The Cell Cellular Level of Organization

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The Cell -Cellular Level of Organization

• A. Cell Theory
•         1. All organisms, both unicellular and
  multicellular, are made up of cells.         2.
  Cells are the smallest units of living matter and
  structural and functional units of all organisms.
         

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• 3. In 1830s, Mathias Schleiden (plants) and
  Theodore Schwann (animals) declared organisms
  were made of cells.
• 4. Cells are capable of self-reproduction Rudolf
  Virchow declared cells come only from preexisting
  cells.

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The smallest unit of life capable of
independently sustaining and reproducing itself.
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Units for measuring size in cells
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Cell Size

•    1. Cells range in size from a frogs egg (one
  millimeter) down to one micrometer.

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Limits to size of cell

• As a cell increases in size (volume), the surface
  area does NOT grow at the same rate
• Because transport through the cell is over a
  longer distance, diffusion of nutrients to
  organelles is a greater distance.  In other
  words, you have very little surface area to feed
  a very large organism.

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• Magnification The relative enlargement of the
  specimen when seen through the microscope.
• Resolution (resolving power) -the least distance
  between two points or lines at which they are
  seen as two, rather than a single blur. .

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Two types of cells
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Prokaryote

• "pro" before, "karyos" nucleus
• Includes bacteria and cyanobacteria (formerly
  called blue-green algae and thought to be
  plants), as well as archaea)
• Simple architecture not understood until EM
  technology in 1940's. View electron micrograph of
  bacteria.
• Bacteria have a single circular DNA molecule
  ("chromosome"), divide by binary fission, not by
  mitosis or meiosis.
• Typical sizes 0.5-5 µm (micrometer) diameter.

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Prokaryote or bacterial shapes

• Rods bacilli (sing. bacillus).
• View Pseudomonas aeruginosa light micrograph,
  gram stain, a common opportunistic pathogen and
  widely distributed soil bacillus.
• View E. coli bacilli in a scanning electron
  micrograph SEM.

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• Spheres cocci (sing. coccus).
• View Staphylococcus light micrograph, gram
  stain.

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• Spiral forms spirilla (sing. spirillum).
• View Borrelia burgdorferi, the bacterium that
  causes Lyme disesase.

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General Characteristics of prokaryotes
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Structures include

• Cell wall is composed of peptidoglycan

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• May be surrounded by capsule and or a gelatinous
  sheath called the slime layer

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• May have flagella which rotate like propellers
  to move through fluid

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• Fimbriae- short appendages that help them attach
  to an appropriate surface
• Pili extensions used in reproduction

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• Plasma membrane- outermost membrane, regulates
  what goes in and what comes out

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• Cytoplasm consist of cytosol, a semi fluid
  medium
• Ribosomes granular inclusions that coordinate
  synthesis of proteins

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• Nucleoid not enclosed in a membrane, contains
  most genes in a circular DNA molecule

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• Plasmids small accessory rings of DNA aside
  from the nucleoid. Used in biotechnology.

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• May contain thylakoids, flattened discs with
  light sensitive pigment molecules, used in
  photosynthesis

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Eukaryotes (you are)
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• Can be unicellular (protists) or multicellular
• First appeared 1.5 billion years ago.

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   Distinguishing characteristics (in comparison
to prokaryotes)..

• Generally larger than prokaryotic cells
•     Eukaryotic DNA works in much the same way as
  prokaryotic DNA and the same code is used,
  however there are some distinct differences as
  well
• a)       Eukaryotic DNA is enclosed by a membrane
  making a well defined nucleus.
• b)       Eukaryotic DNA tends to have protein
  bound to it forming chromatin (chromosomes).
• c)       The DNA has repeated and 'junk' areas
  (e.g. introns).
• d)       The DNA has free ends and is not bound
  to any membranes.
• e)       Replication starts at several points.

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Evolution of Eukaryotes

• Invagination of plasma membrane might explain
  nuclear envelope and Golgi apparatus
• Proposed that mitochondria are aerobic
  heterotrophic bacteria, chloroplasts are
  cyanobacteria this is known as the
  endosymbiotic hypothesis.

