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Cell - structural and functional unit of life -- smallest unit of life
all living things made of cells
cells come from cells
Organismal functions depend on individual and collective cell functions
Biochemical activities of cells dictated by their shapes or forms, and specific
subcellular structures
Continuity of life has cellular basis
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Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions
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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
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Lipid bilayer and proteins in constantly changing fluid mosaic
Plays dynamic role in cellular activity
Separates intracellular fluid (ICF) from extracellular fluid (ECF)
Interstitial fluid (IF) = ECF that surrounds cells
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75% phospholipids (lipid bilayer)
Phosphate heads: polar and hydrophilic
Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b)
5% glycolipids
Lipids with polar sugar groups on outer membrane surface
20% cholesterol
Increases membrane stability
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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; peripheral proteins
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Integral proteins
Firmly inserted into membrane (most are transmembrane)
Have hydrophobic and hydrophilic regions
Can interact with lipid tails and water
Function as transport proteins (channels and carriers), enzymes, or receptors
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Peripheral proteins
Loosely attached to integral proteins
Include filaments on intracellular surface for membrane support
Function as enzymes; motor proteins for shape change during cell division and
muscle contraction; cell-to-cell connections
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1.
Transport
2.
Receptors for signal transduction
3.
Attachment to cytoskeleton and extracellular matrix
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~20% of outer membrane surface
Contain phospholipids, sphingolipids, and cholesterol
More stable; less fluid than rest of membrane
May function as stable platforms for cell-signaling molecules, membrane
invagination, or other functions
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"Sugar covering" at cell surface
Lipids and proteins with attached carbohydrates (sugar groups)
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
glyco = sugar -- calyx = shell
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Some cells "free"
e.g., blood cells, sperm cells
Some bound into communities
Three ways cells are bound:
Tight junctions
Desmosomes
Gap junctions
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Adjacent integral proteins fuse  form impermeable junction encircling cell
Prevent fluids and most molecules from moving between cells
Where might these be useful in body?
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"Rivets" or "spot-welds" that anchor cells together at plaques (thickenings on plasma
membrane)
Linker proteins between cells connect plaques
Keratin filaments extend through cytosol to opposite plaque giving stability to
cell
Reduces possibility of tearing
Where might these be useful in body?
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Transmembrane proteins form pores (connexons) that allow small molecules to pass
from cell to cell
For spread of ions, simple sugars, and other small molecules between cardiac
or smooth muscle cells
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Cells surrounded by interstitial fluid (IF)
Contains thousands of substances, e.g., amino acids, sugars, fatty acids,
vitamins, hormones, salts, waste products
Plasma membrane allows cell to
Obtain from IF exactly what it needs, exactly when it is needed
Keep out what it does not need
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Plasma membranes selectively permeable
Some molecules pass through easily; some do not
Two ways substances cross membrane
Passive processes
Active processes
Passive processes
No cellular energy (ATP) required
Substance moves down its concentration gradient
Active processes
Energy (ATP) required
Occurs only in living cell membranes
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Two types of passive transport
Diffusion
Simple diffusion
Carrier- and channel-mediated facilitated diffusion
Osmosis
Filtration
Usually across capillary walls
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Collisions cause molecules to move down or with their concentration gradient
Difference in concentration between two areas
Speed influenced by molecule size and temperature
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Molecule will passively diffuse through membrane if
It is lipid soluble, or
Small enough to pass through membrane channels, or
Assisted by carrier molecule
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Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid
bilayer
E.g., oxygen, carbon dioxide, fat-soluble vitamins
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Certain lipophobic molecules (e.g., glucose, amino acids, and ions) transported
passively by
Binding to protein carriers
Moving through water-filled channels
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Transmembrane integral proteins are carriers
Transport specific polar molecules (e.g., sugars and amino acids) too large for
channels
Binding of substrate causes shape change in carrier then passage across membrane
Limited by number of carriers present
Carriers saturated when all engaged
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Aqueous channels formed by transmembrane proteins
Selectively transport ions or water
Two types:
Leakage channels
Always open
Gated channels
Controlled by chemical or electrical signals
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Movement of solvent (e.g., water) across selectively permeable membrane
Water diffuses through plasma membranes
Through lipid bilayer
Through specific water channels called aquaporins (AQPs)
Occurs when water concentration different on the two sides of a membrane
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Water concentration varies with number of solute particles because solute particles
displace water molecules
Osmolarity - Measure of total concentration of solute particles
Water moves by osmosis until hydrostatic pressure (back pressure of water on
membrane) and osmotic pressure (tendency of water to move into cell by osmosis)
equalize
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When solutions of different osmolarity are separated by membrane permeable to all
molecules, both solutes and water cross membrane until equilibrium reached
When solutions of different osmolarity are separated by membrane impermeable to
solutes, osmosis occurs until equilibrium reached
Osmosis causes cells to swell and shrink
Change in cell volume disrupts cell function, especially in neurons
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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 concentration than
cytosol
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Energy from solution (heat) = kinetic energy
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Located between plasma membrane and nucleus
Composed of
Cytosol
Water with solutes (protein, salts, sugars, etc.)
