Membrane bounded organelles

Hapuarachchige Dona Thisarani Sankalpana Samarasinghe
Diploma in Bio Medical Science
June 2016

 One of the most important concepts in the biology is that the basic unit of structure and
function in living organisms is the cell. This is known as the cell theory and was proposed
jointly by two scientists, namely Schleiden, a Belgian botanist, in 1838 and Schwann, a German
zoologist, in 1839.
 The discovery was due to the fact that during the nineteenth century there were dramatic
improvements in the quality of lenses for use in microscopy and this in turn led to great
interest in the structure of living organisms.
 An essential part of the cell theory is the idea, first proposed in 1855 that new cells only come
from pre-existing cells.in 1858 Rudolf Virchow explained theory of biogenesis

The nucleus was the first organelle to be discovered by the microscopist Antoine
van Leeuwenhoek (1632–1723).He observed a "lumen", the nucleus, in the red
blood cells of salmon. Eukaryotes usually have a single nucleus, but a few cell
types, such as mammalian red blood cells, have no nuclei, and a few others have
many.
Cell nuclei contain most of the cell's genetic material, organized as
multiple long linear DNA molecules in complex with a large variety of
proteins, such as histones, to form chromosomes

 The nucleus maintains the integrity of genes and controls the activities
of the cell by regulating expression.
 The nucleus is, therefore, the control center of the cell.
 The main structures making up the nucleus are the nuclear envelope, a
double membrane that encloses the entire organelle and isolates its
contents from the cellular cytoplasm, and the nuclear matrix a network
within the nucleus that adds mechanical support, which supports the
cell as a whole.
 Although the interior of the nucleus does not contain any membrane-
bound sub compartments, its contents are not uniform, and a number
of sub-nuclear bodies exist, made up of unique proteins, RNA
molecules, and particular parts of the chromosomes.

 In animal cells, two networks of intermediate filaments provide the nucleus
with mechanical support.
 The nuclear lamina forms an organized meshwork on the internal face of the
envelope, while less organized support is provided on the cytosolic face of the
envelope. Both systems provide structural support for the nuclear envelope and
anchoring sites for chromosomes and nuclear pores.
 The nuclear lamina is composed mostly of lamin proteins.
 Lamins are found inside the nucleoplasm where they form another regular
structure, known as the nucleoplasmic veil that is visible using fluorescence
microscopy

 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.
 Nuclear pores, which provide aqueous channels through the envelope, are composed of
multiple proteins, collectively referred to as nucleoporins.
 The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its
envelope, , each of which contains an eightfold-symmetric ring-shaped structure at a
position where the inner and outer membranes fuse. Attached to the ring is a structure
called the nuclear basket that extends into the nucleoplasm, and a series of filamentous
extensions that reach into the cytoplasm. Both structures serve to mediate binding to
nuclear transport proteins.

 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 familiar from a karyotype. A small fraction of the cell's genes are located
instead in the mitochondria.
 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. During interphase the chromatin organizes itself into discrete individual
patches, called chromosome territories.
 Active genes, which are generally found in the euchromatic region of the chromosome,
tend to be located towards the chromosome's territory boundary.

 The nucleolus is a discrete densely stained structure found in the nucleus.
 It is not surrounded by a membrane, and is sometimes called a sub organelle.
 The main roles of the nucleolus are to synthesize rRNA and assemble
ribosomes.
 The nucleolus disassembles at the beginning of mitosis and begins to
reassemble in telophase
 Besides the nucleolus, the nucleus contains a number of other non-membrane-
delineated bodies. These include Cajal bodies, Gemini of coiled bodies,
paraspeckles, and splicing speckles.
 Although little is known about a number of these domains, they are significant
in that they show that the nucleoplasm is not a uniform mixture, but rather
contains organized functional subdomains.

