Inside the Cell

Role of Subcellular Compartments

• Nucleus: cellular control center
• Endoplasmic reticulum: synthesis and sorting
• Golgi apparatus: sorting and packaging
• Lysosomes: digestive organelles

Nucleus

• One of the largest of the internal membrane-bound organelles which is surrounded by the nuclear envelope
  • Present in all eucaryotic organisms and absent in procaryotes
  • Contains two types of structures:
    • Chromosomes-threadlike bodies that composed of DNA and protein
    • Nucleolus-specialized highly active regions of chromosomes synthesizing rRNA
      

Chromosomes

• Chromosomes are composed of nucleic acid polymers (DNA) and have protein associated with them
• Visible only when highly condensed
  • DNA is the substance of the basic heredity unit, the gene
  • Proteins (histones) provide cores for the DNA to be wound around to form nucleosomes

Condensed Chromosomal Structures

• Chromosomes exist throughout the cell cycle in various states of condensation
  • Condensation is accomplished by winding the DNA around an octet (8) of histone proteins which form a nucleosome
  • Nucleosomes further cluster to form regions of condensed DNA

Flow of Information from the Nucleus to the Cytoplasm

• Information encoded in a region of the chromosome (gene) is transcribed by a protein complex into a RNA copy
  • This copy is messenger RNA (mRNA) which exits the nucleus through a nuclear pore
• In the cytoplasm the mRNA attaches to a ribosome and message encoded in mRNA is translated into the primary sequence of a protein
  

Endomembrane System

• Endoplasmic Reticulum
• Golgi Apparatus
• Lysosomes
• Plasma membrane

Endoplasmic Reticulum

• Synthesis and Sorting of lipids and membrane proteins
  • First isolated in 1938 by centrifugation, and were called "microsomes".
    • Contained large amounts of nucleic acids and phospholipids
  • Microsomes constituted 15-20% of the cell mass
• Microsomes represent fragments to a network membranes (ER)
• ER primarily involved in the synthesis of membrane phospholipids; the synthesis-sorting-packaging of membrane proteins and the storage of calcium
• ER is anatomically divided into two distinct forms
  • Rough endoplasmic reticulum (RER)
  • Smooth endoplasmic reticulum (SER)

RER

• RER represents a series of flattened membrane sacs referred to as cisternae
  • Cisternae probably are interconnected to form a continuous space
  • Rough appearance arises from the association of ribosomes with the surface of the membrane sacs
    • Ribosomes also bind to the outer membrane of the nuclear envelope
  • RER functions as an intracellular transport system, cytoplasmic framework

Sorting of Proteins by ER

• Association of ribosomes with ER is necessary for the synthesis of membrane and many other compartmentalized proteins.
• Integral membrane proteins are embedded in the RER membrane as they are being synthesized and then addressed with the addition of an oligosaccharide "mailing label"
  • Recoginition for synthesis of proteins on the RER requires a Signal Sequence

SER

• Lacks association with ribosomes and has a tubular appearance and a very different protein composition
  • Site of phospholipid synthesis
  • Packages specific proteins in the ER into transport vesicles
  • Site of detoxification enzymes (bariturates, amphetamines, morphine, etc.)
  • Stores and releases calcium (regulatory process)

Golgi Apparatus

• Primary function is the sorting and packaging of materials to be transported to specific cellular locations or for export.
• Numbers of Golgi apparatus correlate with the secretory activity level of the cell
  • Anatomically the Golgi apparatus is a series of membrane sacs that roughly parallel to each other

Golgi Apparatus: Packaging and Sorting

• Materials enter into the golgi complex through the fusion of transport vesicle that originally budded from the SER
  • Vesicle fusion occurs at the membrane face closest to the SER referred to as the cis face
  • Materials progress through a series of medial sacs until they arrive at the trans face
  • At the trans face the modified materials are shipped to specific cellular locations in transport vesicles

Lysosomes: Digestive Organelles

• Membrane bounded compartments that function as storage vesicles for powerful hydrolytic enzymes
  • Lysosomal interior is maintained in a highly acidic condition by specific ion pumps
  • Lysosomes serve as the cellular digestive system for endocytosed materials
    • Many human diseases result from malfunctioning lysosomes

Peroxisomes: Detoxification Organelles

• Membrane bounded structure that contains enzymes which catalyze condensation reactions
• Oxidative removal of amino groups from amino acids, detoxification of alcohols, breakdown of fatty acids and oxidation of peroxides to O2 and H2O
• Membrane lipids are produced by SER and the enzymes are synthesized in the cytoplasm

Mitochondria: Cellular Power Station

Mitochondria carry out cellular respiration, which is the complete breakdown of organic compounds, and make the released energies available for energy requiring reactions

• Double membrane enclosed organelle
  • Outer envelope membrane
  • Inner envelope membrane highly folded and referred to as the cristae
  • Enclosed space is called the matrix

Plastids

• Large cytoplasmic organelles found in plants but not in fungi or animals
• Two principle types
  • Chromoplasts: colored bodies
  • Leucoplasts: white or colorless plastids

Chloroplast: Solar Energy Receptor

• Bounded by to concentric membranes and have a complex internal membrane system

The internal membrane system is referred to as the thylakoid membranes and function in the harvesting of the energy in sunlight.

• Thylakoids have regions that appear to be stack (grana)
• Thylakoids are the site of chlorophyll incorporation
• Region surrounding the thylakoids is the stroma

Leucoplasts: Storage Organs of Plants

• Use primarily for the storage of starch, oils, and proteins
  • Plastids filled with starch are called amyloplasts
    • Found in seeds and storage root and stems such as carrots and potatoes
• All plastids arise from a colorless body called a proplastid
  • Many plastid types are inconvertible

Vacuoles: Mulipurpose Organelles

• Vacuoles are found in both plants and animals
  • Various kinds of vacuoles: Protozoans have both food and contractile vacuoles
    • Difference between vacuoles and vesicles is difficult
    • Vacuoles are longer lived and relatively static
  • Vacuoles in plants
    • Large internal space used both for storage and osmoregulation

• This page is maintained by James C. Pushnik: jpushnik@ecst.csuchico.edu