The Dirt on Black-Market Plants ■ 287cleaning up—so that the cell can function effi-
ciently (review Figure 4.8 for a comparison of
prokaryotic and eukaryotic cells, and Figure 4.9
for an overview of eukaryotic organelles). All of
these organelles take up space, so the diameter
of a eukaryotic cell is, on average, 10 times larger
than that of a prokaryotic cell, and the eukary-
otic cell volume is a thousandfold greater.
This compartmentalization of the cell interior
enables eukaryotic cells to do things that most
prokaryotic cells cannot do. For example, some
eukaryotes engulf their prey and digest them
internally. That’s the way many single-celled
eukaryotes, such as the blob-like amoebas, eat:
they extend gooey cytoplasmic arms to engulf
other cells whole, then digest their prey with an
elaborate system of internal compartments for
digestion, ridding the cell of waste and storing
surplus food (Figure 16.2).
Figure 16.2
Eukaryotes have a true nucleus and compartmentalized cells
The more complex cell structure of eukaryotes enables them to perform
functions that are impossible for prokaryotes. Intracellular compartments
such as food vacuoles enable this amoeba to digest its food, a single-
celled alga. The amoeba expels excess water with the help of its
contractile vacuole, another type of intracellular organelle.Food
vacuolesContractile
vacuole Nucleus AlgaeFungi Play Well with Others
P
lants probably would not have been as successful
on land if they had not entered into a mutualistic
relationship with fungi almost immediately on their
arrival. Today, the vast majority of plants in the wild
have mutualistic fungi, known as mycorrhizal fungi,
associated with their root systems. (The mutualistic
relationships between plants and mycorrhizal fungi are
called “mycorrhizae.”) Truffles, morels, and chanterelles,
all beloved by gourmets, are the reproductive structures
of mycorrhizal fungi.
Mycorrhizal fungi form thick, spongy mats of mycelium on
and in the roots of their plant hosts and also extend into the
surroundings, sometimes permeating several acres of the
soil around the root. Mycelia are thinner, more extensively
branched, and in closer contact with the soil than even the
thinnest branches on a plant root. As a result, a mycelial
mat plumbs far more water and mineral nutrients, such
as phosphorus and nitrogen, than the plant’s root system
could absorb on its own. In return for sharing absorbed
water and mineral nutrients, the fungus obtains sugars that
the plant manufactures through photosynthesis.
Mycorrhizal fungi assist in providing nutrients to orchid
seeds, which are tiny and lack stored food. The embryo
within a newly sprouted orchid seed could not survive
without the mycorrhizal network linking the seedling to
mature photosynthesizing plants, from which the seedling
draws nourishment until it can photosynthesize on its own.
Fungal mutualisms are not found only with other
eukaryotes. A lichen is a mutualistic association betweena photosynthetic prokaryote and a fungus. The fungus
receives sugars and other carbon compounds from
its photosynthetic partner, usually a green alga or a
cyanobacterium. In return, the fungus produces lichen
acids, a mixture of chemicals that scientists believe may
function to protect both the fungus and its partner from
being eaten by predators.
Lichens grow very slowly and are often pioneers in
barren environments. Lichen acids wear down a rocky
surface, facilitating soil formation. Soil particles build up
from the slow weathering of rock, and over time, other
life-forms, including plants, gain a toehold in the newly
made soil.Plants that have no mycorrhizal
fungi associated with their roots
do not grow as well......as those that host
the beneficial fungi.