chapter 4

• The size range of cells

  • most cells are smaller than 0.1mm
  • the smallest cells (some bacteria) are similar in size to large viruses (0.1um)
  • (juicy pieces of a citrus fruit are separate gigantic cells
• Electro microscopy:

  • reached sun Angstrom (A) resolution in year 2000 by resolving the 0.89A spacing between carbon atoms of diamond (1A=10^-10m)
• Composition of a prokaryotic cell

  • the content (cytoplasm) is not separated into compartments
  • DNA and ribosomes are freely floating in the cytoplasm
• Structure of a prokaryotic cell:

  • cell envelope
  • plasma membrane
  • cell wall
  • glycocalyx
  • capsule
• cell envelope includes:

  • plasma membrane
  • cell wall
  • glycocalyx (layer of polysaccharide outside cell wall)
• plasma membrane:

  • a phospholipids bilayer much like the plasma membrane of eukaryotic cells
  • important role in regulating the entrance and the exit of substances into the cytoplasm
• cell wall:

  • composed of a complex molecule peptidoglycan (amino disaccharide and peptide fragments)
  • maintains the overall shape of a bacteria cell (coccus, bacillus and spiral)
  • mycoplasma are bacteria that have no cell wall and therefore have no definite shape
• Glycocalyx:

  • layer of polysaccharide laying outside the cell wall
  • glycocalyx aids against drying out by trapping water and help bacteria to resists a host's immune system
  • it allows that bacterium to attach itself to inert surface (like teeth and rocks), eukaryotes (streptococcus pneumoniae attaches itself to lung cells), or other bacteria (their glycocalyxes can fuse to envelop the colony)
  • is found not only in bacterial cells but also in eukaryotic cells
  • fuzzy gel like, sticky lay made up mainly of proteins and sugars
  • it surrounds the outermost cellular membrane of cells
• capsule:

  • organized layer of polysaccharide
  • protects the bacteria cell and is often associated with pathogenic bacteria
  • serves as a barrier against phagocytosis by white blood cells
• Structure of prokaryotic cells: (Appendages)

  • Bacteria may have the following appendages
  • flagella
  • fimbriae
  • sex pili
• flagella:

  • responsible for most types of bacterial motility
  • flagella are long appendages which rotate by means of a "motor" located just under the plasma membrane
  • bacteria may have 1, a few, or many flagella in different positions on the cell
• fimbriae:

  • small fibers that sprout from the cell surface
  • not involved in motility
  • help bacteria attach to surfaces
• Sex pili:

  • rigid tubular structure used by bacteria to pass DNA from a cell to cell
  • bacteria reproduce asexually by binary fission, but they can exchange DNA through the sex pili
• Bacteria Flagella rotation:

  • it is one of the examples of a wheel principle in nature
• Endoplasmic reticulum:

  • smooth ER
  • synthesis of lipids
  • metabolism of carbohydrates
  • detoxification of drugs and poisons
  • storage of Ca^2+ ions
• endoplasmic reticulum:

  • rough ER
  • synthesis of glycoproteins (carbohydrates + proteins)
  • production of secretory vesicles
  • membrane factory (phospholipids)
• Ribosomes

  • protein synthesis in 3 locations:
  • cytosol (free ribosomes)
  • rough ER
  • nuclear envelope
• Sedimentation

  • svedberg (S):
  • measure of sedimentation speed
  • Sedimentation
  • movement towards the bottom of the centrifuge tube
  • Ribosome 30S subunits sediment with speed of 30 um/s under the force of 10^6g
  • sedimentation speed depends on
  • weight
  • shape
  • temp
  • medium
  • (SVEDBERG UNITS ARE NOT ADDITIVE)
  • Prokaryotic ribosomes: 30S and 50S subunits together have speed of 70S (not 80S)
  • Eukaryotic ribosomes: 40S and 60S
• The golgi apparatus:

  • shipping
  • receiving
  • manufacturing center
• Golgi apparatus functions:

  • modification of polysaccharides, glycoproteins, phospholipids
  • synthesis of many secretory products
  • sorting and shipping to various locations
• Lysosomes:

  • Digestive organelles
  • membrane bounded vesicles produced by the golgi apparatus
  • have a very low pH and contain a powerful hydrolytic digestive enzymes (acid hydrolases) to break down proteins, carbohydrates, nucleic acids and lipids
  • digest food particles, and engulfed viruses or bacteria through endocytosis
  • digest excess or worn out organelles through endoxytosis "autophagy"
  • the membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell
• peroxisomes:

  • contain enzymes that remove hydrogen atoms and transfer them to O2 (hydrogen peroxyde H2O2 is a by product -> name "peroxysome")
  • reaction: (RH2 + O2 -> R + H2O2)
  • conversion of fatty acids to smaller molecules for use as fuel in mitochondria
  • detoxification of alcohol and other poisons in the liver cells
  • thought to have endosymbiotic origin
• peroxisome (pt2)

  • relatively small organelles found in all eukaryotic cells
  • general function to catalyze certain chemical reactions, typically those that break down molecules by removing hydrogen or adding oxygen
  • hydrogen peroxide is immediately broken down to water and oxygen by another peroxysomal enzyme called catalase
  • Enzyme in peroxisome are cell specific
  • in liver, peroxisomes produce bile salts from cholesterol and other break down fats
• types of vacuoles:

  • central vacuole (plants)
  • food vacuoles
  • contractile Vacuoles (protista)
• vacuoles

  • functions varied and differ among cell types and environmental conditions
  • central vacuoles in plants for storage and support
  • contractile vacuoles in protists for expelling excess water
  • phagocytic vacuoles in protists and white blood cells for degradation
• Endomembrane system

  • network of membranes enclosing the
  • nucleus
  • endoplasmic reticulum
  • golgi apparatus
  • lysosomes
  • vacuoles
  • plasma membrane
  • may be directly connected to each other or pass materials via vesicles
  • restrict enzymatic reactions to specific compartments within cell
• mitochondria & chloroplasts

  • The endosymbiont theory
  • mitochondria and chloroplasts derive from ancient prokaryotes captured by a eukaryotic cell
  • proof
  • have their own DNA
  • divide independently
  • have ribosomes of bacterial type (70S)
• Mitochondria

  • mitochondrial matric and inner membrane contain enzymes involved in respiration and production of ATP (energy)
  • 1 to 1000s mitochondria per cell, depending on metabolic activity of the cell
  • can change their shape
  • in metabolically active cells, mitochondria form a network of tubes
• chloroplasts

  • contain enzymes involved in photosynthesis
  • can change their shape
  • fatty acid biosynthesis
  • amino acid biosynthesis
  • plant immune response
• Cytoskeleton

  • is a network of fibers that organize structures and activities in the cell
  • 3 types
  • microtubules (tubulin polymers)
  • microfilaments (actin filaments)
  • intermediate filaments
• Microtubules (tubulin polymers)

  • main functions
  • maintenance of cell shape (compression resisting "girders")
  • cell motility (cilia or flagella)
  • chromosome movements in cell division
  • organelle movements
  • column of tubulin dimers, 25nm width, space in the middle on the tube
• microfilaments (active filaments)

  • main functions
  • maintenance of cell shape (tension-bearing elements)
  • changes in cell shape
  • muscle contraction
  • cytoplasmic streaming
  • cell motility (pseudopodia)
  • cell division (cleavage furrow formation)
  • actin subunits twisted together in a rope form, 7nm width
• intermediate filaments

  • main functions
  • maintenance of cell shape (tension-bearing elements)
  • anchorage of nucleus and certain other organelles
  • formation of nuclear lamina
  • fibrous subunit, keratin coiled together, cord like form, 8-12nm
• Cytoskeleton:

  • serves as internal skeleton that maintains cell shape (construction and organization) and assists in movement of its parts
  • contains 3 types of elements
  • microtubules
  • intermediate filaments
  • actin filaments (also knows as microfilaments)
• microtubules

  • long cylindrical structure composed of polymers of alpha and beta tubulin
  • they have polar structure with a plus end and minus end
  • a single microtubule can oscillate between growing and shortening phases: dynamic instability
  • key roles
  • intercellular transport (associated with dyneins and kinesins, they transport organelles like mitochondria and vesicle)
  • the axoneme of cilia and flagella
  • mitotic spindle
  • synthesis of the cell wall in plants
• intermediate filaments

  • more stable than microtubules and actin filaments, readily polymerize and depolymerize
  • function in the maintenance of cell shape and rigidity by bearing tension
• actin filaments

  • composed of 2 intertwined actin chains
  • actin filaments support the plasma membrane and provide strength and shape to the cell
  • participate in come cell to cell or cell to matric junction
• motor proteins

  • category of cellular proteins that use ATP as a source of energy to promote movement
• motor proteins

  • cellular proteins that use ATP as a source of energy to promote movement
  • consist of 2 domains
  • head
  • hinge
  • tail
  • walking analogy
  • ground is a cytoskeletal filament, your leg is the head of the motor protein, and your hip is the hinge
• 3 different movements for motor proteins

  • moves the cargo from one location to another
  • can remain in place and cause the filament to move
  • attempting to walk (both the motor protein and filament restricted in their movement) exerts a force that causes the filament to bend
  • convert chemical energy into mechanical energy by hydrolysis of ATP