SA

Cell Organelles and Molecular Biology

Cytoskeleton

  • Provides the cell framework and shape; essential for fully functional cells.
  • Components (from smallest to largest): 3 types of fibrous proteins: microfilaments, intermediate filaments, and microtubules.
  • Microfilaments
    • Also called actin filaments; smallest in diameter.
    • Roles: contribute to cell shape, movement, and changes in cell form (e.g., cytokinesis, muscle contraction). Some cells use microfilaments for pseudopod-driven movement.
    • Microvilli are extensions rich in microfilaments to increase surface area for absorption.
  • Intermediate filaments
    • Thickest of the three; provide major structural support (like I-beams in a building).
  • Microtubules
    • Largest in diameter; tubular structures that act as tracks for movement of organelles and vesicles.
    • Essential for mitosis: centrioles organize spindle fibers to move chromosomes.
    • Involved in moving substances inside the cell along microtubule tracks.
  • Centrioles and centrosome
    • Centrosome is the location where centrioles reside; critical for organizing spindle fibers during cell division.
    • Some mature cells lack centrioles (e.g., skeletal muscle cells, certain neurons).
  • Cilia and flagella
    • Also composed of microtubules; cilia: many on a cell surface; flagellum: typically one (e.g., sperm).
    • Motile cilia move substances across surfaces (e.g., respiratory tract); flagellum enables cell movement (sperm).
    • Primary (sensory) cilium: single, non-motile, involved in signal transduction.
  • Quick summary mental model
    • Microfilaments: cell movement and shape change.
    • Intermediate filaments: structural support.
    • Microtubules: transport highways and spindle apparatus.

Nonmembranous organelles

  • Predominantly protein-based, found in most cell types.
  • Include: cytoskeleton components, centrosomes/centrioles, ribosomes, and structures like cilia/flagella.
  • Ribosomes are not membranous; see next section.

Ribosomes

  • All fully functional cells have ribosomes; sites of protein synthesis.
  • Structure: two subunits (large and small); composed of protein + ribosomal RNA (rRNA).
  • Localization and fate of synthesized proteins
    • Free ribosomes: suspended in cytosol; synthesize proteins for use in cytosol.
    • Fixed (bound) ribosomes: attached to rough endoplasmic reticulum; synthesize proteins to be secreted or embedded in membranes.
  • Number varies by cell type depending on protein production needs.

Endoplasmic reticulum (ER)

  • ER is a membranous, mesh-like network inside the cell; the word root “reticulum” = net-like.
  • Rough ER
    • Studded with bound ribosomes; protein synthesis for export or membrane integration.
  • Smooth ER
    • Lacks ribosomes; synthesizes lipids and carbohydrates; detoxifies certain substances; processes some carbohydrates.
  • Distribution varies by tissue (e.g., liver cells have both rough and smooth ER for protein production and detoxification).

Golgi apparatus and vesicles

  • Golgi apparatus is a stack (cisternae) of membranous discs; receives proteins from rough ER.
  • Function: modify, sort, and package proteins for delivery.
  • Vesicles: membrane-bound packages that transport proteins.
    • Transport vesicles from ER fuse with Golgi; contents are processed as they move through the Golgi.
  • Final destinations for processed proteins
    • Secretory vesicles: exocytosis to release contents outside the cell.
    • Membrane-renewal vesicles: fuse with plasma membrane to renew or modify membrane proteins.
    • Lysosome formation: vesicle contents become lysosomes,
  • Lysosome (membranous organelle)
    • Contains digestive enzymes; fuses with worn-out organelles or ingested material to degrade contents.

Membrane flow

  • Concept: continuous movement and exchange of membrane between ER, Golgi, and plasma membrane.
  • Maintains organelle size and membrane renewal; actively secreting cells can replace entire membrane surface in about an hour.

