Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.7 Organelles

The nucleus has a rounded, oval, or elongated shape. A Cell may contain one or multiple nuclei. The Nucleus has a diameter of approximately 10 µm, though nuclear sizes vary across different cell types. Large nuclei are characteristic of young, meristematic Cells, where they can occupy up to 3/4 of the total cell volume.

Externally, the nucleus is enveloped by a double membrane with a perinuclear space in between. The outer membrane features outgrowths that directly transition into the walls of The Endoplasmic reticulum, ensuring continuous communication between the nucleus and the Cytoplasm. The interior of the nucleus contains a granular ground substance (nuclear sap, or nucleoplasm) that houses the Chromosomes and the nucleolus. Chromatin consists of DNA, histone and non-histone Proteins, along with minor amounts of RNA and Lipids. The nucleolus is the site of ribosome material synthesis and The formation of Nuclear Proteins. Chromosomes are built from DNA molecules bound to histone proteins. Their shape varies and is specific to a given Organism species. Chromosomes can be up to 20 µm long, compactly packing a DNA molecule up to 2 cm in length. Specific genes located in distinct chromosomal regions (loci) carry the information required for Protein Synthesis. The nuclear genome contains the Genetic information (encoded by The nucleotide sequence in the DNA molecule) regarding The Structure of all Enzymes (about 10,000 of them), cellular structural RNA proteins, and The regulatory mechanisms governing their synthesis.

The nuclear envelope is perforated by pores 10-20 nm in diameter, through which Nucleic Acids and Proteins are transported. The chromosome number is constant for each species. The nucleoplasm contains enzymes and Cofactors essential for METABOLISM/36.html">DNA Replication and Transcription, alongside various RNA molecules and enzymes.

The nucleus serves as the carrier of The Cell's hereditary information.

Plastids are organelles unique to plant cells. There are three MAIN TYPES OF plastids: colorless leucoplasts, green Chloroplasts, and pigmented chromoplasts containing yellow and red colorants.

The interconversion of one plastid type into another is also possible (such as the greening of potatoes).

Chloroplasts are shaped like biconvex lenses, measuring about 4-6 µm. A single cell may contain 25 to 50 of them. Externally, a chloroplast is enclosed by an envelope consisting of two lipoprotein membranes, with a periplastid space between them. The inner membrane forms thickened invaginations known as thylakoids, which can take the shape of discs called grana thylakoids. Several such thylakoids stack on top of one another to form a granum. Other thylakoids interconnecting the grana are termed stroma thylakoids, alongside fret (intergrana) thylakoids. The membranes forming the grana contain the green pigment chlorophyll.

This is precisely where the light-dependent reactions of Photosynthesis take place—the absorption of light rays by chlorophyll and The conversion of light energy into the energy of excited electrons.

The chemical composition of chloroplasts (as a percentage of dry weight) is as follows: protein — 35-55%, lipids — 20-30%, CARBOHYDRATES — 10%, RNA — 2-3%, DNA — 0.5%, chlorophyll — 9%, carotenoids — 4.5%, Fe — 80%, Zn — 70%, Cu — 50% (with Fe, Zn, and Cu expressed as percentages of total cellular content).

Chloroplasts are motile within the cell and exhibit phototaxis. They possess a degree of autonomy within the cellular system, housing their own Ribosomes, DNA, and enzymes involved in protein synthesis.

Mitochondria are small, spherical or elongated bodies measuring 0.5-1.5 µm. A cell may contain anywhere from 100 to 3,000 mitochondria. They are surrounded by an envelope composed of two lipoprotein membranes with an intermembrane space between them. The inner membrane forms infoldings called cristae. The space enclosed by the inner membrane contains the matrix. The inner mitochondrial membrane and its cristae are constructed from orderly arranged enzyme systems. Thanks to the cristae, the internal working surface area of the mitochondrial membranes is immensely expanded. A number of enzymes are also located in the matrix. Through mitochondrial enzymes, intracellular Respiration and the storage of the resulting energy in the form of ATP are carried out. Mitochondria drive The breakdown of pyruvic acid into CO2 and H2O. Mitochondria are the powerhouse of the cell. Much like chloroplasts, mitochondria act as semi-autonomous genetic centers of the cell, possessing their own ribosomes, DNA, and protein-synthesizing enzymes, which enables them to replicate.

