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3.4b
| Question | Answer |
|---|---|
| Organelles (literally, "little organs") | are intracellular structures that play indispensable roles in cellular metabolism. |
| Organelles (literally, "little organs") | are to the cell what organs are to the body-structures that play individual physiological roles in the survival of the whole. |
| There is no universally agreed definition of the word organelle. Some authorities include cilia, microvilli, and even the plasma membrane in this concept, | whereas others exclude intracellular structures such as ribosomes and centrioles because they're not enclosed in membranes. |
| This book, however, regards organelles as the intracellular structures | above the molecular level of complexity, that play indispensable roles in cellular metabolism. |
| Some organelles are surrounded by membranes and are therefore referred to as | membranous organelles. |
| These are the nucleus, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, and Golgi complex. | membranous organelles. |
| Organelles | without membranes include ribosomes, proteasomes, centrosomes, centrioles, and basal bodies. |
| The nucleus is usually the largest organelle, containing the cell's chromosomes. | It is enclosed in a double membrane, the nuclear envelope, perforated with nuclear pores |
| Inside, the nuclear lamina (a web of intermediate filaments) supports the envelope. | The material inside is nucleoplasm, containing chromatin (DNA and protein) and one or more nucleoli (sites of ribosome assembly). |
| The nucleus (fig. 3.26) | is usually the largest organelle and the only one clearly visible with the light microscope. |
| The nucleus (fig. 3.26) | It contains the cell's chromosomes and is therefore the genetic control center of cellular activity |
| The nucleus (fig. 3.26) | It is typically spheroidal to elliptical in shape and about 5 µm in diameter. |
| The nucleus (fig. 3.26) | Most cells have a single nucleus, but there are exceptions. |
| Mature red blood cells have none; they are anuclear. | A few cell types are multinuclear. |
| Many cells of the liver, urinary bladder, and heart have two nuclei; bone-dissolving cells called osteoclasts have as many as 50; | and a single skeletal muscle cell can have many thousands. |
| The nucleus is enclosed in a double membrane, | the nuclear envelope. |
| The envelope is perforated with nuclear pores | formed by a ring of proteins called the nuclear pore complex. |
| the nuclear envelope. | These proteins regulate molecular traffic through the envelope and act like a rivet to hold the two membrane layers together. |
| Hundreds of molecules pass through the ----- every minute. | nuclear pores |
| Coming into the nucleus are raw materials for ----, enzymes that are made in the cytoplasm but function in the nucleus, and hormones and other chemical messengers that activate certain genes. | DNA and RNA synthesis |
| Going the other way, ---- is made in the nucleus but leaves to perform its job in the cytoplasm | RNA |
| Immediately inside the nuclear envelope is a narrow but densely fibrous zone called the nuclear lamina, composed of a web of ----- | intermediate filaments. |
| It supports the nuclear envelope and pores, provides points of attachment and organization for the chromosomes inside the nucleus, and plays a role in regulating DNA replication and the cell life cycle. | nuclear lamina |
| Abnormalities of its structure or function are associated with certain genetic diseases and premature cell death. | nuclear lamina |
| The material in the nucleus is called | nucleoplasm. |
| This includes ----—fine threadlike matter composed of DNA and protein—and one or more dark-staining masses called nucleoli (singular, nucleolus), where ribosomes are produced. | chromatin |
| Endoplasmic reticulum (ER) | literally means "little network within the cytoplasm. |
| Endoplasmic reticulum (ER) | It is a system of interconnected channels called cisterns [34] enclosed by a unit membrane |
| In areas called ------ the cisterns are parallel, flattened sacs covered with granules called ribosomes. | rough endoplasmic reticulum, |
| Adjacent cisterns | are connected by bridges to create one continuous internal space. |
| The rough ER | is continuous with the outer membrane of the nuclear envelope, and some authorities regard the nuclear envelope as simply a modified extension of it. |
