Chapter 4

1. Chapter 4 Eucaryotic Cell Structure and Function The key to every biological problem must finally be sought in the cell - E. B. Wilson

2. 1. An Overview of Eucaryotic Cell Structure • membrane-delimited nuclei • membrane-bound organelles that perform specific functions • more structurally complex than procaryotic cell • generally larger than procaryotic cell Figure 4.3 The structure of two representative eukaryotic cells

3. 2. The Plasma Membrane and Membrane Structure • the fluid mosaic model is based on eucaryotic membranes • major membrane lipids include phosphoglycerides, sphingolipids and cholesterol • eucaryotic membranes contain microdomains called lipid rafts • they are enriched for certain lipids and proteins • they participate in a variety of cell processes such as cell movement and transduction

4. 3. The Cytoplasmic Matrix, Microfilaments, Intermediate Filaments, and Microtubules • cytoplasmic matrix • the many organelles of eucaryotic cells lie in the cytoplasmic matrix • provides complex environment required for cellular activities • cytoskeleton • vast network of interconnected filaments within the cytoplasmic matrix • filaments that form the cytoskeleton: microfilaments (4-7 nm), microtubules (25 nm), and intermediate filaments (8-10 nm) • plays role in both cell shape and cell movement

5. Microfilaments • minute protein filaments, 4 to 7 nm in diameter • scattered within cytoplasmic matrix or organized into networks and parallel arrays • composed of actin protein • involved in cell motion and shape changes

6. Microtubules • hollow cylinders • shaped like thin cylinders ~ 25 nm in diameter • composed of two kinds of protein subunits (α & β tubulins) • maintain cell shape • involved in cell or organelle movement • participate in intracellular transport of substances

7. Intermediate filaments • heterogeneous elements of the cytoskeleton • ~10 nm in diameter • role in cell is unclear • some shown to form nuclear lamina • others help link cells together to form tissues

8. 4. Organelles of the Biosynthetic-Secretory and Endocytic Pathways • cytoplasmic matrix is permeated with an complex of membranous organelles and vesicles that move materials into the cell from the outside (endocytic) and from the cell to the outside & from location to location within the cell (biosynthetic-secretory)

9. The Endoplasmic Reticulum • irregular network of branching and fusing membranous tubules and flattened sacs (cisternae – s., cisterna)

10. Two types of endoplasmic reticulum (ER) • rough ER • ribosomes attached • synthesis of secreted proteins by ER-associated ribosomes • smooth ER • devoid of ribosomes • synthesis of lipids by ER-associated enzymes

11. Functions of ER • transports & synthesize proteins, lipids, and other materials within cell • major site of cell membrane synthesis • synthesis of lysosomes

12. The Golgi Apparatus • membranous organelle made of cisternae stacked on each other • dictyosomes: stacks of cisternae • involved in modification, packaging, and secretion of materials including proteins movement of materials

13. Lysosomes • membrane-bound vesicles • involved in intracellular digestion • contain hydrolases, enzymes which hydrolyze molecules and function best under slightly acidic conditions • maintain an acidic environment by pumping protons into their interior

14. The Biosynthetic-Secretory Pathway • used to move materials to lysosomes as well as from the inside of the cell to either the cell membrane or cell exterior • proteins synthesized by ribosomes on rough ER released in small vesicles  cis face of Golgi apparatus  trans face of Golgi apparatus • transport vesicles released from trans face of Golgi

15. The biosynthetic-secretory pathway... • after release some vesicles deliver their contents to lysosomes while others deliver to cell membrane • quality assurance mechanism • unfolded or misfolded proteins are secreted into cytosol, targeted for destruction by ubiquitin polypeptides • proteasomes destroy targeted proteins

16. Proteasomes • nonlysosomal protein degradation system by the attachment of small ubiquitins in a ATP-dependent process • observed in eucaryotes, some bacteria, and many archaea • involved in producing peptides for antigen presentation during immunological responses Figure 4.9 Proteasome degradation of proteins

17. The Endocytic Pathway • endocytosis • used to bring materials into the cell • uptake of solutes or particles by enclosing them in vesicles or vacuoles pinched off from the plasma membrane • in most cases materials are then delivered to lysosome and destroyed

18. Types of endocytosis • phagocytosis • use of cell surface protrusions to surround and engulf particles • resulting vesicles called phagosomes • clathrin-dependent endocytosis • involves membrane regions coated on cytoplasmic side with the protein clathrin (coated pits) • coated pits have external receptors that specifically bind macromolecules • pinching off of coated pits forms coated vesicles • called receptor-mediated endocytosis

19. Types of endocytosis... • caveolae–dependent endocytosis • enriched in cholesterol and the membrane protein caveolin • when caveolae pinch off membrane are called caveolar vesicles • do not deliver their contents to lysosomes • may play role in signal transduction, transport of small as well as macromolecules

20. Autophagy • delivery of materials to be digested by route that does not involve endocytosis • involves digestion and recycling of cytoplasmic components • double membrane (may be from endoplasmic reticulum) surrounds cell component forming an autophagosome • autophagosome fuses with late endosome which ultimately becomes a lysosome

21. Figure 4.10 The endocytic pathway. Materials ingested by endocytic processes are delivered to lysosomes. The pathway to lysosomes differs, depending on the type of endocytosis. In addition, cell components are recycled when autophagosomes deliver them to lysosomes for digestion. This process is called autophagy.

