Cellular organelles nucleus, mitochondria & plastids

Cellular Organelles: Nucleus, Mitochondria & Plastids
Cellular Organelles are the membrane-enclosed compartments in a eukaryotic cell that has a
distinct structure, macromolecular composition, and function. Examples are nucleus, mitochondria,
chloroplast, golgi apparatus, ER, vacuole, flagellum etc.

 The nucleus was the first organelle to be discovered.
 We cannot know who first saw the nucleus but we do know that the father of optical
microscopy, Antony van Leeuwenhoeck, who did so with amphibian and avian
erythrocytes in 1710. He observed a "lumen", the nucleus, in the red blood cells
of salmon.
 We do not know how and when the genome of an ancestral cell first became encased in
a primitive nucleus. But we know that Franz Bauer, in 1802, sketched orchid cells and
pointed out the nucleus. Nucleus was described in more detail in 1831 by
Scottish botanist Robert Brown.
 We are uncertain about its evolutionary origin, but we know that the nucleus is
bounded by a double membrane, the nuclear envelope, which in many cells is
connecting with the endoplasmic reticulum.
 Now nucleus can be observed, isolated and is being utilized in the next generation
molecular breeding.

Nucleus: Introduction
 The nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel) is
a membrane-enclosed organelle found in eukaryotic cells.
 Nucleus is most prominent organelle of eukaryotic cells which contains
genetic material.
 Nucleus distinguishes the eukaryotic cell from prokaryotic cell. Eukaryotes
usually have a single nucleus, but a few cell types, such as mammalian red
blood cells, have no nuclei.
 The number of nuclei may vary, they may be uni-nucleate (single nucleus),
bi-nucleate (two nuclei) or even multi-nucleate e.g.: osteoclasts (5
nucleus), coenocytes etc.
 The shape of the nucleus is mostly round, it may be oval, disc shaped
depending on the type of cell.

Typical Structure of Nucleus
Source: Bing Images

(a) Electron micrograph section of interphase nucleus (b) Cutway 3D view of nuclear envelope and pores
Source: Microbiology A system approach

Internal Organization of the Nucleus
• The nucleus is more than a container in
which chromatin, RNAs, and
nuclear proteins move freely in aqueous
solution. Nucleus is the site for Central
Dogma.
• Chromosome, nuclear pore complex,
nuclear matrix, nuclear lamina, nucleolus
are the characteristics of nucleus.
Source: Bing Images

1. The Nuclear Envelope
• The cell nucleus is bound by a double membrane
called the nuclear envelope and is continuous with
the endoplasmic reticulum.
• This membrane separates the contents of the
nucleus from the cytoplasm.
• Like the cell membrane, the nuclear envelope
consists of phospholipids that form a lipid bilayer.
• Perinuclear space is the space between inner and
outer nuclear membrane.
• The envelope helps to maintain the shape of the
nucleus and assists in regulating the flow of
molecules into and out of the nucleus through
nuclear pores. Source: https://www.ncbi.nlm.nih.gov

2. Nuclear Pore Complex
• The nuclear pore complexes are the only channels through which small polar molecules,
ions, and macromolecules (proteins and RNAs) are able to travel between the nucleus and
the cytoplasm.
• The nuclear pore complex is an extremely large structure with a diameter of about 120 nm.
3. Nuclear Matrix and Nuclear Lamina
• The nuclear matrix is the network of fibres found throughout the inside of a cell nucleus.
• The nuclear lamina is a fibrous meshwork underlying the inner membrane.
• The nuclear lamina is a structure near the inner nuclear membrane and the peripheral
chromatin.
• Nuclear lamina is involved in most nuclear activities including DNA replication, RNA
transcription, nuclear and chromatin organization, cell cycle regulation, cell development
and differentiation, nuclear migration, and apoptosis (programmed cell death).

4. Chromosomes
• Chromosomes are located within the nucleus. Chromosomes consists of DNA,
which contains heredity information and instructions for cell growth,
development, and reproduction. When a cell is “resting”, the chromosomes
organized are called chromatin.
• Chromatin becomes highly condensed during mitosis to form the
compact metaphase chromosomes that are distributed to daughter nuclei.
• During interphase, some of the chromatin (heterochromatin) remains highly
condensed and is transcriptionally inactive; the remainder of the chromatin
(euchromatin) is decondensed and distributed throughout the nucleus.
• Much of the heterochromatin is localized to the periphery of the nucleus, possibly
because one of the principal proteins associated with heterochromatin binds to a
protein of the inner nuclear membrane.

