Cells

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Cells

• The Units of Life

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Early Discoveries

• Mid 1600s - Robert Hooke observed and described
  cells in cork
• Late 1600s - Antony van Leeuwenhoek observed
  sperm, microorganisms
• 1820s - Robert Brown observed and named nucleus
  in plant cells

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Cell Theory

• Schleiden and Schwann
• Every organism is composed of one or more cells
• Cell is smallest unit having properties of life
• Virchow
• All exisiting cells arise from pre-existing cells.

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Cell

• Smallest unit of life
• Can survive on its own or has potential to do so
• Is highly organized for metabolism
• Senses and responds to environment
• Has potential to reproduce

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Measuring
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Cells Vary in Size
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Why Are Cells So Small?

• Surface-to-volume ratio
• The bigger a cell is, the less surface area there
  is per unit volume
• Above a certain size, material cannot be moved in
  or out of cell fast enough

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Size is Limited
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Ways to Study CellsMicroscopes

• Create detailed images of something that is
  otherwise too small to see
• Light microscopes
• Simple or compound
• Electron microscopes
• Transmission EM or Scanning EM

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Different Types of Light Microscopy A Comparison
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Limitations of Light Microscopy

• Wavelengths of light are 400-750 nm
• If a structure is less than one-half of a
  wavelength long, it will not be visible
• Light microscopes can resolve objects down to
  about 200 nm in size

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Electron Microscopy

• Uses streams of accelerated electrons rather than
  light
• Electrons are focused by magnets rather than
  glass lenses
• Can resolve structures down to 0.5 nm

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Elecrton Microscopes
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Electron micrographs
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Ways to Study CellsCell Fractionation
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Structure of Cells

• Two types of cells
• Prokaryotic
• Eukaryotic
• All cells have
• Plasma membrane
• Region where DNA is stored
• Cytoplasm

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Prokaryotic Cells

• No nucleus
• Nucleoid area where DNA resides
• No membrane bound organelles.
• 70s ribosomes
• Cell walls contain petidoglycan or pseudomurein
• Prokaryotic Organisms
• Archaeobacteria
• Eubacteria
• Cyanobacteria

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A prokaryotic cell
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E. coli
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Eukaryotic Cells

• Have a nucleus and other organelles
• Eukaryotic organisms
• Protistans
• Fungi
• Plants
• Animals

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Overview of an animal cell
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The nucleus and its envelope 
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Functions of Nucleus

• Keeps the DNA molecules of eukaryotic cells
  separated from metabolic machinery of cytoplasm
• Makes it easier to organize DNA and to copy it
  before parent cells divide into daughter cells

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Cytomembrane System

• Group of related organelles in which lipids are
  assembled and new polypeptide chains are modified
• Products are sorted and shipped to various
  destinations
• Components of the cytomembrane system
• Endoplasmic reticulum
• Golgi apparatus
• vesicles

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Endoplasmic Reticulum

• In animal cells, continuous with nuclear membrane
• Extends throughout cytoplasm
• Two regions - rough and smooth

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Rough ER

• Arranged into flattened sacs
• Ribosomes on surface give it a rough appearance
• Some polypeptide chains enter rough ER and are
  modified
• Cells that specialize in secreting proteins have
  lots of rough ER

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Smooth ER

• A series of interconnected tubules
• No ribosomes on surface
• Lipids assembled inside tubules
• Smooth ER of liver inactivates wastes, drugs
• Sarcoplasmic reticulum of muscle is a specialized
  form

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Endoplasmic reticulum (ER)
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Golgi Bodies

• Put finishing touches on proteins and lipids that
  arrive from ER
• Package finished material for shipment to final
  destinations
• Material arrives and leaves in vesicles

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The Golgi apparatus
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Vesicles

• Membranous sacs that move through the cytoplasm
• Lysosomes
• Peroxisomes

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Lysosomes
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Review relationships among organelles of the
endomembrane system 
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The mitochondrion, site of cellular respiration
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Organelles with no Membranes

