Chapter 3
Cells: The Living Units
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Cell Theory
• The cell is the basic structural and functional unit of life
• Organisms depend on individual and collective activity of cells, dictated by subcellular structures
• Continuity of life has a cellular basis
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Structure of a Generalized Cell
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Plasma Membrane
• Separates intracellular fluids from extracellular fluids
• Plays a dynamic role in cellular activity
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Fluid Mosaic Model
• Double bilayer of lipids with imbedded, dispersed proteins
• Bilayer consists of phospholipids, glycolipids, cholesterol and proteins
• Phospholipid bilayer has hydrophobic (in the middle) and hydrophilic (facing the outside) portions
• Glycolipids are lipids with bound carbohydrates
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Fluid Mosaic Model of Plasma Membrane
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Functions of Membrane Integral Proteins
• Transport –channels or transporters
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Functions of Membrane Peripheral Proteins
• Attach to inner cytoplasm
• Part of glycocalyx outer coating as:
• Receptors
• Enzymes
• Cell identification markers
• Intercellular linkers
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Cell Environment Interactions
• Membrane receptors used for:
• Contact signaling – cell recognition
• Electrical signaling – voltage-regulated “ion gates” in nerve and muscle tissue
• Chemical signaling – neurotransmitters and hormones
• Cell linkers anchor cells, assist in movement of cells past one another, send signals for repair and immunity
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Membrane Potential
• Voltage across a membrane
• Resting membrane potential
• Ranges from –20 to –200 mV
• Results from Na+ and K+ concentration gradients across the membrane
• Differential permeability of the plasma membrane to Na+ and K+
Figure 3.13
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Membrane Transport• The cell membrane is selectively permeable to
substances in the interstitial (extracellular) fluid
• Passive transport processes require no energy
• Active transport processes require energy
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Diffusion
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Passive Membrane Transport: Diffusion
• Simple diffusion –movement from higher to lower concentration
• Nonpolar, lipid-soluble substances diffuse directly through the cell lipid bilayer
• Most small polar substances must go through integral protein channels
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Passive Membrane Transport: Facilitated Diffusion
• Large polar molecules (sugars) combine with integral protein transporters that aid transport across membrane
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Passive Membrane Transport: Osmosis
• Diffusion of water across a semipermeable membrane
• Occurs when the concentration of water is different on opposite sides of a selective, water permeable membrane
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Effect of Membrane Permeability on Osmosis
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Tonicity
• Isotonic – solution has the same solute concentration as that of the cell
• Hypotonic – solution has lesser solute concentration than that of the cell
• Hypertonic – solution has greater solute concentration than that of the cell
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Passive Membrane Transport: Filtration
• The passage of water and solutes through a membrane by pressure
• Pressure gradient pushes substances from a higher-pressure area to a lower-pressure area
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Active Transport (Solute Pump)
• Solutes move against a concentration gradient (uphill)
• Uses ATP to help move solutes across the membrane
• Requires integral transport proteins
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Types of Active Transport
• Symport system – two substances are moved across the membrane in the same direction
• Antiport system – two substances are moved across the membrane in opposite directions
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Types of Active Transport
• Primary active transport – phosphate from ATP causes conformational change of transport protein, which then transports substance
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Types of Active Transport
• Secondary active transport – as Na+ goes across membrane, other solutes (like glucose) “piggyback” across with it
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Vesicular Transport
• Transport of large particles (macromolecules) across plasma membranes
• Exocytosis – moves substance from the cell interior to the extracellular space
• Endocytosis – enables macromolecules to enter the cell
• Phagocytosis – engulf solids and bring them into the cell’s interior
• Pinocytosis – engulf liquids
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Vesicular Transport
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Cytoplasm
• Cytoplasm – material between plasma membrane and the nucleus; contains:
• Cytosol – Colloid of water with dissolved protein, salts, sugars and other solutes
• Inclusions – large chemical substances
• Cytoplasmic organelles – metabolic machinery of the cell
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Cytoplasmic Organelles
• Specialized cellular compartments with specialized functions – division of labor
• Most are membrane - bound
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Cytoskeleton
• The “skeleton” of the cell
• Consists of microfilaments, intermediate filaments and microtubules
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Microfilaments
• Smallest strands like a beaded necklace
• Function to change cell shape, cell movement, endocytosis, exocytosis and support microvilli
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Intermediate Filaments
• Tough, insoluble protein fibers constructed like rope
• Resist pulling forces in the cell
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Microtubules
• Hollow tubes made of a spherical protein
• Determine the overall shape of the cell and distribution of organelles
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Centrioles
• Small barrel-shaped organelles located near the nucleus
• Pinwheel array of microtubules
• Organize mitotic spindle during mitosis
• Form the bases of cilia and flagella
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Cilia
• Hairlike, motile cellular extensions on exposed surfaces of certain cells
• Move substances across cell surface
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Flagella
• “Tail” of sperm cells for cell movement
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Ribosomes
• Granules containing rRNA and protein
