An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 36

of chromosomes into the two daughter cells during cell division. Three types of fibers are
present: actin filaments, intermediate filaments, and microtubules.
Cilia are structures similar to flagella but are shorter and numerous. Only one group of
protozoa, called ciliates, contains cilia. Paramecium species contain nearly 10 4 cilia per cell.
Ciliated organisms move much faster than flagellated ones.
This completes our summary of eukaryotic cell structure. Now let us turn our attention to
the microscopic eukaryotes.
Fungi are heterotrophs that are widespread in nature. Fungal cells are larger than bacterial
cells, and their typical internal structures, such as the nucleus and vacuoles, can be seen
easily with a light microscope. Two major groups of fungi are yeasts and molds.
Yeasts are single small cells of 5- to 10- m m size. Yeast cells are usually spherical, cylindrical,
or oval. Yeasts can reproduce by asexual or sexual means. Asexual reproduction is by either
budding or fission. In budding, a small bud cell forms on the cell, which gradually enlarges
and separates from the mother cell. Asexual reproduction by fission is similar to that of
bacteria. Only a few species of yeast can reproduce by fission. In fission, cells grow to a certain
size and divide into two equal cells. Sexual reproduction of yeasts involves the formation of
a zygote (a diploid cell) from the fusion of two haploid cells, each having a single set of chro-
mosomes. The nucleus of the diploid cells divides several times to form ascospores. Each asco-
spore eventually becomes a new haploid cell and may reproduce by budding and fission. The
life cycle of a typical yeast cell is presented in Fig. 2.9 .
The classification of yeasts is based on reproductive modes (e.g. budding or fission) and
the nutritional requirements of cells. The most widely used yeast, S. cerevisiae, is used in
alcohol formation under anaerobic conditions (e.g. in wine, beer, and whiskey making)
and also for baker's yeast production under aerobic conditions.

Bud appears, and
DNA doubles

Bud scar

Cells divide
forming scar
at division site

Nucleus
replicates

Nucleus separate

Nucleus moves
Into bud

FIGURE 2.9 Cell-division cycle of a typical yeast, Saccharomyces cerevisiae.