Chapter 6: A Tour of the Cell - The Importance of Cells

1. Chapter 6 Chapter 6: A Tour of the Cell

2. The Importance of Cells • All organisms are made of cells • The cell is the simplest collection of matter that can live

3. 10 µm • Cell structure is correlated to cellular function Figure 6.1

4. Microscopy • Scientists use microscopes to visualize cells too small to see with the naked eye • Light microscopes (LMs) • Pass visible light through a specimen • Magnify cellular structures with lenses • Electron microscopes (EMs) • Focus a beam of electrons through a specimen (TEM) or onto its surface (SEM)

5. TECHNIQUE RESULTS 1 µm Cilia (a) Scanning electron micro- scopy (SEM). Micrographs taken with a scanning electron micro- scope 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. • The scanning electron microscope (SEM) • Provides for detailed study of the surface of a specimen Figure 6.4 (a)

6. Longitudinal section of cilium Cross section of cilium 1 µm (b) Transmission electron micro- scopy (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. • The transmission electron microscope (TEM) • Provides for detailed study of the internal ultrastructure of cells Figure 6.4 (b)

7. Homogenization Tissue cells 1000 g (1000 times the force of gravity) 10 min Homogenate Differential centrifugation 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 mem- branes and cells’ internal membranes) Pellet rich in ribosomes Isolating Organelles by Cell Fractionation • Cell fractionation: Takes cells apart and separates the major organelles from one another • - The centrifuge: Is used to fractionate cells into their component parts

8. Eukaryotic cells have internal membranes that compartmentalize their functions • Two types of cells make up every organism • Prokaryotic • Eukaryotic

9. Comparing Prokaryotic and Eukaryotic Cells • All cells have several basic features in common • They are bounded by a plasma membrane • They contain a semifluid substance called the cytosol • They contain chromosomes • They all have ribosomes

10. Prokaryotic cells • Do not contain a nucleus • Have their DNA located in a region called the nucleoid • Eukaryotic cells • Contain a true nucleus, bounded by a membranous nuclear envelope • Are generally quite a bit bigger than prokaryotic cells

11. 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 Bacterialchromosome 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)

12. 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 • 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

13. Outside of cell Hydrophilic region 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. (a) Inside of cell 0.1 µm Hydrophobic region Hydrophilic region Phospholipid Proteins (b) Structure of the plasma membrane • The plasma membrane • Functions as a selective barrier • Allows sufficient passage of nutrients and waste Carbohydrate side chain Figure 6.8 A, B

14. 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

15. Nuclear envelope ENDOPLASMIC RETICULUM (ER) NUCLEUS Nucleolus Rough ER Smooth ER Chromatin Flagelium Plasma membrane Centrosome CYTOSKELETON Microfilaments Intermediate filaments Ribosomes Microtubules Microvilli Golgi apparatus Peroxisome In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm) Lysosome Mitochondrion • A animal cell Figure 6.9

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

17. Nucleus Nucleus 1 µm Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Pore complex Rough ER Surface of nuclear envelope. 1 µm Ribosome 0.25 µm Close-up of nuclear envelope Nuclear lamina (TEM). Pore complexes (TEM). The Nucleus: Genetic Library of the Cell • The nucleus • Contains most of the genes in the eukaryotic cell • The nuclear envelope • Encloses the nucleus, separating its contents from the cytoplasm Figure 6.10

18. Ribosomes Cytosol Free ribosomes Bound ribosomes Large subunit Small subunit 0.5 µm TEM showing ER and ribosomes Diagram of a ribosome Ribosomes: Protein Factories in the Cell • Are particles made of ribosomal RNA and protein • Carry out protein synthesis ER Endoplasmic reticulum (ER)

19. The endomembrane system regulates protein traffic and performs metabolic functions in the cell • The endomembrane system • Includes many different structures

20. Smooth ER Nuclear envelope Rough ER ER lumen Cisternae Ribosomes Transitional ER Transport vesicle 200 µm Smooth ER Rough ER The Endoplasmic Reticulum: Biosynthetic Factory • The endoplasmic reticulum (ER) • Accounts for more than half the total membrane in many eukaryotic cells • Is continuous with the nuclear envelope

21. 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

22. The Golgi Apparatus: Shipping and Receiving Center • The Golgi apparatus • Receives many of the transport vesicles produced in the rough ER • Consists of flattened membranous sacs called cisternae • Functions of the Golgi apparatus include • Modification of the products of the rough ER • Manufacture of certain macromolecules

23. cis face (“receiving” side of Golgi apparatus) 5 3 4 6 2 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 • Functions of the Golgi apparatus Golgi apparatus Figure 6.13 TEM of Golgi apparatus

