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Chapter 3: The Cellular Level of Organization
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KEY CONCEPT Cells: basic structural and functional units of life –respond to their environment –maintain homeostasis at the cellular level –modify structure and function over time
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Figure 3–1 The Cell Performs all life functions
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Sex Cells Sex cells (germ cells): –reproductive cells –male sperm –female oocytes (eggs)
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Somatic Cells Somatic cells (soma = body): –all body cells except sex cells
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Functions of Cell Membrane (1 of 2) Physical isolation: –barrier Regulates exchange with environment: –ions and nutrients enter –waste and cellular products released
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Functions of Cell Membrane (2 of 2) Monitors the environment: –extracellular fluid composition –chemical signals Structural support: –anchors cells and tissues
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The Cell Membrane Contains lipids, carbohydrates, and functional proteins
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Phospholipid Bilayer Double layer of phospholipid molecules: –hydrophilic heads—toward watery environment, both sides –hydrophobic fatty-acid tails—inside membrane –barrier to ions and water soluble compounds
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Membrane Proteins Integral proteins: –within the membrane Peripheral proteins: –inner or outer surface of the membrane
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6 Functions of Membrane Proteins (1 of 2) 1.Anchoring proteins (stabilizers): – attach to inside or outside structures 2.Recognition proteins (identifiers): – label cells normal or abnormal 3.Enzymes: –catalyze reactions
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6 Functions of Membrane Proteins (2 of 2) 4.Receptor proteins: – bind and respond to ligands (ions, hormones) 5.Carrier proteins: – transport specific solutes through membrane 6.Channels: –regulate water flow and solutes through membrane
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Functions of Membrane Carbohydrates Lubrication and protection Anchoring and locomotion Specificity in binding (receptors) Recognition (immune response)
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Cytoplasm All materials inside the cell and outside the nucleus: –cytosol (fluid): dissolved materials: –nutrients, ions, proteins, and waste products –organelles: structures with specific functions
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Types of Organelles Nonmembranous organelles: –no membrane –direct contact with cytosol Membranous organelles: –covered with plasma membrane –isolated from cytosol
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Nonmembranous Organelles 6 types of nonmembranous organelles: –cytoskeleton –microvilli –centrioles –cilia –ribosomes –proteasomes
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Figure 3–3a The Cytoskeleton Structural proteins for shape and strength
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Microfilaments Thin filaments composed of the protein actin: –provide additional mechanical strength –interact with proteins for consistency –Pairs with thick filaments of myosin for muscle movement
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Microtubules Large, hollow tubes of tubulin protein: –attach to centrosome –strengthen cell and anchor organelles –change cell shape –move vesicles within cell (kinesin and dynein) –form spindle apparatus
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Figure 3–3b Microvilli Increase surface area for absorption Attach to cytoskeleton
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Centrioles in the Centrosome Centrioles form spindle apparatus during cell division Centrosome: cytoplasm surrounding centriole Figure 3–4a
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Cilia Power Cilia move fluids across the cell surface Figure 3–4b,c
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Ribosomes Build polypeptides in protein synthesis Two types: –free ribosomes in cytoplasm: proteins for cell –fixed ribosomes attached to ER: proteins for secretion
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Proteasomes Contain enzymes (proteases) Disassemble damaged proteins for recycling
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Membranous Organelles 5 types of membranous organelles: –endoplasmic reticulum (ER) –Golgi apparatus –lysosomes –peroxisomes –mitochondria
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Endoplasmic Reticulum (ER) endo = within, plasm = cytoplasm, reticulum = network Cisternae are storage chambers within membranes
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Functions of ER Synthesis of proteins, carbohydrates, and lipids Storage of synthesized molecules and materials Transport of materials within the ER Detoxification of drugs or toxins
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Smooth Endoplasmic Reticulum (SER) No ribosomes attached Synthesizes lipids and carbohydrates: –phospholipids and cholesterol (membranes) –steroid hormones (reproductive system) –glycerides (storage in liver and fat cells) –glycogen (storage in muscles)
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Rough Endoplasmic Reticulum (RER) Surface covered with ribosomes: –active in protein and glycoprotein synthesis –folds polypeptides protein structures –encloses products in transport vesicles
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Golgi Apparatus Vesicles enter forming face and exit maturing face
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Vesicles of the Golgi Apparatus Secretory vesicles: –modify and package products for exocytosis Membrane renewal vesicles: –add or remove membrane components Lysosomes: –carry enzymes to cytosol
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Transport Vesicles Figure 3–7a Carry materials to and from Golgi apparatus
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Figure 3–7b Exocytosis Ejects secretory products and wastes
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Lysosomes Figure 3–8 Powerful enzyme-containing vesicles: –lyso = dissolve, soma = body
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Lysosome Structures Primary lysosome: –formed by Golgi and inactive enzymes Secondary lysosome: –lysosome fused with damaged organelle –digestive enzymes activated –toxic chemicals isolated
