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Chapter 6

Chapter 6. A Tour of the Cell. Microscopy. Scientists use different types of microscopes to visualize cells and cellular structures that are too small to see with the naked eye. Light microscopes (LMs) Pass visible light through a specimen Magnify cellular structures with lenses

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Chapter 6

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  1. Chapter 6 A Tour of the Cell

  2. Microscopy • Scientists use different types of microscopes to visualize cells and cellular structures that are too small to see with the naked eye

  3. Light microscopes (LMs) Pass visible light through a specimen Magnify cellular structures with lenses Use different methods for enhancing visualization of cellular structures Light microscopes (LMs) 50 µm 50 µm

  4. 1 µm Cilia Longitudinal section of cilium Cross section of cilium 1 µm Electron microscopes (EMs) • Electron microscopes (EMs) • Focus a beam of electrons through a specimen (TEM) or onto its surface (SEM) • The scanning electron microscope (SEM) • Provides for detailed study of the surface of a specimen • The transmission electron microscope (TEM) • Provides for detailed study of the internal ultrastructure of cells

  5. All Organisms Are Made of Cells • A cell is the smallest living unit; it may be the body of a unicellular organism or a part of a multicellular organism • Every cell consists of a boundary, a set of genes, and a cell body • Cells tend to be very tiny, no matter the size of the organism • Subdivision into tiny cells has many advantages

  6. Comparing Prokaryotic and Eukaryotic Cells • Every organism is made of either prokaryotic or eukaryotic cells • Both cell types have several basic features in common • Boundary • They are bounded by a plasma membrane • Set of Genes • They contain chromosomes • Cell Body • They contain a semifluid substance called the cytosol

  7. Prokaryotic cells • Prokaryotic cells • Do not contain a nucleus so their DNA is not separated from the rest of the cytosol by a membrane • The region containing the DNA is called the nucleoid • Smaller and less complex than Eukaryotic cells • Organisms with this cell type are classified in the Domain Archaea or Domain Bacteria • Organisms with this cell type are classified in the Kingdom Monera

  8. Pili: attachment structures on the surface of some prokaryotes Nucleoid: region wherethe 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 Flagella: locomotion organelles of some bacteria (a) A typical rod-shaped bacterium (b) A thin section through the bacterium Bacillus coagulans (TEM) Figure 6.6 A prokaryotic cell

  9. Eukaryotic cells • Eukaryotic cells • Contain a true nucleus, bounded by a membranous nuclear envelope • Larger and more complex than prokaryotic cells • Organisms with this cell type are classified in the Domain Eukarya • Organisms with this cell type are classified in the Kingdom Protista, Kingdom Fungi, Kingdom Plantae, or Kingdom Animalia

  10. Figure 6.2 The size range of cells 10 m 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 Light microscope Most plant and animal cells 10 µm Nucleus nucleus Most bacteria Most bacteria Mitochondrion 1 µm Electron microscope Smallest bacteria 100 nm Viruses Ribosomes 10 nm Proteins Lipids Measurements 1 centimeter (cm) = 102 meter (m) = 0.4 inch 1 millimeter (mm) = 10–3 m 1 micrometer (µm) = 10–3 mm = 106 m 1 nanometer (nm) = 10–3 µm = 10 9 m 1 nm Small molecules Atoms 0.1 nm

  11. 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) 125 125 1 Surface-to-volume ratio (surface area  volume) 6 12 6 Subdivision into tiny cells has many advantages • A smaller cell • Has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell Figure 6.7

  12. The plasma membrane • The plasma membrane • Structure: Phospholipid bilayer embedded with various molecules • Function: Regulates the passage of gases, nutrients, and waste materials in and out of the cell Carbohydrate side chain Outside of cell Hydrophilic region Inside of cell 0.1 µm Hydrophobic region TEM of a plasma membrane. (a) Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane Figure 6.8 A, B

  13. A Panoramic View of the Eukaryotic Cell • Eukaryotic cells • Have extensive and elaborately arranged internal membranes, which form organelles • Plant and animal cells • Have most of the same organelles

  14. Nuclear envelope ENDOPLASMIC RETICULUM (ER) NUCLEUS Nucleolus Rough ER Smooth ER Chromatin Flagellum Plasma membrane Centrosome CYTOSKELETON Microfilaments Ribosomes Microtubules Microvilli Golgi apparatus Peroxisome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) Lysosome Mitochondrion Figure 6.9 Exploring Animal and Plant Cells: Animal Cell Intermediate filaments