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endosymbiotic hypothesis

• Factors in favor of mitochondrial and chloroplast
  endosymbiosis.
• 1.      Outer membrane is similar to the plasma
  membrane.
• 2.      Mitochondria look a lot like bacteria
  chloroplasts look a lot like blue-green algae.
  These organelles are similar to prokaryotes in
  that

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• 3.      Mitochondria and chloroplasts seem to
  divide independently of the rest of the
  eukaryotic cell.
• 4.      Eukaryotes are very good at endocytosis.

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• Simply stated, the theory of endosymbiosis is the
  concept that mitochondria and chloroplasts are
  the result of years of evolution initiated by the
  endocytosis of bacteria and blue-green algae
  which, instead of becoming digested, became
  symbiotic.
• The Evolution of Organelles (see animation)

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Structure of Eukaryote (plant and animal cells )

• The nucleus containerizes the DNA.
• Double membrane
• This double membrane is apparently derived from
  or closely associated with the rough endoplasmic
  reticulum (r.e.r).

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'Nuclear pores'

•   Some ions and some other small molecules can
  not diffuse in easily. Nuclei also have osmotic
  properties suggesting a semipermeable membrane.
• Electron microscopy shows structural and well
  ordered structures in the pores called the
  nuclear pore complex

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Nuclear pore as seen with TEM
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Inside nucleus is chromatin

• Chromatin is threadlike material that coils into
  chromosomes before cell division

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EM chromatin
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Chromosomes

• Rod like structures formed when chromatin is
  coiled or folded prior to cell division

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Nucleoli

• Dark stained spherical bodies, sites where rRNA
  joins proteins to form ribosomes. In this case
  the nucleus has two nucleoli

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• The job of the nucleus is to protect the DNA
  which in turn had the ability to make proteins.
• Remember this dogma
• DNA ---? RNA ----? Protein

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Ribosomes are the sites of Protein Synthesis

• Composed of large and small units
• coordinate assembly of amino acids into proteins
• Some attached to ER (called rough ER)
• Some free float (ribosomes)

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The endomembrane system

• The endomembrane system is composed of a number
  of inter-related membrane sacs within the
  cytoplasm of the cell.
• rough endoplasmic reticulum
• smooth endoplasmic reticulum
• transport vesicles
• golgi apparatus
• lysosomes
• microbodies
• peroxisomes

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• The endomembrane system functions, in part, in
• protein synthesis,
• protein modification,
• protein sorting and
• protein transport.

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• This animation will show the sequence of steps
  used in the endomembrane system to produce
  proteins.
• Protein Secretion
• Go to animation two for additional clarification
  Life  eLearning

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Now lets look at the parts again.

• Endoplasmic reticulum system of membrane
  channels continuous with outer membrane of
  nuclear envelope.

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• Rough Endoplasmic reticulum is studded with
  ribosomes on cytoplasm side of nucleus.
  Polypeptides are process and modified.

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• Smooth ER is continuous with RER but lacks
  ribosomes. Used as detox and storage center..
  Forms transport vesicles for disposal

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Golgi Apparatus

• Part of the system for exocytosis and endocytosis

• Vesicle Budding and Fusing
• Exocytosis move out
• Endocytosis move in

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• Golgi consists of a stack of 3-20 slightly curved
  sacs
• Golgi receives protein filled vesicles that bud
  from ER
• Has cis and tran sides

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Lysosomes

• Membrane bound vesicles produced by Golgi
  apparatus and contain digestive enzymes.
• Macromolecules enter a cell by vesicle formation,
  lysosomes fuse with vesicle and digest . (as seen
  in next slide)

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Put it all together
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Peroxisomes

• Peroxisomes break down fatty acids and amino
  acids. These reactions produce hydrogen peroxide
  which could harm cells if it were allowed to
  persist. An enzyme (catalase) breaks down the
  hydrogen peroxide to water and oxygen, both of
  which can be used by the cell. Peroxisomes also
  breakdown alcohol.

