AQ: Organelles. Answer the following questions after reading Chapter 3 in your t
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Question
AQ: Organelles. Answer the following questions after reading Chapter 3 in your textbook.
Explain the main differences between prokaryotic and eukaryotic cells. F
ill in the following chart: Organelle Function Found in Plants/Animals/Both? Nucleus Provides structure and stability to the cell. Made of the complex carbohydrate cellulose. Plasma Membrane Ribosomes Both The most common type of plastid, where photosynthesis occurs using chlorophyll Plants Facilitates cellular communication and the channeling and processing of materials. Contains Ribosomes Smooth ER Vacuoles Chromoplasts Plants Mitochondria Peroxisomes Cytoskeleton
Compare and contrast the plasma membrane and the cell wall. Be sure to mention ways that they are similar and different.
What are the main organelles involved in the process of mitosis? Explain how each organelle is involved.
)pages 30-44 of book) to answer questions
Figure 3.3 (a) A transmission electron microscope. (b) A scanning electron microscope. (a-b) Courtesy of JEOL-USA, Inc., Peabody, MA
Eukaryotic versus Prokaryotic Cells
Nearly all higher plant and animal cells share most of the various features discussed in this chapter. Some of these features (e.g., nuclei, plastids) are, however, lacking in the cells of some very simple organisms such as bacteria. Cells without nuclei are called prokaryotic (pro = before; karyon = nucleus) to distinguish them from typical eukaryotic (eu = well or good; karyon = nucleus) cells, discussed here. Prokaryotic cells are covered in more detail in Chapter 17. According to endosymbiotic theory, some cell components (chloroplasts and mitochondria) evolved when a large eukaryotic cell engulfed independent prokaryotes. Both prokaryotic and eukaryotic organisms can have cell walls (rigid boundaries of cells), but only eukaryotic cells contain organelles(membrane-bound structures with specialized functions). These features, and other cellular components, are discussed in the sections that follow.
Cell Structure and Communication
Plant cells typically have a cell wall surrounding the protoplasm, which consists of all the living components of a cell. These living components are bounded by a membrane called the plasma membrane. All cellular components between the plasma membrane and a relatively large body called the nucleus are known as cytoplasm. Within the cytoplasm is a souplike fluid called cytosol, in which various bodies called organelles are dispersed. Organelles are persistent structures of various shapes and sizes with specialized functions in the cell; most, but not all, are bounded by membranes (Figs. 3.4and 3.5).
Cell Size
Most plant and animal cells are so tiny they are invisible to the unaided eye. Cells of higher plants generally vary in length between 10 and 100 micrometers.1 Remember that the resolution of a light microscope is 2 micrometers, making it useful for the study of eukaryotic cells. Because there are roughly 25,000 micrometers to the inch, it would take about 500 average-sized cells to extend across 2.54 centimeters (1 inch) of space; 30 of them could easily be placed across the head of a pin. Some prokaryotic (bacterial) cells are less than 0.5 micrometer wide, while cells of the green alga called mermaid’s wineglass (Acetabularia) are mostly between 2 and 5 centimeters long, and fiber cells of some nettles are about 20 centimeters long.
Why are cells so small? Consider that as a cell increases in size, its volume grows much more than its surface area. The increase in surface area of a spherical cell, for example, is equal to the square of its increase in diameter, but its increase in volume is equal to the cube of its increase in diameter. This means that a cell whose diameter increased 10 times would increase in surface area 100 times (10 squared) but in volume 1,000 times (10 cubed). Because all substances enter or leave cells through their surfaces, which are the only contact areas with their surroundings, larger cells are at a disadvantage. Furthermore, the nucleus regulates all aspects of a cell’s activities, and the page 34greater the volume of the cell, the longer it takes for instructions from the nucleus to reach the surface. On the other hand, smaller cells have a clear advantage because they have relatively larger surface area to volume ratios, thereby enabling faster and more efficient communication between the nucleus and other parts of the cell.
