Cell Biology Summary by Joana Costa Campos
Cell Biology Summary by Joana Costa Campos
Cell Biology Summary by Joana Costa Campos
Homework Compilation
• Chapter 4 – Mitochondria
§ Terms: semiautonomous organelle, ATP Synthase Complex, TIM, TOM
1- Please describe the structure characteristics of mitochondria. How can the mitochondria make
ATP?
2- Please describe the characteristics of mitochondrial genome
• Conclusion
Cell Biology: Cell Biology is a subject to study cell structures (including cell membrane,
different organelles and other functional structures), functions, and behaviours (activities), such
as proliferation, differentiation, response, metabolism, movement, senescence, and cell death
(birth, aging, illness and dead). Cell biology study cells at three main types of levels, Molecular
biology level, Light Microscope and Electron Microscope level.
1- Collagen
2- Elastin 14- Cytoplasm
2- Elastin: is a key extracellular matrix (ECM) protein that provides resilience and elasticity
to tissues and organs. Elastin is roughly 1000 times more flexible than collagens; thus, the main
function of elastin is the elasticity of tissues.
3- Proteoglycan complex: The major biological function of proteoglycans derives from the
physicochemical characteristics of the glycosaminoglycan component of the molecule,
which provides hydration and swelling pressure to the tissue enabling it to withstand
compressional forces. Example, hyaluronic acid.
5- Laminin: In both developing and intact tissues, laminins are incorporated into
basement membranes, which separate parenchymal cells from the connective tissue. Laminins
play important roles in tissue morphogenesis and homeostasis by regulating tissue
architecture, cell adhesion, migration and matrix-mediated signaling.
8- Peroxisome: are organelles that sequester diverse oxidative reactions and play
important roles in metabolism, reactive oxygen species detoxification, and signaling.
10- Nucleus (the structure), Nuclear Envelope (the purple layer): The nucleus controls and
regulates the activities of the cell (e.g., growth and metabolism) and carries the genes,
structures that contain the hereditary information. The nuclear envelope separates the contents
of the nucleus from the cytoplasm and provides the structural framework of the nucleus. The
nuclear membranes, acting as barriers that prevent the free passage of molecules between the
nucleus and the cytoplasm, maintain the nucleus as a distinct biochemical compartment.
11- Nuclear Pore: is a large complex of proteins that allows small molecules and ions to
freely pass, or diffuse, into or out of the nucleus. Nuclear pores also allow necessary proteins to
enter the nucleus from the cytoplasm if the proteins have special sequences that indicate they
belong in the nucleus.
12- Rough Endoplasmic Reticulum (the structure), Ribosome (green spots): The rough
endoplasmic reticulum is a membranous organelle that functions to produce proteins with the
help of ribosomes on the membrane surface. A ribosome functions as a micro-machine for
making proteins. Ribosomes are composed of special proteins and nucleic acids. The translation
of information and the linking of amino acids are at the heart of the protein production process.
14- Cytoplasm: is the gel-like fluid inside the cell. It is the medium for chemical reaction. It
provides a platform upon which other organelles can operate within the cell. All of the functions
for cell expansion, growth and replication are carried out in the cytoplasm of a cell.
15- Golgi Complex: functions as a factory in which proteins received from the ER are
further processed and sorted for transport to their eventual destinations: lysosomes, the plasma
membrane, or secretion. In addition, as noted earlier, glycolipids and sphingomyelin are
synthesized within the Golgi.
16- Vacuole: is a membrane-bound cell organelle. In animal cells, vacuoles are generally
small and help sequester waste products. In plant cells, vacuoles help maintain water balance.
Sometimes a single vacuole can take up most of the interior space of the plant cell.
17- Cell: the unit base of life, provide structure for the body, take in nutrients from food,
convert those nutrients into energy, and carry out specialized functions. Cells also contain the
body's hereditary material and can make copies of themselves. Cells have many parts, each with
a different function.
18- Microfilaments: are the smallest filaments of the cytoskeleton. They have roles in cell
movement, muscle contraction, and cell division.
19- Secretory vesicle: play an important role in moving molecules outside of the cell,
through a process called exocytosis. They are crucial for healthy organ and tissue function. For
example, secretory vesicles in the stomach will transport protein-digesting enzymes to help
break down food.