 Ribosomes were first observed in 1950 by Romanian cell biologist George
Emil Palade using an electron microscope as dense particles or granules in
1974.The term "ribosome" was proposed by scientist Richard B. Roberts in
1958.
 The ribosome is a cellular machine which is highly complex. It is made up of
dozens of distinct proteins as well as a few specialized RNA molecules known
as ribosomal RNA (rRNA).these rRNAs do not carry instructions to make
specific proteins like mRNAs.
 Ribosomes consist of two subunits that fit together and work as one to
translate the mRNA into a polypeptide chain during protein synthesis.
Because they are formed from two subunits of non-equal size, they are
slightly longer in the axis than in diameter. The unit of measurement is the
Svedberg unit, a measure of the rate of sedimentation Eukaryotes have 80S
ribosomes, each consisting of a small (40S) and large (60S) subunit

 The mRNA comprises a series of codons that dictate to the ribosome
the sequence of the amino acids needed to make the protein.
 The ribosome contains three RNA binding sites, designated A, P and
E. The A site binds an aminoacyl-tRNA; the P site binds a peptidyl-
tRNA and the E site binds a free tRNA before it exits the ribosome.
 Protein synthesis begins at a start codon AUG near the 5’ cap of the
mRNA. mRNA binds to the P site of the ribosome first. The ribosome is
able to identify the start codon by use of the Shine-Dalgarno sequence
of the mRNA in prokaryotes and Kozak box in eukaryotes.
 The ribosome uses tRNA that matches the current codon on the mRNA
to append an amino acid to the polypeptide chain.
 This is done for each codon on the tRNA, while the ribosome moves
towards the 3' poly-A tail of the mRNA. Usually in bacterial cells,
several ribosomes are working parallel on a single RNA, forming what
is called a polyribosome or polysome

 The endoplasmic reticulum (ER) is a type of organelle in the cells of eukaryotic
organisms that forms an interconnected network of flattened, membrane-enclosed tube-
like structures known as cisternae.
 it is such an extensive network of membranes that it accounts for more than half the total
membrane in many eukaryotic cells.
 The membranes of the ER are continuous with the outer nuclear membrane.
 The general structure of the endoplasmic reticulum is a network of membranes called
cisternae.
 These sac-like structures are held together by the cytoskeleton.
 The phospholipid membrane encloses a space, the cisternal space, which is continuous
with the perinuclear space but separate from the cytosol.
 There are two types of endoplasmic reticulum, rough and smooth.

 A ribosome only binds to the RER once a specific protein-nucleic acid complex forms in
the cytosol. This special complex forms when a free ribosome begins translating the
mRNA of a protein destined for the secretory pathway.
 After secretory proteins are formed, the ER membrane keeps them separate from
proteins that are produced by free ribosomes and that will remain in the cytosol.
 Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud
like bubbles from a specialized region called transitional ER.
 Vesicles in transit from one part of the cell to another are called transport vesicles.
 In addition to making secretory proteins, rough ER is a membrane factory for the cell; it
grows in place by adding membrane proteins and phospholipids to its own membrane.
 The rough endoplasmic reticulum works in concert with the Golgi complex to target new
proteins to their proper destinations.
 A second method of transport out of the endoplasmic reticulum involves areas called
membrane contact sites, where the membranes of the endoplasmic reticulum and other
organelles are held closely together, allowing the transfer of lipids and other small
molecules.

 It synthesizes lipids, phospholipids, and steroids.
 Cells which secrete these products, such as those in the testes,
ovaries, and sebaceous glands have an abundance of smooth
endoplasmic reticulum.
 It also carries out the metabolism of carbohydrates,
detoxification of natural metabolism products and of alcohol
and drugs, attachment of receptors on cell membrane proteins,
and steroid metabolism.
 In muscle cells, it regulates calcium ion concentration.
 Smooth endoplasmic reticulum is found in a variety of cell types
(both animal and plant), and it serves different functions in
each. The smooth ER is especially abundant in mammalian liver
and gonad cells.

