Unit 2 Structures and Functions of Cell Organelles PDF

◤ UNIT-2 STRUCTURES AND FUNCTIONS OF CELL ORGANELLES ◤ ▪ 2.1-Objectives ▪ 2.4- Ribosomes ▪ 2.2-Introduction ▪ 2.4.1-Origin ▪ 2.3-Nucleus ▪ 2.4.2-Structure ▪ 2.3.1-Origin ▪ 2.4.3-Function ▪ 2.3.2-Stru...

◤ UNIT-2 STRUCTURES AND FUNCTIONS OF CELL ORGANELLES ◤ ▪ 2.1-Objectives ▪ 2.4- Ribosomes ▪ 2.2-Introduction ▪ 2.4.1-Origin ▪ 2.3-Nucleus ▪ 2.4.2-Structure ▪ 2.3.1-Origin ▪ 2.4.3-Function ▪ 2.3.2-Structure ▪ 2.5-Nucleoplasm ▪ 2.3.3-Function ▪ 2.5.1-Origin ▪ 2.5.2-Structure ▪ 2.5.3-Function ◤ 2.1 OBJECTIVE ▪ Main objective of this unit is to provide student sufficient knowledge about the cell so that student will be able to- ▪  Define Cytology and Organelles very well. ▪  Able to identify the parts of a cell from models or diagrams given and will get to know their general function. ▪  Know about Organelle such as Endoplasmic reticulum (SER and RER), Chloroplast, Mitochondrion ▪  Know about Nucleus & Nuclear membrane (envelope), Nucleolus ◤ ▪ 2.5-Nucleoplasm ▪ 2.7-Chloroplast ▪ 2.5.1-Origin ▪ 2.7.1-Origin ▪ 2.5.2-Structure ▪ 2.7.2-Structure ▪ 2.5.3-Function ▪ 2.7.3-Function ▪ 2.6-Mitochondria ▪ 2.8-Types of plastids ▪ 2.6.1-Origin ▪ 2.9-Golgi complex ▪ 2.6.2-Structure ▪ 2.10-Endoplasmic reticulum ▪ 2.6.3-Function ◤ ▪ 2.11- Summary ▪ 2.12- Glossary ▪ 2.13-Self Assessment Question ▪ 2.14- References ▪ 2.15-Suggested Readings ▪ 2.16-Terminal Questions ◤ 2.2 INTRODUCTION ▪ Our natural world originated the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells. ▪ Unlike prokaryotic cells, eukaryotic cells have: ▪ (1) a membrane-bound nucleus; ▪ (2) numerous membrane-bound organelles—such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and ▪ (3) several, rod-shaped chromosomes. Because a eukaryotic cell‘s nucleus is surrounded by a membrane, it is often said to have a ―true nucleusǁ. The word ―organelleǁ means ―little organ,ǁ and, organelles have specialized cellular functions, just as the organs of your body have specialized functions. ◤ ▪ Cells are the smallest units of life. ▪ They are a closed system, can self-replicate, and are the building blocks of our bodies. In order to understand how these tiny organisms work, we will look at a cell‘s internal structures. ▪ We will focus on eukaryotic cells, cells that contain a nucleus. ▪ A cell consists of two major regions, the cytoplasm and the nucleus. ▪ The nucleus is surrounded by a nuclear envelope and contains DNA in the form of chromosomes. ▪ The cytoplasm is a fluid matrix that usually surrounds the nucleus and is bound by the outer membrane of the cell. ◤ ▪ Organelles ▪ are small structures within the cytoplasm that carry out functions necessary to maintain homeostasis in the cell. They are involved in many processes, for example energy production, building proteins and secretions, destroying toxins, and responding to external signals. ◤ ▪ Organelles are considered either membranous or non-membranous. ▪ Membranous organelles possess their own plasma membrane to create a lumen separate from the cytoplasm. ▪ This may be the location of hormone synthesis or degradation of macromolecules. ◤ ▪ Non-membranous organelles are not surrounded by a plasma membrane. ▪ Most non-membranous organelles are part of the cytoskeleton, the major support structure of the cell. These include: filaments, microtubules, and centrioles ◤ ▪ Ribosomes, as a site for turning RNA code into protein sequences, and chromosomes, the DNA storage complex, are examples of non-membrane organelles. ▪ These non-membranous organelles are commonly molecular complexes. ▪ They may have complex functions, but the processes by which those functions are done are usually localized to the surfaces of the complex. ▪ They neither require specific isolation nor a large working surface of membrane. ▪ Some functional parts of a eukaryote cell are types of extensions of the external membrane. ▪ They will be treated here as cell extension organelles, although they are not always called "organelles" in some biology books. ◤ ▪ The "soup" inside a cell, often so thick that it becomes a gel, has various names. ▪ In prokaryotes, its protoplasm. ▪ In eukaryotes, the material between the cell membrane and the nuclear envelope is usually called cytoplasm, which sometimes is further divided as cytosol is considered to be just outside the organelles. ▪ The material inside the nucleus is usually called nucleoplasm. ▪ All these organelles along with their structures and functions have been discussed in this unit. ◤ 2.3 NUCLEUS ▪ Nucleus the most prominent organelle of the cell. The number of nuclei may vary, they may be uninucleate (single nucleus), binucleate (two nuclei) or even multi-nucleate. ▪ Certain eukaryotic cells such as the mature sieve tubes of higher plants and mammalian erythrocytes contain no nucleus. ▪ Prokaryotic cells lack nucleus and is complemented by nucleoid. ◤ ▪ The contents of the nucleus are DNA genome, RNA synthetic apparatus, and a fibrous matrix. ▪ It is surrounded by two membranes, each one a phospholipid bilayer containing many different types of proteins. ▪ The inner nuclear membrane defines the nucleus itself. ◤ ▪ In most cells, the outer nuclear membrane is continuous with the rough endoplasmic reticulum, and the space between the inner and outer nuclear membranes is continuous with the lumen of the rough endoplasmic reticulum. ◤ ▪ The two nuclear membranes appear to fuse at nuclear pores, the ring-like complexes composed of specific membrane proteins through which material moves between the nucleus and the cytosol. ▪ It contains cell's genetic material, organized as multiple long linear ◤ ▪ DNA molecules in complex with histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome. ▪ The function is to maintain the integrity of the genes that controls the activities of the cell by regulating gene expression. ◤ History ▪ Nucleus was the first cell organelle to be discovered. ▪ Antonie von Leeuwenhoek (1632 - 1723) observed lumen (nucleus) in the red blood cells of salmon. ▪ The nucleus was also described in 1804 by Franz Bauer (14 March 1758 – 11 December 1840) an Austrian microscopist & botanical artist, and, in more detail in 1831 by Scottish botanist Robert Brown (21 December 1773 – 10 June 1858) in a talk at the Linnean Society of London. ◤ ▪ Brown was studying orchids under microscope when he observed an opaque area, which he called the "areola" or "nucleus", in the