The Nucleus and Endomembrane System

Introduction

The nucleus contains the genetic instructions necessary for manufacturing proteins involved in most cellular functions.
The endomembrane system is a series of membranous organelles functioning in the processing of materials for the cell.

The Nucleus

Defining Characteristic: The presence of a nucleus is a defining characteristic of eukaryotic cells.
Function: Stores genetic information as long chains of DNA.
Chromatin
- A combination of DNA molecules and proteins.
- Surrounded by a semifluid medium called the nucleoplasm.
  - Nucleoplasm differs in pH and composition from the cytoplasm outside the nucleus.
- During cell division, chromatin coils tightly to form visible, long linear structures called chromosomes.
- While uncoiled, individual chromosomes cannot be distinguished; chromatin appears grainy in electron micrographs.
Chromosomes and Genes
- Responsible for transmitting genetic information from one generation to the next.
- Contain collections of genes, which are segments of DNA that hold information for the production of specific proteins.
- These proteins perform many functions and help determine a cell's specificity.
- Gene Expression: Every cell in the body contains the same genes, but cells vary in which genes are activated or deactivated (turned on and off). This enables them to perform their specific functions within a tissue or organism.
Nucleolus
- Often appears as a dark region (or sometimes more than one) of chromatin in micrographs of the nucleus.
- Site where ribosomal RNA (rRNA) is produced.
- Where rRNA joins with proteins to form the subunits of ribosomes.
Nuclear Envelope
- A double membrane that separates the nucleus from the cytoplasm.
- It is continuous with the endoplasmic reticulum (ER), a membranous system of saccules and channels.
Nuclear Pores
- Present in the nuclear envelope.
- Are of sufficient size to allow the passage of ribosomal subunits out of the nucleus and proteins into the nucleus.

Ribosomes

Composition: Organelles composed of proteins and RNA.
Function: Protein synthesis occurs at the ribosomes.
Location
- Often attached to the endoplasmic reticulum.
- May also occur free within the cytoplasm, either singly or in groups called polyribosomes.
Protein Destination: Proteins synthesized at ribosomes attached to the endoplasmic reticulum have a different destination compared to proteins manufactured at ribosomes free in the cytoplasm.

The Endomembrane System

Components: Consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles (tiny membrane sacs).
Function
- Compartmentalizes the cells, restricting chemical reactions to specific regions.
- Vesicles transport molecules from one part of the system to another.

The Endoplasmic Reticulum (ER)

Structure: A system of membranous saccules and channels in the cytoplasm, often with attached ribosomes.
Two Portions:
- Rough ER (RER)
  - Studded with ribosomes on the side of the membrane facing the cytoplasm.
  - Proteins synthesized at these ribosomes enter the interior (lumen) of the ER.
  - Destinations of RER-synthesized proteins:
    - Some are incorporated into the plasma membrane (e.g., channel proteins).
    - Others are packed into vesicles and sent to the Golgi apparatus for additional processing and modifications.
- Smooth ER (SER)
  - Continuous with the rough ER but lacks attached ribosomes.
  - Functions:
    - Synthesizes phospholipids and other lipids found in membranes.
    - Varies depending on the particular cell (e.g., produces testosterone in the testes; helps detoxify compounds like drugs in the liver).
Transport Vesicles: The ER forms transport vesicles in which large molecules are transported to other parts of the cell, often en route to the plasma membrane or the Golgi apparatus.

The Golgi Apparatus

Discoverer: Named for Camillo Golgi, who discovered its presence in cells in the late nineteenth century.
Structure: An organelle consisting of saccules and vesicles.
- Composed of a stack of slightly curved saccules, resembling a stack of pancakes.
Function: Processes, packages, and distributes molecules about or from the cell.
Modifications and Processing
- Proteins and lipids received from the ER are modified here.
- Example: A chain of sugars (carbohydrates) can be added to them, forming glycoproteins and glycolipids.
- These modified molecules are then incorporated into the plasma membrane where they serve in the process of cellular identification.
Secretion and Distribution: Vesicles leaving the Golgi apparatus move to other parts of the cell.
- Some vesicles proceed to the plasma membrane, where they discharge their contents, indicating the Golgi's involvement in secretion.
Overall role: Involved in processing, packaging, and secretion.

Lysosomes

Structure: Membrane-bound vesicles that contain hydrolytic enzymes for digesting macromolecules.
- Membranous sacs produced by the Golgi apparatus.
Prevalence: Found in all cells of the body, but particularly numerous in white blood cells that engulf disease-causing microbes.
Digestion Process
- When a lysosome fuses with an endocytic vesicle (containing engulfed material), its contents are digested by hydrolytic enzymes into simpler subunits.
- These simpler subunits can then enter the cytoplasm for use by the cell.
Autodigestion: Lysosomes can break down parts of a cell during a process called autodigestion.
Lysosomal Storage Diseases
- Some human diseases are caused by the lack of a particular enzyme in the lysosome.
- Tay-Sachs disease: Occurs due to an inactive enzyme in the lysosome.
  - This leads to undigested lipids collecting in nerve cells.
  - Results in developmental problems and ultimately leads to death in early childhood.