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Vacuoles

• Large membranous sac, vesicles are smaller than
  vacuoles

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• Vacuole is more prominent in plant cells and
  provide turgor. Called the central vacuole

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• Plant vacuoles store water, sugars, salts,
  pigments, and toxic substances to protect plant
  from herbivores.

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• Vacuoles are a way for protists to regulate water

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Energy Related Organelles.

• Chloroplast site of photosynthesis.
• 4-6 micrometers in diameter and 1-5 in length
• Bound by double membrane into stacks called
  thylokoids

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• Chlorophyll is located within the thylakoids
  membranes
• The stroma contains enzymes involved in the
  making of carbohydrates

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EM of chloroplasts
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Chloroplast belong to a group of organelles known
as plastids

• Amyloplasts store sugar
• Chromoplasts contain red and orange pigments

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Mitochondria

• The site for cellular respiration
• Are about 0.5 to 1.- micrometers in diameter and
  7 micrometer long.
• Have a double membrane.
• Inner membrane folds called cristae
• The matrix contains rnzymes
• Has own DNA and ribosomes

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Looks like
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Cytoskeleton

• A network of connected filaments and tubules
  extends from nucleus to plasma membrane.
• Maintains cell shape and allows for movement of
  other organelles

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Actin filaments

• Long thin fibers.
• Play a structural role forming a complex web just
  under the plasma membrane

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• In plant cells the actin filaments provides
  tracts along which chloroplasts circulate
• (Note chloroplasts are shown in red)

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• In intestinal cells the actin filament shortens
  and lengthens to extend cell into the intestine

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• Actin firmaments move by interacting with myosin.

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Intermediate filaments

• Intermediate filaments are a very broad class of
  fibrous proteins that play an important role as
  both structural and functional elements of the
  cytoskeleton.

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Microtubules

• Another critical cytoskeletal fiber is the
  microtubules. They are also polymers, and are
  comprised of the protein tubulin. The image
  shows the microtubules in a cow endothelial cell.

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• They participate in a range of motion processes,
  like
• the movements of flagella and cilia, the
  movements of chromosomes during meiosis and
  mitosis and the transport of granules and
  vesicles within the cells that effects cell wall
  formation, shape and specialization of the cells.

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• Microtubules can form regular complex structures.
  All flagella and cilia of eukaryotes are
  characterized by the so-called '92' structure.

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Centrosome regulate microtubules
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centrioles

• In animal and most protists cells centrosomes
  contain two centrioles lying at right angles to
  each other
• Found in pairs
• are at right angles to each other.
• Self-duplicate in S period and move to opposite
  sides during mitosis.

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• Have a 9-0 pattern
• Plant and fungal cells have centrosome but no
  centrioles

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• Function
• As animal cells prepare for cell division these
  two centrioles separate and go to opposite ends
  of the cell. The spindle fibers that pull the
  chromosomes apart during mitosis

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Centrioles and cilia and flagella

• Cilia and flagella are organized from centrioles
  that move to the cell periphery. These are called
  "basal bodies" Basal bodies control the direction
  of movement of the cilia.

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• If there are many of them, they are called cilia
  
• if only one, or a few, they are flagella.
  Flagella also tend to be longer than cilia but
  are otherwise similar in construction.

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• This electron micrograph shows the 92 pattern of
  microtubules in a single cilium seen in cross
  section.

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Cell junctions

• the plasma membranes of adjacent cells are
  pressed together. Four kinds of junctions occur
  in vertebrates
• Tight junctions
• Adherens junctions
• Gap junctions
• Desmosomes

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• Tight Junctions
• they impermeabilize regionsprevents leakage of
  materials between epithelial cells
•  a fibrillar protein network                   

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•  Desmosome - an adhering junction -   (anchors
  cells together)

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• Gap Junctions -   intercellular channels for
  communication           allows ions, electric
  impulses, etc... to pass between

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• Plasmodesma - cytoplasmic strands between plant
  cell walls
•   cells not separated
•  from one another by cell walls or
  membranes.     

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• Introduction to Electron Microscopy Page 1/4 (
  look at different types of junctions via electron
  micrograph)
• Cell Structure and Function ( go down page to
  chart of cell organelles and function. Take a
  look at micrographs