22- Microtubules: Microtubules, with intermediate filaments and microfilaments, are the
components of the cell skeleton which determinates the shape of a cell. Microtubules are
involved in different functions including the assembly of mitotic spindle, in dividing cells, or
axon extension, in neurons.
23- Centrioles: are paired barrel-shaped organelles located in the cytoplasm of animal
cells near the nuclear envelope. Centrioles play a role in organizing microtubules that serve as
the cell's skeletal system. They help determine the locations of the nucleus and other organelles
within the cell.
24- Chromatin: Chromatin fibers are coiled and condensed to form chromosomes.
Chromatin makes it possible for a number of cell processes to occur including DNA replication,
transcription, DNA repair, genetic recombination, and cell division.
25- Nucleolus: The primary function of the nucleolus consists in ribosomal RNA (rRNA)
transcription, rRNA processing and ribosome subunit assembly
26- Extracellular Matrix: it helps cells attach to, and communicate with, nearby cells, and
plays an important role in cell growth, cell movement, and other cell functions. The extracellular
matrix is also involved in repairing damaged tissue.
Liquid crystal state: are a state of matter which has properties between those of
conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a
liquid, but its molecules may be oriented in a crystal-like way.
Unit Membrane: a lipoprotein membrane which encloses many cells and cell organelles
and is composed of two electron-dense layers enclosing a less dense layer. The limiting
membrane of cells and various organelles viewed formerly as a 3- layered structure with an
inner lipid layer and two outer protein layers and currently as a fluid phospholipid bilayer with
intercalating proteins.
Passive Transport: transporter mediated transportation has two types, one is passive
transport, to transport materials from high concentration to low concentration, without using
energy. Passive transport including carrier protein mediated facilitated diffusion and ion
channel mediated ion transporting, carrier protein facilitated diffusion, channel protein
regulated ion transportation. Passive transport does not require energy input. An example of
passive transport is diffusion, the movement of molecules from an area of high concentration
to an area of low concentration. Carrier proteins and channel proteins are involved in
facilitated diffusion.
As the membrane lipid bilayer is a two-dimensional fluid (Liquid crystal state), we can
considerer two important characteristics, Asymmetry and Fluidity
The Asymmetry includes the asymmetry distribution of lipids, proteins and sugar chains
in the cell membrane
The fluidity of the membrane it’s important to guarantee its biological functions. The
fluidity depends on the liquid-crystal state, on the different movements of the lipids that
composed the membrane, and also by the membrane proteins (which have different
movements like lateral diffusion and rotation).
Passive transport is a type of cellular transport in which substances such as ions and
molecules move down their respective concentration gradients. It means that the substance
tends to move from an area of higher concentration to an area of lower concentration.
Substances tend to move towards the region where they are few. Since their movement
is downhill or along their concentration gradient, the process does not require metabolic
energy (ATP) as opposed to active transport, another type of cellular transport that essentially
requires ATP to move substances against their concentration gradient. On this type of
transportation can be or not carrier by protein mediated.
Passive transport including carrier protein mediated, facilitated diffusion and ion channel
mediated ion transporting. The rate of passive transport depends on the permeability of the cell
membrane, which, in turn, depends on the organization and characteristics of the membrane
lipids and proteins.
The four main kinds of passive transport are simple diffusion, facilitated diffusion,
filtration, and osmosis.
Simple Diffusion is the movement of substances from a region of higher concentration to
lower concentration. The difference in the concentration of the two areas is termed as
concentration gradient and the process of diffusion continues until this gradient
neutralizes. Diffusion occurs in liquid and gases because their particles move randomly from
one place to another. It is an important process in living things required for different life
processes. The substances move in and out of the cells by simple diffusion.
Facilitated diffusion is the passive transportation of ions or molecules across the cell
membrane through specific transmembrane integral proteins. The molecules, which are large
and insoluble require a carrier substance for their transportation through the plasma
membrane. This process does not require any cellular or external energy. As an example of
facilitated diffusion, we have the process when glucose is absorbed into the cells by a Glucose
transporter (GLUT4), or also other ion channels and aquaporins.
The cell membrane is permeable only to a few molecules that are smaller in size and non-
polar. Therefore, facilitated diffusion with the help of transmembrane proteins is important.
