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Chapter 6 A Tour of the Cell
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Overview: The Fundamental Units of Life
All organisms are made of cells (Cell Theory). The cell is the simplest collection of matter that can live. Cell structure is correlated to cellular function. All cells are related by their descent from earlier cells (heredity / reproduction). For the Discovery Video Cells, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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To study cells, biologists use microscopes and the tools of biochemistry
Microscopy: Scientists use microscopes to visualize cells too small to see with the naked eye. In a light microscope (LM), visible light passes through a specimen and then through glass lenses, which magnify the image. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The quality of an image depends on
Magnification, the ratio of an object’s image size to its real size. Resolution, the measure of the clarity of the image, or the minimum distance of two distinguishable points. Contrast, visible differences in parts of the sample. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The size range of cells 10 m 1 m 0.1 m 1 cm 1 mm 100 µm 10 µm 1 µm
Human height 1 m Length of some nerve and muscle cells 0.1 m Unaided eye Chicken egg 1 cm Frog egg 1 mm 100 µm Most plant and animal cells Light microscope 10 µm Nucleus Most bacteria 1 µm Mitochondrion Figure 6.2 Smallest bacteria Electron microscope 100 nm Viruses Ribosomes 10 nm Proteins Lipids 1 nm Small molecules 0.1 nm Atoms
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LMs (light microscopes) can magnify effectively to about 1,000 times the size of the actual specimen. Various techniques enhance contrast and enable cell components to be stained or labeled. Most subcellular structures, including organelles (membrane-enclosed compartments), are too small to be resolved by an LM. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Light Microscopy Figure 6.3a-d specimen specimen contrast (Nomarski)
TECHNIQUE RESULTS (a) Brightfield unstained specimen 50 µm (b) Brightfield stained specimen (c) Phase-contrast (d) Differential-interference- contrast (Nomarski) (e )Fluorescence Figure 6.3a-d 50 µm (f) Confocal 50 µm
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TEMs are used mainly to study the internal structure of cells.
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. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Electron microscopy Surface Internal structures Cilia
TECHNIQUE RESULTS 1 µm Cilia (a) Scanning electron microscopy (SEM) Surface Longitudinal section of cilium Cross section of cilium 1 µm Figure 6.4 (b) Transmission electron microscopy (TEM) Internal structures
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Cell Fractionation Cell fractionation takes cells apart and separates the major organelles from one another. Ultracentrifuges fractionate cells and separate their component parts by density. Cell fractionation enables scientists to determine the functions of organelles. Biochemistry and cytology help correlate cell function with structure. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Cell fractionation Homogenization
TECHNIQUE Homogenization Tissue cells Homogenate 1,000 g (1,000 times the force of gravity) 10 min Differential centrifugation : density Supernatant poured into next tube 20,000 g 20 min 80,000 g 60 min Pellet rich in nuclei and cellular debris Figure 6.5 150,000 g 3 hr Pellet rich in mitochondria (and chloro- plasts if cells are from a plant) Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes) Pellet rich in ribosomes
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Differential centrifugation
Cell Fractionation: Part 1 TECHNIQUE Homogenization Figure 6.5 Cell fractionation, part 1 Tissue cells Homogenate Differential centrifugation
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(1,000 times the force of gravity)
Cell Fractionation: Part 2 TECHNIQUE (cont.) 1,000 g (1,000 times the force of gravity) 10 min Supernatant poured into next tube 20,000 g 20 min 80,000 g 60 min Pellet rich in nuclei and cellular debris 150,000 g 3 hr Pellet rich in mitochondria (and chloro-plasts if cells are from a plant) Figure 6.5 Cell fractionation, part 2 Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes) Pellet rich in ribosomes
