CONCEPTUAL LIFE SCIENCE CELLS THE CELL THEORY The word th~ory in science means "explanation." The ceU theory is the accepted explanation about cells. The cell theory was developed after many observations ofliving things by different biologists. The cell theory has two parts. 1. All living things are made ofcells. 2. All cells arise from pre-existing cens. Exceptions to the cell theory. 1. Mitochondria and chloroplasts are self-replicating. 2. Viruses are not true cens. Therearetwobasictypesofcells. Thecellsthatcamefirstarecalled prokaryotic cens. The prefix pro means "before" and the bryo stem means "nucleus." These celIs came before the cells that have a nucleus. The celIs with a nucleus are called euluJryotic cells. The prefix eu means "tnie." So, eukaryotic celIs have a "true nucleus" me~g that the cellular nucleus has a membrane around it. . The word ceU was used to describe these objects because the first ones seen in corkremindedtheobserverofthecellsinamonastery. Cellsaretoosmalltoseewith the unaided eye so we use microscopes. MlCROSCOPES Simple microscope. The simple microscope consists ofonly one lens. The magnifying glass is an exampleofasimplemicroscope. Compound microscope. The compound microscope has two lenses working in series. The objective lens is the lens nearest the slide. It magnifies the object on the slide to produceareal imageoftheobjectinsidethebodyofthemicroscope. The ocUlar lens is the lens nearest the eye. It producesa virtual·image ofthe real imageproducedbytheobjectivelens. Thevirtualimageappearsontheretina ofyom eye. You look into the ocular lens to see the magnified image ofthe specimen. When observing living material with the compound microscope it is possible to see the nucleus, the cytoplasm and, in plant cells, the chloroplasts. This is pretty much all that can be seen with optical microscopes. For further detail, much higher magnificatiOns are possible using the electron microscope. 4-14-2 Electron microscope Theelectronmicroscope usesabeamofelectronsinsteadoflightto magnifythe specimen. Very high magnifications are possible using the electron microscope. These reveal the ultrastructure ofthe cell. Ultrastructme and organelles Ultrastructme is the fine detail observed with the electron microscope. Itis possible to see such structures as membranes which 8re not visible in the light microscope because they are too thin to see. The tiny structures observed in cells are known as organelles because they are small and they serve the functions ofcellular organs. GENERALIZED CELLS AND CELL STRUCTURES Ageneralized cell is a drawing that contains all parts ofthe cell. In reality, there are many cells that lack one or more parts. A red blood cell, for example, contains cytoplasm and a cell membrane ~ut the other parts are missing. Figure 4-1. Typical ultrastructure of an animal cell. . Eukaryotic cells are found in all higher organisms. They contain a Well-defmed nucleus surrounded by a membrane. The prokaryotic organisms, Baeteriaand An:haea, do not have a membrane-bounded nucleus.· .,.I • 4-3 Cell membrane The cell membrane is a structure that surrolDlds each cell. All living cells have a cell membrane. As shown in Figure 4-2, the cell membrane consists oftwo layers of lipid molecules with the polar heads toward the outside and the non-polar fatty acid tails in the center ofthe membrane. This prevents nearly all molecules from passing through the membrane. Only water and a few small neutral molecules can pass through the cell membrane directly. It is the bOlDldary between the living cell and its environment. It has the same function in both plant and animal cells. The cell membrane has transport proteins to assist the passage oflarger moleculesorionizedmolecUles. Thereareavarietyofotherproteinsembedded in the cell membrane. These are known as integralproteins. They help to maintain the structure ofthe cell membrane and other important functions. Sometimes the integral proteins have auxiliary proteins associated with them known as peripheralproteins. These are important for other cellular functions, such as tran~tting messages to parts of the cell. Transport Protein Twa I.,ers at lipidmolecUle. Figure 4.2. Diagram ofcell membrane structure. In the electron microscope, a single membrane, such as illustrated in Figure 4.3, appears astwoparaJlel lines atadistanceof80Angstrom lDlits apart. This is shown in Figure 4-3. ===::i1 IDA Figure 4-3. Width ofmembrane.4-4 The "fluid mosaic" model ofthe cell membrane The proteins in the membrane can move around inside the two layers oflipicls and even protnJde through the SUlface. This gives the smface ofthe membrane the appearance ofa mosaic, apiece of art made oflittle tiles glued together. The cell membrane therefore is referred to as a "fluid mosaic." The Nucleus The nucleus is the central organelle that controls the cell. It is surrounded by a membrane known as the nuclear membrane. It coptains chromosomes and a-~cleolus. r:~---Nuclear Membrene r/*f,~ :~::matln ~» NucleolllB ~ ~)-------Endoplasmic Reticulum ~~;:!--------Ribosome Figure 4-4. Nucleus and Endoplasmic Reticulum Nuclear membrane The nuclear membrane surrounds the nucleus as shown in Figure 4-4. It contains pores and channels that connect to the endoplasmic reticulum, a network ofchannels that is found· in the cytoplasm.. Chromosomes The chromosomes are structures that contain DNA and are found inside the . nucleus ofthe cell. They control the activities ofthe cell. Nucleolus The nucleolus is a large, round structure within·the nucleus. It is responsible for the fonnation ofthe components ofribosomes. . ... '~., . 4-5 The Cytoplasm Thecytoplasmisthepartofthe cellthat is foundoutsideofthenucleus. Itis bounded on the outside by the cell membrane. The cytoplasm contains many different organelles that perfonn various fimctions for the cell. Mitochondria Mitochondria are the structures that produce useful energy for the cell. They contain an internal membrane that is folded to form cristae. Each single crista is an individual fold ofthis inner membrane. See Figure 4-5. IIl}!r----Outer Membrane H~I+---Crist. It.fli-----lnnerMembrane Figuie 4-5. Struetme ofthe mitochondrion. The folds ofthe cristae ofthe mitochondria contain enzymes that perfonn cellular respiration. Cellular respiration is the process by which useful energy is provided to the cell. Endoplasmic reticulum The endoplasmic reticulum (ER) is a series ofchannels in the cytopl8$ID~
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