Organelles – Definition, List, and Functions


Animal Cell Organelles
Organelles are essentially the tiny organs of a cell, each having specific functions.

An organelle is a compartment within a eukaryotic cell that has a specific function. The name “organelle” comes from the idea that these structures are to cells what an organ is to the body. Typically, organelles are enclosed within their own lipid bilayers. They are essential for various cellular functions, including energy production, protein synthesis, and cell division.

Importance of Organelles

Just like an organism depends on organs for survival, a cell depends on organelles. Organelles play critical roles in:

  • Maintaining Cellular Health: They perform specific tasks that are vital for the survival of cells.
  • Energy Conversion: Organelles like mitochondria and chloroplasts convert energy into forms usable by the cell.
  • Protein Synthesis: Organelles such as ribosomes are crucial in making proteins.

History and Etymology

The term “organelle” was coined in the late 19th century. The discovery of organelles dates back to the 1830s with the identification of the nucleus. The development of electron microscopy in the 20th century allowed for the detailed study of organelles.

List of Organelles

Eukaryotic (animal and plant) cells share several organelles in common:

  • Cell Membrane: Separates the cell interior from the external environment.
  • Nucleus: Stores genetic material and coordinates cellular activities.
  • Nucleolus: Found in most eukaryotic cells, it functions in pre-ribosome production.
  • Mitochondria: Powerhouses of the cell, producing ATP through cellular respiration.
  • Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).
  • Golgi Apparatus: Modifies, sorts, and packages proteins for secretion.
  • Lysosomes: Digests excess or worn-out organelles, food particles, and engulfed viruses or bacteria.
  • Ribosomes: Synthesize proteins.
  • Vacuoles: These single-membrane compartments find use in storage and transportation of materials.
  • Vesicles: Single-membrane compartments that perform material transport.
  • Flagellum: Performs sensory or locomotion tasks in some eukaryotes.
  • Peroxisome: Breaks down hydrogen peroxide.
  • Proteasome: Degrades unnecessary proteins.

Animal Cell Organelles

Animal cells have some organelles that are not found in plant cells:

  • Centrosomes and Centrioles: Play a role in cell division.
  • Lysosomes: More prominent in animal cells for digesting materials.
  • Melanosome: Stores pigment in some animal cells.

Plant Cell Organelles

Plant Cell Organelles

Meanwhile, plant cells have certain organelles not found in animal cells:

  • Cell Wall: Plants, fungi, and some protists have a rigid cellulose-based cell wall the keeps the cell rigid and protects it from osmotic pressure.
  • Chloroplasts: Conduct photosynthesis to convert solar energy into chemical energy.
  • Central Vacuole: Stores water, maintains turgor pressure.
  • Glyoxysome: Converts fat into sugars.

Additionally, there are many other organelles found in specific eukaryotic cells that carry out specialized tasks.

Membrane-Bound vs. Non-Membrane-Bound Organelles

One method of classifying organelles is according to whether they are membrane-bound or non-membrane-bound. So, strictly speaking, not all organelles are packaged within membranes.

Membrane-Bound Organelles

  • Examples: Mitochondria, Nucleus, ER, Golgi Apparatus.
  • Characteristics: Enclosed by membranes, have distinct internal environments.

Non-Membrane-Bound Organelles

  • Examples: Ribosomes, Centrosomes.
  • Characteristics: Lack a surrounding membrane, more open to the cytoplasm.

Organelles in Eukaryotic vs. Prokaryotic Cells

  • Eukaryotic Cells: Possess membrane-bound organelles.
  • Prokaryotic Cells: Generally lack membrane-bound organelles, but show some compartmentalization.

