43 Cell Organelles and Features

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4-3 Cell Organelles and Features

• Describe the structure and function of a cells
  plasma membrane.
• Summarize the role of the nucleus.
• List the major organelles found in the cytosol,
  and describe their roles.
• Identify the characteristics of mitochondria.
• Describe the structure and function of the
  cytoskeleton.

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Plasma Membrane / Cell Membrane

• Functions
• Selectively permeable allows only certain
  molecules to enter or leave the cell.
• Separates internal metabolic reactions from the
  external environment
• Allows the cell to excrete wastes
• Allows the cell to interact with its environment

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Membrane Lipids(plasma membrane and organelle
membranes)

• Made primarily of phospholipids
• Polar, hydrophilic phosphate head
  water-loving
• Two nonpolar, hydrophobic fatty acid tails
  water-fearing

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• Phospholipid bilayer Water molecules surround
  the membrane so the phospholipids line up so that
  their heads point outward toward water and tails
  point inward away from water.

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• Sterols eukaryotic cell membrane lipids that
  are located between the tails of the
  phospholipids
• Help make the membrane more firm and prevent the
  membrane from freezing at low temps.
• Ex. Animals cell membranes have cholesterol

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Membrane Proteins

• Integral proteins
• Some emerge from only one side of the membrane.
  Ex. Surface markers
• Some extend across the plasma membrane and are
  exposed to both the cells interior and exterior
  environments. Ex. Receptor proteins and transport
  proteins
• Able to detect environmental signals and transmit
  them to the inside of the cell

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• If exposed to outside surface, may have a
  carbohydrate attached to it. Act as labels on
  cell surface to help cells recognize each other
  and stick together. Can also be used by virus as
  docks for entering and infecting cells.
• Play an important role in actively transporting
  molecules into the cell.
• Can act as channels or pores allowing certain
  substances to pass
• Others bind to a molecule on the outside and then
  transport it through the membrane.
• Others can act as sites where chemical messengers
  such as hormones attach.

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• Peripheral proteins
• Lie only on one side of the membrane and are not
  embedded in it.
• Ex. Enzymes

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Fluid Mosaic Model

• The phospholipid bilayer behaves like a fluid
  more than it behaves like a solid
• The membranes lipids and proteins can move
  laterally within the bilayer like a boat on the
  ocean.
• The pattern or mosaic of lipids and proteins
  constantly changes.

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Nucleus

• Control center of the cell
• Filled with jelly-like liquid called nucleoplasm
  similar to cytosol

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• Houses and protects the cells genetic
  information
• Contains the instructions for the structure and
  function of the organism coded in DNA
• Chromatin threadlike form of DNA when cell is
  not dividing
• Chromosomes condensed, super-coiled form of DNA
  and protein when cell is dividing

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• Site where DNA (deoxyribonucleic acid) is
  transcribed into RNA (ribonucleic acid). RNA then
  moves to the cytoplasm to carry out its function.

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• Nuclear envelope double membrane (phospholipid
  bilayer) that surrounds the nucleus
• Nuclear pores tiny, protein lined holes that
  cover the surface of the nuclear membrane and
  provide passageways for RNA and other materials
  to enter and exit the nucleus

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• Nucleolus one or more denser areas where DNA is
  concentrated when it is in the process of making
  rRNA (ribosomal RNA) which are a component of
  organelles called ribosomes.

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Mitochondria

• Tiny organelles that transfer energy from organic
  molecules to ATP (adenosine triphosphate) which
  ultimately powers most of the cells chemical
  reactions.
• Active cells like muscle cells have more
  mitochondria than other cells that are not as
  active such as fat-storage cells.

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• Have an inner and outer phospholipid bilayer
• Outer layer separates mitochondria from the
  cytosol
• Inner layer many folds called cristae that
  contain proteins that carry out energy-harvesting
  chemical reactions.

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• Mitochondrial DNA
• Mitochondria have their own DNA and can reproduce
  only by the division of preexisting mitochondria.

