Cell Structure and Organization Notes

Cell Structure and Organization

Endomembrane System Function

  • Major site of metabolic reactions.
  • Lipid synthesis.
  • Site of protein synthesis and modification.
  • Storage and recycling of organic molecules.
  • Provides a routing system for transporting cellular and extracellular proteins to their correct destinations.

Membranes: Structure and Function

Key Components of Membranes

  • Phospholipids: Composed of nonpolar fatty acid tails and polar heads. They form a bilayer that is crucial for the structure of plasma membranes and the endomembrane system.
  • Lipids: Made primarily of hydrogen and carbon; hydrophobic and insoluble in water. They include fats, phospholipids, and steroids. Lipids make up ~40% of organic matter in the human body.

Types of Lipids

  1. Fats

    • Triglycerides: Comprised of glycerol bonded to three fatty acids.
    • Essential fatty acids must be obtained through diet (humans cannot synthesize them).
    • Fats serve as energy stores; 1g of fat has twice the energy of 1g of glycogen.
    • Hydrolysis processes release fatty acids from glycerol for energy utilization.

  2. Phospholipids

    • Similar structure to triglycerides, but the third hydroxyl group of glycerol is linked to a phosphate group instead of a fatty acid.
    • Amphipathic properties: have a hydrophilic (polar) head and a hydrophobic (nonpolar) tail, allowing them to form bilayers.

  3. Steroids

    • Consist of four fused carbon rings, often with a polar hydroxyl group attached, forming sterols.
    • Important for the structure of plasma membranes.

Synthesis and Transport of Membrane Lipids

  • Phospholipid synthesis occurs in the cytosol and endomembrane system, particularly at the ER membrane's cytosolic leaflet.
  • Lipids can move between membranes via lateral diffusion, vesicle transport, and lipid exchange proteins.

Protein Sorting

  • Thousands of proteins exist within a cell, each functioning in specific locations.
  • Proteins have sorting signals that direct them to their destinations, often synthesized as the protein is being made.
  • Cotranslational Sorting: occurs when the ER signal peptide attracts the SRP, pausing translation and guiding the ribosome to the rough ER membrane.

Key Processes in Protein Sorting

  1. Cotranslational Translocation: Translates protein while simultaneously guiding it to the ER lumen through a protein channel.
  2. Post-Translational Translocation: Proteins may be released from ribosomes after translation and then transported to the membrane.
  3. Glycosylation: Covalent attachment of carbohydrate groups in the ER lumen to assist in protein folding, stability, and function.

Vesicular Transport of Proteins

  • Proteins are transported in vesicles, utilizing cargo receptors and SNARE proteins to ensure specificity in delivery to target membranes.
  • Once released, proteins either remain in the ER, are modified by the Golgi apparatus, or are transported to lysosomes, vacuoles, or the extracellular environment.

Non-Endomembrane Protein Sorting

  • Proteins destined for organelles like mitochondria or chloroplasts have specific sorting signals and are transported post-translationally in the cytosol.
  • Different regions of mitochondria have unique sorting signals for proper localization.

Summary of Cell Organization

  • Understanding cell biology involves knowledge about the structure and understanding organelles, protein sorting, metabolic pathways, and the roles of different biomolecules (proteins, carbohydrates, lipids, nucleic acids).
  • Key functions of cells include energy generation, waste recycling, and communication within and outside the cellular structure.