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 and chloroplasts are not part of the endomembrane system. Their proteins come primarily from free ribosomes in the cytosol and a few from their own ribosomes. Both organelles have small quantities of DNA that direct the synthesis of the polypeptides produced by these internal ribosomes. Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles.

Found in nearly all eukaryotic cells. 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. Organelles which are the sites of cellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen. Chloroplasts, found in plants and eukaryotic algae, are the site of photosynthesis. They convert solar energy to chemical energy and synthesize new organic compounds from CO2 and H2O. 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 Almost all eukaryotic cells have mitochondria. There may be one very large mitochondrion or hundreds to thousands in individual mitochondria. The number of mitochondria is correlated with aerobic metabolic activity. A typical mitochondrion is 1-10 microns long. Mitochondria are quite dynamic: moving, changing shape, and dividing.

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 The processes in the chloroplast are separated from the cytosol by two membranes. Inside the innermost membrane is a fluid-filled space, the stroma, in which float membranous sacs, the thylakoids. The stroma contains DNA, ribosomes, and enzymes for part of photosynthesis. The thylakoids, flattened sacs, are stacked into grana and are critical for converting light to chemical energy.

Fig. 7.18

Like mitochondria, chloroplasts are dynamic structures. 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 Like mitochondria, chloroplasts are dynamic structures. Their shape is plastic and they can reproduce themselves by pinching in two. Mitochondria and chloroplasts are mobile and move around the cell along tracks in the cytoskeleton.

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. Peroxisomes generate and degrade H2O2 in performing various metabolic functions Peroxisomes contain enzymes that transfer hydrogen from various substrates to oxygen An intermediate product of this process is hydrogen peroxide (H2O2), a poison, but the peroxisome has another enzyme that converts H2O2 to water. Some peroxisomes break fatty acids down to smaller molecules that are transported to mitochondria for fuel. Others detoxify alcohol and other harmful compounds. Specialized peroxisomes, glyoxysomes, convert the fatty acids in seeds to sugars, an easier energy and carbon source to transport. They form not from the endomembrane system, but by incorporation of proteins and lipids from the cytosol.

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 in seeds to sugars, an easier energy and carbon source to transport.(until it is able to produce its own sugar by photosynthesis) Contains enzymes that convert the fatty acids to sugars These biochemical reactions make energy stored in seed oils available for the emerging seedling.

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