Cell Structure and Function

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Cell Structure and Function

• Chapter 4

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Cell Theory

1. Every organism is composed of one or more cells
2. Cell is smallest unit having properties of life
3. Life arises from growth and division of single
   cells (cells come 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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Cell size

• Most cells range in diameter from 1 µm-100 µm
• (1 µm 10-6 m)
• Why are cells so small?

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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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Relationship Between Surface Area Volume
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Structure of Cells

• Two Main Classes of Cells
• Prokaryotic simple organization with no nucleus
  or organelles.
• Eukaryotic highly developed membrane system, DNA
  encapsulated in nucleus, various organelles.

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Prokaryotic Cell
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Prokaryotic Cells

• Archaea and Bacteria
• Contain no nucleus or organelles
• Made up of
• Cell membrane
• Cell wall
• Cytoplasm
• Nucleoid region (DNA)
• Ribosomes
• May contain
• Pili
• Capsule
• flagella

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Prokaryotic Structure
pilus
cytoplasm with ribosomes
DNA
flagellum
capsule
cell wall
plasma membrane
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Eukaryotic cells
Plant Cell
Animal Cell
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Eukaryotic Cellular Organization

• Plasma membrane
• Encapsulates the cell, lipid bilayer
• Nucleus
• Contains the genetic material
• Cytoplasm
• Everything in between
• Contains various organelles

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Organelles

• Small specialized membrane-bound compartments
  found in EUKARYOTIC cells
• Common organelles
• Nucleus
• Endoplasmic reticulum (ER)
• Golgi body
• Lysosomes
• Vesicles
• Vacuoles
• Mitochondria
• Chloroplasts

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Non-Membrane Structures

• Cytoskeleton
• Ribosomes

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Membranes

• Composed of phospholipid bilayer
• Double layer of phospholipids
• Organized with their hydrophobic tails pointing
  inward.
• The hydrophilic heads point out in the solvent
• Also contains proteins
• Channels allow molecules in or out of the cell
  either by passive flow or active pumping.
• Receptors bind signaling molecules given off the
  other cells and trigger changes within the cell.

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Membranes
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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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Components of Nucleus
Nuclear envelope

• Chromatin
• DNA Proteins

• Nucleolus
• Site of ribosome production

Figure 4.11bPage 62
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Nuclear Envelope

• Double lipid bilayer
• Contains Nuclear pores involved in transporting
  material in and out of nucleus

Nuclear pore
bilayer facing cytoplasm
Nuclear envelope
bilayer facing nucleoplasm
Figure 4.12bPage 63
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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
• A system for the transport processing of
  complex molecules running from the nucleus to the
  cells surface

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Components of Cytomembrane System

• Nuclear Envelope
• Smooth Rough Endoplasmic Reticulum (ER)
• Golgi Body
• Vesicles
• Cell membrane

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Endoplasmic Reticulum

• In animal cells, continuous with nuclear membrane
• Extends throughout cytoplasm
• Two regions
• Rough (RER) Produces and processes proteins
• Smooth (SER) Lipid assembly

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Golgi Body

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

budding vesicle
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Vesicles

• Membranous sacs that move through cytoplasm
• Lysosomes - Contain hydrolytic enzymes to break
  down wastes (garbage recycling) require low pH
  (5-6)
• Peroxisomes - contain enzymes to break down
  dangerous oxygen molecules
• Transport vesicles - Carry proteins, etc. to
  different parts of cell
• Vacuoles - storage (water, nutrients, wastes)
  very large vacuole in plants

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Mitochondria

• ATP-producing powerhouses - Site of cellular
  respiration
• Membranes form two distinct compartments
• Matrix Inner compartment
• Cristae folds of the inner membrane

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Specialized Plant Structures

• Cell Wall
• Chloroplast
• Central Vacuole

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Chloroplasts

• Found only in plants and are the location of
  photosynthesis
• Convert sunlight into ATP which is then used to
  convert water and CO2 into sugar.
• Pigments contained within trap light and harness
  its energy

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Cell Wall

• Made of cellulose (polysaccharide made of glucose
  subunits)
• Makes plant cells rigid
• Stores water, nutrients, wastes
• Maintains pressure and rigidity of cell

Central Vacuole
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Mitochondrial and Chloroplast Origins

• Mitochondria resemble bacteria
• Have own DNA, ribosomes
• Divide on their own
• Endosymbiosis May have evolved from ancient
  bacteria that were engulfed but not digested

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Mitochondrial and Chloroplast Origins
Endosymbiosis
http//evolution.berkeley.edu/evolibrary/images/en
dosymbiosis/endosymbiosis.gif
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Mitochondrial and Chloroplast Origins
Endosymbiosis
http//faculty.ircc.edu/faculty/tfischer/images/en
dosymbiosis.jpg
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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
tubulin subunit

• Composed of tubulin
• Arise from microtubule organizing centers (MTOCs)
• Involved in shape, motility, cell division

Figure 4.21Page 71
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Microfilaments

• Thinnest elements
• Composed of actin
• Take part in movement, formation, and maintenance
  of cell shape

actin subunit
Figure 4.21Page 71
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Intermediate Filaments

• Only in animal cells of certain tissues
• Most stable cytoskeletal elements
• Six known groups

one polypeptide chain
Figure 4.21Page 71
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3D View of Cytoskeleton
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Motor Proteins

• Kinesins and dyneins move along microtubules
• Myosins move along microfilaments

kinesin
microtubule
Figure 4.24b, Page 72
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Flagella and Cilia
microtubule

• Structures for cell motility
• 9 2 internal structure

dynein
Figure 4.25Page 73
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Flagella and Cilia

• Both are used for cellular propulsion
• Cilia are also used for food collection or to
  move substances across a cells surface
• Cilia are short and more numerous, while flagella
  are longer (gt2mm), only 1 or 2 are found per cell

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Flagella and Cilia

• Both are constructed similarly w/ a central
  microtubule bundle containing a 92 arrangement

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Cilia and Flagella
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Ribosomes

• Complex made up of proteins and RNA
• Site of protein synthesis
• Made up of two subunits, produced in the
  nucleolus
• Subunits are then exported into cytoplasm
• In the cytoplasm they are assembled into a
  functional ribosome

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Cell-to-Cell Junctions

• Plants
• Plasmodesmata
• Animals
• Tight junctions
• Adhering junctions
• Gap junctions

plasmodesma
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Animal Cell Junctions

• Tight junctions are bands of proteins that link
  together cells of epithelial tissue forming a
  water-tight seal.
• Adherens junctions are found below tight
  junctions are important in tissues that stretch
• Gap junctions link the cytoplasm of neighboring
  cells so that they may communicate or share
  nutrients

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Animal Cell Junctions