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CHAPTER 7
A TOUR OF THE CELL
Other Membranous Organelles
1. Mitochondria and chloroplasts are the main energy transformers of cells
2. Peroxisomes generate and degrade H2O2 in performing various metabolic
functions
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Mitochondria and chloroplasts are the
main energy transformers of cells
 Mitochondria and chloroplasts are organelles that convert
energy acquired from the surrounding into forms useable
for cellular work.
 Enclosed by double membranes.
 Membranes are not part of endomembrane system. Their
membrane proteins are not made in the ER, but by free
ribosomes in the cytosol and by ribosomes located within
the mitochondria and chloroplasts themselves.
 Contain ribosomes and some DNA that programs a small
portion of their own protein synthesis, though most of their
proteins are synthesized in the cytosol programmed by
nuclear DNA.
 Semiautonomous organelles that grow and reproduce
within the cell.
Mitochondria
Mitochondria: Organelles which are the sites of
cellular respiration, a catabolic oxygen-requiring
process that uses energy extracted from organic
macromolecules to produce ATP.
 Found in nearly all eukaryotic cells.
 Number of mitochondria per cell varies and
directly correlates with the cell’s metabolic
activity.
 Are about 1µm in diameter and 1-10 µm in length.
 Are dynamic structure that move, change their
shapes and divide.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Structure of the mitochondrion
 Enclosed by two membranes that have their own unique
combination of proteins embedded in phospholipid
bilayers (Figure 7.17)
 The outer membrane is smooth
 The inner membrane :
 Contains embedded enzymes that are
involved in cellular respiration and ATP
production
 Is Convoluted with many infoldings
or cristae that increase the surface area
available for these reactions to occur
 The inner and outer membranes divide the mitochondrion
into two internal compartments:
1. The intermembrane space:
 Narrow region between the inner and outer
mitochondrial membrane
2. Mitochondrial Matrix
 Compartment enclosed by the inner mitochondrial
membrane.
 Contains enzymes that catalyze some metabolic steps
of cellular respiration.
 Contains mitochondrial DNA and ribosomes
Fig. 7.17
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Chloroplasts
 It is a type of plastids
 Plastids: A group of plant and algal membrane-bound
organelles that include amyloplasts, chromoplasts and
chloroplasts.
Amyloplasts : (Amylo= starch) colorless plastids that
store starch; found in roots and tubers.
Chromoplasts : (chromo=color) plastids containing pigments
other than chlorophyll; responsible for the orange and yellow color
of fruits, flowers and autumn leaves.
Chloroplasts: (Chloro= green) Chlorophyll-containing
plastids which are the sites of photosynthesis
•Found in eukaryotic algae, leaves and other green plant organs
•Are lens-shaped and measure about 2µm by 5µm
•Are dynamic structures that change shape, move and divide.
Structure of the chloroplast
Chloroplasts are divided into three functional compartments by a
system of membranes (Figure 7.18)
1- Intermembrane Space: The chloroplast is bounded by a double
membrane which partitions its contents from the cytosol. A narrow
intermembrane space separates the two membranes.
2- Thylakoid Space: Thylakoids form another membranous system
within the chloroplast. The thylakoid membrane segregates the interior
of the chloroplast into two compartments : thylakoid space and stroma
Thylakoid space: Space inside the thylakoid
Thylakoids: Flattened membranous sacs inside the chloroplast
Fig. 7.18
• Chlorophyll is found in the thylakoid membranes
• Thylakoids function in the steps of photosynthesis that initially
convert light energy to chemical energy
• Some thylakoids are stacked into grana
Grana : (Singular: granum) Stacks of thylakoids in a
chloroplast
3. Stroma: Photosynthetic reactions that convert carbon dioxide to sugar
occur in the stroma.
Stroma: Viscous fluid outside the thylakoids
2. Peroxisomes consume oxygen in various
metabolic functions
 Peroxisomes: Specialized metabolic organelles that
contain peroxide-producing enzymes.
 Bound by a single membrane.
 Contain peroxide-producing enzymes, that transfer hydrogen
from various substrates to oxygen, producing the toxic hydrogen
peroxide (H2O2).
 Contain an enzyme that converts toxic H2O2 to water.
 Are not part of endomemebrane system; they grow in size by
incorporating proteins and lipids made in the cytosol.
 Increase in number by splitting in two.
Fig. 7.19
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Some functions of peroxisomal reactions
 Breakdown of fatty acids (using oxygen) into smaller
molecules. The products are carried to mitochondria as
fuel for cellular respiration.
 Detoxification of alcohol and other harmful compounds.
In the liver, peroxisomal enzymes transfer H2 from poisons
to O2
 Specialized peroxisomes ( glyoxysomes) are found in
fat storing tissues of plant seeds:
 Contains enzymes that convert the fatty acids to sugars
 These biochemical reactions make energy stored in
seed oils available for the emerging seedling.
Figure 7.7
Figure 7.8
Figure 7.9
Figure 7.11
Figure 7.12
Figure 7.14
Figure 7.17
Figure 7.18
Figure 7.21
Figure 7.22)