5. Nucleolus
Contained within the nucleus is a dense
structure composed of RNA and proteins called
the nucleolus.
The most obvious aspect of the internal
organization of the nucleus is the nucleolus,
which is the site at which the rRNA genes are
transcribed and ribosomal subunits are
assembled.
The nucleolus is a ribosome production
factory. The nucleolus contains nucleolar
organizers, which are parts of chromosomes
with the genes for ribosome synthesis on them.
Source: Bing Images

Changes in the Nucleus during Mitosis
• In the living interphase nucleus, no chromosomal structures are visible.
Under certain conditions, this appearance of structure in the living
interphase nucleus is reversible. It has been found that change in the
interphase nucleus is the result of a change in the state of the chromosomes
(Ris and Mirskey, 1949). In the living nucleus the chromosomes are in a
greatly extended state, filling the entire nucleus. But, after injury to the
interphase it showed condensed chromosomes.
• At the beginning of mitosis (Prophase), the chromosomes condense,
the nucleolus disappears, and the nuclear envelope breaks down, resulting in
the release of most of the contents of the nucleus into the cytoplasm. At the
end of mitosis (Telophase), the process is reversed: the chromosomes
decondense, and nuclear envelopes re-form around the separated sets of
daughter chromosomes.

Closed and open mitosis
In closed mitosis (unicellular eukaryotes
e.g.: yeasts), the nuclear envelope remains
intact and chromosomes migrate to opposite
poles of a spindle within the nucleus. In
open mitosis (most of the multicellular
eukaryotes), the nuclear envelope breaks
down and then re-forms around the two sets
of separated chromosomes.
Source: https://www.ncbi.nlm.nih.gov

Breakdown of the nuclear membrane
As the nuclear lamina dissociates, the nuclear membrane fragments into vesicles. The B-type lamins remain bound to
these vesicles, while lamins A and C are released as free dimers.

Re-formation of the nuclear envelop
• The first step in reassembly of the nuclear
envelope is the binding of membrane
vesicles to chromosomes, which may be
mediated by both integral membrane
proteins and B-type lamins (intermediate
filament proteins that form the nuclear
lamina).
• The vesicles then fuse, the nuclear lamina
(provides structural support to the
nucleus) reassembles, and the chromosomes
decondense.

Functions of Nucleus
• It controls the heredity characteristics of an organism.
• It is responsible for protein synthesis, cell division, growth and
differentiation.
• Stores heredity material in the form of deoxy-ribonucleic acid (DNA)
strands.
• Also stores proteins and ribonucleic acid (RNA) in the nucleolus.
• It is a site for transcription process in which messenger RNA (m RNA) are
produced for protein synthesis.
• Aids in exchange of DNA and RNA (heredity materials) between the
nucleus and the rest of the cell.
• Nucleolus produces ribosomes and are known as protein factories.
• It also regulates the integrity of genes and gene expression.

• Nucleus is the site for Central Dogma, the process by which the instructions in DNA
are converted into a functional product . DNA replication, transcription, and RNA
processing all take place within the nucleus, with only the final stage of gene expression
(i.e. translation) occurs in the cytoplasm. Therefore, It is also called the control center
of the cell.

Mitochondria: Energy Generators
of the Cell

• Most eukaryotic cells contain many mitochondria, which occupy up to 25
percent of the volume of the cytoplasm.
• The term "mitochondria" was first coined by Carl Benda in 1898. In
1904, Friedrich Meves, made the first recorded observation of mitochondria
in plants in cells of the white waterlily, Nymphaea alba.
• Although the nucleus is the cell’s control center, none of the cellular
activities it commands could proceed without a constant supply of energy.
• The bulk of the energy is generated in most eukaryotes by mitochondria
and called ‘The Power House of Cell’.
• These organelles generate power by converting energy into forms i.e. ATP,
that are usable by the cell through the cellular respiration.
• When viewed with light microscopy, mitochondria appear as round or
elongated particles scattered throughout the cytoplasm.
• These complex organelles are among the largest organelles, generally
exceeded in size only by the nucleus, vacuoles, and chloroplasts.