| In areas called -------, the cisterns are more tubular, branch more extensively, and lack ribosomes. | smooth endoplasmic reticulum |
| The cisterns of the ----- are continuous with those of the rough ER, so the two are different parts of the same network. | smooth ER |
| The ----- steroids and other lipids, detoxifies alcohol and other drugs, and manufactures nearly all membranes of the cell. | ER synthesizes |
| Rough ER | produces the phospholipids and proteins of the plasma membrane and synthesizes proteins that are either secreted from the cell or packaged in organelles such as lysosomes. |
| Rough ER | is most abundant in cells that synthesize large amounts of protein, such as antibody-producing cells and cells of the digestive glands. |
| In such cells, the ER is often the largest organelle of all, although it isn't visible to the ------because its thin and closely spaced membranes are beyond the LM's limit of resolution. | light microscope (LM) |
| Most cells have only a scanty ----, but it's relatively abundant in cells that engage extensively in detoxification, such as liver and kidney cells. | smooth ER |
| Long-term abuse of alcohol | , barbiturates, and other drugs leads to tolerance partly because the smooth ER proliferates and detoxifies the drugs more quickly. |
| Smooth ER | is also abundant in cells of the testes and ovaries that synthesize steroid hormones. |
| ----- contain extensive networks of smooth ER that store calcium and release it to trigger muscle contraction. | Skeletal and cardiac muscle |
| Ribosomes | are small granules of protein and RNA found in the nucleoli, in the cytosol, in mitochondria, and on the outer surfaces of the rough ER and nuclear envelope. |
| Ribosomes | They "read" coded genetic messages (messenger RNA) and assemble amino acids into proteins specified by the code. |
| Ribosomes | The unattached ribosomes scattered throughout the cytoplasm make enzymes and other proteins for use within the cell. |
| Ribosomes | Free ribosomes within the nucleus and mitochondria make proteins for use in those organelles. |
| Ribosomes | attach to the rough ER when they make proteins destined to be packaged in lysosomes or to be secreted from the cell, such as digestive enzymes, antibodies, and some hormones. |
| The ----- is a small system of cisterns that synthesize carbohydrates and put the finishing touches on protein and glycoprotein synthesis. | Golgi complex (GOAL-jee) |
| The complex resembles a stack of pita bread. | Golgi complex (GOAL-jee) |
| It consists of only a few cisterns, slightly separated from each other; each cistern is a flattened, often curved sac with swollen edges. | Golgi complex (GOAL-jee) |
| The Golgi complex | receives newly synthesized proteins from the rough ER. |
| The Golgi complex | It sorts them, cuts and splices some of them, and adds carbohydrate to some. |
| Finally, the most mature cistern with the finished cell product breaks up into membrane-bounded Golgi vesicles, which are abundant in the neighborhood of the ------ | Golgi complex. |
| Some vesicles become lysosomes, an organelle that contains digestive enzymes; some migrate to the plasma membrane and fuse with it, contributing fresh protein | and phospholipid to the membrane; and some become secretory vesicles that store a cell product, such as breast milk or digestive enzymes, for later release. |
| The roles of the endoplasmic reticulum, ribosomes, | and Golgi complex in protein synthesis and secretion |
| A lysosome\({}^{36}\) (LY-so-some) (fig. 3.30) is a package of enzymes bounded by a membrane. | Although often round or oval, lysosomes vary in shape. |
| lysosome | When viewed with the TEM, they often exhibit dark gray contents devoid of structure, but sometimes show crystals or parallel layers of protein. |
| At least 50 lysosomal enzymes have been identified. | They hydrolyze proteins, nucleic acids, complex carbohydrates, phospholipids, and other substrates. |
| In the liver, lysosomes break down glycogen to release glucose into the bloodstream. | White blood cells use lysosomes to digest phagocytized bacteria |
| Lysosomes | also digest and dispose of surplus or nonvital organelles and other cell components in order to recycle their nutrients to more important cell needs; this process is called autophagy |
| Lysosomes | also aid in a process of "cell suicide." Some cells are meant to do a certain job and then destroy themselves. |