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Eukaryotic cells have internal membranes that compartmentalize their functions
Cell = the basic structural and functional unit of all life. (Cell Theory). The basic structural and functional unit of every organism is one of two types of cells: prokaryotic or eukaryotic. Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells. Protists, fungi, animals, and plants all consist of eukaryotic cells. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Comparing Prokaryotic vs. Eukaryotic Cells
Basic features of all cells: Plasma membrane Semifluid substance called cytosol (cytoplasm) Chromosomes (carry genes) DNA Ribosomes (make proteins) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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No nucleus Prokaryotic cells are characterized by having
DNA in an unbound region called the nucleoid. No membrane-bound organelles. Cytoplasm bound by the plasma membrane. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Prokaryotic Cell Fimbriae Nucleoid Ribosomes Plasma membrane Bacterial
chromosome Cell wall Capsule 0.5 µm Flagella (a) A typical rod-shaped bacterium (b) A thin section through the bacterium Bacillus coagulans (TEM) Figure 6.6
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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. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The plasma membrane is a selective barrier (semi-permeable) that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell. The general structure of a biological membrane is a double layer of phospholipids. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Phospholipid Plasma Membrane (a) TEM of a plasma membrane
Outside of cell Inside of cell 0.1 µm Carbohydrate side chain Hydrophilic region Figure 6.7 The Hydrophobic region Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane
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The surface area to volume ratio of a cell is critical.
The logistics of carrying out cellular metabolism sets limits on the size of cells. The surface area to volume ratio of a cell is critical. As the surface area increases by a factor of n2, the volume increases by a factor of n3 Small cells have a greater surface area relative to volume. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Surface area increases while total volume remains constant
Geometric relationships between surface area and volume Surface area increases while total volume remains constant 5 1 1 Total surface area [Sum of the surface areas (height width) of all boxes sides number of boxes] 6 150 750 Total volume [height width length number of boxes] Figure 6.8 1 125 125 Surface-to-volume (S-to-V) ratio [surface area ÷ volume] 6 1.2 6
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Eukaryotic cell: animal cell
Nuclear envelope ENDOPLASMIC RETICULUM (ER) Nucleolus NUCLEUS Rough ER Smooth ER Flagellum Chromatin Centrosome Plasma membrane CYTOSKELETON: Microfilaments Intermediate filaments Microtubules Ribosomes Figure 6.9 Animal and plant cells—animal cell Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome
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Eukaryotic Cell: Plant Cell
Rough endoplasmic reticulum ( Rough ER) NUCLEUS: Nuclear Envelope Smooth endoplasmic reticulum (Smooth ER) Nucleolus Ribosomes Chromatin Central vacuole Golgi apparatus Microfilaments Intermediate filaments CYTO- SKELETON Microtubules Figure 6.9 Animal and plant cells—plant cell Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Plasmodesmata Wall of adjacent cell
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The Nucleus: Information Central : DNA
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 membrane is a double membrane; each membrane consists of a lipid bilayer. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The nucleus and its envelope
1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope Ribosome 1 µm Figure 0.25 µm Close-up of nuclear envelope Pore complexes (TEM) Nuclear lamina (TEM)
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Pores regulate the entry and exit of molecules from the nucleus.
The shape of the nucleus is maintained by the nuclear lamina, which is composed of protein. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Chromatin condenses to form discrete chromosomes.
