CELL BIO

• Mitochondria


  Bacteria-sized organelles found in large numbers in almost all eukaryotic cells, representing around 25% of the cell volume
• Mitochondria


  • Enclosed by two membranes - a smooth outer membrane and a markedly folded or tubular inner mitochondrial membrane
  • The folds of the inner membrane are known as cristae, and tube-like protrusions are called tubules
  • The intermembrane space is located between the inner and the outer membranes
  • The number and shape of the mitochondria, as well as the numbers of cristae they have, can differ widely from cell type to cell type
  • Tissues with intensive oxidative metabolism have mitochondria with particularly large numbers of cristae
  • The shape of the mitochondria can vary depending on their functional status
  • Mitochondria are mobile, plastic organelles
• Mitochondria probably developed during an early phase of evolution from aerobic bacteria that entered into symbiosis with primeval anaerobic eukaryotes
• Endosymbiont theory


  Supported by many findings, e.g. mitochondria have a ring-shaped DNA and their own ribosomes, the mitochondrial genome became smaller during evolution
• Human mitochondrial DNA spans about 16,569 base pairs, which code for two rRNAs, 22 tRNAs, and 13 proteins
• Only 13 proteins (mostly subunits of respiratory chain complexes) are produced in the mitochondrion
• Outer membrane


  Smooth, composed of equal amounts of phospholipids and proteins, contains porins that allow movement of molecules up to 5000 daltons
• Inner membrane


  More complex, folded into cristae, aids in ATP production, strictly permeable only to oxygen, ATP and metabolites
• Intermembrane space


  Space between the outer and inner membrane, has the same composition as the cell's cytoplasm
• Matrix


  Complex mixture of proteins and enzymes important for ATP synthesis, contains mitochondrial ribosomes, tRNAs and mitochondrial DNA
• Functions of mitochondria


  • Produce energy through oxidative phosphorylation
  • Maintain proper calcium ion concentration in the cell
  • Help build blood and hormones like testosterone and estrogen
  • Detoxify ammonia in liver cells
  • Play a role in apoptosis or programmed cell death
  • Abnormal death of cells due to mitochondrial dysfunction can affect organ function
• Mitochondrial DNA (mtDNA)


  Small amount of DNA present in the mitochondria, separate from nuclear DNA, maternally inherited, susceptible to free oxygen radicals, mutations can lead to mitochondrial diseases
• Mitochondrial disease results from the failure of mitochondria to produce energy needed for life and growth, primarily affecting young and damaging cells in brain, heart, liver, muscles, kidney, respiratory and endocrine systems
• Mitochondria are known as the powerhouse of the cell, responsible for generating ATP through cellular respiration.
• Endoplasmic Reticulum


  The largest single membrane bound intracellular compartment found in almost all plant and animal cells
• The only exceptions are mature erythrocytes and prokaryotes
• Structure of Endoplasmic Reticulum


  • An extensive network of membrane enclosed channels present throughout the cell
  • The enclosed compartment is called the lumen
  • The membrane is continuous with the outer nuclear membrane
• Cisternae


  Broad, flat, membrane bound spaces arranged parallel to each other
• Tubules


  Appear in circles in the endoplasmic reticulum sections
• Vesicles


  Membrane-bound, isolated globose cavities
• Types of Endoplasmic Reticulum


  • Rough Endoplasmic Reticulum
  • Smooth Endoplasmic Reticulum

  
• Rough Endoplasmic Reticulum


  • Composed of flattened sacs-cisternae
  • Outer membrane of the nuclear envelope continues to form RER
  • Ribosomes are present on the cytosolic facade

  
• Smooth Endoplasmic Reticulum


  • Forms highly curved, tubular and interconnecting system
  • Lacks ribosomes

  
• Functions of Endoplasmic Reticulum


  • Mechanical support
  • Synthesis of secretory proteins
  • Glycosylation of proteins
  • Carbohydrate metabolism
  • Lipid synthesis and storage
  • Electron transport and detoxification
  • Circulation and exchange
  • Synthesis of cholesterol and steroid hormones
• The ratio of RER and SER in a cell depends on the cell type and its function
• Pancreatic exocrine cells possess only granular or Rough endoplasmic reticulum
• In epithelial cells of the frog retina and interstitial cells of the testis the endoplasmic reticulum is completely Smooth (SER)
• Endoplasmic reticulum


  Intracellular transport system for various substances, connected to nuclear envelope
• Circulation and exchange in endoplasmic reticulum


  1. Provides extensive internal surface for exchange of material between cytoplasm and lumen

  2. Presence of permeases and carriers on ER membrane facilitates active transportation
• Cholesterol


  Important precursor of steroid hormones
• Endoplasmic reticulum


  • Major site of cholesterol synthesis
  • Involved in synthesis and storage of cholesterol in liver cells
  • Site of synthesis of steroid hormones in testis, ovary and adrenal
• Sarcoplasmic reticulum


  SER of muscles involved in concentration of Ca2+ ions utilizing ATP, release of Ca2+ ions results in muscle contraction
• Oxyntic cells


  Present in stomach ER, implicated in chloride ion secretion
• Endoplasmic reticulum


  Involved in nuclear membrane formation
• Transport of ions, molecules and particles


  May take place through membrane flow, allowing substances like RNA and nuclear proteins to pass out from nucleus
• Plasmodesmata


  Interconnection of cells in plants, formed by endoplasmic reticulum
• Import of proteins into endoplasmic reticulum

  1. Cytosolic proteins intended for secretion or insertion into organelles are first imported into ER lumen

  2. Proteins carry a hydrophobic signal peptide recognized by signal recognition particle (SRP)

  3. SRP binds to growing polypeptide chain and ribosome, directing them to SRP receptor on ER membrane

  4. Polypeptide chain traverses ER membrane through a hydrophilic pore in a protein translocator
• Co-translational import


  Proteins are imported into ER lumen during translation, unlike post-translational import into other organelles
• Soluble proteins


  Pass across ER and into its lumen
• Transmembrane proteins


  Intended for ER membrane or other cell membranes, stay anchored in lipid bilayer