The cell is the basic structural and functional unit of living organisms. While unicellular organisms (e.g., bacteria, protozoa) consist of a single cell capable of sustaining life, multicellular organisms (e.g., animals, land plants) consist of numerous highly specialized and diverse cells organized into various types of tissue. Cells are surrounded by a membrane composed of a lipid bilayer with embedded proteins. Depending on their cell structure, organisms are classified as prokaryotes or eukaryotes. Prokaryotes, which encompass the domains of the Bacteria and the Archaea, are unicellular organisms that lack membrane-bound organelles such as a nucleus and mitochondria (see ). Eukaryotes are unicellular and multicellular organisms with a cell or cells containing various specialized, membrane-bound organelles such as nuclei and mitochondria.
Cell types are classified as either prokaryotic or eukaryotic. Prokaryotes are unicellular organisms that encompass the domains of Bacteria and Archaea. They consist of a single cytoplasm-filled compartment enclosed by a cell membrane. Eukaryotes contain a nucleus and other membrane-bound cell organelles. Eukaryotes encompass all multicellular organisms as well as some unicellular ones (protozoa). Eukaryotic cells are larger (100–10,000-fold) than prokaryotic cells and have a structure that is significantly more complex.
|Nucleus|| || |
Location of DNA
|DNA storage form|| |
|Amount of noncoding DNA|| || |
|Compartmentalization|| || |
|Locomotive structures (flagellum)|| |
Prokaryotic cells do not have a nucleus!
Both prokaryotes and eukaryotes have cell membranes. The cell membrane provides a boundary between the outside environment and the cell interior and is an essential component of living systems. Eukaryotic cells also have intracellular membranes that envelop individual organelles and enable specialized processes to occur in separation from cytoplasmic processes. Furthermore, most prokaryotic and plant cells possess a cell wall, which envelops the cell membrane and protects cells from the outside environment.
Cell membrane structure
Structure: The fundamental building blocks of the lipid bilayer are amphiphilic lipids such as phospholipids or sphingolipids, which possess a polar head (e.g., phosphate, sphingosine) and hydrophobic tails (fatty acids).
- Distribution of nonpolar and polar groups: In an aqueous solution, the nonpolar hydrocarbon tails face inward, while the polar heads form a boundary to water in both directions. As a result, stable lipid bilayers develop, forming a spherical entity (e.g., cells or vesicles).
Distribution of membrane lipids: The different types of lipids are distributed asymmetrically between the two leaflets of the membrane.
- Outer lipid layer: rich in phosphatidylcholine and sphingomyelin
- Inner lipid layer: rich in phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol
- Almost impermeable to polar molecules
- Highly permeable to nonpolar molecules and water
- Fluidity: The fluidity of the membrane lipid bilayer changes depending on the composition of the lipid bilayer and temperature of the environment.
Diffusion (transport): The fluidity of the lipid bilayer allows for movement of individual molecules within the membrane.
- Lateral (parallel) diffusion: Individual lipid molecules diffuse freely within the lipid bilayer.
- Transverse diffusion : very slow; requires enzymatic support by flippases, floppases, or scramblases (phospholipid translocators)
- Facilitated diffusion: diffusion of molecules across the cell membrane via carrier proteins, channel proteins, or ions (e.g., glucose and fructose transport into cells via GLUT transporters)
- Membrane protein content in the lipid bilayer: 20–80%
Types of membrane proteins
- Integral membrane proteins
- Peripheral membrane proteins
- Distribution of membrane proteins: variable composition of the inner and outer membrane surface
|Examples of asymmetrically distributed membrane components|
|Integral membrane proteins||Transmembrane proteins|
|Integral monotopic proteins|| |
|Peripheral membrane proteins||Extracellularly directed|
Because of their fluidity, membranes are also permeable to water and some small molecules like O2, even without the use of specific channels or transporters. Accordingly, they are described as semipermeable.
- Definition: loose glycoprotein-polysaccharide layer covering the outside of the cell membrane in some eukaryotic and prokaryotic cells
- Protects the cell from the external environment
- Transport of substances from the inside to the outside of the cell or from the outside to the inside of the cell
- Signal transduction: conversion of extracellular signals into intracellular reactions
- Every cell expresses specific proteins on its surface that are mostly glycosylated (glycoproteins).