• Site of protein synthesis
• Free ribosomes synthesize soluble proteins
• Membrane-bound ribosomes synthesize proteins to be incorporated into membranes
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Endoplasmic Reticulum (ER)
• Interconnected tubes and parallel membranes enclosing cisterna
• Two varieties – rough ER and smooth ER
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Rough Endoplasmic Reticulum
• External surface studded with ribosomes
• Manufactures and transports protein containing compounds for internal use, integral proteins of plasma membranes and ultimately secretion
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Smooth Endoplasmic Reticulum
• Function to synthesis and internal transport of lipid containing compounds
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Golgi Complex
• Stacked and flattened membranous sacs
• Functions in packaging compounds from the ER to make:
• Secretory vesicles for export from the cell, plasma membrane components or lysosomes
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Role of the Golgi Complex
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Lysosomes
• Spherical sacs containing digestive enzymes:
• Digest ingested bacteria, viruses or toxins
• Breakdown useless tissue
• Degrade nonfunctional organelles
• Cell suicide
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Peroxisomes
• Sacs that contain detoxifying enzymes to detoxify harmful substances, including free radicals – highly reactive chemicals that can damage cells
• Ex: liver & kidney
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Proteasomes
• Contain proteases, enzymes that cut or degrade faulty cellular proteins into small peptides
• 4 stacked rings around a central core
• Too small to see under a light microscope
• In both the cytosol & nucleus
• Malfunction can result in Alzheimer’s & Parkinson’s diseases
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Mitochondria
• Double membrane structure with inner membrane shelves called cristae and matrix between
• Function to provide the cell’s ATP via aerobic cellular respiration
• Contain their own DNA and RNA
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Nucleus
• Gene-containing control center of the cell contains the genetic library with blueprints for nearly all cellular proteins; dictates the kinds and amounts of proteins to be synthesized
• Contains nuclear envelope, nucleolus and chromatin
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Nucleus
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Nuclear Envelope
• Selectively permeable double membrane barrier containing pores
• Pore complex regulates transport of large molecules into and out of the nucleus
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Nucleolus
• Dark-staining spherical body within the nucleus
• Site of ribosome production
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Chromatin
• Threadlike strands of DNA
• Condense and form barlike bodies called chromosomes when the nucleus starts to divide
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Protein Synthesis
• DNA serves as master blueprint for protein synthesis
• Genes are segments of DNA carrying instructions for a polypeptide chain
• Triplets of nucleotide bases form the genetic library; each triplet specifies coding for an amino acid
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Transcription
• Occurs in the nucleus
• Transfer of information from the DNA strand to the mRNA as mRNA is synthesized
• Each DNA triplet codes for a corresponding 3-base sequence of RNA, called a codon
• Each codon corresponds to a DNA triplet
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Transcription
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Genetic Code
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Translation
• Involves all three types of RNA – mRNA, rRNA, and tRNA
• Occurs in the cytoplasm at the ribosomes; mRNA leaves the nucleus and goes to the ribosomes
• rRNA (part of ribosomes) is the anchoring site where mRNA is read and tRNA brings in the various amino acid to build a polypeptide
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Translation
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Roles of the Three Types of RNA
• Messenger RNA (mRNA) carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm
• Ribosomal RNA (rRNA) is a structural component of ribosomes and site of protein synthesis
• Transfer RNAs (tRNAs) bound to amino acids base pair their anticodons with the codons of mRNA at the ribosome to begin the process of protein synthesis
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Information Transfer from DNA to RNA to Make Proteins
• DNA triplets are transcribed into mRNA codons
• Codons base pair with tRNA anticodons at the ribosomes
• Amino acids are peptide bonded at the ribosomes to form polypeptide chains
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Information Transfer from DNA to RNA to Make Proteins
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Cell Life Cycle
• Interphase
• Growth (G1), synthesis (S), growth (G2)
• Cell division is essential for body growth and tissue repair
• Mitosis (nuclear division)
• Cytokinesis (cytoplasm division)
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Interphase Parts• G1 – normal metabolic activity and growth
• If cell gets signal to divide, then
• S (synthesis) – DNA (chromosomes) and centrioles replicate
• G2 – enzyme preparation for division
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DNA Replication
• The DNA double helix unwinds into two complementary nucleotide chains; hydrogen bonds between nucleotides break
• Freed nucleotide strands serve as templates for replication
• Complementary nucleotide strands form: A-T; G-C
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Mitosis Phases
• Prophase
• Metaphase
• Anaphase
• Telophase
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Prophase
• Chromatin condenses into chromosomes
• Nuclear envelope and nucleolus disappear
• Centriole pairs separate and the mitotic spindle forms
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Metaphase
• Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell
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Anaphase
• Sister chromatids split
• Chromosomes are pulled toward poles
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Telophase and Cytokinesis• Chromosomes nearly reach poles
• New sets of chromosomes unwind into chromatin
• New nuclear membrane and nucleolus reappear
• Cytokinesis occurs at the same time
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Cytokinesis
• Cleavage furrow formed in late anaphase by a contractile ring
• Cytoplasm is pinched in two as mitosis ends
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Aging Theories
• Mitosis ceases
• Telemeres (tips of chromosomes) lost each mitosis
• Glucose bridges to proteins
• Free radical damage
• Autoimmunity