Figure 3.4 Anatomy of a young plant cell. (a) Generalized drawing to show cellular components within a young plant cell. (b) Colored transmission electron micrograph section of a young cell from the root-tip meristem of Arabidopsis thaliana: the nucleus (pink) contains a nucleolus (dark); the cytoplasm contains mitochondria (blue), proplastids (juvenile chloroplasts), without developed thylakoid systems (green), dictyosomes (red), and both large and small vacuoles; the cell membrane is also shown with a distinct cell wall around its perimeter. (b) © Biophoto Associates/Science Source
Full-grown organisms have astronomical numbers of cells. For example, it has been calculated that a single mature leaf of a pear tree contains 50 million cells and that the total number of cells in the roots, stems, branches, leaves, and fruit of a full-grown pear tree exceeds 15 trillion. Can you imagine how many cells there are in a 3,000-year-old redwood tree of California, page 35which may reach heights of 90 meters (300 feet) and measure up to 4.5 meters (15 feet) in diameter near the base?
Figure 3.5 Anatomy of a plant cell. Colored scanning electron micrograph of a section through a plant cell, showing internal structures. The cell is encased in by a cell wall composed of cellulose, hemicellulose, and pectin. Inside the cell, are chloroplasts (dark green), the site of photosynthesis, and the nucleus (orange) which contains the cell’s genetic information. At the center of the cell is a large vacuole, which maintains the cell’s shape, stores useful materials, and digests waste products. © Dr. David Furness, Keele University/Getty Images
Some cells are boxlike with six walls, but others assume a wide variety of shapes, depending on their location and function. The most abundant cells in the younger parts of plants and fruits may be more or less spherical, like bubbles, when they are first formed, but as they press against each other, they commonly end up with an average of 14 sides by the time they are mature. These cell types are discussed in Chapter 4.
The Cell Wall
A novelty song of more than 80 years ago listed food items the writer said he disliked. Each verse ended with the line, “I like bananas because they have no bones!” Indeed, bananas and all plants differ from larger animals in having no bones or similar internal skeletal structures. Yet large trees support branches and leaves weighing many tons. They can do this because most plant cells have either rigid walls that provide the support afforded to animals by bones or semi-rigid walls that provide flexibility. At the same time, the walls protect delicate cell contents within. When millions of these cells function together as a tissue, their collective strength is enormous. The redwoods and Tasmanian Eucalyptus trees, which are the largest trees alive today, exceed the mass and volume of the largest land animals, the elephants, by more than a hundred times. The wood of one giant redwood tree could support the combined weight of a thousand elephants.
The first cell structure discovered by Robert Hooke in 1665 was the cell wall, and among plant cell structures observed with a microscope, the cell wall is the most obvious because it defines the shape of the cell. Many of the prepared specimens observed with a microscope in plant biology are merely stained remnants of once-living cells. But the vast diversity of cell walls within and among species tells a story about the structure and function of each cell. For instance, epidermal cells, which form a thin layer on the surfaces of all plant organs, often have unusual shapes and sizes. Some such cells form hairs that may secrete substances that discourage animals from grazing on the plants producing them. Thin-walled cells found beneath the epidermis of leaves are specialized for their function of photosynthesis (discussed in Chapter 10); and thick-walled cells of wood help transport water without collapsing. The structure and function of different plant cells, and the tissues they form, are addressed in Chapter 4.
The main structural component of cell walls is cellulose, which is composed of 100 to 15,000 glucose monomers in long chains and is the most abundant polymer on earth. As a primary food source for grazing animals and at least indirectly for nearly all other living organisms, it could be said that most life on earth relies directly or indirectly on the cell wall. Humans also depend on cell walls because they provide clothing, shelter, furniture, paper, and fuel.
In addition to cellulose, cell walls typically contain a matrix of hemicellulose (a gluelike substance that holds cellulose fibrils together), pectin (the organic material that gives stiffness to fruit jellies), and glycoproteins (proteins that have sugars associated with their molecules).
