The Cell: Organelles

A cell is a complex unit that performs several complex functions. An organelle is a specialized subunit within a cell that fulfills a specific role or function. Organelles are enclosed within their own lipid bilayers or are unbound by membranes. If a cell is viewed as an organism, the organelles are an equivalent of the cell’s internal organs. Cell organelles carry out various functions from maintaining the shape of the cell to reproduction, movement, protein synthesis, energy production, and the transport of substances in and out of the cell.

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Overview

Definition

Organelles are specialized structures within the eukaryotic cell that carry out specific functions (cell’s “internal organs”).

Classification

Membrane-bound organelles:

  • Plasma membrane
  • Nucleus
  • Golgi apparatus
  • Endoplasmic reticulum (ER)
  • Mitochondria
  • Lysosomes
  • Peroxisomes
  • Vacuoles

Non-membrane-bound organelles:

  • Ribosomes
  • Centrioles
  • Nucleoli
  • Proteasomes
  • Flagella/cilia
Cell and its organelles

The cell and its organelles
ER: endoplasmic reticulum

Image by Lecturio.

Membrane-bound Organelles

Plasma membrane

  • A lipid/protein bilayer that surrounds the cell
  • Separates cell from the environment
  • Controls entry and exit of solutes
  • Anchor point for membrane proteins

Nucleus

Structure:

  • Surrounded by a nuclear envelope: a set of 2 lipid bilayers with protein channels (nuclear pores) with a perinuclear space in between
  • The inside of the nucleus is filled with nucleoplasm (similar in composition to cytoplasm).
  • Chromosomes are found within the nucleoplasm.

Functions:

  • Transcription (DNA → RNA) and transcriptional regulation
  • Post-transcriptional RNA modification
  • Transport of RNA into the cytoplasm through nuclear pores
  • DNA replication
  • Protection of DNA from enzymes and metabolic products of the cell
Eukaryotic cell

Eukaryotic cells containing nuclei (darker purple)

Image: “Eukayotic Cell” by Jlipuma1. License: CC BY 4.0

Endoplasmic reticulum

Structure:

  • A network of membrane-enclosed sacs known as cisternae and tubules
  • Held together by cytoskeleton
  • Continuous with the outer membrane of the nuclear envelope
  • The cisternal space is continuous with the perinuclear space.
  • Not found in RBCs and spermatozoa
  • Rough ER (RER): rough appearance because it is studded with ribosomes
  • Smooth ER (SER): lacks ribosomes

Functions:

  • RER:
    • Protein synthesis and folding
    • Packaging cell secretions
    • Production of secretory proteins for cell excretion, and addition of N-linked oligosaccharide to lysosomal and other proteins
    • Synthesis of integral membrane proteins that become part of the plasma membrane
    • Examples:
      • RER in neurons are Nissl bodies: synthesize peptide neurotransmitters for secretion
      • RER in the small intestine: goblet cells that secrete mucus, plasma cells that secrete antibodies
  • SER:
    • Lipid and steroid synthesis for excretion
    • Detoxification of drugs and poisons
    • Examples:
      • Sarcoplasmic reticulum is an SER found in myocytes: stores calcium, which is released to cause muscle contraction
      • Steroid-producing cells like liver hepatocytes, adrenal cortex, and gonads have numerous SER.
Endoplasmic reticulum

Endoplasmic reticulum (ER):
A winding network of thin membranous sacs found in close association with the cell nucleus

Image: “Endoplasmic Reticulum” by OpenStax. License: CC BY 3.0

Golgi complex

Structure:

  • A collection of flattened membrane-bound disks (cisternae)
  • Originate from vesicular clusters that bud off from the ER
  • Usually found near the nucleus

Functions:

  • Collection and dispatch of proteins received from the ER
  • Formation of proteoglycans
  • Sorting, packaging, and processing of cell secretions into vesicles:
    • Cis-Golgi network: 
      • Receiving proteins from the ER into the membrane vesicles
      • Early stages of post-translational protein modification
    • Trans-Golgi network: late modification and packaging of proteins into vesicles for internal use or exocytosis
  • Example: Plasma cells have well-developed Golgi apparatus to produce antibodies.
Golgi apparatus

Golgi apparatus as part of the secretory pathway creating a plasma membrane-bound protein:
(a) Schematic diagram
(b) Electron microscope image

Image: “Golgi Apparatus” by OpenStax. License: CC BY 4.0

Mitochondria

Structure:

  • Bound by a dual membrane (inner and outer lipid bilayers with proteins)
  • Intermembrane space between the 2 layers
  • Inner membrane forms cristae (infoldings).
  • Matrix is the space between the cristae.