 The smooth endoplasmic reticulum also contains the
enzyme glucose-6-phosphatase, which converts glucose-6-
phosphate to glucose, a step in gluconeogenesis.
 It is connected to the nuclear envelope and consists of
tubules that are located near the cell periphery. These
tubes sometimes branch forming a network that is reticular
in appearance.
 In some cells, there are dilated areas like the sacs of rough
endoplasmic reticulum.
 The network of smooth endoplasmic reticulum allows for
an increased surface area to be devoted to the action or
storage of key enzymes and the products of these enzymes.

 The endoplasmic reticulum synthesizes molecules, while the
sarcoplasmic reticulum stores calcium ions and pumps them out into
the sarcoplasm when the muscle fiber is stimulated.
 After their release from the sarcoplasmic reticulum, calcium ions
interact with contractile proteins that utilize ATP to shorten.

 The Golgi apparatus also known as the Golgi complex, Golgi body, or simply the Golgi, is
an organelle found in most eukaryotic cells. It was identified in 1897 by the Italian
physician Camillo Golgi and named after him in 1898.
 Among eukaryotes, the subcellular localization of the Golgi apparatus differs. In
mammals, a single Golgi apparatus complex is usually located near the cell nucleus, close
to the centrosome.
 Tubular connections are responsible for linking the stacks together. Localization and
tubular connections of the Golgi apparatus are dependent on microtubules.
 If microtubules are experimentally depolymerized, then the Golgi apparatus loses
connections and becomes individual stacks throughout the cytoplasm.
 The common feature among Golgi is that they are adjacent to endoplasmic reticulum
(ER) exit sites.

 In most eukaryotes, the Golgi apparatus is made up of a series of
compartments consisting of two main networks: the Cis Golgi network (CGN)
and the Trans Golgi network (TGN).
 The Golgi apparatus tends to be larger and more numerous in cells that
synthesize and secrete large amounts of substances; for example, the antibody-
secreting plasma B cells of the immune system have prominent Golgi
complexes.
 In all eukaryotes, each cisternal stack has a cis entry face and a trans exit face.
These faces are characterized by unique morphology and biochemistry.
 Within individual stacks are assortments of enzymes responsible for selectively
modifying protein cargo. These modifications influence the fate of the protein.
 The compartmentalization of the Golgi apparatus is advantageous for
separating enzymes, thereby maintaining consecutive and selective processing
steps: enzymes catalyzing early modifications are gathered in the cis face
cisternae, and enzymes catalyzing later modifications are found in trans face
cisternae of the Golgi stacks.

 The Golgi apparatus is also involved in lipid transport and lysosome formation.
 This area of the Golgi is the point at which proteins are sorted and shipped to their
intended destinations by their placement into one of at least three different types of
vesicles, depending upon the signal sequence they carry.
.Exocytotic vesicles-Vesicle contains proteins destined for extracellular release. After
packaging, the vesicles bud off and immediately move towards the plasma membrane,
where they fuse and release the contents into the extracellular space in a process known as
constitutive secretion.Ex: Antibody release by activated plasma B cells2.
Secretory vesicle-Vesicles contain proteins destined for extracellular release. After
packaging, the vesicles bud off and are stored in the cell until a signal is given for their
release. When the appropriate signal is received they move toward the membrane and fuse
to release their contents. This process is known as regulated secretion.Ex: Neurotransmitter
release from neurons3.
Lysosomal vesicles-containing many acid hydrolases, or to lysosome-like storage organelles.
These proteins include both digestive enzymes and membrane proteins. The vesicle first
fuses with the late endosome, and the contents are then transferred to the lysosome via
unknown mechanisms.Ex: Digestive proteases destined for the lysosome

 Lysosomes are also known as "suicide bags" or "suicide sacs" of the cells. They were
discovered and named by Belgian biologist Christian de Duve, who eventually received
the Nobel Prize in Physiology or Medicine in 1974.A lysosome is a membrane-bound cell
organelle found in cells. Their sizes can be very different—the biggest ones can be more
than 10 times bigger than the smallest ones.
 Lysosomes contain a variety of enzymes in order to be able to break down the variety of
biomolecules engulfed by the cell, including peptides, nucleic acids, carbohydrates, and
lipids. The enzymes responsible for this hydrolysis require an acidic environment for
optical activity. Besides degradation of polymers, the lysosome is involved in various cell
processes, including secretion, plasma membrane repair, and cell signaling and energy
metabolism
 Lysosomes carry out intracellular digestion in a variety of circumstances. Amoebas and
many other protists eat by engulfing smaller organisms or food particles, a process called
phagocytosis

