GENERAL MICROBIOLOGY - T.P. Pirog - 2004

9. YEASTS

9.2. STRUCTURE OF THE YEAST CELL

A Yeast Cell is a typical Introduction/5.html">Eukaryotic Cell consisting of a Cell wall, Cytoplasm with a Plasma Membrane, and Organelles (Endoplasmic reticulum, dictyosomes, Lysosomes, Microbodies, Mitochondria, Ribosomes, inclusions, vacuoles, Nucleus) (see Chapter 5, Fig. 5.8). The Nucleus is clearly distinct and enclosed by a membrane.

Cell wall. The yeast cell is separated from the external environment by a cell wall with a thickness of 70-350 nm. The main component of the yeast cell wall is Polysaccharides (up to 60-70% of dry weight). In addition to polysaccharides, The Cell wall contains Proteins (6-13%), Lipids (2-9%), and inorganic polyphosphates. The primary structural units of polysaccharides are predominantly glucose and mannose. For example, Fig. 9.1 shows the cell wall of the baker's yeast Saccharomyces cerevisiae, which is composed of phosphomannan, mannan, glucan, and proteins.

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Fig. 9.1. Schematic representation of the cell wall Structure OF THE yeast Saccharomyces cerevisiae (A) and The Plasma Membrane (B)

Besides homopolymers, yeast cell walls may also contain heteropolymers. For instance, galactomannans are constituents of the cell walls in Yeasts of the genera Trichosporon and Torulopsis. Xylomannans have been detected in certain species of basidiomycetous yeasts. The aminopolysaccharide Chitin is also a component of yeast cell walls.

Capsule. Many yeasts inhabiting natural substrates poor in available nutrients form a polysaccharide capsule on their cell surface. It was previously believed that the capsule facilitates cell attachment, preventing them from being washed away, and protects Cells from unfavorable environmental factors. Later studies have shown that the presence of capsule enables the assimilation of nutrients even at low concentrations in the surrounding environment.

Differences in The chemical composition of capsular and extracellular yeast polysaccharides are considered by some researchers to be a taxonomic feature. Thus, phosphomannans are characteristic polysaccharide components in Representatives of the genera Hansenula and Pichia. In yeasts of the genus Rhodotorula, the main component is mannan, whereas in Cryptococcus, it is an acidic heteropolysaccharide consisting of xylose, mannose, and glucuronic acid.

The cytoplasmic membrane consists of a lipid bilayer located between thin protein layers. Protein molecules are distributed not only on the membrane surface but also span entirely through it (see Fig. 9.1). The yeast Plasmalemma is about 7 nm thick, contains approximately equal amounts of lipids and proteins, and significantly fewer CARBOHYDRATES. Lipids are mainly represented by mono-, di-, and triglycerides, Glycolipids, and small amounts of sterols (ergosterol and zymosterol). The carbohydrate content ranges from 2-10% of the membrane mass. Carbohydrate components are present in the form of glycolipids and Glycoproteins.

The nucleus is surrounded by a nuclear membrane (a double-layered, perforated membrane). DNA (the carrier of Genetic information) is distributed among the Chromosomes.

Mitochondria are spherical or rod-shaped structures enclosed by a double membrane. Folds of the inner membrane form cristae. The inner membrane contains Components of the Electron Transport Chain and ATP synthase. In the yeast cell, the nucleus is the primary, but not the only, carrier of genetic information. Part of this information is housed within Mitochondrial DNA.

The endoplasmic reticulum is a system of membranous plates, channels, vesicles, and cisternae surrounded by a unit membrane. Its Functions in the yeast cell include ensuring the Intracellular Transport of nutrients to sites of synthetic reactions, participating in Carbohydrate METABOLISM, lipid synthesis, and other essential metabolic processes.

Lysosomes are intracellular organelles enclosed by a lipoprotein membrane. They contain a set of Enzymes that hydrolyze proteins, polysaccharides, and Nucleic Acids.

Microbodies are a general term for organelles such as Peroxisomes, glyoxysomes, etc. Microbodies contain over 20 different enzymes. For instance, peroxisomes harbor catalase and peroxidase. Additionally, microbodies contain enzymes for fatty acid $\beta$-oxidation, The Glyoxylate cycle, dehydrogenases, etc.

Mature yeast cells contain a large vacuole. Its exact functions have not been definitively established, but it has been shown to contain hydrolytic enzymes, polyphosphates, lipids, low-molecular-weight cellular intermediates, and Metal Ions. It is likely that the vacuole in the yeast cell acts as a reservoir for storing nutrients and hydrolytic enzymes.

Inclusions represent reserve substances, which in yeasts include Glycogen (Saccharomyces, Cryptococcus), polyphosphates (volutin) (Saccharomyces), and fats in the form of individual droplets (Trichosporon) or a single lipid vacuole (Lipomyces).