Organelles
Metabolic machinery of cell; each with specialized function;
either membranous or nonmembranous
Inclusions
Vary with cell type; e.g., glycogen granules, pigments, lipid
droplets, vacuoles, crystals
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Membranous
Membranes allow crucial compartmentalization
Mitochondria
Peroxisomes
Lysosomes
Endoplasmic reticulum
Golgi apparatus
Nonmembranous
Cytoskeleton
Centrioles
Ribosomes
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Double-membrane structure with inner shelflike cristae
Provide most of cell's ATP via aerobic cellular respiration
Requires oxygen
Contain their own DNA, RNA, ribosomes
Similar to bacteria; capable of cell division called fission
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Granules containing protein and rRNA
Site of protein synthesis
Free ribosomes synthesize soluble proteins that function in cytosol or other
organelles
Membrane-bound ribosomes (forming rough ER) synthesize proteins to be
incorporated into membranes, lysosomes, or exported from cell
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Interconnected tubes and parallel membranes enclosing cisterns
Continuous with outer nuclear membrane
Two varieties:
Rough ER
Smooth ER
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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
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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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Stacked and flattened membranous sacs
Modifies, concentrates, and packages proteins and lipids from rough ER
Transport vessels from ER fuse with convex cis face; proteins modified, tagged for
delivery, sorted, packaged in vesicles
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Three types of vesicles bud from concave trans face
Secretory vesicles (granules)
To trans face; release export proteins by exocytosis
Vesicles of lipids and transmembrane proteins for plasma membrane or
organelles
Lysosomes containing digestive enzymes; remain in cell
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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
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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
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down and release glycogen
Destroy cells in injured or nonuseful
tissue (autolysis)
Break down bone to release Ca2+
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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
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Elaborate series of rods throughout cytosol; proteins link rods to other cell structures
Three types
Microfilaments
Intermediate filaments
Microtubules
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Thinnest of cytoskeletal elements
Dynamic strands of protein actin
Each cell has a unique arrangement of strands
Dense web attached to cytoplasmic side of plasma membrane is called terminal web
Gives strength, compression resistance
Involved in cell motility, change in shape, endocytosis and exocytosis
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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
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Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from
centrosome
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
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Protein complexes that function in motility (e.g., movement of organelles and
contraction)
Powered by ATP
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"Cell center" near nucleus
Generates microtubules; organizes mitotic spindle
Contains paired centrioles
Barrel-shaped organelles formed by microtubules
Centrioles form basis of cilia and flagella
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The paired centrioles that are the centrosome make up one of the microtubule
organizing centers (MTOC) of the cell.
