Chapter 6a Cells

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Chapter 6a- Cells Cell Structures

• Two main types of organisms
• - Single celled one cell
• - multicellular up to many billions/trillions
• Many different-looking cells both within and
  among organisms. However, there are more
  similarities among cells than differences.

bacteria
algae
Unicellular ?
Cells in plant leaf
Animal cells, liver
Multicellular ?
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Early Cell studies

• R.Hooke (17th century)- First to use the term
  cell to describe little boxes that made dry
  plant parts (cork).
• A.van Leeeuwenhoek- observed microorganisms
• T.Schwann (1800s)- Animals are also made of
  cells.
• R.Virchow- First to propose the cell theory, as
  All cells come from cells

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The Cell Theory (some implications)

• The modern cell Theory
• 1- Cells, or products made by cells are the
  units of structure and function in organisms
• 2- All cells come from pre-existing cells
• 2a- Cells contain the hereditary information,
  and this
• information is passed from parent to
  daughter cell.
• ______________________________________________
• Some implications
• Whatever their size, all organisms are composed
  of cells, the basic unit of life.
• There is no such thing as spontaneous generation.

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Disproving Spontaneous Generation
Louis Pasteur (1860s) demonstrated that
spontaneous generation does not occur
Pre-existing cells are needed as inoculum.
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Lifes origins still controversial

• Some hold views that favor creationism.
• Others do not see a need for creation of a
  creator.
• Russian Biochemist Oparin proposed that organic
  compounds dont have to be made by organisms.
  Miller (1950s) demonstrated that Oparin was
  partialy correct.
• If certain materials are present (particularly
  hydrogen, oxygen, carbon, and nitrogen) and in
  certain arrangements (water), and if a source of
  energy is provided, many organic compounds can be
  formed.
• Thus, organic compounds can be formed both by
  living organism, or in their absence.
• The early Earth had water (ancient sea), and the
  4 elements listed above. However, free oxygen was
  not present in free form in the early atmosphere.

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Cell studies and microscopy

• Technological advances have been a pre-requisite
  of cellular advances.
• Light microscopes light passing through an
  specimen bends (refracts) to form an enlarged
  view. The lens is a quartz glass.
• Transmission Electron Microscopes (TEM) A
  magnetic field act as a lens, channeling e-
  through an specimen, and into a focal point.
• _________________________________
• Scanning Electron Microscopes (SEM) A narrow
  beam of e- moves back and forth over the surface
  of a metal-coated specimen, and e- from the
  specimens are detected onto an screen forming an
  image.
• Scanning Tunneling microscopes (STM) e- already
  in the sample are used, rather than an e- beam.
  Surface features of the specimen are viewed by
  having a prove emit a current that is sensitive
  to surface features.

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Light, SEM, TEM microscopy

• Traditionally light microscopy was characterized
  by viewing through eyepiece lenses. In contrast,
  electron microscopy used tv-like or computer
  screens from very early models. Today, more and
  more research-level light microscopy includes
  viewing through computarized images, rather than
  eyepieces.
• In Electron microscopy, most eyepieces are used
  to position and manipulate specimen-holding
  devices, rather than viewing specimens.
• TEM images (top) allow viewing the inside of
  samples, by transmitting e- through them.
• SEM images (bottom) allow viewing surface
  features. Internal features can only seen if the
  specimens are sectioned to allow scanning the
  interior.

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Sperm samples viewed with light, TEM SEM
Light
TEM
SEM
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Cell and organisms sizes

• A um (micron) is 1/1000 of a millimeter (mm), or
  10-6 m (meters)
• Cells average 10-20 um in diameter. However,
  there are extremes An ostrich egg (largest cell
  known) is larger than a softball. In general,
  eggs are among the largest cells.
• Some bacteria are just about 1 um long (or
  diameter).
• In general, higher magnification can be achieved
  with electron microscopes than w/ light
  microscopes. However, SEM is often used also at
  fairly low magnifications, even for samples than
  can be seen (with less detail) without
  microscopes.

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2 types of cells Pro- and Eukaryotic

• Prokaryotes (the bacteria) are the simplest
  living cells. They have no nucleus and do not
  have several other cellular structures
  (organelles) of more complex cells. Generally
  small, the largest about 5 um across.
• Eukaryotes are larger (10-50 um on the average,
  and more complex. They have a nucleus and many
  more organelles, with different functions.

