Cell Structure

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Cell Structure

• Monday, July 14

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Studying Cells

• Cells can be viewed by microscopy which magnifies
  the image
• Light microscopy
• Brightfield (unstained/stained)
• Fluorescence
• Phase-contrast
• Confocal
• Electron microscopy
• Scanning (SEM)
• Transmission (TEM)

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Studying Organelles

• Cells contain membrane bound structures called
  organelles
• These compartments can be isolated by cell
  fractionation and studied
• Different organelles are separated on the basis
  of weight by centrifugation

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Figure 7.4 A prokaryotic cell
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Figure 7.5 Geometric relationships explain why
most cells are microscopic
As an object grows, its volume grows more than
its surface area the smaller the object, the
greater its surface area to volume ratio
A greater surface area to volume ratio
facilitates the exchange of materials between a
cell and its environment
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Organelles of a Cell

• Plasma membrane (PM ) surround cell and allows
  exchange of materials
• Nucleus contains genetic material
• Cytoplasm fluid substance between the PM and
  nucleus
• Endoplasmic Reticulum (ER)
• Smooth lipid synthesis
• Rough protein synthesis
• Golgi Complex synthesis, sorting, and secretion
  of cell products
• Mitochondria/Chloroplasts cellular respiration
• Lysosomes digestive
• Cytoskeleton cell movement

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Figure 7.7 Overview of an animal cell
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Figure 7.8 Overview of a plant cell
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Nucleus

• Nuclear envelope an outer and inner membrane
  encloses the nucleus
• Perforated with nuclear pore complexes which
  regulate the entry and exit of large molecules
• Nucleus contains
• Chromosomes (DNA protein)
• Nucleolus site of ribosome synthesis
• Ribosome site of protein synthesis

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Figure 7.9 The nucleus and its envelope 
Netlike array of protein filaments
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Ribosomes

• Structure
• Ribosomal RNA (rRNA) and protein
• Complex of a large and small subunit
• Function
• Protein synthesis
• Free ribosome
• Cytosolic proteins
• Proteins contained in organelles (mitochondria,
  chloroplasts)
• Bound ribosomes rough ER, nuclear envelope
• Secreted proteins
• Proteins inserted into membranes
• Proteins contained in organelles (golgi,
  lysosomes)

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Endomembrane System
Rough ER
Rough ER
Secretory pathway
Constitutive or regulated
Endocytic pathway
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Endoplasmic Reticulum

• Network of membranous tubules and sacs called
  cisternae, continuous with the nuclear membrane
• Smooth ER
• Lipid synthesis
• Detoxify drugs and poisons
• Muscle contraction (sequester Ca ions)
• Rough ER (studded with ribosomes)
• Synthesis of secretory proteins
• Threaded through the ribosome into the ER
• Attachment of carbohydrate groups (glycoproteins)
• Transported to PM in transport vesicles
• Synthesis of integral membrane proteins and
  phospholipids

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Figure 7.11 Endoplasmic reticulum (ER)
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Golgi Apparatus

• Structure flattened membranous sacs
• Polarity ER ? cis-Golgi ? trans-Golgi ? PM
• Functions
• Modification of proteins, glycoproteins, and
  phospholipids from the ER
• Synthesis of polysaccharides
• Molecules are modified or synthesized in steps as
  they travel through the Golgi
• This is accomplished by each cisternae containing
  its own set of specialized enzymes
• Sorting of molecules to their appropriate
  destination by molecular identification tags
• Ship molecules in transport vesicles to
  destination by tags on the vesicles that
  recognize docking sites

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Figure 7.12 The Golgi apparatus
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Lysosomes

• Digestive compartments membrane-bound sac
  containing hydrolytic enzymes (acid hydrolases)
• Can digest all 4 kinds of macromolecules
• Acidic environment (pH5)
• Membrane pumps in H ions from cytosol
• Functions
• Phagocytosis engulfing pathogens
• Autophagy recycle organic material
• Apoptosis programmed cell death