A middle lamella, which consists of a layer of pectin, is first produced when new cell walls are formed. This middle
lamella is normally shared by two adjacent cells and is so thin that it may not be visible with an ordinary light microscope unless it is specially stained. A flexible primary wall, consisting of a fine network of cellulose, hemicellulose, pectin, and glycoproteins, is laid down on either side of the middle lamella (Fig. 3.6a). Reorganization, synthesis of new molecules, and insertion of new wall polymers lead to rearrangement of the cell wall during growth. Secondary walls, which are produced inside the primary walls, are derived from primary walls by thickening and inclusion of lignin, a complex polymer.
Figure 3.6a A small portion of a cell wall of the green alga Chaetomorpha melagonium, showing how cellulose microfibrils are laid down. Each microfibril is composed of numerous molecules of cellulose. ×24,000. (a) © Biophoto Associates/Science Source
Secondary cell walls of plants generally contain more cellulose (40% to 80%) than primary walls. As the cell ages, wall thickness can vary, occupying from as little as 5% to more than 95% of the volume of the cells. During secondary wall formation, cellulose microfibrils become embedded in lignin, much as steel rods are embedded in concrete to form prestressed concrete (Fig. 3.6b).
Communication between Cells
Cells that manufacture, process, or store food have thin walls, while those involved in support usually have relatively thick walls. Although each living cell is capable of independently carrying on complex activities, it is essential that these activities be coordinated through some means of communication among all the living cells of an organism. Fluids and dissolved substances can pass through primary walls of adjacent cells via plasmodesmata (singular: plasmodesma), which are tiny strands of cytoplasm that extend between the cells through minute openings (see Fig. 3.20). The translocation of sugars, amino acids, ions, and other substances occurs through the plasmodesmata. The middle lamellae and most cell walls are, however, permeable and permit slower movement of water and dissolved substances between cells.
Cellular Components
Most chemical reactions that take place in cells occur in the protoplasm, as part of a dynamic series of events that make the plant a living entity. Each organelle within the protoplast has a primary function, and the flow of metabolites (products of chemical synthesis or breakdown) from one organelle to another is necessary for a balance of events that take place.
Envision a journey through the plant cell as an exciting voyage in which information is stored primarily in the nucleus, processed in the cytoplasm, and sent on to different parts of the cell. This information can bring about the synthesis of proteins in the cytoplasm, where they become involved in metabolic reactions (see Chapter 10), or they may be destined for use in other cellular locations. The packaged proteins may be incorporated in membranes or organelles, and other compounds may be manufactured in specific organelles or enter from an adjacent cell.
The Plasma Membrane
The outer boundary of the living part of the cell, the plasma membrane, is roughly eight-millionths of a millimeter thick. To get an idea of how incredibly thin that is, consider that it would take 12,500 such membranes neatly stacked in a pile to achieve the thickness of an ordinary piece of writing paper. Yet this delicate, semipermeable structure is of vital importance in regulating the movement of substances into and out of the cell. While the plasma membrane may inhibit movement of some substances, it can otherwise allow free movement and can even control movement of other substances into and out of the cell. As a result, the proportions and makeup of chemicals within a cell become quite different from those outside the cell. The plasma membrane is page 37also involved in the production and assembly of cellulose for cell walls.
Figure 3.6b Secondary cell wall structure. Components are arranged so that the cellulose microfibrils and hemicellulose chains are embedded in lignin. (Reproduced by permission of the Oklahoma Academy of Science.)
Evidence obtained since the early 1970s indicates that the plasma membrane and other cell membranes are composed of phospholipids arranged in two layers, with proteins interspersed throughout (Fig. 3.7). This fluid mosaic model for the plasma membrane implies a dynamic structure with numerous components, some of which can migrate and interact directly with each other. Covalent bonds link carbohydrates to both lipids and proteins on the outer surfaces of membranes. Some proteins extend across the entire width of the membrane, while others are embedded or apparently are loosely bound to the outer surface.
