Chapter 6

1. Chapter 6 A Tour of the Cell

2. 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, or on the surface of a specimen

3. 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)

4. 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)

5. Concept 6.2: Eukaryotic cells have internal membranes that compartmentalize their functions • Two types of cells make up every organism

6. Outside of cell (a) Inside of cell 0.1 µm Phospholipid Proteins (b) Structure of the plasma membrane Comparing Prokaryotic and Eukaryotic Cells • All cells have several basic features in common • They are enclosed by a plasma membrane • Functions as a selective barrier • Allows sufficient passage of nutrients and waste

7. Comparing Prokaryotic and Eukaryotic Cells • They contain a semifluid substance called the • They contain • In prokaryotes, everything inside the cell • In eukaryotes, everything between the plasma membrane and the nucleus • They contain chromosomes • They all have ribosomes

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

9. 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

10. 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 An animal cell Figure 6.9

11. 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 Cell wall Plasmodesmata Figure 6.9

12. Concept 6.3: • The eukaryotic cell’s genetic instructions are housed in the and carried out by the ribosomes • The nuclear envelope • Encloses the nucleus, separating its contents from the cytoplasm

13. Ribosomes: Protein Factories in the Cell • Ribosomes • Are particles made of ribosomal RNA and protein • Manufactured in the • Site of protein synthesis

14. Concept 6.4: • The endomembrane system regulates protein traffic and performs metabolic functions in the cell • The endomembrane system includes: • Endoplasmic reticulum (ER) • Golgi apparatus • Lysosomes • Vacuoles • (plasma membrane)

15. Smooth ER Nuclear envelope Rough ER ER lumen Cisternae Ribosomes Transitional ER Transport vesicle 200 µm Smooth ER Rough ER The endomembrane system • The ER membrane is continuous with the nuclear envelope • There are two distinct regions of ER • which lack ribosomes • with attached ribosomes Figure 6.12

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

17. Functions of Rough ER • The rough ER • Modifies proteins and produces membranes, which are distributed by

18. The Golgi Apparatus: Shipping and Receiving Center • The Golgi apparatus • Receives many of the transport vesicles produced in the rough ER • Modifies some of the products of the rough ER • Manufactures many polysaccharides

19. 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

20. Functions of the Golgi apparatus • Transport vesicles from the Golgi are shipped to various locations: • i.e. -

21. Lysosomes: Digestive Compartments • A lysosome • Is a membranous sac of hydrolytic enzymes • Can digest all kinds of macromolecules

22. 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 Lysosomes carry out intracellular digestion initiates the process Figure 6.14 A

23. Lysosome containing two damaged organelles 1 µ m Mitochondrion fragment Peroxisome fragment Lysosome fuses with vesicle containing damaged organelle Hydrolytic enzymes digest organelle components Lysosome Digestion Vesicle containing damaged mitochondrion (b) Autophagy: lysosome breaking down damaged organelle Lysosomes carry out intracellular digestion • Autophagy Figure 6.14 B

24. Vacuoles: Diverse Maintenance Compartments • Food vacuoles • Are formed by phagocytosis • Contractile vacuoles • Pump excess water out of

25. 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 • May contain poison or dye • Enlarges as plant cell ages Figure 6.15

26. Concept 6.5: Mitochondria and chloroplasts • Mitochondria • Are the sites of cellular respiration • Abnormal functioning may cause mitochondrial disease • Chloroplasts • are the sites of photosynthesis • Sugar is produced

27. 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

28. Chloroplast Ribosomes Stroma Chloroplast DNA Inner and outer membranes Granum 1 µm Thylakoid Chloroplasts • Are found in leaves and other green organs of plants and in algae Figure 6.18

29. Peroxisomes: contain degrading enzymes • Peroxisomes • i.e. breaks down fatty acids, breaks down toxins • Produce hydrogen peroxide as a by-product and convert it to water

30. Microtubule Microfilaments 0.25 µm Figure 6.20 Concept 6.6: The cytoskeleton • Is a network of fibers extending throughout the cytoplasm

31. Roles of the Cytoskeleton: • Support, Motility, and Regulation • Three types • Microtubules • Microfilaments • Intermediate filaments

32. Centrosome • The centrosome • Is considered to be a “microtubule-organizing center” • Involved in cell division

33. Cilia and Flagella • Cilia and flagella • Are locomotor appendages of some cells • Contain specialized arrangements of

34. 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

35. 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

37. Microvillus Plasma membrane Microfilaments (actin filaments) Intermediate filaments 0.25 µm Microfilaments • Are found in microvilli • Are built from molecules of the protein Figure 6.26

38. Muscle Contraction

39. 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 filaments Figure 6.27 B

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

41. Concept 6.7: Extracellular components • Plant cell walls • Are made of cellulose fibers embedded in other polysaccharides and protein • Animal cells • Lack cell walls • Are covered by an extracellular matrix, ECM

42. Polysaccharide molecule EXTRACELLULAR FLUID Collagen A proteoglycan complex Carbo- hydrates Core protein Fibronectin Proteoglycan molecule Plasma membrane Integrins CYTOPLASM Micro- filaments Integrin Figure 6.29 The ECM • Is made up of glycoproteins and proteoglycans

43. Functions of the ECM include • Support • Adhesion • Movement • Regulation

44. Cell walls Interior of cell Interior of cell 0.5 µm Plasmodesmata Plasma membranes Figure 6.30 Intercellular Junctions • Are channels that connect adjacent plant cells

45. Animals: Tight Junctions, Desmosomes, and Gap Junctions • In animals, there are three types of intercellular junctions • Tight junctions • Desmosomes • Gap junctions

46. TIGHT JUNCTIONS At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continu- ous seals around the cells, tight junctions prevent leakage of extracellular fluid across A layer of epithelial cells. Tight junction Tight junctions prevent fluid from moving across a layer of cells 0.5 µm DESMOSOMES Desmosomes (also called anchoring junctions) function like rivets, fastening cells Together into strong sheets. Intermediate Filaments made of sturdy keratin proteins Anchor desmosomes in the cytoplasm. Tight junctions Intermediate filaments Desmosome Gap junctions 1 µm GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for commu- nication between cells in many types of tissues, including heart muscle and animal embryos. Extracellular matrix Space between cells Gap junction Plasma membranes of adjacent cells Figure 6.31 0.1 µm Types of intercellular junctions in animals