Cell Structure & Function
Centrioles
Centrioles are found in the cytoplasm, close to the nucleus, in a region of the cell called the microtubule organizing center (MTOC) or centrosome. Centrioles resemble a short cylinder and come in pairs that are arranged at right angles to each other. Each cylindrical structure contains nine groups of three fused peripheral microtubules. Each triplet is held to the next by a set of proteinaceous fibers. During interphase of the cell cycle, the centrosome and centriole pair duplicate, and by the end of prophase of mitosis, they are at the poles of the dividing cell. Attached by their minus (-) ends at the MTOC, a radial array of microtubules called an aster projects outward toward the cell periphery to form the mitotic spindle, a complex that is essential for the proper separation of the metaphase chromosomes into their respective daughter cells during cytokinesis.
Eukaryotic (Animal) Cells: Have Membrane-Bound Organelles
Organelles
Organelles are highly specialized subunits that carry out specific functions within each of the 200 or so different types of cells in the human body. Many are enclosed within their own separate single- or double-membrane systems; others are not. The larger organelles are readily seen using light microscopy, while the smaller organelles become apparent with the use of electron microscopy. Outlined below is an alphabetized list of the major organelles in a typical animal cell, along with their structure and function.
Centrioles
Cilia (and Flagella)
Endoplasmic Reticulum Endosomes
Golgi Apparatus
Intermediate Filaments
Lysosomes Microfilaments Microtubules Mitochondria
Nucleus Peroxisomes
Plasma Membrane Ribosomes
Vacuoles Microvilli
Nucleolus Vesicles
Centriole pair within the centrosome during
G1 of the cell cycle Aster
Replicated centrioles within the MTOC during metaphase of mitosis Centrosome
G2 S
Mitotic spindle MTOC
Cilia (and Flagella)
Cilia and flagella have different characteristic motions, even though they have a common structural design. Cilia beat backward and forward in a whiplike manner about 5-10 times each second to help move fluid over the surface of a cell. They are quite numerous on the epithelial cells lining the human respiratory tract and aid in the movement of particulate matter and mucus toward the oral cavity, where it can be eliminated. Cilia also facilitate the movement of eggs along the length of the Fallopian tube in the female reproductive tract.
Flagella are much longer than cilia and display a beat similar to that of a sinusoidal wave.
A classic example is the flagellum of the sperm, which has a wave frequency of about 30-40 each second, directed away from the cell and allowing it to be propelled forward.
Top-down view of a centriole
Triplet of microtubules Protein fiber
connection
Endoplasmic Reticulum (ER)
Cilia and flagella are built from a collection of protein connective elements and microtubules arranged in a 9 + 2 array. This core, called the axoneme, is surrounded by an extension of the cell’s plasma membrane. At the center of an axoneme are two singlet microtubules; at the periphery are nine doublet microtubules. Numerous proteins are associated with the axoneme. Some act as links (e.g., nexin) that bind the microtubules together;
others generate the forces (e.g., dynein) needed to give the structure its characteristic movement.
The endoplasmic reticulum is organized into a connecting labyrinth of flattened sacs and tubules that permeate the cytoplasm of all eukaryotic cells. The complex architecture of the ER provides a very large surface area and in the average animal cell accounts for slightly more than 50% of the cell’s total membrane. Variations in the organization of this membrane-bound organelle can be found in different cell types as well as within different regions of the ER itself. The ER is continuous with the outer nuclear membrane.
Rough Endoplasmic Reticulum (RER)
Ribosomes
Flattened sacs
Tubules
An ER coated with membrane-bound ribosomes is referred to as rough endoplasmic reticulum. If a ribosome is synthesizing a protein that has an ER signal peptide at its N-terminus, then that signal helps direct the ribosome to a docking protein embedded in the membrane of the ER.