cells of the flower's outer layer. ▪ It was discovered and named by Robert Brown in 1833 in the plant cells and is recognized as a constant feature of all animal and plant cells. ◤ Nucleus Definition ▪ In cell biology, the nucleus (plural-nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane-enclosed organelle found in eukaryotic cells. ▪ 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. ▪ Human skeletal muscle cells have more than one nucleus, as do eukaryotes like fungi. ◤ ▪ 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 genes within these chromosomes are the cell's nuclear genome and are structured in such a way to promote cell function. ▪ The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. ◤ 2.3.1- Origin ▪ A study of the comparative genomics, evolution and origins of the nuclear membrane led to the proposal that the nucleus emerged in the primitive eukaryotic ancestor (the ―prekaryoteǁ), and was triggered by the archaeo-bacterial symbiosis. ▪ Several ideas have been proposed for the evolutionary origin of the nuclear membrane. ◤ ▪ These ideas include the invagination of the plasma membrane in a prokaryote ancestor, or the formation of a genuine new membrane system following the establishment of proto-mitochondria in the archaeal-host. ▪ The adaptive function of the nuclear membrane may have been to serve as a barrier to protect the genome from reactive oxygen species (ROS) produced by the cells' pre-mitochondria. ◤ 2.3.2- Nucleus Structure ▪ The nucleus is the largest organelle of the cell. It occupies about 10% of the total volume of the cell. In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. ▪ The viscous liquid within it is called nucleoplasm (karyolymph), and is similar in composition to the cytosol found outside the nucleus. ◤ ▪ Generally there is a single nucleus per cell (Mononucleate conditions), but more than one nucleus (Polynucleate condition) may be found in certain special cases. ▪ There are many nuclei in a syncytium which is formed due to fusion of cells. ▪ A similar multinucleate situation is found in coenocytes commonly found in plants. ▪ A coenocyte results by repeated nuclear divisions without cytokinesis ◤ ▪ There are also variations with respect to shape and size of nucleus. ▪ It may be spherical oval to flattened lobe or irregular in shape. ▪ Shape of nucleus also depends on the cell. The spheroid, cuboid or polyhedral cells, nucleus is usually spheroid. In cylindrical, prismatic or fusiform cells, nucleus is ellipsoid. ◤ Nuclear Envelope ▪ 1. The nuclear envelope is also known as the nuclear membrane. ▪ 2. It is made up of two membranes the outer membrane and the inner membrane. ▪ 3. The outer membrane of the nucleus is continuous with the membrane of the rough endoplasmic reticulum. ▪ 4. The space between these layers is known as the Perinuclear space. ▪ 5. The nuclear envelope encloses the nucleus and separates the genetic material of the cell from the cytoplasm of the cell. ▪ 6. It also serves as a barrier to prevent passage of macro-molecules freely between the nucleoplasm and the cytoplasm. ◤ Nuclear Pore ▪ 1. The nuclear envelope is perforated with numerous pores called nuclear pores. ▪ 2. The nuclear pores are composed of many proteins known as nucleoproteins. ▪ 3. The nuclear pores regulate the passage of the molecules between the nucleus and cytoplasm. ▪ 4. The pores allow the passage of molecules of only about 9nm wide. The larger molecules are transferred through active transport. ▪ 5. Molecules like of DNA and RNA are allowed into the nucleus. But energy molecules (ATP), water and ions are permitted freely. ◤ Chromosomes (Chromatin structure) ▪ 1. The nucleus of the cell contains majority of the cells genetic material in the form of multiple linear DNA molecules. ▪ 2. These DNA molecules are organized into structures called chromosomes. ▪ 3. The DNA molecules are in complex with a large variety of proteins (histones) which form the chromosome ◤ ▪ 4. In the cell they are organized in a DNA-protein complex known as chromatin. ▪ a. Chromatin = DNA + Histone + DNA binding proteins. ▪ b. Two type of chromatin are present. ▪ (i) Euchromatin ▪ (ii) Heterochromatin ▪ 5. During cell-division the chromatin forms well-defined chromosomes. ▪ 6. The genes within the chromosomes consists of the cells nuclear genome. ▪ 7. Mitochondria of the cell also contains a small fraction of genes. ▪ 8. Human cells has nearly 6 feet of DNA, which is divided into 46 individual molecules. ◤ Nucleolus ▪ 1. The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus. ▪ 2. The nucleoli are formed around the nuclear organizer regions. ▪ 3. It synthesizes and assembles ribosomes and r-RNA. ◤ ▪ 4. The number of nucleoli is different from species to species but within a species the number is fixed. ▪ 5. During cell division, the nucleolus disappears. ▪ 6. Studies suggest that nucleolus may be involved in cellular aging and senescence. ◤ ▪ In the nucleolus seems to proceed from center to periphery 3 distinct region are: ▪ i) Fibriller Center (FC): Where r-RNA genes of nucleolus organizer region (NOR) are located, the transcription of r-RNA genes also takes place in this region. ▪ ii) Dense Fibriller Component (DFC): Which surround the fibriller genes and where RNA synthesis progress. The 80S ribosomal proteins also bind to the transcripts in this region. ▪ iii) Cortical Granular Component(CGC): It is the inner-most region and where processing and maturation of pre ribosomal particles occurs. ▪ Therefore, these region roles in ―ribosome formation. ◤ 2.3.3- Functions of the Nucleus ▪ Speaking about the functions of a cell nucleus, it controls the hereditary characteristics of an organism. This organelle is also responsible for the protein synthesis, cell division, growth, and differentiation. Some important functions carried out by a cell nucleus are: ▪ 1. Storage of hereditary material, the genes in the form of long and thin DNA (deoxyribonucleic acid) strands, referred to as chromatins. ▪ 2. Storage of proteins and RNA (ribonucleic acid) in the nucleolus. ▪ 3. It is responsible for protein synthesis, cell division, growth and differentiation. ▪ 4. Nucleus is a site for transcription in which messenger RNA (mRNA) are produced for the protein synthesis. ◤ ▪ 5. It controls the heredity characteristics of an organism. Exchange of hereditary molecules (DNA and RNA) between the nucleus and rest of the cell. ▪ 6. During the