Mitochondrion Anatomy
• The internal ultrastructure reveals that a single mitochondrion is bound by a double
membrane. Each of these membranes is a phospholipid bilayer with embedded proteins.
• The outermost membrane is smooth while the inner membrane has many folds. These folds
are called cristae. The folds enhance the "productivity" of cellular respiration by increasing
the available surface area.
• The cristae membranes hold the enzymes and electron carriers of aerobic respiration and
stores energy in the form of high-energy molecules, or ATP.
• The spaces around the cristae are filled with a chemically complex fluid called the matrix,
which holds ribosomes, DNA, and the pool of enzymes and other compounds involved in
the metabolic cycle.
• The mitochondrial matrix contains circular mitochondrial DNA (mtDNA), ribosomes
(prokaryotic sized 70S type), and enzymes. Several of the steps in cellular respiration,
including the Citric Acid Cycle and oxidative phosphorylation, occur in the matrix due to its
high concentration of enzymes.

Fig: Mitochondrion Detailed 3D Structure Source: Microbiology A system approach

Mitochondria: Distinguishing Characteristics
• Mitochondria have a distinctive oblong or oval shape and are bounded by a double
membrane.
• They are found in both animal and plant cells.
• The number of mitochondria within a cell varies depending on the type and
function of the cell.
• Some cells, such as mature red blood cells, do not contain mitochondria at all.
• The absence of mitochondria and other organelles leaves room for the millions of
hemoglobin molecules needed in order to transport oxygen throughout the body.
• Muscle cells, on the other hand, may contain thousands of mitochondria needed to
provide the energy required for muscle activity. Mitochondria are also abundant
in fat cells and liver cells.
• Porins are found in mitochondria and chloroplasts which are beta
barrel proteins that cross a cellular membrane and act as a pore through
which molecules can diffuse.

Mitochondrial genome or DNA
• Mitochondria contain their own genetic system, which is separate and distinct from
the nuclear genome of the cell.
• Mitochondria have their own circular DNA, ribosomes and can make their
own proteins.
• The genomes of human and most other animal mitochondria are only about 16 kb, but
substantially larger mitochondrial genomes are found in yeasts (approximately 80 kb)
and plants (more than 200 kb).
• Mitochondrial DNA (mtDNA) encodes for proteins that are involved in electron
transport and oxidative phosphorylation, which occur in cellular respiration.
• Proteins synthesized from mtDNA also encode for the production of the RNA
molecules: transfer RNA and ribosomal RNA.
• Mitochondrial DNA differs from DNA found in the cell nucleus in that it does not
possess the DNA repair mechanisms that help prevent mutations in nuclear DNA.
• As a result, mtDNA has a much higher mutation rate than nuclear DNA. Exposure to
reactive oxygen produced during oxidative phosphorylation also damages mtDNA.
• Diseases caused by mutation in the mtDNA include Kearns-Sayre syndrome, MELAS
syndrome and Leber's hereditary optic neuropathy etc.

Mitochondrial Replication and Inheritance
• Mitochondria are semi-autonomous in that they are only partially dependent
on the cell to replicate and grow.
• They have their own DNA, ribosomes, make their own proteins, and have
some control over their reproduction. Similar to bacteria, mitochondria have
circular DNA and replicate by a reproductive process called binary fission.
• Prior to replication, mitochondria merge together in a process called fusion.
Fusion is needed in order to maintain stability, as without it, mitochondria
will get smaller as they divide. These smaller mitochondria, if produced, are
not able to produce sufficient amounts of energy needed for proper cell
function.
• An individual's mitochondrial genes are not inherited by the same
mechanism as nuclear genes. Typically, the mitochondria are inherited from
one parent only.

Mitochondrial Fission and Fusion
(A) Generalized figure of fission and fusion (B) Electron Micrograph of Mitochondrial fission and fusion

Functions of Mitochondria
• The most important function of the mitochondria is to produce energy. The
simpler molecules of nutrition are sent to the mitochondria to be processed
and to produce charged molecules. These charged molecules combine with
oxygen and produce ATP molecules. This process is known as oxidative
phosphorylation.
• Mitochondria help the cells to maintain proper concentration of calcium
ions which increase mitochondrial uptake up to 10-1000folds and begin to
shape dynamics within the compartments of the cell.
• The mitochondria also help in building certain parts of blood and hormones
like testosterone and estrogen.
• The liver cells mitochondria have enzymes that detoxify ammonia.
• The mitochondria also play important role in the process of apoptosis or
programmed cell death. Abnormal death of cells due to the dysfunction of
mitochondria can affect the function of organ.

