Cell structure

AS- Level Biology
Unit-1
Cell Structure
By- Suman Tiwari

What is Cell?
• The cell (from Latin cella, meaning "small
room") is the basic structural, functional,
and biological unit of all known organisms.
A cell is the smallest unit of life. Cells are
often called the "building blocks of life".
The study of cells is called cell biology,
cellular biology, or cytology.
• Cells were discovered by Robert Hooke in
1665

• Cell theory:
•first developed in 1839 by Matthias Jakob
Schleiden and Theodor Schwann, states:
•All organisms are composed of one or more
cells, that cells are the fundamental unit of
structure and function in all living organisms.
•All cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years
ago
Cell Theory

Organelles are parts of the cell which are adapted
and/or specialized for carrying out one or more
vital functions, analogous to the organs of the
human body (such as the heart, lung, and
kidney, with each organ performing a different
function).Both eukaryotic and prokaryotic cells
have organelles, but prokaryotic organelles are
generally simpler and are not membrane-bound.
Organelles

•There are several types of organelles in a cell.
Some (such as the nucleus and golgi apparatus)
are typically solitary,
•while others
Such,as mitochondria, chloroplasts, peroxisomes
and lysosomes) can be numerous (hundreds to
thousands).
•The cytosol/cytoplasm is the gelatinous fluid
that fills the cell and surrounds the organelles.
Organelles

•The endomembrane system (endo = “within”) is a group
of membranes and organelles in eukaryotic cells that
works together to modify, package, and transport lipids
and proteins. It includes the nuclear envelope,
lysosomes, and vesicles, which we have already
mentioned, and the endoplasmic reticulum and Golgi
apparatus, which we will cover shortly. Although not
technically within the cell, the plasma membrane is
included in the endomembrane system because, as you
will see, it interacts with the other endomembranous
organelles. The endomembrane system does not include
either mitochondria or chloroplast membranes.
Endomembrane
system

Membrane Systems
and organelles of a cell

•Many types of prokaryotic and eukaryotic cells
have a cell wall. The cell wall acts to protect the
cell mechanically and chemically from its
environment, and is an additional layer of
protection to the cell membrane. Different types
of cell have cell walls made up of different
materials; plant cell walls are primarily made up
of cellulose, fungi cell walls are made up
of chitin and bacteria cell walls are made up
of peptidoglycan.
Cell walls and
Plasmodesmata

•It can be tough, flexible, and sometimes rigid. It
provides the cell with both structural support
and protection, and also acts as a filtering
mechanism.
•Cell walls are present in
most prokaryotes (except mollicute bacteria),
in algae, fungi and eukaryotes including plants b
ut are absent in animals. A major function is to
act as pressure vessels, preventing over-
expansion of the cell when water enters.
Cell wall

Cell wall of land plants is composed of the
polysaccharides cellulose, hemicelluloses and
pectin. Often, other polymers such
as lignin, suberin or cutin.
•Lignins are particularly important in the formation of cell
walls, especially in wood and bark, because they lend
rigidity and do not rot easily.
•Suberin is a major constituent of cork. Its main function
is as a barrier to movement of water and solutes.
•Cutin is one of two waxy polymers that are the main
components of the plant cuticle, which covers all aerial
surfaces of plants.
Cell wall

•Algae possess cell walls made of glycoproteins
and polysaccharides such
as carrageenan and agar that are absent from
land plants.
•Diatoms are a major group
of algae specifically microalgae, found in the
oceans, waterways and soils of the world. Living
diatoms make up a significant portion of the
Earth's biomass: they generate about 20 to 50
percent of the oxygen produced on the planet.
They have a cell wall composed of biogenic
silica.
Cell wall

•Cells interact though plasmodesmata, which are
inter-connecting channels of cytoplasm that
connect to the protoplasts of adjacent cells
across the cell wall.
• https://www.khanacademy.org/science/biology/structure-of-a-
cell/cytoskeleton-junctions-and-extracellular-structures/v/plant-
cell-walls
• http://www.biology4kids.com/files/cell_membrane.html
Cell wall

