Chapter 6 A Tour of the Cell 6 1 Biologists use microscopes and the tools of biochemistry to study cells In the hierarchy of biological organization the cell is the simplest collection of matter that can be alive some forms of life are single celled but most are multicellular Cell walls were first see by Robert Hooke in 1655 Light microscopes LM visible light is passed through the specimen and then through glass lenses Three important parameters in microscopy are 1 Magnification the ratio of an object s image size to its real size 2 Resolution 3 Contrast Organelles a measure of the clarity of the image accentuates differences in parts of the sample the membrane enclosed structures within eukaryotic cells Electron Microscope EM focuses a beam of electrons through a specimen or onto its surface Scanning Electron Microscope SEM useful for detailed study of the topography of a specimen Transmission Electron Microscope TEM used to study the internal structure of cells cytology the study of cell structure Cell fractionation takes cells apart and separates major organelles and other subcellular structures from one another this is done with a centrifuge enables researchers to prepare specific cell components in bulk and identify their functions 6 2 Eukaryotic cells have internal membranes that compartmentalize their functions Bacteria and Archaea consist of prokaryotic cells Protists Fungi animals and plants all consist of eukaryotic cells All cells share certain basic features they are all bounded by a selective barrier called the plasma membrane inside all cells is a semifluid jellylike substance called cytosol in which subcellular components are suspended all cells contain chromosomes which carry genes in the form of DNA all cells have ribosomes which are tiny complexes that make proteins according to instructions from the genes There are some differences though The location of the DNA is different Eukaryotic cells most of the DNA is in the nucleus which is bounded by a double membrane Prokaryotic cells the DNA is concentrated in a region that is not membrane enclosed called the nucleotide prokaryotic cells have no nuclei The interior of either type of cell is called the cytoplasm in eukaryotic cells this term refers only to the region between the nucleus and the plasma membrane a variety of organelles are suspended in the cytoplasm these membrane bounded organelles are absent in prokaryotic cells Eukaryotic cells are much larger than prokaryotic cells At the boundary of every cell the plasma membrane functions as a selective barrier that allows passage of enough oxygen nutrients and wastes to service the entire cell A eukaryotic cell has extensive and elaborately arranged internal membranes that divide the cell into compartments the organelles The basic fabric of most biological membranes is a double layer of phospholipids and other lipids embedded in this lipid bi layer or attached to its surfaces are diverse proteins 6 3 The eukaryotic cell s genetic instructions are housed in the nucleus and carried out by ribosomes The nucleus contains most of the genes in the eukaryotic cells the nuclear envelope encloses the nucleus separating it from the cytoplasm double membrane each a lipid bi layer with associated proteins the envelope is perforated by pore structures at the lip of each pore the inner and outer membranes of the nucleus envelope are continuous the nuclear side of the envelope is lined by the nuclear lamina a netlike array of protein filaments that maintain the shape of the nucleus within the nucleus the DNA is organized into discrete units called chromosomes structures that carry the genetic information each chromosome contains one long DNA molecule associated with many proteins chromatin the complex of DNA and proteins making up chromosomes each eukaryotic species has a certain number of chromosomes humans have 46 nucleolus densely stained granules and fibers adjoining part of the chromosomes ribosomal RNA rRNA is synthesized here proteins are also imported here from the cytoplasm and are assembled with rRNA into large and small subunits of ribosomes these subunits then exit the nucleus through the pores and then assemble into ribosomes in the cytoplasm the nucleus directs protein synthesis by synthesizing messenger RNA mRNA according to instructions from RNA the mRNA then goes into the cytoplasm through the nuclear pores once there the ribosomes translate the genetic message into the primary structure of a specific polypeptide Ribosomes ribosomes are complexes made of ribosomal RNA and protein they are the cellular components that carry out protein synthesis Ribosomes build proteins in two cytoplasmic locales free ribosomes are suspended in the cytosol bound ribosomes are attached to the outside of the endoplasmic reticulum bound and free ribosomes are structurally identical and they can alternate between the two roles most of the proteins made on free ribosomes function within the cytosol bound ribosomes generally make proteins that are destined for insertion into membranes 6 4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell Many of the different membranes of the eukaryotic cell are part of the endomembrane system includes the nuclear envelope the endoplasmic reticulum ER the golgi apparatus lysosomes vesicles and vacuoles and the plasma membrane Performs a variety of tasks synthesis of proteins transport of proteins into membranes and organelles or out of the cell metabolism and movement of lipids detoxification of poisons Endoplasmic Reticulum ER accounts for half of the total membrane in eukaryotic cells consists of a network of membranous tubules and sacs called cisternae the ER separates the internal compartment of the ER ER lumen or cisternal space from the cytosol the ER membrane is continuous with the nuclear envelope There are two distinct regions of the ER differ in structure and function 1 Smooth ER called so because its outer layer lacks ribosomes functions in diverse metabolic processes synthesis of lipids metabolism of carbohydrates detoxification of drugs and poisons storage of calcium ions enzymes of the smooth ER help detoxify drugs and poisons liver cells in muscle cells the smooth ER membrane pumps calcium ions from the cytosol into the ER lumen 2 Rough ER studded with ribosomes on the outer surface of the membrane proteins from the ribosomes enter the ER lumen and fold into its shape most secretory
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