MICROBIOLOGY Study Guide - 2012

CHAPTER 5. FUNGI

5.5. STRUCTURE OF THE YEAST CELL

The Yeast Cell possesses all the fundamental structures inherent to any Introduction/5.html">Eukaryotic Cell: a Cell wall, Plasma Membrane, Cytoplasm, Nucleus, Mitochondria, Ribosomes, Lysosomes, vacuoles, nutrient reserves, and membranous structures (Fig. 16).

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Fig. 16. Structure OF THE yeast cell (Schlegel, 1987):

CW — cell wall; PM — plasma membrane; Cyt — cytoplasm containing ribosomes; Nuc — nucleolus; Mit — mitochondrion; ER — Endoplasmic reticulum; D — dictyosome; P — polyphosphate granules; L — lipid droplet; Sc — bud scar

The cell wall is a sturdy, elastic structure ranging from 40 to 250 nm in thickness, typically consisting of three layers. The outer layer is lipoproteinaceous; adjacent to it is the mannan-protein layer, followed by the third layer—composed of β-glucan—which lies next to The Plasma Membrane. This innermost layer plays a primary role in maintaining The Cell's constant shape. Chitin, a polymer consisting of N-acetylglucosamine residues, has also been detected in the yeast cell wall. It forms a ring around the daughter scars of the mother cell. Enzymes of the hydrolase and oxidase classes, which catalyze substrate Breakdown and Oxidation reactions, are concentrated within the glucan layers.

The cell wall performs essential Functions: it gives the cell a defined shape, maintains turgor pressure, and exhibits selective permeability—allowing only low-molecular-weight compounds to enter the cell.

The cytoplasmic membrane or Plasmalemma (CPM) is a 6–8 nm thick structure that lies tightly against the inner surface of the cell wall. It consists of a phospholipid bilayer with embedded protein molecules. Peripheral Proteins are located on the inner and outer surfaces of the membrane. In Saccharomyces cerevisiae, the predominant Phospholipids are phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine, which account for up to 90% of all Membrane Lipids. In addition to phospholipids, the CPM also contains sterol-type lipids, primarily ergosterol. The COMPOSITION OF THE cell membrane from phospholipid molecules imparts two mutually opposing properties to the structure: while the glycerol backbone with its phosphorus group attracts Water (being hydrophilic), the tails of the fatty acid residues repel water (being hydrophobic). As a result, an impermeable bilayer is formed without covalent bonds between the phospholipid molecules.

Functions of the CPM include regulating cell wall Biosynthesis, transporting nutrients and K⁺ and Na⁺ ions into the cell, and excreting metabolic products. Membrane components act as an ion exchanger, regulating the passage of charged ions across the membrane.

The cytoplasm of the yeast cell (Cytosol) occupies more than 50% of the cell volume and represents a complex colloidal system whose viscosity varies depending on the cell's age. Young Cells exhibit active cyclosis—circular cytoplasmic streaming that ensures efficient transport of metabolic products from one organelle to another. All Cellular Structures and nutrient reserve granules are located within the cytoplasm. According to electron microscopic studies, the structural foundation of the cytoplasm is a homogeneous network of fine filamentous and globular molecules.

The nucleus plays a central role in the cell. Its primary function is the storage of Genetic information. It controls PROTEIN AND NUCLEIC acid synthesis, cell reproduction, and The formation of new cellular structures. The yeast nucleus is a spherical body 1–2 µm in diameter, separated from the cytoplasm by a double, porous membrane. Possessing a unique structure, the nuclear membrane directly participates in METABOLISM/36.html">DNA Replication and can communicate with the cytoplasm. The main component of The Nucleus is Chromatin, which consists of DNA and Histones. Chromatin may exist in a diffuse state during interphase. Prior to reproduction, all chromatin condenses into Chromosomes, the division of which transmits the Organism's hereditary traits. The chromosome number in the nuclei of various yeast species can vary, ranging from 2 (in Candida utilis) to 16 (in Saccharomyces cerevisiae). Inside the nucleus lies the nucleoplasm with a nucleolus, which synthesizes high-molecular-weight RNA for ribosomes.