•They are present in the cells of plants, fungi and
certain protists,animals and bacteria.
•Vacuoles are essentially enclosed
compartments which are filled with water
containing inorganic and organic molecules
including enzymes in solution, though in certain
cases they may contain solids which have been
engulfed.Vacuoles are formed by the fusion of
multiple membrane vesicles and are effectively
just larger forms of these.
Vacuoles

• They have one large vacuole that typically occupies
more than 30% of the cell's volume, and that can occupy
as much as 80% . A vacuole is surrounded by a
membrane called the tonoplast and filled with cell
sap. Tonoplast also called the vacuolar membrane. The
main role of the central vacuole is to
maintain turgor pressure against the cell wall.
•Another function of a central vacuole is that it pushes all
contents of the cell's cytoplasm against the cellular
membrane, and thus keeps the chloroplasts closer to
light.
Vacuoles in Plant

•vacuoles perform mostly subordinate roles, assisting in
larger processes of exocytosis and endocytosis.
•Exocytosis is the extrusion process of proteins and lipids
from the cell. These materials are absorbed into
secretory granules within the Golgi apparatus before
being transported to the cell membrane and secreted
into the extracellular environment.
•Endocytosis is the reverse of exocytosis and can occur in
a variety of forms. Phagocytosis ("cell eating") is the
process by which bacteria, dead tissue, or other bits of
material visible under the microscope are engulfed by
cells.
Salmonella is able to survive and reproduce in the
vacuoles of several mammal species after being
engulfed.
Vacuoles in Animals

•Isolating materials that might be harmful or a
threat to the cell
•Containing waste products
•Containing water in plant cells
•Maintaining internal hydrostatic
pressure or turgor within the cell
(https://microbenotes.com/vacuoles-structure-
types-and-functions/)
Function of Vacuoles

•Maintaining an acidic internal pH
•Exporting unwanted substances from the cell
•Allows plants to support structures such as
leaves and flowers due to the pressure of the
central vacuole
•By increasing in size, allows the germinating
plant or its organs (such as leaves) to grow very
quickly and using up mostly just water.
•In seeds, stored proteins needed for germination
are kept in 'protein bodies', which are modified
vacuoles.
Function of Vacuoles

•Chloroplasts are organelles that
conduct photosynthesis, where the
photosynthetic pigment chlorophyll captures
the energy from sunlight, converts it, and stores
it in the energy-storage
molecules ATP and NADPH while
freeing oxygen from water
in plant and algal cells. They then use the ATP
and NADPH to make organic molecules
from carbon dioxide in a process known as
the Calvin cycle. Chloroplasts carry out a number
of other functions, including fatty acid synthesis,
much amino acid synthesis, and the immune
response in plants.
Chloroplast

•A chloroplast is a type of organelle known as
a plastid, characterized by its two
membranes and a high concentration
of chlorophyll. Other plastid types, such as
the leucoplast and the chromoplast, contain
little chlorophyll and do not carry out
photosynthesis.
•Chloroplasts, like mitochondria, contain their
own DNA.
Chloroplast

They are oval-shaped.
•They are made up of two surface membranes, i.e outer
and inner membrane and an inner layer known as the
thylakoid layer has two membranes.
•The outer membrane forms the external lining of the
chloroplast while the inner membrane is below the
outer layer.
•The membranes are separated by thin membranous
space and within the membrane, there is also a space
known as the stroma. The stroma houses the
chloroplast.
Chloroplast

•The third layer known as the thylakoid layer is
extensively folded making the appearance of a
flattened disk known as thylakoids which have
large numbers of chlorophyll and carotenoids
and the electron transport chain, defined as the
light-harvesting complex, used during
photosynthesis.
•Thylakoids are piled on top of each other in
stacks known as grana.
Chloroplast

Nucleus
•The nucleus is the largest organelle in animal
cells.
•Nucleus is a specialized structure occurring in
most cells (except bacteria and blue-green
algae).
•The contents of the nucleus are held in
the nucleoplasm similar to the cytoplasm in the
rest of the cell. The fluid component of this is
termed the nucleosol, similar to the cytosol in
the cytoplasm.