24. 1 µm Nucleus Lysosome Hydrolytic enzymes digest food particles Food vacuole fuses with lysosome Lysosome contains active hydrolytic enzymes Digestive enzymes Lysosome Plasma membrane Digestion Food vacuole (a) Phagocytosis: lysosome digesting food - Is a membranous sac of hydrolytic enzymes - Can digest all kinds of macromolecules - Phagocytosis: intracellular digestion Figure 6.14 A

25. Vacuoles: Diverse Maintenance Compartments • A plant or fungal cell • May have one or several vacuoles • Food vacuoles • Are formed by phagocytosis • Contractile vacuoles • Pump excess water out of protist cells

26. Central vacuole Cytosol Tonoplast Nucleus Central vacuole Cell wall Chloroplast 5 µm • Central vacuoles • Are found in plant cells • Hold reserves of important organic compounds and water Figure 6.15

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

28. Mitochondria and chloroplasts change energy from one form to another • Mitochondria • Are the sites of cellular respiration • Chloroplasts • Found only in plants, are the sites of photosynthesis

29. Mitochondrion Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Inner membrane Cristae Matrix Mitochondrial DNA 100 µm • Mitochondria are enclosed by two membranes • A smooth outer membrane • An inner membrane folded into cristae Figure 6.17

30. Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Chloroplasts: Capture of Light Energy • Chloroplasts • Are found in leaves and other green organs of plants and in algae • Thylakoids, membranous sacs • Stroma, the internal fluid Figure 6.18

31. Chloroplast Peroxisome Mitochondrion 1 µm Peroxisomes: Oxidation • Peroxisomes • Produce hydrogen peroxide and convert it to water Figure 6.19

32. Microtubule Microfilaments 0.25 µm Figure 6.20 • The cytoskeleton • Is a network of fibers extending throughout the cytoplasm Figure 6.20

33. Vesicle ATP Receptor for motor protein Motor protein (ATP powered) Microtubule of cytoskeleton (a) Motor proteins that attach to receptors on organelles can “walk” the organelles along microtubules or, in some cases, microfilaments. Vesicles Microtubule 0.25 µm (b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, two  vesicles can be seen moving along a microtubule. (A separate part of the experiment provided the evidence that they were in fact moving.) Figure 6.21 A, B • Is involved in cell motility, which utilizes motor proteins

34. Table 6.1 • There are three main types of fibers that make up the cytoskeleton

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

36. 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

37. (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 flagellatelocomotion (LM). Direction of swimming 1 µm Cilia and Flagella • Flagella beating pattern Figure 6.23 A

38. (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). Figure 6.23 B • Ciliary motion 15 µm

39. Outer microtubule doublet Plasma membrane 0.1 µm Dynein arms Central microtubule Outer doublets cross-linking proteins inside Microtubules Radial spoke Plasma membrane Basal body (b) 0.5 µm 0.1 µm (a) Triplet (c) Figure 6.24 A-C Cross section of basal body • Cilia and flagella share a common ultrastructure

40. Microtubule doublets ATP Dynein arm Powered by ATP, the dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again. If the two microtubule doublets were not attached, they would slide relative to each other. (a) Figure 6.25 A • The protein dynein • Is responsible for the bending movement of cilia and flagella

41. ATP Outer doublets cross-linking proteins Anchorage in cell (b) In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two of the nine outer doublets in Figure 6.24b are shown here.) Figure 6.25 B

42. 1 3 2 Localized, synchronized activation of many dynein arms probably causes a bend to begin at the base of the Cilium or flagellum and move outward toward the tip. Many successive bends, such as the ones shown here to the left and right, result in a wavelike motion. In this diagram, the two central microtubules and the cross-linking proteins are not shown. (c) Figure 6.25 C

43. Microfilaments (Actin Filaments) • Microfilaments • Are built from molecules of the protein actin

44. Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm Figure 6.26 • Are found in microvilli

45. Muscle cell Actin filament Myosin filament Myosin arm (a) Figure 6.27 A Myosin motors in muscle cell contraction. • Microfilaments that function in cellular motility • Contain the protein myosin in addition to actin

46. Cortex (outer cytoplasm): gel with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium (b) Amoeboid movement • Amoeboid movement • Involves the contraction of actin and myosin filaments Figure 6.27 B

47. Nonmoving cytoplasm (gel) Chloroplast Streaming cytoplasm (sol) Parallel actin filaments Cell wall (b) Cytoplasmic streaming in plant cells • Cytoplasmic streaming • Is another form of locomotion created by microfilaments Figure 6.27 C

48. Intermediate Filaments • Intermediate filaments • Support cell shape • Fix organelles in place

49. Extracellular components and connections between cells help coordinate cellular activities

50. Central vacuole of cell Plasma membrane Secondary cell wall Primary cell wall Central vacuole of cell Middle lamella 1 µm Central vacuole Cytosol Plasma membrane Plant cell walls Plasmodesmata Figure 6.28 Cell Walls of Plants • Plant cell walls • Are made of cellulose fibers embedded in other polysaccharides and protein • May have multiple layers