  15. Figure 6.9 Exploring Animal and Plant Cells: Plant Cell Nuclear envelope Rough endoplasmic reticulum Nucleolus NUCLEUS Chromatin Smooth endoplasmic reticulum Centrosome Ribosomes ( small brown dots ) Central vacuole Tonoplast Golgi apparatus Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Plasma membrane Chloroplast Cell wall Plasmodesmata In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata Wall of adjacent cell

  16. The Nucleus:Genetic Library of the Cell • The nucleus • Structure: Conspicuous membrane-bound spherical body • Function: Contains most of the genes in the eukaryotic cell • The nuclear envelope • Encloses the nucleus, separating its contents from the cytoplasm Figure 6.10

  17. 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. Ribosome 1 µm 0.25 µm Close-up of nuclear envelope Figure 6.10 The nucleus and its envelope Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope. Pore complexes (TEM). Each pore is ringed by protein particles.

  18. Ribosomes:Protein Factories in the Cell • Ribosomes • Structure: Small, round bodies made of ribosomal RNA and protein subunits • Function: Carry out protein synthesis

  19. ER Ribosomes Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit Small subunit 0.5 µm TEM showing ER and ribosomes Diagram of a ribosome Figure 6.11 Ribosomes

  20. The Endomembrane System • The endomembrane system • Includes many different structures: • Endoplasmic Reticulum • Golgi Apparatus • Lysosomes • Vacuoles • Regulates protein traffic and performs metabolic functions in the cell

  21. The Endoplasmic Reticulum: Biosynthetic Factory • The endoplasmic reticulum (ER) • Structure: extensive dynamic, membrane-lined channels continuous with the nuclear envelope; may be rough or smooth • Accounts for more than half the total membrane in many eukaryotic cells • Function: modifies proteins, synthesizes lipids, metabolizes carbohydrates

  22. Smooth ER Rough ER Nuclear envelope ER lumen Cisternae Ribosomes Transitional ER Transport vesicle 200 µm Smooth ER Rough ER Figure 6.12 Endoplasmic reticulum (ER)

  23. Smooth ER and Rough ER • There are two distinct regions of ER • Smooth ER, which lacks ribosomes • Synthesizes lipids • Metabolizes carbohydrates • Stores calcium • Detoxifies poison • Rough ER, which contains ribosomes • Produces proteins and membranes, which are distributed by transport vesicles

  24. The Golgi Apparatus:Shipping and Receiving Center • The Golgi apparatus • Structure: Consists of flattened membranous sacs called cisternae • Function: Modification of the products of the rough ER and manufacture of certain macromolecules

  25. Golgi apparatus cis face (“receiving” side of Golgi apparatus) 4 2 5 6 3 1 Vesicles coalesce to form new cis Golgi cisternae Vesicles move from ER to Golgi 0.1 0 µm Vesicles also transport certain proteins back to ER Cisternae Cisternal maturation: Golgi cisternae move in a cis- to-trans direction Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma mem- brane for secretion trans face (“shipping” side of Golgi apparatus) Vesicles transport specific proteins backward to newer Golgi cisternae TEM of Golgi apparatus Figure 6.13 The Golgi apparatus

  26. Lysosomes:Digestive Compartments • A lysosome • Structure: a membranous sac of hydrolytic enzymes • Function: intracellular digestion, recycling of organic materials, cell destruction

  27. 1 µm Nucleus Lysosome containing two damaged organelles 1 µ m Mitochondrion fragment Peroxisome fragment Lysosome Lysosome contains active hydrolytic enzymes Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Hydrolytic enzymes digest food particles Food vacuole fuses with lysosome Digestive enzymes Lysosome Lysosome Lysosome Plasma membrane Digestion Digestion Vesicle containing damaged mitochondrion Food vacuole (a) Phagocytosis: lysosome digesting food (b) Autophagy: lysosome breaking down damaged organelle Figure 6.14 Lysosomes

  28. Vacuoles: Diverse Maintenance Compartments • Vacuoles • Structure: membrane enclosed sac larger than a vesicle • Function: diverse functions depending on type (know these 3: food, contractile, central) • Food vacuoles (see Fig 6.14) • Are formed by phagocytosis • Contractile vacuoles • Pump excess water out of protist cells • Central vacuoles (see Fig 6.15) • Are found in plant cells • Hold reserves of important organic compounds and water

