Cell Organelles Study Notes

CELL ORGANELLES

  • Organelles are specialized structures within cells that perform distinct functions necessary for cellular life.

Key Cell Organelles

  • Nucleus
    • Contains the genome (excluding mitochondrial and chloroplast DNA).
    • Principal site for DNA and RNA synthesis.
  • Cytoplasm
    • Composed of cytosol and organelles suspended within it.
    • Cytosol accounts for just over half the cell's total volume.
    • Site of protein synthesis and degradation, as well as intermediary metabolism.

Endoplasmic Reticulum (ER)

  • About half the total area of the membrane in eukaryotic cells is ER.
  • Rough ER
    • Contains ribosomes on its cytosolic surface.
    • Ribosomes synthesize soluble and integral membrane proteins, mainly for secretion or other organelles.
    • Proteins are transported to other organelles post-synthesis completion.
  • Smooth ER
    • Lacks ribosomes and functions in lipid production and Ca²⁺ ion storage.

Golgi Apparatus

  • Organized stacks of disc-like compartments known as Golgi cisternae.
  • Modifies and dispatches lipids and proteins received from the ER to various destinations within the cell.

Mitochondria and Chloroplasts

  • Generate most of the ATP for cellular energy needs.
    • Mitochondria: powerhouse of the cell.
    • Chloroplasts: specialized plastids in plants and algae for photosynthesis and storage.

Lysosomes and Endosomes

  • Lysosomes
    • Contain digestive enzymes for degrading defunct organelles and macromolecules.
  • Endosomes
    • Intermediate vesicular compartments that transport material destined for lysosomes.

Peroxisomes

  • Small vesicular compartments containing enzymes for oxidative reactions.

Cellular Structure and Adaptation

  • The abundance and shape of organelles adapt to cellular needs.
    • Example: Plasma cells have amplified rough ER for antibody synthesis.
  • Membrane-enclosed organelles have characteristic distributions driven by interactions with the cytoskeleton.

Evolution of Eukaryotic Cells

  • Evidence suggests the nuclear genome of eukaryotes evolved from an ancient archaeon.
    • Eukaryotic cells are thought to have emerged from a symbiotic relationship between anaerobic archaea and aerobic bacteria around 1.6 billion years ago.
    • The nuclear envelope likely originated from invagination of the plasma membrane for chromosome protection.

Nuclear Envelope and Compartmentalization

  • The double-layered nuclear envelope is punctuated by nuclear pore complexes allowing selective molecular transit.
  • Four distinct intracellular compartments are identified in eukaryotic cells:
    1. Nucleus and cytosol.
    2. Organelles functioning in the secretory and endocytic pathways.
    3. Mitochondria.
    4. Plastids in plants.

Protein Transport Mechanisms

  • Gated Transport
    • Movement of proteins and RNA through nuclear pore complexes between the cytosol and nucleus.
  • Transmembrane Transport
    • Direct translocation of proteins across membranes into specific organelle spaces (e.g., ER, mitochondria).
  • Vesicular Transport
    • Transport vesicles ferry proteins between topologically equivalent compartments, maintaining orientation and asymmetry of membrane proteins.

Membrane Structure and Function

  • Plasma Membrane
    • Regulates substance passage and signaling between cells.
    • Composed of a lipid bilayer with embedded proteins, primarily phospholipids, organized with polar regions at the surfaces.
  • Cholesterol
    • Imparts stability and fluidity to the membrane; prevents solidification of the lipid bilayer.

Membrane Proteins

  • Two classes:
    • Integral Membrane Proteins: Embed within the lipid bilayer; may span the entire membrane.
    • Peripheral Membrane Proteins: Associate with the membrane surfaces, often connected to cytoskeletal elements.

Functions of Membrane Carbohydrates

  • Carbohydrates on the extracellular surface form the glycocalyx, assisting in cell recognition and protection against mechanical damage.

Nucleus and Its Components

  • Acts as the information processing center of the cell, coordinating activities related to metabolism and reproduction.
  • Nucleoplasm: The matrix within the nucleus containing chromatin.
  • Nucleoli: Non-membrane-bound organelles for ribosome production; the quantity can vary depending on species.

Nucleolus Formation and Structure

  • Forms when chromosomes gather at nucleolar organizing regions during cell division, disappearing during mitosis.
  • Composed of granular and fibrillar components related to ribosomal RNA synthesis.

Cytoplasm and Its Properties

  • A complex system that enables movement of particles via the cytoskeleton and motor proteins; it can exhibit both viscous and elastic properties depending on metabolic activities.

Cytoskeleton

  • Provides structure, shape, and allows movement through microtubules, intermediate filaments, and microfilaments.
    • Microtubules: Hollow tubes involved in transport and mitosis, formed from tubulin dimers, dynamic in nature.
    • Intermediate Filaments: Provide mechanical support, vary by cell type, contribute to tension resilience.
    • Microfilaments: Comprised of actin, essential for cell movement and shape.

Mitochondria

  • Evolved from engulfed aerobic bacteria, crucial for energy production through oxidative phosphorylation, and contain their own DNA (mtDNA).
  • Human mtDNA codes for 13 proteins essential for ATP production.
  • Maternal inheritance elucidates evolutionary descent and tracks mutations linked to diseases.

Lysosomes

  • Membrane-bound structures containing hydrolytic enzymes, functioning best at an acidic pH (around 5.0).
  • Serve to digest internal and external materials; dysfunction can lead to lysosomal storage diseases.