After the polypeptide is translocated across the ER membrane through a pore, a peptidase then cleaves the signal peptide on the lumenal side of the membrane and releases the nascent
protein into the lumen or cisternal space. Lumen
RER
SER
Smooth Endoplasmic Reticulum (SER)
An ER that is not coated with membrane-bound ribosomes is referred to as smooth endoplasmic reticulum. In general, the membranous region of the SER is more convoluted and has more branch points than the RER, forming a network that resembles a sponge. The SER is involved in lipid synthesis and the detoxification of lipid-
soluble drugs and toxic chemical compounds produced by metabolism. In many cell types, specialized regions of the SER (e.g., the sarcoplasmic reticulum of skeletal muscle cells) can store and release Ca2+. In some cell types (e.g., neurons and hepatocytes), the SER appears to occupy a different region of the cytoplasm from the RER, indicating a unique spatial organization.
Endosomes
Endocytosis is the process by which the cell membrane of a cell invaginates to form a pit and subsequently pinches off as an internalized endosome that contains material from the extracellular medium. Material entering the cell usually follows one of three distinct but similar pathways: phagocytosis, bulk-phase endocytosis, or receptor-mediated endocytosis.
Labeling experiments show that there are two types of internalized structures, an early endosome and a late endosome, and their interior is acidic (pH ≈ 5-6). Endosomes act as sorting stations that pass material to either the lysosome or the Golgi apparatus.
membraneCell
Doublet microtubule
Nexin Dynein
arms
Singlet microtubule
Golgi Complex
The Golgi complex consists of a series of flattened, membrane-bound sacs (cisternae) that resembles a stack of dinner plates. It is usually located close to the nucleus of the cell.
Intermediate Filaments
Three types of dynamic cytoskeletal elements lend support and stability to a cell’s shape and structure: intermediate filaments, micofilaments, and microtubules. Intermediate filaments (IF) have an average diameter of 10 nm and crisscross the cell’s cytoplasm from the nucleus to the plasma membrane. They can be attached to specialized structures called desmosomes, which mediate cell-to-cell adhesion. The structural unit of an IF is a coiled-coil dimer of two polypeptide chains. Two dimers are next arranged in a staggered and antiparallel fashion to form a tetramer. End-to-end aggregation of tetramers forms a protofilament. A pair of protofilaments associate laterally to form a protofibril, and lateral association of four protofibrils forms a mature IF, which in cross-section would have 32 individual polypeptide chains.
Lysosomes
Lysosomes contain many different hydrolytic enzymes, such as proteinases, nucleases, lipases, and phosphatases, to name just a few. The enzymes found within this membrane- bound organelle are capable of degrading all major classes of biological macromolecules as well as cellular organelles through a digestive process called autophagy. The pH of the Transport vesicles leaving regions of the RER that do
not display ribosomes travel to and fuse with the cis face of the Golgi complex. As proteins and lipids pass from the cis region, to the medial region, and then to the trans region of the complex, they are covalently modified in an ordered series of reactions (e.g., glycosylation) by enzymes unique to each compartment. With the exception of those resident proteins that remain in the Golgi complex, all others are sorted in the trans region according to their structures and final destinations.
Vesicles budding from the trans face transport these modified proteins to the plasma membrane (either for fusion or exocytosis), lysosomes, and secretory vesicles.
faceCis
Trans face Secretory
vesicle Transfer vesicle
from RER
regionCis Trans region Medial
region
Coiled-coil dimer
N N
C C
Tetramer Protofilament
C C
N N N
N
C C
Protofibril Intermediate filament
One protofibril with 8 polypeptide chains
lysosomal lumen is between 4.5 and 5.0, while the pH of the cytoplasm is about 7.0. In order for the interior of the lysosome to become acidic, an ATP-powered proton pump moves protons from the cytoplasm into the lumen. An equal number of anions (e.g., Cl-) follows. If a large number of lysosomes rupture within a cell, it can lead to increased cytoplasmic acidity and cell death. Tay-Sachs, a fatal lysosomal storage disease, occurs when a specific ganglioside cannot be degraded by the lysosome and accumulates in the nerve cells of the brain. The release of lysosomal enzymes into the extra- cellular fluid by bone cells is thought to be a factor in arthritis.
Cytoplasm Cl- H+
ATP
ADP + Pi
Lumen H+
Cl- H+
Cl-
Microtubules
All eukaryotic microtubules have a similar structural design that allows them to participate in cellular organization through the microtubule-organizing center (MTOC), in cellular movement through the beating of cilia and flagella, and in the distribution of membrane- bound vesicles in the cytoplasm. Microtubules have a diameter of about 25 nm. They are constructed from tubulin subunits, heterodimers of the globular proteins α- and β-tubulin.
