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Chapter 3
Cells: The Living Units
Cell Theory
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Cell - structural and functional unit of life
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Biochemical activities of cells dictated by their shapes or
forms, and specific subcellular structures
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Continuity of life has cellular basis
Organismal functions depend on individual and collective cell
functions
•Inner lining (nuclear lamina) maintains shape of nucleus;
scaffold to organize DNA
•Pores allow substances to pass; nuclear pore complex line
pores; regulates transport of large molecules into and out of
nucleus
Nucleoli
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Dark-staining spherical bodies within nucleus
Generalized Cell
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Usually one or two per cell
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Chromatin
All cells have some common structures and functions
Human cells have three basic parts:
– Plasma membrane—flexible outer boundary
– Cytoplasm—intracellular fluid containing organelles
– Nucleus—control center
Nucleus
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Largest organelle; genetic library with blueprints for nearly all
cellular proteins
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Responds to signals; dictates kinds and amounts of proteins
synthesized
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Most cells uninucleate; skeletal muscle cells, bone
destruction cells, and some liver cells are multinucleate; red
blood cells are anucleate
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Three regions/structures
The Nuclear Envelope
•Double-membrane barrier; encloses nucleoplasm
•Outer layer continuous with rough ER and bears ribosomes
Involved in rRNA synthesis and ribosome subunit assembly
Associated with nucleolar organizer regions
–Contains DNA coding for rRNA
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Threadlike strands of DNA (30%), histone proteins (60%),
and RNA (10%)
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Arranged in fundamental units called nucleosomes
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Condense into barlike bodies called chromosomes when cell
starts to divide
Histones pack long DNA molecules; involved in gene
regulation
Plasma Membrane
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Lipid bilayer and proteins in constantly changing fluid
mosaic
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Plays dynamic role in cellular activity
Separates intracellular fluid (ICF) from extracellular fluid
(ECF)
– Interstitial fluid (IF) = ECF that surrounds cells
Membrane Lipids
The Glycocalyx
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75% phospholipids (lipid bilayer)
– Lipids and proteins with attached carbohydrates (sugar
5% glycolipids
20% cholesterol
groups)
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Allow communication with environment
½ mass of plasma membrane
Most specialized membrane functions
Some float freely
Some tethered to intracellular structures
Two types:
– Integral proteins
Every cell type has different pattern of sugars
– Specific biological markers for cell to cell recognition
– Allows immune system to recognize "self" and "non self"
– Cancerous cells change it continuously
Membrane Proteins
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"Sugar covering" at cell surface
Cellular Extensions
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Microvilli
–Minute, fingerlike extensions of plasma membrane
–Increase surface area for absorption
–Core of actin filaments for stiffening
Plasma Membrane
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Cells surrounded by interstitial fluid (IF)
– Contains thousands of substances, e.g., amino acids,
– Peripheral proteins
sugars, fatty acids, vitamins, hormones, salts, waste
products
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Plasma membrane allows cell to
– Obtain from IF exactly what it needs, exactly when it is
needed
Six Functions of Membrane Proteins
1. Transport
2. Receptors for signal transduction
3. Attachment to cytoskeleton and extracellular matrix
4.Enzymatic activity
5. Intercellular joining
6. Cell-cell recognition
– Keep out what it does not need
Cytoplasm
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Located between plasma membrane and nucleus
–Composed of
• Cytosol
• Organelles
• Inclusions
Cytoplasmic Organelles
•Membranous
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Rough ER
• Nonmembranous
Mitochondria
Peroxisomes
Lysosomes
Endoplasmic reticulum
Golgi apparatus
 Cytoskeleton
 Centrioles
 Ribosomes
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External surface studded with ribosomes
Manufactures all secreted proteins
Synthesizes membrane integral proteins and phospholipids
Assembled proteins move to ER interior, enclosed in vesicle,
go to Golgi apparatus
•Membranes allow crucial compartmentalization
Smooth ER
Mitochondria
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Double-membrane structure with inner shelflike cristae
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Contain their own DNA, RNA, ribosomes
