1-ORGANIZATION-AND-STRUCTURE-OF-CELLS (1).pdf

Introduction to the Study of Cell and Molecular Biology The Discovery of Cells One of Robert Hooke’s more ornate compound (double‐lens) microscopes. (Inset) Hooke’s drawing of a thin slice of cork, showing the honeycomb‐like network of “cells.” (b) Single‐lens microscope used by Antonie...

Introduction to the Study of Cell and Molecular Biology The Discovery of Cells One of Robert Hooke’s more ornate compound (double‐lens) microscopes. (Inset) Hooke’s drawing of a thin slice of cork, showing the honeycomb‐like network of “cells.” (b) Single‐lens microscope used by Antonie van Leeuwenhoek to observe bacteria and other microorganisms. The biconvex lens, which was capable of magnifying an object approximately 270 times and providing a resolution of approximately 1.35 μm, was held between two metal plates.  The First Observations 1665 – Robert Hooke, observed a thin slice of cork through a crude microscope life’s smallest structural units were “little boxes” or cells Cell theory, all living things are composed of cells. A drawing of the microscope used by Robert Hooke in 1664.  1673 – 1723 – Anton van Leeuwenhoek, probably the first to observe live microorganisms through the magnifying lenses of the more than 400 microscopes. - He wrote “animalcules” - He made detailed drawings of organisms he found in rainwater, feces, and material scraped from teeth. The van Leeuwenhoek microscope. (a) A replica of Antoni van Leeuwenhoek’s microscope. (b) Van Leeuwenhoek’s drawings of bacteria, published in 1684. Even from these simple drawings we can recognize several shapes of common bacteria: A, C, F, and G, rods; E, cocci; H, packets of cocci. (c) Photomicrograph of a human blood smear taken through a van Leeuwenhoek microscope. Red blood cells are clearly apparent. Microscopy. (a) A compound light microscope (inset photomicrograph of unstained cells taken through a phase-contrast light microscope). (b) Path of light through a compound light microscope. Besides 10*, ocular lenses are available in 15–30* magnifications. Light microscopy - refers to the use of any kind of microscope that uses visible light to observe specimens Several types of light microscopy. 1. Compound Light Microscope (LM) 2. Bright-field 3. Phase-contrast 4. Differential interference contrast 5. Dark-field 6. Fluorescence Resolution (resolving power) - is the ability of the lenses to distinguish fine detail and structure. Specifically, it refers to the ability of the lenses to distinguish two points that are a specified distance apart. Compound Light Microscope (LM) - has series of lenses and uses visible light as its source of illumination - examines very small specimens How to calculate the total magnification of a specimen? Multiply the objective lens magnification (power) by the ocular lens magnification (power). ▪ Objective lenses – 10X (low power), 40X (high power), and 100X (oil immersion). ▪ Most ocular lenses magnify specimens by a factor of 10 What is the total magnification of a compound light microscope with objective lens magnification of 40X and ocular lens of 10X? Probing Cell Structure: Electron Microscopy Electron microscopes - use electrons instead of visible light (photons) to image cells and cell structures. In the electron microscope, electromagnets function as lenses, and the whole system operates in a vacuum. ✓Two types of electron microscopy are transmission and scanning The electron microscope. This instrument encompasses both transmission and scanning electron microscope functions. Transmission Electron Microscopy The transmission electron microscope (TEM) – used to examine cells and cell structure at very high magnification and resolution. The resolving power of a TEM is much greater than that of the light microscope, even allowing one to view structures at the molecular level. resolving power of a TEM is about 0.2 nanometer to view the internal structure of a cell, thin sections of the cell are needed, and the sections must be stabilized and stained with various chemicals to make them visible. To obtain sufficient contrast, the sections are treated with stains such as osmic acid, or permanganate, uranium, lanthanum, or lead salts. Electron micrographs. (a) Micrograph of a thin section of a dividing bacterial cell, taken by transmission electron microscopy (TEM). The cell is about 0.8 mm wide. Scanning Electron