84
UNIT ONE
and weak, fl
accid muscles. Some cells, such as red
blood cells, lack mitochondria.
Mitochondria provide glimpses into the past. These organelles are
passed to offspring from mothers only, because the mitochon-
dria are excluded from the part of a sperm that enters an egg cell.
Mitochondrial DNA sequences are consulted to trace human origins,
back to a long-ago group of common ancestors of us all metaphori-
cally called “mitochondrial Eve.” Going even farther back, mitochon-
dria are thought to be the remnants of once free-living bacterialike
cells that entered primitive eukaryotic cells. These bacterial passen-
gers remain in our cells today, where they participate in energy reac-
tions. Mitochondria physically resemble bacteria.
6.
Lysosomes.
Lysosomes (li
so-so
¯mz) are the “garbage
disposals” of the cell, where enzymes dismantle
debris. Lysosomes can be difF
cult to identify because
their shapes vary so greatly, but they often appear
as tiny, membranous sacs
(f
g. 3.13)
. These sacs
contain powerful enzymes that break down proteins,
A mitochondrion (mi
to-kon
dre-on) has two
layers—an outer membrane and an inner membrane.
The inner membrane is folded extensively in, forming
shelfl
ike partitions called
cristae
(f
g. 3.12)
.
This
organization dramatically increases the surface
area on which chemical reactions can occur. Small,
stalked particles that contain enzymes are connected
to the cristae. These enzymes and others dissolved
in the fl
uid in the mitochondrion, called the matrix,
control many of the chemical reactions that release
energy from glucose and other nutrients. The
mitochondrion captures and transfers this newly
released energy into special chemical bonds of
the molecule
adenosine triphosphate
(ATP), that
cells can readily use (chapter 4, p. 119). ±or this
reason, the mitochondrion is sometimes called the
“powerhouse” of the cell.
A typical cell has about 1,700 mitochondria,
but cells with very high energy requirements, such
as skeletal muscle cells, have many thousands of
mitochondria. This is why common symptoms of
illnesses affecting mitochondria are exercise intolerance
ER membrane
Membranes
Ribosomes
Ribosomes
Membranes
(a)
(b)
(c)
FIGURE 3.9
The endoplasmic reticulum is the site of protein and lipid synthesis, and serves as a transport system. (
a
) A transmission electron
micrograph of rough endoplasmic reticulum (ER) (28,500×). (
b
) Rough ER is dotted with ribosomes, whereas (
c
) smooth ER lacks ribosomes.
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