In the nucleus, DNA and proteins form genetic material called chromatin. Chromatin condenses to form discrete chromosomes. The nucleolus is located within the nucleus and is the site of ribosomal RNA rRNA synthesis. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Ribosomes: Protein Factories
Ribosomes are particles made of ribosomal RNA and protein. Ribosomes carry out protein synthesis in two locations: In the cytosol (free ribosomes) On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes - ER). For the Cell Biology Video Staining of Endoplasmic Reticulum, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Ribosomes Cytosol Endoplasmic reticulum (ER) Free ribosomes
Bound ribosomes Large subunit Figure 6.11 Small subunit 0.5 µm TEM showing ER and ribosomes Diagram of a ribosome
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Components of the endomembrane system:
The endomembrane system regulates protein traffic and performs metabolic functions in the cell Components of the endomembrane system: Nuclear envelope Endoplasmic reticulum: ER Golgi apparatus Lysosomes Vacuoles Plasma membrane These components are either continuous or connected via transfer by vesicles. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The Endoplasmic Reticulum: Biosynthetic Factory
The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells. The ER membrane is continuous with the nuclear envelope. There are two distinct regions of ER: Smooth ER, which lacks ribosomes. Rough ER, with ribosomes studding its surface. For the Cell Biology Video ER and Mitochondria in Leaf Cells, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Endoplasmic Reticulum Smooth ER ER Rough ER Nuclear envelope ER lumen
Cisternae Transitional ER Ribosomes Transport vesicle 200 nm Smooth ER Rough ER Figure 6.12
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Functions of Smooth ER The smooth ER Synthesizes lipids
Metabolizes carbohydrates Detoxifies poison Stores calcium Ca+ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Functions of Rough ER The rough ER
Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates). Distributes transport vesicles, proteins surrounded by membranes. Is a membrane factory for the cell. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The Golgi Apparatus: Shipping and Receiving Center
The Golgi apparatus consists of flattened membranous sacs called cisternae. Functions of the Golgi apparatus: Modifies products of the ER Manufactures certain macromolecules Sorts and packages materials into transport vesicles. For the Cell Biology Video ER to Golgi Traffic, go to Animation and Video Files. For the Cell Biology Video Golgi Complex in 3D, go to Animation and Video Files. For the Cell Biology Video Secretion From the Golgi, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Golgi apparatus cis face trans face
(“receiving” side of Golgi apparatus) 0.1 µm Cisternae Figure 6.13 The trans face (“shipping” side of Golgi apparatus) TEM of Golgi apparatus
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Lysosomes: Digestive Compartments
A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules. Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. Helps to breakdown and recycle cell material. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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A lysosome fuses with the food vacuole and digests the molecules.
Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole. A lysosome fuses with the food vacuole and digests the molecules. Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy. For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Lysosomes Lysosome Lysosome Nucleus 1 µm Vesicle containing
two damaged organelles 1 µm Mitochondrion fragment Peroxisome fragment Lysosome Digestive enzymes Lysosome Lysosome Plasma membrane Peroxisome Figure 6.14a Digestion Food vacuole Mitochondrion Digestion Vesicle (a) Phagocytosis :engulfs (b) Autophagy : recycles
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Vacuoles: Diverse Maintenance Compartments
Food vacuoles are formed by phagocytosis. Contractile vacuoles, found in many freshwater protists, pump excess water out of cells: osmoregulation. Central vacuoles, found in many mature plant cells, hold organic compounds and water. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The plant cell vacuole Central vacuole Central vacuole
Tonoplast = central vacuole membrane Cytosol Nucleus Central vacuole Figure 6.15 Cell wall Chloroplast 5 µm
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The Endomembrane System
Nucleus Rough ER Smooth ER cis Golgi Figure 6.16 Review: relationships among organelles of the endomembrane system Plasma membrane trans Golgi
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Mitochondria and Chloroplasts change energy from one form to another
Mitochondria are the sites of aerobic cellular respiration, a metabolic process that generates ATP. Chloroplasts, found in plants and algae, are the sites of photosynthesis. Peroxisomes are oxidative organelles. For the Cell Biology Video ER and Mitochondria in Leaf Cells, go to Animation and Video Files. For the Cell Biology Video Mitochondria in 3D, go to Animation and Video Files. For the Cell Biology Video Chloroplast Movement, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Mitochondria and Chloroplasts:
Are not part of the endomembrane system Have a double membrane Have proteins made by free ribosomes Contain their own DNA. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Mitochondria: Chemical Energy Conversion
Mitochondria are in nearly all eukaryotic cells. They have a smooth outer membrane and an inner membrane folded into cristae. The inner membrane creates two compartments: intermembrane space and mitochondrial matrix. Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix. Cristae present a large surface area for enzymes that synthesize ATP. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The mitochondrion = site of cellular respiration
Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Figure 6.17 Matrix 0.1 µm
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Chloroplasts: Capture of Light Energy
The chloroplast is a member of a family of organelles called plastids. Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis: water + carbon dioxide = sugar + oxygen. Chloroplasts are found in leaves and other green organs of plants and in algae. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Chloroplast structure includes:
Thylakoids, membranous sacs, stacked to form a granum : light dependent reactions. Stroma, the internal fluid: Calvin Cycle = light independent reactions. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The chloroplast = site of photosynthesis Chloroplast
Ribosomes Stroma Inner and outer membranes Granum 1 µm Thylakoid Figure 6.18
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Peroxisomes: Oxidation
Peroxisomes are specialized metabolic compartments bounded by a single membrane. Peroxisomes produce hydrogen peroxide and convert it to water. Oxygen is used to break down different types of molecules. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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It is composed of three types of molecular structures:
The cytoskeleton is a network of fibers that organizes structures and activities in the cell The cytoskeleton is a network of fibers extending throughout the cytoplasm. It organizes the cell’s structures and activities, anchoring many organelles. It is composed of three types of molecular structures: Microtubules Microfilaments Intermediate filaments For the Cell Biology Video The Cytoskeleton in a Neuron Growth Cone, go to Animation and Video Files For the Cell Biology Video Cytoskeletal Protein Dynamics, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The cytoskeleton Microtubule Figure 6.20 Microfilaments 0.25 µm
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Roles of the Cytoskeleton: Support, Motility, and Regulation
The cytoskeleton helps to support the cell and maintain its shape. It interacts with motor proteins to produce motility. Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton. Recent evidence suggests that the cytoskeleton may help regulate biochemical activities. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The cytoskeleton Vesicle Receptor for motor protein
ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton (a) Microtubule Vesicles 0.25 µm Figure 6.21 (b)
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Components of the Cytoskeleton
Three main types of fibers make up the cytoskeleton: Microtubules are the thickest of the three components of the cytoskeleton Microfilaments, also called actin filaments, are the thinnest components Intermediate filaments are fibers with diameters in a middle range. For the Cell Biology Video Actin Network in Crawling Cells, go to Animation and Video Files. For the Cell Biology Video Actin Visualization in Dendrites, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Table 6-1 10 µm 10 µm 10 µm Column of tubulin dimers Keratin proteins
Actin subunit Fibrous subunit (keratins coiled together) 25 nm 7 nm 8–12 nm Tubulin dimer
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10 µm Table 6-1a Column of tubulin dimers 25 nm Tubulin dimer
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10 µm Table 6-1b Actin subunit 7 nm
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Fibrous subunit (keratins coiled together)
5 µm Table 6-1c Keratin proteins Fibrous subunit (keratins coiled together) 8–12 nm
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Microtubules Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long Functions of microtubules: Shaping the cell Guiding movement of organelles Separating chromosomes during cell division For the Cell Biology Video Transport Along Microtubules, go to Animation and Video Files. For the Cell Biology Video Movement of Organelles in Vivo, go to Animation and Video Files. For the Cell Biology Video Movement of Organelles in Vitro, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Centrosomes and Centrioles
In many cells, microtubules grow out from a centrosome near the nucleus The centrosome = “microtubule-organizing center” = MTOC In animal cells, the centrosome has a pair of centrioles 9x3 Each centriole has nine triplets of microtubules arranged in a ring. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Centrosome Centrioles Microtubule 0.25 µm
Figure 6.22 Centrosome containing a pair of centrioles Longitudinal section of one centriole Microtubules Cross section of the other centriole
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Cilia and flagella differ in their beating patterns.