•Nuclear envelope
•The nuclear envelope, otherwise known as nuclear
membrane, consists of two cellular membranes, an inner
and an outer membrane, arranged parallel to one
another and separated by 10 to 50 nanometres (nm).
The nuclear envelope completely encloses the nucleus
and separates the cell's genetic material from the
surrounding cytoplasm, serving as a barrier to
prevent macromolecules from diffusing freely between
the nucleoplasm and the cytoplasm.
•The outer nuclear membrane is continuous with the
membrane of the rough endoplasmic reticulum (RER),
and is similarly studded with ribosomes.
•The space between the membranes is called the
perinuclear space and is continuous with the RER lumen.
Nucleus

•Nuclear pore
•It provide aqueous channels through the
envelope, are composed of multiple proteins,
collectively referred to as nucleoporins.
•It allows the passage of small water-soluble
molecules while preventing larger molecules,
such as nucleic acids and larger proteins, from
inappropriately entering or exiting the nucleus.
These large molecules must be actively
transported into the nucleus instead.
•The nucleus of a typical mammalian cell will
have about 3000 to 4000 pores throughout its
envelope
Nucleus

•Chromosomes
•The cell nucleus contains the majority of the
cell's genetic material in the form of multiple
linear DNA molecules organized into structures
called chromosomes. Each human cell contains
roughly two meters of DNA. During most of
the cell cycle these are organized in a DNA-
protein complex known as chromatin, and
during cell division the chromatin can be seen to
form the well-defined chromosomes.
Nucleus

•There are two types of
chromatin. Euchromatin is the less compact DNA
form, and contains genes that are
frequently expressed by the cell.
•The other type, heterochromatin, is the more
compact form, and contains DNA that is
infrequently transcribed.
•Chromosomes contain DNA, which is organised
into functional units called genes.
Nucleus

•Nucleolus
•The nucleolus is the largest of the discrete
densely stained, membraneless structures
known as nuclear bodies found in the nucleus. It
forms around tandem repeats of rDNA, DNA
coding for ribosomal RNA (rRNA). These regions
are called nucleolar organizer regions (NOR).
•The main roles of the nucleolus are to synthesize
rRNA and assemble ribosomes.
Nucleus

•Other nuclear bodies
•Besides the nucleolus, the nucleus contains a
number of other nuclear bodies. These
include Cajal bodies, gemini of Cajal bodies,
polymorphic interphase karyosomal association
(PIKA), promyelocytic leukaemia (PML)
bodies, paraspeckles, and splicing speckles.
Nucleus

•A cell normally contains only one nucleus. Under
some conditions, however, the nucleus divides
but the cytoplasm does not. This produces a
multinucleate cell (syncytium) such as occurs
in skeletal muscle fibres. Some cells—e.g., the
human red blood cell—lose their nuclei upon
maturation.
• white blood cell, the nucleus is lobated and can
be bi-lobed, tri-lobed or multi-lobed.
Nucleus

•The endoplasmic reticulum (ER) is an
important organelle in eukaryotic cells. It plays a
major role in the production, processing, and
transport of proteins and lipids. The ER produces
transmembrane proteins and lipids for its
membrane and many other cell components
including lysosomes, secretory vesicles,
the Golgi appatatus, the cell membrane,
and plant cell vacuoles.
Endoplasmic reticulum

•The endoplasmic reticulum is a network of tubules and
flattened sacs that serve a variety of functions in plant
and animal cells.
•The two regions of the ER differ in both structure and
function. Rough ER has ribosomes attached to the
cytoplasmic side of the membrane. Smooth ER lacks
attached ribosomes. Typically, the smooth ER is a tubule
network and the rough ER is a series of flattened sacs.
•The space inside of the ER is called the lumen. The ER is
very extensive extending from the cell
membrane through the cytoplasm and forming a
continuous connection with the nuclear envelope. Since
the ER is connected with the nuclear envelope, the
lumen of the ER and the space inside the nuclear
envelope are part of the same compartment.

