Organelles are specialized structures within the eukaryotic cell that carry out specific functions (cell’s “internal organs”).
- Plasma membrane
- Golgi apparatus
- Endoplasmic reticulum (ER)
- A lipid/protein bilayer that surrounds the cell
- Separates cell from the environment
- Controls entry and exit of solutes
- Anchor point for membrane proteins
- 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.
- 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
- 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
- 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
- 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
- Lipid and steroid synthesis for excretion
- Detoxification of drugs and poisons
- 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.
- A collection of flattened membrane-bound disks (cisternae)
- Originate from vesicular clusters that bud off from the ER
- Usually found near the nucleus
- 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
- Cis-Golgi network:
- Example: Plasma cells have well-developed Golgi apparatus to produce antibodies.
- 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.
- 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
- 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
- Large membrane-bound sacs
- Formed by the coalescence of small membrane-bound vesicles
- Storage of waste, water, solutes, and enzymes
- Isolating toxins from the rest of the cell
- Maintaining pH
- Assist exocytosis and endocytosis
- 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
- 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
- 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
- Translation of RNA and protein synthesis
- Protein folding
- Largest structure within the nucleus
- Made up of DNA, RNA, and proteins
- Ribosome biogenesis
- Formation of signal-recognition particles
- 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.
- Cylindrical organelle composed of the protein, tubulin
- Short microtubule triplets arranged in a cylinder
- A bound pair of centrioles forms a centrosome.
- 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
- 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.
- 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.
- Campbell, N., Reece, J., Mitchell, L. (2002). Biology (6th ed.). Benjamin Cummings.
- 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/