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Chapter 6

Chapter 6. A Tour of the Cell. Overview: The Importance of Cells. All organisms are made of cells 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.

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Chapter 6

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  1. Chapter 6 A Tour of the Cell

  2. Overview: The Importance of Cells • All organisms are made of cells • 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

  3. Concept 6.1: To study cells, biologists use microscopes and the tools of biochemistry • Though usually too small to be seen by the unaided eye, cells can be complex

  4. 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 (can view living cells) • The minimum resolution (measure of clarity) of an LM is about 200 nanometers (nm), the size of a small bacterium

  5. LE 6-2 10 m Human height 1 m Length of some nerve and muscle cells Unaided eye 0.1 m Chicken egg 1 cm Frog egg 1 mm 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 100 µm Most plant and animal cells Light microscope 10 µm Nucleus Most bacteria Mitochondrion 1 µm Electron microscope Smallest bacteria 100 nm Viruses Ribosomes 10 nm Proteins Lipids 1 nm Small molecules Atoms 0.1 nm

  6. LMs 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, or organelles, are too small to be resolved by a LM

  7. LE 6-3a Brightfield (unstained specimen) 50 µm Brightfield (stained specimen) Phase-contrast

  8. LE 6-3b Differential- interference- contrast (Nomarski) Fluorescence 50 µm Confocal 50 µm

  9. (2)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 3D • Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen • TEMs are used mainly to study the internal ultrastructure of cells

  10. LE 6-4 1 µm Scanning electron microscopy (SEM) Cilia Longitudinal section of cilium Transmission electron microscopy (TEM) Cross section of cilium 1 µm

  11. Isolating Organelles by Cell Fractionation • (3)Cell fractionation takes cells apart and separates the major organelles from one another • Ultracentrifuges fractionate cells into their component parts • Size and weight determines which layer an organelle end up in • Cell fractionation enables scientists to determine the functions of organelles

  12. LE 6-5a Homogenization Tissue cells Homogenate Differential centrifugation

  13. LE 6-5b 1000 g (1000 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) Pellet rich in “microsomes” (pieces of plasma membranes and cells’ internal membranes) Pellet rich in ribosomes

  14. 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 • Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells • Protists, fungi, animals, and plants all consist of eukaryotic cells

  15. Comparing Prokaryotic and Eukaryotic Cells • Basic features of all cells: • Plasma membrane • Semifluid substance called the cytosol • Chromosomes (carry genes) • Ribosomes (make proteins)

  16. Prokaryotic cells have no nucleus • In a prokaryotic cell, DNA is in an unbound region called the nucleoid • Prokaryotic cells lack membrane-bound organelles such as endoplasmic reticulum, golgi bodies and nucleus.

  17. LE 6-6 Pili Nucleoid Ribosomes Plasma membrane Cellwall Bacterial chromosome Capsule 0.5 µm Flagella A typical rod-shaped bacterium A thin section through the bacterium Bacillus coagulans (TEM)

  18. Eukaryotic cells have DNA in a nucleus that is bounded by a membranous nuclear envelope • Eukaryotic cells have membrane-bound organelles • Eukaryotic cells are generally much larger than prokaryotic cells • The logistics of carrying out cellular metabolism sets limits on the size of cells

  19. LE 6-7 Surface area increases while Total volume remains constant 5 1 1 Total surface area (height x width x number of sides x number of boxes) 750 6 150 Total volume (height x width x length X number of boxes) 125 125 1 Surface-to-volume ratio (surface area  volume) 6 6 1.2

  20. The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of the cell • The general structure of a biological membrane is a double layer of phospholipids • Often call fluid mosaic model.

  21. LE 6-8 Outside of cell Carbohydrate side chain Hydrophilic region Inside of cell 0.1 µm Hydrophobic region Hydrophilic region Phospholipid Proteins Structure of the plasma membrane TEM of a plasma membrane

  22. A Panoramic View of the Eukaryotic Cell • A eukaryotic cell has internal membranes that partition the cell into organelles • Plant and animal cells have most of the same organelles

  23. LE 6-9a ENDOPLASMIC RETICULUM (ER Nuclear envelope Flagellum Rough ER Smooth ER NUCLEUS Nucleolus Chromatin Centrosome Plasma membrane CYTOSKELETON Microfilaments Intermediate filaments Microtubules Ribosomes: Microvilli Golgi apparatus Peroxisome Mitochondrion Lysosome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm)

  24. LE 6-9b Nuclear envelope Rough endoplasmic reticulum NUCLEUS Nucleolus Chromatin Smooth endoplasmic reticulum Centrosome Ribosomes (small brown dots) Central vacuole Golgi apparatus Microfilaments Intermediate filaments CYTOSKELETON Microtubules Mitochondrion Peroxisome Chloroplast Plasma membrane Cell wall Plasmodesmata Wall of adjacent cell In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata

  25. Concept 6.3: The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes • The nucleus contains most of the DNA in a eukaryotic cell (some genes located in the mitochondria and chloroplasts) and is the most conspicuous organelle

  26. The Nucleus: Genetic Library of the Cell • The nuclear envelope encloses the nucleus, separating it from the cytoplasm • Double membrane perforated by pores • Pores allow compounds such as ribosomal RNA, messenger RNA out of nucleus and allow proteins synthesized in the cytoplasm that are part of ribosomes into the nucleus.