Microtubules control the beating of cilia and flagella, locomotor appendages of some cells. Cilia and flagella differ in their beating patterns. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Direction of organism’s movement
Comparison of the beating of flagella and cilia. Direction of swimming (a) Motion of flagella 5 µm Direction of organism’s movement Figure 6.23a Power stroke Recovery stroke (b) Motion of cilia 15 µm
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Cilia and flagella share a common ultrastructure:
A core of microtubules sheathed by the plasma membrane. A basal body that anchors the cilium or flagellum. A motor protein called dynein, which drives the bending movements of a cilium or flagellum. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Basal body Ultrastructure of a eukaryotic flagellum or motile cilium
Outer microtubule doublet Plasma membrane 0.1 µm Dynein proteins Central microtubule Radial spoke Protein cross-linking outer doublets Microtubules (b) Cross section of cilium 9x2 Plasma membrane Basal body 0.5 µm (a) Longitudinal section of cilium 0.1 µm Figure Triplet (c) Cross section of basal body 9x3
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The motor protein, dynein, moves flagella and cilia by “walking” :
Dynein arms alternately grab, move, and release the outer microtubules. Protein cross-links limit sliding. Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum. For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files. For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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How dynein “walking” moves flagella and cilia
Microtubule doublets ATP Dynein protein (a) Effect of unrestrained dynein movement ATP Cross-linking proteins inside outer doublets Anchorage in cell Figure 6.25 (b) Effect of cross-linking proteins 1 3 2 (c) Wavelike motion
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Microfilaments (Actin Filaments)
Microfilaments are solid rods about 7 nm in diameter, built as a twisted double chain of protein actin subunits. The structural role of microfilaments is to bear tension, resisting pulling forces within the cell. They form a 3-D network called the cortex just inside the plasma membrane to help support the cell’s shape. Bundles of microfilaments make up the core of microvilli of intestinal cells. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Microfilaments that function in cellular motility contain the protein myosin in addition to actin.
In muscle cells, thousands of actin filaments are arranged parallel to one another. Thicker filaments composed of myosin interdigitate with the thinner actin fibers. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Microfilaments and motility Figure 6.27 Muscle cell Actin filament
Myosin filament Myosin arm (a) Myosin motors in muscle cell contraction Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement Nonmoving cortical cytoplasm (gel) Figure 6.27 Chloroplast Streaming cytoplasm (sol) Vacuole Parallel actin filaments Cell wall (c) Cytoplasmic streaming in plant cells
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Sarcomere – muscle cell unit of contraction
Actin filament Myosin filament Myosin arm Figure 6.27a Microfilaments and motility (a) Myosin motors in muscle cell contraction
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Cortex (outer cytoplasm): gel with actin network
Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement Nonmoving cortical cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Figure 6.27b,c Microfilaments and motility Vacuole Parallel actin filaments Cell wall (c) Cytoplasmic streaming in plant cells
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Localized contraction brought about by actin and myosin also drives amoeboid movement.
Pseudopodia (cellular extensions) extend and contract through the reversible assembly and contraction of actin subunits into microfilaments. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Cytoplasmic streaming is a circular flow of cytoplasm within cells.
This streaming speeds distribution of materials within the cell. In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Intermediate Filaments
Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules. They support cell shape and fix organelles in place. Intermediate filaments are more permanent cytoskeleton fixtures than the other two classes. For the Cell Biology Video Interphase Microtubule Dynamics, go to Animation and Video Files. For the Cell Biology Video Microtubule Sliding in Flagellum Movement, go to Animation and Video Files. For the Cell Biology Video Microtubule Dynamics, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Intercellular junctions
Extracellular components and connections between cells help coordinate cellular activities Most cells synthesize and secrete materials that are external to the plasma membrane These extracellular structures include: Cell walls of plants The extracellular matrix (ECM) of animal cells Intercellular junctions For the Cell Biology Video Ciliary Motion, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Prokaryotes, fungi, and some protists also have cell walls.
