cells, have many nuclei. The nucleus is enclosed by a
double membrane called the nuclear envelope. The
nuclear envelope has numerous openings called nucle-
ar pores, which control the movement of substances
into and out of the nucleus. Inside the nucleus is a
large, round structure called the nucleolus (
noo-KLE-
o-lu
s), which is made of DNA, RNA, and proteins. The
nucleolus makes ribosomes. The nucleus also houses
genetic material, which contains all the information
necessary to control cell activities and make new cells.
In a non-dividing cell, the genetic material is spread
out in the form of chrom atin (
KRO-ma-tin
). In a divid-
ing cell, the genetic material is condensed into struc-
tures called chromosomes (
KRO-mo-somz
), which we
will talk about when we discuss cell division later in this
chapter.
CONCEPT CHECK
1.
What
are the functions of the plasma membrane?
2.
What
organelle consists of a smooth outer
membrane and an inner folded membrane?
3.
What
part of the nucleus controls the flow of
substances into and out of the nuclear
membrane?
Cells Carry Out Many Processes
LEARNING OBJECTIVES
1.
Outline
the various processes of membrane
transport.
2.
Explain
the steps of protein synthesis.
ells must carry out several processes to func-
tion properly. Materials must be able
to
move across cell and organelle membranes
in a controlled manner. The various proteins
that carry out many cell functions must be made, using
instructions encoded in the cell’s genes. Finally, cells
must be able to make copies of themselves, whether to
grow, repair, or reproduce. Here we will discuss all these
processes. Let’s begin with membrane transport.
Membranes Transport Substances
Movement of materials across membranes is essential
to the life of a cell. There are essentially two fluid com-
partments important to cellular function: intracellular
fluid (inside the cell) and extracellular fluid (outside
the cell). The name of the extracellular fluid is based
on its location:
3.
Compare
and contrast the processes of mitosis
and meiosis.
Interstitial fluid—Fluid between cells within a tissue
Plasma—Fluid within a blood vessel
Lymph—Fluid within a lymphatic vessel
Cerebrospinal fluid—Fluid surrounding the brain
and spinal cord
As they move across and within the cells, substances—in-
cluding gases, nutrients, and ions—are dissolved in the
various fluids. As you learned in Chapter 2, the following
terms apply to solutions:
Solute—A substance that is dissolved within a fluid
Solvent—A fluid (or gas) in which a solute is dissolved
Concentration—The amount of solute dissolved in a
given volume of solvent
C oncentration
gradient—The
difference
in
concentration of a substance between two areas
Let’s begin with the simplest process by which substanc-
es move from one place to another, a process called
diffusion.
Cells Carry Out Many Processes 57
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