Chapter 6: A Tour of the Cell. 1. Studying Cells 2. Intracellular Structures 3. The Cytoskeleton 4. Extracellular Structures

Transcription

1 Chapter 6: A Tour of the Cell 1. Studying Cells 2. Intracellular Structures 3. The Cytoskeleton 4. Extracellular Structures

2 1. Studying Cells

3 Concepts of Microscopy MAGNIFICATION factor by which the image produced is larger than the actual object (e.g. 100X ) RESOLUTION minimum distance across which 2 points can be resolved or seen distinctly (limited by wavelength) CONTRAST degree to which objects differ from background Limits of Resolution 10 m 1 m 0.1 m 1 cm 1 mm 100 µm 10 µm 1 µm 100 nm 10 nm 1 nm Chicken egg Frog egg Nucleus Ribosomes Proteins Lipids 0.1 nm Atoms Human height Length of some nerve and muscle cells Most plant and animal cells Most bacteria Mitochondrion Smallest bacteria Viruses Small molecules Unaided eye Light microscope Electron microscope

4 Light Microscopy a typical compound microscope such as used in your lab

5 Bright Field Microscopy TECHNIQUE RESULTS (a) Brightfield (unstained specimen) Standard form of Light Microscopy, poor contrast 50 µm (b) Brightfield (stained specimen) Staining increases contrast, though the staining process usually kills the specimen

6 Phase Contrast Microscopy TECHNIQUE RESULTS (c) Phase-contrast Enhances misalignment of light waves to create contrast Reveals internal detail without staining, useful for viewing live specimens (d) Differential-interferencecontrast (Nomarski) A variation of phase-contrast microscopy involving a more complex combination of filters and prisms.

7 Fluorescence Microscopy TECHNIQUE RESULTS (e) Fluorescence Fluorescent dyes or antibodies with a fluorescent tag stick to specific targets which then fluoresce under UV light. 50 µm Only the objects or structures that fluoresce are visible. objects that bind the fluorescent stain or antibody objects that are naturally fluorescent

8 Confocal Fluorescence Microscopy Only light from a given depth or plane is transmitted, out of focus light is excluded

9 TECHNIQUE (a) Scanning electron microscopy (SEM) view of whole specimen, reveals surface features RESULTS Cilia 1 µm Electron Microscopy Electromagnetic lenses focus electron beam onto heavy metalstained specimen. (b) Transmission electron microscopy (TEM) specimen cut in thin sections, higher resolution Longitudinal section of cilium Cross section of cilium 1 µm electron beams have very short wavelengths allows far greater resolution than with light microscopy

10 Fractionation by TECHNIQUE Centrifugation Homogenization In addition to microscopic examination, cells and their structures are also studied biochemically: Tissue cells 1,000 g (1,000 times the force of gravity) 10 min Differential centrifugation Supernatant poured into next tube 20,000 g 20 min Homogenate in order to study a cellular compartment biochemically, it must be separated from the rest of the cell this is accomplished through successive centrifugation steps at increasing speeds Pellet rich in nuclei and cellular debris Pellet rich in mitochondria (and chloroplasts if cells are from a plant) 80,000 g 60 min 150,000 g 3 hr Pellet rich in microsomes (pieces of plasma membranes and cells internal membranes) Pellet rich in ribosomes

11 Why are cells the Surface area increases while total volume remains constant size they are? Total surface area [Sum of the surface areas (height width) of all boxes sides number of boxes] Total volume [height width length number of boxes] Surface-to-volume (S-to-V) ratio [surface area volume]

12 Why aren t cells bigger? 1) Cell size is limited by the rate of diffusion: if cells get too large, it takes too much time for nutrients, wastes, etc, to disperse in the cell 2) And also by the surface to volume ratio (S/V): surface area of sphere = 4pr 2 volume of sphere = (4/3)pr 3 *Surface Area increases by square of radius *Volume increases by cube of radius ***The larger the cell, the smaller the S/V ratio***

13 2. Intracellular Structures

14 Cells come in 2 basic types: 1. Prokaryotic cells ( before nucleus) lack a nucleus & other organelles small, unicellular diameter ~1-10 µm organisms in the following domains: Bacteria Archaea

15 2. Eukaryotic cells ( true nucleus) have a nucleus, subcellular organelles unicellular or multicellular large (diameter ~10 µm 1 mm) Eukarya: Protists, Fungi, Plants & Animals

16 Prokaryotic Cells Fimbriae Nucleoid Ribosomes Plasma membrane Bacterial chromosome (a) A typical rodshaped bacterium Cell wall Capsule Flagella 0.5 µm (b) A thin section through the bacterium Bacillus coagulans (TEM) Have intracellular organization despite no organelles.

