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Chapter 3: The Cellular Level of Organization
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The Cell Performs all life functions Figure 3–1
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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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Organelle Functions Table 3–1 (1 of 2)
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Organelle Functions Table 3–1 (2 of 2)
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Functions of Cell Membrane (1 of 2)
Physical isolation Monitors & Regulates exchange with environment: extracellular fluid composition chemical signals ions and nutrients enter waste and cellular products released Structural support: anchors cells and tissues
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Structures and functions of the cell membrane
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The Cell Membrane Contains lipids, carbohydrates, and functional proteins 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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6 Functions of Membrane Proteins (1 of 2)
Anchoring proteins (stabilizers): attach to inside or outside structures Recognition proteins (identifiers): label cells normal or abnormal Enzymes: catalyze reactions
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6 Functions of Membrane Proteins (2 of 2)
Receptor proteins: bind and respond to ligands (ions, hormones) Carrier proteins: transport specific solutes through membrane Channels: regulate water flow and solutes through membrane
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Membrane Carbohydrates
Proteoglycans, glycoproteins, and glycolipids: extend outside cell membrane form sticky “sugar coat” (glycocalyx)
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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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What are cell organelles and their functions?
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Types of Organelles Nonmembranous organelles: Membranous 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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The Cytoskeleton Structural proteins for shape and strength
Figure 3–3a
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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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Intermediate Filaments
Mid-sized between microfilaments and thick filaments: durable (collagen) strengthen cell and maintain shape stabilize organelles stabilize cell position
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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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Microvilli Increase surface area for absorption Attach to cytoskeleton
Figure 3–3b
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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)
Figure 3–5a
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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 Figure 3–6a
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Golgi Apparatus Vesicles enter forming face and exit maturing face
PLAY Functions of the Golgi Apparatus
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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 Carry materials to and from Golgi apparatus
Figure 3–7a
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Exocytosis Ejects secretory products and wastes Figure 3–7b
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Lysosomes Powerful enzyme-containing vesicles:
lyso = dissolve, soma = body Figure 3–8
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Lysosome Structures Primary lysosome: Secondary 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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Peroxisomes Are enzyme-containing vesicles:
break down fatty acids, organic compounds produce hydrogen peroxide (H2O2) replicate by division
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Membrane Flow A continuous exchange of membrane parts by vesicles:
all membranous organelles (except mitochondria) allows adaptation and change
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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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Mitochondrion Structure
Figure 3–9a
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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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Aerobic Cellular Respiration
Aerobic metabolism (cellular respiration): mitochondria use oxygen to break down food and produce ATP
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glucose + oxygen + ADP carbon dioxide + water + ATP
The Reactions glucose + oxygen + ADP carbon dioxide + water + ATP Glycolysis: glucose to pyruvic acid (in cytosol) Tricarboxylic acid cycle (TCA cycle): pyruvic acid to CO2 (in matrix)
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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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How does the nucleus control the cell?
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The Nucleus Is the cell’s control center Figure 3–10a
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Structure of the 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: Nucleoplasm: Nuclear matrix:
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: Chromatin: Chromosomes:
DNA coiled around histones Chromatin: loosely coiled DNA (cells not dividing) Chromosomes: tightly coiled DNA (cells dividing)
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What is genetic code?
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DNA and Genes DNA: Gene: instructions for every protein in the body
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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How do DNA instructions become proteins?