The remaining cell contents usually push the plasma membrane up against the cell wall because of pressures developed by osmosis (see Chapter 9), but the membrane is quite flexible and often forms folds, which may, in turn, become little, hollow spheres or vesicles that float off into the cell. In fact, experiments have shown that by adding detergents to a continuous membrane, it can be broken up and dispersed, yet it can partially reform when the detergents are removed. The membrane may even shrink away from the wall temporarily, but if it ever ruptures, the cell soon dies.
The Nucleus
The nucleus is the control center of the cell. In some ways, it functions like a combination of a computer program and a dispatcher that sends coded messages or “blueprints” originating from DNA in the nucleus with information that will ultimately be used in other parts of the cell. In other words, page 38the DNA in the nucleus provides the original information needed to fulfill the cell’s needs. This nuclear information contributes to growth, differentiation, and the myriad activities of the complex cell “factory.” The nucleus also stores hereditary information, which is passed from cell to cell as new cells are formed.
Figure 3.7 A model of a small portion of a plasma membrane, showing its fluid mosaic nature. The proteins, which are coiled chains of polypeptides, are either embedded or on the surfaces. Some of the embedded proteins extend all the way through and may serve as conduits for diffusion of certain ions. In cells, and other places where there are watery fluids, a double layer of phospholipids forms. The heads point outward toward the water. The tails, which are long-chain fatty acids, are hydrophobic (i.e., they “dislike” water) and point inward away from the water. The membrane is about 8 nanometers thick.
The nucleus often is the most conspicuous object in a living cell, although in green cells, chloroplasts may obscure it. In living cells without chloroplasts, the nucleus may appear as a grayish, spherical to ellipsoidal lump, sometimes lying against the plasma membrane to one side of the cell or toward a corner. Some nuclei are irregular in form, and they can vary greatly in size. They are, however, generally from 2 to 15 micrometers or larger in diameter. Certain fungi and algae have numerous nuclei within a single extensively branched cell, but the cells of more complex plants usually have a single nucleus.
Each nucleus is bounded by two membranes, which together constitute the nuclear envelope. Structurally complex pores, about 50 to 75 nanometers apart, occupy up to one-third of the total surface area of the nuclear envelope (Fig. 3.8). Proteins that act as channels for molecules are embedded within the pores. The pores apparently permit only certain kinds of molecules (for example, proteins being carried into the nucleus and RNA being carried out) to pass between the nucleus and the cytoplasm.
The nucleus contains a granular-appearing fluid called nucleoplasm, which is packed with short fibers that are about 10 nanometers in diameter; several different larger bodies are suspended within it. Of the larger nuclear bodies, the most noticeable are one or more nucleoli (singular: nucleolus), which are composed primarily of RNA and associated proteins.
Other important nuclear structures, which are not apparent with light microscopy unless the cell is stained or is in the process of dividing, include thin strands of chromatin. When a nucleus divides, the chromatin strands coil, becoming shorter and thicker, and in their condensed condition, they are called chromosomes. Chromatin is composed of protein and DNA (discussed in Chapter 13). Each cell of a given plant or animal species has its own fixed number and composition of chromosomes; the cells involved in sexual reproduction have half the number found in other cells of the same organism. The number of chromosomes present in a nucleus normally bears no relation to the size and complexity of the organism. Each body cell of a radish, for example, has 18 chromosomes in its nucleus, while a cell of one species of goldenweed has four, and a cell of a tropical adder’s tongue fern has over 1,000. Humans have 46 chromosomes in each body cell.
The Endoplasmic Reticulum
The outer membrane of the nucleus is connected to and continuous with the endoplasmic reticulum. The endoplasmic reticulum facilitates cellular communication and channeling of materials. Many important activities, such as the synthesis of membranes for other organelles and modification of proteins from components assembled from elsewhere within the cell, occur either on the surface of the endoplasmic reticulum or within its compartments.