•Rough Endoplasmic Reticulum
•The rough endoplasmic reticulum manufactures membranes
and secretory proteins. The ribosomes attached to the
rough ER synthesize proteins by the process of translation.
In certain leukocytes (white blood cells), the rough ER
produces antibodies. In pancreatic cells, the rough ER
produces insulin.
•The rough and smooth ER are usually interconnected and
the proteins and membranes made by the rough ER move
into the smooth ER to be transferred to other locations.
Some proteins are sent to the Golgi apparatus by special
transport vesicles. After the proteins have been modified in
the Golgi, they are transported to their proper destinations
within the cell or exported from the cell by exocytosis.

•Smooth Endoplasmic Reticulum
•The smooth ER has a wide range of functions
including carbohydrate and lipid synthesis. Lipids such
as phospholipids and cholesterol are necessary for the
construction of cell membranes. Smooth ER also serves
as a transitional area for vesicles that transport ER
products to various destinations.
•In liver cells the smooth ER produces enzymes that help
to detoxify certain compounds. In muscles the smooth
ER assists in the contraction of muscle cells, and
in brain cells it synthesizes male and female hormones.
•The sarcoplasmic reticulum is a specialized type of SER
that regulates calcium ion concentration in
the cytoplasm of striated muscle cells.

•Ribosomes were first observed by George Emil Palade,
using an electron microscope and is also called as
Palade’s particles. Old name of ribosome was
microsome.
•They are non-membranous structure.
•In eukaryotes, ribosomes are present
in mitochondria (sometimes called mitoribosomes) and
in plastids such as chloroplasts (also called
plastoribosomes).
•They are found in both eukaryotic and prokaryotic cells
except in mammalian sperm cell and RBCs.
Ribosome

•Ribosomes are typically composed of two subunits:
a large subunit and a small subunit. Eukarotic ribosomes
(80S), such as those in plant cells and animal cells, are
larger in size than prokaryotic ribosomes (70S), such as
those in bacteria. Ribosomal subunits are synthesized in
the nucleolus and cross over the nuclear membrane to
the cytoplasm through nuclear pores.
•Both ribosomal subunits join together when the
ribosome attaches to messenger RNA (mRNA)
during protein synthesis. Ribosomes along with another
RNA molecule, transfer RNA (tRNA), help to translate the
protein-coding genes in mRNA into proteins. Ribosomes
link amino acids together to form polypeptide chains,
which are further modified before becoming
functional proteins.
Ribosome

•S-sedimentation coefficient or Svedberg's constant
•Larger subunit is always bound ribosomes and they attach
to ER by a protein called ribophorin.
70S
60-65% rRNA
35-40% Protein
80S
45% rRNA
55% Protein
Ribosome

Ribosome
Ribosomal Assembly formation:
Joining of two sub units takes place at 0.001 Molar
concentration of Mg ions.
At concentration less than 0.001 Molar ribosomes
dissociates.
At concentration more than 0.001 Molar they make
diamer ribosomal units.

•There are two places where ribosomes commonly exist
within a eukaryotic cell: suspended in the cytosol are
called as free ribosomes and bound to the endoplasmic
reticulum called as bound ribosomes.
•ribosomes usually form aggregates called polysomes or
polyribosomes during protein synthesis. Polyribosomes
are clusters of ribosomes that attach to a mRNA
molecule during protein synthesis. This allows for
multiple copies of a protein to be synthesized at once
from a single mRNA molecule.
Ribosome

•Their main function is to convert genetic code into an
amino acid sequence and to build protein polymers from
amino acid monomers. This is called as protein
biogenesis or translation.
Ribosome Function

