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Chap 6 - The Cell.docx

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University of Lethbridge
BIOL 1010
Brent Sellinger

Reece et al. 9 Edition Chapter 6 Page ▯ ATour of the Cell Recall • Cells are the fundamental unit of life (i.e., in the hierarchy of biological organization, cells are the simplest collection of matter that can be alive) • Cells are the basic unit an organisms basic unit of structure and function • All cells arise from preexisting cells • Cells show an incredible diversity in structure and function and yet share many common features I. Tools to Study Cells 1. Microscopy The invention of the light microscope led to the discovery of cells. Three important concepts in microscopy are magnification, resolution and contrast Magnification = ratio of size of object image to real size of object • How much larger an object is made to appear compared to its real size Resolving power • Resolution is a measure of clarity of the image; it is the minimum distance two points can be separated by and still be distinguished as two separate points • Resolving power is inversely related to wavelength of electromagnetic radiation used by the microscope Contrast • Accentuate different parts of the specimen Types of Microscopes a. Light microscope • Visible light is focused on the specimen with a condenser. • Light passing through the specimen is refracted through an objective lens and an ocular lens producing an inverted magnified image. • 1000 x magnification and 0.2 µm resolution (limited by wavelength of visible light) for conventional light microscopy such as brightfield microscopy. • Above 1000 x magnification objects become increasingly blurry – This is often referred to as “empty magnification” because no useful information is gained from increasing magnification beyond 1000 x. • Contrast my be created due to natural pigmentation of specimens, staining or labeling cell components, or specialized microscopes with light modifying condensers. b. Electron microscope • These instruments use electromagnets as lens to focus a beam of electrons on specimen. Reece et al. 9 Edition Chapter 6 Page ▯ • Electron microscopes have a much higher resolving power. Theoretically it is around 2 pm for transmission electron microscopy due to the much shorter wavelength of the electron beam. However, the practical limit for biological systems is 2 nm. • The development of electron microscopes allowed researchers to study internal cell ultrastructure (e.g., intracellular organelles) using transmission electron microscopy. i) Transmission electron microscopy (TEM) • Electrons transmitted through the specimen • Specimens are stained with heavy metals resulting in regions with different electron densities. When exposed to a beam of electrons, regions with high electron densities allow fewer electrons to pass through. Displayed images represent a pattern of transmitted electrons • TEM is used to study internal cellular ultrastructure ii) Scanning electron microscopy (SEM) • Specimens are coated with a think film of gold.A scanning beam of electrons excites secondary electrons on gold specimen surface. These are collected and focused onto a viewing screen resulting in a 3-D image with great depth of field. • SEM used for studying the surface of a specimen. What are the advantages and disadvantages of the different types of microscopy? 2. Cell Fractionation and BiochemicalAnalysis II. Cell Size (Figure 6.2) • Cells come in a variety of sizes & shapes • Size limit is set by the logistics required to carry out metabolism • The smallest cells are nanobacteria and mycoplasmas (diameter of 0.1 to 1.0 µm). • Most bacterial cells are 10 times larger than mycoplasmas (1 to 5 µm in diameter) • Eukaryotic cells are typically 10 times larger than bacteria (10 - 100 µm in diameter). Reece et al. 9 Edition Chapter 6 Page ▯ What factors constrain the lower size limit of cells? What factors constrain the upper size limit of cells? Hint: Fig. 6.7 Size does not always correlate with class of cells (i.e., prokaryotic vs eukaryotic cells) • Generally prokaryotic cells are smaller than eukaryotic cells. But there are exceptions. e.g., Epulopiscium fishelsoni Thiomargarita namibiensis • Generally cells are microscopic. But there are exceptions to this as well • Loligo -Atlantic squid has neurons with axon diameters as large as 1.0 mm. • Human motor neurons • Ostrich egg Cell shapes vary in shape from sphere and cylinders to very irregular nerve cells. III. Cell Types You should be able to compare and contrast prokaryotic and eukaryotic cells. Similarities • Cells are the basic unit of life. • Cells are highly organized structures • Cells are separated from their external environment by a plasma membrane. • All cells use DNA as their genetic information. The DNA is found in chromosomes (although there are significant differences between prokaryotic and eukaryotic chromosomes) • All cells have ribosomes • All present day cells have apparently evolved from the same