17 * not in plant cells * Flagellum ENDOPLASMIC RETICULUM (ER) Rough ER Smooth ER Nuclear envelope Nucleolus Chromatin NUCLEUS * Centrosome Plasma membrane CYTOSKELETON: Microfilaments Intermediate filaments Microtubules Ribosomes * Microvilli Peroxisome Golgi apparatus Mitochondrion Lysosome * Animal Cell

18 The Nucleus Where genetic material (DNA) is stored, gene expression begins. Nucleus 1 µm Nucleolus Chromatin Nucleolus Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex where ribosomal subunits are assembled from rrna & proteins Surface of nuclear envelope Ribosome Rough ER 1 µm Chromatin 0.25 µm Close-up of nuclear envelope complex of DNA & histone proteins Pore complexes (TEM) Nuclear lamina (TEM)

19 Ribosomes Carry out protein synthesis by the process of translation. Cytosol Endoplasmic reticulum (ER) Free ribosomes Bound ribosomes Large subunit 0.5 µm TEM showing ER and ribosomes Small subunit Diagram of a ribosome

20 Smooth ER Rough ER Nuclear envelope Endoplasmic Reticulum ER lumen Cisternae Ribosomes Transport vesicle Smooth ER Rough ER Transitional ER 200 nm Rough ER (RER) ribosomes on cytoplasmic face of ER membrane synthesize proteins across ER membrane into lumen of ER beginning of the secretory pathway Smooth ER (SER) has membrane-associated enzymes that catalyze new lipid synthesis (also found in RER), neutralizing toxins storage of calcium ions

21 The Golgi apparatus cis face ( receiving side of Golgi apparatus) Cisternae 0.1 µm trans face ( shipping side of Golgi apparatus) TEM of Golgi apparatus proteins destined to leave ER are transported to the Golgi where they are modified, sorted and sent to various destinations. polysaccharides are produced in the Golgi apparatus as well

22 Lysosomes Nucleus 1 µm Vesicle containing two damaged organelles 1 µm Mitochondrion fragment Lysosome Peroxisome fragment Lysosome Digestive enzymes Lysosome Plasma membrane Digestion Peroxisome Food vacuole Vesicle Mitochondrion Digestion (a) Phagocytosis (b) Autophagy acidic compartments full of enzymes for the breakdown or digestion of foreign or waste material

23 The Endomembrane System Nucleus Rough ER Smooth ER cis Golgi aka the Secretory Pathway trans Golgi Plasma membrane

24 Mitochondria Have ribosomes that resemble those of prokaryotes Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix 0.1 µm ATP production via cellular respiration convert energy from glucose, fatty acids, etc, to energy in ATP

25 NUCLEUS Nuclear envelope Nucleolus Chromatin Rough endoplasmic reticulum Smooth endoplasmic reticulum * not in animal cells Ribosomes Golgi apparatus Central vacuole Microfilaments Intermediate filaments Microtubules * CYTO- SKELETON Mitochondrion Peroxisome Plasma membrane * Cell wall Wall of adjacent cell Plasmodesmata Chloroplast * * Plant Cell

26 Central Vacuole Nucleus Cell wall Chloroplast Central vacuole Cytosol Central vacuole 5 µm Plant organelle that stores water and various ions Source of turgor pressure that maintains rigidity of plant cells swells when water is plentiful due to osmosis cell wall provides support, prevents lysis

27 Chloroplasts Site of photosynthesis in plant cells. Chloroplast Like mitochondria, have ribosomes and other components that resemble those of prokaryotes Stroma Inner and outer membranes Granum Intermembrane space production of glucose from CO 2 and H 2 O using sunlight the basis of essentially all ecosystems