cell division, chromatins are arranged into chromosomes in the nucleus. ▪ 7. Production of ribosomes (protein factories) in the nucleolus. ▪ 8. Selective transportation of regulatory factors and energy molecules through nuclear pores. ▪ 9. It also regulates the integrity of genes and gene expression. ◤ Animal Cell Nucleus ▪ Animal cell nucleus is a membrane bound organelle. It is surrounded by double membrane. ▪ The nucleus communicates with the surrounding cell cytoplasm through the nuclear pores. ▪ The DNA in the nucleus is responsible for the hereditary characteristics and protein synthesis. The active genes on the DNA are similar, but some genes may be turned on or off depending on the specific cell type. ▪ This is the reason why a muscle cell is different from a liver cell. Nucleolus is a prominent structure in the nucleus. This aids in ribosomes production and protein synthesis. ◤ Plant Cell Nucleus ▪ Plant cell nucleus is a double-membrane bound organelle. It controls the activities of the cell and is known as the master mind or the control center of the cell. ▪ The plant cell wall has two layers -the outer membrane and the inner membrane, which encloses a tiny space known as perinuclear space. ◤ ▪ The nucleus communicates to the cell cytoplasm through the nuclear pores present in the nuclear membrane. ▪ The nuclear membrane is continuous with the endoplasmic reticulum. ▪ The DNA is responsible for cell division, growth and protein synthesis. ◤ Bacterial Cell Nucleus ▪ Bacteria are minute single-celled microorganisms under the domain Prokaryota. ▪ Interestingly, they are believed to be the direct descendants of the first ever organisms that thrive on Earth about 3.5 billion years ago. ▪ While they seem to be invisible with the naked eye, under powerful microscopes, the structures within bacteria can be observed. ◤ ▪ The bacterial cell does not contain any nucleus. ▪ The bacterial chromosome is not enclosed in a membrane bound nucleus. ▪ The bacterial chromosome is circular and located in the cytoplasm ◤ 2.4 RIBOSOME ▪ Proteins are necessary for the cells to perform cellular functions. ▪ Ribosomes are the cellular components that make proteins from all amino acids. Ribosomes are made from complexes of RNAs and proteins. ▪ The number of ribosomes in a cell depends on the activity of the cell. ◤ ▪ Ribosomes are freely suspended in the cytoplasm or attached to the endoplasmic reticulum forming the rough endoplasmic reticulum. ▪ On an average in a mammalian cell there can be about 10 million ribosomes ◤ ▪ When the ribosomes are attached to the same mRNA strand, this structure is known as Polysome. ▪ The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units separate and is reused or broken up. ◤ ▪ Amino acids are joined by the ribosomes at a rate of 200 per minute. ▪ Therefore small proteins can be made quickly but two or three hours are needed for proteins which are as large as 30,000 amino acids. ◤ ▪ The ribosomes present in the prokaryotes function differently in protein production than the ribosomes of the eukaryote organisms. ▪ The ribosomes of bacteria, archaea and eukaryotes differ significantly from each other in structure and RNA sequences. ◤ ▪ The differences in the ribosomes allows the antibiotic to kill the bacterial ribosome by inhibiting the activity of the bacterial ribosomes, the human ribosome remain unaffected. ▪ The ribosomes of the eukaryotic cells are similar to the ribosomes of the bacterial cells, showing the evolutionary origin of the organelle. ◤ Ribosomes Definition ▪ Ribosomes are small particles, present in large numbers in all the living cells. ▪ They are sites of protein synthesis. ◤ ▪ The ribosome word is derived - 'ribo' from ribonucleic acid and 'somes' from the Greek word 'soma' which means 'body’. ▪ The ribosomes link amino acids together in the order that is specified by the messenger RNA molecules. ◤ ▪ The ribosomes are made up of two subunits - a small and a large subunit. ▪ The small subunit reads the mRNA while the large subunit joins the amino acids to form a chain of polypeptides. ◤ ▪ Ribosomal subunits are made of one or more rRNA (ribosomal RNA) molecules and various proteins. ▪ The ribosomes and associated molecules are also known as the translational apparatus. ◤ ▪ Ribosomes were first observed in the mid-1950s by Romanian-American cell biologist George Emil Palade, using an electron microscope, as dense particles or granules. ▪ The term "ribosome" was proposed by scientist Richard B. Roberts in the end of 1950s. ◤ Types of Ribosomes ▪ Ribosomes are classified into two types based on their sedimentation coefficient, 70S and 80S. ▪ S stands for "Svedberg unit" and related to sedimentation rate (sedimentation depends on mass and size). ▪ Thus, the value before S indicates size of ribosome. ◤ 2.4.1- Origin ▪ The ribosome may have first originated in an RNA world, appearing as a self-replicating complex that only later evolved the ability to synthesize proteins when amino acids began to appear. ▪ Studies suggest that ancient ribosomes constructed solely of rRNA could have developed the ability to synthesize peptide bonds. ◤ ▪ In addition, evidence strongly points to ancient ribosomes as self-replicating complexes, where the rRNA in the ribosomes had informational, structural, and catalytic purposes because it could have coded for tRNAs and proteins needed for ribosomal self-replication. ◤ ▪ In the prokaryotes, the ribosome originates in the cytoplasm as there is no nucleolus ▪ In eukaryotes, the ribosome is partly nucleolar (rRNA) and partly cytoplasmic (proteins) in origin ◤ 2.4.2- Structure ▪ 1. Ribosomes are tiny particles about 200 Ã. ▪ Ribosomes in a cell are located in two regions of the cytoplasm. ▪ They are found scattered in the cytoplasm and some are attached to the endoplasmic reticulum. ◤ ▪ 2. When the ribosomes are bound to the ER there are known as the Rough Endoplasmic Reticulum (RER). ▪ The bound and the free ribosomes are similar in structure and are involved in protein synthesis ◤ ▪ 4. Ribosome is made up of two subunits. The subunits of ribosomes are named according to their ability of sedimentation on a special gel which the Svedberg Unit. ◤ ▪ 5. Prokaryotes have 70S ribosomes, each subunit consisting of small subunit is of 30S and the large subunit is of 50S. ▪ Eukaryotes have 80S ribosomes, each consisting of small (40S) and large (60S) subunit. ◤ ▪ 6. The ribosomes found in the chloroplasts of mitochondria of eukaryotes consists of large and small subunits bound together with proteins into one 70S particle. ◤ ▪ 7. The ribosomes share a core structure which is similar to all ribosomes despite differences in its size. ◤ ▪ 8. The RNA is organized in various tertiary structures. ▪ The RNA in the larger ribosomes are into several continuous insertion as they form loops out of the core structure without disrupting or changing it.  ◤ ▪ 9. The catalytic activity of the ribosome is carried out by the RNA, the proteins reside on the surface and stabilize the structure. ◤ 2.4.3-Function ▪ 1. Nearly all the proteins required by cells are synthesized by ribosomes. Ribosomes are found free‘ in the cell cytoplasm and also attached to rough endoplasmic reticulum. ▪ 2. Ribosomes receive information from the cell nucleus and construction materials from the cytoplasm. ▪ 3. Ribosomes translate information encoded in messenger ribonucleic acid (mRNA). ▪ 4. They link together specific amino acids to form polypeptides and they export these to the cytoplasm. ▪ 5. A mammalian cell may contain as many as 10 million ribosomes, but each ribosome has only a temporary existence. ◤ ▪ 6. Ribosomes can link up amino acids at a rate of 200 per minute. ▪ 7. Ribosomes are formed from the locking of a small sub-unit on to a large sub-unit. The sub-units are normally available in the cytoplasm, the larger one being about twice the size of the smaller one. ▪ 8. Each ribosome is a complex of Ribonucleoprotein with two-thirds of its mass is composed of ribosomal RNA and about one-third ribosomal protein. ▪ 9. Protein production takes place in three stages: (1) Initiation, (2) elongation, and (3) termination. ◤ ▪ 10. During peptide production the ribosome moves along the mRNA in an intermittent process called translocation. ▪ 11. Antibiotic drugs such as streptomycin can be used to attack the translation mechanism in prokaryotes. This is very useful. Unfortunately some bacterial toxins and viruses can also do this. ▪ 12. After they leave the ribosome most proteins are folded or modified in some way. This is called ‗post translational modification‘. ◤ 2.5 NUCLEOPLASM ▪ The nucleus of most cells contains a substance that suspends structures inside the nuclear membrane. ▪ Just like the cytoplasm found inside a cell, the nucleus contains nucleoplasm, also known as Karyoplasm ◤ ▪ The nucleoplasm is a type of protoplasm that is made up mostly of water, a mixture of various molecules, and dissolved ions. It is completely enclosed within the nuclear membrane or nuclear envelope. ▪ It is a highly gelatinous, sticky liquid that supports the chromosomes and nucleoli. ◤ ▪ The soluble, fluid component of the nucleoplasm is called the Nucleosol or Nuclear Hyaloplasm. ▪ The nucleoplasm includes the chromosomes and nucleoli. Many substances such as nucleotides (necessary for purposes such as the replication of DNA) and enzymes (which direct activities that take place in the nucleus) are dissolved in the nucleoplasm ◤ ▪ The term "nucleoplasm" was coined by embryologist, cytologist and marine biologist Edouard van Beneden (1875), while "karyoplasm" was by Walther Flemming (1878) a German biologist and a founder of Cytogenetics. ◤ 2.5.1- Origin ▪ It arises from the nuclear content and chromatin content contained in a cell nucleus. ◤ 2.5.2- Structure ▪ The nucleoplasm consists of a viscous mix of water, in which various substances and structures are dissolved or carried, and an underlying intranuclear ultrastructure. ▪ The nucleoplasm is especially rich in protein enzymes and protein constituents involved in the synthesis of deoxyribonucleic acid (DNA) and the various types of ribonucleic acid (RNA), the precursor molecules of RNA, and the nucleotides from which they are assembled. ◤ ▪ Some of these proteins direct initial transcription, while others function in the further modification of the RNA molecules for packaging and transport to the cytoplasm ◤ ▪ Prominent structures located within the interphase nucleoplasm (the resting cell or the non replicating cell) include organelles called nucleoli and the unwound DNA, called chromatin. ▪ The nucleoli resemble miniature nuclei and are the sites of synthesis of precursor RNA molecules and their assembly. ◤ ▪ The other major components in nucleoplasm include the DNA chromosomes seen during mitosis. ▪ During cell interphase most of the DNA chromosomes exist as unwound chromatin that extend through the nucleoplasm. ▪ Two distinct types of chromatin are recognized. Diffuse, or uncondensed, chromatin is called Euchromatin and exists as thin threads that extend throughout much of the nucleoplasm. ◤ 2.5.3- Function ▪ 1. The nucleoplasm acts as a suspension medium for components of the nucleus including the nucleolus, and chromatin. ▪ 2. Nucleotides required for DNA replication and enzymes involved in other nuclear processes are also found dissolved within the nucleoplasm. ▪ 3. The nucleoplasm plays a role in the maintenance of the shape and structure of the nucleus ◤ ▪ 4. The nuclear matrix is present within the nuclear hyaloplasm, the liquid component of the nucleoplasm. ▪ 5. One other function is that it is responsible for the transport of materials that are vital to metabolism and cell function. ◤ 2.6 MITOCHONDRIA ▪ Mitochondria are well-defined cytoplasmic organelles of the cell which take part in a variety of cellular metabolic functions. Survival of the cells requires energy to perform different functions. ▪ The mitochondria are important as the fact that these organelles supply all the necessary biological energy of the cell, and they obtain this energy by oxidizing the substrates of the Krebs cycle. ◤ ▪ Energy of the cell is got from the enzymatic oxidation of chemical compounds in the mitochondria. Hence, the mitochondria re referred to as the "power houses of the cell". ▪ Almost all the eukaryotic cell have mitochondria, though they are lost in the later stages of development of cell like in the red blood cells or in elements of phloem sieve tube ◤ ▪ In 1890, mitochondria were first described by Richard Altmann (12 March 1852 – 8 December 1900) an German pathologist and histologist, and he called them as "bioblasts". ◤ ▪ Carl Benda, another German scientist (one of the first microbiologists) in the year 1897 coined the term "mitochondrion". ▪ In the 1920s, a biochemist Warburg found that oxidative reactions takes place in most