Tubulin dimers link in a head-to-tail fashion to form a longitudinal protofilament, and 13 protofilaments interact laterally--in the same direction--to form a cylindrical microtubule.
This ensures that the microtubule has polarity: α-tubulin at the minus (-) end and β-tubulin at the plus (+) end. The (-) end of most microtubules lies near the MTOC.
Microfilaments
Microfilaments, also called actin filaments, consist of two chains of actin subunits wound into an 8-nm diameter double helix that makes one turn (i.e., a pitch) every 71 nm. This is about twenty-one times longer than a pitch of B-DNA, which is 3.4 nm. Each actin subunit has two lobes that are separated by a deep cleft. The globular form of this protein is called G-actin, whereas the filamentous form is called F-actin. Microfilament assembly requires the binding of ATP and Mg2+ to the cleft in G-actin. During nucleation, a few subunits get together to form a stable oligomer. Actin monomers then add to both ends of the oligomer during elongation. When equilibrium is reached, actin monomers are exchanging with subunits at both ends of the filament. The net mass of the filament does not change during this steady state. As the subunits add to the ends of the filament, they all align themselves to point in the same direction, giving polarity to the structure. If the cleft of G-actin is exposed at the end of a filament, that end is designated the minus (-) end. The plus (+) end finds the cleft contacting its neighbor. Even though microfilaments are dynamic protein structures that extend throughout the cell, they are found in higher concentrations just below the cell’s plasma membrane. Since they are constantly assembling and disassembling themselves, microfilaments change the morphology of the cell.
end(+)
end(-) Steady state
Elongation Nucleation
G-actin
F-actin
Cleft
Oligomer
Tubulin dimer
β α
1
9 5 Rotate left 90º Protofilament
Microtubule End-on view
Microvilli
Microvilli are fingerlike extensions of the plasma membrane. They are found on the apical (lumenal) brush border surface of many animal cells, including those in the small intestine,
end(-) (+)
end
kidney, inner ear, ova, and taste buds. They increase the surface area of cells, and they participate in absorption, secretion, cellular adhesion, and mechanotransduction.
Each microvillus is about 100 nm in diameter and has at its core about 30 parallel actin microfilaments. Microvilli are covered with glycocalyx, a meshwork of glycolipids and glycoproteins that house digestive enzymes and help
absorb nutrient compounds like amino acids and glucose. Microvillus Glycocalyx
Microvilli Microfilaments Cell
Mitochondria
Mitochondria are found in almost all eukaryotic cells. Some exceptions include mature red blood cells and microsporidia (found within the kingdom Fungi). They are believed to have been derived from endosymbiotic prokaryotes about 2 billion years ago. Mitochondria divide by binary fision. The number of these double membrane-bound organelles varies widely in human beings by tissue type. Most cells contain mitochondria numbering in the tens to hundreds; other cells have thousands.
Nucleolus
The nucleolus of a diploid interphase cell is a densely structured subregion of the nucleus.
It is not bounded by a lipid membrane. Inside the nucleolus are loops of chromatin fibers from five chromosome pairs (of the 46 chromosomes in a diploid cell). At the tips of the chromosomes are ribosomal RNA (rRNA) genes that undergo transcription to form a 45S rRNA precursor transcript. This large precursor is split into 18S, 5.8S, and 28S segments of rRNA. Proteins and a 5S rRNA transcript destined to be assembled into ribosomal subunits are synthesized in the cytoplasm and transported into the nucleus through the nuclear pores. Once inside the nucleolus, some of the imported proteins and the 18S rRNA combine to form the small (40S) ribosomal subunit. Other imported proteins and the 5S, 5.8S, and 28S rRNAs combine to form the large (60S) ribosomal subunit. The finished ribosomal subunits leave the nucleolus and pass through the nuclear pores to the cytosol, where they combine for use in protein synthesis.
Nucleus
The nucleus contains the cell’s genetic information (DNA) and is surrounded by a double membrane system (i.e., the nuclear envelope). It is the largest organelle in a eukaryotic animal cell, occupying about 10% of the cell’s volume. The outer membrane is continuous with the RER and is covered with ribosomes. The inner membrane’s nucleoplasmic face is lined with intermediate filaments. The region between the two membranes, the perinuclear The outer mitochondrial membrane is composed
of roughly 50 percent protein and 50 percent lipid.