ancestor Reece et al. 9 Edition Chapter 6 Page ▯ Differences (Note this is not an exhaustive list but a good starting point) Prokaryotic (before nucleus) cell Eukaryotic (true nucleus) cell • no membrane bound nucleus - membrane bound nucleus • no membrane bound organelles - membrane bound organelles • simple cell structure - complex cell structure • generally smaller than eukaryotic cells - generally larger than prokaryotic cells • Domains: Bacteria &Archaea - Domain: Eukarya 1. Prokaryotic cells • Generally single cell organisms (Figure 6.5) • Life cycle can be as short as twenty minutes. • Vegetative reproduction in which an individual produces two daughter cells by a process called binary fission. Prokaryotic cell structure i) Cell or plasma membrane (discussed in more detail in Chapter 7) • every cell is surrounded by this selective barrier • allows passage of sufficient nutrients, oxygen, and wastes to service the cell • consists of a lipid bilayer composed of phospholipids and other lipids • proteins serving a variety of functions (enzymes, receptors, attachment, transport…) are embedded or attached to the surface of the lipid bilayer • membrane composition varies depending on the type of cell and composition is related to function ii) Nucleoid region - region where the chromosome (DNA) of a prokaryotic cell is located. This region is NOT membrane bound. iii) Cytoplasm – interior of the prokaryotic cell iv) Cell wall – rigid structure outside the plasma membrane v) Capsule - a gelatinous or slimy layer external to the cell wall composed of polypeptides or carbohydrates vi) Pilus (pl. Pili) and fimbria (pl. fimbriae) - attachment structures on the cell surface Reece et al. 9 Edition Chapter 6 Page ▯ vii) Flagellum (pl - Flagella) - locomotory organelle viii) Ribosomes - site of protein synthesis Mesosome - at one time thought to be involved in cell division, now thought to be an artifact. Note Prokaryotic cells lack a nucleus and membrane bound organelles 2. Eukaryotic Cells How do eukaryotic cells differ from prokaryotic cells? Structure of a Eukaryotic Cell i) Cell Membrane - separates cell from the surrounding environment. Described above and will be covered extensively in chapter 7. ii) Nucleus • contains most of a cell's genetic material • usually the most prominent organelle in an eukaryotic cell. • surrounded by nuclear envelope composed of a double membrane. Pores in the nuclear envelope regulate the movement of proteins and RNAs into and out of the nucleus. The inside of the nuclear envelope is lined by the nuclear lamina (a network of protein filaments that maintain the structure of the nucleus). • Chromosomes in the nucleus are formed of chromatin (complex of DNA and protein). Reece et al. 9 Edition Chapter 6 Page ▯ • When a cell prepares to divide the chromatin condenses and forms into structures that are visible with the light microscope. The number of chromosomes is a characteristic of a eukaryotic species • Nucleolus • Nuclear pore • Nuclear matrix iii) Cytoplasm - cellular contents excluding the nucleus - strip away membrane, everything that is left excluding the nucleus. iv) Cytosol – semi-fluid (i.e., gel) substance in which the organelles are found. v) Ribosomes • sites of protein synthesis • found free within the cytoplasm and associated with the endoplasmic reticulum and nuclear envelope as well as within mitochondria and chloroplasts • number varies depending on the level of protein synthesis going on within the cell • two types of ribosomes based on local. The structures of these ribosomes are identical. Free ribosomes - in the cytosol - produce proteins that will function within the cytosol. Also most of the mitochondrial and chloroplast proteins are produced by these ribosomes. Bound ribosomes - attached to the outer surface of the nuclear envelope and endoplasmic reticulum. Produce proteins that are to be included in membranes, packaged into membrane bound organelles or exported from the cell (secretion). The Endomembrane System (Fig 6.15) • Anumber of related membranous structures including nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and plasma membrane • Believed to have arisen by invagination of the plasma membrane. • Carries out a variety of functions, including synthesis of proteins, transport of protein into membranes and organelles or out of the cell, metabolism and movement of lipoids and detoxification of toxins Reece et al. 9 Edition Chapter 6 Page ▯ vi) Endoplasmic reticulum (“within cytoplasm network”) = ER • extensive membrane network of tubules and sacs (cisternae; singular - cisterna) that is continuous with the nuclear envelope • the ER represents up to half of an eukaryotic cell’s total membrane Two types of ER that differ in structure and function though interconnected Smooth ER • lacks ribosomes Functions (dependent upon cell type) • synthesis of lipids (fatty acids, phospholipids and steroids) • metabolism of carbohydrates • detoxification of drugs and poisons
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