CHAPTER 6 A Tour of the Cell CH 6 Learning Objectives 1 Identify how common techniques of microscopy and biochemistry are used in cell biology 2 Compare and contrast 1 prokaryotic and eukaryotic cells and 2 animal and plant cells 4 3 Describe the structure of the nucleus chromosomes and ribosomes and explain their functions Identify the parts of the endomembrane system and describe their roles in the cell 5 Compare and contrast the structures and functions of mitochondria and 6 Describe the subunits and structures of the three types of cytoskeletal chloroplasts fibers and list their functions 7 Compare and contrast 1 the extracellular components of plant and animal cells and 2 the cell junctions of plant and animal cells 8 Explain how the parts of a cell work together to enhance cellular function I would suggest completing the crossword puzzle to help you understand the terminology and correlate how the terms relate to topics covered in this chapter Figure 6 1b CONCEPT 6 1 Biologists use microscopes and biochemistry to study cells Microscopes are used to visualize cells In a light microscope LM visible light is passed through a specimen and then through glass lenses Lenses refract bend the light so that the image is magnified Three important parameters of microscopy 1 Magnification the ratio of an object s image size to its real size 2 Resolution the measure of the clarity of the image or the minimum distance of two distinguishable points 3 Contrast visible differences in brightness between parts of the sample Figure 6 2 CLICK ME interactive website Light microscopes can magnify effectively to about 1 000x the size of actual specimen Various techniques enhance contrast and enable cell components to be stained or labeled The resolution of standard light microscopy is too low to study organelles the membrane enclosed structures in eukaryotic cells Two basic types of electron microscopes EMs are used to study subcellular structures 1 Scanning electron microscopes SEMs focus a beam of electrons onto the surface of a specimen providing images that look 3D 2 Transmission electron microscopes TEMs focus a beam of electrons through a specimen TEMs are used mainly to study the internal structure of cells Recent advances in light microscopy Labeling individual cells with fluorescent markers improves the level of detail that can be seen Confocal microscopy and deconvolution microscopy provide sharper images of 3D tissues and cells Cryo electron microscopy cryo EM allows preservation of specimens at very low temperatures This allows visualization of structures in their cellular environment with no need for preservatives This method is used to complement X ray crystallography in revealing protein complexes and subcellular structures Microscopes are important tools of cytology the study of cell structure Cell Fractionation Cell fractionation takes cells apart and separates the major organelles from one another Centrifuges fractionate cells into their component parts differential centrifugation Cell fractionation enables scientists to determine the functions of organelles Biochemistry and cytology help correlate cell function with structure CONCEPT 6 2 Eukaryotic cells have internal membranes that compartmentalize functions The basic structural and functional unit of every organism is one of two types of cells prokaryiotic or eukaryotic Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells Protists fungi animals and plants all consist of eukaryotic cells Comparing Prokaryotic and Eukaryotic Cells Basic features of ALL cells Plasma membrane Semifluid substance called cytosol Chromosomes carry genes Ribosomes make proteins Cells are classified as either PROKARYOTIC i e bacteria or EUKARYOTIC i e animal Viruses do not fall into either category and are technically NOT alive Prokaryotic cells are characterized by having No nucleus DNA in an unbound region called the nucleoid No membrane bound organelles Cytoplasm bound by the plasma membrane Eukaryotic cells are characterized by having DNA in a nucleus that has a double membrane Membrane bound organelles Cytoplasm in the region between the plasma membrane and nucleus Eukaryotic cells are generally much larger than prokaryotic cells Metabolic requirements set upper size limits of cells The plasma membrane is a selective barrier that allows sufficient passage of oxygen nutrients and waste to service the volume of every cell The surface area to volume ratio of a cell is critical As a cell increases in size its volume grows proportionately more than its surface area As a spherical cell enlarges its innermost parts get farther away from the plasma membrane Once a cell gets too large diffusion which is relatively slow can take too long to supply important processes deep within the cell r 1 0 12 6 4 2 3 0 distance to center r surface area 4 r2 Volume 4 3 r3 area volume r 2 0 50 3 33 5 1 5 r 4 0 201 1 268 1 0 75 A Panoramic View of the Eukaryotic Cell A eukaryotic cell has internal membranes that divide the cell into compartments the organelles The cell s compartments provide different local environments so that incompatible processes can occur in a single cell The basic fabric of biological membranes is a double layer of phospholipids and other lipids Plant and animal cells have most of the same organelles Figure 6 8 CONCEPT 6 3 The eukaryotic cell s genetic instructions are housed in the nucleus and carried out by the ribosomes The nucleus contains most of the cell s genes and is usually the most conspicuous organelle The nuclear envelope encloses the nucleus separating it from the cytoplasm The nuclear envelope is a double membrane each membrane consist of a lipid bilayer Ribosomes use the information from the DNA to make proteins Figure 6 9 Pores lined with a structure called a pore complex regulate the entry exit of molecules from the nucleus The nuclear side of the envelope is lined by the nuclear lamina which is composed of proteins and maintains the shape of the nucleus In the nucleus DNA is organized into discrete units called chromosomes Each chromosome contains one DNA molecule associated with proteins called chromatin Chromatin condenses to form discrete chromosomes as a cell prepares to divide The nucleolus located within the nucleus is the site of ribosomal RNA rRNA synthesis Ribosomes Protein Factories Ribosomes are complexes made of ribosomal RNA and protein Ribosomes build proteins in two locations In the cytosol
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