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• Scientists think that mitochondria originated
  from prokaryotic cells that were incorporated
  into ancient eukaryotic cells. (endosymbiosis)
• Symbiotic relationship provided the prokaryotic
  invaders with a protected place to live and
  provided the eukaryotic cell with an increased
  supply of ATP

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Ribosomes

• Small, roughly spherical organelles
• Responsible for building proteins
• Do not have a membrane
• Made of protein and RNA molecules

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• Ribosome assembly begins in the nucleus and is
  completed in the cytoplasm
• One small subunit and one large subunit make a
  functioning ribosome
• Some float freely within cytosol, others are
  attached to rough endoplasmic reticulum

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Endoplasmic Reticulum (ER)

• System of membranous tubes and sacs called
  cisternae
• Functions as an intercellular highway a path
  along which molecules move from one part of the
  cell to another.
• Amount of ER inside a cell fluctuates depending
  on cell activity

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Types of ER (continuous interconnected network)

• Rough ER covered with ribosomes.
• Produces phospholipids and proteins.
• Certain types of proteins are made on the rough
  ERs ribosomes which are later exported outside
  of the cell or inserted into one of the cells
  own membranes
• Ex. Rough ER ribosomes make digestive enzymes
  which accumulate inside the ER. Little sacs or
  vesicles pinch off from the ends of rough ER and
  store the digestive enzymes until they are
  released from the cell
• Most abundant

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• Smooth ER not covered with ribosomes
• Not abundant
• Builds lipids such as cholesterol
• In ovaries and testes produces steroid hormones
  estrogen and testosterone
• Releases calcium in skeletal and heart muscle
  cells to stimulate contraction
• Helps detoxify drugs and poisons in kidneys and
  liver. Long-term abuse of alcohol and other drugs
  causes these cells to produce more smooth ER. Can
  lead to drug tolerance

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Golgi Apparatus

• System of flattened membranous sacs
• Receive vesicles from ER containing newly made
  proteins and lipids

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• Vesicles are modified as they move from one part
  of the golgi apparatus to the next
• Carbohydrate labels are added to proteins which
  direct them to various other parts of the cell

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Vesicles

• Small, spherically shaped sacs that are
  surrounded by a single membrane
• Types are classified by their contents
• Often migrate to and merge with the plasma
  membrane to release their contents to the outside
  of the cell

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Lysosomes

• Vesicles that bud from the golgi apparatus and
  contain digestive enzymes
• Break down large molecules such as proteins,
  nucleic acids, carbohydrates, and phospholipids

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• Examples
• In the liver, lysosomes break down glycogen to
  release glucose into the blood stream.
• White blood cells use lysosomes to break down
  bacteria
• Autophagy digest worn-out organelles within a
  cell
• Autolysis the digestion of damaged or extra
  cells by the enzymes of their own lysosomes.
  Important in organisms health by destroying old
  cells or ones that no longer function properly

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Peroxisomes

• Similar to lysosomes but contain different
  enzymes and are not produced by golgi apparatus
• Abundant in liver and kidney cells.
• Neutralize free radicals (oxygen ions that can
  damage cells) and detoxify alcohol and other
  drugs.
• Named for hydrogen peroxide H2O2 that they
  produce when breaking down alcohol or killing
  bacteria
• Break down fatty acids which mitochondria use as
  an energy source.

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Other Vesicles

• Glyoxysomes specialized peroxisomes found in
  seeds of some plants. Break down stored fats to
  provide energy for developing embryo
• Endosome pocket that buds off into a cell
  forming a vesicle as the cell engulfs material by
  surrounding it with the plasma membrane.
  Lysosomes fuse with it and digest its contents.

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• Food vacuoles vesicles that store nutrients for
  a cell
• Contractile vacuoles vesicles that contract and
  dispose of excess water

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Protein Synthesis

• Major function of a cell
• Steps
• Proteins are assembled by ribosomes on the rough
  ER
• Vesicles transport proteins from rough ER to the
  golgi apparatus
• Proteins are modified in the Golgi apparatus and
  packaged in new vesicles.
• Some vesicles release their proteins outside the
  cell
• Vesicles containing enzymes remain inside the
  cell as lysosomes, peroxisomes, endosomes, or
  other types of vesicles.

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Cytoskeleton

• Network of thin tubes and filaments that
  crisscrosses the cytosol
• Give shape to the cell
• Also acts as a system of internal tracks on which
  items move around inside the cell

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• Structural elements
• Microtubules hollow tube made of a coiled
  protein called tubulin which consists of two
  subunits.
• Radiate out from central point near nucleus
  called centrosome
• Hold organelles in place, maintain cell shape,
  act as tracks that guide organelles and molecules
  as they move within cell

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• Microfilaments finer strands of long beadlike
  intertwined proteins called actin
• Contribute to cell movement (crawling of white
  blood cells, muscle contraction), maintenance and
  changing of cell shape, cell division, movement
  of cytoplasm.
• Intermediate Filaments rods or protein fibers
  coiled into cables
• Anchor nucleus and other organelles in place.
• Maintain internal shape of nucleus
• Make up hair shaft in hair-follicle cells

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Cilia and Flagella

• Hairlike structures that extend from the surface
  of a cell
• Assist in movement
• Have a membrane on their surface and an internal
  structure of 9 pairs of microtubules surrounding
  a central pair