•The Golgi apparatus, or complex, plays an important role
in the modification and transport of proteins within the
cell.
•The Golgi apparatus is the "manufacturing and shipping
center" of a eukaryotic cell.
•The Golgi apparatus, sometimes called the Golgi
complex or Golgi body, is responsible for manufacturing,
warehousing, and shipping certain cellular products,
particularly those from the endoplasmic reticulum (ER).
Depending on the type of cell, there can be just a few
complexes or there can be hundreds. Cells that specialize
in secreting various substances typically have a high
number of Golgi.
Golgi body

•A Golgi apparatus is composed of flat sacs known as
cisternae. The sacs are stacked in a bent, semicircular
shape. Each stacked grouping has a membrane that
separates its insides from the cell's cytoplasm. Golgi
membrane protein interactions are responsible for their
unique shape. These interactions generate the force that
shapes this organelle.
•The Golgi apparatus is very polar. Membranes at one end
of the stack differ in both composition and in thickness
from those at the other end. One end (cis face) acts as
the "receiving" department while the other (trans face)
acts as the "shipping" department. The cis face is closely
associated with the ER.
•
Golgi body

•The Golgi apparatus modifies many products from
the ER including proteins and phospholipids.
•The Golgi apparatus contains processing enzymes, which
alter molecules by adding or
removing carbohydrate subunits.
•It is involved in the formation of lysosomes and other
enzyme-containing cellular inclusions, and in the
formation of secretory granules in cells such as those
found in the pancreas, pituitary and mammary glands,
and mucous-secreting glands of the intestine and in
many other cell types.
Golgi body Function

•Golgi bodies during mitotic cell division form a cell plate at the
centre of spindle.
•Golgi bodies of plant cells synthesize all polysaccharides such as
pectin, hemicellulose and microfibrils of a-cellulose.
•Golgi body in endocrine cells helps in secretion of hormones.
Golgi body function

•The Golgi apparatus or Golgi complex is capable of
disassembly and reassembly. During the early stages
of mitosis, the Golgi disassembles into fragments which
further break down into vesicles.
•As the cell progresses through the division process, the
Golgi vesicles are distributed between the two forming
daughter cells by spindle microtubules. The Golgi
apparatus reassembles in the telophase stage of mitosis.
Golgi body

Lysosomes
Lysosomes are organelles that are found in most animal cells
and act as the digesters of a eukaryotic cell.

•Lysosomes are spherical membranous sacs of enzymes.
These enzymes are acidic hydrolase enzymes that can
digest cellular macromolecules. The lysosome
membrane helps to keep its internal compartment acidic
and separates the digestive enzymes from the rest of
the cell. Lysosome enzymes are made by proteins from
the endoplasmic reticulum and enclosed within vesicles
by the Golgi apparatus. Lysosomes are formed by
budding from the Golgi complex.
Lysosomes

•Lysosome Enzymes
•Lysosomes contain various hydrolytic enzymes (around
50 different enzymes) that are capable of digesting
nucleic acids, polysaccharides, lipids, and proteins. The
inside of a lysosome is kept acidic as the enzymes within
work best in an acidic environment. If a lysosome's
integrity is compromised, the enzymes would not be
very harmful in the cell's neutral cytosol.
Lysosome

•Lysosomes act as the "garbage disposal" of a cell. They
are active in recycling the cell's organic material and in
the intracellular digestion of macromolecules. Some
cells, such as white blood cells, have many more
lysosomes than others. These cells destroy bacteria,
dead cells, cancerous cells, and foreign matter through
cell digestion. Macrophages engulf matter by
phagocytosis and enclose it within a vesicle called a
phagosome. Lysosomes within the macrophage fuse
with the phagosome releasing their enzymes and
forming what is known as a phagolysosome. The
internalized material is digested within the
phagolysosome. Lysosomes are also necessary for the
degradation of internal cell components such as
organelles.
Lysosome

