Page 116 - Fundamentals of anatomy physiology

cytosol reabsorbs released nutrients, and the remaining mate-
rial is expelled from the cell.
Lysosomes also destroy bacteria (as well as liquids and
organic debris) that enter the cell from the extracellular fluid.
The cell encloses these substances in a small portion of the
plasma membrane, which is then pinched off to form a trans-
port vesicle, or endosome, in the cytoplasm. (We will discuss
this method of transporting substances into the cell, called
endocytosis, later in this chapter.) Then a primary lysosome
fuses with the vesicle, forming a secondary lysosome. Activated
enzymes inside break down the contents and release usable
substances, such as sugars or amino acids. In this way, the cell
both protects itself against harmful substances and obtains
valuable nutrients.
Lysosomes also do essential cleanup and recycling inside
the cell. For example, when muscle cells are inactive, lyso-
somes gradually break down their contractile proteins. (This
mechanism accounts for the reduction in muscle mass that ac-
companies aging.) The process is usually precisely controlled,
but in a damaged or dead cell, the regulatory mechanism fails.
Lysosomes then disintegrate, releasing enzymes that become
activated within the cytosol. These enzymes rapidly destroy the
cell’s proteins and organelles in a process called autolysis (aw-
TOL-i-sis; auto-, self). We do not know how to control lyso-
somal activities or why the enclosed enzymes do not digest the
lysosomal membranes unless the cell is damaged.
Problems in producing lysosomal enzymes cause more
than 30 serious diseases affecting children. In these condi-
tions, called lysosomal storage diseases, the lack of a specific ly- The cells of all living things require energy to carry out the
sosomal enzyme results in the buildup of waste products and
debris that lysosomes normally remove and recycle. Affected
individuals may die when vital cells, such as those of the heart,
can no longer function.
Peroxisomes
Peroxisomes are smaller than lysosomes and carry a dif-
ferent group of enzymes. In contrast to lysosomes, which
are produced at the Golgi apparatus, new peroxisomes are
produced by the growth and subdivision of existing peroxi-
somes. Their enzymes are produced at free ribosomes and
transported from the cytosol into the peroxisomes by carrier
proteins.
Peroxisomes absorb and break down fatty acids and
other organic compounds. As they do so, peroxisomes gener-
ate hydrogen peroxide (H2O2), a potentially dangerous free
radical. p. 60 Catalase, the most abundant enzyme within
the peroxisome, then breaks down the hydrogen peroxide to
oxygen and water. In this way, peroxisomes protect the cell
from the potentially damaging effects of free radicals pro-
duced during catabolism. Peroxisomes are present in all cells,
but their numbers are highest in metabolically active cells,
such as liver cells.
Chapter 3 The Cellular Level of Organization 103
Membrane Flow
When the temperature changes markedly, you change your
clothes. Similarly, when a cell’s environment changes, it
3changes the structure and properties of its plasma membrane.
With the exception of mitochondria, all membranous organ-
elles in the cell are either interconnected or in communication
through the movement of vesicles. The membranes of the RER
and SER are continuous and are connected to the nuclear en-
velope. Transport vesicles connect the ER with the Golgi ap-
paratus, and secretory vesicles link the Golgi apparatus with
the plasma membrane. Finally, vesicles forming at the exposed
surface of the cell remove and recycle segments of the plasma
membrane. This continuous movement and exchange of
membrane segments is called membrane flow. In an actively
secreting cell, an area equal to the entire membrane surface
may be replaced each hour.
Membrane flow is an example of the dynamic nature of
cells. It provides a way for cells to change the characteristics of
their plasma membranes by altering their lipids and the pro-
teins serving as receptors, channels, anchors, and enzymes—
as they grow, mature, or respond to a specific environmental
stimulus.
Mitochondria
functions of life. The organelles that produce energy are the
mitochondria (mı.-to. -KON-dre. -u. h; singular, mitochondrion;
mitos, thread + chondrion, granule). These small structures
vary widely in shape, from long and slender to short and fat.
The number of mitochondria in a particular cell varies with
the cell’s energy demands. These organelles may account for
30 percent of the volume of a heart muscle cell, yet are absent
in red blood cells.
Mitochondria have an unusual double membrane (Fig-
ure 3–9a). The outer membrane surrounds the organelle. The
inner membrane contains numerous folds called cristae,
which surround the fluid contents, or matrix, of the mito-
chondrion. Cristae increase the membrane surface area in
contact with the matrix and the number of attached protein
complexes and enzymes involved in making ATP from ADP
and P. Metabolic enzymes in the matrix catalyze reactions that
release carbon dioxide and provide some additional energy
for cellular functions.
Mitochondria contain their own DNA (mtDNA) and ri-
bosomes. The mtDNA codes for small numbers of RNA and
polypeptide molecules. The polypeptides are used in en-
zymes required for energy production. Although mitochon-
dria contain their own genetic system, their functions de-
pend on imported proteins coded by nuclear DNA.
Most of the chemical reactions that release energy take
place in the mitochondria, yet most of the cellular activities