UNIT FOUR A TOUR OF THE CELL 4 1 The fundamental Units of Life cell is simplest collection of matter that can be alive all cells related by their descent from earlier cells differ from one another but also share common features 1 Microscopy 1590 inventions of microscopes refined in 1600 s cell walls first seen by Robert Hooke 1665 only observed dead cells Antoni van Leeuwenhoek crafted lenses that allowed living cells to be seen called microorganisms animalcules microscopes 1st used by Renaissance a scientists light microscope LM visible light passed through the specimen and then through glass lenses Lenses bend refract light and magnify the observed subject 3 important parameters in microscopy magnification resolution contrast Magnification ratio of objects image size to its real size Resolution measure of image clarity Contrast difference in brightness between light and dark areas of an image until recently resolution barrier prevented using light microscopy to see organelles 1950 s electron microscope invented uses electron microscope EM focuses beam of electrons through specimen resolution inversely related to wavelength of radiation a microscope uses for imaging electron beams have shorter wavelengths than light Transmission Electron Microscope TEM study internal structure of cells aims electron beam through very thin section of a specimen specimen has been stained with atoms of heavy metals uses electromagnets as lenses to bend paths of electrons Scanning Electron Microscope SEM useful for topography of specimen electromagnetic lenses like TEM scans surface that is usually coated in a thin film of gold beam excites electrons on surface and secondary electrons are detected by a device that translates their patterns into to a video screen Electron microscopes have revealed many organelles and other subcellular structures although the methods to prepare the specimen kill the cells 2 Cell Fractionation useful in studying cell structure and function broken cells place in tube that s spun in centrifuge cause largest cell components to settle to bottom of tube forming a pellet allows researchers to prepare components in bulk and identify functions 4 2 Eukaryotic Cells Have Internal Membranes that Compartmentalize Their Functions 1 Comparing Prokaryotic and Eukaryotic Cells All cells similar in that bound by selective barrier Plasma Membrane Selective membrane Internal semifluid jellylike called Cytosol All cells contain chromosomes carrying genes in form of DNA All cells have ribosomes to make proteins Eukaryotic Cell most DNA is in the organelle Nucleus which is bound by a double membrane Eukaryotic true nucleus usually much larger than prokaryotes Prokaryotic Cell DNA is concentrated in the Nucleoid region not bound by a membrane Prokaryotic before nucleus size is a general feature of cell structure that relates to function metabolic requirements have upper limits on size as a cell grows in size its volume grows proportionately more than its surface area smaller cells have greater ratio of surface area to volume larger organisms don t have larger cells they have more cells 2 A Panoramic View of the Eukaryotic Cell Eukaryotic cell has extensively and elaborately arranged membranes membranes dived the cell into compartments this way incompatible processes can occur simultaneously plasma membrane and organelle membrane participate in cell metabolism many enzymes built right into membranes basic fabric of biological membranes double layer of phospholipids and other lipids but each type of membrane has a unique composition of lipids suited to its specific function 4 3 The Eukaryotic Cell s genetic instructions are housed in the nucleus and carried out by the ribosomes 1 The Nucleus Information Cell Nucleus contains most of the genes in the Eukaryotic cell some genes located in mitochondria and chloroplasts nucleus is most conspicuous organelle in Eukaryotic cell Nuclear Envelope double membrane that encloses the nucleus separating its contents from the cytoplasm Two membranes lipid bilayer associate w proteins separated by a space of 20 40nm envelope is perforated by pore structures lip of each pore inner and outer membranes of the nuclear envelope are continuous Pore Complex intricate protein structure that lines each pore regulating entry and exit of proteins RNA s and large complexes of Macromolecules Nuclear lamina a netlike array of protein filaments that line the nuclear side of envelope that maintains shape of nucleus by supporting nuclear envelope Chromosomes structures that carry genetic info inside nucleus contains 1 long DNA molecule associated w proteins Chromatin complex of DNA and Proteins that make up chromosomes appears as a diffuse mass when cell not dividing as cell prepares to divide chromosomes coil condense further becoming thick enough to separate structures each Eukaryotic species has specific of chromosomes ex typical human cell has 46 chromosomes in nucleus exceptions sex cells which each have 23 nucleolus prominent structure within the non dividing nucleus Ribosomal RNA rRNA is synthesized here from DNA instructions proteins imported from cytoplasm assemble w rRNA into subunits of subunits then exit nucleus through nuclear pores to cytoplasm where a large and small unit can assemble into an actual ribosome Ribosomes complexes made of rRNA and protein are cellular components that carry out protein synthesis ribosomes the more ribosomes the higher the rate of protein synthesis build proteins into 2 cytoplasmic locales free ribosomes suspended in cytosol bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope most proteins made on free ribosomes function within cytosol example enzymes catalyzing sugar breakdown bound ribosomes make proteins used for insertion into membranes or for packaging within certain organelles such as lysosomes 4 4 The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions in the Cell 1 Endoplasmic Reticulum Biosynthetic Factory Endomembrane System includes nuclear envelope endoplasmic reticulum golgi apparatus various kinds of vacuoles and vesicles lysosomes and plasma membrane carries out variety of tasks synthesis of proteins Transport of proteins Metabolism and movement of lipids Detoxification of poisons membranes of this system related either through direct physical continuity or transfer of membrane segments known as vesicles vesicles sacs of membrane various membranes