Figure 3.8 Drawing of the nucleus showing the inner and outer membranes, nuclear pores, and nucleolus. The electron micrographs show detail of the nuclear pores, which are about 60 nanometers in diameter. © Don W. Fawcett/Science Source; Courtesy of Ron Milligan
The endoplasmic reticulum (often referred to simply as ER) is an enclosed space consisting of a network of flattened sacs and tubes that form channels throughout the cytoplasm, the amount and form varying considerably from cell to cell. Transmission electron micrographs of sectioned ER give it the appearance of a series of parallel membranes that resemble long, narrow tubes or sacs, creating subcompartments within the cell.
Ribosomes (discussed in the section that follows) may be distributed on the outer surface (i.e., the surface in contact with the cytoplasm) of the endoplasmic reticulum. Such endoplasmic reticulum is said to be rough and is primarily associated with the synthesis, secretion, or storage of proteins (Fig. 3.9; see also Chapter 13). This contrasts with smooth endoplasmic reticulum, which has few, if any, ribosomes lining the surface and is associated with lipid secretion. Both types of endoplasmic reticulum can occur in the same cell and can be interconverted, depending on the demands of the cell. Many enzymes involved in the process of cellular respiration are synthesized on the surface of the endoplasmic reticulum. The enzymes, however, enter other organelles (primarily mitochondria, which are discussed later in this chapter) without passing through the endoplasmic reticulum. The endoplasmic reticulum also appears to be the primary site of membrane synthesis within the cell.
Ribosomes
Ribosomes are tiny bodies that are visible with the aid of an electron microscope. They are typically roughly ellipsoidal in shape with apparently varied and complex surfaces. Each ribosome is composed of two subunits that are composed of RNA and proteins; the subunits, upon close inspection, can be differentiated by a line or cleft toward the center. Ribosomes average only about 20 nanometers in diameter in most plant cells. Unattached ribosomes often occur in clusters of five to 100, particularly when they are involved in linking amino acids together in the construction of the large, complex protein molecules that are a basic part of all living organisms.
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Figure 3.9 Transmission electron micrograph of the cytoplasm in a root tip cell of corn (Zea mays). The endoplasmic reticulum, with attached ribosomes, can be seen slightly toward the right portion of the photograph. ×60,000. © Dr. Jeremy Burgess/Science Source
Ribosomal subunits are assembled within the nucleolus, are released, and in association with special RNA molecules initiate protein synthesis. Once assembled, complete ribosomes may line the outside of the endoplasmic reticulum but can also occur unattached in the cytoplasm, chloroplasts, or other organelles. About 55 kinds of protein are found in each ribosome of prokaryotic cells and a slightly higher number in those of eukaryotic cells (see the discussion of various types of RNA in Chapter 13). Unlike other organelles, ribosomes have no bounding membranes, and because of this, some scientists prefer not to call them organelles.
Dictyosomes
Stacks of flattened discs or vesicles known as dictyosomes may be scattered throughout the cytoplasm of a cell. Dictyosomes are often bounded by branching tubules that originate from the endoplasmic reticulum but are not directly connected to it (Fig. 3.10). Five to eight dictyosomes per cell are typical, but up to 30 or more may be found in cells of simpler organisms. Aggregations page 41of dictyosomes, constituting the Golgi apparatus, occur in protein-secreting animal cells and a few plant cells with similar function. In animal cells, the term Golgi body is used to describe dictyosomes, which are named after Camillo Golgi, who discovered the Golgi apparatus in 1898.
Figure 3.10 A dictyosome from Euglena, a waterweed. ×40,000. Courtesy of John Z. Kiss
Dictyosomes are involved in the modification of carbohydrates attached to proteins that are synthesized and packaged in the endoplasmic reticulum. Complex polysaccharides are also assembled within the dictyosomes and collect in small vesicles(tiny, blisterlike bodies) that are pinched off from the margins. These vesicles migrate to the plasma membrane, fuse with it, and secrete their contents outside the cell. Substances secreted by vesicles may include cell-wall polysaccharides, floral nectars, and essential oils found in herbs.
The enzymes needed for the packaging of proteins are produced by the endoplasmic reticulum and further modified within the dictyosomes. One might describe dictyosomes as collecting, packaging, and delivery centers or, perhaps, as “post offices” of the cell.