It is quite permeable to molecules of 6,000 daltons or less, due to the transmembrane channel protein porin. The inner membrane, thrown into folds called cristae to increase surface area, is composed of about 80 percent protein and 20 percent lipid, and is much less permeable. Scattered along the inner membrane are the proteins of the electron transport chain and
The space enclosed by the inner membrane is called the matrix. Within this space are 5 to 10 identical copies of mitochondrial DNA (mtDNA), a double-stranded circular genome containing 16,569 base pairs. Ribosomes, tRNA, and enzymes involved in the oxidation of pyruvate, fatty acids (e.g., β-oxidation), and the Krebs cycle are also found here.
the FoF1 ATPases involved in oxidative phosphorylation and the subsequent synthesis of ATP. Between these two membranes lies the intermembrane space.
Outer membrane Inner membrane
Intermembrane space Matrix
Electron transport chain FoF1 ATPase
space, is continuous with the lumen of the RER.
The nuclear envelope is perforated with about 4,000 nuclear pores, each having an outer diameter of about 90 nm. Each pore is filled in with a doughnut- shaped annular material that limits the internal diameter to about 9 nm and allows for selective diffusion between the cytoplasm and nucleoplasm.
Macromolecules, like the ribosomal subunits and DNA polymerase, are actively transported across the pores.
Nucleolus
Pores
Chromatin Nucleoplasm
Nuclear envelope
Ribosomes
Peroxisomes
Peroxisomes (i.e., microbodies) have a single lipid bilayer, contain oxidative enzymes, and are found in almost all eukaryotic cells. Oxidases in the organelle can utilize oxygen as an electron acceptor to oxidize organic molecules, producing the toxic substance hydrogen peroxide (H2O2) as a by-product: R-H2 + O2→ R + H2O2. The peroxide that is produced is quickly converted into water and oxygen by catalase: 2H2O2 → 2H2O + O2. As self- replicating organelles, peroxisomes are also involved in the breakdown of fatty acids by the process of β-oxidation.
Plasma (Cell) Membrane
Membranes contain lipids, proteins, and carbohydrates. They organize biological processes by compartmentalizing them.
Lipids are soluble in organic solvents like methanol, but essentially insoluble in water.
The major classes of lipids include fatty acids, triacylglycerols, glycerolphospholipids, sphingolipids, and cholesterol.
Fatty Acids are long-chain hydrocarbons attached to a carboxylic acid. In animal cells, the most common fatty acids are those with a length of 16 or 18 carbon atoms.
Palmitic Acid (C16 saturated fatty acid: CH3(CH2)14COOH)
Oleic Acid (C18 unsaturated fatty acid: CH3(CH2)7CH=CH(CH2)7COOH)
Melting Points of saturated fatty acids increase with molecular mass, because there is little steric interference between adjacent methylene groups and because of the increased van der Waals interactions between those groups.
Melting Points of unsaturated fatty acids (which are primarily in the cis configuration) decrease as the number of double bonds increase. This is due to reduced van der Waals interactions.
Triacylglycerols (aka triglycerides) are synthesized in fat cells (adipocytes). These molecules function as energy stores and are the most abundant class of lipids in animals. They are not components of biological membranes.
Fats are stored in an anhydrous form and provide more energy upon oxidation than carbohydrates or proteins.
Men have a fat content of about 20% and women about 25%. These fat stores can fend off starvation for about 2 to 3 months. The body’s reserves of glycogen, however,
last just a day or two. Triacylglycerol
Glycerolphospholipids (aka phosphoglycerides) are the primary lipid components of biological membranes. They are amphiphilic, having a nonpolar hydrocarbon
Glycerophospholipid
Sphingolipids are also a major component of membranes.
They are based on sphingosine, an amino alcohol. Ceramide, a precursor of all sphingolipids, is produced by attaching a fatty acyl group (-COCH2(CH)15CH3) to the amino group of sphingosine. Attachment of an R group to the C-1 hydroxyl of ceramide gives one of three types of sphingolipids:
tail and a polar phosphoryl head.The X-moiety on the head group can be -H, -CH2CH2NH3+, -CH2CH2N(CH3)3+, or a number of other polar alcohols.