Plastids: Sites for Photosynthesis and Storage and
Pigmentation

• Plastids are thought to have originated from endosymbiotic cyanobacteria.
• The term plastid was derived from the Greek word plastikas meaning
formed or moulded. This term was coined by Schimper in 1885.
• Plastids are membrane-bound organelles that convert energy from
foodstuffs (non-photosynthetic plastids) or sunlight (chloroplasts) into
cellular energy.
• Chloroplasts and other non- photosynthetic plastids develop from cells
called proplastids.
• Proplastids are immature, undifferentiated cells that develop into different
types of plastids.
• A proplastid that develops into a chloroplast, only does so in the presence of
light.

Types of Plastids
The different types of plastids are frequently classified according to the kinds of
pigments they contain.
• Chloroplasts: Plastids which contain chlorophyll, takes part in photosynthesis and
also called green plastids.
• Chromoplasts: Plastids which lack chlorophyll but contain carotenoids; they are
responsible for the yellow, orange, and red colors of some flowers and fruits,
although their precise function in cell metabolism is not clear.
• Leucoplasts: Plastids which are non-pigmented plastids, which store a variety of
energy sources in non-photosynthetic tissues. Amyloplasts and elaioplasts are
examples of leucoplasts that store starch and lipids, respectively. Proteinoplasts
(sometimes called proteoplasts, aleuroplasts, and aleuronaplasts) are specialized
organelles found only in plant cells. They contain crystalline bodies of protein and
can be the sites of enzyme activity involving those proteins. Tannosomes are
another kind of leucoplasts for synthesizing and producing tannins and
polyphenols.

Chloroplast: Site for Photosynthesis

Chloroplast : Introduction
• A chloroplast is a type of plant cell organelle in which photosynthesis
occurs. It is 5 to 10 μm long (1μm = 10-6 m.) with double membrane
envelope.
• A chloroplast contains a green pigment called chlorophyll, which
absorbs light energy for photosynthesis. Hence, the name chloroplast
indicates that these structures are chlorophyll containing plastids.
• Chloroplasts are also responsible for producing amino acids and
lipid components needed for chloroplast membrane production.
• Chloroplasts can also be found in other photosynthetic organisms such
as algae.

Chloroplast: Structure
• Plant chloroplasts develop mainly in cells located in plant leaves.
Chloroplast consists following structural components with their specific
functions:
• Membrane Envelope - contains an inner and outer membranes that act as
protective coverings and keep chloroplast structures enclosed.
• Thylakoid Membrane - internal membrane system consisting of flattened
sac-like membrane structures called thylakoids that serve as the sites of
conversion of light energy to chemical energy.
• Grana (singular granum) - dense layered stacks of thylakoid sacs that
serve as the sites of conversion of light energy to chemical energy.
• Stroma - dense fluid within the chloroplast that lies inside the envelope but
outside the thylakoid membrane. This is the site of conversion of carbon
dioxide to carbohydrates(sugar).
• Chlorophyll - a green photosynthetic pigment within the chloroplast that
absorbs light energy and plays a vital role in photosynthesis.

Chloroplast: Structure
Source: Bing Images

Chloroplast: Photosynthesis
• In photosynthesis, the sun's solar energy is converted to chemical energy. The
chemical energy is stored in the form of glucose (sugar).
• Carbon dioxide, water, and sunlight are used to produce glucose, oxygen, and
water. Photosynthesis occurs in two stages. These stages are known as the light
reaction stage and the dark reaction stage.
• The light reaction stage takes place in the presence of light and occurs within the
chloroplast grana. The primary pigment used to convert light energy into chemical
energy is chlorophyll a. Other pigments involved in light absorption include
chlorophyll b, xanthophyll, and carotene. In the light reaction stage, sunlight is
converted to chemical energy in the form of ATP (free energy containing
molecule) and NADPH (high energy electron carrying molecule). This process is
done by electron transport chain reaction in thylakoid membranes of chloroplast.
• Both ATP and NADPH are used in the dark reaction stage to produce sugar.
The dark reaction stage is also known as the carbon fixation stage or the Calvin
cycle. Dark reactions occur in the stroma. The stroma contains enzymes which
facilitate a series of reactions that use ATP, NADPH, and carbon dioxide to
produce sugar. The sugar can be stored in the form of starch, used during
respiration, or used in the production of cellulose.

