Compartmentalization

Why Compartmentalize the Cell?

  • Membrane Surface Area:

    • Increased surface area for membrane-based processes.

    • Greater membrane area to cell volume ratio enhances efficiency.

  • Segregates Specialized Functions:

    • Enables incompatible reactions to occur without interference (e.g., synthesis vs degradation).

    • Management of proteins entering and exiting the cell.

    • Designated storage sites for different substances.

    • Differentiation between acidic and non-acidic environments.

  • Membrane-bound Organelles Structure:

    • Cytoplasmic Face: The face of the bilayer that faces the cytoplasm.

    • Luminal Face: The face of the bilayer that faces the lumen of the organelle.

    • Lumen: The interior space of the membrane-bound organelle.

Importance of Organization

  • Healthy cellular function relies on organized structures.

  • Disruption in compartmentalization can lead to cellular dysfunction.

Organelles and Protein Synthesis

  • Organelles:

    • Rough Endoplasmic Reticulum (RER): Site of protein modifications.

    • Smooth Endoplasmic Reticulum (SER): Involved in lipid synthesis and detoxification.

    • Golgi Apparatus: Processes and sorts proteins and lipids.

    • Lysosomes: Contain enzymes for digestion.

    • Mitochondria: Powerhouse of the cell, ATP production.

    • Peroxisomes: Involved in oxidation reactions.

    • Plastids: Found in plants for manufacturing and storage (e.g., chloroplasts for photosynthesis).

Basic Set of Organelles in Eukaryotic Cells

  • Common Organelles:

    • Nuclear membrane, Rough and Smooth ER, Golgi apparatus, Lysosomes, Endosomes, Peroxisomes, Mitochondria, and Plastids.

    • All organelles arise from pre-existing ones, contributing to cell specialization in multicellular organisms.

Intracellular Compartment Volumes

  • Organelles occupy approximately half of the cell's volume.

  • Relative Volume in a Hepatocyte:

    • Cytosol: 54%

    • Mitochondria: 22%

    • Rough ER: 9%

    • Smooth ER: 6%

    • Nucleus: 6%

    • Peroxisomes, Lysosomes, Endosomes: 1% each.

Membrane Types and Distribution

  • Different percentages of membrane types in liver hepatocytes versus pancreatic exocrine cells.

  • Key Membrane Types:

    • Rough ER membrane: ~35% in hepatocytes

    • Smooth ER membrane: ~16% in hepatocytes

    • Total estimated membrane areas vary significantly between different cell types.

Topological Relationships Among Organelles

  • Four Organelles Families:

    1. Nucleus + Cytosol (connected by nuclear pores)

    2. Endomembrane System: ER, Golgi, endosomes, lysosomes, peroxisomes

    3. Mitochondria

    4. Plastids (e.g., chloroplasts in plants)

  • Evolutionary origins likely explain the close relationships among certain organelles.

Endomembrane System

  • Communicates with the outside of the cell without crossing membranes.

  • Molecules move between compartments via vesicles.

Mitochondria and Plastids

  • Maintain separate compartments with no fusion or vesicular traffic with the endomembrane system, displaying independent functions.

Differential Protein Distribution

  • Cells contain around 10 billion protein molecules with around 10,000 different types produced mainly by cytosolic ribosomes, with some synthesized in mitochondria and plastids.

Mechanisms of Protein Targeting

  • Each organelle has specific functions and requires distinct proteins and lipids.

  • Proteins possess intrinsic signals for localization, as discovered by Günter Blobel (1999 Nobel Prize).

  • Key Signal Sequences:

    • Typically 15-60 amino acids and can be continuous or discontinuous (signal patch).

Protein Transport Mechanisms

  • Three Major Mechanisms:

    1. Gated Transport: Between cytosol and nucleus, utilizing nuclear pore complexes.

    2. Transmembrane Transport: Proteins inserted into membranes and lumen during translation.

    3. Vesicular Transport: Moves proteins between endomembrane system components.

Nuclear Transport: Gated Transport

  • Description of nuclear envelope structure and its bidirectional transport capabilities concerning molecules like mRNA, tRNA, and ribosomal RNA (rRNA).

Nuclear Pore Complexes (NPC)

  • Composed of ~30 different proteins, enabling rapid bidirectional transport of proteins and RNA.

  • Transport Types:

    1. Passive diffusion for small molecules.

    2. Active transport for larger proteins and RNA.

Nuclear Localization Signals

  • Direct nuclear proteins require a sorting signal, receptor, and energy for importation into the nucleus.

  • Signals usually rich in positive amino acids, positioning can vary within the protein structure.

Nuclear Import Receptors

  • Bind to nuclear localization signals and facilitate the transport of proteins through the NPC.

  • Importins form complexes that operate based on specific NLS types, enabling selective binding and transport.

Nuclear Export Mechanism

  • Functions similarly to import but in a reverse process using nuclear export signals.

  • Exportins bind to proteins and facilitate transport through the nuclear pore complex based on energy provided by GTP hydrolysis.

Ran GTPase Role in Nuclear Transport

  • Establishes directionality of transport across the NPC, maintaining concentration gradients of GTP-bound and GDP-bound forms.

  • Energy Source: Concentration gradients of Ran GTP provides necessary energy for transport mechanisms.

Summary of the Transport Process

  • The gradients of Ran-GTP and Ran-GDP in the nucleus and cytosol are crucial for nuclear import and export efficiency, influencing localization and cellular function.