Prokaryotic cells are typically simpler and smaller than eukaryotic cells. While prokaryotic cells do not have membrane-bound organelles, they have some internal organization and compartmentalization that is analogous to organelles. Here’s a comprehensive list of these structures, their functions, and examples of prokaryotic organisms that contain them:

  • Nucleoid
    • Function: Region in the cell where the genetic material (DNA) is located. Unlike a nucleus, it is not enclosed by a membrane.
    • Example Organisms: Escherichia coli, Bacillus subtilis.
  • Ribosomes
    • Function: Sites of protein synthesis. Prokaryotic ribosomes are smaller (70S) compared to eukaryotic ribosomes (80S).
    • Example Organisms: All prokaryotes including Streptococcus pneumoniae, Mycobacterium tuberculosis.
  • Plasmids
    • Function: Small, circular DNA molecules separate from chromosomal DNA. They often carry genes beneficial for survival under specific conditions, like antibiotic resistance.
    • Example Organisms: Agrobacterium tumefaciens (carries plasmids used in genetic engineering of plants).
  • Cell Wall
    • Function: Provides structural support and protection. Made of peptidoglycan in bacteria.
    • Example Organisms: Staphylococcus aureus (Gram-positive bacteria), E. coli (Gram-negative bacteria).
  • Plasma Membrane
    • Function: Controls the movement of substances in and out of the cell.
    • Example Organisms: All prokaryotes.
  • Cytoplasm
    • Function: Gel-like substance inside the cell membrane containing enzymes, nutrients, and other molecules needed for cell survival.
    • Example Organisms: All prokaryotes.
  • Pili and Fimbriae
    • Function: Hair-like structures that help in attachment to surfaces and in conjugation (transfer of genetic material between bacteria).
    • Example Organisms: Neisseria gonorrhoeae (uses pili for attachment to host cells).
  • Flagella
    • Function: Long, whip-like structures used for movement.
    • Example Organisms: Salmonella typhi (uses flagella to move).
  • Endospores
    • Function: Resistant, dormant structures formed under stress conditions, ensuring survival.
    • Example Organisms: Bacillus anthracis (forms endospores).
  • Capsule
    • Function: A thick polysaccharide layer for protection against environmental hazards and in some cases, helps in evading the host immune system.
    • Example Organisms: Streptococcus pneumoniae (has a capsule that contributes to its pathogenicity).
  • Inclusion Bodies
    • Function: Storage of nutrients, enzymes, or metabolic end products.
    • Example Organisms: Many cyanobacteria store glycogen, lipids, or other compounds in inclusion bodies.
  • Carboxysomes
    • Function: Microcompartments that contain enzymes for carbon fixation in photosynthetic bacteria.
    • Example Organisms: Cyanobacteria like Synechococcus.
  • Magnetosomes
    • Function: Organelles in some bacteria containing iron oxide, aiding in navigation by orienting the bacteria in line with Earth’s magnetic field.
    • Example Organisms: Magnetospirillum magnetotacticum.
  • Gas Vesicles
    • Function: Hollow structures that provide buoyancy in aquatic environments.
    • Example Organisms: Halobacterium salinarum.
  • Thylakoids
    • Function: Membrane systems in photosynthetic bacteria where photosynthesis occurs.
    • Example Organisms: Cyanobacteria.

Origin of Organelles

The prevailing theory for the origin of organelles is endosymbiosis. This suggests that organelles like mitochondria and chloroplasts were once independent prokaryotic organisms that were taken inside another cell and evolved into the organelles we see today.

References

  • Alberts, B.; Johnson, A.; et al. (2014). Molecular Biology of the Cell (6th ed.). Garland Science. ISBN 978-0815345244.
  • Campbell, N.A.; Reece, J.B.; Mitchell, L.G. (2002). Biology (6th ed.). Benjamin Cummings. ISBN 978-0-8053-6624-2.
  • Kerfeld, C.A.; Sawaya, M.R.; et al. (2005). “Protein structures forming the shell of primitive organelles”. Science. 309 (5736): 936–8. doi:10.1126/science.1113397
  • Mullins, C. (2004). “Theory of Organelle Biogenesis: A Historical Perspective”. The Biogenesis of Cellular Organelles. Springer Science+Business Media, National Institutes of Health. ISBN 978-0-306-47990-8.
  • Murat, Dorothee; Byrne, Meghan; Komeili, Arash (2010). “Cell Biology of Prokaryotic Organelles”. Cold Spring Harbor Perspectives in Biology. 2 (10): a000422. doi:10.1101/cshperspect.a000422