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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)
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Centrioles forming base called basal bodies
Cilia movements alternate between power stroke and recovery stroke  current at
cell surface
Primary cilia
Single, nonmotile projection on most cells
Probe environment for molecules receptors can recognize; coordinate
intracellular pathways
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Microvilli
Minute, fingerlike extensions of plasma membrane
Increase surface area for absorption
Core of actin filaments for stiffening
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Largest organelle; genetic library with blueprints for nearly all cellular proteins
Responds to signals; dictates kinds and amounts of proteins synthesized
Most cells uninucleate; skeletal muscle cells, bone destruction cells, and some liver
cells are multinucleate; red blood cells are anucleate
Three regions/structures
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Double-membrane barrier; encloses nucleoplasm
Outer layer continuous with rough ER and bears ribosomes
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
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Dark-staining spherical bodies within nucleus
Involved in rRNA synthesis and ribosome subunit assembly
Associated with nucleolar organizer regions
Contains DNA coding for rRNA
Usually one or two per cell
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Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%)
Arranged in fundamental units called nucleosomes
Histones pack long DNA molecules; involved in gene regulation
Condense into barlike bodies called chromosomes when cell starts to divide
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Defines changes from formation of cell until it reproduces
Includes:
Interphase
Cell grows and carries out functions
Cell division (mitotic phase)
Divides into two cells
Period from cell formation to cell division
Nuclear material called chromatin
Three subphases:
G1 (gap 1)—vigorous growth and metabolism
Cells that permanently cease dividing said to be in G0 phase
S (synthetic)—DNA replication occurs
G2 (gap 2)—preparation for division
Prior to division cell makes copy of DNA
DNA helices separated into replication bubbles with replication forks at each end
Each strand acts as template for complementary strand
DNA polymerase begins adding nucleotides at RNA primer
DNA polymerase continues from primer
Synthesizes one leading, one lagging strand
DNA polymerase only works in one direction
Leading strand synthesized continuously
Lagging strand synthesized discontinuously into segments
DNA ligase splices short segments of discontinuous strand together
End result: two identical DNA molecules formed from original
During mitotic cell division one complete copy given to new cell; one retained
in original cell
Process is called semiconservative replication
Each DNA composed of one old and one new strand
Meiosis - cell division producing gametes
Mitotic cell division - produces clones
Essential for body growth and tissue repair
Occurs continuously in some cells
•Skin; intestinal lining
None in most mature cells of nervous tissue, skeletal muscle, and cardiac
muscle
Repairs with fibrous tissue
Mitosis—division of nucleus
Four stages ensure each cell receives copy of replicated DNA
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis—division of cytoplasm by cleavage furrow
Chromosomes become visible, each with two chromatids joined at centromere
Centrosomes separate and migrate toward opposite poles
Mitotic spindles and asters form
Nuclear envelope fragments
Kinetochore microtubules attach to kinetochore of centromeres and draw them
toward equator of cell
Polar microtubules assist in forcing poles apart
Centromeres of chromosomes aligned at equator
Plane midway between poles called metaphase plate
Shortest phase
Centromeres of chromosomes split simultaneously—each chromatid becomes a
chromosome
Chromosomes (V shaped) pulled toward poles by motor proteins of kinetochores
Polar microtubules continue forcing poles apart
Begins when chromosome movement stops
Two sets of chromosomes uncoil to form chromatin
New nuclear membrane forms around each chromatin mass
Nucleoli reappear
Spindle disappears
Begins during late anaphase
Ring of actin microfilaments contracts to form cleavage furrow
Two daughter cells pinched apart, each containing nucleus identical to original
"Go" signals:
Critical volume of cell when area of membrane inadequate for exchange
Chemicals (e.g., growth factors, hormones)
Availability of space–contact inhibition
To replicate DNA and enter mitosis requires
Cyclins–regulatory proteins
Accumulate during interphase
Cdks (Cyclin-dependent kinases)–bind to cyclins  activated
Enzyme cascades prepare cell for division
Cyclins destroyed after mitotic cell division
"Go" signals
G1 checkpoints (restriction point) most important
If doesn't pass  G0–no further division
Late in G2 MPF (M-phase promoting factor) required to enter M phase
"Other Controls" signals
Repressor genes inhibit cell division
E.g., P53 gene
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DNA is master blueprint for protein synthesis
Gene - segment of DNA with blueprint for one polypeptide. (In other words, a specific
location on the DNA molecule.)