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Protein Synthesis Transcription: Translation:
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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Codons Table 3–2
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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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Translation (5 of 6) Enzymes join amino acids with peptide bonds
Polypeptide chain has specific sequence of amino acids Figure 3–13 (Step 4)
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Translation (6 of 6) At stop codon, components separate
Protein Synthesis: Sequence of Amino Acids in the Newly Synthesized Polypeptide PLAY Figure 3–13 (Step 5)
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KEY CONCEPT Genes: Protein synthesis requires:
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 semipermeable: 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 Membrane Transport: Fat- and Water-Soluble Molecules PLAY
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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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Factors Affecting Diffusion Rates
Gradient size: the difference between high and low concentration Electrical forces: opposites attract, like charges repel
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Diffusion and the Cell Membrane
Diffusion can be simple or channel-mediated Figure 3–15
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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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Factors in Channel-Mediated Diffusion
Passage depends on: size charge interaction with the channel
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Osmosis Osmosis is the diffusion of water across the cell membrane
Figure 3–16
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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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Tonicity The osmotic effect of a solute on a cell:
2 fluids may have equal osmolarity, but different tonicity Figure 3–17a
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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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Special transport mechanisms
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Special Transport Mechanisms
Carrier-mediated transport of ions and organic substrates: facilitated diffusion active transport
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Characteristics of Carrier-Mediated Transport
Specificity: 1 transport protein, 1 set of substrates Saturation limits: rate depends on transport proteins, not substrate Regulation: cofactors such as hormones
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Special Transport Mechanisms
Cotransport 2 substances move in the same direction at the same time Countertransport 1 substance moves in while another moves out
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Facilitated Diffusion
Passive Carrier mediated 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+, Mg2+) exchange pump countertransports 2 ions at the same time
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Sodium-Potassium Exchange Pump
Figure 3–19
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Receptor-Mediated Endocytosis
Receptors (glycoproteins) bind target molecules (ligands) Coated vesicle (endosome) carries ligands and receptors into the cell
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Pinocytosis Pinocytosis (cell drinking)
Endosomes “drink” extracellular fluid Figure 3–22a
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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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Exocytosis Is the reverse of endocytosis Figure 3–7b
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Summary The 7 methods of transport Table 3–3
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What is transmembrane potential?
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Electrical Charge Inside cell membrane is slightly negative, outside is slightly positive Unequal charge across the cell membrane is transmembrane potential Resting potential ranges from —10 mV to —100 mV, depending on cell type
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Cell Life Cycle Figure 3–3
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Cell Life Cycle Most of a cell’s life is spent in a nondividing state (interphase)
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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 G1 phase—cell growth, organelle duplication, protein synthesis S phase—DNA replication and histone synthesis G2 phase—finishes protein synthesis and centriole replication
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DNA Replication DNA strands unwind
DNA polymerase attaches complementary nucleotides Figure 3–24
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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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Features of Prophase Nucleoli disappear
Centriole pairs move to cell poles Microtubules 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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Features of Cytokinesis
Division of the cytoplasm Cleavage furrow around metaphase plate Membrane closes, producing daughter cells
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Long Life, Short Life Muscle cells, neurons rarely divide
Exposed cells (skin and digestive tract) live only days or hours Normally, cell division balances cell loss
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Factors Changing Cell Division
Increases cell division: internal factors (MPF) extracellular chemical factors (growth factors) Decreases cell division: repressor genes (faulty repressors cause cancers) worn out telomeres (terminal DNA segments)
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Cell Differentiation Cells specialize or differentiate:
to form tissues (liver cells, fat cells, and neurons) by turning off all genes not needed by that cell
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KEY CONCEPT All body cells, except sex cells, contain the same 46 chromosomes Differentiation depends on which genes are active and which are inactive
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SUMMARY (1 of 4) Structures and functions of human cells
Structures and functions of membranous and nonmembranous organelles
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SUMMARY (2 of 4) ATP, mitochondria, and the process of aerobic cellular respiration Structures and functions of the nucleus: control functions of nucleic acids structures and replication of DNA DNA and RNA in protein synthesis
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SUMMARY (3 of 4) Structures and chemical activities of the cell membrane: diffusion and osmosis active transport proteins vesicles in endocytosis and exocytosis electrical properties of plasma membrane
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SUMMARY (4 of 4) Stages and processes of cell division:
DNA replication mitosis cytokinesis
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