Provide most of cell's ATP via aerobic cellular respiration
– Requires oxygen
Similar to bacteria; capable of cell division called fission
Ribosomes
Network of tubules continuous with rough ER
Its enzymes (integral proteins) function in
– Lipid metabolism; cholesterol and steroid-based hormone
synthesis; making lipids of lipoproteins
– Absorption, synthesis, and transport of fats
– Detoxification of drugs, some pesticides, carcinogenic
chemicals
– Converting glycogen to free glucose
– Storage and release of calcium
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Granules containing protein and rRNA
Golgi Apparatus
Site of protein synthesis
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Membrane-bound ribosomes (forming rough ER)
synthesize proteins to be incorporated into membranes,
lysosomes, or exported from cell
Free ribosomes synthesize soluble proteins that function in
cytosol or other organelles
Endoplasmic Reticulum (ER)
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Interconnected tubes and parallel membranes enclosing
cisterns
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Continuous with outer nuclear membrane
Two varieties:
Stacked and flattened membranous sacs
Modifies, concentrates, and packages proteins and lipids
from rough ER
Lysosomes
–Spherical membranous bags containing digestive enzymes
(acid hydrolases) "Safe" sites for intracellular digestion
•Digest ingested bacteria, viruses, and toxins
•Degrade nonfunctional organelles
•Metabolic functions, e.g., break down and release glycogen
•Destroy cells in injured or nonuseful tissue (autolysis)
Break down bone to release Ca2+
Peroxisomes
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Membranous sacs containing powerful oxidases and
catalases
Detoxify harmful or toxic substances
Catalysis and synthesis of fatty acids
Neutralize dangerous free radicals (highly reactive
chemicals with unpaired electrons)
– Oxidases convert to H2O2 (also toxic)
– Catalases convert H2O2 to water and oxygen
Endomembrane System
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Overall function
– Produce, degrade, store, and export biological molecules
– Degrade potentially harmful substances
Includes ER, Golgi apparatus, secretory vesicles, lysosomes,
nuclear and plasma membranes
Cytoskeleton
Intermediate Filaments
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Tough, insoluble, ropelike protein fibers
Composed of tetramer fibrils
Resist pulling forces on cell; attach to desmosomes
E.g., neurofilaments in nerve cells; keratin filaments in
epithelial cells
Microtubules
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Largest of cytoskeletal elements; dynamic hollow tubes; most
radiate from centrosome
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Composed of protein subunits called tubulins
Determine overall shape of cell and distribution of organelles
Mitochondria, lysosomes, secretory vesicles attach to
microtubules; moved throughout cell by motor proteins
Centrosome and Centrioles
Elaborate series of rods throughout cytosol; proteins link rods
to other cell structures
–Three types:
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"Cell center" near nucleus
Microfilaments
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Centrioles form basis of cilia and flagella
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Thinnest of cytoskeletal elements
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Involved in cell motility, change in shape, endocytosis and
exocytosis
Generates microtubules; organizes mitotic spindle
Contains paired centrioles
–Barrel-shaped organelles formed by microtubules
Dynamic strands of protein actin
Cellular Extensions
Each cell has a unique arrangement of strands
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Dense web attached to cytoplasmic side of plasma
membrane is called terminal web
–Gives strength, compression resistance
Cilia and flagella
–Whiplike, motile extensions on surfaces of certain cells
–Contain microtubules and motor molecules
–Cilia move substances across cell surfaces
– Longer flagella propel whole cells (tail of sperm)
• Carrier- and channel-mediated facilitated diffusion
• Osmosis
Cell Diversity
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– Filtration
Over 200 different types of human cells
• Usually across capillary walls
Types differ in size, shape, subcellular components, and
functions
Tonicity
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Tonicity: Ability of solution to alter cell's water volume
– Isotonic: Solution with same non-penetrating solute
concentration as cytosol
– Hypertonic: Solution with higher non-penetrating solute
concentration than cytosol
– Hypotonic: Solution with lower non-penetrating solute
Membrane Transport
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concentration than cytosol
Plasma membranes selectively permeable
– Some molecules pass through easily; some do not
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Two ways substances cross membrane
– Passive processes
– Active processes
Types of Membrane Transport
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Membrane Transport: Active Processes
