Biology (Holt)

Prokaryotes

The smallest and simplest cells are prokaryotes. A (proh
KAIR ee oht) is a single-celled organism that lacks a nucleus and other
internal compartments. Without separate compartments to isolate
materials, prokaryotic cells cannot carry out many specialized func-
tions. Early prokaryotes lived at least 3.5 billion years ago. For nearly
2 billion years, prokaryotes were the only organisms on Earth. They
were very simple and small (1–2 μm in diameter). Like their ancestors,
modern prokaryotes are also very small (1–15 μm). The familiar
prokaryotes that cause infection and cause food to spoil belong to
a subset of all prokaryotes that is commonly called bacteria.

Characteristics of Prokaryotes

Prokaryotes can exist in a broad range of environmental conditions.
Many prokaryotes, including some bacteria that cause infection in
humans, grow and divide very rapidly. Some prokaryotes do not need
oxygen to survive. Other prokaryotes cannot survive in the presence
of oxygen. Some prokaryotes can even make their own food.
The cytoplasm of a prokaryotic cell includes everything inside the
cell membrane. As Figure 6 shows, a prokaryote’s enzymes and ribo-
somes are free to move around in the cytoplasm because there are
no internal structures that divide the cell into compartments. In
prokaryotes, the genetic material is a single,
circular molecule of DNA. This loop of pro-
karyotic DNA is often located near the center
of the cell, suspended within the cytoplasm.
Prokaryotic cells have a sur-
rounding the cell membrane that provides
structure and support. The cells of fungi and
plants also have cell walls; only animal cells
and some protists lack cell walls. Prokaryotes
lack an internal supporting skeleton, so they
depend on a strong cell wall to give the cell
shape. A prokaryotic cell wall is made of
strands of polysaccharides connected by
short chains of amino acids. Some prokary-
otic cell walls are surrounded by a structure
called a capsule, which is also composed
of polysaccharides. The capsule enables
prokaryotes to cling to almost anything,
including teeth, skin, and food.
Many prokaryotes have (fluh JEL
uh),which are long, threadlike structures
that protrude from the cell’s surface and
enable movement. Prokaryotic flagella
rotate, propelling the organism through its
environment at speeds of up to 20 cell
lengths per second. Figure 6 shows a
prokaryote with several flagella.

flagella

cell wall

prokaryote

SECTION 2 Cell Features 57

Magnification: 61,850

Figure 6 Prokaryotes.

Prokaryotic cells have little

internal structure. Many also

have a capsule and flagella.

Reading Effectively

For many words ending in

• um,the plural is formed by
  changing the -umto -a. For
  example, the plural of bac-
  teriumis bacteria,and the
  plural of flagellumis flagella.