Plastids
Most living plant cells have several kinds of plastids, with the chloroplasts (Fig. 3.11a) being the most conspicuous. They occur in a variety of shapes and sizes, such as the beautiful corkscrewlike ribbons found in cells of the green alga Spirogyra (see Fig. 18.6) and the bracelet-shaped chloroplasts of other green algae, such as Ulothrix (see Figs. 18.2d and 18.5). The chloroplasts of higher plants, however, tend to be shaped somewhat like two Frisbees glued together along their edges, and when they are sliced in median section, they resemble the outline of a rugby football.
Although several algae and a few other plants have only one or two chloroplasts per cell, the number of chloroplasts is usually much greater in a green cell of a higher plant. Seventy-five to 125 is quite common, with the green cells of a few plants having up to several hundred. The chloroplasts may be from 2 to 10 micrometers in diameter, and each is bounded by an envelope consisting of two delicate membranes. The outer membrane apparently is derived
from endoplasmic reticulum, while the inner membrane is believed to have originated from the cell membrane of a cyanobacterium (discussed in Chapter 17).
Figure 3.11 Drawing of leaf mesophyll cell chloroplast and transmission electron micrographs showing variation in chloroplast structure. (a) A chloroplast. ×20,000. (b) Cutaway of a chloroplast. (c) Grana. ×40,000. (d) A few thylakoids. (a) © Dr. Kari Lounatmaa/Getty Images; (c) © Biology Pics/Science Source
Within the chloroplast are numerous grana (singular: granum), which are formed from membranes and have the appearance of stacks of coins with double membranes. There are usually about 40 to 60 grana linked together by arms in each chloroplast, and each granum may contain from two or three to more than 100 stacked thylakoids. In reality, thylakoids are part of an overlapping and continuous membrane system suspended in the liquid portion of the chloroplast (Fig. 3.11b). The thylakoid membranes contain green chlorophyll and other pigments. These “coin stacks” of grana are vital to life as we know it, for it is within the thylakoids that the first steps of the important process of photosynthesis (see Chapter 10) occur. In photosynthesis, green plants convert water and carbon dioxide (from the air) to simple food substances, harnessing energy from the sun in the process. The existence of human and all other animal life depends on the activities of the chloroplasts.
The liquid portion of the chloroplast is a colorless fluid matrix called stroma, which contains enzymes involved in photosynthesis. Genes in the nucleus dictate most of the activities of chloroplasts, but each chloroplast contains a small, circular DNA molecule that encodes for production of certain proteins related to photosynthesis and other activities in the chloroplast and cell. The chloroplast also contains RNA and ribosomes, which facilitate some protein synthesis. Some plastids (e.g., those of tobacco) store proteins. There are usually four or five starch grains in the stroma, as well as oil droplets and enzymes.
Chromoplasts are another type of plastid found in some cells of more complex plants. Although chromoplasts are similar to chloroplasts in size, they vary considerably in shape, often being somewhat angular. They sometimes develop from chloroplasts through internal changes that include the disappearance of chlorophyll. Chromoplasts are yellow, orange, or red in color due to the presence of carotenoid pigments, which they synthesize and accumulate. They are most abundant in the yellow, orange, or some red parts of plants, such as ripe tomatoes, carrots, or red peppers (Fig. 3.12). These carotenoid pigments, which are lipid soluble, are not, however, the predominant pigments in most red flower petals. The anthocyanin pigments of most red flower petals are water soluble.
Leucoplasts are yet another type of plastid common to cells of higher plants. They are essentially colorless and include amyloplasts, which synthesize starches, and elaioplasts, which synthesize oils. If exposed to light, some leucoplasts will develop into chloroplasts, and vice versa.
Plastids of all types develop from proplastids, which are small, pale green or colorless organelles having roughly the size and form of mitochondria (discussed in the next section). They are simpler in internal structure than plastids and have fewer thylakoids, the thylakoids not being arranged in grana stacks. Proplastids frequently divide and become distributed throughout the cell. After a cell itself divides, each daughter cell has a proportionate share. Plastids also arise through the division of existing mature plastids.
