Nucleus and Endomembrane System Notes

3.4 Nucleus and Endomembrane System

Learning Outcomes

  • Describe the structure of the nucleus and explain its role as the storage place of genetic information.

  • Summarize the functions of the organelles of the endomembrane system.

  • Explain the role and location of the ribosomes.

Nucleus

  • Contains genetic instructions necessary for the production of proteins involved in most cellular functions.

  • Presence of a nucleus is a defining characteristic of eukaryotic cells (Fig. 3.13).

  • Function: stores genetic information as long chains of DNA.

Chromatin

  • Combination of DNA molecules and proteins.

  • Surrounded by nucleoplasm, a semifluid medium with different pH and composition from the cytoplasm.

  • During cell division, chromatin coils tightly to form visible, long, linear structures called chromosomes.

  • Uncoiled, individual chromosomes cannot be distinguished.

  • Appears grainy in electron micrographs.

  • Chromosomes are responsible for transmitting genetic information from one generation to the next (Section 19.1).

  • Genes: segments of DNA that contain information for the production of specific proteins.

  • Proteins determine a cell's specificity.

  • Every cell contains the same genes, but cells vary in which genes are turned on and off.

Nucleolus

  • Dark region of chromatin within the nucleus.

  • Site of ribosomal RNA (rRNA) production.

  • rRNA joins with proteins to form the subunits of ribosomes.

Nuclear Envelope

  • Double membrane separating the nucleus from the cytoplasm.

  • Continuous with the endoplasmic reticulum.

  • Nuclear pores allow passage of ribosomal subunits out of the nucleus and proteins into the nucleus.

Ribosomes

  • Organelles composed of proteins and RNA.

  • Site of protein synthesis.

  • Attached to the endoplasmic reticulum or free within the cytoplasm (singly or in groups called polyribosomes).

  • Proteins synthesized at ribosomes attached to the endoplasmic reticulum have different destinations than those manufactured by free ribosomes.

Endomembrane System

  • Consists of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles (tiny membranous sacs; Fig. 3.14).

  • Compartmentalizes the cell, restricting chemical reactions to specific regions.

  • Vesicles transport molecules from one part of the system to another.

Endoplasmic Reticulum (ER)

  • Two portions:

    • Rough ER: Studded with ribosomes on the side of the membrane facing the cytoplasm (Fig. 3.13).

      • Proteins synthesized at these ribosomes enter the interior of the ER.

      • Some proteins 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: Continuous with the rough ER, but without attached ribosomes.

      • Synthesizes phospholipids and other lipids that occur in membranes.

      • Performs various other functions depending on the cell type.

        • Example: Produces testosterone in the testes.

        • Example: Detoxifies compounds (e.g., drugs) in the liver.

      • Forms transport vesicles in which large molecules are transported to other parts of the cell.

      • Vesicles often travel to the plasma membrane or the Golgi apparatus.

Golgi Apparatus

  • Named for Camilo Golgi, who discovered it in the late 19th century.

  • Consists of a stack of slightly curved saccules (resembling a stack of pancakes).

  • Proteins and lipids received from the ER are modified.

    • A chain of sugars (carbohydrates) may be added, forming glycoproteins and glycolipids.

    • These are incorporated into the plasma membrane to serve in cellular identification.

  • Vesicles leaving the Golgi apparatus move to other parts of the cell.

  • Some vesicles proceed to the plasma membrane, where they discharge their contents.

  • Functions: processing, packaging, and secretion.

Lysosomes

  • Membranous sacs produced by the Golgi apparatus that contain hydrolytic enzymes.

  • Found in all cells but particularly numerous in white blood cells.

  • When a lysosome fuses with an endocytic vesicle, its contents are digested by hydrolytic enzymes into simpler subunits, which then enter the cytoplasm.

  • Autodigestion: parts of a cell may be broken down by the lysosomes.

  • Some human diseases are caused by the lack of a particular enzyme in the lysosome.

    • Tay-Sachs disease: caused by an inactive enzyme in the lysosomes, leading to undigested lipids collecting in nerve cells, causing developmental problems and death in early childhood.