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Lysosome Functions Clean up inside cells: –break down large molecules –attack bacteria –recycle damaged organelles –ejects wastes by exocytosis
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Autolysis Self-destruction of damaged cells: –auto = self, lysis = break –lysosome membranes break down –digestive enzymes released –cell decomposes –cellular materials recycle
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Mitochondrion Structure Have smooth outer membrane and folded inner membrane (cristae) Matrix: –fluid around cristae
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Mitochondrial Function Mitochondrion takes chemical energy from food (glucose): –produces energy molecule ATP Figure 3–9b
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KEY CONCEPT Mitochondria provide cells with energy for life: –require oxygen and organic substrates –generate carbon dioxide and ATP
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Figure 3–10a The Nucleus Is the cell’s control center
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Structure of the Nucleus Nucleus: –largest organelle Nuclear envelope: –double membrane around the nucleus Perinuclear space: –between 2 layers of nuclear envelope Nuclear pores: –communication passages
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Within the Nucleus DNA: –all information to build and run organisms Nucleoplasm: –fluid containing ions, enzymes, nucleotides, and some RNA Nuclear matrix: –support filaments
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Nucleoli in Nucleus Are related to protein production Are made of RNA, enzymes, and histones Synthesize rRNA and ribosomal subunits
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Organization of DNA Nucleosomes: –DNA coiled around histones Chromatin: –loosely coiled DNA (cells not dividing) Chromosomes: –tightly coiled DNA (cells dividing)
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DNA and Genes DNA: –instructions for every protein in the body Gene: –DNA instructions for 1 protein
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Genetic Code The chemical language of DNA instructions: –sequence of bases (A, T, C, G) –triplet code: 3 bases = 1 amino acid
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KEY CONCEPT The nucleus contains chromosomes Chromosomes contain DNA DNA stores genetic instructions for proteins Proteins determine cell structure and function
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Protein Synthesis Transcription: –copies instructions from DNA to mRNA (in nucleus) Translation: –ribosome reads code from mRNA (in cytoplasm) –assembles amino acids into polypeptide chain
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Protein Synthesis Processing: –by RER and Golgi apparatus produces protein
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mRNA Transcription A gene is transcribed to mRNA in 3 steps: –gene activation –DNA to mRNA –RNA processing
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Step 1: Gene Activation Uncoils DNA, removes histones Start (promoter) and stop codes on DNA mark location of gene: –coding strand is code for protein –template strand used by RNA polymerase molecule
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Step 2: DNA to mRNA Enzyme RNA polymerase transcribes DNA: –binds to promoter (start) sequence –reads DNA code for gene –binds nucleotides to form messenger RNA (mRNA) –mRNA duplicates DNA coding strand, uracil replaces thymine
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Step 3: RNA Processing At stop signal, mRNA detaches from DNA molecule: –code is edited (RNA processing) –unnecessary codes (introns) removed –good codes (exons) spliced together –triplet of 3 nucleotides (codon) represents one amino acid
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Translation (1 of 6) mRNA moves: –from the nucleus –through a nuclear pore Figure 3–13
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Translation (2 of 6) mRNA moves: –to a ribosome in cytoplasm –surrounded by amino acids Figure 3–13 (Step 1)
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Translation (3 of 6) mRNA binds to ribosomal subunits tRNA delivers amino acids to mRNA Figure 3–13 (Step 2)
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Translation (4 of 6) tRNA anticodon binds to mRNA codon 1 mRNA codon translates to 1 amino acid Figure 3–13 (Step 3)
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Figure 3–13 (Step 4) Translation (5 of 6) Enzymes join amino acids with peptide bonds Polypeptide chain has specific sequence of amino acids
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Figure 3–13 (Step 5) Translation (6 of 6) At stop codon, components separate
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Nucleus Controls Cell Structure and Function Direct control through synthesis of: –structural proteins –secretions (environmental response) Indirect control over metabolism through enzymes
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KEY CONCEPT Genes: –are functional units of DNA –contain instructions for 1 or more proteins Protein synthesis requires: –several enzymes –ribosomes –3 types of RNA
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KEY CONCEPT Mutation is a change in the nucleotide sequence of a gene: –can change gene function Causes: –exposure to chemicals –exposure to radiation –mistakes during DNA replication
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Overcoming the Cell Barrier The cell membrane is a barrier, but: –nutrients must get in –products and wastes must get out
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Permeability Permeability determines what moves in and out of a cell: A membrane that: –lets nothing in or out is impermeable –lets anything pass is freely permeable –restricts movement is selectively permeable
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Selective Permeability Cell membrane is selectively permeable: –allows some materials to move freely –restricts other materials
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Restricted Materials Selective permeability restricts materials based on: –size –electrical charge –molecular shape –lipid solubility
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Transport Transport through a cell membrane can be: –active (requiring energy and ATP) –passive (no energy required)
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3 Categories of Transport Diffusion (passive) Carrier-mediated transport (passive or active) Vesicular transport (active)
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Solutions All molecules are constantly in motion Molecules in solution move randomly Random motion causes mixing
