Lecture Review: Cell Structure and Function

Overview of Cellular Structures

• Discussion about the importance of cell structures in understanding cellular function.

• Key components to focus on: nucleus, ribosomes, endoplasmic reticulum (ER), Golgi apparatus, and vesicular transport.

Endomembrane System

• Definition: The endomembrane system includes membrane-bound compartments within cells.

• Major components:

  • Nucleus

  • Endoplasmic Reticulum (ER)

    • Rough ER: Studded with ribosomes; involved in protein synthesis.

    • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Other components:

  • Vesicular Transport: Involves structures such as endosomes and lysosomes, facilitating transport within cells.

Compartmentalization of Cells

• Origin of membrane-bound organelles:

  • Derived from invaginations of the plasma membrane in ancient prokaryotic cells.

  • Eukaryotic cells exhibit greater complexity than prokaryotic cells due to compartmentalization.

  • Mitochondria theory: Endosymbiont theory suggests mitochondria evolved from ancient prokaryotic cells incorporated into eukaryotic cells.

  • Mitochondria have both their own DNA and the ability to replicate independently of the cell cycle.

The Nucleus

• Structure:

  • Largest organelle in the cell.

  • Surrounded by a nuclear envelope with two membranes (inner and outer) separated by a perinuclear space.

• Functions:

  • Stores genetic information (DNA).

  • Site for regulation of gene expression and DNA transcription.

  • Contains chromatin (DNA + proteins) and is involved in ribosome production (nucleolus).

Chromatin

• Two forms of chromatin:

  • Euchromatin: Less condensed form of chromatin; active regions of transcription.

  • Heterochromatin: Tightly packed; inactive regions that can inhibit gene expression.

• Chromatin can change shape during the cell cycle:

  • Interphase Chromosomes: Extended, diffuse form seen in non-dividing cells.

  • Metaphase Chromosomes: Condensed form during cell division.

Ribosomes

• Function: Site of protein synthesis (translation).

• Composition: Complex aggregates of rRNA and ribosomal proteins.

• Types:

  • Free Ribosomes: Synthesize proteins that function in the cytoplasm.

  • Membrane Bound Ribosomes: Found on rough ER, synthesize proteins for export or to be integrated into membranes.

• Histological stain reveals ribosome-rich areas in certain cell types (e.g., nerve cells).

Endoplasmic Reticulum (ER)

• Rough ER: Contains ribosomes; involved in synthesizing and transporting proteins destined for secretion.

• Smooth ER: Involved in lipid biosynthesis, storage, and detoxification processes.

• Location & structure: Continuous with the outer membrane of the nucleus, folded membranes known as cisternae and tubules.

• Sarcoplasmic Reticulum: Modified form found in muscle cells, crucial for calcium storage and release during contraction.

Golgi Apparatus

• Structure: Stack of flattened membranous sacs, receives vesicles from the ER.

• Function: Modifies proteins and lipids (e.g., glycosylation) and packages them for transport.

• Vesicular transport routes: Molecules can be sent to lysosomes or secreted from the cell.

Vesicular Transport and Lysosomes

• Process: Transport of molecules via vesicles created from the ER.

• Lamellar structure of vesicles consists of lipid bilayer enclosing cargo.

• Lysosomes: Contain enzymes to break down macromolecules, serve as the cell's digestive system.

Cellular Transport Mechanisms

• Vesicles interact with target membranes through specific proteins, enabling accurate transport.

• Two types of secretion:

  • Constitutive Secretion: Constantly produces and releases products.

  • Regulated Secretion: Releases products in response to specific stimuli.

Key Points on Insulin Secretion

• Insulin produced by specialized pancreatic cells in response to blood glucose levels.

• Secretion process involves vesicle fusion with the plasma membrane, controlled by cellular mechanisms.

Histological Techniques and Artifacts

• Histological processing alters tissue structure, causing artifacts.

• Understanding artifacts helps in interpreting histological images correctly.

• Expected to recognize differences between preserved tissues versus living tissues.