Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Microbial Self 83

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

4.1.6 Bacterial individuality

Motile bacteria engage in “random walk” (by tumbling) in search of
nutrients. When they detect a positive gradient in the environment,
they stop tumbling and swim in the direction of the food. But once the
gradient disappears, random walk resumes. Even if genetically identi-
cal, some bacteria appear to be more “nervous” and tumble more often,
while others do not jitter as much. The tumbling frequency is correlated
with the sensitivity to an attractant and differs from cell to cell.^10

4.2 Protists: The One-cell Eukaryotes

In contrast to prokaryotes, eukaryotes possess well-formed structures
inside the cell called organelles. These include the nucleus, mitochon-
dria, lysosomes and Golgi apparatus. Compared to prokaryotes, their
strategy for survival and preservation of self is much more sophisticated.
They can literally swallow other organisms alive by phagocytosis (cell
eating) and take in liquid material by pinocytosis (cell drinking). Even
for the simplest form of eukaryotes, the protists, a plethora of behavior
is developed to interact with the environment and for the procurement
of food. Here we shall look at the behavior of some protists.

4.2.1 Amoeba feeding behavior

Amoebas are unicellular organisms that live at the bottom of ponds or
other moist places including soil. A typical amoeba is about 0.3 mm in
size. It consists of a cell membrane that surrounds a fluid-like cytoplasm
made up of an outer thin ectoplasm and a bulky endoplasm. The amoeba
moves along a solid surface by a rolling motion of the cell body and
the extension and retraction of finger-like projections of the cytoplasm
called pseudopodia. They ingest solid food by surrounding it with the
pseudopodia and incorporating it into a round structure called food vac-
uole, which contains the digestive enzymes. After absorption, the resi-
due in the food vacuole is extruded and left behind in the track of the