22. Once lysosome is formed... • digestion occurs without release of lysosome enzymes into cytoplasmic matrix • as contents are digested, products leave lysosome and can be used as nutrients • resulting lysosome called a residual body which can release contents to cell exterior by process called lysosome secretion

23. Eucaryotic Ribosomes • 80S in size • a dimer of 60S + 40S subunits • may be attached to ER or free in cytoplasm

24. Eucaryotic ribosomes… • ER-associated ribosomes • synthesize integral membrane proteins • synthesize proteins that are secreted • free ribosomes • synthesize nonsecretory proteins and nonmembrane proteins • some proteins are inserted into organelles • polyribosomes (polysomes) • complexes of mRNA with numerous ribosomes

25. Mitochondria • site of tricarboxylic acid (TCA) cycle activity • site where ATP is generated by electron transport and oxidative phosphorylation

26. Mitochondrial structure • outer membrane • contains porins similar to the outer membrane of gram negative bacteria • inner membrane • highly folded to form cristae (s., crista) • contains enzymes & electron carriers for electron transport and oxidative phosphorylation • small spheres (F1 particles = ATP synthase): synthesize ATP during cellular respiration

27. Mitochondrial structure... • matrix • contains ribosomes, mitochondrial DNA, and large calcium phosphate granules • uses its DNA & ribosomes to synthesize its own proteins • contains enzymes of the tricarboxylic acid cycle and the β-oxidation pathway for fatty acids

28. Figure 4.14 Mitochondrial structure

29. 7. Chloroplasts • type of plastid • cytoplasmic pigment (chlorophyll)-containing organelles observed in plants and algae • site of photosynthetic reactions and storage of food reserves • surrounded by double membrane

30. Chloroplast structure • double membrane system encompasses chloroplast • stroma (matrix) • contains DNA, ribosomes, lipid droplets, starch granules, and thylakoids • site of dark reactions of photosynthesis (formation of carbohydrates from water and carbon dioxide)

31. Chloroplast structure... • thylakoids • flattened, membrane-delimited sacs • grana (s., granum) – stacks of thylakoids • site of light reactions (trapping of light energy to generate ATP, NADPH & oxygen) • algal chloroplasts many contain a pyrenoid • participates in polysaccharide synthesis in algae

32. Figure 4.16 Chloroplast structure

33. 8. The Nucleus and Cell Division • nucleus • membrane-bound structure that houses genetic material of eucaryotic cell

34. Nuclear Structure • chromatin • dense fibrous material within nucleus • contains DNA • exists in a dispersed condition in non-dividing cells, but condenses to form chromosomes during cell division

35. Nuclear structure... • nuclear envelope • double membrane structure that delimits nucleus • penetrated by nuclear pores • pores allow materials to be transported into or out of nucleus

36. No Nu The Nucleolus •  1 nucleolus/nucleus • not membrane enclosed • granular & fibrillar regions • important in ribosome synthesis • directs synthesis and processing of rRNA • directs assembly of rRNA and ribosomal proteins to form ribosomes 20× 40×

37. Mitosis and Meiosis • mitosis • one component of cell cycle • the process of nuclear division and chromosome distribution in eucaryotic cells • distributes DNA to 2 new nuclei • ploidy (number of sets of chromosomes) of daughter cells is the same as the mother cell • after mitosis, a diploid organism remains diploid

38. period of cell growth G2: preparation for mitosis and cell division synthesis period (S): rapid synthesis & doubling of nuclear DNA and histones gap 1 period (G1): synthesis of RNA, ribosomes, and cytoplasmic constituents Figure 4.17 The eucaryotic cell cycle

39. Mitosis and meiosis... • meiosis • complex, two-stage process of nuclear division • number of chromosomes in the resulting daughter cells is reduced by 1/2: diploid  haploid • haploid cells act as gametes and fuse to reform diploid organisms

40. Figure 4.19 Generalized eucaryotic life cycle

41. 9. External Cell Coverings • cell wall • rigid covering • variable make-up • algae cell walls contain cellulose, pectin, or silica ... • fungal cell walls – chitin, cellulose, or glucan • pellicle • relatively rigid layer of components just beneath plasma membrane • common in protozoa and some alage • not as strong or rigid as cell wall • provides characteristic shape to cell

42. 10. Cilia and Flagella • cilia (s., cilium) • 5-20 μm long • beat with two phases, working like oars • flagella (s., flagellum) • 100-200 μm long • move in undulating fashion (helical waves)

43. 11. Comparison of Procaryotic and Eucaryotic Cells Figure 4.24 Comparison of procaryotic and eucayotic cell structure

45. The molecular unity of procaryotes and eucaryotes • same basic chemical composition • same genetic code • same basic metabolic processes