The endoplasmic reticulum (ER) is a system of membrane-bounded channels and cisternae measuring 5-6 nm in thickness. The endoplasmic reticulum can be smooth (agranular) or rough (granular). Ribosomes are attached to the outer surface of the rough ER channels. Because enzymes are embedded in ER membranes, it Functions as a assembly line for the enzymatic transformation and synthesis of substances, as well as a highway for Intracellular Transport. The reticulum originates from the outer nuclear membrane, branches out, extends toward various cytoplasmic organelles, and reaches the Plasmalemma, thereby interconnecting all PARTS OF THE cell. Furthermore, the channels of the endoplasmic reticulum pass through plasmodesmata, connecting the reticulum of adjacent cells. The membranes of the endoplasmic reticulum partition the cell into compartments and serve as pathways for bioelectric currents—signals that alter selective membrane permeability and enzyme activity.

The Golgi apparatus is represented by dictyosomes. Each dictyosome constitutes a stacked system of membranes. The spaces between membranes appear either as narrow slits and flattened sacks (cisternae) or as vesicles. Their shape changes during the organelle's activity and depends on the degree to which the intermembrane space is filled with secreted and accumulating substances. Evidently, numerous cellular vacuoles surrounded by a tonoplast are products of Golgi activity; they originate as vesicles that pinch off from the Golgi apparatus and subsequently increase in size. The Golgi apparatus is particularly well-developed in secretory cells that store or export various substances, and it synthesizes and secretes the components that form The Cell wall.

Peroxisomes and glyoxysomes are spherical organelles with a diameter of 0.2-1.5 µm, enclosed by a single membrane and containing a granular matrix. Peroxisomes perform the function of oxidizing glycolic acid synthesized in chloroplasts during photosynthesis to produce The amino acid Glycine, which is subsequently converted into Serine in the mitochondria.

In the leaves of higher plants, peroxisomes participate in Photorespiration. Glyoxysomes convert Fatty acids into sugars with the help of specific enzymes. During enzymatic transformations within peroxisomes and glyoxysomes, hydrogen peroxide is generated, which is then broken down by the enzyme catalase.

Ribosomes are spherical particles with a diameter of 20-30 nm. Some are attached to the outer surface of the endoplasmic reticulum membranes, while others remain free in the hyaloplasm. A cell may contain up to 5 million ribosomes, which serve as the machinery for protein synthesis. Ribosomes are also found in the nucleus, mitochondria, and chloroplasts, where they synthesize the proteins that make up those organelles. Each ribosome consists of two nucleoprotein subunits. These ribosomal subunits are assembled in the nucleolus and then transported into the cytoplasm, where functional ribosomes are formed on mRNA molecules.

Microtubules are tubular structures with internal channels, having an outer diameter of 25 nm (250 Å), and occasionally appearing as double tubules. The walls of microtubules are built from protein subunits. Microtubules are believed to be associated with the contractile activity of the cytoplasm and its derivatives. During Cell Division, microtubules form the spindle fibers. In cells lacking a rigid cell wall, microtubules perform a supportive function, forming the cell's internal Cytoskeleton.

Spherosomes are highly light-refractive spherical bodies with a diameter of 0.5 µm. Containing lipids, they are frequently referred to as lipid droplets (oleosomes). Enzymes such as lipase and esterase have been detected within them. During the germination of oilseeds, spherosomes function in coordination with glyoxysomes to break down complex fats.

A vacuole is a cavity filled with cell sap and enclosed by a membrane (the tonoplast), representing a quintessential feature of plant cells. Vacuoles are formed by the fusion of endoplasmic reticulum cisternae. The vacuole contains cell sap in which mineral salts, organic acids, sugars, Amino Acids, and proteins are dissolved. In addition to these substances, vacuolar cell sap contains phenols, Tannins, Alkaloids, and anthocyanins. Vacuoles are rich in enzymes (predominantly Hydrolases). The vacuole serves as the site where cellular nutrients are accumulated and stored, as well as where harmful metabolic byproducts are neutralized by enzymes.

Cytoplasmic inclusions include lipid droplets, starch grains, and aleurone grains. Typically, cytoplasmic inclusions function as reserve nutrient stores.