MR

CH3 CELL PT 2 - Organelles: Nucleus and Protein Synthesis Pathway

Inclusions

  • An inclusion is typically a clump of material that is not soluble in the cytosol.
  • Inclusions often serve as storage material in the cell.
  • Examples mentioned:
    • Fat
    • Glycogen (storage form of glucose)
    • Pigments such as melanin
  • Inclusions are not a membrane-bound organelle; they are distinct accumulations within the cytoplasm.

Membranous vs Nonmembranous Organelles

  • Membranous organelles are surrounded by a phospholipid bilayer, similar to the plasma membrane.
  • Nonmembranous organelles lack a surrounding phospholipid bilayer and are typically in contact with the cytosol.
  • The nucleus is a membranous organelle with a notable feature: it is surrounded by a double phospholipid bilayer (two bilayers, i.e., four layers total).
  • Nuclear pores are embedded in the nuclear envelope to allow passage of molecules in and out of the nucleus.

The Nucleus

  • The nucleus is a large membranous organelle that serves as the control center of the cell.
  • The nuclear envelope (membrane) is unique for its double bilayer and nuclear pores.
  • Outer membrane of the nucleus is continuous with the endoplasmic reticulum (ER).
  • Inside the nucleus, DNA exists in different forms depending on the cell cycle stage:
    • In a non-dividing cell: DNA is found as chromatin (long, thread-like form).
    • When a cell prepares to divide: chromatin condenses into chromosomes (more compact form).
  • Chromatin structure: DNA is double-stranded and wound around histone proteins; this organization allows DNA to fit inside the nucleus and be accessible for transcription.
  • There are 46 strands of DNA present in the nucleus of a human cell.
  • Nucleolus: one or more small gray bodies within the nucleus.
    • The nucleolus is responsible for making ribosomal subunits.
    • Ribosomal subunits are assembled here; they are composed in part of RNA synthesized in the nucleus.

Ribosomes and Protein Synthesis

  • Ribosomes are the cellular factories that synthesize proteins.
  • There are two locations where ribosomes operate:
    • Free ribosomes: located in the cytosol; synthesize proteins for use inside the cytosol.
    • Bound ribosomes: attached to the rough endoplasmic reticulum (rough ER); synthesize proteins that are destined for export or for incorporation into membranes.
  • Ribosome structure: ribosomes consist of two subunits; there are 2 subunits per ribosome; subunits are made of RNA and proteins; the RNA components are synthesized in the nucleus by the nucleolus.
  • Messenger RNA (mRNA): a copy of the genetic instructions from DNA that can exit the nucleus through nuclear pores.
    • mRNA carries the code to ribosomes so that amino acids can be assembled in the proper order.
  • The role of the ribosome: to make proteins by linking amino acids in the sequence specified by mRNA (primary structure of the protein).
  • Primary structure: the linear sequence of amino acids in a protein produced by ribosomes.
  • Dehydration synthesis: the chemical process by which amino acids are linked to form a polypeptide chain (peptide bonds).
  • Location-specific protein synthesis:
    • Proteins made at free ribosomes generally function within the cytosol.
    • Proteins made at ribosomes on the rough ER generally enter the secretory pathway (often exported from the cell or incorporated into membranes).

Endoplasmic Reticulum (ER) and Secretory Pathway

  • Rough endoplasmic reticulum (rough ER): a membranous network with ribosomes attached on its cytoplasmic surface; ribosomes here synthesize proteins destined for export or membrane insertion.
  • The rough ER is continuous with the outer membrane of the nuclear envelope.
  • After synthesis, proteins enter the rough ER lumen and are packaged into transport vesicles for delivery to the Golgi apparatus.
  • Transport vesicles transport proteins between organelles and membranes; the vesicle membrane is part of the phospholipid bilayer and can fuse with target membranes.

Golgi Apparatus

  • The Golgi apparatus (Golgi complex) receives transport vesicles from the rough ER.
  • It further modifies proteins (e.g., cleavage of amino acids, addition of sugar moieties).
  • After modification, proteins exit the Golgi in secretory vesicles destined for export or other destinations.
  • Secretory vesicles are vesicles that carry proteins to the plasma membrane for release outside the cell (exocytosis).

Secretory Vesicles and Membrane Flow

  • Secretory vesicles fuse with the plasma membrane, releasing their protein contents to the outside of the cell.
  • Membrane flow concept: membranes are dynamic and continually exchanged among organelles. During vesicle trafficking, portions of the ER membrane become part of the Golgi membrane, and portions of the Golgi membrane pinch off to form secretory vesicles, which may fuse with the plasma membrane. This continual flow helps transport proteins while maintaining membrane organization.

