Mitochondria animal cell

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

Voelker, D.R., 1989, Phosphatidylserine translocation to the mitochondrion is an ATP-dependent process in permeabiUzed animal cells. Proc. Natl. Acad Sci. USA. 86 . 9921-9925. 

Eukaryotic cells also have organelles, mitochondria (Fig. 24-6) and chloroplasts, that contain DNA. Mitochondrial DNA (mtDNA) molecules are much smaller than the nuclear chromosomes. In animal cells, mtDNA contains fewer than 20,000 bp (16,569 bp in human mtDNA) and is a circular duplex. Each mitochondrion typically has two to ten copies of this mtDNA molecule, and the number can rise to hundreds in certain cells when an embryo is undergoing cell differentiation. In a few organisms (trypanosomes, for example) each mitochondrion contains thousands of copies of mtDNA, organized into a complex and interlinked matrix known as a kinetoplast. Plant cell mtDNA ranges in size from... 

Chloramphenicol blocks translation in bacteria by inhibiting peptidyltransferase of the large ribosomal subunit. It does not interfere with peptidyltransferase in the large subunit of eukaryotic ribosomes. However, the mitochondrion of animal cells contains ribosomes that are similar to bacterial ribosomes, and chloramphenicol can block protein synthesis in this organelle. This could contribute to the side effects of this drug when used in the treatment of animals. 

Although fatty acid synthesis occurs within the cytoplasm of most animal cells, liver is the major site for this process. (Recall, for example, that liver produces VLDL. See p. 349.) Fatty acids are synthesized when the diet is low in fat and/or high in carbohydrate or protein. Most fatty acids are synthesized from dietary glucose. As discussed, glucose is converted to pyruvate in the cytoplasm. After entering the mitochondrion, pyruvate is converted to acetyl-CoA, which condenses with oxaloacetate, a citric acid cycle intermediate, to form citrate. When mitochondrial citrate levels are sufficiently high (i.e., cellular energy requirements are low), citrate enters the cytoplasm, where it is cleaved to form acetyl-CoA and oxaloacetate. The net reaction for the synthesis of palmitic acid from acetyl-CoA is as follows ... 

Figure 1 Major intracellular compartments in an animal cell. 1. Nucleus 2. cytosol 3. mitochondrion 4. lysosome ...

FIGURE 21.18 A portion of an animal cell, showing the sites of various aspects of fatty-acid metabolism. The cytosol is the site of fatty-acid anabolism. It is also the site of formation of acyl-CoA, which is transported to the mitochondrion for catabolism by the P-oxidation process. Some chainlengthening reactions (beyond Cjg) take place in the mitochondria. Other chain-lengthening reactions take place in the endoplasmic reticulum (ER), as do reactions that introduce double bonds. 

Oxidative phosphorylation occurs on membranes. In bacteria, chemiosmotic ATP synthesis occurs at the cytoplasmic membrane. In plant and animal cells, these reactions occur in the mitochondrion, a double-membraned organelle (Figure 11-1). The ancestor of mitochondria was a bacterial cell incorporated into a nucleated cell, which subsequently lost much (although not all) of its DNA. Most mitochondrial proteins are encoded by nuclear DNA. Some respiratory proteins, along with mitochondrial ribosomal RNA and transfer RNAs, are encoded by mitochondrial DNA. 

Within the cytoplasm are specialized strnctnres called organelles that carry out specific functions in the cell. The ribosomes are the sites of protein synthesis. The mitochondria are the energy-prodncing factories of the cells. A mitochondrion has an outer and an inner membrane, with an intermembrane space between them. The fluid section surrounded by the inner membrane is called the matrix. Enzymes located in the matrix and along the inner membrane catalyze the oxidation of carbohydrates, fats, and amino acids. All these oxidation pathways eventnally produce CO2, H2O, and energy, which is used to form energy-rich componnds. Table 18.1 summarizes some of the functions of the components in animal cells. 

Homeostatic mechanisms also allow animals to control their intracellular pH very strictly. In humans for example, blood pH (usually taken as a reliable but indirect measure of cellular pH) is 7.4 0.04. At 37 °C cytosolic pH is actually slightly lower at about 7.0 but different compartments within the eukaryotic cells may have quite different pH, for example, lysosomes have an internal pH of about 5 the inside of a mitochondrion is more alkaline than the outside whilst the inside of a phagosome in a white blood cell is more acidic than its surrounding cytosol, both situations arising due to proton pumping across a membrane. 

Each mitochondrion contains several molecules of DNA (mtDNA), usually in a closed, circular form, as well as the ribosomes, tRNA molecules, and enzymes needed for profein synthesis. With rare exceptions almost all of the mitochondrial DNA in a human cell is inherited from the mother. The size of the DNA circles varies from 16-19 kb in animals to over 200 kb in many higher plants. Complete sequences of many mitochondrial DNAs are known. Among these are the 16,569 bp human mtDNA, the 16,338 bp bovine mtDNA, the 16,896 bp mtDNA of fhe wallaroo Macropus robustus, and the 17,533 bp mtDNA of the amphibian Xenopus laevis The sea urchin Paracentotus lividus has a smaller 15,697 bp genome. However, the order of the genes in this and other invertebrate mtDNA is different from that in mammalian mitochondria. Protozoal mtDNAs vary in size from 5900 bp for the... 

With regard to the high (positive) value of and to the short half-life values, escape of OH and from the sphere immediately surrounding mitochondrion can be virtually excluded. Yet the neutral molecule H Oj is considered to be movable one, which can escape as from the body of the mitochondrion as well as from the cell body itself. It is comprehensible that in some tissues the actual concentrations may reach 100 pM or more as for example, in human and other animal aqueous and vitreous humors. The hydroperoxide levels at or below 20-50 pM seem, however, to have limited cytotoxicity to many cell types [32],... 

---