Functions:

  • Important for oxidative phosphorylation and production of energy in the form of ATP
  • Site for numerous biochemical processes including:
    • Krebs cycle (citric acid cycle): key process in ATP generation
    • Fatty acid oxidation (β-oxidation)
    • Acetyl-CoA production
    • Ketogenesis
  • Heat production:
    • Through proton leak (also known as mitochondrial uncoupling)
    • Facilitated diffusion of protons across the mitochondrial membrane down the electrochemical gradient liberates heat (brown adipose tissue)
  • Calcium storage:
    • Short term 
    • Mitochondria act as cytosolic buffers of calcium ions
  • Regulation of cell proliferation through ATP production

Vacuoles

Structure:

  • Large membrane-bound sacs
  • Formed by the coalescence of small membrane-bound vesicles

Functions:

  • Storage of waste, water, solutes, and enzymes
  • Isolating toxins from the rest of the cell
  • Maintaining pH 
  • Assist exocytosis and endocytosis

Lysosomes

  • Membrane-bound spherical vesicles containing hydrolytic enzymes
  • Can break down proteins, nucleic acids, carbohydrates, and lipids
  • Waste-disposal system
  • Digest materials that are inside (autophagy) and outside (endocytosis) the cell

Peroxisomes

  • Membrane-bound oxidative organelles
  • Function in the reduction of reactive oxygen species (ROS):
    • Hydrogen peroxide is formed from molecular oxygen and hydrogen from organic compounds.
    • Hydrogen peroxide is used by catalase to reduce other compounds, and water is produced.
  • Breakdown of fatty acids:
    • β-oxidation of very-long-chain fatty acids (VLCFAs)
    • α-oxidation of branched-chain fatty acids
  • Detoxification of ethanol, phenol, formaldehyde, and other substances
  • Synthesis of plasmalogen: precursor of myelin
  • Synthesis of bile acids

Non-membrane-bound Organelles

Ribosomes

Structure:

  • Consist of small (30S) and large (50S) ribosomal subunits
  • Each subunit consists of ribosomal RNA and proteins.
  • Present in the cytosol, as part of RER, and in mitochondria

Function:

  • Translation of RNA and protein synthesis
  • Protein folding
Ribosome

Ribosome translating polypeptide chain off the matrix RNA

Image: “Ribosome” by DataBase Center for Life Science. License: CC BY 4.0

Nucleolus

  • Largest structure within the nucleus
  • Made up of DNA, RNA, and proteins
  • Functions:
    • Ribosome biogenesis
    • Formation of signal-recognition particles
Nucleolus

Microphotography of the nucleus and nucleolus

Image: “Nucleus&Nucleolus”. License: Public Domain

Proteasomes

  • Protein complexes
  • Contain enzymes proteases
  • Degrade unneeded or damaged proteins

Flagella and cilia

  • Microtubule-based cytoskeletal structures
  • Responsible for movement and sensory functions
  • Sperm use flagella for propulsion
  • Epithelial cells use cilia for chemo-, thermo-, and mechanosensation.

Centrioles

Structure:

  • Cylindrical organelle composed of the protein, tubulin
  • Short microtubule triplets arranged in a cylinder 
  • A bound pair of centrioles forms a centrosome.

Functions:

  • Cell division (spindle formation in mitosis)
  • Cellular organization:
    • Anchoring the cytoskeleton
    • Organization of microtubules in the cytosol and determination of spatial arrangement
  • Production, arrangement, and function of cilia and flagella
Centriole

Schematic diagram of centrioles: cylindrical organelles comprising microtubule triplets

Image: “Figure 04 03 08” by CNX OpenStax. License: CC BY 4.0

Clinical Relevance

  • Ethanol metabolism: acetaldehyde dehydrogenase, the enzyme involved in the final step of the pathway in the conversion of ethanol to acetate, is found in the mitochondria. Disulfiram, a drug used to worsen the effects of alcohol hangover to discourage alcohol use, inhibits acetaldehyde dehydrogenase. Metronidazole, an antibiotic, causes a disulfiram-like reaction.
  • Lysosomal storage diseases: a group of inherited metabolic disorders that result from defects in lysosomal function. Lysosomal storage disorders are usually caused by deficiency of a single enzyme involved in the metabolism of lipids, glycoproteins (sugar-containing proteins), or mucopolysaccharides.
  • Mitochondrial myopathies: a set of diseases that occur as a result of biochemical failure in mitochondrial function, which may lead to muscle weakness. The symptoms and their severity vary depending on the specific subtype. A muscle biopsy is required for diagnosis.
  • Zellweger syndrome: a peroxisome-related disease that causes hypotonia, hepatomegaly, seizures, and early death. Zellweger syndrome is an autosomal recessive disorder of peroxisome biogenesis. Cells are unable to break down VLCFAs and branched-chain fatty acids.
  • Malignant hyperthermia: an autosomal dominant disorder of the regulation of calcium transport in the skeletal muscles that results in a hypermetabolic crisis. Dantrolene is a drug treatment for malignant hyperthermia. Dantrolene prevents the release of calcium ions from the sarcoplasmic reticulum and inhibits muscle contractility.

References

  1. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., Walter, P., Wilson, J., Hunt, T. (2014). Molecular Biology of the Cell (6th ed.). Garland Science. 
  2. Campbell, N., Reece, J., Mitchell, L. (2002). Biology (6th ed.). Benjamin Cummings.
  3. Lodish, H., Berk, A., Zipursky, S., et al. (2000). Molecular Cell Biology. 4th edition. New York: W. H. Freeman. Section 5.4, Organelles of the Eukaryotic Cell. https://www.ncbi.nlm.nih.gov/books/NBK21743/

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