tissues in small parts of the cell. ◤ Mitochondria Definition ▪ Mitochondria is a membrane bound cellular structure and is found in most of the eukaryotic cells. ▪ The term 'mitochondrion' is derived from a Greek word mitos which means "thread" and chondrion which means "granule" or "grain-like". ▪ Mitochondria are commonly between 0.75 and 3 μm in diameter but vary considerably in size and structure. ◤ ▪ The mitochondria are sometimes described as power plants of the cells. These organelles generate most of the energy of the cell in the form of adenosine triphosphate (ATP) and it is used a source of chemical energy. ▪ The mitochondria also involved in other cellular activities like signaling, cellular differentiation, cell senescence and also control of cell cycle and cell growth. ◤ 2.6.1- Origin ▪ Mitochondria also affect human health, like mitochondrial disorder and cardiac dysfunction and they also play important role in the aging process. ◤ ▪ 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 ◤ ▪ Unlike any other organelle, except for chloroplasts, mitochondria appear to originate only from other mitochondria. ▪ They contain their own DNA, which is circular as is true with bacteria, along with their own transcriptional and translational machinery. ▪ Mitochondrial ribosomes and transfer RNA molecules are similar to those of bacteria, as are components of their membrane. ◤ 2.6.2- 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. ▪ 1. Outer mitochondrial membrane, ▪ 2. Intermembrane space (the space between the outer and inner membranes), ▪ 3. Inner mitochondrial membrane, ◤ ▪ 4. Cristae space (formed by infoldings of the inner membrane), and ▪ 5. Matrix (space within the inner membrane).Mitochondria stripped of their outer membrane leaving the inner membrane intact are called Mitoplasts ◤ Outer Membrane ▪ 1. It is smooth and is composed of equal amounts of phospholipids and proteins. ▪ 2. It has a large number of special proteins known as the Porins. ▪ 3. The Porins are integral membrane proteins and they allow the movement of molecules that are of 5000 daltons or less in weight to pass through it. ▪ 4. The outer membrane is freely permeable to nutrient molecules, ions, oxygen, pyruvate, energy molecules like the ATP and ADP molecules ◤ Intermembrane Space ▪ 1. It is the space between the outer and inner membrane of the mitochondria, it has the same composition as that of the cell's cytoplasm. ▪ 2. It has a high proton concentration. This is due to the electron transport system of the inner mitochondrial membrane. ◤ Inner Membrane ▪ 1. The inner membrane of mitochondria is more complex in structure. ▪ 2. It has many invaginations and is known as the Cristae. ▪ 3. This folding help to increase the surface area inside the organelle. ◤ ▪ 4. Many of the chemical reactions that take place within mitochondria occur on the inner membrane. It contains the electron transport system and the ATPase complex: ▪ (i) Electron transport system - generates a proton gradient. ▪ (ii) ATPase complex - uses proton gradient to produce adenosine triphosphate (ATP) from adenosine diphosphate (ADP). ▪ 5. Hence the inner mitochondrial membrane is the site of oxidative phosphorylation. ◤ ▪ 6. Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable only to oxygen, ATP and it also helps in regulating transfer of metabolites across the membrane. ◤ Cristae Space ▪ 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. ▪ Cristae are covered with many tiny "stalked particles" called inner membrane spheres that are also known as simply "spheres" or "knobs". ◤ Matrix Space ▪ The matrix of the mitochondria is a complex mixture of proteins and enzymes. ▪ These enzymes are important for the synthesis of ATP molecules, mitochondrial ribosomes, tRNAs and mitochondrial DNA. ◤ Plant Cell Mitochondria ▪ Like in other eukaryotic cells, the mitochondria in plants play an important role in the production of ATP via the process of oxidative phosphorylation. ▪ Mitochondria also play essential roles in other aspects of plant development and performance. ▪ It also has various properties which allows the mitochondria to interact with special features of metabolism in plant cell. ◤ Animal Cell Mitochondria ▪ Mitochondria are known as "power houses" of the cells, they are unusual organelles and are surrounded by a double membrane. These organelles have their own small genome. ▪ They divide independently by simple fission. ◤ ▪ The division of the mitochondria is a result of the energy demand, so the cells with high need of energy have greater number of mitochondria. ▪ A typical animal cell may have about 1000 to 2000 mitochondria. The process creating energy for the cell is known as cellular respiration. ▪ Most of the chemical reactions of this process happen in the mitochondria ◤ 2.6.3- Function ▪ Functions of mitochondria depend on the cell type in which they are present. ▪ 1. 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 ◤ ▪ 2. Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of the cell. ▪ 3. The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen. ▪ 4. The liver cells mitochondria have enzymes that detoxify ammonia. ◤ ▪ 5. 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. ◤ 2.7 CHLOROPLAST ▪ The word chloroplast is derived from the Greek word chloros meaning "green" and plastesmeaning "the one who forms". ▪ Chloroplasts are organelles, specialized compartments, 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" ◤ ▪ Chloroplasts are organelles present in plant cells and some eukaryotic organisms. ▪ Chloroplasts are the most important plastids found in plant cells. It is the structure in a green plant cell in which photosynthesis occurs. ▪ Chloroplast is one of the three types of plastids. ▪ The chloroplasts take part in the process of photosynthesis and it is of great biological importance. ▪ Animal cells do not have chloroplasts ◤ ▪ All green plant take part in the process of photosynthesis which converts energy into sugars and the byproduct of the process is oxygen that all animals breathe. ▪ This process happens in chloroplasts. ▪ The distribution of chloroplasts is homogeneous in the cytoplasm of the cells and in certain cells chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. A typical plant cell might contain about 50 chloroplasts per cell ◤ 2.7.1- Origin ▪ Chloroplasts are unique organelles and are said to have originated as endosymbiotic