Chapter 6 A Tour of the Cell 6 1 Biologists use microscopes and the tools of biochemistry to study cells Light microscope LM visible light is passed through the specimen and then through glass lenses the lenses refract bend the light in such a way that the image of the specimen is magnified as it is projected into the eye or into a camera o Magnification the ratio of an object s image size to its real size o o Resolution a measure of the clarity of the image it is the minimum distance two points can be separated and still be distinguished as two points Contrast accentuates differences in parts of the sample Organelles membrane enclosed structures within eukaryotic cells Electron microscope EM focuses a beam of electrons through the specimen or onto its surface used to study subcellular structures Scanning electron microscope SEM especially useful for detailed study of the topography of a specimen the electron beam scans the surface of the sample usually coated with a thin film of gold and provides images that appear 3 D Transmission electron microscope TEM used to study the internal structure of cells focuses a beam of electrons through a specimen Cytology the study of cell structure Biochemistry the study of the chemical processes metabolism of cells Cell fractionation takes cells apart and separates major organelles and other subcellular structures from one another Centrifuge spins test tubes holding mixtures of disrupted cells at a series of increasing speeds at each speed the resulting force causes a fraction of the cell components to settle to the bottom of the tube forming a pellet Biochemistry and cytology complement each other in correlating cell function with structure Microsomes pieces of plasma membranes and cells internal membranes found in pellets after centrifuge 6 2 Eukaryotic cells have internal membranes that compartmentalize their functions Cytosol a semifluid jellylike substance inside all cells in which subcellular components are suspended Chromosomes carry genes in the form of DNA Ribosomes tiny complexes that make proteins according to the instructions from the genes Eukaryotic cell most of the DNA is in an organelle called the nucleus which is bounded by a double membrane protists fungi animals and plants true nucleus membrane bound organelles cytoplasm is in the region between the plasma membrane and nucleus Prokaryotic cell no nucleus DNA is concentrated in a region that is not membrane enclosed called the nucleoid no membrane bound organelles Bacteria and Archaea before nucleus reflecting the fact that prokaryotic cells evolved before eukaryotic cells cytoplasm is bound by the plasma membrane Cytoplasm interior of either type of cell in eukaryotic cells this term refers only to the region between the nucleus and the plasma membrane Size is a general feature of a cell structure that relates to cell function eukaryotic cells are generally much larger than prokaryotic cells 1 Plasma membrane at the boundary of every cell functions as a selective barrier that allows passage of enough oxygen nutrients and wastes to service the entire cell the general structure of a biological membrane is a double layer of phospholipids Larger organisms do not generally have larger cells than smaller organisms they simply have more cells Microvilli long thin projections from cells that exchange a lot of material with their surroundings like intestinal cells which increase surface area without an appreciable increase in volume Mitochondria function in cellular respiration In animal cells but not plant cells In plant cells but not animal cells o o o o o o o Lysosomes digestive organelles where macromolecules are hydrolyzed Centrosomes with centrioles region where the cell s microtubules are initiated contains a pair of centrioles Flagella motility structure present in some animal cells composed of a cluster of microtubules within an extension of the plasma membrane present in some plant sperm Chloroplasts photosynthetic organelle converts energy of sunlight to chemical energy stored in sugar molecules Central vacuole prominent organelle in older plant cells functions include storage breakdown of waste products hydrolysis of macromolecules enlargement of vacuole is a major mechanism of plant growth Cell wall outer layer that maintains cell s shape and protects cell from mechanical damage made of cellulose other polysaccharides and protein Plasmodesmata cytoplasmic channels through cell walls that connect the cytoplasms of adjacent cells 6 3 The eukaryotic cell s genetic instructions are housed in the nucleus and carried out by the ribosomes Nucleus contains most of the genes in the eukaryotic cell Nuclear envelope double membrane that encloses the nucleus separating its contents from the cytoplasm Pore complex intricate protein structure that lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs as well as large complexes of macromolecules Nuclear lamina a netlike array of protein filaments that lines the nuclear side of the envelope except at the pores and maintains the shape of the nucleus by mechanically supporting the nuclear envelope Nuclear matrix a framework of protein fibers extending throughout the nuclear interior Chromosomes structures of DNA within the nucleus that carry out the genetic information each chromosome is composed of a single DNA molecule associated with proteins Chromatin the complex of DNA and proteins making up chromosomes condenses to form discrete chromosomes as a cell prepares to divide Nucleolus where a type of RNA called ribosomal RNA rRNA is synthesized from instructions in the DNA proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes Ribosomes complexes made of ribosomal RNA and protein the cellular components that carry out protein synthesis in the cytosol free ribosomes and on the outside of the endoplasmic reticulum or nuclear envelope bound ribosomes o Free and bound ribosomes are structurally identical and can alternate between the two roles 2 6 4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell Endomembrane system includes the nuclear envelope the endoplasmic reticulum the Golgi apparatus lysosomes various kinds of vesicles and vacuoles and the plasma membrane these components are either continuous or connected via transfer or vesicles Vesicles sacs made of membrane Endoplasmic reticulum
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