- These glycoproteins are highly specific for each cell type and allow self cells to be distinguished from one another as well as from foreign cells.
- Generation of an electrochemical gradient across the membrane creates a membrane potential.
- Excitation activates voltage-gated ion channels, temporarily decreasing the negative membrane potential (depolarization).
- Cell junctions: : formed by anchor proteins (cell adhesion molecules), which are anchored to the and protrude outside of the cell
Cellular organelles are compartments within cells that are enveloped by a membrane and have a highly specific function. Eukaryotes contain numerous organelles, whereas prokaryotes lack compartmentalization.
|Overview of the most important cell organelles|
|Endoplasmic reticulum (ER)|| || |
|Golgi apparatus|| || |
|Mitochondria|| || |
|Lysosomes|| || |
The nucleus is the control center of the cell. It is surrounded by a double membrane and contains all of the cell's genetic material, except for the mitochondrial DNA.
- Outer nuclear membrane: contains numerous ribosomes
- Inner nuclear membrane: covered by the nuclear lamina, a network of intermediate filaments (lamins) that stabilizes the membrane
- Nuclear pores: The inner and outer nuclear membranes fuse at some points and form nuclear pores with the aid of large protein complexes.
- Chromatin: complex of DNA, histones, and nonhistone proteins
- Nucleolus: site of rRNA synthesis and ribosomal subunit assembly
- Storage of the entire genetic information of an organism in the form of chromatin (except mitochondrial DNA)
- Duplication of genetic information before cell division ( ): See for further information.
- Transcription: initial step of
- Synthesis of nucleolus in the
- Packaging and protection of inactive DNA by histones
The endoplasmic reticulum (ER) is an extensive network of membranes that is directly connected to the outer nuclear membrane. The ER forms a channel system of elongated cavities. The most important function is the synthesis of cellular components and cell export products. The ER can be microscopically and functionally differentiated into the rough and smooth ER.
- Membranous channel system
- In direct contact with the outer nuclear membrane
- Composed of two microscopic and functionally different regions
- Synthesis of membrane, secretory, and lysosomal proteins ()
- Packaging of newly synthesized proteins into vesicles to transport to the Golgi apparatus (for further processing) or directly to a specific location
- Staining of negatively-charged ribosomes with cationic (basic) dyes: e.g., rER appears as Nissl bodies in the soma and dendrites of neurons (light microscopy) when stained with Nissl stain
- Cells rich in rER include cells of the exocrine pancreas, antibody-secreting , and mucus-secreting goblet cells.
The Golgi apparatus receives vesicles, mainly from the ER, and is responsible for further distribution of the substances (proteins and lipids) to their specific target structure, e.g., the lysosomes or the cell membrane.
- Enveloped, disc-shaped, slightly curved vesicle system with two sides
- Cis-Golgi face (convex side): bends slightly around the ER
- Trans-Golgi face (concave side): towards the cell membrane
- Modification of glycoproteins and hormone precursors received from the ER as well as of membrane proteins recycled from the plasma membrane by endocytosis
- Activation of hormones and other proteins
- Sorting of proteins according to their target sequence or attached oligosaccharides
- Synthesis of lysosomes and their loading with enzymes
- Reprocessing of membrane components
Vesicular trafficking proteins
- COPI protein: retrograde transportation of vesicles. Examples:
COPII protein: anterograde transportation of vesicles. Example:
- Endoplasmic reticulum → cis-Golgi
- Clathrin: formation of coated vesicles for transport within cells. Examples:
Two steps forward (COPII - anterograde transport) and one step back (COPI - retrograde transport)
Mitochondria are often described as the powerhouses of the cell because of their central role in the synthesis of , a vital source of energy for the body. They are composed of a double membrane, intramembranous space, and matrix. Various mitochondrial types can be differentiated based on the inner membrane structure.
The structure and DNA of mitochondria resemble the structure and DNA of prokaryotes. Mitochondria are believed to have been prokaryotes originally that evolved into endosymbionts living inside eukaryotes (see ).