bacteria. ▪ They develop from colourless precursors, called Proplastids or Eoplasts. ▪ They are semi autonomous in nature and arise from pre existing chloroplast as they have their own machinery to synthesize the required proteins. ◤ ▪ This is very clear in algae, where one chloroplast divides into two during cell division. ▪ In higher plants, the division of chloroplasts is very difficult to observe as. the number of chloroplast is very high. Still, some-times the dividing chloroplast is seen under the phase contrast microscope as in Spinach ◤ 2.7.2- Structure ▪ 1. Chloroplasts found in higher plants are generally biconvex or planoconvex shaped. ▪ In different plants chloroplasts have different shapes, they vary from spheroid, filamentous saucer-shaped, discoid or ovoid shaped. ◤ ▪ 2. They are vesicular and have a colorless center. Some chloroplasts are in shape of club, they have a thin middle zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band. ◤ ▪ 3. The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4-6 microns in diameter and 1-o microns in thickness. ◤ ▪ 4. The chloroplasts are double membrane bound organelles and are the site of photosynthesis. ▪ The chloroplasts have a system of three membranes: The Outer Membrane, The Inner Membrane and The Thylakoid system. ▪ The outer and the inner membrane of the chloroplast enclose a semigel-like fluid known as the Stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma. ◤ Components of Chloroplast ▪ Outer Membrane: It is a semi-porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins. ▪ Intermembrane Space: It is usually a thin intermembrane space about 10-20 nanometers and it is present between the outer and the inner membrane of the chloroplast. ▪ Inner Membrane: The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane. ◤ ▪ Stroma: Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA, chloroplast ribosomes and the thylakoid system, starch granules and many proteins are found floating around the stroma. ◤ ▪ Thylakoid System: It is suspended in the stroma. The Thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as Grana. Each granum contains around 10-20 thylakoids ◤ General Features of Thylakoid System ▪ 1. Thylakoids are interconnected small sacks, the membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place. The word thylakoid is derived from the Greek word "thylakos" which means 'sack'. ▪ 2. Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons. ◤ ▪ 3. The molecules present in the thylakoid membrane use the electrons that are energized to pump hydrogen ions into the thylakoid space, this decrease the pH and become acidic in nature. ▪ A large protein complex known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma. ◤ ▪ 4. Thylakoids are of two types - Granal Thylakoids and Stromal Thylakoids. Granal thylakoids arranged in the grana, are pancake shaped circular discs, which are about 300-600 nanometers in diameter. The Stromal thylakoids are in contact with the stroma and are in the form of helicoid sheets. ◤ ▪ 5. The Granal thylakoids contain only Photosystem II protein complex, this allows them to stack tightly and form many granal layers with granal membrane. This structure increases stability and surface area for the capture of light. ▪ 6. The Photosystem I and ATP synthase protein complexes are present in the stroma. These protein complexes act as spacers between the sheets of Stromal thylakoids ◤ Transport of proteins into Chloroplast ▪ 1. For import of protein across double membrane, the chloroplast seems to employ ATP hydrolysis. ▪ 2. Signal peptide on chloroplast, proteins can be recognized by receptor on chloroplast membrane. ▪ 3. In first step the protein passes through chloroplast double membrane to reach stroma. ▪ 4. From where in the second step they are transported to Thylakoid space. ▪ 5. After the protein reaches the stroma, the chloroplast signal peptide cleaved by Stromal peptidase which facilitates transport to Thylakoid ◤ 2.7.3- Functions of Chloroplast ▪ 1. In plants all the cells participate in plant immune response as they lack specialized immune cells. The chloroplasts with the nucleus and cell membrane and ER are the key organelles of pathogen defense. ▪ 2. The most important function of chloroplast is absorption of light energy and conversion of it into biological energy, making food by the process of photosynthesis. Food is prepared in the form of sugars. The chloroplast is very important as it is the cooking place for all the green plants ◤ ▪ 3. During the process of photosynthesis sugar and oxygen are made using light energy, water, and carbon dioxide. Conversion of PGA (phosphoglyceric acid) into different sugars and store as starch. ▪ 4. Like the mitochondria, Chloroplasts use the potential energy of the H+ ions or the hydrogen ion gradient to generate energy in the form of ATP ◤ ▪ 5. Light reactions takes place on the membranes of the thylakoids. Production of NADPH2 and evolution of oxygen through the process of photolysis of water. ▪ 6. The dark reactions also known as the Calvin cycle takes place in the stroma of chloroplast. ◤ ▪ 7. Enzymes for carbon dioxide fixation and other dark reactions are present in the stroma and the enzymes for light reactions are present in the thylakoids. Two separate ways for carbon dioxide fixation are observed in higher plants which are broadly classified into C3 and C4 plants. ▪ 8. Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid by the utilization of assimilatory powers (NADPH2 and ATP). ◤ 2.8 TYPES OF PLASTID ▪ Plastids are double membraned organelles which are found in plant cells only. ▪ They are usually spherical or discoidal in shape and their average size is 4-6 µm. ▪ A plastid shows two distinct regions- Grana and Stroma. ◤ ▪ Grana are stacks of membrane-bound, flattened, discoid sacs containing chlorophyll molecules. ▪ These molecules are responsible for the production of food by ▪ the process of photosynthesis. They are, therefore, called "Kitchen of the cell". ▪ They are the main functional units of the chloroplast. ◤ ▪ The homogenous matrix in which grana are embedded is known as Stroma. ▪ A variety of photosynthetic enzymes and starch grains are present in the stroma. ▪ The stroma is colourless, whereas the grana contain the pigments. ▪ Plastids are living and multiply by division of the pre-existing plastids called Proplastids. ◤ Types of Plastids ▪ 1. Leucoplasts: ▪ These are