Chapter 11: Bioenergetics and Metabolism
Chapter Summary
MITOCHONDRIA
Organization and Function of Mitochondria: Mitochondria, which play a critical role in the generation of metabolic energy, are surrounded by a double-membrane system. The matrix contains the enzymes of the citric acid cycle; the inner membrane contains protein complexes involved in electron transport and oxidative phosphorylation. In contrast to the inner membrane, the outer membrane is freely permeable to small molecules.
The Genetic System of Mitochondria: Mitochondria contain their own genomes, which encode rRNAs, tRNAs, and some of the proteins that are involved in oxidative phosphorylation.
Protein Import and Mitochondrial Assembly: Most mitochondrial proteins are encoded by the nuclear genome. These proteins are translated on free ribosomes and imported into mitochondria as completed polypeptide chains. Positively charged presequences target proteins for import to the mitochondrial matrix. Phospholipids are carried to mitochondria from the endoplasmic reticulum by phospholipid transfer proteins.
THE MECHANISM OF OXIDATIVE PHOSPHORYLATION
The Electron Transport Chain: Most of the energy derived from oxidative metabolism comes from the transfer of electrons from NADH and FADH2 to O2. In order to harvest this energy in usable form, electrons are transferred through a series of carriers organized into four protein complexes in the inner mitochondrial membrane.
Chemiosmotic Coupling: The energy-yielding reactions of electron transport are coupled to the generation of a proton gradient across the inner mitochondrial membrane. The potential energy stored in this gradient is harvested by a fifth protein complex, ATP synthase, which couples ATP synthesis to the energetically favorable return of protons to the mitochondrial matrix.
Transport of Metabolites across the Inner Membrane: In addition to driving ATP synthesis, potential energy stored in the proton gradient drives the transport of ATP, ADP, and other metabolites into and out of mitochondria.
CHLOROPLASTS AND OTHER PLASTIDS
The Structure and Function of Chloroplasts: Chloroplasts are large organelles that function in photosynthesis and a variety of other metabolic activities. Like mitochondria, chloroplasts are bounded by a double-membrane envelope. In addition, chloroplasts have an internal thylakoid membrane, which is the site of electron transport and the chemiosmotic generation of ATP.
The Chloroplast Genome: Chloroplast genomes contain more than 100 genes, including genes encoding rRNAs, tRNAs, some ribosomal proteins, and some proteins involved in photosynthesis.
Import and Sorting of Chloroplast Proteins: Most chloroplast proteins are synthesized on free ribosomes in the cytosol and targeted for import to chloroplasts by amino-terminal transit peptides. Most proteins incorporated into the thylakoid lumen are first imported into the chloroplast stroma and then targeted for transport across the thylakoid membrane by several different pathways.
Other Plastids: The chloroplast is only one member of a family of related organelles, all of which contain the same genome. Other plastids serve to store energy sources, such as starch and lipids, and function in other aspects of plant metabolism.
PHOTOSYNTHESIS
Electron Flow through Photosystems I and II: During photosynthesis, energy from sunlight is harvested and converted to usable forms of potential chemical energy. Absorption of light by chlorophylls excites electrons to a higher energy state. These high-energy electrons are then transferred through a series of carriers organized into two photosystems and the cytochrome bf complex in the thylakoid membrane. The sequential flow of electrons through both photosystems is coupled to the synthesis of ATP at photosystem II and the reduction of NADP+ to NADPH at photosystem I. Both ATP and NADPH are then used in the synthesis of carbohydrates from CO2, which takes place in the chloroplast stroma.
Cyclic Electron Flow: The alternative pathway of cyclic electron flow allows light energy harvested at photosystem I to be converted to ATP, rather than NADPH.
ATP Synthesis: The chemiosmotic synthesis of ATP is driven by a proton gradient across the thylakoid membrane.
PEROXISOMES
Functions of Peroxisomes: Peroxisomes are small organelles, bounded by a single membrane, that contain enzymes involved in a variety of metabolic reactions, including fatty acid oxidation, the glyoxylate cycle, and photorespiration.
Peroxisome Assembly: Peroxisome assembly begins on the ER with the formation of specific vesicles. However, most peroxisomal proteins are synthesized on free ribosomes in the cytosol and imported to peroxisomes as complete polypeptide chains. At least two types of signals target proteins to the interior of peroxisomes, but the mechanism of protein import is not well understood.
