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 significantly more complex structure.
|Overview of the eukaryote and prokaryote cell structure|
|Factor||Eukaryotes (humans, protozoa, animals, and plants)||Prokaryotes (archaea and bacteria)|
|Nucleus|| || |
Location of DNA
|DNA storage form|
|Amount of noncoding DNA|| || |
|Mitochondria|| || |
|Ribosomes|| || |
|Cell wall|| |
|Compartmentalization|| || |
|Locomotive structures (flagellum)|| |
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, stabilizes, and protects cells from the outside environment.
Cell membrane structure
The cell membrane (or plasma membrane) is composed of an asymmetric lipid bilayer with embedded or attached membrane proteins. The synthesis of membrane components occurs in the smooth endoplasmic reticulum (SER).
Structure: consists of 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 bilayer and the 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)
- Flippases: move phospholipids from the outer to the inner surface
- Floppases: move phospholipids from the inner to the outer surface
- Scramblases: move phospholipids in both directions
Facilitated diffusion: diffusion of molecules across the cell membrane via carrier proteins, channel proteins, or ions. Examples include:
- Glucose and fructose transport into cells via GLUT transporters
- Transport of glucose from the blood into the pancreatic beta cell via GLUT2
- Water transport via aquaporin channels in principal cells of the kidney
- Bicarbonate reabsorption in the early proximal convoluted tubule
- Reabsorption of urea in the inner collecting ducts via urea transporters
- Calcium transport from the sarcoplasmic reticulum into the cytosol via voltage-gated calcium channels
- 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|| || |
- 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 (translation) and their modification (e.g., )
- Packaging of newly synthesized proteins into vesicles to transport to the Golgi apparatus (for further processing) or directly to a specific location
- All proteins found within cell organelles (e.g., Golgi apparatus, lysosomes, ER) have their origin in the RER.
- Cells rich in RER include exocrine pancreas cells, antibody-secreting , and mucus-secreting goblet cells.
- Nissl bodies: the RER found in the soma and dendrites of neurons
- Synthesis of phospholipids, fatty acids, cholesterol, and steroids
- Biotransformation of drugs, alcohol, and toxins in the liver
- Storage and release of carbohydrates
- Location of glucose 6-phosphatase
- Calcium storage
- Cells rich in SER include hepatocytes, and steroid-secreting cells (e.g., adrenal cortex or gonadal cells)
Enveloped, disc-shaped, slightly curved vesicle system with two sides:
Cis-Golgi face (convex side)
- Bends slightly around the ER
- Membrane vesicles from the ER that are loaded with proteins are received at the cis-Golgi side.
- Trans-Golgi face (concave side)
- Synthesis of lysosomes and their loading with enzymes
- Recycling of plasma membrane proteins via endocytosis
- Activation of hormones and other proteins
- Modification of glycoproteins and hormone precursors received from the RER
- O-oligosaccharides to serine or threonine: attachment of
- Modification of N-oligosaccharides on aspargine after RER in the
- Phosphorylation: Mannose residues on glycoproteins (e.g., lysosomal acid hydrolases) are phosphorylated to mannose-6-phosphate, allowing them to be trafficed to lysosomes. (Defects in this process lead to .)
- Sorting of proteins according to their target sequence or attached oligosaccharides
Vesicular trafficking proteins
- COPI protein: trans-Golgi network (TGN) → cis-Golgi network (CGN) → endoplasmic reticulum (retrograde trafficking)
- COPII protein: endoplasmic reticulum → CGN → TGN (anterograde trafficking)
- Clathrin: formation of coated vesicles (endosomes) for transport within cells
To remember that COPII facilitates anterograde (forward) transport from the rough endoplasmatic reticulum to the Golgi apparatus and COPI facilitates retrograde (backward) transport, think: “Two cops (COPII) go for (forward) a coffee to go (to the Golgi apparatus). One cop (COPI) goes back (backward) to the rough (rough ER) neighborhood.”
- Vesicular, membrane-enclosed cell organelles originating from the of the
- Subclassified into early and late endosomes depending on their stage of maturation
- Intracellular sorting and transport system
- Early endosomes
- Late endosomes: fuse with lysosomes and thereby allow for lysosomal degradation of endosomal content
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 )
Types of inner mitochondrial membranes
- Thin invaginations (cristae) of the inner membrane
- Present in most cells
- Inner membrane forms tubules
- Mainly in cells that produce steroids
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: See section “Apoptosis” in the article on for more information.
“If you cite (cytoplasm) my article, I might (mitochondria) give you a HUG”: Heme synthesis, the Urea cycle, and Gluconeogenesis take place in both, the cytoplasm and the mitochondria, think: “If you cite (cytoplasm) my article, I might (mitochondria) give you a HUG”.
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.
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
- Fatty acid oxidation
- Hydrogen peroxide metabolism
- Biosynthetic function
- Catabolic function: amino acids and ethanol metabolism
Cytosol and ribosomes
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
- Membrane-bound ribosomes: bound to the RER
- Definition: a network of filaments (protein fibers) that extends throughout the cytosol.
- Stability and movement of the cell and its organelles
- Transport processes within the cell
- Essential for cell division
- Accessory proteins
Actin filaments (microfilaments)
|Intermediate filaments (IFs)|| |
| || |
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 (adhering junctions)
Anchoring junctions are mechanical attachments between cells. Several forms can be differentiated according to function.
Adherens junction (zonula adherens, belt desmosome)
- Description: tightly connects cells across a broader belt-shaped area
- 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).
- Function: connects, e.g., epithelial cells and endothelial cells in a continuous, belt-like manner
Desmosomes (macula adherens, spot desmosome)
- Description: linking of two cells via intermediate filaments
- Description: : does not connect two cells, but attaches cells to the extracellular matrix
- Structure: : Integrins connect the intracellular cytoskeleton (keratin) with molecules of the basement membrane (laminin, fibronectin, and collagen).
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
- Connexon: composed of six membrane-spanning proteins (connexins) with a central pore
- Structure: formed by the interaction of the connexons of two neighboring cells
- action potentials are transmitted from a presynaptic to a postsynaptic structure (e.g., neurons, muscle) : areas where signals or