colorless plastids. They store the food of the plant body in the form of starch, protein and lipids. They occur most commonly in the storage cells of roots and underground stems ▪ 2. Chloroplasts: ▪ These are green plastids because of the presence of chlorophyll. Chloroplasts occur abundantly in green leaves, and also to some extent in green parts of the shoot. ▪ 3. Chromoplasts: ▪ These are variously colored plastids. They are mostly present tin flowers and fruits.One form of plastid can change into another. For example, leucoplasts can change into chloroplasts when the former are exposed to light for a long period. ◤ Functions of Plastids: ▪ 1. By trapping solar energy, green plastids manufacture food through photosynthesis ▪ 2. Chromoplasts provide colored to various flowering parts. ▪ 3. Leucoplasts help in storage of protein, starch and oil. ◤ ▪ On the basis of presence of pigments, the plastids are of two types: ▪ 1. Chromoplasts: The chromoplasts may be further divided on the basis of colour of the pigment and these are of the following types ▪ A. Chloroplasts: It is the most common plastid which contains chlorophyll a and b pigments, and DNA and RNA. Chloroplasts are found mainly in the cells of the leaves of higher plants and algae. It is the most biologically important plastid. By the process of photosynthesis, they produce oxygen and the most of the chemical energy used by living organisms ◤ ▪ B. Phaeoplast: These are yellow or brown plastids found in brown algae, diatoms and dinoflagellates. Fucoxanthin is a carotenoid pigment which masks the colour of chlorophyll a, which is also present. It also absorbs light and transfer the energy to chlorophyll a ◤ ▪ C. Rhodoplasts: These are red coloured plastids. It is found in red algae and its red colour is due to phycoerythrin. It also absorbs light. ◤ ▪ D. Chromatophores: ▪ These are present in the blue-green algae. ▪ The term chromatophore is used instead of plastid, since the pigments are not organized within a discrete plastid body but are often arranged on lamellar structures in concentric rings or plates within algal cell. ▪ Blue-green colour of this algae is due to phycocyanin and phycobilins. These accessory pigments do not participate in photosynthesis. ◤ Non-photosynthetic chromoplasts: ▪ (i) A variety of accessory pigments is also found which do not appear to be directly involved in photosynthesis or energy transfer. ▪ (ii) Chromoplasts may develop from chloroplasts by accumulation of non-photosynthetic pigments, e.g., red carotenoid, Lycopene in tomatoes. Genes for synthesis of pigments lie in the nucleus. ◤ ▪ 2. Leucoplasts: These plastids are devoid of pigment and are membranous structures. ▪ They serve to store starches, oils and proteins. These are of the following types A. Amyloplasts: These are food storage cells and store starch. These are generally found in storage tubers, cotyledons and endosperm. These are found in regions of little or no illumination. Amyloplasts have nucleoids and ribosomes. ▪ B. Elaioplasts: These are found in certain monocotyledons and their function is to store oils. ▪ C. Proteinoplasts: Also known as Aleuroneplasts. These are found in seeds of Ricinus and Brazil nut, and store proteins. Epidermal cells of Helleborus also possess Proteinoplast. ◤ ▪  Plastid differentiation depends upon the metabolic requirements of the cell. The chloroplasts may develop from leucoplasts, and chromoplasts, which are considered end forms of plastid differentiation, may develop from either leucoplasts or chloroplasts. ▪  Proplastids can differentiate into one of three types of plastids and since, in certain cases, one type of plastid can differentiate into another, it has been generally assumed that all plastids are essentially the same in structure, having the ability to differentiate in various ways, depending upon the requirements of the cells. ◤ 2.9 GOLGI COMPLEX ▪ Golgi apparatus or Golgi complex is a cytoplasmic organelle of smooth membranes sac or cisternae, tubules and vesicles. ▪ It was identified in 1897 by the Italian scientist Camillo Golgi, in the nerve cells of barn owl and cat by means of impregnation method, and named after him in 1898. ▪ With the aids of special staining techniques the Golgi bodies were seen as densely stained region of the cytoplasm under the optical microscope. ▪ Under the electron microscope the Golgi apparatus is seen to be composed of stacks of flattened structures which contains numerous vesicles containing secretory granules ◤ ▪ The Golgi apparatus is the processing, packaging and secretion organelle of the cell. It is found in all eukaryotic cells with the exception of mammalian erythrocytes, sieve tube elements. ▪ Prokaryotic cell do not contain the apparatus. In plants Golgi apparatus is formed of a number of unconnected units called Dictyosomes. ◤ ▪ The newly synthesized proteins, found in the channels of the rough endoplasmic reticulum are moved to the Golgi body where the carbohydrates are added to them and these molecules are enveloped in a part of the Golgi membrane and then the enveloped molecules leave the cell. ▪ The Golgi apparatus hence acts as the assembly factory of the cell where the raw materials are directed to the Golgi apparatus before being passed out from the cell ◤ Golgi apparatus Definition ▪ An organelle, consisting of layers of flattened sacs, that takes up and processes secretory and synthetic products from the endoplasmic reticulum and then either releases the finished products into various parts of the cell cytoplasm or secretes them to the outside of the cell. ◤ ▪ The Golgi complex is responsible inside the cell for packaging of the protein molecules before they are sent to their destination. ▪ This organelles helps in processing and packaging the macromolecules like proteins and lipids that are synthesized by the cell, sometimes referred as "post office" of the cell. ◤ Origin ▪ The intracellular origin of Golgi bodies has been a hotly debated subject for many years. Among the proposed sources of new Golgi bodies are: ▪ (i) Vesicles dis-patched from the endoplasmic reticulum, ▪ (ii) Vesicles dispatched from the outer membrane of the nuclear envelope, ▪ (iii) Vesicles formed by invaginations of the plasma membrane, and ▪ (iv) Division of Golgi bodies al-ready present in the cell ◤ ▪ The most widely accepted view is that Golgi bodies are formed from vesicles dis-patched from the ER. ▪ These vesicles