Essential Cell Biology 5th edition

Membrane-Enclosed Organelles
497
mitochondrion
Golgi
apparatus
endoplasmic
reticulum with
membrane-bound
ribosomes
endosome
lysosome
cytosol
peroxisome
Figure 15–2 A cell from the lining of
the intestine contains the basic set
of membrane-enclosed organelles
found in most animal cells. The nucleus,
endoplasmic reticulum (ER), Golgi
apparatus, lysosomes, endosomes,
mitochondria, and peroxisomes are distinct
compartments separated from the cytosol
by at least one selectively permeable
membrane. Ribosomes are shown bound
to the cytosolic surface of portions of the
ER, called the rough ER; the ER that lacks
ribosomes is called smooth ER. Additional
ribosomes can be found free in the cytosol.
free ribosomes
nucleus
plasma membrane
15 µm
known as the nuclear envelope, and communicates with the cytosol via
nuclear pores that perforate the envelope. The outer nuclear membrane
is continuous with the membrane of the endoplasmic reticulum (ER), a
system of interconnected membranous sacs and tubes that often extends
throughout most of the cell. The ER is the major site of synthesis of new
membranes in the cell. Large ECB5 areas m12.01/15.02 of the ER have ribosomes attached
to the cytosolic surface and are designated rough endoplasmic reticulum
(rough ER). The ribosomes are actively synthesizing proteins that are
inserted into the ER membrane or delivered to the ER interior, a space
called the lumen. The smooth endoplasmic reticulum (smooth ER) lacks
ribosomes. It is scanty in most cells but is highly developed for performing
particular functions in others: for example, it is the site of steroid
hormone synthesis in some endocrine cells of the adrenal gland and the
site where a variety of organic molecules, including alcohol, are detoxified
in liver cells. In many eukaryotic cells, the smooth ER also sequesters
Ca 2+ from the cytosol; the release and reuptake of Ca 2+ from the ER is
involved in muscle contraction and other responses to extracellular signals
(discussed in Chapters 16 and 17).
TABLE 15–1 THE MAIN FUNCTIONS OF MEMBRANE-ENCLOSED
ORGANELLES OF A EUKARYOTIC CELL
Compartment
Main Function
Cytosol contains many metabolic pathways (Chapters 3 and 13);
protein synthesis (Chapter 7); the cytoskeleton (Chapter 17)
Nucleus
Endoplasmic
reticulum (ER)
Golgi apparatus
Lysosomes
Endosomes
contains main genome (Chapter 5); DNA and RNA synthesis
(Chapters 6 and 7)
synthesis of most lipids (Chapter 11); synthesis of proteins for
distribution to many organelles and to the plasma membrane
(this chapter)
modification, sorting, and packaging of proteins and lipids for
either secretion or delivery to another organelle (this chapter)
intracellular degradation (this chapter)
sorting of endocytosed material (this chapter)
Mitochondria ATP synthesis by oxidative phosphorylation (Chapter 14)
Chloroplasts (in
plant cells)
Peroxisomes
ATP synthesis and carbon fixation by photosynthesis
(Chapter 14)
oxidative breakdown of toxic molecules (this chapter)