| The uterus for example, weighs about 900 g at full-term pregnancy and shrinks to 60 g within 5 or 6 weeks after birth. | This shrinkage is due to autolysis,\({}^{38}\) the digestion of surplus cells by their own lysosomal enzymes. |
| Peroxisomes (fig. 3.30) | resemble lysosomes but contain different enzymes. |
| They are produced by collaboration between the endoplasmic reticulum and mitochondria and by fission of preexisting -----. | peroxisomes |
| Their general function is to use molecular oxygen (O2) to oxidize organic molecules. | peroxisomes |
| These reactions produce hydrogen peroxide (H202) | hence, the name of the organelle. H202 is then used to oxidize other molecules, and the excess is broken down to water and oxygen by an enzyme called catalase. |
| Peroxisomes | occur in nearly all cells but are especially abundant in liver and kidney cells. |
| Peroxisomes | They neutralize free radicals and detoxify alcohol, other drugs, and a variety of blood-borne toxins. |
| Peroxisomes | also decompose fatty acids into two-carbon fragments that the mitochondria use as an energy source for ATP synthesis. |
| Cells must tightly control the concentration of proteins in their cytoplasm. | Therefore, they must not only synthesize new proteins, but also dispose of those no longer needed. |
| Cells | also need to rid themselves of damaged and nonfunctional proteins and foreign proteins introduced by such events as viral infection. |
| Protein synthesis, we have seen, is the domain of the ribosomes; protein disposal is the function of another structurally simple organelle called a -----. | proteasome |
| Proteasomes | are hollow, cylindrical complexes of proteins located in both the cytoplasm and nucleus (fig. 3.31). |
| Proteasomes | A cell tags undesirable proteins for destruction and transports them to a proteasome. |
| Proteasomes | As the undesirable protein passes through the core of this organelle, the proteasome's enzymes unfold it and break it down into short peptides and free amino acids. |
| Proteasomes | These can be used to synthesize new proteins or be presented to the immune system for action. Proteasomes degrade more than 80% of a cell's proteins. |
| Mitochondria (singular, mitochondrion) | are organelles specialized for synthesizing ATP. They have a variety of shapes—spheroidal, rod-shaped, kidney-shaped, or threadlike |
| Mitochondria (singular, mitochondrion) | They are quite mobile, squirming and changing shape continually. |
| Mitochondria (singular, mitochondrion) | They sometimes undergo fusion (two mitochondria joining to become one) and fission (one mitochondrion dividing in two). |
| Like the nucleus, a ----- is surrounded by a double membrane. | mitochondrion |
| The inner membrane usually has folds called -----, which project like shelves across the organelle. | cristae |
| The space between the cristae, called the matrix, contains ribosomes; enzymes used in ATP synthesis; | and a few small, circular DNA molecules called mitochondrial DNA (mtDNA). |
| The principal function of mitochondria is to serve as the | "powerhouses" of the cell. |
| Energy isn't made here, | but it is extracted from organic compounds and transferred to ATP, primarily by enzymes located on the cristae. |
| Liver mitochondria | also play a role in detoxifying ammonia (a waste product of protein metabolism) and in converting amino acids and other organic molecules to glucose |
| It is virtually certain that mitochondria evolved from bacteria that invaded another primitive cell, survived in its cytoplasm, and became permanent residents. | The double membranes around the mitochondrion suggest that the original bacterium provided the inner membrane, and the host cell's phagosome provided the outer membrane when the bacterium was phagocytized. |
| Several comparisons show the apparent relationship of mitochondria to bacteria. | Their ribosomes are more like bacterial ribosomes than those of eukaryotic (nucleated) cells. |
| Mitochondrial DNA (mtDNA) | is a small, circular molecule that resembles the circular DNA of bacteria, not the linear DNA of the cell nucleus. |
| Mitochondrial DNA (mtDNA) | It replicates independently of nuclear DNA. |
| Mitochondrial DNA (mtDNA) | codes for some of the enzymes employed in ATP synthesis. |
| Mitochondrial DNA (mtDNA) | It consists of 16,569 base pairs (explained in chapter 4), comprising 37 genes, compared with over 3 billion base pairs and over 22,000 genes in nuclear DNA. |