  27. LE 6-10 Nucleus Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope Ribosome 1 µm 0.25 µm Close-up of nuclear envelope Pore complexes (TEM) Nuclear lamina (TEM)

  28. Ribosomes: Protein Factories in the Cell • Ribosomes use the information from the DNA to make proteins • Ribosomes are particles made of ribosomal RNA and protein • Ribosomes carry out protein synthesis in two locations:

  29. In the cytosol (free ribosomes) • Make proteins that will stay in cytosol • On the outside of the endoplasmic reticulum (ER) or the nuclear envelope (bound ribosomes) • Make proteins destined for export

  30. LE 6-11 Ribosomes ER Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit Small subunit 0.5 µm TEM showing ER and ribosomes Diagram of a ribosome

  31. Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell • Components of the endomembrane system: • Nuclear envelope • Endoplasmic reticulum • Golgi apparatus • Lysosomes • Vacuoles • Plasma membrane • These components are either continuous or connected via transfer by vesicles

  32. 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

  33. LE 6-12 Smooth ER Nuclear envelope Rough ER ER lumen Cisternae Ribosomes Transitional ER Transport vesicle 200 nm Rough ER Smooth ER

  34. Functions of Smooth ER • The smooth ER • Synthesizes lipids • Metabolizes carbohydrates • Stores calcium • Detoxifies poison

  35. Functions of Rough ER • The rough ER • Has bound ribosomes • Produces proteins and membranes, which are distributed by transport vesicles • Is a membrane factory for the cell • Membrane flow generally flows from RER  vesicles  Golgi  plasma membrane

  36. 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

  37. LE 6-13 Golgi apparatus cis face (“receiving” side of Golgi apparatus) Vesicles coalesce to form new cis Golgi cisternae Vesicles move from ER to Golgi 0.1 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 Vesicles transport specific proteins backward to newer Golgi cisternae trans face (“shipping” side of Golgi apparatus) TEM of Golgi apparatus

  38. Lysosomes: Digestive Compartments • A lysosome is a membranous sac of hydrolytic enzymes • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids • Lysosomes also use enzymes to recycle organelles and macromolecules, a process called autophagy

  39. LE 6-14a 1 µm Nucleus Lysosome Lysosome contains active hydrolytic enzymes Hydrolytic enzymes digest food particles Food vacuole fuses with lysosome Digestive enzymes Plasma membrane Lysosome Digestion Food vacuole Phagocytosis: lysosome digesting food

  40. LE 6-14b Lysosome containing two damaged organelles 1 µm Mitochondrion fragment Peroxisome fragment Hydrolytic enzymes digest organelle components Lysosome fuses with vesicle containing damaged organelle Lysosome Digestion Vesicle containing damaged mitochondrion Autophagy: lysosome breaking down damaged organelle

  41. Vacuoles: Diverse Maintenance Compartments • Vesicles and vacuoles (larger versions of vacuoles) are membrane-bound sacs with varied functions • A plant cell or fungal cell may have one or several vacuoles

  42. Food vacuoles are formed by phagocytosis • Contractile vacuoles, found in many freshwater protists, pump excess water out of cells • Central vacuoles, found in many mature plant cells, hold organic compounds and water

  43. LE 6-15 Central vacuole Cytosol Tonoplast Central vacuole Nucleus Cell wall Chloroplast 5 µm

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

  45. LE 6-16-1 Nucleus Rough ER Smooth ER Nuclear envelope

  46. LE 6-16-2 Nucleus Rough ER Smooth ER Nuclear envelope cis Golgi Transport vesicle trans Golgi

  47. LE 6-16-3 Nucleus Rough ER Smooth ER Nuclear envelope cis Golgi Transport vesicle Plasma membrane trans Golgi

  48. Concept 6.5: Mitochondria and chloroplasts change energy from one form to another • Mitochondria are the sites of cellular respiration • Chloroplasts, found only in plants and algae, are the sites of photosynthesis • Mitochondria and chloroplasts are not part of the endomembrane system • Peroxisomes are oxidative organelles

  49. 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

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