MEMBRANE BOUNDEDORGANELLES IN

EUKARYOTIC CELL

Hapuarachchige Dona Thisarani Sankalpana Samarasinghe

Diploma in Bio Medical ScienceJune 2016

Introduction.The cell concept 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

Ultrastructure of Animal cell

Nucleus: Information CentralThe 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

Nuclear lamina

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.

Nuclear envelope and nuclear pores

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.

Chromosome

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.

Nucleolus

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

Ribosomes: Protein Factories

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.

Endoplasmic reticulum: Biosynthetic Factory

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.

Rough endoplasmic reticulum (RER)

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.

Smooth endoplasmic reticulum.

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.

Sarcoplasmic reticulum (SR)

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.

Golgi apparatus: Shipping and Receiving Center

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: Digestive Compartments

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.

Vacuoles: Diverse Maintenance

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 cell2. • Containing waste products3. • Containing water in plant cells4. • Maintaining internal hydrostatic pressure or turgor within the cell5. • Maintaining an acidic internal pH6. • Containing small molecules7. • Exporting unwanted substances from the cell8. • Allows plants to support structures such as leaves and flowers due to the

pressure of the central vacuole9. • 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: Chemical Energy Conversion

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: Capture of Light Energy

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.

Peroxisome: Oxidation

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.

Glyoxysome

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