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Prokaryotic cells - Bacteria

• Unicellular, but can associate in groups, films,
  chains, etc.
• Most with rigid cell wall (as in plant cells),
  but without cellulose.
• Nucleoid (nuclear area with nuclear functions)
  but no nucleus. Contains 1 chromosome
  (double-stranded DNA), attached to plasma
  membrane.
• Plasmids circular DNA molecules attached to
  membrane, containing genes.
• No mitochondria, no chloroplasts.
• Flagella enable cells to move. Some sense
  characteristics of their environment and move
  accordingly.
• Many are autotrophs, either photo- or
  chemoautotrophs.
• Many are harmful, responsible for serious
  diseases.
• Most are beneficial (producers, decomposers,
  nitrogen fixers, bacteria in gut of higher
  organisms, bacteria used in industrial and
  medical processes).

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Eukaryotic cells

• Contain many organelles (portion of the cell with
  its own structure and function)
• Compartmentation (with organelles) allow
  different reactions and concentration of
  materials in different portions of the eukaryotic
  cell, making it more efficient.
• Many cells have cilia (cilli) short flagella
  arranged in rows. Allow cells to move, or to move
  materials around them.
• Nucleus is the most telling distinction between
  pro- and eukaryotic cells. Chromosomes (with DNA)
  are in nuclei, as well as nucleoli (contain RNA)
• Cytoplasm includes organelles, floating in the
  cytosol, which is an organized system called
  cytoskeleton.
• Other organelles Ribosomes, Endoplasmic
  reticulum, Golgi apparatus, Vesicles, Lysosomes,
  Vacuoles, Centrioles, Mitochondria

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Typical plant cell
nucleus
Endoplasmic reticulum
chloroplast
nucleolus
Plasma membrane
Cell wall
mitochondria
vacuole
Golgi apparatus
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Typical animal cell
RER
SER
vacuole
nucleus
Golgi apparatus
nucleolus
Lysosome
Plasma membrane
mitochondria
centrioles
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Review of facts of cell structures

• 1- Cell size - Limited by surface area/volume
  limitations. With an increase in radius, area
  increases as a power of 2, and volume as a power
  of 3. Cells must be able to supply all its volume
  of needed materials and remove wastes. Unless the
  cell is relatively inactive (i.e., eggs made of
  mostly storage material), cells need to have
  maximum sizes of about 100 microns. Another
  compromise is having multiple nuclei.
• 2- Cell wall outermost layer in some cells
• - in plants. Also in algae and bacteria (without
  cellulose)
• - Made of various carbohydrates and protein
• - Provides rigidity
• 3- Plasma membrane outermost layer in some
  cells
• - Double layer (phospholipid and protein)
• - semipermeable
• - allows cell-to-cell recognition thru
  glycoproteins.
• - transport proteins inserted in membrane
• - Cytoskeleton of cytosol is attached to plasma
  membrane
• 4- Cytoplasm The fluid material inside the cell
  (cytosol) the organelles
• - Cytosol Network of fibers and microtubules
  (cytoskeleton) that maintain organelles in
  relative positions to one another, but also
  allows movement of organelles and other cytoplasm
  materials inside the cell. Also, it plays a role
  in cell movement.
• - Not present in prokaryotic cells (bacteria)

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Continued (2)

• 5- Nucleus Control center (contains genetic
  information for protein synthesis)
• - double membrane
• - single long DNA molecule
• - one or more Nucleoli (RNA drops) sites of
  synthesis of RNA.
• 6- Ribosomes small, membrane-bound bodies
• - Main function is protein synthesis, using
  recipes contained in chromosomes.
• - contain RNA and protein
• - All cells have them (Prokaryotic- a few
  thousand Eukaryotic-millions)
• - More abundant in cells ins charge of active
  protein/enzyme synthesis
• - Free ribosomes (not attached to membranes)
  Make proteins for use within the cell. Bound
  ribosomes (attached to membranes such as those of
  Rough Endoplasmic Reticulum) Make proteins for
  export out of cell, or for use in membranes.
• ? ENDOMEMBRANE SYSTEM (ER, Golgi, Lysosomes) ?
• 7- Endoplasmic reticulum (general) System of
  internal membranes
• - Provide structure and compartmentalization to
  the cell, defining areas with various functions
  and concentrations of materials.
• - Serves as a communication network and contains
  the nucleus with the exterior
• - Continuous/connected with nuclear membrane
• - System in charge of synthesis of
  macromolecules and transport/delivery to various
  parts of the cell.
• - Not in prokaryotic cells.