 Lysosomes also use their hydrolytic enzymes to recycle the cells own organic material, a
process called Autophagy.
 During autophagy, a damaged organelle or small amount of cytosol becomes surrounded
by a double membrane and a lysosome fuses with the outer membrane of this vesicle .The
lysosomal enzymes dismantle the enclosed material, and the organic monomers are
returned to the cytosol for reuse.
 With the help of lysosomes, the cell continually renews itself. A human liver cell, for
example, recycles half of its macromolecules each weekThe cells of people with inherited
lysosomal storage diseases lack a functioning hydrolytic enzyme normally present in
lysosomes.
 The lysosomes become engorged with indigestible substrates, which begin to interfere
with other cellular activities. In Tay-Sachs disease, for example, a lipid-digesting enzyme
is missing or inactive, and the brain becomes impaired by an accumulation of lipids in
the cells. Fortunately, lysosomal storage diseases are rare in the general population

 Vacuoles are large vesicles derived from the endoplasmic reticulum and Golgi
apparatus. Thus, vacuoles are an integral part of a cell s endomembrane system.
Like all cellular membranes, the vacuolar membrane is selective in transporting
solutes; as a result, the solution inside a vacuole differs in composition from the
cytosol.
 Vacuoles perform a variety of functions in different kinds of cells.
 Food vacuoles, formed by phagocytosis. Many freshwater protists have
contractile vacuoles that pump excess water out of the cell, thereby
maintaining a suitable concentration of ions and molecules inside the cell in
plants and fungi, certain vacuoles carry out enzymatic hydrolysis, a function
shared by lysosomes in animal cells and in plants, smaller vacuoles can hold
reserves of important organic compounds, such as the proteins stockpiled in
the storage cells in seeds.

 Mature plant cells generally contain a large central vacuole, which
develops by the coalescence of smaller vacuoles. The solution inside
the central vacuole, called cell sap, is the plant cell s main repository of
inorganic ions, including potassium and chloride.
 The central vacuole plays a Major role in the growth of plant cells,
which enlarge as the vacuole absorbs water, enabling the cell to become
larger with a minimal investment in new cytoplasm.
 The cytosol often occupies only a thin layer between the central
vacuole and the plasma membrane, so the ratio of plasma membrane
surface to cytosolic volume is sufficient, even for a large plant cell.

 The function and significance of vacuoles varies greatly according to the type of cell in
which they are present, having much greater prominence in the cells of plants, fungi and
certain protists than those of animals and bacteria. In general, the functions of the
vacuole include:
1. • Isolating materials that might be harmful or a threat to the cell
2. • Containing waste products
3. • Containing water in plant cells
4. • Maintaining internal hydrostatic pressure or turgor within the cell
5. • Maintaining an acidic internal pH
6. • Containing small molecules
7. • Exporting unwanted substances from the cell
8. • Allows plants to support structures such as leaves and flowers due to the pressure
of the central vacuole
9. • In seeds, stored proteins needed for germination are kept in 'protein bodies',
which are modified vacuoles.
10. Vacuoles also play a major role in autophagy, maintaining a balance between
biogenesis and degradation, of many substances and cell structures in certain
organisms.