Mitochondria are obligatory structures of The Eukaryotic Cell. Depending on yeast cultivation conditions, mitochondria may be spherical, rod-shaped, or filamentous. Mitochondria are bounded by a double membrane: an outer smooth membrane and an inner membrane that forms numerous folds or tubules called cristae. The internal content of the mitochondria is called the matrix. Mitochondria consist of 30–40% lipids and 60–70% protein.

The enzymes of The Tricarboxylic Acid Cycle are localized in the mitochondrial matrix, while the enzymes of the Respiratory Chain reside in the inner membrane. Mitochondria also contain Mitochondrial DNA (mtDNA), which encodes certain matrix enzymes. The number of mitochondria per cell varies from 1 to 20 depending on the growth stage and yeast cultivation conditions. A critical function of mitochondria is the synthesis of ATP (adenosine triphosphate) during Respiration.

The endoplasmic reticulum (ER) is a system of double membranes (tubules, vesicles, or cisternae) that is in close contact with the CPM, nuclear membrane (nuclear envelope), mitochondria, and other structures. The ER permeates the entire cytoplasm, and its surface is studded with ribosomes. The primary function of the ER is to ensure the orderly delivery of various substances to their sites of utilization. The large active surface area of the membranes facilitates the localization of diverse enzyme systems. Chains of ribosomes are arranged along The endoplasmic reticulum.

Dictyosomes (derived from the Greek words diktyon meaning net and soma meaning body) are tightly stacked arrays of 3–5 flat sacs, also known as Golgi cisternae. At the periphery, these sacs expand into a rim and are surrounded by vesicles. They perform excretory and secretory functions, participate in the formation of new membranes, and contribute to the growth of cell walls. The Golgi apparatus is the source for the formation of specialized structures known as lysosomes.

Lysosomes are electron-dense granules bounded by a lipoprotein membrane. They contain active enzymes of the hydrolase class that rid the cell of foreign and irreversibly damaged endogenous subcellular components.

Ribosomes are nucleoprotein particles about 20 nm in diameter, containing 40–45% RNA and 55–60% protein. They are located in the cytoplasm either freely or bound to the membranes of the endoplasmic reticulum. Ribosomes consist of two subunits—large and small—each built from a ribosomal RNA molecule and a specific set of proteins. The primary function of ribosomes is Protein Synthesis.

Vacuoles are Organelles separated from the cytoplasm by a vacuolar membrane. The vacuole plays a crucial role in cellular metabolism, concentrating sodium, potassium, calcium, and magnesium salts, as well as various enzymes, polyphosphates, CARBOHYDRATES, and proteins. These substances are utilized by The Cell as needed. The size of the vacuole depends on the age and physiological state of the yeast cell: in young cells, vacuoles are small, whereas in older cells, they occupy the majority of the cell volume.

Glycogen is a reserve nutrient and polysaccharide whose structural unit is α-glucose. Glycogen granules appear in yeast cells at the beginning of Fermentation when carbohydrates are abundant in the medium. The glycogen content can increase to over 30% of the yeast dry weight. By the end of fermentation, the intracellular glycogen content decreases. Along with other reserve substances, the disaccharide trehalose is stored in the yeast cell. It is enclosed in small membrane-bound vesicles. Yeasts grown under anaerobic conditions accumulate primarily glycogen, whereas those grown under aerobic conditions accumulate trehalose.

Metachromatin (volutin) is a cellular reserve substance composed of Lipoproteins, polyphosphates, magnesium, and RNA. It accumulates in vacuoles or in the cytoplasm as granules. Metachromatin is absent in yeasts grown in phosphorus-deficient media.

Lipids accumulate in the cytoplasm and serve as a reserve substance. They represent a mixture of triglycerides, phospholipids, and ergosterol. In older cells, fat droplets are clearly distinguishable due to light refraction.

Bud scars appear On the surface of the mother cell after the daughter cell detaches during budding, the asexual method of yeast reproduction. The age of the cell can be determined by counting its scars.