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Continued (3)

• 7a- Rough ER- (with ribosomes)
• - Synthesis and delivery of proteins for use
  outside of cell (mainly in secretory cells).
• - Proteins usually are stored in the interior of
  SER, then go outside via the Golgi apparatus.
• 7b- Smooth ER (without ribosomes)
• - Synthesis of carbohydrates, lipids,
  phospholipids and other materials. Materials may
  be used inside or outside of the cell.
• - Also, it detoxifies some toxins.
• 8- Golgi apparatus
• - Series of membranous sacks.
• - connected with ER via vesicles that are not
  permanent
• - Exports materials synthesized elsewhere,
  taking them outside of the cell.
• - Also it modifies materials secreted elsewhere,
  attaching additional components. Secretes new
  membrane.
• - Makes vesicles (lysosomes) fro transport of
  materials.
• - Serve as storage sites.
• - Present in all eukaryotic cells. Particularly
  abundant in plants cells (apparently play role in
  synthesis of cellulose cell walls)

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Continued (4)

• 9- Lysosomes Golgi-made vesicles that are for
  internal cell use, not for export
• - Contain hydrolytic enzymes that digest cells
  macromolecules. Thus, they need to be bound (by
  membrane) at all times. Their membrane is
  resistant to the enzymes.
• - May function is intracellular digestion of
  various materials, including re-utilization of
  older molecules.
• - Enzymes in lysosomes have been synthesized in
  RER, then transported to Golgi apparatus where
  they are modified, and incorporated in vesicles.
• - Not in prokaryotes.
• __________________________________________________
  ________________________
• Mitochondria and chloroplasts have many features
  in common
• 10- Chloroplasts Photosynthesis reactions
  (energy metabolism).
• - Double membrane
• - Have their own DNA and their own ribosomes
• - Responsible for their own reproduction and
  must arise from pre-existing chloroplasts.
• - Have membrane expansions for electron
  transport systems.
• - Only in photo-autotrophic eukaryotes (not in
  prokaryotes)
• 11- Mitochondria ATP synthesis
• - Double membrane
• - Have their own DNA and their own ribosomes
• - Responsible for their own reproduction and
  must arise from pre-existing mitochondria.
• - Have membrane expansions for electron
  transport systems.
• - Not in prokaryotes

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Continued (5)

• 12- centrioles Unclear function in cells (not
  fully understood)
• - Found only in organisms that have cilia or
  flagella.
• - Not present in higher plants (flowering
  plants).
• - When present, they occur as pairs. Each
  centriole from a pair is arranged at right angle
  (perpendicular) to the other.
• - Involved in the formation of the mitotic
  spindle (web-like structure that forms during
  cell division). However, cells that dont have
  centrioles still develop a spindle.
• - Centrioles replicate themselves.
• 13- Flagella and cilia-
• - Important in locomotion (movement) of either
  the cell themselves, or of materials around them.
• - Both are microtubules that extend from the
  cell towards the ouside. They are actually
  bundles of microtubules, arranged in specific
  patterns.
• - When flagella are present, usually they are a
  single one, or a pair, and are long.
• - If they are short and numerous they are called
  cilia.
• - Cilia may be in unicellular organisms, or in
  the surfaces of the cells of multicellular
  organisms.

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A few facts about evolution (relationship of
prokaryotes and eukaryotes)

• No one knows exactly when the first cells
  appeared on Earth.
• Currently available evidence supports the
  following arguments
• The first cells were prokaryotic.
• The first cells probably were heterotrophic (did
  not synthesize organic molecules by themselves).
  The organic molecules they fed own probably were
  formed spontaneously (not made by other
  organisms).
• Because there was no oxygen, the first cells
  probably used anaerobic pathways, such as
  fermentation.
• The first autotrophs probably resulted from
  mutations of heterotrophic forms, and became able
  to use sunlight to split water or H2S (hydrogen
  sulfide) and obtain electrons to synthesize ATP.
• After simple autotrophs release sufficient oxygen
  into the early atmosphere, the first aerobic
  organisms appeared.
• Eukaryotic cells probably came from associations
  of prokaryotic organisms. This is the
  ENDOSYMBIOTIC THEORY Chloroplasts and
  mitochondria were once independent prokaryotes,
  that eventually associated themselves in to
  larger, more complex cells, which through further
  increases in complexity became eukaryotic cells.
  Alternativelly, large prokaryotic cells (lacking
  chloroplasts and mitochondria) may have ingested
  but not digested them, incorporating these
  organelles as endosymbionts.