 The first observations of intracellular structures that probably represented mitochondria
were published in the 1840s. Richard Altman, in 1894, established them as cell organelles
and called them "bioblasts. The term "mitochondria" was coined by Carl Benda in 1898
 The mitochondrion is a double membrane-bound organelle found in all eukaryotic
organisms, although some cells in some organisms may lack them (e.g. Red blood cells). A
number of organisms have reduced or transformed their mitochondria into other structures
 In addition to supplying cellular energy, mitochondria are involved in other tasks, such as
signaling, cellular differentiation, and cell death, as well as maintaining control of the cell
cycle and cell growth. Mitochondrial biogenesis is in turn temporally coordinated with
these cellular processes
 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

 Mitochondria have their own genetic material, and the machinery to manufacture their own RNAs
and proteins
 Mitochondria play a central role in many other metabolic tasks, such as:
 • Signaling through mitochondrial reactive oxygen species
 • Regulation of the membrane potential
 • Calcium signaling
 • Regulation of cellular metabolism
 • Certain heme synthesis reactions
 • Steroid synthesis.
 • Hormonal signaling,
 Mitochondria are sensitive and responsive to hormones, in part by the action of mitochondrial
estrogen receptors (mtERs). These receptors have been found in various tissues and cell types,
including brain and heart
 Some mitochondrial functions are performed only in specific types of cells. For example,
mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of
protein metabolism. A mutation in the genes regulating any of these functions can result in
mitochondrial diseases

 Chloroplasts are organelles, specialized subunits, in plant and algal cells. Their
discovery inside plant cells is usually credited to Julius von Sachs (1832–1897),
an influential botanist and author of standard botanical textbooks –
sometimes called "The Father of Plant Physiology”.
 These lens-shaped organelles, about 36 m in length. They are considered to
have originated from cyanobacteria through endosymbiosis. When a
eukaryotic cell engulfed a photosynthesizing cyanobacterium that became a
permanent resident in the cell.
 The contents of a chloroplast are partitioned from the cytosol by an envelope
consisting of two membranes separated by a very narrow intermembrane
space.
 Inside the chloroplast is another membranous system in the form of flattened,
interconnected sacs called thylakoids. In some regions, thylakoids are stacked
like poker chips; each stack is called a granum.
 The fluid outside the thylakoids is the stroma, which contains the chloroplast
DNA and ribosomes as well as many enzymes.

 Chloroplasts are highly dynamic. They circulate and are moved around within
plant cells, and occasionally pinch in two to reproduce. Their behavior is
strongly influenced by environmental factors like light color and intensity.
Chloroplasts, like mitochondria, contain their own DNA, which is thought to
be inherited from their ancestor.
 Chloroplasts' main role is to conduct photosynthesis, where the photosynthetic
pigment chlorophyll captures the energy from sunlight and converts it and
stores it in the energy-storage molecules ATP and NADPH while freeing oxygen
from water.
 Chloroplasts carry out a number of other functions, including fatty acid
synthesis, much amino acid synthesis, and the immune response in plants.

 Peroxisomes (also called microbodies) are organelles found in virtually all eukaryotic
cells.
 Peroxisomes were identified as organelles by the Belgian cytologist Christian de Duve in
1967.after they had been first described by a Swedish doctoral student, J. Rhodin in 1954.
 Peroxisomes contain oxidative enzymes, such as catalase, D-amino acid oxidase, and uric
acid oxidase.
 Catalase uses H2O2 to oxidize other substrates, including phenols, formic acid,
formaldehyde, and alcohol, by means of the peroxidation reaction. Thus eliminating the
poisonous hydrogen peroxide in the process.
 This reaction is important in liver and kidney cells, where the peroxisomes detoxify
various toxic substances that enter the blood. About 25% of the ethanol alcohol humans
drink is oxidized to acetaldehyde in this way. In addition, when excess H2O2 accumulates
in the cell, catalase converts it to H2O.

 Glyoxysomes are specialized peroxisomes found in plants (particularly
in the fat storage tissues of germinating seeds) and also in filamentous
fungi.
 Thus, glyoxysomes (as all peroxisomes) contain enzymes that initiate
the breakdown of fatty acids and additionally possess the enzymes to
produce intermediate products for the synthesis of sugars by
gluconeogenesis.
 The seedling uses these sugars synthesized from fats until it is mature
enough to produce them by photosynthesis.
 Glyoxysomes also participate in photorespiration and nitrogen
metabolism in root nodules.

Membrane bounded organelles

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Membrane bounded organelles