Cells: The Living Units

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Chapter 3

• Cells The Living Units
• J.F. Thompson, Ph.D. J.R. Schiller, Ph.D. G.
  Pitts, Ph.D.

2
The Cell Theory

• The Cell is the fundamental structural and
  functional unit of living organisms
• The activity of an organism is dependent on both
  the individual and collective activities of its
  cells
• Cell actions are determined and made possible by
  specific subcellular structures The Principle
  of Complementarity
• Cells come from cells

3
The Cellular Basis of Life

• Overview
• The cell is the unit of life it contains
  everything needed to survive.
• Loss of cellular homeostasis ? disease
• Made of complex organic molecules (containing
  mostly carbon, hydrogen, oxygen, nitrogen and
  traces of others elements)
• Organized into multiple structures called
  organelles
• Many different shapes and sizes

20 µm - 1 m
5-24 µm
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Not All Cells Contain Every Component

• 3 basic parts
• Nucleus
• Cytoplasm
• all cellular contents between plasma membrane and
  nucleus
• organelles are specialized internal structures
• Plasma membrane

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Plasma Membrane Structure Fluid Mosaic Model

• Fluid membrane components can move within the
  membrane
• Mosaic a mix of molecular components
• Lipids phospholipid bilayer foundation with some
  cholesterol mixed in
• Proteins diverse, can be varied and regulated to
  alter membrane functions
• Carbohydrates combined with lipids/proteins make
  up the glycocalyx
• sticky surface that functions in cell
  recognition
• Individual molecules are recycled often

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Plasma Membrane Structure

• Membrane Chemistry and Anatomy
• 50-50 ratio by weight lipid/protein
• Far more lipid molecules than protein molecules
  because of protein molecules larger sizes

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Plasma Membrane Chemistry

• 50-50 ratio by weight lipid/protein
• Far more lipid molecules than protein molecules
  because of protein molecules larger sizes

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Plasma Membrane lipids

• Phospholipids 75
• Bilayer
• Phosphate heads are polar, thus hydrophilic,
  point out towards interstitial fluid and in
  toward cytoplasm
• Non polar fatty acid tails are hydrophobic, make
  up middle of membrane
• Glycolipids 5
• Contribute to glycocalyx
• Cholesterol 20
• Increases mobility (fluidity) of phospholipids

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Plasma Membrane Proteins

• Determine the functions a cell can perform
• Composition varies widely among cell types
• Integral proteins located within
• the membrane
• channels
• transporters
• receptors
• intracellular junctions
• enzymes
• cytoskeleton anchors
• cell identity markers (glycoproteins)
• peripheral proteins located on either
• face of the membrane
• A similar list of many functions

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Membrane Junctions
Tight Junctions Impermeable junctions that
encircle adjacent cells. Desmosones Anchoring
junctions between cells. Gap Junctions
Communicating junctions that allow substance to
pass from one cell to another.
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Membrane Transport

• Selective permeability - allows passage of some
  substances limits others
• Dependent on
• molecular size
• lipid solubility
• charge
• membranes impermeable to all charged molecules
• Ions only move through a membrane through
  channels
• the presence of channels transporters is very
  specific

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Passive Transport

• Moves materials across cell and organelle
  membranes without expending cellular energy
• Simple Diffusion
• kinetic energy is everywhere - allows mixing or
  diffusion
• diffusion requires a concentration gradient
• high concentration in one area, lower
  concentration in another
• if areas are continuous (connected), particles
  move with (down) the concentration gradient
• eventually it reaches equal concentration
  everywhere - equilibrium

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Factors Affecting Diffusion

• Increased temperature increases diffusion rate
• Greater concentration gradients increase
  diffusion rate
• Larger surface area increases diffusion rate
• Smaller particle sizes increase diffusion rate
• Time - diffusion decreases as concentrations
  equalize

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Simple Diffusion

• water and lipid-soluble molecules move freely
  through the membrane

(a)
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Facilitated Diffusion

• Integral proteins allow larger molecules or
  charged/polar ions to diffuse across membrane
• Passive transport because substances diffuse
  down the concentration gradient - no cellular
  energy required
• May be regulated by hormones
• example insulin will increase cellular
  glucose uptake

Channel
Carrier
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Osmosis

• The diffusion of water from an area of higher
  H2O to lower H2O
• Water concentration 1/solute concentration
• Polar water molecules move through aquaporin
  channels (AQP)
• Or (perhaps) wiggle through phospholipids

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Osmolarity and Osmotic Pressure

• Osmolarity the total concentration of all
  solutes
• Osmotic pressure (s) the net pressure effect of
  individual particles in solution
• Hydrostatic pressure (h) fluid pressure is
  created by osmosis

h
s
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Tonicity

• The ability of a solution to change the shape
  (tone) of cells by altering their internal
  water volume
• Isotonic solutions
• Solute concentration is the same inside and
  outside the cells
• No net diffusion of water (e.g., normal saline
  solution is isotonic to blood plasma)

