Organelles – Definition, List, and 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.

Key Takeaways: Organelles

• An organelle is a specialized cell structure that performs a particular function.
• Major eukaryotic organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, vacuoles, and ribosomes.
• Plant cells contain chloroplasts and a large central vacuole, while typical animal cells contain centrosomes with centrioles and prominent lysosomes.
• Some organelles are membrane-bound, while others, including ribosomes and the nucleolus, are not.
• Mitochondria and chloroplasts contain their own DNA and arose through endosymbiosis.
• Prokaryotes lack a nucleus and the membrane-bound organelles characteristic of eukaryotic cells, but many have specialized internal compartments and structures.

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 and lipids 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.

The cell membrane, cytoplasm, cytoskeleton, and plant cell wall are also essential cell structures, although they are not always classified as organelles.

Animal Cell Organelles

Animal cells have some organelles that are not found in most 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

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

Different organelles have different evolutionary origins. The best-known example is the endosymbiotic origin of mitochondria and chloroplasts.

According to endosymbiotic theory, an ancestral cell engulfed bacteria that survived inside the host rather than being digested. Over evolutionary time, these bacteria became permanent cellular components. An aerobic bacterium gave rise to mitochondria, while a photosynthetic cyanobacterium gave rise to chloroplasts and other plastids.

Several lines of evidence support endosymbiosis. Mitochondria and chloroplasts have their own DNA, contain bacterial-like ribosomes, reproduce by division, and are surrounded by double membranes.

Endosymbiosis does not explain the origin of all organelles. The nucleus, endoplasmic reticulum, Golgi apparatus, and other components of the endomembrane system have different evolutionary histories.

Common Misconceptions About Organelles

All organelles are membrane-bound.
Not under the broad definition of an organelle. Ribosomes, nucleoli, centrosomes, and several other specialized structures lack surrounding membranes.

Only eukaryotic cells have organized internal structures.
Prokaryotes lack the extensive membrane-bound organelle system of eukaryotes, but they can contain specialized compartments and highly organized internal structures.

The cell wall is an organelle.
A cell wall is an important cellular structure, but it is generally not classified as an organelle.

Plant cells do not have mitochondria.
Plant cells contain mitochondria as well as chloroplasts. Chloroplasts capture light energy, while mitochondria perform cellular respiration and produce much of the cell’s usable ATP.

Only animal cells have vacuoles.
Both plant and animal cells can contain vacuoles. The large central vacuole is particularly characteristic of mature plant cells.

Mitochondria and chloroplasts are completely independent cells.
Although they retain some bacterial characteristics and their own DNA, mitochondria and chloroplasts depend on numerous proteins encoded by nuclear genes and cannot live independently of their host cells.

Frequently Asked Questions

What are the main organelles in a cell?

Major eukaryotic organelles include the nucleus, nucleolus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, vacuoles, and vesicles. Plant cells also contain chloroplasts, while typical animal cells have centrosomes containing centrioles.

Which organelle is the powerhouse of the cell?

The mitochondrion is commonly called the powerhouse of the cell because mitochondria generate most cellular ATP through aerobic cellular respiration.

Which organelle controls the cell?

The nucleus is often described as the cell’s control center because it contains most of the cell’s DNA and plays a central role in regulating gene expression. However, cell activity results from interactions among many organelles and cellular systems.

Which organelles contain DNA?

The nucleus, mitochondria, and chloroplasts contain DNA. Most of a eukaryotic cell’s genetic material occurs in the nucleus, while mitochondria and chloroplasts contain much smaller genomes of their own.

Are ribosomes organelles?

It depends on the definition. Under a broad definition, ribosomes are non-membrane-bound organelles because they are specialized cellular structures with a specific function. Definitions that restrict the word “organelle” to membrane-bound compartments exclude ribosomes.

Do prokaryotes have organelles?

Prokaryotes lack a nucleus and most of the membrane-bound organelles characteristic of eukaryotic cells. However, many prokaryotes have specialized cellular compartments and structures, and some researchers use the term “prokaryotic organelle” for certain examples.

What is the largest organelle in a cell?

The answer depends on the type of cell. The nucleus is often one of the largest organelles in an animal cell. In a mature plant cell, the central vacuole may occupy most of the cell’s volume.

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