Cell Walls of Plants The cell wall is an extracellular structure that distinguishes plant cells from animal cells. Prokaryotes, fungi, and some protists also have cell walls. The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water. Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Plant cell walls may have multiple layers:
Primary cell wall: relatively thin and flexible Middle lamella: thin layer between primary walls of adjacent cells Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall. Plasmodesmata are channels between adjacent plant cells. For the Cell Biology Video E-cadherin Expression, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Plant cell walls Secondary cell wall
Primary cell wall outermost of the cell walls Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Figure 6.28 Plant cell walls Plasmodesmata
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The Extracellular Matrix (ECM) of Animal Cells
Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM). The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin. ECM proteins bind to receptor proteins in the plasma membrane called integrins. For the Cell Biology Video Cartoon Model of a Collagen Triple Helix, go to Animation and Video Files. For the Cell Biology Video Staining of the Extracellular Matrix, go to Animation and Video Files. For the Cell Biology Video Fibronectin Fibrils, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Extracellular matrix (ECM) of an animal cell
Collagen Proteoglycan complex Polysaccharide molecule EXTRACELLULAR FLUID Carbo- hydrates Fibronectin Core protein Integrins Proteoglycan molecule Plasma membrane Proteoglycan complex Figure 6.30 Micro- filaments CYTOPLASM
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ECM of Animal cell Proteoglycan complex Collagen EXTRACELLULAR FLUID
Fibronectin Integrins Plasma membrane Figure 6.30 Extracellular matrix (ECM) of an animal cell, part 1 Micro-filaments CYTOPLASM
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Functions of the ECM: Support Adhesion Movement Regulation
Identification “ID” cell-to-cell recognition Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Intercellular Junctions
Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact Intercellular junctions facilitate this contact There are several types of intercellular junctions *Plasmodesmata = channels / plant cells *Gap junctions = channels / animal cells Tight junctions – waterproof Desmosomes - anchors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Plasmodesmata in Plant Cells
Plasmodesmata are channels that perforate plant cell walls. Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Plasmodesmata between plant cells
Cell walls Interior of cell Interior of cell Figure 6.31 0.5 µm Plasmodesmata Plasma membranes
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Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells
At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid… waterproof. desmosomes (anchoring junctions) fasten cells together into strong sheets. gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Intercellular junctions
in animal tissues Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm Tight junction Intermediate filaments Desmosome Desmosome Gap junctions 1 µm Figure 6.32 Extracellular matrix Space between cells Gap junction Plasma membranes of adjacent cells 0.1 µm
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The Cell: A Living Unit Greater Than the Sum of Its Parts
Cells rely on the integration of structures and organelles in order to function. For example, a macrophage’s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The emergence of cellular functions –> Macrophages’s ability to engulf and destroy bacteria
Figure 6.33
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Cell Structures & Functions Cell Component Structure Function
Concept 6.3 Nucleus Surrounded by nuclear envelope (double membrane) perforated by nuclear pores. The nuclear envelope is continuous with the endoplasmic reticulum (ER). Houses chromosomes, made of chromatin (DNA, the genetic material, and proteins); contains nucleoli, where ribosomal subunits are made. Pores regulate entry and exit of materials. The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes (ER) Ribosome Two subunits made of ribo- somal RNA and proteins; can be free in cytosol or bound to ER Protein synthesis Concept 6.4 Endoplasmic reticulum Extensive network of membrane-bound tubules and sacs; membrane separates lumen from cytosol; continuous with the nuclear envelope. Smooth ER: synthesis of lipids, metabolism of carbohy- drates, Ca2+ storage, detoxifica-tion of drugs and poisons The endomembrane system regulates protein traffic and performs metabolic functions in the cell (Nuclear envelope) Rough ER: Aids in synthesis of secretory