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Chapter 6: A Tour of the Cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A cell and its skeleton viewed by fluorescence microscopy 10 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 6.1: Biologists use microscopes and the tools of biochemistry to study cells • Though usually too small to be seen by the unaided eye, cells can be complex • Scientists use microscopes to visualize cells too small to see with the naked eye • 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 © 2011 Pearson Education, Inc. The size range of cells 10 m Human height Length of some nerve and muscle cells Unaided eye 1m 0.1 m Chicken egg 1 cm Frog egg Most plant and animal cells 10 µm 1 µm 100 nm nucleus Nucleus Most bacteria Most bacteria Mitochondrion Smallest bacteria Viruses Ribosomes 10 nm Electron microscope 100 µm Light microscope 1 mm Proteins Lipids 1 nm 0.1 nm Small molecules Atoms Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Measurements 1 centimeter (cm) = 102 meter (m) = 0.4 inch 1 millimeter (mm) = 10–3 m 1 micrometer (µm) = 10–3 mm = 106 m 1 nanometer (nm) = 10–3 µm = 10 9 m Research Method Light Microscopy TECHNIQUE RESULTS (a) Brightfield (unstained specimen). Passes light directly through specimen. Unless cell is naturally pigmented or artificially stained, image has little contrast. [Parts (a)–(d) show a human cheek epithelial cell.] 50 µm (b) Brightfield (stained specimen). Staining with various dyes enhances contrast, but most staining procedures require that cells be fixed (preserved). (c) Phase-contrast. Enhances contrast in unstained cells by amplifying variations in density within specimen; especially useful for examining living, unpigmented cells. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (d) Differential-interference-contrast (Nomarski). Like phase-contrast microscopy, it uses optical modifications to exaggerate differences in density, making the image appear almost 3D. (e) Fluorescence. Shows the locations of specific molecules in the cell by tagging the molecules with fluorescent dyes or antibodies. These fluorescent substances absorb ultraviolet radiation and emit visible light, as shown here in a cell from an artery. 50 µm (f) Confocal. Uses lasers and special optics for “optical sectioning” of fluorescently-stained specimens. Only a single plane of focus is illuminated; out-of-focus fluorescence above and below the plane is subtracted by a computer. A sharp image results, as seen in stained nervous tissue (top), where nerve cells are green, support cells are red, and regions of overlap are yellow. A standard fluorescence micrograph (bottom) of this relatively thick tissue is blurry. 50 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Two basic types of electron microscopes (EMs) are used to study subcellular structures • Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a specimen, providing images that look 3-D • Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen • TEMs are used mainly to study the internal structure of cells © 2011 Pearson Education, Inc. Research Method Electron Microscopy TECHNIQUE (a) Scanning electron microscopy (SEM). Micrographs taken with a scanning electron microscope show a 3D image of the surface of a specimen. This SEM shows the surface of a cell from a rabbit trachea (windpipe) covered with motile organelles called cilia. Beating of the cilia helps move inhaled debris upward toward the throat. RESULTS Cilia Longitudinal section of cilium (b) Transmission electron microscopy (TEM). A transmission electron microscope profiles a thin section of a specimen. Here we see a section through a tracheal cell, revealing its ultrastructure. In preparing the TEM, some cilia were cut along their lengths, creating longitudinal sections, while other cilia were cut straight across, creating cross sections. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm Cross section of cilium 1 µm Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions • The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic • Organisms of the domains Bacteria and Archaea consist of prokaryotic cells • Protists, fungi, animals, and plants all consist of eukaryotic cells © 2011 Pearson Education, Inc. Comparing Prokaryotic and Eukaryotic Cells • Basic features of all cells – – – – Plasma membrane Semifluid substance called cytosol Chromosomes (carry genes) Ribosomes (make proteins) © 2011 Pearson Education, Inc. A prokaryotic cell Pili: attachment structures on the surface of some prokaryotes Nucleoid: region where the cell’s DNA is located (not enclosed by a membrane) Ribosomes: organelles that synthesize proteins Plasma membrane: membrane enclosing the cytoplasm Cell wall: rigid structure outside the plasma membrane Capsule: jelly-like outer coating of many prokaryotes 0.5 µm (a) A typical rod-shaped bacterium Flagella: locomotion organelles of some bacteria Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) A thin section through the bacterium Bacillus coagulans (TEM) • 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 © 2011 Pearson Education, Inc. Exploring Animal and Plant Cells Animal Cell (Eukaryotic Cells) ENDOPLASMIC RETICULUM (ER) Rough ER Smooth ER Nuclear envelope Nucleolus NUCLEUS Chromatin Flagelium Plasma membrane Centrosome CYTOSKELETON Microfilaments Intermediate filaments Ribosomes Microtubules Microvilli Golgi apparatus Peroxisome Mitochondrion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysosome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) • Eukaryotic cells are characterized by having – DNA in a nucleus that is bounded by a membranous nuclear envelope – Membrane-bound organelles – Cytoplasm in the region between the plasma membrane and nucleus • Eukaryotic cells are generally much larger than prokaryotic cells © 2011 Pearson Education, Inc. Geometric relationships between surface area and volume Surface area increases while total volume remains constant 5 1 1 Total surface area (height width number of sides number of boxes) 6 150 750 Total volume (height width length number of boxes) 1 125 125 Surface-to-volume ratio (surface area volume) 6 1.2 6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The plasma membrane Outside of cell Carbohydrate side chain Hydrophilic region Inside of cell 0.1 µm Hydrophobic region (a) TEM of a plasma membrane. The plasma membrane, here in a red blood cell, appears as a pair of dark bands separated by a light band. Hydrophilic region Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phospholipid Proteins (b) Structure of the plasma membrane Animal and Plant Cells: Plant Cell Nuclear envelope Nucleolus Chromatin NUCLEUS Centrosome Rough endoplasmic reticulum Smooth endoplasmic reticulum Ribosomes ( small brown dots ) Central vacuole Tonoplast Golgi apparatus Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Plasma membrane Chloroplast Cell wall Plasmodesmata Wall of adjacent cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata The nucleus and its envelope Nucleus Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope. TEM of a specimen prepared by a special technique known as freeze-fracture. 