Lysosomes and Digestion

  • Lysosome: a membranous organelle containing digestive enzymes.
  • The digestive enzymes are produced in the rough ER, processed in the Golgi apparatus, and packaged into vesicles that become lysosomes.
  • Lysosomes remain inside the cell and digest proteins, fats, and carbohydrates; they also help destroy old cellular components and bacteria.
  • Importantly, digestive enzymes are contained within lysosomes to prevent them from digesting cellular components in the cytosol.
  • Mechanism summary:
    • Enzymes synthesized in rough ER → packaged into vesicles → travel to Golgi → modified and sorted → vesicles become lysosomes (membrane derived from Golgi) → lysosomes digest unwanted material intracellularly.

Relationship Between Organelles and Function

  • The abundance and prominence of rough ER, Golgi, and nucleoli reflect a cell's specialized function (e.g., export-heavy cells have more rough ER and Golgi; more ribosome production may require more nucleoli).
  • Different cells tailor organelle content to meet their functional demands (e.g., protein export vs. cytosolic enzymatic activity).

Recap: Key Pathways and Roles

  • DNA in nucleus codes for proteins; DNA code is too large to exit the nucleus.
  • mRNA copies this code and exits through nuclear pores to reach ribosomes.
  • Ribosomes synthesize proteins:
    • Free ribosomes: proteins stay in cytosol.
    • Rough ER ribosomes: proteins are processed in ER and typically exported or inserted into membranes.
  • Proteins synthesized on rough ER are transported via transport vesicles to the Golgi for modification and sorting.
  • Golgi modification results in secretory vesicles that fuse with the plasma membrane to export proteins from the cell.
  • Some proteins, like digestive enzymes, are routed via the Golgi into lysosomes, not secreted, to keep them contained.
  • The overall membrane flow ensures that membranes and proteins are correctly distributed and compartmentalized inside the cell.

Foundational Connections and Concepts

  • The nucleus houses DNA and uses a double bilayer with nuclear pores for selective transport—vital for regulating which macromolecules access the nucleus.
  • Chromatin vs chromosomes reflect the cell’s state: chromatin when not dividing; condensed chromosomes during division.
  • The nucleolus produces ribosomal subunits, which are essential for ribosome assembly.
  • The ribosome’s two subunits, made of RNA, are assembled in the nucleolus and then transported to sites of protein synthesis in the cytosol or on the rough ER.
  • The central dogma steps reflected here: DNA -> RNA (transcription) -> protein (translation).

Quantitative and Structural Details (Numerical References)

  • DNA strands in a human nucleus: 46 strands.
  • Nuclear envelope structure: Double phospholipid bilayer (two bilayers, four layers total) surrounding the nucleus.
  • Ribosome subunits: 2 subunits per ribosome.
  • Amino acids: there are 20 standard amino acids used to build proteins.
  • Membrane continuity: the outer nuclear membrane is continuous with the rough ER.
  • Membrane flow: dynamic transfer of membrane components between ER, Golgi, and secretory vesicles, culminating in the plasma membrane.

Terminology Summary (quick glossary)

  • Inclusion: non-soluble storage material in the cytosol (e.g., fat, glycogen, melanin).
  • Membranous organelle: organelle surrounded by a phospholipid bilayer (e.g., nucleus, ER, Golgi, lysosomes).
  • Nonmembranous organelle: organelle lacking a surrounding bilayer and in direct contact with cytosol.
  • Chromatin: DNA wrapped around histones in a non-dividing cell.
  • Chromosome: condensed DNA during cell division.
  • Nucleolus: nuclear substructure that assembles ribosomal subunits.
  • Ribosome: molecular machine that synthesizes proteins; two subunits; RNA-based.
  • Messenger RNA (mRNA): RNA copy of DNA that guides protein synthesis.
  • Endoplasmic Reticulum (ER): membranous network; rough ER has ribosomes; rough ER participates in protein synthesis for export.
  • Golgi apparatus: modifies, sorts, and packages proteins for secretion or delivery to lysosomes or membranes.
  • Secretory vesicle: vesicle carrying proteins to the plasma membrane for release outside the cell.
  • Lysosome: organelle with digestive enzymes for intracellular digestion.
  • Dehydration synthesis: chemical process that links amino acids to form a polypeptide chain.