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Tonicity

• Hypertonic solution
• a solution with higher solute than inside cells
  
• water moves out of cells cells shrink
  (crenation)
• Hypotonic solution
• a solution with lower solute than inside cells
• water moves into cells, cells swell may rupture

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Active Transport Processes

• Energy must be expended for active processes
  (they require the energy derived from splitting
  ATP energy of hydrolysis)
• Some substances cannot move passively because
  they
• are too big
• have the wrong charge
• must be moved against concentration gradient
• Two mechanisms
• active transport
• vesicular transport

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1. Active Transport

• Integral membrane proteins (pumps or
  transporters) use ATP hydrolysis energy to move
  substances against their concentration gradient
• Two types of Active Transport
• Primary active transport pump uses ATP energy to
  transport substances directly.
• Secondary active transport energy from ATP
  creates concentration gradient that is used to
  drag or push transport of another
  substance(s) by facilitated transport

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a. Primary Active Transport

• Primary active transport pump uses ATP energy to
  transport substances directly.
• Na and Cl- almost always moved by primary active
  transport
• Many other substances, K, CA2, H, glucose,
  amino acids, etc. may be transported by primary
  active transport
• Na/ K ATPase
• pumps 3 Na out 2 K in during each cycle
• because Na K ions always leaks across the
  membrane, the pump must always operate

See also Fig. 3.10, p. 76
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b. Secondary Active Transport

• energy from ATP drives ion pump to create
  concentration gradient then carrier protein uses
  energy of concentration gradient to drag or
  push transport of another substance(s) by
  facilitated transport
• Symport/symporter transports two substances in
  the same direction (co-transport)
• Antiport/antiporter transports two substances
  in opposite directions (counter-transport)

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Vesicular Transport

• The transfer of large molecules and fluids across
  membranes via vesicles. This type of transport
  mechanism includes exocytosis and endocytosis.

Exocytosis
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Endocytosis
Phagocytosis
Clathrin-mediated endocytosis
Receptor-mediated endocytosis
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Resting Membrane Potential

• Generating/maintaining a resting membrane
  potential
• all cells are polarized
• negatively charged inside
• positively charged outside
• Na/K ATPase creates the unequal charge
  distribution
• The sodium-potassium pump transports 3 Na out
  2 K in with each cycle, using the energy of one
  ATP hydrolysis
• More K tends to diffuse out through leak
  channels than Na
• More negatively charged proteins (A-) are located
  within the cell
• This creates the charge differential
• Electrochemical gradient
• the net effect of all charged ions on either side
  of the membrane

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Cell-Environment Interactions

• Cells interact with their environment, including
  other cells, via various molecules in their cell
  membrane
• Cell adhesion molecules Attach cells to their
  environment
• Membrane receptors Detect contact and chemical
  signals
• Contact inhibition regulates cell proliferation
• Hormones, neurotransmitters, and other ligands
• Voltage-sensitive membrane channel proteins
  Detect voltage changes
• Essential for nerve and muscle function

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Cell-Environment Interactions Membrane Receptors

• contact signaling - identifying neighbor cells
• contact inhibition - regulates cell proliferation
• electrical signaling - channels responding to
  voltage changes (changes in the concentrations of
  charged ions)
• chemical signaling various signal compounds
  neurotransmitters, hormones, local hormones, and
  other ligands

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Chemical Signaling G-Protein Linked Receptors

• A chemical signaling mechanism
• Ligand binds to receptor
• Receptor activates G protein that activates an
  enzyme
• The enzyme activates a second messenger
• Second messenger activates other enzymes

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The Cytoplasm

• Cytosol viscous, semitransparent fluid in which
  other cytoplamic elements are suspended
• Cytoplasmic organelles you should know
• Mitochondria
• Ribosomes
• Rough endoplasmic reticulum (Rough ER)
• Smooth endoplasmic reticulum (Smooth ER)
• Golgi apparatus
• Lysosomes Peroxisomes
• Cytoskeleton
• Nucleus
• Nucleoli
• Inclusion bodies substances
• not enclosed in membrane

• Centrioles
• Cilia
• Flagella

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The Cytoplasm

• Cytosol viscous, semitransparent fluid in which
  other cytoplamic elements are suspended
• Cytoplasmic organelles you should know
• Mitochondria
• Ribosomes
• Rough endoplasmic reticulum (Rough ER)
• Smooth endoplasmic reticulum (Smooth ER)
• Golgi apparatus
• Lysosomes Peroxisomes
• Cytoskeleton
• Nucleus
• Nucleoli
• Inclusion bodies substances
• not enclosed in membrane

• Centrioles
• Cilia
• Flagella

not membrane-bound
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Mitochondria

• The powerhouse of the cell
• Generates most ATP
• Have 2 membranes
• Contain mitochondrial DNA (passed from mother
  only)
• Arose from bacterial infection