Microscopy In scanning electron microscopy, the specimen is coated with a thin film of a heavy metal, typically gold. In the SEM, even fairly large specimens can be observed, and the depth of field (the portion of the image that remains in sharp focus) is extremely good. Electron micrographs taken by either TEM or SEM are originally taken as black- and-white images. (b) TEM of negatively stained molecules of hemoglobin. Each hexagonal shaped molecule is about 25 nanometers (nm) in diameter and consists of two doughnut-shaped rings, a total of 15 nm wide. (c) Scanning electron micrograph of bacterial cells. A single cell is about 0.75 mm wide. ORGANIZATION AND STRUCTURE OF CELLS  PROKARYOTIC CELL ORGANIZATION Prokaryotes - bacteria or green algae, most abundant organisms on earth. A prokaryotic cell does not contain a membrane-bound nucleus. Bacteria are either cocci (spheroidal), bacilli (rod like) or spirilla (helically coiled) in shape, and fall into two groups, the eubacteria and the archaebacteria. Electron micrographs of E. coli cells. (a) Stained to show internal structure. (b) Stained to reveal flagella and pili. [a: CNRI/Photo Researchers; b: Courtesy of Howard Berg, Harvard University.] Schematic diagram of a prokaryotic cell.  Bacteria (singular: bacterium) - are relatively simple, single-celled (unicellular organisms). - Prokaryotes - Bacterial cells generally appear in one of several shapes. - Bacillus (rodlike), coccus (spherical or ovoid), and spiral (corkscrew or curved) - Some bacteria are star shaped or square Bacillus subtilis - Individual bacteria may form pairs, chains, clusters, or other groupings, such formations are usually characteristic of a particular genus or species of bacteria. - Enclosed in cell walls that are largely composed of a carbohydrate and protein complex called peptidoglycan. - Bacteria are classified according to their cell wall as Gram-positive or Gram- negative. - In Gram-positive bacteria, the peptidoglycan forms a thick (20-80 nm) layer external to the cell membrane and may contain other macromolecules. - In Gram-negative species, the peptidoglycan layer is thin (5-10 nm) and is overlaid by an outer membrane are lipopolysaccharides and lipoprotein. Bacterial cell walls – the peptidoglycan (protein and oligosaccharide) cell wall protects the prokaryotic cell from mechanical and osmotic pressure. A Gram-positive bacterium has a thick cell wall surrounding the plasma membrane, whereas Gram-negative bacteria have a thinner cell wall and an outer membrane. Between the outer membrane and the cell wall is the periplasm, a space occupied by the proteins secreted by the cell. Cell structure – each prokaryotic cell is surrounded by a cell membrane (plasma membrane) which consists of a lipid bilayer containing embedded proteins that control the passage of molecules in and out of the cell and catalyze a variety of reactions. The cell has no subcellular organelles, only infoldings of the plasma membrane called mesosomes. The deoxyribonucleic acid (DNA) is condensed within the cytosol to form the nucleoid. Some prokaryotes have tail-like flagella. - Bacteria generally reproduce by dividing into two equal cells (binary fission) - For nutrition, most bacteria use organic chemicals, which in nature can be derived from either dead or living organisms. Cell wall structure of gram-negative and gram-positive bacteria Construction of the cell walls of Gram-positive and Gram-negative bacteria. Scale drawings of some prokaryotic cells. Archaea - Consist of prokaryotic cells - Walls lack peptidoglycan - Often found in extreme environments and divided into three main groups. - Methanogens – produce methane as waste product from respiration - Extreme halophiles (halo = salt; philic = loving) – live in extremely salty environments such as the Great Salt Lake and Dead Sea. - Extreme thermophiles (therm = heat) live in hot sulfurous water, such as hot springs at Yellowstone National Park. - Not known to cause disease in humans. Stansbury Island in the Great Salt Lake, northern Utah, with salt deposits in the foreground. © Johnny Adolphson/Shutterstock.com Thermophiles, or heat-loving microscopic organisms, are nourished by the extreme habitat at hydrothermal features in Yellowstone National Park. They also color hydrothermal features shown here at Firehole Spring. NPS/Jim Peaco EUKARYOTIC CELL ORGANIZATION Schematic diagram