Essential Cell Biology 5th edition

22 CHAPTER 1 Cells: The Fundamental Units of Life
Figure 1–26 The cytosol is extremely
crowded. This atomically detailed model
of the cytosol of E. coli is based on the
sizes and concentrations of 50 of the most
abundant large molecules present in the
bacterium. RNAs, proteins, and ribosomes
are shown in different colors (Movie 1.2).
(From S.R. McGuffee and A.H. Elcock, PLoS
Comput. Biol. 6:e1000694, 2010.)
25 nm
QUESTION 1–5
Suggest a reason why it would be
advantageous for eukaryotic cells to
evolve elaborate internal membrane
systems that allow them to import
substances from the outside, as
shown in Figure 1–25.
liquid solution (Figure 1–26). The cytosol is the site of many chemical
reactions that are fundamental to the cell’s existence. The early steps in
the breakdown of nutrient molecules take place in the cytosol, for example,
and it is here that most proteins are made by ribosomes.
The Cytoskeleton Is Responsible ECB5 n1.100/1.26 for Directed Cell
Movements
The cytosol is not just a structureless soup of chemicals and organelles.
Using an electron microscope, one can see that in eukaryotic cells the
cytosol is criss-crossed by long, fine filaments. Frequently, the filaments
are seen to be anchored at one end to the plasma membrane or to radiate
out from a central site adjacent to the nucleus. This system of protein
filaments, called the cytoskeleton, is composed of three major filament
types (Figure 1–27). The thinnest of these filaments are the actin filaments;
they are abundant in all eukaryotic cells but occur in especially large
numbers inside muscle cells, where they serve as a central part of the
machinery responsible for muscle contraction. The thickest filaments in
the cytosol are called microtubules (see Figure 1−7B), because they have
the form of minute hollow tubes; in dividing cells, they become reorganized
into a spectacular array that helps pull the duplicated chromosomes
Figure 1–27 The cytoskeleton is a
network of protein filaments that can
be seen criss-crossing the cytoplasm of
eukaryotic cells. The three major types of
filaments can be detected using different
fluorescent stains. Shown here are (A)
actin filaments, (B) microtubules, and
(C) intermediate filaments. Intermediate
filaments are not found in the cytoplasm of
cells with cell walls, such as plant cells.
(A, Molecular Expressions at Florida State
University; B, courtesy of Nancy Kedersha;
C, courtesy of Clive Lloyd.)
(A)
20 µm
(B)
(C)