Cell Compartmentalization 1

Intracellular Compartments and Protein Sorting

Introduction

  • Study content from Chapter 12 on intracellular compartments and protein sorting in eukaryotic cells.

  • Understanding compartmentalization is essential for cell organization and function.

Why Compartmentalize the Cell?

  1. Membrane Surface Area

    • More surface area available for membrane-based processes.

    • Higher membrane area to cell volume ratio.

  2. Segregates Specialized Functions

    • Separates incompatible reactions (e.g., synthesis vs. degradation).

    • Distinguishes proteins coming into versus leaving the cell.

    • Creates storage sites with specific environments (e.g., acidic vs. non-acidic).

  3. Distinct Parts of Membrane-Bound Organelles

    • Cytoplasmic face of the bilayer.

    • Luminal face of the bilayer.

    • The lumen, or interior, of the organelle.

Organelles and Their Functions

  • Organelles are specialized parts of a cell that perform distinct functions.

  • Major organelles include:

    • Nucleus

    • Rough Endoplasmic Reticulum (ER)

    • Mitochondria

    • Lysosomes

    • Peroxisomes

Eukaryotic Cell Organization

  • Compartmentalization is key to understanding eukaryotic cells.

  • Different organelles enable specialization of cellular activities.

Overview of Chapter Topics

  • Origins and topological relationships of eukaryotic organelles.

  • Mechanisms of protein sorting and transport, focusing on:

    • Gated transport (nuclear pore structure and function).

    • Post-translational protein translocation into mitochondria and chloroplasts.

    • Structure and function of the ER and how proteins are sorted and modified.

    • Lipid synthesis.

Intracellular Volume and Membrane Types

  • Organelles occupy a significant volume of cells (e.g., ~50% in liver cells).

  • Breakdown of volume occupied by major compartments in a liver cell:

    • Cytosol: 54%

    • Mitochondria: 22%

    • Rough ER: 9%

    • Nucleus: 6%

Relative Amounts of Membranes

  • Distribution of membranes in different eukaryotic cells (e.g., liver and pancreatic cells).

    • Plasma membrane: 2% (liver) vs. 5% (pancreas).

    • Rough ER membrane: 35% (liver) vs. 60% (pancreas).

Topological Relationships Among Organelles

  1. Nucleus and Cytosol connected by nuclear pores.

  2. Endomembrane System includes:

    • ER, Golgi apparatus, endosomes, lysosomes, and peroxisomes.

  3. Mitochondria and Plastids (in plants only).

Gated Transport Mechanisms

  • Gated transport occurs through nuclear pores.

  • Process allows selective movement of macromolecules into the nucleus, such as mRNA and ribosomal proteins.

Directionality of Transport

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  • Ran GTPase gradient maintains directionality in nuclear transport:

    • High Ran-GTP in the nucleus and low in the cytoplasm.

    • Ensures proteins are effectively imported and exported according to cellular needs.

Summary of GTPase Function

  • Ran-GTP functions as a switch that regulates protein transport through nuclear pore complexes.

  • Involved in both import (in nucleus) and export (to cytoplasm) of proteins.

Importance of Access Control

  • Regulation of nuclear transport is crucial for the expression of transcription factors and overall cellular function.