Cellular Organelles Study Notes

Overview of Cellular Organelles and Endomembrane System

  • Textbook Online Resources and Required Reading

    • Review glossary, flashcards, quizzes, and practice questions online.

    • Refer to animations, specifically for Mitochondria (Chapter 5, Figure 18.13).

    • Required Reading Sections:

    • 12.3 – Structure of the Nuclear Envelope

    • 8.3 – Endoplasmic Reticulum

    • 8.4 – Golgi Complex

    • 8.7 – Lysosomes

    • 5.7 – Peroxisomes

    • 5.1 – Mitochondrial Structure and Function

Compartmentalization in Cells

  • Compartmentalization Functions

    1. Define Boundaries: Establishes limits for cells and organelles.

    2. Scaffolding for Biochemical Activities: Organizes enzymes for effective interactions.

    3. Selective Permeability Barrier: Regulates the exchange of substances between compartments.

    4. Solute Transport: Membrane proteins facilitate movement of substances.

    5. Response to External Stimuli: Membrane receptors transduce signals from ligands.

    6. Cell-Cell Communication: Mediates recognition and interactions between cells.

    7. Energy Transduction: Allows for conversion of chemical energy into ATP and stores energy.

Major Intracellular Compartments of Animal Cells

  • Eukaryotic cells possess membrane-bound compartments known as organelles.

    • Each organelle has distinct functions, enzyme sets, and structural properties.

    • There is a complex distribution system for moving molecules between compartments.

    • Unicellular Organisms: Must perform all life functions in a single cell.

    • Multicellular Organisms: Have specialized cells for particular functions.

Nuclear Envelope and Nucleus Structure

  • Nuclear Envelope: Comprises two membranes:

    • Inner Membrane: Defines the nucleus.

    • Outer Membrane: Continuous with the endoplasmic reticulum (ER).

  • Nuclear Pores: Fuse the two membranes, acting as conduits for transport between cytoplasm and nucleoplasm.

  • Function: The nucleus contains DNA organized into chromosomes, with rRNA synthesized in the nucleolus which is critical for ribosome assembly.

Endoplasmic Reticulum (ER)

  • The ER is a network of membranes in the cytoplasm of nearly all eukaryotic cells.

  • Structure:

    • Composed of tubular membranes and flattened sacs named cisternae.

    • Internal space known as lumen and is continuous with the outer nuclear membrane.

Rough Endoplasmic Reticulum (RER)

  • Appearance: Studded with ribosomes on the cytoplasmic side, giving it a "rough" appearance.

  • Function:

    • Primary site for protein biosynthesis.

    • Ribosomes synthesize polypeptides that are inserted into the ER membrane or transported into the lumen.

Smooth Endoplasmic Reticulum (SER)

  • Characteristics: Lacks ribosomes (smooth appearance).

  • Functions:

    • Synthesis of lipids and steroid hormones (especially in gonad and adrenal cortex cells).

    • Detoxification of organic compounds (e.g., barbiturates, ethanol) in liver.

    • Sarcoplasmic reticulum: Specialized smooth ER for calcium storage and release during muscle contraction.

Golgi Complex

  • Structure:

    • Composed of stacked flattened vesicles known as cisternae, closely associated with the ER.

  • Function:

    • Processes and packages secretory proteins.

    • Modifies and synthesizes complex polysaccharides.

    • Proteins from the rough ER are processed as they transit through the Golgi, with vesicles budded off from the trans Golgi network for delivery to their final destinations.

Lysosomes

  • Structure:

    • A single membrane surrounds ~50 hydrolytic enzymes (hydrolases) that digest biological molecules.

    • Enzymes are synthesized in the ER, transported to the Golgi, and targeted to lysosomes.

  • Function:

    • Maintain a low internal pH via an ATP-driven H+ pump.

    • Sequestered hydrolases prevent digestion of the cell's contents.

    • Involved in three degradation pathways:

    1. Endocytosis

    2. Phagocytosis

    3. Autophagy: Degrades intracellular organelles, increasing under starvation conditions.

Peroxisomes

  • Characteristics:

    • Small vesicles (0.1-10 µm), surrounded by a single membrane; crucial in liver and kidney cells.

  • Functions:

    • Detoxify harmful compounds (e.g., methanol, ethanol).

    • Catabolize long-chain fatty acids (fatty acid breakdown mainly occurs in mitochondria).

    • Contain oxidases that generate H2O2, which is then degraded by catalase.

Mitochondria

  • Structure:

    • Contain two membranes:

    • Outer Membrane: Permeable; contains porins allowing small molecules to pass.

    • Inner Membrane: Impermeable; extensively folded into cristae, embedded with respiratory proteins.

    • Matrix: Enzyme-rich interior containing DNA, ribosomes, and additional proteins from nuclear genes.

  • Function:

    • Primary site of ATP production in non-photosynthetic cells; number and location vary based on cells' energy demands (1-100,000 mitochondria).

    • Play roles in regulating apoptosis.

Endosymbiont Theory

  • Explains the evolutionary origin of mitochondria (and chloroplasts) from ancient bacteria via phagocytosis without digestion.

  • Similarities with bacteria:

    • Size, shape, double membranes, circular DNA without histones, similarities in rRNA and protein synthesis mechanisms.

Summary Points

  • Understand the basic organization, characteristics of plasma membranes, organelle structures, and functions.

  • Learn the endomembrane system's roles in endocytosis and exocytosis.

  • Grasp the fundamentals of the endosymbiont theory for cellular evolution.