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• Cilia short and present in large numbers on
  certain cells
• On cells of inner ear. Vibrate and help detect
  sound
• On Protist help row through water environment
  or help sweep water and food particles into
  mouthlike opening

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• Flagella longer and less numerous
• Used by many Protists and by human sperm cells
  for propulsion

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Centrioles

• Consist of two short cylinders of microtubules at
  right angles to each other.
• Situated in the cytoplasm near the nuclear
  envelope
• Occur in animal cells (plant cells lack
  centrioles) organize the microtubules of the
  cytoskeleton that pull chromosomes apart during
  cell division
• Basal Bodies have same structure as centrioles.
  Located at the base of cilia and flagella and
  help with their development

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4-4 Unique Features of Plant Cells

• List three structures that are present in plant
  cells but not in animal cells.
• Compare the plasma membrane, the primary cell
  wall, and the secondary cell wall.
• Explain the role of the central vacuole.
• Describe the roles of plastids in the life of a
  plant.
• Identify features that distinguish prokaryotes,
  eukaryotes, plant cells, and animal cells.

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Plant Cells

• Three additional structures cell walls, large
  central vacuoles, and plastids
• Plants lifestyle differs from an animals.
• Make their own food by photosynthesis

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Cell Wall

• Rigid layer that lies outside the cells plasma
  membrane
• Cellulose (a carbohydrate) is embedded in a
  matrix of protein and other carbohydrates that
  form a stiff box around the cell
• Pores in cell wall allow water, ions, and some
  molecules to enter and exit

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• Primary Cell Wall
• Cellulose is made directly on the surface of the
  plasma membrane by enzymes.
• Other components of the cell wall are made by ER
  which move in vesicle to the golgi apparatus and
  then to the cell surface.
• Secondary Cell Wall
• In some plants, when cell stops growing, the
  secondary cell wall which is more rigid is
  secreted between the plasma membrane and the
  primary cell wall. Very strong but can no longer
  expand.

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Central Vacuole

• Large, fluid-filled organelle that stores water,
  enzymes, metabolic wastes, and other materials.
• Forms as smaller vacuoles fuse together.

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• Can make up to 90 of a plant cells volume. Push
  all of the other organelles into a thin layer
  against the plasma membrane.

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• When plant is hydrated the vacuole is full and
  plant is rigid and stands upright
• When plant is dehydrated, the vacuoles lose
  water, cells shrink and plant wilts.

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Other Vacuoles

• Some vacuoles store toxic materials
• Acacia tree stores poisons as a defense against
  plant-eating animals
• Tobacco plant stores the toxin nicotine in a
  storage vacuole.
• Other vacuoles store plant pigments such as those
  found in rose petals.

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Plastids

• Surrounded by a double membrane and contain their
  own DNA (like mitochondria)
• Several types chloroplasts, chromoplasts,
  leucoplasts. All arise from same precursor called
  a proplastid.

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Chloroplasts

• Use light energy to make carbohydrates from
  carbon dioxide and water during photosynthesis
• Contains a system of flattened membranous sacs
  called thylakoids
• Thylakoids contain the green pigment chlorophyll,
  the main light absorbing molecule that captures
  light energy for the cell

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• Can be found in green plants and eukaryotic algae
  such as seaweed
• Chloroplast DNA is very similar to DNA of certain
  photosynthetic bacteria
• Plant cell chloroplasts can arise only by the
  division of preexisting chloroplasts.
• Suggests that chloroplasts are descendents of
  ancient prokaryotic cells that were incorporated
  into plant cells through a process called
  endosymbiosis

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Chromoplasts

• Plastids that contain colorful pigments that may
  or may not take place in photosynthesis
• Carrot root cells contain chromoplasts filled
  with orange pigment called carotene
• Other chromoplasts in flower petal cells contain
  red, purple, yellow, or white pigments

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Other Plastids

• Share general features of chloroplasts but differ
  in content.
• Ex. Amyloplasts store starch

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Comparing Cells

• Prokaryote vs. Eukaryote
• Prokaryotes lack a nucleus and membrane bound
  organelles.
• Prokaryotes have a nucleoid region in which
  genetic material is concentrated but lack an
  internal membrane system.
• Prokaryotes are smaller than eukaryotes.
• Prokaryotes are thought to have evolved earlier.

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• Plant cells vs. Animal cells
• Plant cells have a cell wall. Animal cells do
  not.
• Plant cells contain a large central vacuole.
  Animals cells have several smaller vacuoles.
• Plant cells contain a variety of plastids.
  Animals cells do not.
• Animal cells have centrioles. Plant cells do not.