Yeast Cell Envelopes Biochemistry Biophysics and Ultrastructure

Volume I

Leo H Arnold

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315898698
• eBook ISBN: 9781351094696
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

A comprehensive review of the yeast cell envelope has not appeared previously and therefore this book is timely. The title of this volume was chosen to reflect the three major areas of contribution to our current understanding of the cell envelope, but we have not attempted to group chapters into subdivisions. The approach was to describe phenomena, to review the literature and to illuminate outstanding problems. It was also attempted to generate working hypotheses which may stimulate further studies. The some of these ideas be of germinal value is of more concern to us than that all of the hypotheses should stand the test of further experimentation.

Chapter 1

INTRODUCTION

W. N. Arnold

A working definition of the class of organisms collectively known as “yeasts” is not easily postulated. They are unicellular, eukaryotic microorganisms that typically reproduce by budding. So much is true for the majority of yeasts but one soon runs into exceptions; there are species with a propensity for mycelium formation, others with a fission type of cell division, and still another genus that bears conidia on sterigmata. As discussed by Lodder¹ (Table 1) the term “yeasts” developed historically and really includes a heterogeneous group of microorganisms. Phaff et al.,² have reviewed the etymology of “yeast”, and equivalent terms in other languages, and point out the consistent relationship to fermentation. However, they also conclude that the name embraces a heterogeneous collection of fungi.

The number of defined yeast species is now somewhat in excess of 500. In addition there are a number of fungi with “yeast-like” cells as part of their cell cycle. At the other extreme some biochemists incorrectly use the term yeast as synonymous with Saccharomyces cerevisiae. One of the themes that will emerge in the following chapters is that a rich variety of additional species awaits further biological investigation and that in some circumstances the more exotic species may offer advantages for experimental study.

This book is devoted to the yeast cell envelope. The accumulated primary literature on various aspects of this subject now merits this degree of specialization. More general references on yeast include volumes edited by Cook,³ and by Rose and Harrison.⁴ The text by Phaff et al.,² includes an introduction to yeast biology.

The cell envelope consists of the plasma membrane, the periplasmic space, cell wall, and (for some species) a slime layer. A schematic representation is given in Figure 1, primarily to establish relative location of components rather than actual dimensions. The cell envelope is thus bounded on the inside by cytoplasm, and on the outside by the medium.

The definitions that follow were developed with the assistance of contributors and other colleagues and have been applied consistently. In the majority of cases I have chosen not to belabor synonymy. Also, inappropriate terms or incorrect applications of terms in current usage have been avoided. One outstanding example of the latter from the older literature is the generic term “membrane” which was sometimes used to describe any membrane of the protoplasm (including the plasma membrane), the cell wall, or even the cell envelope. In the current setting this is, of course, intolerably confusing.

The plasma membrane is the boundary of the cell protoplasm and is microscopically discernible as a bilayer. The plasma membrane is the primary site for solute regulation between the cytoplasm and the medium. Plasma membrane, plasmalemma, ectoplast, protoplasmic membrane, and cytoplasmic membrane are used synonymously in the literature. Semantic arguments against the last two terms have been registered; plasma membrane is in vogue although plasmalemma is frequently encountered. Ectoplast is valid but has not been popularly embraced.

Cytoplasm plus plasma membrane is defined as protoplasm. The protoplasm of a cell gives rise to a protoplast, i.e., an unnaturally produced entity derived from a cell by denuding the cell of cell wall (and slime layer).

Table 1
ABBREVIATIONS FOR YEAST GENERA

Bullera	B.
Candida	C.
Cryptococcus	Cr.
Debaryomyces	Deb.
Endomycopsis	E.
Hanseniaspora	H’spora
Hansenula	H.
Issatchenkia	I.
Kloeckera	Kl.
Kluyveromyces	K.
Leucosporidium	Leu.
Lodderomyces	Lod.
Lipomyces	L.
Melschnikowia	M.
Nadsonia	N.
Nematospora	Nem
Pichia	P.
Pityrosporum	Pit.
Rhodotorula	Rh.
Saccharomyces	Sacch.
Saccharomycodes	S’codes
Saccharomycopsis	S.
Schizosaccharomyces	Schiz.
Sporidiobolus	Sporid.
Sporobolomyces	Sp.
Sterigmatomyces	St.
Torulopsis	T.
Trichosporon	Tr.
Trigonopsis	Trig.

Following Lodder, J., The Yeasts, North-Holland, Amsterdam, 1971.

The periplasmic space is a region of the cell envelope which is bounded by the plasma membrane and the inner aspect of the cell wall. This space includes invaginations in the plasma membrane and outward excursions into the inner aspect of the cell wall. The periplasmic space is the locale for several digestive enzymes that act upon substrates that require hydrolysis prior to assimilation by the protoplasm.⁵ Physical measurements⁶ indicate that reversible changes in the size of the periplasmic space attend the temporary exposure of cells to high osmotic pressure in the bathing medium.

The cell wall is an integument to the cytoplasm and is that part of the cell envelope which confers mechanical stability and dictates shape. For the majority of yeast species the cell wall is the outermost anatomical region. Under normal conditions the yeast cell is turgid and the osmotic potential of the protoplasm is counteracted by wall pressure.

The slime layer is a peripheral region of the envelope which is associated more or less firmly with the outer aspect of the cell wall. ...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Introduction
• Chapter 2 Atlas of Cell Morphology
• Chapter 3 Physical Aspects of the Yeast Cell Envelope
• Chapter 4 Transport Processes in the Plasma Membrane
• Chapter 5 Nature and Disposition of Polysaccharides within the Cell Envelope
• Chapter 6 Antigenic Determinants
• Chapter 7 Lipids
• Subject Index
• Taxonomic Index