A Tour of the Cell

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A Tour of the Cell

• Chapter 4

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History of the Microscope

• The microscope was invented in the 17th century
• Using a microscope, Robert Hooke discovered cells
  in 1665
• Cell Theory- All living things are made of cells,
  and all cells come from other cells

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The Microscope

• Light Microscope (LM)
• Pass visible light through a specimen
• Magnification- an increase in the apparent size
  of an object
• Resolving power- a measure of the clarity of an
  image
• Light microscope cannot resolve detail finer than
  0.2 micrometer (0.2/1000 mm)
• Cannot show the details of the cells internal
  structure

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The Light Microscope
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Electron Microscopes

• Electron microscopes were invented in the 1950s
• They use a beam of electrons instead of light
• The greater resolving power of electron
  microscopes
• allows greater magnification
• reveals cellular details

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Scanning Electron Microscopy (SEM)

• Used to study the architecture of cell surfaces
• Object is covered with metal
• Electrons are bounced off of the surface
• Bounced electrons are collected and formed into
  an image

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Transmission Electron Microscopy (TEM)

• Used to study the details of internal cell
  structure
• Object cut into extremely thin sections
• Electron beam shot through the specimen
• Electromagnets are used to focus and magnify the
  image

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Electron Microscopy

• Excellent tool for studying internal structures
  of cells
• Good for studying 3D anatomy of cells and tissues
  at a greater magnification
• Cannot replace the light microscope
• Cant use living specimens
• Requires much prep time
• Requires special training
• Expensive

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Cell Size

• Cell size and shape are related to cell function
• Chicken eggs are large because they hold
  nutrients
• Muscle cells are long to help pull body parts
  together
• Blood cells are small to help them navigate
  through tiny blood vessels

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Natural Laws Limit Cell Size

• At minimum, a cell must be large enough to house
  the parts it needs to survive and reproduce
• The maximum size of a cell is limited by the
  amount of surface needed to obtain nutrients from
  the environment and dispose of wastes

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Surface Area and Cell Size

• A small cell has a greater ratio of surface area
  to volume than a large cell of the same shape
• Muscle cells can be very thin because they are
  long and have more surface area

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Prokaryotic Cells

• Bacteria and Archaea
• Most prokaryotic cells range from 2-8?m in length
• Lacks a nucleus, DNA is coiled into a nucleoid
  that does not have a membrane
• Plasma membrane surrounds the cell, and a
  prokaryotic cell wall is outside the plasma
  membrane

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Prokaryotic Cells

• May also contain a sticky outer coat called a
  capsule
• Helps them stick to things
• Some have pili to help stick
• Some have flagella to aid in liquid movement

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Eukaryotic Cells

• All other life forms are made up of one or more
  eukaryotic cells
• Plants, animals, fungi
• These are larger and more complex than
  prokaryotic cells
• Eukaryotes are distinguished by the presence of a
  true nucleus
• Variety of structures (organelles) in the
  cytoplasm. Cytoplasm- fluid-filled region between
  the nucleus and the plasma membrane

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An Animal Cell
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Cell Membranes

• The plasma membrane controls the cells contact
  with the environment
• The cytoplasm contains organelles
• Many organelles have membranes as boundaries
• These compartmentalize the interior of the cell
• This allows the cell to carry out a variety of
  activities simultaneously
• Increase total membrane area of the cell

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A Plant Cell

• All of the membrane-bound organelles present in
  animal cells are also in plant cells except the
  lysosome
• No centriole or flagellum
• Plant cells have structures that animal cells do
  not have
• Cell wall- protect cells and maintain shape
• Chloroplasts where photosynthesis occurs
• Large central vacuole carry out cellular
  digestion

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A Plant Cell
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The Nucleus

• Genetic control center of a eukaryotic cell
• Nuclear DNA is attached to proteins forming very
  long fibers called chromatin, each fiber
  constitutes a chromosome
• When cell reproduction occurs, the chromosome
  coils up

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The Nucleus

• The nucleus is enclosed by a nuclear envelope
• Double membrane with pores to control in and
  outflow of material into the cell
• Nucleolus- within the nucleus mass of fibers
  and granules where the components of ribosomes
  are made

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The Nucleus
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Endomembrane System

• A biological membrane system in organelles that
  runs throughout the cell
• Some of the membranes are connected, some are not
• Many organelles work together in the synthesis,
  storage, and export of important molecules
• Rough and smooth endoplasmic reticulum, the golgi
  aparatus, lysosomes, and vacuoles

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Endoplasmic Reticulum

• Two kinds of ER
• Rough ER and smooth ER
• These two types of organelles differ in structure
  and function, but a continuous membrane runs
  between them
• Membranes of rough ER are continuous with the
  plasma membrane
• Space within the ER is separated from the
  cytoplasm

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Rough ER

• Refers to the appearance of this organelle in
  electron micrographs
• Roughness results from ribosomes which stud the
  membranes of the organelle
• Two main functions
• Make more membrane
• Make proteins that are secreted by the cell

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Smooth ER

• Continuous with rough ER
• Lacks ribosomes embedded in the membrane
• Activity results from enzymes embedded in the
  membrane
• Synthesizes lipids (fatty acids, phospholipids,
  and steroids)
• In some cells (liver), it regulates carbohydrate
  metabolism and breaks down toxins and drugs

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Smooth ER

• Drug Tolerance
• When Liver cells respond to certain drugs over
  and over they make more ER and become tolerant to
  the drug
• Becomes resistant to the drug and its relatives