  29. Central vacuole Cytosol Tonoplast Central vacuole Nucleus Cell wall Chloroplast 5 µm Figure 6.15 The plant cell vacuole

  30. The Endomembrane System: A Review • The endomembrane system • Is a complex and dynamic player in the cell’s compartmental organization

  31. 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 3 Golgi pinches off transport vesicles and other vesicles that give rise to lysosomes and vacuoles Plasma membrane trans Golgi Lysosome available for fusion with another vesicle for digestion 4 Transport vesicle carries proteins to plasma membrane for secretion 5 Plasma membrane expands by fusion of vesicles; proteins are secreted from cell 6 Figure 6.16 Review: relationships among organelles of the endomembrane system

  32. Mitochondria and Chloroplasts • Mitochondria and chloroplasts change energy from one form to another • Mitochondria • Found in nearly all eukaryotic cells, are the sites of cellular respiration • Chloroplasts • Found only in plant cells, are the sites of photosynthesis

  33. Mitochondria: Chemical Energy Conversion • Mitochondrion (s.), Mitochondria (pl.) • Structure: enclosed by two membranes: • A smooth outer membrane • An inner membrane folded into cristae • The central compartment is called the matrix • Function: site of cellular respiration

  34. Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA 100 µm Figure 6.17 The mitochondrion, site of cellular respiration

  35. Chloroplasts: Capture of Light Energy • Chloroplasts • Structure: enclosed by two membranes: • A smooth outer membrane • An inner membrane • membranous sacs called thylakoids; these are stacked into grana • The internal fluid is called the stroma • Function: Site of photosynthesis

  36. Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Figure 6.18 The chloroplast, site of photosynthesis

  37. Chloroplast Peroxisome Mitochondrion 1 µm Peroxisomes: Oxidation • Peroxisomes • Produce hydrogen peroxide (H2O2) and convert it to water (H2O) Figure 6.19

  38. Cytoskeleton • The cytoskeleton • Structure: a network of fibers (3 types) extending throughout the cytoplasm • Microtubules • Microfilaments • Intermediate filaments • Function: Provides structural support, functions in cell motility and regulation.

  39. Table 6.1 The Structure and Function of the Cytoskeleton

  40. Microtubules • Microtubules • Made of Tubulin • Shape the cell • Guide movement of organelles • Help separate the chromosome copies in dividing cells

  41. Centrosome Microtubule Centrioles 0.25 µm Longitudinal section of one centriole Cross section of the other centriole Microtubules Figure 6.22 Centrosomes and Centrioles • The centrosome • Is considered to be a “microtubule-organizing center” • Contains a pair of centrioles

  42. Cilia and Flagella • Cilia and flagella • Contain a “9+2” arrangement of microtubules • Function in locomotion for some cells

  43. Figure 6.24 Ultrastructure of a eukaryotic flagellum or cilium Outer microtubule doublet Plasma membrane 0.1 µm Dynein arms Central microtubule Outer doublet cross-linking proteins Microtubules Radial spoke Plasma membrane (b) A cross section through the cilium shows the ”9 + 2“ arrangement of microtubules (TEM). Basal body 0.5 µm 0.1 µm (a) A longitudinal section of a cilium shows micro- tubules running the length of the structure (TEM). Triplet Cross section of basal body

  44. Cilia and Flagella • Cilia and flagella • Differ in length, number, and movement pattern: • Cilia are short, Flagella are long • Cilia are numerous, Flagella are few • Cilia move back and forth, Flagella move in an S-like pattern

  45. (a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellate locomotion (LM). Direction of swimming 1 µm (b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this colpidium, a freshwater protozoan (SEM). Direction of organism’s movement Direction of active stroke Direction of recovery stroke 15 µm Figure 6.23 A comparison of the beating of flagella and cilia

  46. Microfilaments(Actin Filaments) • Microfilaments • Made from Actin • Are found in microvilli • Microfilaments that function in cellular motility • Contain the protein myosin in addition to actin

  47. 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 cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Parallel actin filaments Cell wall (b) Cytoplasmic streaming in plant cells. Figure 6.27 Microfilaments and motility

  48. Intermediate Filaments • Intermediate filaments • Made of fibrous proteins (Keratin) • Support cell shape • Fix organelles in place

  49. Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm Figure 6.26 A structural role of intermediate filaments

  50. Extracellular components • Plants have cell walls • Animals have an extracellular matrix (ECM)

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