Sphingosine
Sphingomyelins (R = phosphate and then choline) Cerebrosides (R = D-galactose or D-glucose)
Gangliosides (R = several sugar residues and sialic acid)
H3N CH2
C HO
H C C H C
H
(CH2)12 CH3
+
H HO
R group attaches here
O C R3
O O C R2
O
H2C H2C HC
O C R1 O
O P O O O C R2
O
H2C H2C HC
O C R1
O
X O
2 1
3
Proteins are a linear polymer of amino acids linked together by peptide bonds and then folded into a globular shape. Proteins have a wide variety of functions, ranging from an enzyme catalyzing a metabolic reaction to an actin filament lending support to a cell’s cytoskeleton.
pKa values are important for acid-base equilibria in biochemistry, for the stability of proteins, and for the activity of enzymes. The larger the pKa value, the less likely a proton (H+) is to dissociate from an acid (HA). Since the pKa values of the standard 20 amino acids range between about 2 and 13, they are considered to be weak acids. At a certain pH in solution, referred to as the isoelectric point (pI), an amino acid bears no overall net charge. This species is called the zwitterion.
Isoelectric points can be used to separate proteins on a polyacrylamide gel. Isoelectric focusing takes advantage of the fact that the pH of a protein’s surroundings changes its charge. If the pH of the gel is below the pI of the protein, the protein bears apositive charge and migrates to the cathode.
Amino acids have a backbone that is made up of an α-amino and an α-carboxyl group, each with a specific pKa value. Except for glycine, the α-carbon of the standard 22 amino acids is chiral and can exist as either the D or L optical isomer.
In the vast majority of proteins, however, the L-amino acid predominates. In terms of absolute stereochemistry, almost all of the α-carbons in the amino acids of a protein are designated by an S stereocenter. Glycine is achiral and does not have a designation. Cysteine has a (heavy) sulfur atom in its side chain, so its α-carbon is designated as R. Amino acids can be classified by the side chain attached to the α-carbon atom. Some are acidic, while others are basic; some are polar, while others are nonpolar. (Note: Selenocysteine (U) and pyrrolysine (O) are recent additions.) Cholesterol is an important structural component of biological membranes in eukaryotic cells. Animal cell plasma membranes contain almost as much cholesterol as phospholipids. Cholesterol has a polar (hydroxyl) head group and a nonpolar tail consisting of four fused rings, two methyl groups, and an aliphatic hydrocarbon moiety. As this sterol intercalates among the phospholipids in the membrane, the polar head groups and the nonpolar tails of the phospholipids and cholesterol interact with one another, leading to a decrease in membrane fluidity and making them less vulnerable to phase transitions (e.g., crystallization). Cholesterol is also a precursor to numerous steroids, including aldosterone, cortisol, progesterone, testosterone, and the estrogens. It also leads to the synthesis of bile and vitamin D.
Peptide bonds are formed when the α-carboxyl group of one amino acid reacts with the α-amino group of another amino acid, releasing a water molecule in a condensation reaction. The peptide bond that is formed is also an amide (CONH).
The native conformation is the functional form of a protein; it is determined by four distinct aspects of its structure:
Primary (1º): the linear amino acid sequence.
Secondary (2º): stabilization by hydrogen bonds (α-helix, β-sheet, turns).
Tertiary (3º): relationship of 2º structures to one another (disulfide bonds).
Quaternary (4º): noncovalent arrangement of 3º structures (Hb, pores).
