Organelles and compartmentalisation

• Organelles


  Discrete subunits of cells with a membrane that are adapted to perform a specific metabolic function
• Not organelles


  • Not membrane bound/don't have a specific function
  • Cytoskeleton + cell wall
  • Cytoplasm
• Cell fractionation


  1. Homogenization - tissue with cells is broken up in a blender

  2. Filtered to remove large cell debris
• Ultracentrifugation


  1. Spun at low speeds

  2. Densest organelles (nucleus) forms a pellet at the bottom

  3. Pellet is removed, process repeated at different speeds
• Post transcriptional changes occur before translation in eukaryotes before the mRNA meets ribosomes in the cytoplasm
• In prokaryotes, mRNA immediately meets ribosomes
• Signals sent to the nucleus

  Message passed to change gene expression through an increase or decrease in transcription
• Altering gene expression


  Cell produces different types/amounts of proteins in response to changing conditions
• Specific reactions occur in specific places
• Macrophages use phagocytic vacuoles to break down pathogens
• Enzymes would damage other cell parts
• Small space means the concentration is high, faster and more efficient reaction
• Advantages of compartmentalization


  • Efficient metabolism - concentrated enzymes and substrates
  • Localized conditions - pH at optimum
  • Isolate toxic substances
• Mitochondria


  • Matrix - high concentration of enzymes and molecules for the Krebs cycle
  • Intermembrane space - small, allows high concentration of molecules such as protons, high concentration gradient across the inner membrane
  • 70s ribosomes - synthesize proteins for aerobic respiration
  • Chromosome - single circular chromosome, genetic material for proteins
  • Outer membrane - not permeable to protons, build up in the intermembrane space protein channels allows pyruvate to enter from the cytoplasm
  • Inner membrane - highly folded, contain cristae, increase surface area
• Chloroplasts


  • 3 membranes -> inner, outer and thylakoid
  • 3 compartmentalized areas (intermembrane space, stroma, thylakoid space)
  • Thylakoid membrane - stacks of grana, chlorophyll, electron transport chain, ATP synthase
  • Thylakoid space - within the lumen for buildup of protons
  • Stroma - chromosome, 70s ribosomes and enzymes for the Calvin cycle
  • Chromosome - genetic info for synthesizing proteins (enzymes for photosynthesis)
• Role of the nucleus in gene expression

  • Membrane separates mRNA production from the cytoplasm
  • Nucleus has the ideal conditions for transcription
  • mRNA enters through the nuclear pore
  • Inner membrane controls entry and exit of signaling molecules and transcription factors
  • Regulates gene expression
• Role of the nucleus in cell division

  • Nuclear envelope breaks down during cell division to allow for the separation of chromosomes
  • After cell division, it must reassemble to enclose the newly formed nucleus
  • Initiated by the formation of small membrane bound vesicles from the ER
  • Vesicles contain proteins + lipids specific to the nuclear membrane, bind to the chromosome
  • Vesicles join and surround chromosomes
  • Fuse together to form the double membrane
  • Maintains genetic material integrity and ensures cellular processes occur
• Protein transport


  Protein -> nuclear pore -> rough er -> golgi -> vesicles -> plasma membrane
• Bound ribosomes


  • Joined to the ER
  • Proteins end up outside the RER
  • More of them than free ones
  • mRNA that codes for proteins that need to be exported out is transcribed with an ER signal
  • When joined with a ribosome, the signal directs it towards the ER membrane
  • Cell directs proteins to correct location
• Ribosomes


  • Synthesize proteins by translating mRNA
  • Composed of large ribosomal subunit and small ribosomal subunit
  • Proteins and rRNA
• RER


  • Interconnected membranes that form flattened sacs + tubes
  • ER lumen is inside the flat sacs
  • Rough due to attached ribosomes
  • Bound ribosomes synthesize proteins, transported to the lumen for processing and modification
  • RER enzymes modify proteins
• Golgi apparatus


  • Vesicles transport protein to Golgi apparatus
  • Piece of RER membrane breaks off to form a vesicle and travels to the Golgi apparatus
  • Stack of flattened membrane bound sacs that sorts proteins based on their final destination
• Exocytosis


  Removal of bulk substances from a cell due to vesicles fusing with the membrane and contents released
• Endocytosis


  Invagination of the membrane, bulk substances are taken into a cell via a membrane into vesicles
• Roles of vesicles


  • Transport vesicles - materials from one cell to another
  • Secretory vesicles - store + transport molecules to be secreted outside the cell
  • Lysosomes - hydrolytic enzymes break down macromolecules
  • Peroxisomes - dif set of enzymes, detoxify harmful compounds + lipid metabolism
• Clathrin


  1. Clathrin forms a cage-like structure through polymerization of clathrin around future vesicle area

  2. Membrane invaginates, clathrin surrounds it, forms a clathrin coated pit

  3. Coated pit is scaffold for vesicle formation

  4. Once a vesicle has formed, the clathrin coat breaks down through hydrolysis into individual pieces