- Structure: smooth
- Permeability: interspersed with pores, highly permeable for various molecules
- Structure: convoluted
- Permeability: impermeable, especially to ions; however the inner membrane contains many different highly specific transport proteins
- Characteristic component: cardiolipin (stabilizes the enzymes of )
Carriers of the inner mitochondrial membrane
Specific transporters regulate the transport of substances through the inner membrane.
- Functional mechanism: antiporter of two molecules
- Contains mitochondrial DNA (mtDNA) and ribosomes responsible for the synthesis of ∼ 15% of the mitochondrial proteins
- The remaining mitochondrial proteins are encoded in the nucleus and are transported into the mitochondria in an unfolded state, where they take on their final folded structure.
- Energy production: The inner mitochondrial membrane contains the enzymes of the respiratory chain and the ATP synthase that together produce ATP (oxidative phosphorylation).
- Other metabolic pathways in the matrix
- Initiation of apoptosis
The DNA and ribosomes of mitochondria and prokaryotes have many similarities. The discovery of this resulted in the endosymbiotic theory of mitochondrial evolution, which is that mitochondria were originally independent prokaryotic bacteria with the special ability to produce energy through oxidative phosphorylation and were eventually engulfed by eukaryotic cells. As a result, the prokaryotic cells lost parts of their DNA and their ability to live independently, while the eukaryotic host cell became dependent on the energy produced by the incorporated bacterium.
Lysosomes can be regarded as the cell's waste disposal system. Their main function is intracellular digestion, e.g., the degradation of polymers into monomers.
- Small, spherical organelles that are surrounded by a lipid bilayer and filled with digestive hydrolytic enzymes, which are responsible for the degradation of macromolecules
Hydrolytic enzymes: lipases, glucosidases, acidic phosphatases, nucleases, endoproteases (e.g., cathepsins )
Origin of hydrolytic enzymes
- Enzymes are synthesized at the ribosomes of the rough ER and then transported to the Golgi apparatus.
- A mannose 6-phosphate molecule is attached to the enzymes after their translation by N-acetylglucosaminyl-1-phosphotransferase in the Golgi apparatus.
- The enzymes tagged with mannose 6-phosphate are packaged into vesicles (primary lysosomes).
- Origin of hydrolytic enzymes
- Acidic environment (pH value of ∼ 5)
- Hydrolytic enzymes: lipases, glucosidases, acidic phosphatases, nucleases, endoproteases (e.g., cathepsins )
Intracellular degradation of macromolecules
- Primary lysosomes are vesicles with newly synthesized hydrolytic enzymes that bud from the Golgi apparatus.
- They fuse with vesicles that contain digestive materials, e.g., endosomes, phagosomes, and thereby form secondary lysosomes.
- The hydrolytic enzymes in the secondary lysosomes degrade the macromolecules.
- Cleavage products are emptied into the cytosol and can be reused for new synthesis processes.
- Residual bodies: lipid-rich, undigested material (lipofuscin) left over from macromolecule degradation is expelled from the cell or stored in the cytosol in residual bodies.
Origin of macromolecules
- Autophagy: Autophagosomal membranes fuse and form an autophagosome that sequesters intracellular debris, e.g., proteins, lipids, cell organelles. It later fuses with lysosomes in order to degrade the macromolecules.
- Autolysis: In the event of severe cellular damage, lysosomes release their contents into the cytosol, causing the cell to disintegrate ( ).
Lysosomes play an important role in adaptive immunity. Antigen-presenting cells (e.g., macrophages, dendritic cells) internalize antigens and degrade them through proteolysis within lysosomes. Afterwards, the resulting peptides are loaded onto MHC class II molecules, delivered to the cell surface and presented to naive T cells.
- Relatively small, round, membrane-enclosed vesicles
- Degradation (beta-oxidation) of very long chain fatty acids up to octanoyl-coenzyme A (CoA) ; :
- Degradation of hydrogen peroxide
- Biosynthetic function: : substeps of, e.g, steroid hormone synthesis, bile acid synthesis, and plasmalogen formation
- Catabolism of amino acids, branched-chain fatty acids (α-oxidation), and ethanol
Refsum disease is an autosomal recessive disorder involving insufficient α-oxidation of branched-chain fatty acids: Phytanic acid cannot be metabolized to pristanic acid, which leads to the onset of progressive ataxia, sensorineural hearing loss, scaling skin, cataracts, and night blindness in adolescence.
