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106 Unit 1 Levels of Organization
Figure 3–8 Lysosome Functions. Primary lysosomes, formed at the Golgi apparatus, contain inactive
enzymes. They may be activated under any of the three basic conditions indicated here.
3 Lysosome activation occurs when:
Golgi 31 Damaged organelle 1
apparatus Primary A primary lysosome fuses with the
lysosome Secondary membrane of another organelle, such
Autolysis liberates lysosome as a mitochondrion
digestive enzymes 2
Reabsorption 2
Reabsorption A primary lysosome fuses with an
endosome containing fluid or solid
materials from outside the cell
3
The lysosomal membrane breaks
down during autolysis following injury
to, or death of, the cell
Secondary Endosome
lysosome
Extracellular
solid or fluid
Endocytosis
Exocytosis Exocytosis
ejects residue ejects residue
that require energy occur in the surrounding cytoplasm. For Clinical Note
this reason, cells must store energy in a form that can be moved
from place to place. Recall from Chapter 2 that cellular energy Inheritable Mitochondrial Disorders Several inherit-
is stored and transferred in the form of high-energy bonds. The able disorders result from abnormal mitochondrial activ-
best example is the high-energy bond that attaches a phos- ity. Mitochondria are not totally self-sufficient, but they
phate group (PO43−) to adenosine diphosphate (ADP), creat- do contain DNA and ribosomes to manufacture many of
ing the high-energy compound adenosine triphosphate (ATP). their own specific proteins. The mitochondria involved in
Cells can then break this high-energy bond under controlled congenital diseases contain abnormal DNA. As a result,
conditions, reconverting ATP to ADP and phosphate and re- the enzymes they produce reduce the efficiency of ATP
leasing energy for the cell’s use when and where it is needed. production. Cells throughout the body may be affected.
Symptoms involving muscle cells, neurons, and the recep-
Mitochondrial Energy Production. Most cells generate ATP tor cells in the eye are most common because these cells
and other high-energy compounds as they break down carbo- have especially high energy demands.
hydrates, especially glucose. We will examine the entire pro-
cess in Chapter 25, but a few basic concepts now will help you remainder then enters the citric acid cycle (also known as the
follow discussions of muscle contraction, neuron function, Krebs cycle and the tricarboxylic acid cycle or TCA cycle). The cit-
and endocrine function in Chapters 10–18. ric acid cycle is an enzymatic pathway that breaks down the
absorbed pyruvate.
Most ATP is produced inside mitochondria, but the first
steps take place in the cytoplasm (Figure 3–9b). A reaction se- The remnants of pyruvate molecules contain carbon, oxy-
quence called glycolysis (glycos, sugar + -lysis, a loosening) gen, and hydrogen atoms. The carbon and oxygen atoms are
breaks down a glucose molecule into two molecules of pyru-
vate. Mitochondria then absorb the pyruvate molecules.
In the mitochondrial matrix, a carbon dioxide (CO2) mol-
ecule is removed from each absorbed pyruvate molecule. The
