Ultrastructure of cells

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Ultrastructure of cells

  1. 1. Cells are small and complex. It is hard to see their structure, hard to discover their molecular composition, and harder still to find out how their various components function. What we can learn about cells depends on the tools at our disposal. Principal methods in microscopy used to study cells. Understanding the structural organization of cells is an essential prerequisite for understanding how cells function. Optical microscopy will be our starting point. An important advantage of optical microscopy is that light is relatively nondestructive Light microscopy is limited in the fineness of detail that it can reveal. A typical animal cell is 10-20 μm in diameter, which is about one-fifth the size of the smallest particle visible to the naked eye. It was not until good light microscopes became available in the early part of the nineteenth century that all plant and animal tissues were discovered to be aggregates of individual cells. Animal cells are not only tiny, they are also colorless and translucent. Consequently, the discovery of their main internal features depended on the development, in the latter part of the nineteenth century, of a variety of stains that provided sufficient contrast to make those features visible. Similarly, the introduction of the far more powerful electron microscope in the early 1940s required the development of new techniques for preserving and staining cells before the full complexities of their internal fine structure could begin to emerge. To this day, microscopy depends as much on techniques for preparing the specimen as on the performance of the microscope itself. To make a permanent preparation that can be stained and viewed at leisure in the microscope, one first must treat cells with a fixative so as to immobilize, kill, and preserve them. In chemical terms, fixation makes cells permeable to staining reagents and cross-links their macromolecules so that they are stabilized and locked in position. Some of the earliest fixation procedures involved immersion in acids or in organic solvents, such as alcohol. There is little in the contents of most cells (which are 70% water by weight) to impede the passage of light rays. Thus, most cells in their natural state, even if fixed and sectioned, are almost invisible in an ordinary light microscope. One way to make them visible is to stain them with dyes. In the early nineteenth century, the demand for dyes to stain textiles led to a fertile period for organic chemistry. Some of the dyes were found to stain biological tissues and, unexpectedly, often showed a preference for particular parts of the cell the nucleus or mitochondria, for example making these internal structures clearly visible. Today a rich variety of organic dyes is available, with such colorful names as Malachite green, Sudan black, and Coomassie blue, each of which has some specific affinity for particular subcellular components. The dye hematoxylin, for example, has an affinity for negatively charged molecules and therefore reveals the distribution of DNA, RNA, and acidic proteins in a cell. The chemical basis for the specificity of many dyes, however, is not known.
  2. 2. Figure 9-7. Two ways to obtain contrast in light microscopy. (A) The stained portions of the cell reduce the amplitude of light waves of particular wavelengths passing through them. A colored image of the cell is therebyobtained that is visible in the ordinary way. (B) Light passing through the unstained, living cell undergoes very little change in amplitude, and the structural details cannot be seen even if the image is highly magnified. The phase of the light, however, is altered by its passage through the cell, and small phase differences can be made visible by exploiting interference effects using a phase-contrast or a differential-interference-contrast microscope. Internal Structure of the Cell Membrane Membrane protein. Special proteins inserted in cellular membranes create pores that permit the passage of molecules across them These have a polar head group and two hydrophobic hydrocarbon tails. The tails are usually fatty acids, and they can differ in length (they normally contain between 14 and 24 carbon
  3. 3. atoms). One tail usually has one or more cis-double bonds (i.e., it is unsaturated), while the other tail does not (i.e., it is saturated) Intracellular compartment and Protein sorting Unlike a bacterium, which generally consists of a single intracellular compartment surrounded by a plasma membrane, a eucaryotic cell is elaborately subdivided into functionally distinct, membrane-enclosed compartments. Nucleus - contains the main genome and is the principal site of DNA and RNA synthesis. The surrounding cytoplasm consists of the cytosol and the cytoplasmic organelles suspended in it. The ER has many ribosomes bound to its cytosolic surface; these are engaged in the synthesis of both soluble and integral membrane proteins, most of which are destined either for secretion to the cell exterior or for other organelles. We shall see that whereas proteins are translocated into other organelles only after their synthesis is complete, they are translocated into the ER as they are synthesized Golgi apparatus consists of organized stacks of disclike compartments called Golgi cisternae; it receives lipids and proteins from the ER and dispatches them to a variety of destinations, usually covalently modifying them en route. Mitochondria and (in plants) chloroplasts generate most of the ATP used by cells to drive reactions that require an input of free energy; chloroplasts are a specialized version of plastids, which can also have other functions in plant cells, such as the storage of food or pigment molecules Lysosomes contain digestive enzymes that degrade defunct intracellular organelles, as well as macromolecules and particles taken in from outside the cell by endocytosis. On their way to lysosomes, endocytosed material must first pass through a series of organelles called endosomes. Peroxisomes are small vesicular compartments that contain enzymes utilized in a variety of oxidative reactions. Cytoskeleton Cells have to organize themselves in space and interact mechanically with their environment. They have to be correctly shaped, physically robust, and properly structured internally. Many of them also have to be able to change their shape and move from place to place. All of them have to be able to rearrange their internal components as they grow, divide, and adapt to changing circumstances. All these spatial and mechanical functions are developed to a very high degree in eucaryotic cells, where they depend on a remarkable system of filaments called the cytoskeleton. The cytoskeleton pulls the chromosomes apart at mitosis and then splits the dividing cell into two. It drives and guides the intracellular traffic of organelles, ferrying materials from one part of the cell to another. It supports the fragile plasma membrane and provides the mechanical linkages that let the cell bear stresses and strains without being ripped apart as the environment
  4. 4. shifts and changes. It enables some cells, such as sperm, to swim, and others, such as fibroblasts and white blood cells, to crawl across surfaces. It provides the machinery in the muscle cell for contraction and in the neuron to extend an axon and dendrites. It guides the growth of the plant cell wall and controls then amazing diversity of eucaryotic cell shapes. Vacuole membrane-bound sac with liquid + dissolved salts, ions, pigments, and waste products maintains cell shape (turgid) temporary storage area A vacuole may occupy 90% of the cell when the plant cell is mature the membrane of the vacuole is tonoplast like how a plasma membrane works turgid cell is swollen or firm because of water uptake Onion bulb for plant cell Cheek cell for animal cell Root nodules of Makahiya plant(Mimosa pudica) or yakult for live bacterial cells Bread molds for fungi Methylene blue and iodine solution Glass slides and cover slips Knife or cutter Tissue paper Alcohol lamp Inoculating loop Compound microscope

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