Adrenoleukodystrophy is an X-linked recessive disorder of β-oxidation that is caused by a mutation in the ABCD1 gene: Very long chain fatty acids (VLCFA) build up in the adrenal glands, white matter of the brain, and testicles, leading to adrenal insufficiency, cognitive impairment, and progressive vision, hearing, and motor deterioration.
The cytosol, also termed matrix, is part of the cytoplasm and enclosed by the cell membrane. In prokaryotes, almost all metabolic pathways occur directly in the cytosol. In eukaryotes, several of these processes occur in cell organelles that are separated from the cytosol by a membrane (compartmentalization).
- Water, dissolved ions, and small molecules (70%)
- Proteins, e.g., enzymes involved in metabolic pathways (30%)
Ribosomes are very large molecule complexes of RNA and proteins that are located in the cytosol, on the cytosolic side of the rough endoplasmic reticulum (rER) and within the mitochondria. The ribosome is the site of protein synthesis (translation).
- Mass: The mass of the ribosomal subunits is measured using the sedimentation coefficient (unit: Svedberg, or S).
- Cytosolic ribosomes: Free ribosomes in the cytosol or bound to the cytoskeleton
- Membrane-bound ribosomes: bound to the rER
- Translation: Ribosomes constitute the structural prerequisites for protein synthesis and are catalytically active. The RNA components of ribosomes (rRNA) interact with mRNA and tRNA and catalyze peptide bond formation.
- Stability and movement of the cell and its organelles
- Transport processes within the cell
- Essential for cell division
Structure: composed of filaments and accessory proteins
- Filaments: elongated cell structures composed of monomers
- Accessory proteins: responsible for various functions of the cytoskeleton (motion, attachment and detachment of monomers, etc.)
The most important cytoskeletal elements
Actin filaments (microfilaments)
|Intermediate filaments (IFs)|| |
Intermediate filaments can be used as to detect the origin of a neoplasm.
To remember drugs that disrupt microtubules, think “Microtubules Get Constructed Very Poorly”: Mebendazole, Griseofulvin, Colchicine, Vincristine/Vinblastine, Paclitaxel!
Negative end Near Nucleus, while Positive end Points to the Periphery: The negative end of the microtubule is oriented towards the nucleus and the positive end is oriented towards the periphery of the cell.
Kin (keen) to go out (anterograde), Dying to come back home (retrograde). Kinesin transports anterograde (from – → +) along the microtubule. Dynein transports retrograde (from + → –) along the microtubule.
The cells of the body are connected to other cells and the surrounding structures by cell-cell junctions and cell-matrix junctions. The type and number of junctions varies between different cell types. While red blood cells do not form cell junctions, epithelial cells are tightly connected to one another and to the basal lamina.
Tight junction (zonula occludens): sealing contact that forms an intercellular barrier between epithelial cells
- Localization: usually at the apical surface between epithelial cells
Anchoring junctions are mechanical attachments between cells. Several forms can be differentiated according to function.
Desmosomes (macula adherens, spot desmosome): linking of two cells with support by IFs
- Occurrence/function: primarily connect cells subject to high levels of mechanical stress (e.g., epithelial cells and cardiomyocytes)
- Hemidesmosome: does not connect two cells, but attaches cells to the extracellular matrix
Adherens junction (zonula adherens, belt desmosome): tightly connects cells across a broader belt-shaped area
- Structure: Vinculin and catenin are located on the intracellular side of the cell membrane and connect the intracellular actin filaments with transmembrane adhesion proteins such as cadherins (mainly E-cadherin).
- Occurrence/function: connects, e.g., epithelial cells and endothelial cells in a continuous, belt-like manner
Communicating junctions permit the passage of electrical or chemical signals.
Gap junction (nexus): intercellular channels that connect two cells
- Structure: formed by the interaction of the connexons of two neighboring cells
- Occurrence/function: primarily cardiomyocytes; control the passage of electrical stimulus in cardiomyocytes as well as epithelial and retinal cells