| When a sperm fertilizes an egg, any mitochondria introduced by the sperm are destroyed and only those provided by the egg are passed on to the developing embryo. | Therefore, mtDNA is inherited exclusively through the mother. |
| While nuclear DNA is reshuffled in every generation by sexual reproduction, ----- remains unchanged except by random mutation. | mtDNA |
| Because of the known pace of such mutations, biologists and anthropologists can use ---- in humans and other species. | mtDNA as a "molecular clock" to trace evolutionary lineages |
| The amount of difference between the ---- of related species affords a record of how much time has passed since they diverged from their last common ancestor. | mtDNAs |
| Anthropologists have gained evidence from mtDNA that of all the females who lived in Africa 200,000 years ago, only one has left an unbroken line of female descent to today. | Other females from that time, however, may have descendants alive today through sons and the subsequent male line. |
| (Similarly, all males living today are thought to be descended from a single male who lived about 300,000 years ago. This was traced not through mtDNA, of course, but through the Y chromosome.) | Mitochondrial DNA has also been used as evidence in criminal law and to identify the remains of soldiers killed in combat. It was used in 2001 to identify the remains of the famed bandit Jesse James, who was killed in 1882. |
| Mutations | in mtDNA are responsible for various rare hereditary diseases and death in early childhood. |
| Mutations | Tissues and organs with the highest energy demands are the most vulnerable to mitochondrial dysfunctions—nervous tissue, the heart, the kidneys, and skeletal muscles, for example. |
| Mitochondrial myopathy is a degenerative muscle disease in which the muscle displays "ragged red fibers," cells with abnormal mitochondria that stain red with a particular histological stain | Another mtDNA disease is Leber hereditary optic neuropathy (LHON), a form of blindness that usually appears in young adulthood as a result of damage to the optic nerve. |
| Kearns-Sayre syndrome (KSS) involves paralysis of the eye muscles, degeneration of the retina, heart disease, hearing loss, diabetes, and kidney failure. | Damage to mtDNA has also been implicated as a possible factor in Alzheimer disease, Huntington disease, and other degenerative diseases of old age. |
| Some women known to carry mtDNA mutations can now avoid passing such diseases to their children through in vitro fertilization (IVF) with | mitochondrial replacement therapy (MRT) |
| The techniques of MRT are complex and diverse, but the essence of it is to provide an egg of the patient with healthy mitochondria from another egg donor; | fertilize the recipient egg with the father's sperm; and implant this in the patient's uterus. |
| The successful result is a ----- who has the father's nuclear DNA, the birth mother's nuclear DNA, and the mitochondria donor's mtDNA | "three-parent baby" |
| A centriole (SEN-tree-ole) is a short cylindrical assembly of microtubules, arranged in nine groups of three microtubules each (fig. 3.33). | Near the nucleus, most cells have a small, clear patch of cytoplasm called the centrosome 41 containing a pair of mutually perpendicular centrioles |
| Centrioles play a role in cell division described in the next chapter. | Each basal body of a flagellum or cilium is a single centriole oriented perpendicular to the plasma membrane. |
| Basal bodies | originate in a centriolar organizing center and migrate to the plasma membrane. Two microtubules of each triplet then elongate to form the nine pairs of peripheral microtubules of the axoneme. |
| A ----- can grow to its full length in less than an hour. | cilium |
| How does a centriole resemble the axoneme of a cilium? How does it differ? | Centrioles and axonemes are similar in that both feature a cylinder composed of nine microtubule groups. |
| centriole | However, they differ in structure: centrioles consist of nine triplets of microtubules without a central pair, while axonemes typically have nine doublets and a central pair. |
| Inclusions are of two kinds: accumulated cell products such as glycogen granules, pigments, and oil droplets (see fig. 3.28b); and foreign bodies such as viruses, bacteria, and dust particles and other debris phagocytized by a cell. | Inclusions are never enclosed in a membrane, and unlike the organelles and cytoskeleton, they are not essential to cell survival. |
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