•The mitochondrion (plural mitochondria) is a
semiautonomous double-membrane-bound organelle found in
most eukaryotic organisms.
•Also termed as the power house of the cell.
•It was discovered by Kolliker in flight muscles of an insect.
•Term mitochondria was given by Benda.
•It is a semiautonomous organelle.
• Some cells in some multicellular organisms may, however, lack
mitochondria (for example, mature mammalian red blood
cells). whereas liver cells can have more than 2000.
•5 Lacks of mitochondria in flight muscle cell.
Mitochondria

•Structure
•A mitochondrion contains outer and inner membranes
composed of phospholipid bilayers and proteins.
•The two membranes have different properties. Because
of this double-membraned organization, there are five
distinct parts to a mitochondrion. They are:
•the outer mitochondrial membrane,
•the intermembrane space (the space between the outer
and inner membranes),
•the inner mitochondrial membrane,
•the cristae space (formed by infoldings of the inner
membrane), and
•the matrix (space within the inner membrane).
Mitochondria

•Outer membrane
• It has a protein-to-phospholipid ratio similar to that of
the cell membrane. It contains large numbers of integral
membrane proteins called porins.
•It has less protein
•It is fully permeable.
•The outer membrane also contains enzymes involved in
such diverse activities as the elongation of fatty
acids, oxidation of epinephrine, and
the degradation of tryptophan.
Mitochondria

•Intermembrane space/Peri mitochondrial space
•The mitochondrial intermembrane space is the space
between the outer membrane and the inner membrane.
It is also known as perimitochondrial space. Because the
outer membrane is freely permeable to small molecules,
the concentrations of small molecules, such as ions and
sugars, in the intermembrane space is the same as in
the cytosol.
•The protein composition of this space is different from
the protein composition of the cytosol.
Mitochondria

•Inner membrane
•The inner mitochondrial membrane contains more
proteins and are selectively permeable with five types
of functions:
•Those that perform the redox reactions of oxidative
phosphorylation
•ATP synthase, which generates ATP in the matrix
•Specific transport proteins that
regulate metabolite passage into and out of
the mitochondrial matrix
•Protein import machinery
•Mitochondrial fusion and fission protein
Mitochondria

•It contains more than 151 different polypeptides, and
has a very high protein-to-phospholipid ratio.
•The inner membrane is home to around 1/5 of the total
protein in a mitochondrion.
•In addition, the inner membrane is rich in an unusual
phospholipid, cardiolipin. This phospholipid was
originally discovered in cow hearts, and is usually
characteristic of mitochondrial and bacterial plasma
membranes.
•Unlike the outer membrane, the inner membrane does
not contain porins, and is highly impermeable to all
molecules. Almost all ions and molecules require special
membrane transporters to enter or exit the matrix.
Mitochondria

•Cristae
•The inner mitochondrial membrane is
compartmentalized into numerous cristae, which expand
the surface area of the inner mitochondrial membrane,
enhancing its ability to produce ATP. For typical liver
mitochondria, the area of the inner membrane is about
five times as large as the outer membrane.
•These folds are studded with small round bodies known
as F1 particles or oxysomes.
•Mitochondria has high denaturation temperature as they
have DNA with more C-G pairs.
Mitochondria

•Matrix
•The matrix is the space enclosed by the inner
membrane. It contains about 2/3 of the total proteins in
a mitochondrion. The matrix is important in the
production of ATP with the aid of the ATP synthase
contained in the inner membrane.
•The matrix contains a highly concentrated mixture of
hundreds of enzymes for cellular respiration, and
divalent ions of Mg, Mn, Fe.
•Special mitochondrial ribosomes, tRNA, and several
copies of the mitochondrial DNA genome.
•Ribosomes in mammalian cells mitochondria are 55S
type.
Mitochondria

•Functions
•ATP synthesis through respiration, and to regulate
cellular metabolism. The central set of reactions involved
in ATP production are collectively known as the citric
acid cycle, or the Krebs cycle.
•Nebenkern( Mitochondria in sperm cell in the middle
part of sperm the spiral shape, they produce ATP) helps
tail to swim fast.
•Thermiogenesis,ie production of heat.
•Cytoplasmic Inheritance.
•Vitellogenesis- Yolk synthesis.( mitochondria and Golgi
body).
•Synthesis of heme part of hemoglobin and myoglobin.
Mitochondria