Filtration is the process of separating solids from liquids and gases. The selective
absorption of nutrients in the body is an example of filtration. This process does not require any
energy and takes place along the concentration gradient. The kidneys are an example of a
biological filter. The blood is filtered by the glomerulus and the necessary molecules are
Student: Joana Costa Campos
Nº: 202116410026
Grade: 2021
reabsorbed. In the process of filtration, the cell membrane permits only those substances which
are soluble and could easily pass through its pores.
In the process of osmosis, water and other molecules pass through a selectively
permeable membrane in order to balance the concentration of other substances.
Osmosis is affected by the concentration gradient and temperature. The greater the
concentration gradient, the faster is the rate of osmosis. Also, the rate of osmosis increases with
the increase in temperature.
There is a theory of conflict about the process of osmosis. Few biologists suggest that
osmosis is an active transport and not passive transport.
Summarising, Simple and facilitated diffusions refer to the net movement of molecules
from higher to lower concentrations. Osmosis refers to the diffusion of a solvent (usually water
molecules) through a semipermeable membrane from lower to higher solute concentrations.
Filtration is the movement of water and solute molecules across the cell membrane driven by
hydrostatic pressure that is generated by the cardiovascular system.
Besides the examples that I gave above, this ones are more simple to understand the
passive transportation:
o Ethanol enters our body and hits the bloodstream. This happens because the ethanol
molecules undergo simple diffusion and pass through the cell membrane without any
external energy.
o Reabsorption of nutrients by the intestines by separating them from the solid waste and
transporting the nutrients through the intestinal membrane into the bloodstream.
o When a raisin is soaked in water the water moves inside the raisin by the process of
osmosis and it swells.
But sometimes our body needs to move molecules against their gradient. In this case, we
have the active transportation, which transport materials from low concentration to high
concentration, by using biological energy (ATP). In another words, Active transport is the
movement of all types of molecules across a cell membrane against its concentration gradient.
Active transport uses cellular energy, unlike passive transport, which does not use cellular
energy.
The definitions of Antiport, Symport and cotransport were given anteriorly on the terms
designations part, and so, I will skip that explanation.
There are 3 main types of active transport include different kind ion pumps and two bulk
mechanisms (vesicle transport), exocytosis and endocytosis.
For the ion pumps, the Ion pump hydrolyze ATP for active transport including P-class ion
pump, V-class proton pump, F-class proton pump, ABC transport. However, there is a very
classic example, the sodium-potassium pump, which moves both types of ions against the
concentration gradient.
Basically, the way one of these sodium-potassium pumps work is that 3 sodium ions from
inside the cell bind to a carrier protein in the cell membrane. Then, an ATP molecule is broken
down by an enzyme called sodium-potassium-ATPase, releasing energy and causing the
protein to change shape, pushing the sodium ions out of the cell.
Exocytosis is when a transport vesicle inside the cell fuses with the cell membrane and
then whatever is inside the transport vesicle can be released out into the extracellular
fluid. There is three types of endocytosis, phagocytosis, pinocytosis, and receptor-mediated
endocytosis.
When the receptors bind to their specific target molecule, endocytosis is triggered, and
the receptors and their attached molecules are taken into the cell in a vesicle. The coat proteins
participate in this process by giving the vesicle its rounded shape and helping it bud off from
the membrane. Receptor-mediated endocytosis allows cells to take up large amounts of
molecules that are relatively rare (present in low concentrations) in the extracellular fluid.
Signal peptide hypothesis: The signal hypothesis proposes that proteins destined for
secretion, which involves the movement of the protein across a biological membrane, are
originally manufactured with an initial sequence of amino acids that may or may not present in
the mature protein.
The site of synthesis of a protein is determined by the sequence of amino acids in the N-
terminal portion of the polypeptide, which is the first part to emerge from the ribosome during
protein synthesis (Sabatini & Dobberstein, 1970).
1. Secretory proteins contain a signal sequence at their N- terminus that directs the
emerging polypeptide and ribosome to the ER membrane.
2. The polypeptide moves into the cisternal space of the ER through a protein lined,
aqueous channel in the ER membrane. It was proposed that the polypeptide moves through the
membrane as it is being synthesized, that is, co-translationally.