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Endoplasmic Reticulum

• Interconnected tubes parallel membranes
  enclosing cisternae
• Rough ER
• Studded with ribosomes
• Manufactures secreted proteins
• Manufactures integral proteins and phospholipids

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Rough Endoplasmic Reticulum
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Smooth Endoplasmic Reticulum

• Does not contain ribosomes
• Not involved in protein synthesis
• Catalyzes chemical reactions
• Lipid metabolism
• Steroid synthesis
• Absorption, synthesis, and transport of fats
• Drug detoxification
• Breakdown of glycogen
• Ca2 ion storage

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Golgi Apparatus

• The protein traffic director.
• Modifies and packages proteins and lipids made in
  the RER.
• Produces lysosomes and secretory
  vesicles/granules.

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Endomembrane Transport
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Lysosomes

• Disintegrator bodies
• Spherical organelles containing digestive enzymes
  under acidic conditions
• Digest
• Bacteria, viruses, toxins
• Non-functional organelles
• Non-useful tissues
• bone

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Peroxisomes

• Peroxide bodies
• Detoxify alcohols and formaldehyde
• Neutralize free radicals (chemicals with unpaired
  electrons)

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Cytoskeleton I

• Microtubules
• Large diameter, hollow tubes made of tubulins
• Extend outward from the centrosome
• Anchor and transport organelles
• Very dynamic

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Cytoskeleton II

• Intermediate filaments
• Medium sized filament
• Act as guy wires

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Cytoskeleton III

• Microfilaments
• The smallest filament
• Composed of actin
• Involved in cell motility and cell shape
• Responsible for muscle contraction when actin
  interacts with myosin

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Centrosome Centromere

• Centrosome
• Microtubule organizing center
• Contains centrioles
• Centrioles
• Involved in mitosis
• Give rise to cilia and flagella

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The Nucleus

• Cells control center
• Usually visible
• Nuclear envelope
• double membrane
• nuclear pores in membrane allow passage of
  substances between cytoplasm and nucleus
• Contains the hereditary material (DNA)
• DNA carries instructions for making proteins
• determines cell structure, coordinates activities
  of the cell

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The Nucleolus

• Nucleoli
• Darker staining, oval/spherical bodies within the
  nucleus
• Clusters of DNA, RNA, and protein (not
  membrane-bound)
• The site of ribosome assembly

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Chromatin

• Grainy threadlike material seen in the nucleus
• DNA molecules organize into large, compact
  visible chromosomes before each cell division
• Chromosomes contain DNA coiling proteins
• DNA is first wrapped around histone proteins -
  "beads on a string
• Higher levels of DNA packaging (supercoiling)

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Chromatin Organization

• DNA is packaged by various levels of supercoiling

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Chromatin Organization
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The Cell Cycle

• Interphase
• normal cell growth
• three subphases
• G1
• growth and metabolism
• protein replication
• S (synthesis of DNA)
• preparation for division
• DNA replication
• G2
• more metabolism
• further preparation
• enzymes, proteins for mitosis
• Mitosis nuclear division

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Mitosis - Nuclear Division
P

• Prophase
• Metaphase
• Anaphase
• Telophase

M
A
T
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DNA Replication I
Unwinding of DNA by helicase
Replication fork
Replication bubble
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DNA Replication II
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DNA Replication Summary

• semi-conservative
• helicase unwinds DNA
• DNA polymerase
• one strand is the template
• builds a complementary strand
• bases pair with hydrogen bonds
• A-T
• C-G

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Protein Synthesis

• Transcription (in nucleus)
• DNA gene blue print
• template
• triplet code - 3 bases/AA
• exons - expressed
• introns excised
• RNA tools for protein synthesis
• mRNA
• tRNA
• rRNA
• Translation (in cytoplasm)
• Ribosome
• codons are read to build a primary protein
  structure

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Transcription I
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Transcription II
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Translation I
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Translation at the Ribosome
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Gene Expression Review

• DNA contains a sequence of nitrogenous bases
  which codes for the sequence of amino acids in a
  protein
• A triplet code, in which each codon is composed
  of 3 bases, forms the genetic code
• During transcription
• one strand of DNA serves as a template for
  formation of messenger RNA
• mRNA has bases complementary to the base series
  in the DNA
• Messenger RNA is processed, with intron removal,
  before leaving the nucleus

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Gene Expression Review (cont.)

• mRNA carries the codon sequence to the ribosomes
  (rRNA and protein) in the cytoplasm
• Each tRNA carries a particular kind of amino acid
• each tRNA also carries a 3-base anticodon which
  pairs complementarily to a codon of the mRNA
• During translation
• the linear sequence of codons in the mRNA
  determines the order of tRNAs and their attached
  amino acids
• sequential peptide bond formation produces the
  primary structure of the protein at the ribosome

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End Chapter 3