Figure 3.12 Chromoplasts in the flesh of a red pepper. ×400. © James E. Bidlack
Mitochondria
Mitochondria (singular: mitochondrion) are often referred to as the powerhouses of the cell, for it is within them that energy is released from organic molecules by the process of cellular respiration (the role of mitochondria in respiration is further discussed in Chapter 10). This energy is needed to keep the individual cells, and the plant as a whole, functioning. Carbon skeletons and fatty acid chains are also rearranged within mitochondria, allowing for the building of a wide variety of organic molecules. Mitochondria are numerous and tiny, typically measuring from 1 to 3 or more micrometers in length and having a width of roughly 0.5 micrometer; they are barely visible with light microscopes. They appear to be in constant motion in living cells and tend to accumulate in groups where energy is needed. They often divide in two; in fact, they all originate from the division of existing mitochondria.
Mitochondria typically are shaped like cucumbers, paddles, rods, or balls. A sectioned mitochondrion resembles a scooped-out watermelon, with inward extensions of the rind forming mostly incomplete partitions perpendicular to the surface (Fig. 3.13). The appearance of incomplete partitions results from the fact that each mitochondrion is bounded by two membranes, with the inner membrane forming numerous platelike folds called cristae. The cristae greatly increase the surface area available to the enzymes contained in a matrix fluid. The number of cristae, as well as the number of mitochondria themselves, can change over time, depending on the activities taking place within the cell. The matrix fluid also contains DNA, RNA, ribosomes, proteins, and dissolved substances.
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Microbodies
Various small bodies distributed throughout the cytoplasm tend to give it a granular appearance. Examples of such components include types of small, spherical organelles called microbodies, which contain specialized enzymes and are bounded by a single membrane. Peroxisomes, for instance, contain enzymes needed by some plants to survive during hot conditions in a process called photorespiration (discussed in Chapter 10), whereas glyoxisomes contain enzymes that aid in the conversion of fats to carbohydrates during, for example, the germination of seeds containing fats. If present, peroxisomes are generally found associated with chloroplasts, and glyoxisomes usually are located near mitochondria. During a plant’s life cycle, peroxisomes and glyoxisomes may increase in number at stages when the need for them is greatest.
Figure 3.13 A mitochondrion greatly enlarged and cut away to show the cristae (folds of the inner membrane). A mitochondrion is about 2 micrometers long.
At one time, lipid, fat, or wax droplets commonly found in cytoplasm were believed to be bounded by a membrane; recent evidence, however, suggests no membrane is present, and some, therefore, do not consider them true organelles. Another organelle, called a lysosome, stores enzymes that digest proteins and certain other large molecules but is apparently confined to animal cells. The digestive activities of lysosomes are similar to those of the vacuoles of plant cells (discussed next).
Vacuoles
In a mature living plant cell, 90% or more of the volume may be taken up by one or two large central vacuoles that are bounded by vacuolar membranes (tonoplasts) (Fig. 3.14). The vacuolar membranes, which constitute the page 44inner boundaries of the living part of the cell, are similar in structure and function to plasma membranes.
Figure 3.14 A small portion of a root cap cell of tobacco. ×100,000. V = vacuole; T = vacuolar membrane (tonoplast); G = dictyosome with vesicles (arrows); M = mitochondrion; ER = endoplasmic reticulum; PM = plasma membrane; CW = cell wall. Courtesy of John Z. Kiss
The vacuole evidently received its name because of a belief that it was just an empty space; hence, its name has the same Latin root as the word vacuum (from vacuus—meaning “empty”). Vacuoles, however, have a variety of functions, including maintenance of cell pressure and pH, as well as storage of numerous cell metabolites and waste products. Inside the vacuole is a watery fluid called cell sap, which is slightly to moderately acidic. Cell sap, which helps maintain pressures within the cell (see the discussion of osmosis in Chapter 9), contains dissolved substances, such as salts, sugars, organic acids, and small quantities of soluble proteins. It also frequently contains water-soluble pigments. These pigments, called anthocyanins, are responsible for many of the red, blue, or purple colors of flowers and some reddish leaves. In some instances, anthocyanins accumulate to a greater extent in response to cold temperatures in the fall. They should not be confused, however, with the red and orange carotenoid pigments confined to the chromoplasts. Yellow carotenoid pigments (carotenes) also play a role in fall leaf coloration (discussed in Chapter 7).