A hypothesis to explain how ribosomes become attached to membranes within cells in
order to deliver the appropriate proteins to cell organelles, such as mitochondria and
chloroplasts, or transport proteins outside the cell membrane. It proposes that the leading end
of the nascent polypeptide chain consists of a signal peptide. This sticks out from the ribosome
and is recognized by a ribonucleoprotein particle called a signal recognition particle (SRP).
When the complex of ribosome and SRP encounters a membrane, the SRP binds to a docking
protein (signal recognition particle receptor) on the membrane surface. Synthesis of the
polypeptide, which has hitherto been stalled, now resumes, and the polypeptide (or fully formed
protein) passes into the membrane, where the signal peptide is removed by a signal peptidase
enzyme. Once translation is completed, the ribosome dissociates and is freed from the
membrane. It is thought that the signal sequence tags the protein for insertion at particular sites,
by interacting with membrane-bound glycoproteins (signal sequence receptors). If the signal
sequence is not the correct one, the ribosome is released before delivering its protein. The
hypothesis, which was formulated in the early 1970s by workers including Gunter Blobel and
César Milstein, is now widely accepted.
GOLGY COMPLEX
Student: Joana Costa Campos
Nº: 202116410026
Grade: 2021
1- What is the characteristics of Golgi Complex in morphology, what is the
function of Golgi Complex?
The Golgi complex, also known as a Golgi apparatus, is a cell organelle that helps process
and package proteins and lipid molecules, especially proteins destined to be exported from the
cell. Named after its discoverer, Camillo Golgi, the Golgi body appears as a series of stacked
membranes.
The Golgi apparatus is a series of stacked membranes that are located within the
cytoplasm (gel-like fluid held in the cell membrane) in all eukaryotic cells (complex cells). We
can divide the membrane system into three parts, cis Golgi network, Golgi stack (composed by
three cisternas, cis, medial and trans), and trans Golgi network. It can typically be found
adjacent to the nucleus and rough endoplasmic reticulum. Its chemical components are 40%
lipids, 60% proteins. We can also find many kind of enzymes (glycosyltransferase, which is the
marker enzyme, phospholipase , oxidoreductase) and sugar chains (increasing from the cis to
trans part of the complex).
The Golgi Complex has several functions:
o A transition station for endomembrane protein transportation
o Golgi Complex provide the sites for protein and lipid modification (glycoprotein
glycosylation, glycolipid glycosylation, protein glycosylation)
o the place for Glycolipid formation and modification (the sugar chain of glycolipid is
added in Golgi complex)
o A place for protein hydrolysis or cutting (maturation)
o Protein sorting (the process of lysosomal enzyme protein formation, the
membrane proteins are also from ER and Golgi)
o an important site for membrane lipid transportation and renewing
In Golgi complex the N-linked glycosylation initially occurs in the cis Golgi part, to after
passing through all the parts of this complex. The O-glycosylation occurs post-translationally
on serine and threonine side chains in the Golgi apparatus.
Lysosomal enzymes are recognized by an enzyme in the cis cisternae that transfers a
phosphorylated N -acetylglucosamine from a nucleotide sugar donor to one or more mannose
residues of N -linked oligosaccharides. The glucosamine moiety is then removed in a second
step by a second enzyme, leaving mannose 6-phosphate residues as part of the oligosaccharide
chain.
Student: Joana Costa Campos
Nº: 202116410026
Grade: 2021
Schematic diagram showing the pathways followed by a lysosomal enzyme (black) from
its site of synthesis in the ER to its delivery to a lysosome. The mannose residues of the
lysosomal enzyme are phosphorylated in the Golgi cisternae and then selectively incorporated
into a clathrin- coated vesicle at the TGN.
The mannose 6-phosphate receptors are thought to have a dual role, they interact
specifically with the lysosomal enzymes on the luminal side of the vesicle, and they interact
specifically with adaptors on the cytosolic surface of the vesicle. The mannose 6-phosphate
receptors separate from the enzymes and are returned to the Golgi complex. The lysosomal
enzymes are delivered to an endosome and eventually to a lysosome. Mannose 6-phosphate
receptors are also present in the plasma membrane, where they capture lysosomal enzymes
that are secreted into the extracellular space and return the enzymes to a pathway that directs
them to a lysosome.