+ H3N C H R2
C O + OH H3N C
H R1
C O + OH
α H3N C
H R1
C + O
α N C
H R2
C O
OH AmideH
Amino acid H2O
H2O
pK1 + pK2
pI = 2
pKa≈ 2.3
SeH
N N
N
OH N
N N
S
HO N
OH N
N HS
OH OH
O O O O O
pKa≈ 9.6
255 g/mol 75 g/mol
Increasing Molecular Weight
Cholesterol
CH3
CH3
sugar is an aldose; if it is a ketone, the sugar is a ketose. Glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose) are the simplest monosaccharides. Glyceraldehyde has a chiral carbon, and therefore exists in two enantiomeric forms that have the absolute configuration (+)-glyceraldehyde (OH on the right) and (-)-glyceraldehyde (OH on the left). Early in the last century the D-L system of stereochemical designation was introduced and uses the (+) and (-) configuration of glyceraldehyde as a standard for all monosaccharides. If the highest- numbered chiral carbon (reference carbon) of a monosaccharide has the same configuration as D-(+)-glyceraldehyde, then it is called a D sugar.
Monosaccharides with the opposite stereochemistry at this center represent an L sugar.
Since the OH group on the highest-numbered chiral carbon in the Fisher projection of glucose points to the right, that monosaccharide is a D sugar. An intramolecular reaction of the C-5 hydroxyl group with the aldehyde group of D-glucose creates a new chiral center at C-1 and gives two cyclic hemiacetals (shown in the Haworth projections).
These diastereomers are designated as an α anomer or a β anomer, and C-1 is called the anomeric carbon.
Monosaccharides have the general formula (CH2O)n, where n is an interger from 3 (triose) to 7 (heptose), and they have either an aldehyde or a ketone and two or more hydroxyl groups. If the carbonyl group on a monosaccharide is an aldehyde, the Carbohydrates are hydrates of carbon. The most common forms are sugars, starches, and fibers. The basic building block of every carbohydrate is a sugar molecule (i.e., a monosaccharide). Starches and fibers are composed of chains of monosaccharides held together in specific linkages to create polysaccharides (or oligosaccharides).
Proteins can be associated with membranes in a variety of ways. Transmembrane proteins are amphipathic and extend through the lipid bilayer. The hydrophobic regions of these proteins interact with the hydrophobic tails of the bilayer, while their hydrophilic regions interact with the water on either side of the bilayer. Proteins that do not span the membrane but rather are attached to either face of the bilayer are referred to as peripheral membrane proteins. Some of these proteins can be released from the face of the bilayer by a variety of extraction methods, including exposure to changes in pH or variable ionic solutions. If a protein cannot be removed from a bilayer without damaging the membrane, it is referred to as an integral membrane protein.
Cytosol Extracellular
space
Lipid bilayer Integral membrane
proteins
Peripheral membrane protein
H C C O
H
Glyceraldehyde OH CH2OH Chiral
carbon
CH2OH C
Dihydroxyacetone O CH2OH
Highest-numbered chiral carbon (Reference carbon) H C
D-Glucose OH CH2OH
H C O
H C OH HO C H
H C OH 1 2
Disaccharides like sucrose (glucose-fructose), maltose (glucose-glucose), and lactose (galactose-glucose) undergo acid-catalyzed hydrolysis to give their respective monosaccharide components. The enzymes sucrase, maltase, and lactase hydrolyze the O-glycosidic linkages in these disaccharides to give the same results.
3 4 5 6
Anomeric carbon
O HOCH2
OH
OH HO OH
1 6
2 3 4
5
β anomer
Ribosomes
Ribosomes are found in the cytoplasm of both prokaryotic and eukaryotic cells. They are also found in the matrix of the mitochondrion and bound to the membrane of the RER.
These organelles are oblate spheroid granules about 25 nm in diameter. They are composed of rRNA and protein and are the sites of translation (protein synthesis).
Prokaryotic ribosomes have a sedimentation coefficient of 70S and contain about two-thirds rRNA and one-third protein. The sedimentation coefficients of the large subunit and small subunit are 50S and 30S, respectively. A bacterial cell like E. coli is about 500 nm by 1,500 nm and contains about 20,000 ribosomes.
Svedberg (S) is a unit that describes how fast a molecule sediments in a density gradient of CsCl in an ultracentrifuge. Molecules suspended in this rotating gradient experience a centrifugal force balanced by a frictional force. The value is expressed in terms of time, where 1S = 10-13 seconds. The larger the value of S, the larger the size of the molecule. Sedimentation coefficients are not additive.
Eukaryotic ribosomes have a sedimentation coefficient of 80S. The sedimentation coefficients of the large subunit and small subunit are 60S and 40S, respectively.