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Golgi Apparatus

• Unconnected, flattened stacks
• of golgi sacks correlates with how active the
  cell is at secreting proteins
• Works with ER by receiving and modifying
  substances manufactured by the ER
• Modifies them and marks them for their
  destination
• The golgi vesicles then bud off and the molecules
  are shipped to the plasma membrane

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Golgi Apparatus
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Lysosomes

• Digestive enzymes enclosed in a membranous sac
• Come from the golgi apparatus
• Compartmentalize digestive enzymes so they wont
  harm the cell
• Digest food vacuoles in order to digest them
• Destroy harmful bacteria
• Recycle damaged organelles
• Play important roles in embryonic development

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Lysozymes
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Vacuoles

• Membranous sacs that have a variety of functions
• Food and chemical storage
• Central vacuole of plants aids in plant growth,
  and chemical and waste storage
• Also contain pigments for flower color and
  poisons that protect plants from predation
• In protists they collect water to prevent dilution

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Vacuoles
Protists
Plants
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Endomembrane System
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Chloroplasts

• The photosynthesizing organelles of plants and
  protists
• Most of the living world runs on the energy
  provided by photosynthesis
• Internal membranes partition the chloroplast into
  3 compartments
• Intermembrane space
• Stroma
• Granum

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Chloroplasts

• Intermembrane space- between outer and inner
  membrane of the chloroplast
• Stroma- Network of tubules and interconnected
  hollow disks formed of membranes
• Granum- Stacks of hollow disks that are the
  chloroplasts solarpower packs

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Mitochondria

• Organelles that convert energy from one chemical
  form to another
• Carry out cellular respiration
• Sugars are converted to the chemical energy of a
  molecule of ATP
• Enclosed by two membranes
• Composed of two compartments
• Intermembrane space
• Mitochondrial matrix

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Mitochondria

• Intermembrane space
• Fluid-filled compartment
• Encloses mitochondrial matrix
• Mitochondrial matrix-
• Many of the rxns of cellular respiration occur
  here
• Inner membrane highly folded into christae that
  increase the surface area
• Embedded in the cristae are the enzymes that make
  ATP

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Cytoskeleton

• Cytoskeleton- A supportive meshwork of fine
  fibers contained in eukaryotic cells
• Fibers extend throughout the cell
• Also involved in cell movement
• May help regulate cellular activity by
  transmitting signals from the cells exterior to
  the interior
• Three main types of fibers
• Microfiliments, intermediate filiments,
  microtubules

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Cytoskeleton

• Microfilaments
• Solid helical rods composed mainly of actin
• Twisted double chain
• Can help cells change shape by assembling and
  disassembling
• Interact with other protein filaments to make
  cells contract
• Intermediate filaments
• Fibrous proteins with a rope-like structure
• Reinforcing rods and anchor organelles

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Cytoskeleton

• Microtubules- straight, hollow tubes composed of
  globular proteins called tubulin
• May disassemble and reassemble to provide
  rigidity and shape to the cell
• Anchorage for organelles and tracks for organelle
  movement within the cytoplasm

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Cilia and Flagella

• Cilia- short, numerous appendages
• Flagella- longer, less numerous appendages
• Core of microtubules wrapped in an extension of
  the plasma membrane
• 9 microtubule doublets surrounds a central pair
  (92)
• At the base microtubules form an anchoring
  structure called the basal body where the central
  core mts disappear
• Cilia and flagella provide support and contribute
  to movement

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Locomotion with Cilia and Flagella

• Protein knobs (dynein arms) attached to each
  microtubule doublet
• Bending is done by the dynein arms grabbing onto
  the adjacent mt and walking along it so that
  the arms slide along each other

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Cell surfaces

• Most cells have an additional surface coating
  surrounding the plasma membrane
• Prokaryotes-capsules
• Interact mainly with non-cellular surroundings
• Plants- cell walls
• Protect cells and provide structural support
• Eukaryotes- composed of many cells organized into
  a single, functional organism

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Plant cell walls

• Consist of fibers of the polysaccharide cellulose
  embedded in a matrix of other polysaccharides and
  proteins
• Cell walls are multi-layered
• Between the walls of adjacent cells is a layer of
  sticky polysaccharides that bonds the cells
  together

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Plant cell walls

• Cell walls are not totally isolated they have
  cell junctions called plasmodesmata that connect
  them to one another
• Plasmodesmata are channels between adjacent cells
  that form a circulatory and communication system
  connecting the cells in plant tissues

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Plant cell walls

• Cytoplasmic fluid of the cells extends through
  the plasmadesmata so that water and small
  molecules can pass from cell to cell
• Nourishment, water, chemical signals

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Animal cells

• Animal cells secrete and are embedded in a sticky
  layer of glycoproteins called the extracellular
  matrix
• Helps hold cells together
• Protective and supportive functions
• Regulates cell behavior
• Adjacent cells in many tissues also connect by
  cell junctions

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Three general types of Cell Junctions

• Tight junctions- bind cells together forming a
  leak-proof sheet
• Digestive tract
• Anchoring junctions- attach cells together in the
  extracellular matrix
• Muscle cells
• Communicating junctions- channels similar to
  plasmodesmata
• Animal embryos

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Three general types of Cell Junctions
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Functional categories of organelles

• Organelles can be grouped into four categories
  according to function
• Manufacture- synthesis of molecules and
  transport from cell to cell
• Breakdown- breakdown and recycle materials that
  are harmful or no longer needed
• Energy processing
• Support, movement and intercellular communication

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