Tertiary Lysosome: are those lysosomes in which only indigestible food materials have
been left, so is a state of post enzymatic digestion. In this stage the enzyme activity is almost
exhausted. The primary role of lysosome is to process the “garbage” inside the cell for recycle.
Intracellular digestion results in complete breakdown of the phagocytosed material or
indigestible residues that persist in the form of residual bodies. Residual bodies sometimes
consist of tightly packed membranes (indicated by arrows), reflecting the difficulty lysosomal
enzymes have with lipid digestion. The residual body in this micrograph is the end result
of autophagy rather than phagocytosis. The swirls of lipid are the remains of the organelle
membranes. All cells use lysosomes for the removal of cellular components as they undergo
routine turnover. Because lysosome is able to clean heterophogosome, it also has the function
to protect the cell. Besides, lysosome involves thyroid hormone formation and development
during acrosomal reaction.
COPII coated vesicles: a type of coated vesicle which is formed in Endoplasmic Reticulum
that generally move from ER to Golgi Complex.
COPI coated vesicles: a type of coated vesicle which are used between parts of the Golgi
apparatus as well as to form vesicles going from the Golgi back to the ER, in order to sending
back the escape retention proteins to ER.
Clathrin coated vesicles: a type of coated vesicle that can be formed in the trans Golgi
network of Golgi complex and also formed from the plasma membrane for endocytosis.
Vesicle transport is very important for cell’s correct functions and life. Starting point that
the vesicle transport is a basic way for intercellular materials exchange. Proteins and membrane
lipids are transported by vesicles. Therefore, we can verify that the membrane lipid
transportation depends on this process. But, for proteins transportation is a little bit more
complex because the wrong “delivery” of proteins in our body can cause severe disease. That’s
the reason why this process is assured and dependent on signal peptide, to in order to mark the
target sequency, and the vesicle is an enclosed transportation.
In the same order, transportation is highly ordered and strictly controlled logistic
transportation, to assure the right orientation of the transportation. The targeted proteins for
transportation is selected and controlled. This one is a closed vehicle transportation. As an
example we have the role of the chaperones in proteins maturation.
The vesicle transportation is also important due the fact that there is a specific recognition
and induced fusion guarantees the unloading properly. Related to the membrane proteins
recognition, to the V-SNARE and T-SNARE process (the words SNARE means soluble N-ethyl
maleimide-sensitive factor attachment protein receptors). v-SNARE and t-SNARE recognition is
essential for vesicle docking and fusion or unloading
The free Ribosomes can synthetize enzyme proteins and membrane proteins of
peroxisome protein, majority of mitochondrial proteins, cytosol and nucleus proteins. In
general, free Ribosomes synthesize proteins that remain in the cell, such as hemoglobin in red
blood cells or contractile proteins in muscle cells. Polyribosomes attached to the endoplasmic
reticulum synthesize proteins for export from the cell (example, digestive enzymes) and for
placement within the plasma membrane (example, ion channel proteins).
In the attached ribosomes the proteins synthetized are membranes proteins, secretion
proteins, retention proteins, and lysosome enzyme proteins and membrane proteins.
Attached ribosomes produce proteins which are exported from the cell to the outside.
These proteins include digestive enzymes, polypeptide hormones, cell surface receptors, cell
signaling molecules, etc. These proteins are secreted from the cell using secretory vesicles.
The table below describes the main difference in this two types of Ribosomes.
ATP Synthase Complex: ATP synthase is a protein that catalyses the formation of the
energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP)
and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation
of P-O bond (phosphodiester bond). The formation of ATP from ADP and Pi is energetically
unfavourable and would normally proceed in the reverse direction. In order to drive this
reaction forward, ATP synthase couples ATP synthesis during cellular respiration to an
electrochemical gradient created by the difference in proton (H+) concentration across the
inner mitochondrial membrane in eukaryotes or the plasma membrane in bacteria. During
photosynthesis in plants, ATP is synthesized by ATP synthase using a proton gradient created
in the thylakoid lumen through the thylakoid membrane and into the chloroplast stroma.
In mitochondria we need to consider two important steps, the Citric Acid Cycle and the
Oxidative Phosphorylation but to master the ATP production we should start from the early
beginning.
The APT production, also named cellular respiration process, starts outside of the
mitochondria, in the cytosol. The glycolysis is the process where the simple sugar is broken
down to for Pyruvate.