The production of eukaryotic ribosomes begins in the nucleolus, continues in the nucleus, and is finalized in the cytoplasm with the joining of the large and small subunits during protein synthesis. A mammalian cell contains millions of ribosomes.
Ribosomes that participate in protein synthesis are made of a large and a small subunit of unequal size. The formation of a peptide bond involves three stages: initiation, elongation, and termination. Initiation involves the binding of mRNA to the small subunit. Once the mRNA stand is in position, an initiator tRNA with its amino acid binds to the P (peptidyl) site. The large subunit joins the small subunit and elongation begins with the binding of a new tRNA with its attached amino acid to the A (aminoacyl) site. As a peptide bond is being formed between the two amino acids, the ester linkage of the initiator tRNA with its amino acid is being hydrolyzed. The initiator tRNA moves into the E (exit) site before leaving the ribosome. The newly formed dipeptide (attached to the second tRNA) moves into the P site. This translocation frees up the A site for an incoming tRNA with its amino acid. Protein synthesis is an expensive process as each peptide bond costs about four phosphodiester bonds. Termination of translation occurs when a stop codon (e.g., UGA) on the mRNA is read by a release factor. The large and small ribosomal subunits dissociate, and the newly formed polypeptide is released into the cytoplasm.
5’ 3’
P site A site
5’ 3’
AA1
Small subunit
Large subunit
tRNA AA1
tRNA-AA2
AA2
E site
5’ 3’
AA1
HO
Peptide bond
Peptide formationbond
Translocation
mRNA
AA2 β-(1→4)
O-glycosidic linkage
4 1 OH CH2OH O
H H OH H
HO H
H CH2OH O H H HO OH
O OH
H H H
Lactose
D-glucose D-galactose
4 1
D-galactose
OH CH2OH O H H
OH H
HO H H
4 1 OH
D-glucose
4H CH2OH O 1 H H HO OH
OH H H
HO H3O+ or
β-D-galactosidase (intestinal lactase)
β anomer
Vesicles
Proteins synthesized by the RER enter the Golgi apparatus at its cis face. As these proteins move toward the trans face, they are modified in a stepwise fashion, eventually becoming mature proteins that are sorted and distributed throughout the cell. The structures in which these proteins are transported between the RER, the Golgi apparatus, and their final destinations within the cell are referred to as vesicles. They are distinguished from one another by a protein coat that surrounds their single lipid bilayer. Vesicles that transport proteins to the plasma membrane are initially coated with clathrin, a three-legged protein that forms a polyhedral cage around the vesicle as it develops from the Golgi apparatus.
It seems that the formation of the clathrin coat helps the vesicle bud out from the Golgi’s membrane. Soon after the clathrin-coated vesicle (CCV) buds off the membrane, the clathrin is released and recycled. Vesicles that travel long distances to the plasma membrane are transported along a highway of microtubules by an ATP-driven mechanism involving dynein and kinesin. Dynein moves cargo toward the minus-end of a microtubule, located near the center of a cell, while kinesin moves cargo toward the plus-end of a microtubule, located toward the periphery of a cell. CCVs not only participate in excotysosis (e.g., the release of neurotransmitters at the synaptic cleft), they also engulf certain items from the extracellular matrix by endocytosis.
Vacuoles
Vacuoles are present in some eukaryotic animal and prokaryotic cells. These membrane- bound organelles act as storage compartments for metabolites and function in exocytosis and endocytosis. In animal cells they are thought to originate from the Golgi apparatus through cytoplasmic vesicle fusion. Other possible origins include pathways from the endoplasmic reticulum, the nuclear membrane, and even the plasma membrane. Vacuoles are filled with water rich in inorganic substances, giving it a higher ionic concentration than in the cytoplasm. Membrane-bound H+-ATPases help to acidify a wide variety of intracellular organelles, including vacuoles.
Centrioles
Endoplasmic
reticulum Golgi
apparatus
Lysosome
Mitochondrion Nucleolus
Nucleus
Plasma membrane
Ribosome Vesicle
Eukaryotic (Animal) Cell Summary
Only the highlights of the organelles just discussed are shown in the diagram below.
Transmembrane protein Peripheral
protein
Polypeptide Trans face Cis face
Protein channel