•There are two hypotheses about the origin of
mitochondria: endosymbiotic and autogenous.
•The endosymbiotic hypothesis suggests that
mitochondria were originally prokaryotic cells, capable
of implementing oxidative mechanisms that were not
possible for eukaryotic cells; they
became endosymbionts living inside the eukaryote.
•In the autogenous hypothesis, mitochondria were born
by splitting off a portion of DNA from the nucleus of the
eukaryotic cell at the time of divergence with the
prokaryotes; this DNA portion would have been enclosed
by membranes, which could not be crossed by proteins.
Since mitochondria have many features in common
with bacteria, the endosymbiotic hypothesis is more
widely accepted.
Mitochondria

Microvilli (singular: microvillus) are microscopic cellular
membrane protrusions that increase the surface area for
diffusion and minimize any increase in volume, and are
involved in a wide variety of functions,
including absorption, secretion, cellular adhesion,
and mechanotransduction (convert mechanical
stimulus into electrochemical activity. This form of sensory
transduction is responsible for a number of senses and physiological
processes in the body like, touch, balance, and hearing)
Microvilli

•Structure
•Microvilli are covered in plasma
membrane, which
encloses cytoplasm and microfilaments
(actin protein filament). Though these are
cellular extensions, there are little or no
cellular organelles present in the
microvilli.
•Each microvillus has a dense bundle of
cross-linked actin filaments, which serves
as its structural core. 20 to 30 tightly
bundled actin filaments are cross-linked
by bundling proteins fimbrin (or plastin-
1), villin and espin to form the core of the
microvilli.
Microvilli

•Location
•Thousands of microvilli form a structure called the brush
border that is found on the apical surface of
some epithelial cells, such as the small intestines.
(Microvilli should not be confused with intestinal villi,
which are made of many cells. Each of these cells has
many microvilli.) Microvilli are observed on the plasma
surface of eggs, aiding in the anchoring of sperm cells
that have penetrated the extracellular coat of egg cells.
•Microvilli are also of importance on the cell surface
of white blood cells, as they aid in the migration of white
blood cells.
Microvilli

•Function
•Greatly increase the surface area of the cell surface
membrane.
•It helps for absorption in the gut and for reabsorption in
the proximal convoluted tubules of the kidney.
•Primary surface of nutrient absorption in the
gastrointestinal tract. Because of this vital function, the
microvillar membrane is packed with enzymes that aid in
the breakdown of complex nutrients into simpler
compounds that are more easily absorbed. For example,
enzymes that digest carbohydrates
called glycosidases are present at high concentrations on
the surface of enterocyte microvilli.
Microvilli

Microtubules (MTOCs)
https://www.khanacademy.org/test-
prep/mcat/cells/cytoskeleton/v/microtubules-2

•These straight, hollow cylinders are found throughout
the cytoplasm of all eukaryotic cells (prokaryotes don't
have them) and carry out a variety of functions, ranging
from transport to structural support. Microtubules,
which are about 25 nanometers in diameter, form part
of the cytoskeleton that gives structure and shape to a
cell, and also serve as conveyor belts moving other
organelles throughout the cytoplasm. In addition,
microtubules are the major components of cilia and
flagella, and participate in the formation of spindle fibers
during cell division (mitosis). The length of microtubules
in the cell varies between 200 nanometers and 25
micrometers, depending upon the task of a particular
microtubule and the state of the cell's life cycle.

•Microtubules are biopolymers that are composed of
subunits made from an abundant globular cytoplasmic
protein known as tubulin. Each subunit of the
microtubule is made of two slightly different but closely
related simpler units called alpha-tubulin and beta-
tubulin that are bound very tightly together to
form heterodimers. In a microtubule, the subunits are
organized in such a way that they all point the same
direction to form 13 parallel protofilaments. This
organization gives the structure polarity, with only
the alpha-tubulin proteins exposed at one end and
only beta-tubulin proteins at the other.

