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Lecture 10(2) - Lecture 10 Eukes Cells the...

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Unformatted text preview: Lecture 10 Eukes Cells the Fundamental Units of Life The plasma membrane: •  Is a selec%vely permeable barrier •  Allows cells to maintain a constant internal environment •  Is important in communica%on and receiving signals •  OCen has proteins for binding and adhering to adjacent cells Cells the Fundamental Units of Life Two types of cells: Prokaryo)c and eukaryo)c Bacteria and Archaea are prokaryotes. The first cells were probably prokaryoIc. Eukarya are eukaryotes̶the DNA is in a membrane ­enclosed compartment called the nucleus. EukaryoIc Cells EukaryoIc cells are up to ten Imes larger than prokaryotes. EukaryoIc cells have membrane ­enclosed compartments called organelles. Each organelle has a specific role in cell funcIoning. EukaryoIc Cells Inner life of the cell Figure 5.7 EukaryoIc Cells (Part 1) Figure 5.7 EukaryoIc Cells (Part 2) Figure 5.7 EukaryoIc Cells (Part 3) Figure 5.7 EukaryoIc Cells (Part 4) EukaryoIc Cells The nucleus is usually the largest organelle. •  Contains the DNA •  Site of DNA replicaIon •  Site where gene transcripIon is turned on or off •  Assembly of ribosomes begins in a region called the nucleolus Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 1) EukaryoIc Cells The nucleus is surrounded by two membranes—the nuclear envelope. Nuclear pores in the envelope control movement of molecules between nucleus and cytoplasm. Figure 5.8 The Nucleus Is Enclosed by a Double Membrane (Part 2) EukaryoIc Cells Some large molecules (e.g., proteins) must have a certain amino acid sequence known as a nuclear localiza%on signal (NLS) to cross the nuclear envelope. EukaryoIc Cells In the nucleus, DNA combines with proteins to form chroma)n in long, thin threads called chromosomes. Before cell division, chromaIn condenses, and individual chromosomes are visible in the light microscope. Figure 5.9 ChromaIn and Chromosomes EukaryoIc Cells Nucleoplasm surrounds the chromaIn, and a network of structural proteins (nuclear matrix) helps organize the chromaIn. The nuclear lamina a^aches to both the chromaIn and the nuclear envelope and maintains nuclear shape. EukaryoIc Cells The endomembrane system includes the plasma membrane, nuclear envelope, endoplasmic reIculum, Golgi apparatus, and lysosomes. Tiny, membrane ­surrounded vesicles shu^le substances between the various components. EukaryoIc Cells Endoplasmic re)culum (ER): network of interconnected membranes in the cytoplasm; has large surface area. Rough endoplasmic re)culum (RER): ribosomes are a^ached. Newly made proteins enter the RER lumen where they are modified, folded, and transported to other regions. EukaryoIc Cells Smooth endoplasmic re)culum (SER): more tubular, no ribosomes •  Chemically modifies small molecules such as drugs and pesIcides •  Hydrolysis of glycogen in animal cells •  Synthesis of lipids and steroids EukaryoIc Cells The Golgi apparatus is composed of fla^ened sacs (cisternae) and small membrane ­enclosed vesicles. •  Receives proteins from the RER—can further modify them •  Concentrates, packages, sorts proteins •  In plant cells, polysaccharides for cell walls are synthesized here EukaryoIc Cells The cis region receives vesicles (a piece of the ER that “buds” off) from the ER. At the trans region, vesicles bud off from the Golgi apparatus and are moved to the plasma membrane or other organelles. Figure 5.10 The Endomembrane System (Part 1) Figure 5.10 The Endomembrane System (Part 2) EukaryoIc Cells Primary lysosomes originate from the Golgi apparatus. They contain digesIve enzymes— macromolecules are hydrolyzed into monomers. EukaryoIc Cells Food molecules enter the cell by phagocytosis—a phagosome is formed. Phagosomes fuse with primary lysosomes to form secondary lysosomes. Enzymes in the secondary lysosome hydrolyze the food molecules. Figure 5.11 ysosomes Isolate DigesIve Enzymes from the Cytoplasm (Part 1) Figue 5.11 Lysosomes Isolate DigesIve Enzymes from the Cytoplasm (Part 2) EukaryoIc Cells Lysosomes also digest cell materials (autophagy). Cell components are frequently destroyed and replaced by new ones. EukaryoIc Cells In the mitochondria, energy in fuel molecules is transformed to the bonds of energy ­rich ATP: Cellular respira%on. Cells that require a lot of energy have a lot of mitochondria. EukaryoIc Cells Mitochondria have two membranes. The inner membrane folds inward to form cristae. This creates a large surface area for proteins involved in cellular respiraIon reacIons. The mitochondrial matrix contains enzymes, DNA, and ribosomes. Figure 5.12 A Mitochondrion Converts Energy from Fuel Molecules into ATP EukaryoIc Cells Plas)ds occur only in plants and some proIsts. Any plant organelle that house photosyntheIc biochemical pathways Chloroplasts: Site of photosynthesis—light energy is converted to the energy of chemical bonds Chloroplasts have a double membrane. EukaryoIc Cells Grana are stacks of thylakoids—made of circular compartments of the inner membrane. Thylakoids contain chlorophyll and other pigments that harvest light energy for photosynthesis. Stroma—fluid in which grana are suspended. The stroma contains DNA and ribosomes. Figure 5.13 Chloroplasts Feed the World Figure 5.14 Chloroplasts Are Everywhere EukaryoIc Cells Other plasIds: Chromoplasts contain red, orange, and yellow pigments—gives color to flowers. Leucoplasts store starches and fats. Figure 5.15 Chromoplasts and Leucoplasts EukaryoIc Cells Peroxisomes: