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 Eukaryotic Eukaryotes can be single-celled or multicellular organisms and include plants, animals, fungi, and protozoa. Eukaryotic cells contain a well-organized nucleus contained by a membrane, along with other membrane-bound organelles. Cell Types: Eukaryotic versus Prokaryotic 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 Transcription Transcription of genetic information is the first step in gene expression. Transcription is the process by which DNA is used as a template to make mRNA. This process is divided into 3 stages: initiation, elongation, and termination. Stages of Transcription ( DNA DNA The molecule DNA is the repository of heritable genetic information. In humans, DNA is contained in 23 chromosome pairs within the nucleus. The molecule provides the basic template for replication of genetic information, RNA transcription, and protein biosynthesis to promote cellular function and survival. DNA Types and Structure RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure) and transcriptional regulation
  • Post-transcriptional RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure modification
  • Transport of RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure into the cytoplasm through nuclear pores
  • DNA replication DNA replication The entire DNA of a cell is replicated during the S (synthesis) phase of the cell cycle. The principle of replication is based on complementary nucleotide base pairing: adenine forms hydrogen bonds with thymine (or uracil in RNA) and guanine forms hydrogen bonds with cytosine. DNA Replication
  • Protection of DNA DNA The molecule DNA is the repository of heritable genetic information. In humans, DNA is contained in 23 chromosome pairs within the nucleus. The molecule provides the basic template for replication of genetic information, RNA transcription, and protein biosynthesis to promote cellular function and survival. DNA Types and Structure from enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of 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 Cytoskeleton A cell's cytosol is the liquid inside the cell membrane that surrounds the organelles and cytoskeleton. The cytosol is a complex solution where many biochemical processes take place. The Cell: Cytosol and 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 Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: goblet cells that secrete mucus, plasma cells that secrete antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins
  • 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 Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. 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 Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins.
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 Citric acid cycle The citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or the Krebs cycle, is a cyclic set of reactions that occurs in the mitochondrial matrix. The TCA cycle is the continuation of any metabolic pathway that produces pyruvate, which is converted into its main substrate, acetyl-CoA. 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 Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or "brite" adipose tissue, which is a transitional form. 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 Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes
  • Isolating toxins from the rest of the cell
  • Maintaining pH 
  • Assist exocytosis and endocytosis

Lysosomes

  • Membrane-bound spherical vesicles containing hydrolytic enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes
  • Can break down proteins, nucleic acids Nucleic Acids Nucleic acids are polymers of nucleotides, organic molecules composed of a sugar, a phosphate group, and a nitrogenous base. Nucleic acids are responsible for storage, replication, and expression of genetic information. The 2 nucleic acids most commonly seen in eukaryotic cells are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic Acids, carbohydrates Carbohydrates Carbohydrates are one of the 3 macronutrients, along with fats and proteins, serving as a source of energy to the body. These biomolecules store energy in the form of glycogen and starch, and play a role in defining the cellular structure (e.g., cellulose). Basics of 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 RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure and proteins.
  • Present in the cytosol, as part of RER, and in mitochondria

Function:

  • Translation Translation Translation is the process of synthesizing a protein from a messenger RNA (mRNA) transcript. This process is divided into three primary stages: initiation, elongation, and termination. Translation is catalyzed by structures known as ribosomes, which are large complexes of proteins and ribosomal RNA (rRNA). Stages and Regulation of Translation of RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure and protein synthesis
  • Protein folding
Ribosome

Ribosome translating polypeptide chain off the matrix RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure

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

Nucleolus

  • Largest structure within the nucleus
  • Made up of DNA DNA The molecule DNA is the repository of heritable genetic information. In humans, DNA is contained in 23 chromosome pairs within the nucleus. The molecule provides the basic template for replication of genetic information, RNA transcription, and protein biosynthesis to promote cellular function and survival. DNA Types and Structure, RNA RNA Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer of nucleotides that is essential to cellular protein synthesis. Unlike DNA, RNA is a single-stranded structure containing the sugar moiety ribose (instead of deoxyribose) and the base uracil (instead of thymine). RNA generally carries out the instructions encoded in the DNA but also executes diverse non-coding functions. RNA Types and Structure, 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 Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of 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 Cytoskeleton A cell's cytosol is the liquid inside the cell membrane that surrounds the organelles and cytoskeleton. The cytosol is a complex solution where many biochemical processes take place. The Cell: Cytosol and Cytoskeleton
    • Organization of microtubules in the cytosol Cytosol A cell's cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. . The Cell: Cytosol and Cytoskeletonand 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 Ethanol metabolism Ethanol is a chemical compound that is produced in small amounts within the small intestine and is also ingested from alcoholic drinks. Ethanol's digestion involves a complex catabolic pathway that mainly takes place in the liver. Ethanol is turned into acetaldehyde, then to acetate, and finally into acetyl-CoA. 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 Mitochondrial myopathies Mitochondrial myopathies are conditions arising from dysfunction of the mitochondria (the energy-producing structures) and are characterized by prominent muscular symptoms and accompanied by various symptoms from organs with high energy requirements. The organs disproportionately affected include the skeletal muscles, brain, and heart. 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 Zellweger syndrome Zellweger syndrome (ZWS), also called cerebrohepatorenal syndrome, is a rare congenital peroxisome biosynthesis disorder and is considered an inborn error of metabolism. Zellweger syndrome is the most severe form of a spectrum of conditions called Zellweger spectrum disorder (ZSD), and is characterized by the reduction or absence of functional peroxisomes. Zellweger Syndrome: a peroxisome-related disease that causes hypotonia, hepatomegaly, seizures Seizures A seizure is abnormal electrical activity of the neurons in the cerebral cortex that can manifest in numerous ways depending on the region of the brain affected. Seizures consist of a sudden imbalance that occurs between the excitatory and inhibitory signals in cortical neurons, creating a net excitation. The 2 major classes of seizures are focal and generalized. Seizures, and early death. Zellweger syndrome Zellweger syndrome Zellweger syndrome (ZWS), also called cerebrohepatorenal syndrome, is a rare congenital peroxisome biosynthesis disorder and is considered an inborn error of metabolism. Zellweger syndrome is the most severe form of a spectrum of conditions called Zellweger spectrum disorder (ZSD), and is characterized by the reduction or absence of functional peroxisomes. Zellweger Syndrome is an  autosomal recessive Autosomal recessive Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal recessive diseases are only expressed when 2 copies of the recessive allele are inherited. Autosomal Recessive and Autosomal Dominant Inheritancedisorder of peroxisome biogenesis. Cells are unable to break down VLCFAs and branched-chain fatty acids.
  • Malignant hyperthermia Malignant hyperthermia An important complication of anesthesia is malignant hyperthermia, an autosomal dominant disorder of the regulation of calcium transport in the skeletal muscles resulting in a hypermetabolic crisis. Malignant hyperthermia is marked by high fever, muscle rigidity, rhabdomyolysis, and respiratory and metabolic acidosis. Malignant Hyperthermia: an autosomal dominant Autosomal dominant Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal dominant diseases are expressed when only 1 copy of the dominant allele is inherited. Autosomal Recessive and Autosomal Dominant Inheritance 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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