• Ribosomes
• Function in protein synthesis
• Cytoskeleton
• Function in maintenance of cell shape and
  positioning of organelles
• Centrioles (animals only)
• Function during cell division

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Ribosomes
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Cytoskeleton

• Present in all eukaryotic cells
• Basis for cell shape and internal organization
• Allows organelle movement within cells and, in
  some cases, cell motility

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Cytoskeletal Elements
intermediate filament
microtubule
microfilament
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Microtubules

• Largest elements
• Composed of the protein tubulin
• Arise from microtubule organizing centers (MTOCs)
• Polar and dynamic
• Involved in shape, motility, cell division

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Microtubules

• Largest elements
• Composed of the protein tubulin
• Arise from microtubule organizing centers (MTOCs)
• Polar and dynamic
• Involved in shape, motility, cell division

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Microfilaments

• Thinnest cytoskeletal elements
• Composed of the protein actin
• Polar and dynamic
• Take part in movement, formation and maintenance
  of cell shape

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Intermediate Filaments

• Present only in animal cells of certain tissues
• Most stable cytoskeletal elements
• Six known groups
• Different cell types usually have 1-2 different
  kinds

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Cell Junctions
tight junctions
gap junction
adhering junction
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Cell Membranes

• Structure and Function

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lipid bilayer
fluid
fluid
one layer of lipids
one layer of lipids
Fig. 4.3, p. 52
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Lipid Bilayer

• Main component of cell membranes
• Gives the membrane its fluid properties
• Two layers of phospholipids

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Figure 8.2 Two generations of membrane models
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The detailed structure of an animal cells plasma
membrane, in cross section
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Functions of Membrane Proteins

• Transport
• Enzymatic activity
• Receptors for signal transduction

Figure 3.4.1
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Functions of Membrane Proteins

• Intercellular adhesion
• Cell-cell recognition
• Attachment to cytoskeleton and extracellular
  matrix

Figure 3.4.2
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oligosaccharide groups
cholesterol
phospholipid
EXTRACELLULAR ENVIRONMENT
(cytoskeletal pro-teins beneatch the plasma
membrane)
open channel protein
LIPID BILAYER
RECEPTOR PROTEIN
gated channel proten (open)
active transport protein
gated channel proten (closed)
ADHESION PROTEIN
RECOGNITION PROTEIN
(area of enlargment)
CYTOPLASM
TRANSPORT PROTEINS
PLASMA MEMBRANE
Fig. 4.4, p. 53
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The fluidity of membranes
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Cell Membranes Show Selective Permeability
O2, CO2, and other small nonpolar moleculesand
H2O
C6H12O6, and other large, polar (water-soluble)
molecules ions such as H, Na, CI-, Ca plus
H2O hydrogen-bonded to them
X
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Membrane Crossing Mechanisms

• Diffusion across lipid bilayer
• Passive transport
• Active transport
• Endocytosis
• Exocytosis

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Diffusion

• The net movement of like molecules or ions down a
  concentration gradient
• Although molecules collide randomly, the net
  movement is away from the place with the most
  collisions (down gradient)

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Concentration Gradient

• Means the number of molecules or ions in one
  region is different than the number in another
  region
• In the absence of other forces, a substance moves
  from a region where it is more concentrated to
  one one where its less concentrated - down
  gradient

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Factors Affecting Diffusion Rate

• Steepness of concentration gradient
• Steeper gradient, faster diffusion
• Molecular size
• Smaller molecules, faster diffusion
• Temperature
• Higher temperature, faster diffusion
• Electrical or pressure gradients

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The diffusion of solutes across membranes
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Osmosis A Special Case of Simple Diffusion

• Diffusion of water molecules across a selectively
  permeable membrane
• Direction of net flow is determined by water
  concentration gradient
• Side with the most solute molecules has the
  lowest water concentration