Mitochondria

  • Energy production: ATP synthesis; powerhouses of the cell.
  • Endosymbiotic origin: mitochondria originated from bacteria-like ancestors; retain some circular DNA and replicate independently.
  • Structure: double membrane with inner folds called cristae, increasing surface area for ATP production.
  • Metabolic pathway: glycolysis (cytosol) → pyruvate import into matrix → Krebs cycle (citric acid cycle) → electron transport chain (ETC) in inner membrane; most ATP produced in ETC.
  • Oxygen role: final electron acceptor in aerobic metabolism; without O2, ATP production collapses.
  • Maternal inheritance: mitochondria are inherited from the mother via the egg.

Nucleus

  • Nuclear envelope: double membrane surrounding the nucleus (two layers of membrane).
  • Nuclear pores: regulate traffic of RNA and proteins between nucleus and cytoplasm.
  • Nucleolus: region where ribosomes are assembled; contained within the nucleus.
  • Nucleoplasm: internal nuclear content; high DNA concentration organized as chromatin.
  • Chromatin and nucleosomes
    • DNA wrapped around histone proteins forming nucleosomes; condensation into chromatin helps protect DNA while remaining accessible.
  • Chromosomes
    • During cell division, chromatin condenses into chromosomes for protection and easier segregation.
  • Nucleus in cells
    • Nearly all cells have a nucleus; some exceptions include multi-nucleated skeletal muscle cells and anucleate mature red blood cells.
  • Histones
    • Protein cores around which DNA winds to form chromatin; packaging proteins help stabilize DNA.

DNA and RNA basics

  • Nucleic acids: DNA and RNA; DNA uses deoxyribose; RNA uses ribose.
  • DNA structure: double helix with a phosphate-sugar backbone and nitrogenous bases projecting outward.
  • Base pairing (hydrogen bonds)
    • Adenine (A) pairs with Thymine (T) in DNA; Adenine pairs with Uracil (U) in RNA.
    • Cytosine (C) pairs with Guanine (G).
  • DNA organization
    • Gene: basic unit of inheritance; contains instructions to build a protein, plus regulatory regions.
    • Human genome: roughly 2\times 10^4 genes, encoding about 20,000 proteins (simplified view).
  • Genetic code
    • Triplets (codons): three nucleotide bases encode one amino acid.
    • Universal: same code used across organisms; enables cross-species gene transfer with relevant caveats.
    • Coding vs template strands
    • Coding strand indicates the amino acid sequence.
    • Template strand is used to synthesize messenger RNA (mRNA).
  • Messenger RNA (mRNA)
    • Single-stranded copy of the coding sequence; carries information to ribosomes.
    • During transcription, RNA polymerase reads the DNA template strand and synthesizes complementary mRNA.
  • Translation overview
    • Ribosome reads codons on mRNA and matches them with transfer RNA (tRNA) anticodons.
    • Each tRNA carries a specific amino acid corresponding to its anticodon.
    • Example: Codon AGC codes for Serine; tRNA with anticodon UCG carries Serine.
  • RNA vs DNA specifics
    • RNA uses Uracil (U) instead of Thymine (T).
    • DNA uses Thymine (T).
  • Transcription vs translation locations
    • Transcription: nucleus (DNA to mRNA).
    • Translation: cytoplasm at ribosomes.

Protein synthesis and gene expression (quick recap)

  • Transcription: DNA -> mRNA via RNA polymerase; happens in the nucleus.
  • Translation: mRNA -> protein at ribosome; involves tRNA anticodons and codons.
  • Relationship: RNA is the intermediary that carries genetic information from DNA to the ribosome to build proteins.

Quick callouts and exceptions

  • Not every cell has the same organelles in the same amounts; some cells lack centrioles or have multiple nuclei.
  • Red blood cells lose their nucleus during maturation.
  • Mitochondria and chloroplasts (in plants) highlight endosymbiotic origins and their own DNA.
  • DNA integrity is crucial; nucleus provides a protective environment to minimize damage and enable repair; damage without repair can lead to loss of function or cancer.