114 CHAPTER 6
outside of the cell by membrane-located ion pumps,
using energy derived from the dissociation of ATP.
The ions then re-enter the cell through specific types
of membrane proteins termed symporters, which
simultaneously transport an organic molecule into
the cell. This seems to apply generally in fungi, because
their plasma membranes have been shown to have
H+-coupled transport systems for sugars, amino acids,
nitrate, ammonium, phosphate, and sulfate, as well as
other ion channels such as stretch-activated channels
for calcium, and nonselective channels for both cations
and anions (Garrill 1995).
But, fungi are thought to differ from most other
organisms in one significant respect: in most cell types
the ion pumps and the symport proteins probably
occur in close proximity to one another, whereas in
tip-growing fungi the ion pumps are thought to be most
active behind the apex, whereas the symport proteins
are active close to the tip. This spatial separation
would explain the external electrical field (Fig. 6.3)
and it makes sense intuitively. The nutrient-uptake
symporters would be of most value at the hyphal
tip, which extends continuously into fresh zones
of nutrients, while immediately behind the tip there
is a dense zone of mitochondria that could supply
the ATP needed for the proton pumps (see Figs 3.2,
3.4, 3.15).Enzyme secretion
The need to obtain nutrients has, literally, shaped the
way that fungi grow. The rate of diffusion of soluble
organic nutrients to a stationary cell would always
tend to be growth-limiting, and the need to release
degradative enzymes imposes even greater constraints.
Enzymes are large molecules, about 20,000–60,000
Daltons in the case of fungal cellulases, so they do not
diffuse far from the hyphal surface. As a result, fungi
create localized zones of substrate erosion (nutrient
depletion) when growing in substrates such as cellulose,
and the hyphaemust extend continuously into fresh
zones (Fig. 6.4). This explains why yeast cells never pro-
duce depolymerase enzymes, because they would have
no way of escaping from the erosion zones they had
created. Instead, yeasts occur in environments rich
in simple soluble nutrients – on leaf, fruit and root
surfaces, or on mucosal membranes in the case of
Candida albicans. They depend either on a continuous
supply of nutrients from the underlying substratum, or
on water currents to bring fresh supplies of nutrients.
The large size of enzymes creates a potential prob-
lem for their release through the hyphal wall, because
this would require the presence of continuous pores
of sufficiently large size. Estimates of wall porosity
can be obtained by studying the permeability of walls
to molecules of known molecular mass such as com-
mercially available polyethylene glycols and dextrans.
These studies suggest a cut-off of wall porosity at
between 700 and 5000 Da, which is much lower than
the size of most enzymes. These findings apply mainly
to yeasts, and might be different for mycelial fungi.
However, recent studies indicate that enzymes are
released mainly and perhaps exclusively in the regions
of new wall growth. This was first shown when a
cellulase gene was cloned into yeast (S. cerevisiae) and
the enzyme was found to be released from the growth
sites of the bud. Also, a glucoamylase has been shownFig. 6.4Fungal strategies for growth on insol-
uble polymers. (a) Hyphae extend continuously
at the apex, drawing protoplasm forward to
evacuate the zones of enzyme erosion of the
substrate. Yeasts do not utilize insoluble
(nondiffusible) polymers because they would
become trapped in their own substrate erosion
zones. (b) Suggested defense of a substrate
by a hypha of a polymer-degrading fungus.
Enzymes are secreted at the apex to degrade the
polymer, and the soluble nutrients released
by these enzymes are absorbed subapically.
Antibiotics or other inhibitors (shown as arrow-
heads) may be released subapically into the
substrate erosion zone to prevent competing
organisms from using the enzyme digestion
products.