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Tonicity

• Refers to relative solute concentration of two
  fluids
• Hypertonic - having more solutes
• Isotonic - having same amount
• Hypotonic - having fewer solutes

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Tonicity and Osmosis
2 sucrose
water
10 sucrose
2 sucrose
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The water balance of living cells
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The contractile vacuole of Paramecium an
evolutionary adaptation for osmoregulation
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Increase in Fluid Volume
compartment 1
compartment 2
membrane permeable to water but not to solutes
fluid volume increases In compartment 2
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Passive Transport

• Faciltiated Diffusion
• Flow of solutes through the interior of passive
  transport proteins down their concentration
  gradients
• Passive transport proteins allow solutes to move
  both ways
• Does not require any energy input

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Transport Proteins

• Span the lipid bilayer
• Interior is able to open to both sides
• Change shape when they interact with solute
• Play roles in active and passive transport

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Two models for facilitated diffusion
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Facilitated Diffusion
solute
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Active Transport

• Net diffusion of solute is against concentration
  gradient
• Transport protein must be activated
• ATP gives up phosphate to activate protein
• Binding of ATP changes protein shape and affinity
  for solute

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Active Transport
High solute concentration
Low solute concentration

• ATP gives up phosphate to activate protein
• Binding of ATP changes protein shape and affinity
  for solute

P
ATP
ADP
P
P
P
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The sodium-potassium pump a specific case of
active transport
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Review passive and active transport compared
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Bulk Transport Another form of Active Transport
Exocytosis
Endocytosis
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The three types of endocytosis in animal cells
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Mitosis

• How to clone a nucleus

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Roles of Mitosis

• Multicelled organisms
• Growth
• Cell replacement
• Some protistans, fungi, plants, animals
• Asexual reproduction

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Chromosome

• A DNA molecule attached proteins
• Duplicated in preparation for mitosis

one chromosome (unduplicated)
one chromosome (duplicated)
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Cell Cycle

• Cycle starts when a new cell forms
• During cycle, cell increases in mass and
  duplicates its chromosomes
• Cycle ends when the new cell divides

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Interphase

• Usually longest part of the cycle
• Cell increases in mass
• Number of cytoplasmic components doubles
• DNA is duplicated

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Mitosis

• Period of nuclear division
• Usually followed by cytoplasmic division
• Four stages
• Prophase
• Metaphase
• Anaphase
• Telophase

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Control of the Cycle

• Once S begins, the cycle automatically runs
  through G2 and mitosis
• The cycle has a built-in molecular brake in G1
• Cancer involves a loss of control over the cycle,
  malfunction of the brakes

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Stopping the Cycle

• Some cells normally stop in interphase
• Neurons in human brain
• Arrested cells do not divide
• Adverse conditions can stop cycle
• Nutrient-deprived amoebas get stuck in interphase

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Stages of Mitosis

• Prophase
• Metaphase
• Anaphase
• Telophase

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Early Prophase - Mitosis Begins

• Duplicated chromosomes begin to condense

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Late Prophase

• New microtubules are assembled
• One centriole pair is moved toward opposite pole
  of spindle
• Nuclear envelope starts to break up

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Transition to Metaphase

• Spindle forms
• Spindle microtubules become attached to the two
  sister chromatids of each chromosome

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Metaphase

• All chromosomes are lined up at the spindle
  equator
• Chromosomes are maximally condensed

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Anaphase

• Sister chromatids of each chromosome are pulled
  apart
• Once separated, each chromatid is a chromosome

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Telophase

• Chromosomes decondense
• Two nuclear membranes form, one around each set
  of unduplicated chromosomes

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Results of Mitosis

• Two daughter nuclei
• Each with same chromosome number as parent cell
• Chromosomes in unduplicated form

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Cytoplasmic Division

• Usually occurs between late anaphase and end of
  telophase
• Two mechanisms
• Cell plate formation (plants)
• Cleavage (animals)

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Animal Cell Division