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1. 3 Cells: The Living Units: Part A

2. Erythrocytes Fibroblasts Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Nerve cell Skeletal Muscle cell (e) Cell that gathers information and control body functions Smooth muscle cells (b) Cells that move organs and body parts Sperm Macrophage (f) Cell of reproduction Fat cell (c) Cell that storesnutrients (d) Cell that fights disease Figure 3.1

3. Nuclear envelope Chromatin Nucleolus Nucleus Smooth endoplasmic reticulum Plasma membrane Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements • Microtubule • Intermediate filaments Peroxisome Figure 3.2

4. Outer mitochondrial membrane Ribosome Mitochondrial DNA Inner mitochondrial membrane (a) Cristae Matrix (c) Enzymes (b) Figure 3.17

5. Smooth ER Nuclear envelope Rough ER Ribosomes (a) Diagrammatic view of smooth and rough ER Figure 3.18a

6. 1 2 The mRNA-ribosome complex isdirected to the rough ER by the SRP.There the SRP binds to a receptor site. Once attached to the ER, the SRP is releasedand the growing polypeptide snakes through theER membrane pore into the cisterna. ER signalsequence 3 The signal sequence is clipped off by anenzyme. As protein synthesis continues, sugargroups may be added to the protein. Ribosome mRNA 4 In this example, the completedprotein is released from the ribosomeand folds into its 3-D conformation,a process aided by molecular chaperones. Signalrecognitionparticle(SRP) Signalsequenceremoved Receptor site 5 The protein is enclosed within aprotein (coatomer)-coated transportvesicle. The transport vesicles maketheir way to the Golgi apparatus,where further processing of theproteins occurs (see Figure 3.19). Growingpolypeptide Sugargroup Releasedprotein Rough ER cisterna Coatomer-coatedtransport vesicle Transport vesiclepinching off Cytoplasm Figure 3.39

7. 1 Protein- containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi apparatus. Rough ER Phagosome ER membrane Plasma mem- brane Proteins in cisterna 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 Pathway B: Vesicle membrane to be incorporated into plasma membrane Golgi apparatus Pathway A: Vesicle contents destined for exocytosis Secretion by exocytosis Extracellular fluid Figure 3.20

8. Coated pit ingestssubstance. 1 Extracellular fluid Plasmamembrane Protein coat(typicallyclathrin) Cytoplasm 2 Protein-coatedvesicledetaches. 3 Coat proteins detachand are recycled toplasma membrane. Transportvesicle Endosome Uncoatedendocytic vesicle 4 Uncoated vesicle fuseswith a sorting vesiclecalled an endosome. 5 Transportvesicle containingmembrane componentsmoves to the plasmamembrane for recycling. Lysosome 6 Fused vesicle may (a) fusewith lysosome for digestionof its contents, or (b) deliverits contents to the plasmamembrane on theopposite side of the cell(transcytosis). (b) (a) Figure 3.12

9. Nuclear envelope Nucleus Smooth ER Rough ER Vesicle Golgi apparatus Plasma membrane Transport vesicle Lysosome Figure 3.22

10. (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 3.23a

11. (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 3.23b

12. (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 3.23c

13. Vesicle ATP Receptor for motor molecule Motor molecule (ATP powered) Microtubule of cytoskeleton (a) Motor molecules can attach to receptors onvesicles or organelles, and “walk” the organellesalong the microtubules of the cytoskeleton. ATP Motor molecule (ATP powered) Cytoskeletal elements (microtubules or microfilaments) (b) In some types of cell motility, motor molecules attached to oneelement of the cytoskeleton can cause it to slide over anotherelement, as in muscle contraction and cilia movement. Figure 3.24

14. Outer microtubule doublet Dynein arms The doublets also have attached motor proteins, the dynein arms. Central microtubule Cross-linking proteins inside outer doublets The outer microtubule doublets and the two central microtubules are held together by cross-linking proteins and radial spokes. Radial spoke TEM A cross section through the cilium shows the “9 + 2” arrangement of microtubules. Microtubules Cross-linking proteins inside outer doublets Radial spoke Plasma membrane Plasma membrane Cilium Triplet Basal body TEM TEM Basal body (centriole) A longitudinal section of a cilium shows microtubules running the length of the structure. A cross section through the basal body. The nine outer doublets of a cilium extend into a basal body where each doublet joins another microtubule to form a ring of nine triplets. Figure 3.26