of an animal cell accompanied by electron micrographs of its organelles. Drawing of a plant cell accompanied by electron micrographs of its organelles. Eukaryotes - eukaryotic cells have a membrane-bound nucleus and a number of other membrane- bound subcellular (internal) organelles, each of which has a specific function Plasma membrane – surrounds the cell, separating it from the external environment. Selectively permeable barrier due to the presence of specific transport proteins. Involved in receiving information when ligands bind to the receptor proteins on its surface, and in the processes of exocytosis and endocytosis. Nucleus – stores the cell’s genetic information as DNA in chromosomes. Bounded by a double membrane but pores in this membrane allow molecules to move in and out of the nucleus. The nucleolus within the nucleus is the site of ribosomal ribonucleic acid (rRNA) synthesis. Endoplasmic reticulum – this interconnected network of membrane vesicles is divided into two distinct parts. Rough endoplasmic (RER), which is studded with ribosomes, is the site of membrane and secretory protein biosynthesis and their post- transcriptional modification. Soft endoplasmic (SER), is involved in phospholipid biosynthesis and in the detoxification of toxic compounds. Golgi apparatus – a system of flattened membrane- bound sacs, is the sorting and packaging center of the cell. It receives membrane vesicles from the RER, further modifies the proteins within them, and then packages the modified proteins in other vesicles which eventually fuse with the plasma membrane or other subcellular organelles. Mitochondria – have an inner and an outer membrane separated by the intermembrane space. Outer membrane is more permeable than the inner membrane due to the presence of porin proteins. Inner membrane, which is folded to form cristae, is the site of oxidative phosphorylation, which produces ATP. The central matrix is the site of fatty acid degradation and the citric acid cycle. Lysosomes – lysosomes in animal cells are bounded by a single membrane. They have an acidic internal pH (pH 4-5), maintained by proteins in the membrane that pump in H+ ions. Within the lysosomes are acid hydrolases, enzymes involved in the degradation of macromolecules, including those internalized by endocytosis. SOURCE: From D. J. Des Marais, Science 289:1704, 2001. Copyright © 2000. Reprinted with permission from AAAS. Cytosol – is the soluble part of the cytoplasm where a large number of metabolic reactions take place. Within the cytosol is the cytoskeleton, a network of fibers (microtubules, intermediate filaments, and microfilaments) that maintain the shape of the cell. Peroxisomes – contain enzymes involved in the breakdown of amino acids and fatty acids, a by-product of which is hydrogen peroxide. This toxic compound is rapidly degraded by the enzyme catalase, also found within the peroxisomes. Plant cell wall – the cell wall surrounding a plant cell is made up of the polysaccharide cellulose. In woody plants, the phenolic polymer called lignin gives the cell wall additional strength and rigidity. Chloroplast – chloroplasts in plant cells are surrounded by a double membrane and have an internal membrane system of thylakoid vesicles that are stacked up to form grana. Thylakoid vesicles contain chlorophyll and are the site of photosynthesis Carbon dioxide (CO2) fixation takes place in the stroma, the soluble matter around the thylakoid vesicles. Plant cell vacuole – the membrane- bound vacuole is used to store nutrients and waste products, has an acidic pH and, due to the influx of water, creates turgor pressure inside the cell as it pushes out against the cell wall.  Plant and Animal Cell Within Eukaryotic cells are plant and animal cells Structural Similarities – Plasma membrane – Genetic mechanisms – Most organelles Structural Differences – Plants have choloroplasts, a large central vacuole and a cell wall – Plant cells do not have centrioles – Plant cells have plasmodesmata – Animal cells have gap junctions Physiological Differences – Plant cells have photosynthesis in addition to respiration – During mitosis a cell plate is formed in plant cells – Starch is molecule for energy storage while in animal cells it is glycogen – Large central vacuole stores more water and carbohydrates then animal cell vacuoles Animal and Plant Cells Have More Similarities Than Differences