The reactions of photosynthesis in a chloroplast:
• Water is oxidized and oxygen is released in
the photosynthetic electron-transfer reactions
(Light Reaction) while carbon dioxide is
assimilated (fixed) to produce sugars and a
variety of other organic molecules (Dark
Reactions) in the carbon-fixation reactions.
• The energy thus released during those reactions
is stored in the form of proton gradient in the
thylakoid membrane of chloroplast and is
utilized later by cells.
• A total of 3 molecules of ATP and 2 molecules
of NADPH are consumed for each
CO2 molecule converted into carbohydrate.
• The net equation is:

The Chloroplast Genome and Replication
• Like mitochondria, chloroplasts contain their own genetic system,
reflecting their evolutionary origins from photosynthetic bacteria.
• The genomes of chloroplasts are similar to those of mitochondria in
that they consist of circular DNA molecules present in multiple copies
per organelle.
• However, chloroplast genomes are larger and more complex than those
of mitochondria, ranging from 120 to 160 kb and containing
approximately 120 genes.
• Like mitochondria, chloroplasts have their own DNA, are responsible
for energy production, and reproduce independently from the rest of
the cell through a division process similar to bacterial binary fission.

Protein Import and Sorting in Mitochondria and Plastids
• The production of mitochondrial and chloroplast proteins occur by
two separate genetic systems. Most of the proteins in these
organelles are encoded by the nucleus and must be imported from
the cytosol while other organelle proteins are encoded by organelle
DNA.
• But, no known proteins are exported from mitochondria or
chloroplasts to the cytosol.
• The multiple copies of mitochondrial and chloroplast DNA
contained within the matrix or stroma of these organelles are usually
distributed in several clusters, called nucleoids.
• Nucleoids are thought to be attached to the inner mitochondrial
membrane. Although it is not known how the DNA is packaged, the
DNA structure in nucleoids is likely to resemble that in bacteria
rather than that in eukaryotic chromatin. As in bacteria, for example,
there are no histones.
• Although chloroplasts encode more of their own proteins than
mitochondria, about 90% of chloroplast proteins are still encoded by
nuclear genes. Source: https://www.ncbi.nlm.nih.gov

Chemiosmotic generation of ATP in chloroplasts and mitochondria
• In mitochondria, electron transport generates a
proton gradient across the inner membrane, which is
then used to drive ATP synthesis in the matrix. 34
ATP molecules are generated from single glucose
molecule.
• In chloroplasts, the proton gradient is generated
across the thylakoid membrane and used to drive
ATP synthesis in the stroma.
• This proton gradient serves as an energy store that
can be used to drive ATP synthesis by the ATP
synthase enzyme.
• Pump proteins maintains the proton gradient and
harness the energy of electron transfer and ATP
synthase synthesizes ATP from ADP in presence of
inorganic phosphate (Pi).

Characteristics: Mitochondria vs.
Chloroplasts
• Chloroplast contains thylakoid membranes and pigment molecules,
whereas the mitochondria membranes contain respiratory enzymes
not found in chloroplast membranes.
• Chloroplasts are found in plants only while mitochondria are found in
both plants and animals.
• Besides production of ATP, chloroplasts help in photosynthesis and
produce food molecules like sugars, amino acids, fatty acids, lipids
whereas mitochondria generates energy in the form of ATP by
degradation of food molecules.

References
https://www.ncbi.nlm.nih.gov (date accessed: 19th to 24th September 2016)
https://biology.about.com (date accessed: 19th to 24th September 2016)
https://en.wikipedia.org (date accessed: 19th to 24th September 2016)
Cowan, M. K., 2012. Microbiology - A Systems Approach (3rd Ed). McGraw-Hill Publications, New
York. P. 108-138
Lodish, H., Berk, A., Zipursky, S.L. 2000. Molecular Cell Biology, 4th edition. New York: W. H.
Freeman

Cellular organelles nucleus, mitochondria & plastids

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