15. Power, or propulsive, stroke Recovery stroke, when cilium is returning to its initial position 1 2 3 4 5 6 7 (a) Phases of ciliary motion. Layer of mucus Cell surface (b) Traveling wave created by the activity ofmany cilia acting together propels mucusacross cell surfaces. Figure 3.27

16. Nuclear pores Nuclear envelope Nucleus Chromatin (condensed) Nucleolus Cisternae of rough ER (a) Figure 3.29a

17. 1 DNA double helix (2-nm diameter) Histones 2 Chromatin (“beads on a string”) structure with nucleosomes Linker DNA Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double helix) (a) 3 Tight helical fiber (30-nm diameter) 4 Looped domain structure (300-nm diameter) 5 Chromatid (700-nm diameter) Metaphase chromosome (at midpoint of cell division) (b) Figure 3.30

18. Extracellular fluid (watery environment) Cholesterol Polar head of phospholipid molecule Glycolipid Glycoprotein Carbohydrate of glycocalyx Outward- facing layer of phospholipids Integral proteins Filament of cytoskeleton Peripheral proteins Bimolecular lipid layer containing proteins Inward-facing layer of phospholipids Nonpolar tail of phospholipid molecule Cytoplasm (watery environment) Figure 3.3

19. (a) Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Figure 3.4a

20. (b) Receptors for signal transduction Signal A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. Receptor Figure 3.4b

21. (c) Attachment to the cytoskeleton and extracellular matrix (ECM) Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Figure 3.4c

22. (d) Enzymatic activity Enzymes A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. Figure 3.4d

23. (e) Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. CAMs Figure 3.4e

24. (f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Figure 3.4f

25. Microvilli Plasma membranes of adjacent cells Intercellular space Basement membrane Interlocking junctional proteins Intercellular space (a) Tight junctions:Impermeable junctions prevent molecules from passing through the intercellular space. Figure 3.5a

26. Microvilli Plasma membranes of adjacent cells Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Linker glycoproteins (cadherins) (b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers. Figure 3.5b

27. Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (connexon) (c) Gap junctions: Communicating junctions allow ions and small mole- cules to pass from one cell to the next for intercellular communication. Figure 3.5c

28. Extracellular fluid Lipid- soluble solutes Cytoplasm (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer Figure 3.7a

29. Water molecules Lipid billayer Aquaporin (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7d

30. (a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged Left compartment: Solution with lower osmolarity H2O Solute Solute molecules (sugar) Membrane Figure 3.8a

31. (b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move Left compartment Right compartment H2O Solute molecules (sugar) Membrane Figure 3.8b

32. (a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9

33. Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7b

34. Small lipid- insoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7c

35. Extracellular fluid Na+ Na+-K+ pump Na+ bound K+ ATP-binding site Cytoplasm 1 Cytoplasmic Na+ binds to pump protein. P ATP K+ released ADP 6 2 K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats. Binding of Na+ promotesphosphorylation of the protein by ATP. Na+ released K+ bound P Pi K+ 5 3 K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation. Phosphorylation causes the protein tochange shape, expelling Na+ to the outside. P 4 Extracellular K+ binds to pump protein. Figure 3.10

36. Endocytosis • Fluid-phase endocytosis (pinocytosis)—plasma membrane infolds, bringing extracellular fluid and solutes into interior of the cell • Nutrient absorption in the small intestine

37. The process of exocytosis Plasma membrane SNARE (t-SNARE) Extracellular fluid Fusion pore formed 1 The membrane- bound vesicle migrates to the plasma membrane. Secretory vesicle Vesicle SNARE (v-SNARE) 3 The vesicle and plasma membrane fuse and a pore opens up. Molecule to be secreted Cytoplasm 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). 4 Vesicle contents are released to the cell exterior. Fused v- and t-SNAREs Figure 3.14a