Sometimes, large crystals of waste products form within the cell sap after certain ions have become concentrated there. Vacuoles in newly formed cells are usually tiny and numerous. They increase in size and unite as the cell matures. In addition to accumulating the various substances and ions mentioned previously, vacuoles are apparently involved in the recycling of certain materials within the cell and even aid in the breakdown and digestion of organelles, such as plastids and mitochondria.
The Cytoskeleton
The cytoskeleton is involved in movement within a cell and in a cell’s architecture. It is an intricate network constructed mainly of two kinds of fibers—microtubules and microfilaments.
Microtubules control the addition of cellulose to the cell wall (Fig. 3.15). They are also involved in cell division, movement of cytoplasmic organelles, control of the movement of vesicles containing cell-wall components assembled by dictyosomes, and movement of the tiny, whiplike flagella and cilia possessed by some cells (see the section on plant movements in Chapter 11).
Microtubules are unbranched, thin, hollow, tubelike structures that resemble tiny straws. They are composed of proteins called tubulins and are of varying lengths, most being between 15 and 25 nanometers in diameter. They are most commonly found just inside the plasma membrane. Microtubules are also found in the special fibers that form the spindles and phragmoplasts of dividing cells, discussed later in this chapter.
Microfilaments, which play a major role in the contraction and movement of cells in multicellular animals, are present in nearly all cells. They are three or four times thinner than microtubules and consist of long, fine threads of protein with an average diameter of 6 nanometers. They are often in bundles and appear to play a role in the cytoplasmic streaming (sometimes referred to as cyclosis) that occurs in all living cells. When cytoplasmic streaming is occurring, a microscope reveals the apparent movement of organelles as a current within the cytoplasm carries them around within the walls. This streaming probably facilitates exchanges of materials within the cell and plays a role in the movement of substances from cell to cell. The precise nature and origin of cytoplasmic streaming are still not known, but there is evidence that bundles of microfilaments may be responsible for it. Other evidence suggests that it may be related to the transport of cellular substances by microtubules.
Explanation / Answer
1.Ans-
3.Ans-
Similarities-
Differences-
4. Ans-
The main organelles involved in the mitosis include- the smooth endoplasmic reticulum, the mitochondria, and the centrosomes.
Smooth ER Makes Membranes
SER involved in producing phospholipids for the membranes of a cell. The phospholipids amount in the cell become double during the S phase of interphase for dividing the cells.
Mitochondria Fuse
The mitochondria are called powerhouse of cells and are bean-shaped in structure. During mitosis, the cells require ATP before entering into S phase. So, these mitochondria are fuse together to form a network which produces more ATP than individual mitochondria.
Centrosomes and Spindle Fibers
Centrosomes are the proteins and involved in the formation of spindle fibers and these fibers are called microtubules. Centrosomes become double during S phase of interphase because, during mitosis, each daughter cell has one centrosome. Before mitosis, the main function of centrosomes is to divide the cells into both poles which are joined by spindle fibers.
2. Fill in the following chart:
Organelle Function Found in Plants/Animals/Both? Nucleus
Provides structure and stability to the cell.
Made of the complex carbohydrate cellulose. Plasma Membrane Ribosomes Both The most common type of plastid, where photosynthesis occurs using chlorophyll Plants Facilitates cellular communication and the channeling and processing of materials. Contains Ribosomes Smooth ER Vacuoles Chromoplasts Plants Mitochondria Peroxisomes Cytoskeleton
Note- Kindly, write the question properly, it’s unable to understand for me.