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exponentially growing cultures (where one cell produces
two in a unit of time, two produce four in the next
unit of time, and so on) one half of all the cells will
be first-time mothers, one quarter will be second-time
mothers, and an infinitessimally small number will be
very old mothers. In fact, the typical chitin content of a
yeast population seldom exceeds more than 1– 2% of
the total wall material, and almost all of this chitin is
found in the bud scars, not in the rest of the yeast cell
wall. We return to this topic in Chapter 4, because some
important advances have been made in understand-
ing growth polarity, based on the use of cell-cycle
mutants in the laboratory of the Nobel Laureate, Sir Paul
Nurse.
The taxonomy of yeasts is complicated by the lack
of obvious morphological features, so in recent years
it has relied heavily on molecular approaches. There
are probably many hundreds, if not thousands, of
yeast species that have yet to be clearly demarcated and
described. Recent evidence from the ascomycetous
budding yeasts suggests that these are a monophyletic
group – all with a common ancestor – and they are
not simply reduced forms derived from the mycelial
Ascomycota. They are also distinct from the “fission
yeasts” such as Schizosaccharomyces species, which
do not bud but instead form filaments that fragment
by septation into brick-like cells (arthrospores, or
arthroconidia).

Fungal walls and wall components

In recent years it has become clear that fungal walls
serve many important roles, quite apart from the obvi-
ous role of providing a structural barrier. For example,
the way in which a fungus grows – whether as cylin-
drical hyphae or as yeasts – is determined by the wall
components and the ways in which these are assem-
bled and bonded to one another. The wall is also the
interface between a fungus and its environment: it

protects against osmotic lysis, it acts as a molecular sieve

regulating the passage of large molecules through the

wall pore space, and if the wall contains pigments such

as melanin it can protect the cells against ultraviolet

radiation or the lytic enzymes of other organisms.

In addition to these points, the wall can have several

physiological roles. It can contain binding sites for

enzymes, because many disaccharides (e.g. sucrose and

cellobiose) and small peptides need to be degraded

to monomers before they can pass through the cell

membrane, and this is typically achieved by the actions

of wall-bound enzymes (Chapter 6). The wall also

can have surface components that mediate the inter-

actions of fungi with other organisms, including plant

and animal hosts. All these features require a detailed

understanding of wall structure and architecture.

The major wall components

The primary approach to investigating the wall com-

position of fungi is to disrupt fungal cells and purify

the walls by using detergents and other mild chemical

treatments, then use acids, alkalis and enzymes to

degrade the walls sequentially. Although relatively few

fungi have been analysed in detail, these treatments

show that fungal walls are predominantly composed

of polysaccharides, with lesser amounts of proteins

and other components. The major wall components can

be categorized into two major types: (i) the structural

(fibrillar) polymersthat consist predominantly of

straight-chain molecules, providing structural rigidity,

and (ii) the matrix componentsthat cross-link the

fibrils and that coat and embed the structural polymers.

The main wall polysaccharides differ between the

majorfungal groups, as shown in Table 3.1. The Chy-

tridiomycota, Ascomycota, and Basidiomycota typically

have chitin and glucans (polymers of glucose) as their

major wall polysaccharides. Chitin consists of long,

straight chains of β-1,4 linked N-acetylglucosamine

FUNGAL STRUCTURE AND ULTRASTRUCTURE 55

Taxonomic group Structural (fibrillar) components Matrix components

Chytridiomycota Chitin Glucan?

Glucan

Zygomycota Chitin Polyglucuronic acid

Chitosan Mannoproteins

Ascomycota Chitin Mannoproteins

b-(1→3), b-(1→6)-glucan a(1→3)-glucan

Basidiomycota Chitin Mannoproteins

b-(1→3), b-(1→6)-glucan a(1→3)-glucan

Oomycota b-(1→3), b-(1→6)-glucan Glucan

(not true fungi) Cellulose

Table 3.1The major components of
fungal walls.