0.25 µm Ribosome 1 µm Close-up of nuclear envelope Pore complexes (TEM). Each pore is ringed by protein particles. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope. Ribosomes Ribosomes ER Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Small subunit Diagram of a ribosome Endoplasmic reticulum (ER) Smooth ER Rough ER Nuclear envelope ER lumen Cisternae Ribosomes Transport vesicle Smooth ER Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transitional ER Rough ER 200 µm The Golgi apparatus Golgi apparatus cis face (“receiving” side of Golgi apparatus) 1 Vesicles move 2 Vesicles coalesce to 6 Vesicles also from ER to Golgi form new cis Golgi cisternae transport certain Cisternae proteins back to ER 3 Cisternal maturation: Golgi cisternae move in a cisto-trans direction 5 Vesicles transport specific proteins backward to newer Golgi cisternae 0.1 0 µm 4 Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma membrane for secretion trans face (“shipping” side of Golgi apparatus) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TEM of Golgi apparatus Lysosomes Nucleus 1 µm Lysosome containing two damaged organelles 1µm Mitochondrion fragment Peroxisome fragment Lysosome Lysosome contains Food vacuole fuses Hydrolytic active hydrolytic enzymes digest with lysosome enzymes food particles Digestive enzymes Lysosome fuses with vesicle containing damaged organelle Lysosome Plasma membrane Lysosome Lysosome Hydrolytic enzymes digest organelle components Digestion Food vacuole (a) Phagocytosis: lysosome digesting food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Digestion Vesicle containing damaged mitochondrion (b) Autophagy: lysosome breaking down damaged organelle The plant cell vacuole Central vacuole Cytosol Tonoplast Nucleus Central vacuole Cell wall Chloroplast 5 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review: relationships among organelles of the endomembrane system 1 Nuclear envelope is connected to rough ER, which is also continuous with smooth ER Nucleus Rough ER Smooth ER Nuclear envelope 3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review: relationships among organelles of the endomembrane system 1 Nuclear envelope is connected to rough ER, which is also continuous with smooth ER Nucleus Rough ER 2 Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi Smooth ER cis Golgi Nuclear envelope Transport vesicle 3 Golgi pinches off transport vesicles and other vesicles that give rise to lysosomes and vacuoles trans Golgi 4 Lysosome available 5 Transport vesicle carries for fusion with another proteins to plasma vesicle for digestion membrane for secretion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review: relationships among organelles of the endomembrane system 1 Nuclear envelope is connected to rough ER, which is also continuous with smooth ER Nucleus Rough ER 2 Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi Smooth ER cis Golgi Nuclear envelope Transport vesicle 3 Golgi pinches off transport vesicles and other vesicles that give rise to lysosomes and vacuoles trans Golgi Plasma membrane 4 Lysosome available 5 Transport vesicle carries 6 Plasma membrane expands for fusion with another proteins to plasma by fusion of vesicles; proteins vesicle for digestion membrane for secretion are secreted from cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolutionary Origins of Mitochondria and Chloroplasts • Mitochondria and chloroplasts have similarities with bacteria – Enveloped by a double membrane – Contain free ribosomes and circular DNA molecules – Grow and reproduce somewhat independently in cells © 2011 Pearson Education, Inc. • The Endosymbiont theory – An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host – The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion – At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts © 2011 Pearson Education, Inc. The mitochondrion, site of cellular respiration Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 100 µm The chloroplast, site of photosynthesis Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Peroxisomes Chloroplast Peroxisome Mitochondrion 1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The cytoskeleton Microtubule 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Microfilaments Centrosome containing a pair of centrioles Centrosome Microtubule Centrioles 0.25 µm Longitudinal section Microtubules Cross section of one centriole of the other centriole Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasmodesmata between plant cells Cell walls Interior of cell Interior of cell 0.5 µm Plasmodesmata Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plasma membranes Exploring Intercellular Junctions in Animal Tissues TIGHT JUNCTIONS At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continuous seals around the cells, tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells. Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm DESMOSOMES Desmosomes (also called anchoring junctions) function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm. Tight junctions Intermediate filaments Desmosome Gap junctions Space between Plasma membranes cells of adjacent cells 1 µm Extracellular matrix Gap junction 0.1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, including heart muscle and animal embryos.