28 Peroxisomes Chloroplast Peroxisome Mitochondrion Contain enzymes that oxidize (i.e., remove H) various organic molecules thus forming H 2 O 2 from O 2 H 2 O 2 is then converted to O 2 and H 2 O 1 µm Involved in the detoxification of toxic substances, breakdown of fatty acids

29 3. The Cytoskeleton

30 The Cytoskeleton A complex, highly dynamic intracellular network of protein filaments largely responsible for: cell shape, rigidity cell movement, motility movement of vesicles (and organelles) Microtubule localization of organelles 0.25 µm Microfilaments dynamics of cell division

31 3 Basic Cytoskeletal Filaments Actin filaments/microfilaments (MF) intermediate filaments (IF) microtubules (MT) MF IF MT

32 Microfilaments 10 µm Actin subunit 7 nm

33 Intermediate Filaments 5 µm Keratin proteins Fibrous subunit (keratins coiled together) 8 12 nm

34 Microtubules 10 µm Column of tubulin dimers 25 nm a b Tubulin dimer

35 ATP Vesicle Receptor for motor protein Vesicle Transport (a) Microtubule Motor protein (ATP powered) Vesicles Microtubule of cytoskeleton 0.25 µm a variety of motor proteins are involved in binding and transporting vesicles along cytoskeletal fibers to their destination (b)

36 Centrosome Centrosomes Centrioles Microtubule 0.25 µm centrosomes contain a pair of centrioles (animal cells only) these structures are involved in the formation of the mitotic spindle or spindle fibers that play such an important role in cell division Longitudinal section of one centriole Microtubules Cross section of the other centriole

37 Flagella & Cilia Direction of swimming (a) Motion of flagella Direction of organism s movement Power stroke (b) Motion of cilia Recovery stroke 5 µm 15 µm FLAGELLA are involved in cell motility, are very long, and cells have relatively few (1 or several) CILIA are involved in motility, moving material across the cell surface, and are present on the cell surface in high numbers

38 4. Extracellular Structures

39 Plant Cell Walls Cell walls Interior of cell Interior of cell 0.5 µm Plasmodesmata Plasma membranes Secondary cell wall Primary cell wall Middle lamella Plant cell walls contain fibers of cellulose and other polysaccharides as well as proteins one or more layers of secondary cell wall may be produced in some plant cells 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata

40 Intercellular Tight junction Junctions Tight junctions prevent fluid from moving across a layer of cells TIGHT JUNCTIONS are impenetrable seals connecting adjacent cells that prevent fluid and other materials from passing between the cells DESMOSOMES are strong connections between cells that create a very strong sheet of cells Intermediate filaments Tight junction Desmosome Gap junctions Desmosome 0.5 µm 1 µm GAP JUNCTIONS (animal) & PLASMODESMATA (plant) provide channels through which ions & other small molecules can pass from cell to cell Space between cells Plasma membranes of adjacent cells Extracellular matrix Gap junction 0.1 µm

41 The Extracellular Matrix A meshwork of protein fibers and polysaccharides that retain fluid and produce gel-like matrix that holds cells together in tissues. Polysaccharide molecule Collagen EXTRACELLULAR FLUID Proteoglycan complex Carbohydrates Fibronectin Core protein Integrins Plasma membrane Proteoglycan molecule Microfilaments CYTOPLASM Proteoglycan complex

42 Key Terms for Chapter 6 magnification, resolution, contrast bright field, phase contrast, fluorescent, confocal, transmission & scanning electron microscopy prokaryotic vs eukaryotic cell wall, capsule, flagella, nucleoid, cytoplasm nucleus, nucleolus, endoplasmic reticulum, ribosome Golgi apparatus, lysosome, peroxisome, vesicle endomembrane system, central vacuole mitochondria, chloroplasts

43 cytoskeleton, cilia, flagella, centrosome, centriole microtubules, microfilaments, intermediate filaments motor proteins tight junctions, gap junctions, desmosomes, plasmodesmata extracellular matrix, proteoglycan Relevant Chapter Questions 1-9