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Concentration Gradient Concentration is the amount of solute in a solvent Concentration gradient: –more solute in 1 part of a solvent than another
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Function of Concentration Gradient Diffusion: –molecules mix randomly –solute spreads through solvent –eliminates concentration gradient
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Diffusion Solutes move down a concentration gradient
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Factors Affecting Diffusion Rates Distance the particle has to move Molecule size: –smaller is faster Temperature: –more heat, faster motion
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Diffusion and the Cell Membrane Figure 3–15 Diffusion can be simple or channel- mediated
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Simple Diffusion Materials which diffuse through cell membrane: –lipid-soluble compounds (alcohols, fatty acids, and steroids) –dissolved gases (oxygen and carbon dioxide)
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Channel-Mediated Diffusion Materials which pass through transmembrane proteins (channels): –are water soluble compounds –are ions
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Osmosis Figure 3–16 Osmosis is the diffusion of water across the cell membrane
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How Osmosis Works More solute molecules, lower concentration of water molecules Membrane must be freely permeable to water, selectively permeable to solutes
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Osmosis Water Movement Water molecules diffuse across membrane toward solution with more solutes Volume increases on the side with more solutes
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Osmotic Pressure Is the force of a concentration gradient of water Equals the force (hydrostatic pressure) needed to block osmosis
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Isotonic Solutions A solution that does not cause osmotic flow of water in or out of a cell iso = same, tonos = tension
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Hypotonic Solutions hypo = below Has less solutes Loses water through osmosis
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Cells and Hypotonic Solutions A cell in a hypotonic solution: –gains water –ruptures (hemolysis of red blood cells) Figure 3–17b
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Hypertonic Solutions hyper = above Has more solutes Gains water by osmosis
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Cells and Hypertonic Solutions A cell in a hypertonic solution: –loses water –shrinks (crenation of red blood cells) Figure 3–17c
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KEY CONCEPT (1 of 2) Concentration gradients tend to even out In the absence of membrane, diffusion eliminates concentration gradients
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KEY CONCEPT (2 of 2) When different solute concentrations exist on either side of a selectively permeable membrane, osmosis moves water through the membrane to equalize the concentration gradients
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Facilitated Diffusion Passive Carrier mediated Membrane Transport: Facilitated Diffusion PLAY Figure 3–18
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How Facilitated Diffusion Works Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids): –molecule binds to receptor site on carrier protein –protein changes shape, molecules pass through –receptor site is specific to certain molecules
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Active Transport Active transport proteins: –move substrates against concentration gradient –require energy, such as ATP –ion pumps move ions (Na +, K +, Ca +, Mg 2 + ) –exchange pump countertransports 2 ions at the same time
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Figure 3–22a Pinocytosis Pinocytosis (cell drinking) Endosomes “drink” extracellular fluid
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Phagocytosis Phagocytosis (cell eating) –pseudopodia (psuedo = false, podia = feet) –engulf large objects in phagosomes Figure 3–22b
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Figure 3–7b Exocytosis Is the reverse of endocytosis
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Cell Life Cycle Most of a cell’s life is spent in a nondividing state (interphase)
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Interphase, Mitosis, and Cytokinesis PLAY 3 Stages of Cell Division Body (somatic) cells divide in 3 stages: –DNA replication duplicates genetic material exactly –Mitosis divides genetic material equally –Cytokinesis divides cytoplasm and organelles into 2 daughter cells
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Interphase The nondividing period: –G-zero phase—specialized cell functions only –G 1 phase—cell growth, organelle duplication, protein synthesis –S phase—DNA replication and histone synthesis –G 2 phase—finishes protein synthesis and centriole replication
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DNA Replication Figure 3–24 DNA strands unwind DNA polymerase attaches complementary nucleotides
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Mitosis Mitosis divides duplicated DNA into 2 sets of chromosomes: –DNA coils tightly into chromatids –chromatids connect at a centromere –protein complex around centromere is kinetochore
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Figure 3–25 (Stage 1) Stage 1: Prophase
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Features of Prophase Nucleoli disappear Centriole pairs move to cell poles Microtubules (spindle fibers) extend between centriole pairs Nuclear envelope disappears Spindle fibers attach to kinetochore
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Features of Metaphase Chromosomes align in a central plane (metaphase plate)
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Features of Anaphase Microtubules pull chromosomes apart Daughter chromosomes groups near centrioles
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Features of Telophase Nuclear membranes reform Chromosomes uncoil Nucleoli reappear Cell has 2 complete nuclei
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KEY CONCEPT Mitosis duplicates chromosomes in the nucleus for cell division
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Figure 3–25 (Stage 4, 2 of 2) Stage 4: Cytokinesis Division of the cytoplasm
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Features of Cytokinesis Cleavage furrow around metaphase plate Membrane closes, producing daughter cells
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