Triplets (three sequential DNA nitrogen bases) form genetic library
Bases in DNA are A, G, T, and C
Each triplet specifies coding for number, kind, and order of amino acids in
polypeptide
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Genes composed of exons and introns
Exons code for amino acids
Introns–noncoding segments
Role of RNA
DNA decoding mechanism and messenger
Three types–all formed on DNA in nucleus
Messenger RNA (mRNA); ribosomal RNA (rRNA); transfer RNA (tRNA)
RNA differs from DNA
Uracil is substituted for thymine
Messenger RNA (mRNA)
Carries instructions for building a polypeptide, from gene in DNA to ribosomes
in cytoplasm
Ribosomal RNA (rRNA)
Structural component of ribosomes that, along with tRNA, helps translate
message from mRNA
Transfer RNAs (tRNAs)
Bind to amino acids and pair with bases of codons of mRNA at ribosome to
begin process of protein synthesis
Transcription = DNA into RNA
Translation = RNA in protein
Transfers DNA gene base sequence to complementary base sequence of mRNA
Transcription factors–gene activators
Loosen histones from DNA in area to be transcribed
Bind to promoter-DNA sequence specifying start site of gene on template
strand
Mediate binding of RNA polymerase (enzyme synthesizing mRNA) to
promoter
Three phases
Initiation
RNA polymerase separates DNA strands
Elongation
RNA polymerase adds complementary nucleotides
Termination
Termination signal indicates "stop"
Transcription factors–gene activators
Loosen histones from DNA in area to be transcribed
Bind to promoter-DNA sequence specifying start site of gene on template
strand
Mediate binding of RNA polymerase (enzyme synthesizing mRNA) to
promoter
Initiation
RNA polymerase separates DNA strands
Elongation
RNA polymerase adds complementary nucleotides
Termination
Termination signal indicates "stop"
mRNA edited and processed before translation
Introns removed by spliceosomes
mRNA complex proteins associate to guide export, ensure accuracy for
translation
Converts base sequence of nucleic acids into amino acid sequence of proteins
Involves mRNAs, tRNAs, and rRNAs
Each three-base sequence on DNA (triplet) represented by codon
Codon—complementary three-base sequence on mRNA
Some amino acids represented by more than one codon
45 different types
Binds specific amino acid at one end (stem)
Anticodon at other end (head) binds mRNA codon at ribosome by hydrogen bonds
E.g., if codon = AUA, anticodon = UAU
Ribosome coordinates coupling of mRNA and tRNA; contains three sites
Aminoacyl site; peptidyl site; exit site
Translation is the process in which genetic information carried by an mRNA is
decoded in the
ribosome to form a particular polypeptide.
Three phases that require ATP, protein factors, and enzymes
Initiation
Elongation
Termination
Small ribosomal subunit binds to initiator tRNA and mRNA to be decoded;
scans for start codon
Large and small ribosomal units attach, forming functional ribosome
At end of initiation
tRNA in P site; A site vacant
New amino acids added by other tRNAs as ribosome moves along mRNA
Initial portion of mRNA can be "read" by additional ribosomes
Polyribosome
multiple ribosome-mRNA complex
Produces multiple copies of same protein
Three steps
Codon recognition
tRNA binds complementary codon in A site
Peptide bond formation
Amino acid of tRNA in P site bonded to amino acid of tRNA in A
site
Translocation
tRNAs move one position–A  P; P  E
When stop codon (UGA, UAA, UAG) enters A site
Signals end of translation
Protein release factor binds to stop codon  water added to chain 
release of polypeptide chain; separation of ribosome subunits;
degradation of mRNA
Protein processed into functional 3-D structure
mRNA–ribosome complex directed to rough ER by signal-recognition particle
(SRP)
Forming protein enters ER
Sugar groups may be added to protein, and its shape may be altered
Protein enclosed in vesicle for transport to Golgi apparatus
Complementary base pairing directs transfer of genetic information in DNA into
amino acid sequence of protein
DNA triplets  mRNA codons
Complementary base pairing of mRNA codons with tRNA anticodons ensures
correct amino acid sequence
Anticodon sequence identical to DNA sequence except uracil substituted for
thymine
Intron ("junk") regions of DNA code for other types of RNA:
Antisense RNA
Prevents protein-coding RNA from being translated
MicroRNA
Small RNAs that silence mRNAs made by certain exons
Riboswitches
Folded RNAs that act as switches regulating protein synthesis in
response to environmental conditions
Autophagy
Cytoplasmic bits and nonfunctional organelles put into autophagosomes;
degraded by lysosomes
Ubiquitins
Tag damaged or unneeded soluble proteins in cytosol
Digested by soluble enzymes or proteasomes
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
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 increases cell numbers when needed
Atrophy (decreased size) results from loss of stimulation or use