2 PART 1

1. 2 PART 1 Cells: The Living Units Pages 22-40

2. Introduction to Cells • Several important scientists made discoveries about cells • Robert Hooke • Matthias Schleiden and Theodor Schwann • Rudolf Virchow • Cells—the smallest living units in our bodies • Organelles—“little organs”—carry on essential functions of cells

3. Introduction to Cells • Cells have three main components • Plasma membrane—the outer boundary • Cytoplasm—contains most organelles • Nucleus—controls cellular activities

4. Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 2.1

5. The Plasma Membrane • Plasma membrane defines the extent of the cell • Structure of membrane • Fluid mosaic model (lipid bilayer) • Types of membrane proteins • Integral proteins—firmly imbedded in, or attached to lipid bilayer • Short chains of carbohydrates attach to integral proteins • Form the glycocalyx • Peripheral proteins—attach to membrane surface • Support plasma membrane from the cytoplasmic side

6. The Plasma Membrane Polar head of phospholipid molecule Extracellular fluid (watery environment) Glycolipid Cholesterol Nonpolar tail of phospholipid molecule Glycoprotein Carbohydrate of glycocalyx Bimolecular lipid layer containing proteins Outward- facing layer of phospholipids Inward-facing layer of phospholipids Integral proteins Cytoplasm (watery environment) Filament of cytoskeleton Peripheral proteins Figure 2.2

7. The Plasma Membrane • Functions – relate to location at the interface of cell’s exterior and interior • Provides barrier against substances outside cell • Some plasma membranes act as receptors • Determines which substances enter or leave the cell • Membrane is selectively permeable

8. Membrane Transport • Simple diffusion—tendency of molecules to move down their concentration gradient • Osmosis—diffusion of water molecules across a membrane

9. Figure 2.3 Membrane transport mechanisms. Extracellular fluid Water soluble solutes Water molecules Lipid- soluble solutes Solute Lipid bilayer ATP Cytoplasm Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer down their concentration gradient Osmosis, diffusion of water through the lipid bilayer Facilitated diffusionAn integral protein that spans the plasma membrane enables the passage of a particular solute across the membrane. Active transport Some transport proteins use ATP as an energy source to actively pump substances across the plasma membrane against their concentration gradient.

10. Membrane Transport Mechanisms • Facilitated diffusion—movement of molecules down their concentration gradient through an integral protein • Active transport—integral proteins move molecules across the plasma membrane against their concentration gradient

11. Endocytosis • Endocytosis • Mechanism by which particles enter cells • Phagocytosis—“cell eating” • Pinocytosis—“cell drinking”

12. Receptor-mediated Endocytosis • Receptor-mediated endocytosis • Plasma proteins bind to certain molecules • Invaginates and forms a coated pit • Pinches off to become a coated vesicle

13. Three Types of Endocytosis (b) Pinocytosis The cell “gulps” drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein- coated. (a) Phagocytosis The cell engulfs a large particle by forming pro- jecting pseudopods (”false feet”) around it and en- closing it within a membrane sac called a phagosome. The phagosome then combines with a lysosome, and its contents are digested. Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles. Vesicle Phagosome (c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of protein-coated pits, enabling the cell to ingest and concentrate specific substances in protein-coated vesicles. The ingested substance may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles. Vesicle Receptor recycled to plasma membrane Figure 2.4

14. Exocytosis • Exocytosis—a mechanism that moves substances out of the cell • Substance is enclosed in a vesicle • The vesicle migrates to the plasma membrane • The lipid layers from both membranes bind, and the vesicle releases its contents to the outside of the cell

15. Exocytosis (a) The process of exocytosis Plasma membrane SNARE (t-SNARE) Extracellular fluid 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs 1 The membrane- bound vesicle migrates to the plasma membrane. Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm Fusion pore formed 4 Vesicle contents are released to the cell exterior. 3 The vesicle and plasma membrane fuse and a pore opens up. Figure 2.5

16. The Cytoplasm • Cytoplasm—lies internal to plasma membrane • Consists of cytosol, organelles, and inclusions • Cytosol • Jelly-like fluid in which other cellular elements are suspended • Consists of water, ions, and enzymes

17. Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 2.1

18. Cytoplasmic Organelles • Ribosomes—constructed of proteins and ribosomal RNA; not surrounded by a membrane • Site of protein synthesis • Assembly of proteins is called translation • Are the “assembly line” of the manufacturing plant

19. Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 2.1

20. Cytoplasmic Organelles • Endoplasmic reticulum—“network within the cytoplasm” • Rough ER—ribosomes stud the external surfaces • Smooth ER—consists of tubules in a branching network • No ribosomes are attached; therefore no protein synthesis

21. The Endoplasmic Reticulum and Ribosomes Smooth ER Nuclear envelope Rough ER Ribosomes Cisternae (a) Diagrammatic view of smooth and rough ER (b) Electron micrograph of smooth and rough ER (85,000) Figure 2.6

22. Cytoplasmic Organelles • Golgi apparatus—a stack of three to 10 disk-shaped envelopes • Sorts products of rough ER and sends them to proper destination • Products of rough ER move through the Golgi from the convex (cis) to the concave (trans) side • Is the “packaging and shipping” division of the manufacturing plant

23. Golgi Apparatus New vesicles forming Cis face— “receiving” side of Golgi apparatus Transport vesicle from rough ER Cisternae New vesicles forming Transport vesicle from trans face Trans face— “shipping” side of Golgi apparatus Secretory vesicle Transport vesicle from the Golgi apparatus Golgi apparatus (a) Many vesicles in the process of pinching off from the membranous Golgi apparatus (b) Electron micrograph of the Golgi apparatus (90,000) Figure 2.7

24. Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 2.1

25. The sequence of events from protein synthesis on the rough ER to the final distribution of these proteins (1 of 2). Rough ER ER membrane Phagosome Plasma membrane Proteins in cisterna 1 Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi apparatus. Pathway C: Lysosome containing acid hydrolase enzymes 2 Proteins are modified within the Golgi compartments. Vesicle becomes lysosome 3 Proteins are then packaged within different vesicle types, depending on their ultimate destination. Secretory vesicle Golgi apparatus Pathway B: Vesicle membrane to be incorporated into plasma membrane Pathway A: Vesicle contents destined for exocytosis Secretion by exocytosis Extracellular fluid Figure 2.8

26. Cytoplasmic Organelles • Lysosomes—membrane-walled sacs containing digestive enzymes • Digest unwanted substances

27. Figure 2.9 Electron micrograph of lysosomes (27,000), artificially colored. Lysosomes Light green areas are regions where materials are being digested.

28. Mitochondria • Mitochondria—generate most of the cell’s energy; most complex organelle • More abundant in energy-requiring cells, like muscle cells and sperm • “Power plant” of the cell

29. Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Figure 2.1

31. Peroxisomes • Peroxisomes—membrane-walled sacs of oxidase enzymes • Enzymes neutralize free radicals and break down poisons • Break down long chains of fatty acids • Are numerous in the liver and kidneys • Are the toxic waste removal system

32. Cytoplasmic Organelles • Cytoskeleton—“cell skeleton”—an elaborate network of rods • Contains three types of rods: • Microtubules—cylindrical structures made of proteins • Microfilaments—filaments of contractile protein actin • Intermediate filaments—protein fibers

33. Cytoskeleton: Microfilaments (a) Microfilaments Strands made of spherical protein subunits called actins Actin subunit 7 nm Microfilaments form the blue network surrounding the pink nucleus in this photo. Figure 2.11a

34. Cytoskeleton: Intermediate filaments (b) Intermediate filaments Tough, insoluble protein fibers constructed like woven ropes Fibrous subunits 10 nm Intermediate filaments form the purple batlike network in this photo. Figure 2.11b

35. Cytoskeleton: Microtubules (c) Microtubules Hollow tubes of spherical protein subunits called tubulins Tubulin subunits 25 nm Microtubules appear as gold networks surrounding the cells’ pink nuclei in this photo. Figure 2.11c

36. Cytoplasmic Organelles • Centrosomes and centrioles • Centrosome—a spherical structure in the cytoplasm • Composed of centrosome matrix and centrioles • Centrioles—paired cylindrical bodies • Consists of 27 short microtubules • Act in forming cilia • Necessary for karyokinesis (nuclear division)

38. The Nucleus • The nucleus—“little nut” or “kernel”—control center of cell • DNA directs the cell’s activities • Nucleus is approximate 5µm in diameter

39. The Nucleus Surface of nuclear envelope. Fracture line of outer membrane Nuclear pores Nuclear envelope Nucleus Chromatin (condensed) Nucleolus Nuclear lamina. The netlike lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear envelope. Nuclear pore complexes. Each pore is ringed by protein particles Cisternae of rough ER (b) (a) Figure 2.13

40. The Nucleus • Nuclear envelope—two parallel membranes separated by fluid-filled space • Nuclear pores penetrate the nuclear envelope • Pores allow large molecules to pass in and out of the nucleus

41. The Nucleus • Nucleolus—“little nucleus”—in the center of the nucleus • Contains parts of several chromosomes • Site of ribosome subunit assembly

42. Chromatin and Chromosomes • DNA double helix is composed of four subunits: • Thymine (T), adenine (A), cytosine (C), and guanine (G) • DNA is packed with proteins • DNA plus the proteins form chromatin

43. Figure 2.14 Molecular structure of DNA. Hydrogen bond Nucleotides Deoxyribose sugar Sugar-phosphate backbone Phosphate Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

44. Chromatin and Chromosomes • DNA is packed with proteins • DNA plus the proteins form chromatin • Chromosomes—highest level of organization of chromatin • Contains a long molecule of DNA • 46 chromosomes are in a typical human cell

45. Figure 2.15 Chromatin and chromosome structure. 1 2 3 4 5 6 DNAdouble helix (2-nm diameter) Histones Extended chromatin structure with nucleosomes Linker DNA Nucleosome(10-nm diameter; eight histone proteins wrappted by two winds of the DNA double helix) Condensed chromatin; a tight helical fiber (30-nm diameter) Looped domain structure (300-nm diameter) Chromatid (700-nm diameter) Chromosome in metaphase (at midpoint of cell division) consists of two sister chromatids

46. The Cell Life Cycle • The cell life cycle is the series of changes a cell goes through • Interphase • G1 phase—growth 1 or Gap 1 phase • The first part of interphase • Cell metabolically active—growth—make proteins • Variable in length from hours to YEARS • Centrioles begin to replicate near the end of G1

47. The Cell Life Cycle • S (synthetic) phase—DNA replicates itself • Ensures that daughter cells receive identical copies of the genetic material (chromatin extended) • G2 phase—growth 2 or Gap 2 • Centrioles finish copying themselves • Enzymes needed for cell division are synthesized in G2

48. The Cell Life Cycle G1 checkpoint (restriction point) Interphase S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M Mitosis Prophase Metaphase Cytokinesis Telophase Anaphase Mitotic phase (M) G2 checkpoint Figure 2.16

49. The Cell Life Cycle • Cell division • M (mitotic) phase—cells divide during this stage • Follows interphase (G1, S, and G2)

50. The Cell Life Cycle • Cell division involves: • Mitosis—division of the nucleus during cell division • Chromosomes are distributed to the two daughter nuclei • Cytokinesis—division of the cytoplasm • Occurs after the nucleus divides