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Figure 7.13 Lysosomes
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Figure 7.16 Review relationships among
organelles of the endomembrane system 
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Mitochondria and Chloroplasts

• Convert energy to usable forms
• Mitochondria cellular respiration
• Glucose Oxygen ? ATP CO2 Water
• Chloroplasts photosynthesis
• Light CO2 Water ? Glucose Oxygen
• Semi-autonomous organelles
• Contain own DNA for protein synthesis
• Imports proteins from cytosol
• Structure
• Mitochondria smooth outer membrane, folded
  inner membrane (cristae) enclosing matrix
• Chloroplasts two membranes enclose fluid
  (stroma) and thylakoids (flat, stacked membranes)

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Figure 7.17 The mitochondrion
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Figure 7.18 The chloroplast
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Peroxisomes

• Metabolic functions reactions that transfer
  hydrogen from various substrates to oxygen,
  forming hydrogen peroxide (H2O2) as a byproduct
• Converts H2O2 to water

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Cytoskeleton

• Structure Network of fibers extending through
  the cell
• Functions
• Maintain organization and shape
• Disassemble and reassemble rapidly to change cell
  shape
• Cell motility cilia and flagella movement
• Vesicular traffiking vesicles travel to
  destination along tubules
• Transmission of mechanical signals

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Figure 7.22 Centrosome containing a pair of
centrioles
Organize Microtubules (MT) Assembly MTs grow out
of a centrosome (located near nucleus) to form a
pair of centrioles which are composed of 9 sets
of triplet MT organized in a ring
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Figure 7.23 A comparison of the beating of
flagella and cilia
Animal sperm
Unicellular organisms
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Figure 7.24 Ultrastructure of a eukaryotic
flagellum or cilium
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Figure 7.21 Motor molecules and the cytoskeleton
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Figure 7.25 How dynein walking moves cilia and
flagella
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Functions of microfilaments

• Structural support
• Microvilli bundles of microfilaments (MF)
• Increase surface area for membrane transport

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Cell Motility
Localized contraction near the trailing end
squeezing the interior fluid to the front
Myosin motor pulls the actin causing the
filaments to slide pass one another
Cytoplasm circulates through the cell to
distribute materials by the interaction of actin
and myosin
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Figure 7.x4 Actin and keratin
Intermediate filaments (keratin) are more
permanent structures in the cell than MF and MT
and they are important in cell shape and
organelle positioning
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Cell Surfaces plants

• Cell walls are microfibrils of cellulose embedded
  in a matrix of other polysaccharides and proteins
• Cells are linked together by the middle lamella
  composed of a sticky polysaccharide called pectin

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Cell Surfaces animals

• Extracellular Matrix (ECM) is composed of
  glycoproteins
• Proteoglycans weaves a network
• Collagen strong fibers embedded in network
  (most abundant)
• Fibronectins bind to receptors (integrins) on
  the plasma membrane
• ECM communicates with the cell by transmitting
  signals
• Integrin receptors span the membrane and are
  bound to MF on the cytoplasmic side of the cell
• Cytoskeletal components may then trigger chemical
  signaling pathways

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Figure 7.29 Extracellular matrix (ECM)
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Intercellular Junctions plants

• Neighboring cells adhere, interact, and
  communicate through special patches of direct
  physical contact
• Plants cell walls are perforated with
  plasmodesmata that connect cells
• Allows passage of cytoplasm
• Plasma membrane is continuous

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Intercellular Junctions animals

• Tight junctions
• continuous belt of fused membranes (fluid
  impermeable)
• Desmosomes (anchoring junctions)
• fasten cells into strong sheets strengthened by
  Ifs
• Gap junctions (communicating junctions)
• Cytoplasmic channels between cells that are
  surrounded by proteins

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Figure 7.30 Intercellular junctions