Collect and break down toxic byproducts of metabolism such as H2O2, using specialized enzymes Glyoxysomes: Only in plants—lipids are converted to carbohydrates for growth EukaryoIc Cells Plant and proIst cells have vacuoles: •  Store waste products and toxic compounds; some may deter herbivores •  Provide structure for plant cells̶water enters the vacuole by osmosis, creaIng turgor pressure. EukaryoIc Cells Vacuoles (conInued): •  Store anthocyanins (pink and blue pigments) in flowers and fruits; the colors a^ract pollinators •  Vacuoles in seeds have digesIve enzymes to hydrolyze stored food for early growth Figure 5.16 Vacuoles in Plant Cells Are Usually Large EukaryoIc Cells Freshwater proIsts may have contrac3le vacuoles to expel excess water. The vacuoles take in excess water that enters the cell by osmosis; then expel it by contracIng, forcing water out through a pore. EukaryoIc Cells The cytoskeleton: •  •  •  •  •  Supports and maintains cell shape Holds organelles in posiIon Moves organelles Involved in cytoplasmic streaming Interacts with extracellular structures to hold cell in place EukaryoIc Cells The cytoskeleton has three components: •  Microfilaments •  Intermediate filaments •  Microtubules Figure 5.17 The Cytoskeleton EukaryoIc Cells Microfilaments: •  Help a cell or parts of a cell to move •  Determine cell shape •  Made from the protein ac3n •  AcIn has + and – ends and polymerizes to form long helical chains (reversible) EukaryoIc Cells In muscle cells, acIn filaments are associated with the “motor protein” myosin; interacIons between the two result in muscle contracIon. Microfilaments are also involved in the formaIon of pseudopodia (pseudo, “false”; podia, “feet”). Figure 5.18 Microfilaments and Cell Movements EukaryoIc Cells In some cells, microfilaments form a meshwork just inside the plasma membrane. This provides structure, for example in the microvilli that line the human intesIne. Figure 5.19 Microfilaments for Support EukaryoIc Cells Intermediate filaments: •  Many different kinds in six molecular classes •  Tough, ropelike protein assemblages •  Anchor cell structures in place •  Resist tension EukaryoIc Cells Microtubules: •  Form rigid internal skeleton in some cells •  Act as a framework for motor proteins •  Made from the protein tubulin—a dimer •  Have + and – ends •  Can change length rapidly by adding or losing dimers EukaryoIc Cells Cilia and eukaryoIc flagella are made of microtubules in “9 + 2” array. •  Cilia—short, usually many present, move with sIff power stroke and flexible recovery stroke •  Flagella—longer, usually one or two present, movement is snakelike Figure 5.20 Cilia EukaryoIc Cells Centrioles are idenIcal to basal bodies. Involved in formaIon of the mitoIc spindle ̶to which chromosomes a^ach during cell division. EukaryoIc Cells Motor proteins: undergo reversible shape changes powered by ATP hydrolysis. Dynein binds to microtubule doublets and allows them to slide past each other. Nexin can cross ­link the doublets and prevent them from sliding, and the cilium bends. Figure 5.21 A Motor Protein Moves Microtubules in Cilia and Flagella EukaryoIc Cells The motor protein kinesin binds to a vesicle and “walks” it along by changing shape. Figure 5.22 A Motor Protein Drives Vesicles along Microtubules EukaryoIc Cells Experiments to determine the funcIon of cellular components fall into two categories: Inhibi%on: A drug that inhibits a structure or process̶does the funcIon sIll occur? Muta%on: Examine a cell that lacks the gene for the structure or process Figure 5.23 The Role of Microfilaments in Cell Movement— Showing Cause and Effect in Biology Roles of Extracellular Structures Extracellular structures are secreted to the outside of the plasma membrane. Example: The pepIdoglycan cell wall of bacteria. In eukaryotes, extracellular structures have a prominent fibrous macromolecule in a gel ­ like medium. Roles of Extracellular Structures Plant cell walls: Cellulose fibers are embedded in other complex polysaccharides and proteins. Adjacent plant cells are connected by plasma membrane ­lined channels called plasmodesmata. Figure 5.24 The Plant Cell Wall Roles of Extracellular Structures Many animal cells are surrounded by an extracellular matrix, composed of fibrous proteins such as collagen, gel ­ like proteoglycans (glycoproteins), and other proteins. Figure 5.25 An Extracellular Matrix Roles of Extracellular Structures The extracellular matrix: •  Holds cells together in Issues •  Contributes to properIes of bone, carIlage, skin, etc. •  Filters materials passing between different Issues •  Orients cell movements in development and Issue repair •  Plays a role in chemical signaling How Did EukaryoIc Cells Originate? The endomembrane system and cell nucleus may have originated from the inward folds of plasma membrane of prokaryotes. Enclosed compartments would have allowed chemicals to be concentrated and chemical reacIons to proceed more efficiently. Figure 5.26 The Origin of Organelles (A) How Did EukaryoIc Cells Originate? Some organelles may have arose by symbiosis (“living together”). The endosymbiosis theory proposes that mitochondria and plasIds arose when one cell engulfed another cell. Figure 5.26 The Origin of Organelles (B) How Did EukaryoIc Cells Originate? Support for the endosymbiosis theory: The discovery of a a single ­celled eukaryote, Hatena, that ingests a green alga, Nephroselmis The green alga loses most of its structures and acts as a chloroplast Figure 5.27 Endosymbiosis in AcIon ...
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