and other proteins from bound ribosomes; adds carbohydrates to glycoproteins; produces new membrane Golgi apparatus Stacks of flattened membranous sacs; has polarity (cis and trans faces) Modification of proteins, carbo- hydrates on proteins, and phos- pholipids; synthesis of many polysaccharides; sorting of Golgi products, which are then released in vesicles. Lysosome Membranous sac of hydrolytic enzymes (in animal cells) Breakdown of ingested substances, cell macromolecules, and damaged organelles for recycling Vacuole Large membrane-bounded vesicle in plants Digestion, storage, waste disposal, water balance, cell growth, and protection Concept 6.5 Mitochondrion Bounded by double membrane; inner membrane has infoldings (cristae) Cellular respiration Mitochondria and chloro- plasts change energy from one form to another Chloroplast Typically two membranes around fluid stroma, which contains membranous thylakoids stacked into grana (in plants) Photosynthesis Peroxisome Specialized metabolic compartment bounded by a single membrane Contains enzymes that transfer hydrogen to water, producing hydrogen peroxide (H2O2) as a by-product, which is converted to water by other enzymes in the peroxisome
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The eukaryotic cell’s Genetic Instructions are housed in
Cell Component Structure Function Concept 6.3 Nucleus Surrounded by nuclear envelope (double membrane) perforated by nuclear pores. The nuclear envelope is continuous with the endoplasmic reticulum (ER). Houses chromosomes, made of chromatin (DNA, the genetic material, and proteins); contains nucleoli, where ribosomal subunits are made. Pores regulate entry and exit os materials. The eukaryotic cell’s Genetic Instructions are housed in the nucleus and carried out by the ribosomes (ER) Ribosome Two subunits made of ribo- somal RNA and proteins; can be free in cytosol or bound to ER Protein synthesis
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The endomembrane system regulates protein traffic and
Cell Component Structure Function Concept 6.4 Endoplasmic reticulum Extensive network of membrane-bound tubules and sacs; membrane separates lumen from cytosol; continuous with the nuclear envelope. Smooth ER: synthesis of lipids, metabolism of carbohy- drates, Ca2+ storage, detoxifica- tion of drugs and poisons The endomembrane system regulates protein traffic and performs metabolic functions in the cell (Nuclear envelope) Rough ER: Aids in sythesis of secretory and other proteins from bound ribosomes; adds carbohydrates to glycoproteins; produces new membrane Golgi apparatus Stacks of flattened membranous sacs; has polarity (cis and trans faces) Modification of proteins, carbo- hydrates on proteins, and phos- pholipids; synthesis of many polysaccharides; sorting of Golgi products, which are then released in vesicles. Breakdown of ingested sub- stances cell macromolecules, and damaged organelles for recycling Lysosome Membranous sac of hydrolytic enzymes (in animal cells) Vacuole Large membrane-bounded vesicle in plants Digestion, storage, waste disposal, water balance, cell growth, and protection
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Mitochondria and chloroplasts change energy from
Cell Component Structure Function Concept 6.5 Mitochondrion Bounded by double membrane; inner membrane has infoldings (cristae) Cellular respiration Mitochondria and chloroplasts change energy from one form to another Chloroplast Typically two membranes around fluid stroma, which contains membranous thylakoids stacked into grana (in plants) Photosynthesis Peroxisome Specialized metabolic compartment bounded by a single membrane Contains enzymes that transfer hydrogen to water, producing hydrogen peroxide (H2O2) as a by-product, which is converted to water by other enzymes in the peroxisome
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You should now be able to:
Distinguish between the following pairs of terms: magnification and resolution; prokaryotic and eukaryotic cell; free and bound ribosomes; smooth and rough ER. Describe the structure and function of the components of the endomembrane system. Briefly explain the role of mitochondria, chloroplasts, and peroxisomes. Describe the functions of the cytoskeleton. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Describe the structure of a plant cell wall.
Compare the structure and functions of microtubules, microfilaments, and intermediate filaments. Explain how the ultrastructure of cilia and flagella relate to their functions. Describe the structure of a plant cell wall. Describe the structure and roles of the extracellular matrix in animal cells. Describe four different intercellular junctions. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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