•By adding or removing globular tubulin proteins, the
length of polymeric microtubules can be increased or
decreased. Because the two ends of a microtubule are
not the same, however, the rate at which growth or
depolymerization occurs at each pole is different. The
end of a polarized filament that grows and shrinks the
fastest is known as the plus end and the opposing end is
called the minus end. For all microtubules, the minus
end is the one with exposed alpha-tubulins. In an animal
cell, it is this end that is located at the centriole-
containing centrosome found near the nucleus, while
the plus end, comprised of exposed beta-units, is
projected out toward the cell's surface. Microtubules are
continuously being assembled and disassembled so that
tubulin monomers can be transported elsewhere to
build microtubules when needed.

•Presented in Figure 2 is a digital image of the
microtubule network found in an embryonic mouse cell
as seen through a fluorescence optical microscope.

•In addition to their structural support role, microtubules also
serve as a highway system along which organelles can be
transported with the aid of motor proteins. For instance, the
microtubule network interconnects the Golgi apparatus with
the plasma membrane to guide secretory vesicles for export,
and also transports mitochondria back and forth in the
cytoplasm. Another example is the translocation of vesicles
containing neurotransmitters by microtubules to the tips of
nerve cell axons.
https://study.com/academy/lesson/microtubules-definition-
functions-structure.html

•The motor proteins involved in organelle transport
operate by altering their three-dimensional
conformation using adenosine triphosphate (ATP) as fuel
to move back and forth along a microtubule. With each
step, the motor molecule releases one portion of the
microtubule and grips a second site farther long the
filament. Motor proteins, which are grouped into several
distinct classes, attach to organelles through specialized
receptors.

•Since eukaryotic cells greatly depend upon the integrity
of microtubules and other cytoskeletal filaments to
maintain their structure and essentially to survive, many
plants produce natural toxins aimed at disrupting the
microtubule network as a means of self-defense. Taxol,
for example, is a toxic substance produced by a species
of yew trees that increases microtubule polymerization
(building a macromolecule) by binding to the filament
and stabilizing it.

•Other natural toxins, such as the colchicine produced by
the meadow saffron, destabilize microtubules and
hinder their polymerization. Both kinds of events can be
fatal to the affected cell, though in some circumstances,
this can be beneficial to animals, as demonstrated by
taxol, which is commonly used as a cancer medication.
•The assembly of microtubules from tubulin molecules is
controlled by special location in cells called microtubule
organising centres(MTOCs).
•Centrioles found at the bases of cilia and flagella, where
they are known as basal bodies, do act as MTOCs. The
microtubules that extend from the basal bodies into cilia
and flagella are essential for the beating movements of
these organelles.

Two fundamentally
different types of cell

Viruses vary in their individual structure, but all viruses
contain two structures.
•Nucleic acid: DNA or RNA
•Capsid: a protein coat that gives a virus its shape. It
makes up 95% of the virus. Capsid is made up of
separate protein molecules, each of which is called a
capsomere.
•Other structures a virus could have are:
Envelopes: consist of lipids and are found only in some
viruses; it is believed to come from the host cell when
the virus exits in the cell.
•Tail: viruses that infect bacteria have a tail used for
attachment.
•
Viruses Structure

Viruses can be three shapes:
•Helical (rod shaped)
•Cubic or polyhedral (many sided)
•Complex (neither of the others)
Viruses

•All viruses are parasitic because they can only reproduce
by infecting and taking over the protein synthesising
machinery of the host cell, Which then helps to make
new virus particles.
Viruses

shows measurement
of scale bar in mm ,
multiplication by
1000 , division by
535 ;
displays answer as a
whole number

• https://www.alevelbiologytutor.com/tutoring-
blog/2018/1/18/magnification-calculations-and-graticules

Cell structure

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