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– Active transport (solute pumping)
– Vesicular transport
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Passive processes
Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
Passive Processes
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Two types of passive transport
– Diffusion
• Simple diffusion
Both require ATP to move solutes across a living plasma
membrane because
– Solute too large for channels
– Solute not lipid soluble
– Solute not able to move down concentration gradient
– No cellular energy (ATP) required
– Substance moves down its concentration gradient
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Two types of active processes
Active Transport
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Requires carrier proteins (solute pumps)
– Bind specifically and reversibly with substance
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Moves solutes against concentration gradient
– Requires energy
Sodium-Potassium Pump
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Na+ and K+ channels allow slow leakage down concentration
gradients
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Na+-K+ pump works as antiporter
– Pumps against
K+
Na+
K+
and
gradients to maintain high
concentration and high extracellular Na+
intracellular
concentration
• Maintains electrochemical gradients essential for
functions of muscle and nerve tissues
• Allows all cells to maintain fluid volume
Vesicular Transport
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Transport of large particles, macromolecules, and fluids
across membrane in membranous sacs called vesicles
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Requires cellular energy (e.g., ATP)
Vesicular Transport
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Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
• Phagocytosis
• Pinocytosis
– Transcytosis
– transport into, across,
and then out of cell
– Vesicular trafficking
– transport from one area
or organelle in cell to another
Cell Life Cycle
Cells have two major periods
Interphase
Cell division
Interphase
Events of Cell Division
Mitosis—division of the nucleus
Results in the formation of two daughter nuclei
Cytokinesis—division of the cytoplasm
Begins when mitosis is near completion
Results in the formation of two daughter cells
Stages of Mitosis
Prophase
Centrioles migrate to the poles to direct assembly of
mitotic spindle fibers
DNA appears as double-stranded chromosomes
Nuclear envelope breaks down and disappears
Metaphase
Chromosomes are aligned in the center of the cell on
the metaphase plate
Anaphase
Chromosomes are pulled apart and toward the opposite
ends of the cell
Cell begins to elongate
Telophase
Chromosomes uncoil to become chromatin
Nuclear envelope reforms around chromatin
Spindles break down and disappear
Cytokinesis
Begins during late anaphase and completes during telophase
A cleavage furrow forms to pinch the cells into two parts
Protein Synthesis
Gene—DNA segment that carries a blueprint for building one
protein
Proteins have many functions
Building materials for cells
Act as enzymes (biological catalysts)
RNA is essential for protein synthesis
Role of RNA
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)
Messenger RNA (mRNA)
Transcription
Transfer of information from DNA’s base sequence to the
complimentary base sequence of mRNA
Three-base sequences on mRNA are called codons
Translation
Base sequence of nucleic acid is translated to an amino acid
sequence
Amino acids are the building blocks of proteins
Extracellular Materials
•Body fluids—interstitial fluid, blood plasma, and cerebrospinal
fluid
•Cellular secretions—intestinal and gastric fluids, saliva, mucus,
and serous fluids
•Extracellular matrix–most abundant extracellular material
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Developmental Aspects of Cells
•All cells of body contain same DNA but cells not identical
•Chemical signals in embryo channel cells into specific
developmental pathways by turning some genes on and others
off
•Development of specific and distinctive features in cells called
cell differentiation
Apoptosis and Modified Rates of Cell Division
•During development more cells than needed produced (e.g., in
nervous system)
•Eliminated later by programmed cell death (apoptosis)
–Mitochondrial membranes leak chemicals that activate
caspases  DNA, cytoskeleton degradation  cell death
–Dead cell shrinks and is phagocytized
•Organs well-formed and functional before birth
•Cell division in adults to replace short-lived cells and repair
wounds
•Hyperplasia
•Atrophy
Theories of Cell Aging
•Wear and tear theory—Little chemical insults and free radicals
have cumulative effects
•Mitochondrial theory of aging–free radicals in mitochondria
diminish energy production
•Immune system disorders—autoimmune responses;
progressive weakening of immune response; C-reactive protein
of acute inflammation causes cell aging
•Most widely accepted theory
–Genetic theory-cessation of mitosis and cell aging
programmed into genes
•Telomeres (strings of nucleotides protecting ends of
chromosomes) may determine number of times a cell can divide
•Telomerase lengthens telomeres
–Found in germ cells; ~ absent in adult cells