Page 63 - Anatomy and Physiology of Farm Animals, 8th Edition

48 / Anatomy and Physiology of Farm Animals

area for attachment of enzymes (Figs. 1‐2 synthesis of proteins and lipids and have
their own ribosomes. The first, and rate
and 2‐7). Studies of fragmented mito­
VetBooks.ir chondria indicate that all of the enzymes limiting, step in steroid hormone synthesis
is side‐chain cleavage of cholesterol to
associated with oxidation of nutrients to
carbon dioxide, ATP, and water are found in pregnenolone in the mitochondria.
the mitochondria. Thus, all of the enzymes
and coenzymes involved in the tricarbo- Lysosomes
xylic acid cycle (also called the Krebs
cycle, or citric acid cycle) are largely local Lysosomes are membrane‐bound vesicles
to the mitochondria. of digestive (hydrolytic) enzymes. They are
Oxidation during the tricarboxylic acid
cycle releases carbon dioxide and hydro­ larger than ribosomes but smaller than
mitochondria, ranging in diameter from
gen atom pairs (H ). The H furnishes its 0.25 to 0.75 μm. Lysosomes originate from
2
2
electrons to the mitochondrial electron the endoplasmic reticulum and the Golgi
transport system to drive a series of reduc­ apparatus. They contain a variety of
tion reactions, culminating in the forma­ enzymes that degrade all types of biologic
tion of water and storage of the energy in molecules. Normally, the membrane of
the form of ATP. The ATP is formed by the lysosome prevents lysosomal enzymes
the oxidative phosphorylation of ADP from acting on molecules within the cyto­
(adenosine diphosphate), which adds plasm. However, in certain conditions, the
one inorganic phosphate molecule to ADP, enzymes are released into the cytosol,
creating a higher‐energy compound. The which may then lyse (destroy) the cell itself.
energy incorporated into ATP becomes Cytoplasmic vesicles formed by the
available for any cellular activity that requires phagocytosis of extracellular material may
energy, such as protein synthesis, muscle fuse with lysosomes, thereby permitting
contraction, and active transport. Energy is the enzymatic digestion of the contents of
released during the reconversion of ATP to the vesicle while protecting the cell itself
ADP and an inorganic phosphate. Most of from lysis. White blood cells, which act as
the cellular processes requiring energy take scavenger cells by phagocytizing bacteria,
place outside of the mitochondria. dead tissue, and damaged cell debris,
Since the mitochondria produce the
energy for the cell, it follows that the more contain many lysosomes. Lysosomes also
engulf and degrade intracellular organelles.
mitochondria in a cell, the more active the This is a means by which individual cells
cell can be. Mitochondrial ATP production can remove and recover components of
depends on oxygen, so highly active cells damaged parts of themselves. The only
also need a ready supply of oxygen. The mammalian cells that are known not to
enzymes of the glycolytic pathway, using contain lysosomes are the red blood cells.
glucose as a substrate, can produce ATP When cells contain inactive lysosomes,
without oxygen in the cytoplasm. However, disease can follow. An example is Pompe’s
this pathway is less efficient than mito­ disease, in which the lysosomes cannot
chondrial production; it produces less ATP digest glycogen. Also, changes caused by
per molecule of substrate. sunburn occur when the ultraviolet light of
Mitochondria contain their own DNA
and RNA for reproducing themselves. This the sun ruptures lysosomes in the skin
cells.
DNA is inherited at the time of fertilization
from the cytoplasmic contribution of the
oocyte, hence it is referred to as maternal Other Structures
DNA. Mitochondrial reproduction can be
stimulated by increased demands for cell Peroxisomes are smaller than lysosomes
energy and is not dependent on cellular and are most numerous in cells of the liver
division. Mitochondria also carry on partial and kidney. Peroxisomes contain enzymes