38. (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 is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein- coated but has receptors capable of binding to microorganisms or solid particles. Phagosome Figure 3.13a

39. (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. Vesicle Figure 3.13b

40. (c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of coated pits, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands 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 3.13c

41. Old strand acts as a template for synthesis of new strand DNA polymerase Free nucleotides Chromosome Leading strand Two new strands (leading and lagging) synthesized in opposite directions Lagging strand Old DNA Helicase unwinds the double helix and exposes the bases Replication fork Adenine Thymine Cytosine DNA polymerase Old (template) strand Guanine Figure 3.32

42. G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

43. Nuclear envelope DNA Transcription RNA Processing Pre-mRNA mRNA Nuclear pores Ribosome Translation Polypeptide Figure 3.34

44. RNA polymerase Coding strand DNA Terminationsignal Promoterregion Template strand 1 Initiation: With the help of transcription factors, RNApolymerase binds to the promoter, pries apart the two DNA strands,and initiates mRNA synthesis at the start point on the template strand. Template strand mRNA Coding strand of DNA Rewindingof DNA Unwindingof DNA 2 Elongation: As the RNA polymerase moves along the templatestrand, elongating the mRNA transcript one base at a time, it unwindsthe DNA double helix before it and rewinds the double helix behind it. RNA nucleotides Direction oftranscription Templatestrand mRNA transcript DNA-RNA hybrid region mRNA RNApolymerase 3 Termination: mRNA synthesis ends when the termination signalis reached. RNA polymerase and the completed mRNA transcript arereleased. The DNA-RNA hybrid: At any given moment, 16–18 base pairs ofDNA are unwound and the most recently made RNA is still bound toDNA. This small region is called the DNA-RNA hybrid. Completed mRNA transcript RNA polymerase Figure 3.35

45. RNA polymerase Coding strand DNA Template strand Terminationsignal Promoterregion 1 Initiation: With the help of transcription factors, RNApolymerase binds to the promoter, pries apart the two DNA strands,and initiates mRNA synthesis at the start point on the template strand. Figure 3.35 step 1

46. Template strand mRNA 2 Elongation: As the RNA polymerase moves along the templatestrand, elongating the mRNA transcript one base at a time, it unwindsthe DNA double helix before it and rewinds the double helix behind it. mRNA transcript Figure 3.35 step 2

47. 3 Termination: mRNA synthesis ends when the termination signalis reached. RNA polymerase and the completed mRNA transcript arereleased. RNApolymerase Completed mRNA transcript Figure 3.35 step 3

48. Coding strand of DNA Rewindingof DNA Unwindingof DNA RNA nucleotides Direction oftranscription Templatestrand DNA-RNA hybrid region mRNA RNApolymerase The DNA-RNA hybrid: At any given moment, 16–18 base pairsof DNA are unwound and the most recently made RNA is stillbound to DNA. This small region is called the DNA-RNA hybrid. Figure 3.35 step 4

49. Energized by ATP, the correct amino acid is attached to each species of tRNA by aminoacyl- tRNA synthetase enzyme. Nucleus RNA polymerase mRNA Template strand of DNA Leu Amino acid 1 After mRNA synthesis in the nucleus, mRNA leaves the nucleus and attaches to a ribosome. Nuclear pore tRNA Nuclear membrane A G A Released mRNA Aminoacyl-tRNA synthetase Figure 3.37 step 1

50. SECOND BASE U C A G U UUU UCU UAU UGU Tyr Cys Phe C UUC UCC UAC UGC U Ser A UUA UCA UAA Stop UGA Stop Leu G UUG UCG UAG Stop UGG Trp U CUU CCU CAU CGU His C CUC CCC CAC CGC C Leu Pro Arg A CUA CCA CAA CGA Gln G CUG CCG CAG CGG U AUU ACU AAU AGU Asn Ser C Ile AUC ACC AAC AGC A Thr A AUA ACA AAA AGA Lys Arg Met or G AUG ACG AAG AGG Start U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly A GUA GCA GAA GGA Glu G GUG GCG GAG GGG Figure 3.36