastids found in plants grown in the dark. They can be observed in cotyledons and embryonic leaves that remain underground. They contain carotenoid and protochlorophyll pigments, ribosomes, and DNA. An etioplast features a double-membrane envelope containing 1–4 prolamellar bodies. The latter consist of four tubules interconnected at a single point in a tetrahedral arrangement. Upon exposure to light, etioplasts in etiolated plants rapidly transform into chloroplasts, and the prolamellar bodies develop into a system of thylakoid membranes.

Chromoplasts are plastids whose Thylakoid membranes contain carotenoids, which impart a yellow or red color to the pericarp of fleshy fruits (rose hips, rowan berries, tomatoes), flower petals, and root crops (carrots). Chromoplasts most frequently develop from chloroplasts and are therefore similar to them in size, shape, and structure. Based on their internal Organization, five types of chromoplasts are distinguished: globular, consisting of lipid droplets (plastoglobules) containing carotenoids (petals of sunflower, beggar-ticks, tansy, coltsfoot); membranous, possessing up to 25 sets of concentric membranes with carotenoids (petals of the daffodil); tubular, characterized by the presence of carotenoid-bearing fibrils (fruits of bell peppers); reticulo-tubular, formed by a dense network of branched, non-parallel tubules (the spathe of certain Araceae inflorescences); and crystalline, containing carotenoids in the form of crystals (fruits of tomatoes, rowan berries, rose hips).

Amyloplasts are colorless plastids filled with starch grains. They are found in the storage Tissues of the endosperm or seed cotyledons, tubers, etc. Amyloplasts are surrounded by a double membrane enclosing the stroma. The stroma may contain a single large starch grain (in potato tuber amyloplasts) or several small ones (in root cap Cell amyloplasts), along with DNA nucleoids and ribosomes. In storage tissues, amyloplasts synthesize starch from sucrose delivered by photosynthetic tissues, storing it until mobilization during germination.

In the root cap and the starch Sheath of the primary stem cortex, amyloplasts play a crucial role in the perception of gravity and the processes of geotropism.

Plastids are capable of interconversion. Chloroplasts, etioplasts, chromoplasts, and amyloplasts can all develop directly from proplastids. The reversion of plastids into proplastids can be observed when differentiated tissue cells transform back into meristematic cells. Etioplasts develop into chloroplasts upon exposure to light. Amyloplasts are formed from either chloroplasts or etioplasts, and when exposed to light, amyloplasts can transform back into chloroplasts. In carrot root vegetables, amyloplasts transform into chromoplasts.

In the cells of meristematic tissues, the number of plastids increases through division, which coincides with Cell Division. However, in young leaf cells, plastid division continues even after cell division has ceased. Besides proplastids, chloroplasts are also capable of division.

In the course of evolution, chloroplasts most likely originated from cyanobacteria.