During these two processes 36 ATP molecules are produced. So, all together there are 38
molecules of ATP produced in aerobic respiration plus 2 ATP are formed outside the
mitochondria.
There are two basic types of microtubule motors, plus-end motors and minus-end motors,
depending on the direction in which they "walk" along the microtubule cables within the cell.
The best prominent example of a motor protein is the muscle protein myosin which
"motors" the contraction of muscle fibers in animals. Motor proteins are the driving force behind
most active transport of proteins and vesicles in the cytoplasm. Kinesins and cytoplasmic
dyneins play essential roles in intracellular transport such as axonal transport and in the
formation of the spindle apparatus and the separation of the chromosomes during mitosis and
meiosis. Axonemal dynein, found in cilia and flagella, is crucial to cell motility, for example in
spermatozoa, and fluid transport, for example in trachea.
The primary function of intermediate filaments is to create cell cohesion and prevent the
acute fracture of epithelial cell sheets under tension. This is made possible by extensive
interactions between the constituent protofilaments of an intermediate filament, which
enhance its resistance to compression, twisting, stretching and bending forces. These
properties also allow intermediate filaments to help stabilize the extended axons of nerve cells,
as well as line the inner face of the nuclear envelope where they help harness and protect the
cell’s DNA.
General, it as a diameter of approximately 10 nm, which is intermediate between that of
microtubules and microfilaments. It has no polar. It is comparatively tough, not easily be
depolymerized. Sometimes, some special members, phosphorylation will make it
depolymerized, such as lamina in nucleus.
Functions of Microfilaments
Form a band just beneath the plasma membrane that:
1. To provides mechanical strength to the cell cell-cell connection: anchored junction
anchors the centrosomes at opposite poles of the cell during mitosis
2. Cell movement
3. muscle movement: interact with myosin ("thick") filaments in skeletal muscle fibers
to provide the force of muscular contraction
Therefore, to summarise its functions, the nucleolus is the site of ribosomal RNA synthesis
and ribosome assembly. As rDNA being transcripted into rRNA, Ribosomal proteins are imported
from the cytoplasm and conjugated with ribosomal RNA to form the ribosomal subunits which
then pass through the nuclear pore complex into the cytoplasm before being assembled into
fully active ribosomes.
Euchromatin is defined as the area of the chromosome which is rich in gene concentration
and actively participates in the transcription process. It is scattered throughout the nucleus and
not visible under light microscope, which is the active form of chromatin where the genetic
material of the DNA molecules is being transcribed into RNA.
These are loosely packed form of chromatin. These are active during transcription. It
contains 90% of the entire human genome. Housekeeping genes are one of the forms of
euchromatin.
Sex Chromatin is the number of chromosomes in somatic cells is specific for the species
and is called the Genome, the total genetic makeup. In humans the genome is made up of 46
chromosomes representing 23 homologous pairs of chromosomes. Of the 23 pairs 22 are called
autosomes, whereas the remaining pair that determines gender is the sex chromosomes.
Barr Body: microscopic study of interphase nuclei of cells from female displays a very
tightly coiled clump of chromatin, the sex chromatin (Barr Body), the inactive counterpart of the
two X Chromosomes. It is a facultative heterochromatin, which is one of the two X
chromosomes becoming condensed randomly in female cells.
From here, we can notice three distinct nucleolar regions that can be distinguished
morphologically. The bulk of the nucleolus consists of a granular component (gc), which
contains ribosomal subunits in various stages of assembly.
Embedded within the granular regions are fibrillar centers (fc) that are surrounded by a
denser fibrillar component (dfc).
The inset shows a schematic drawing of these parts of the nucleolus. According to one
model, the fc contains the DNA that codes for ribosomal RNA, and the dfc contains the nascent
pre-rRNA transcripts and associated proteins. According to this model, transcription of the pre-
rRNA precursor takes place at the border between the fc and dfc.
Summarizing, under electron microscope we can observe:
o Fibrillar center (FC): containing rRNA chromatin , which composes the
nucleolus center
o Dense fibrillar component (DFC): containing nucleolar RNAs being transcribed
o Granular component (GC): in which maturing ribosomal subunits are assembled
o The nucleolar matrix: a network of fibers that participates in nucleolar
organization