are called transi-tion vesicles Transition vesicles migrate to the forming face of the Golgi body, fuse there with existing cisterna membranes, and in so doing contribute to the organelle's growth ◤ Structure ▪ Shape and size of Golgi complex is largely dependent upon type of cell and its physiological state. It is small in muscle cell but it is well developed in secretary cells. Further, it can be compact stack of fenestrated saccules or a diffuse network of lamellae. It posses four types of components: cisternae, tubules, vesicles and vacuoles. ▪ 1. The Golgi apparatus is a major organelle in most of the eukaryotic cells. They are membrane bound organelles, which are sac-like. They are found in the cytoplasm of plant and animal cells. ▪ 2. The Golgi complex is composed of stacks of membrane-bound structures, these structures are known as the cisternae. An individual stack of the cisternae is sometimes referred as Dictyosome. ◤ ▪ 3. In a typical animal cell, there are about 40-100 stacks. In a stack there are about four to eight cisternae. ▪ Each cisternae is a disc enclosed in a membrane, it possess special enzymes of the Golgi which help to modify and transport of the modified proteins to their destination. ◤ ▪ 4. The flat sacs of the cisternae are stacked and is bent and semicircular in shape. ▪ Each group of stacks is membrane bound and its insides are separated from the cytoplasm of the cell. The interaction in the Golgi membrane in responsible for the unique shape of the apparatus. ◤ ▪ 5. The Golgi complex is polar in nature. The membranes of one end of the stack is different in composition and thickness to the membranes at the other end. ▪ 6. One end of the stack is known as the Cis-face, it is the "receiving department" while the other end is the Trans-face and is the "shipping department". The Cis-face of the Golgi apparatus is closely associated with the endoplasmic reticulum. ◤ Golgi apparatus Function ▪ 1. The cell synthesizes a huge amount of variety of macromolecules. The main function of the Golgi apparatus is to modify, sort and package the macromolecules that are synthesized by the cells for secretion purposes or for use within the cell. ▪ 2. 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 ◤ ▪ 3. They are also involved in the transport of lipid molecules around the cell. The Golgi complex is thus referred as post office where the molecules are packaged, labeled and sent to different parts of the cell. ◤ ▪ 4. It mainly modifies the proteins that are prepared by the rough endoplasmic reticulum. The enzymes in the cisternae have the ability to modify proteins by the addition of carbohydrates and phosphate by the process of glycosylation and phosphorylation respectively ◤ ▪ 5. Carbohydrates are synthesized in the Golgi body. The process of carbohydrate synthesis involves production of polysaccharides and glycosaminoglycans (GAGs). ▪ 6. The long, unbranched polysaccharides and GAGs are attached to proteins in order to form proteoglycans, the molecules that are present in the extracellular matrix of the animal cells ◤ ▪ 7. Sulfation process of certain molecules is an important task carried out by the Golgi body. The sulfating of substances passing through the lumen of Golgi body is carried out with the help of sulfotransferases. ▪ 8. To carry out the glycosylation and phosphorylation processes, nucleotide sugars and ATP are imported by the Golgi apparatus from cytosol. ▪ 9. Golgi apparatus plays an important role in the prevention of destruction of cells (or apoptosis). The Bcl-2 genes present in the Golgi are used for this purpose. ◤ 2.10 ENDOPLASMIC RETICULUM ▪ Endoplasmic reticulum is a continuous membrane, which is present in both plant cells, animal cells and absent in prokaryotic cells. ▪ It is the membrane of network tubules and flattened sacs, which serves a variety of functions within the cell. The space, which is present in the endoplasmic reticulum, is called as the Lumen. ▪ The word reticulum, which means "network", was applied to describe the fabric of membranes. ▪ It can be defined as a eukaryotic organelle, which forms a network of tubules, vesicles and cisternae within the cells. ◤ ▪ There are two regions of the Endoplasmic reticulum, which differ in both structure and function. ▪ One region is called as Rough Endoplasmic Reticulum, as it contains ribosome attached to the cytoplasmic side of the membrane and they are the series of flattened sacs. ▪ The other region is called as Smooth Endoplasmic Reticulum as it lacks the attached ribosome and they are tubule network ◤ ▪ The electron microscope reveals an extensive membrane system in the cytoplasm called Endoplasmic reticulum (ER). ▪ It was first reported by Keith R. Porter (a Canadian-American cell biologist) in 1945. This continuous membrane system joins the nuclear membrane on one end and the cell membrane on the other ◤ Types of Endoplasmic Reticulum ▪ Two types of ER, such as smooth walled and rough walled, have been recognized. They may be present in the same or different types of cells. ▪ (i) Smooth Endoplasmic Reticulum (SER): The surface of this type of reticulum is smooth as ribosomes not attached. Smooth ER is present in cells, which are actively engaged in steroid synthesis, carbohydrate metabolism, pigment production etc. ▪ (ii) Rough Endoplasmic Reticulum: The rough ER have ribosomes attached throughout the surface. These are present in cells, which are active in protein synthesis. ◤ Plant Cell Endoplasmic Reticulum ▪ In plant cell, the endoplasmic reticulum acts as a port for the entry of proteins into the membrane. ▪ It also plays a vital role in the biosynthesis and storage of lipids. ▪ There are number of soluble membrane, which are associated with the enzymes and the molecular chaperones. ▪ The general functions of the endoplasmic reticulum in plant cell are protein synthesis and maturation. ◤ ▪ Endoplasmic reticulum of plant cell possesses some additional functions, which is not found in animal cells. ▪ The additional function involves cell to cell communication between specialized cells and also it serves as a storage site for proteins. ▪ Endoplasmic reticulum of plant cell contains enzymes and structural proteins, which are involved in the process of oil body biogenesis and lipid storage. ▪ In plants, the endoplasmic reticulum is connected between the cells via the plasmodesmata ◤ Animal Cell Endoplasmic Reticulum ▪ In animal cells, the endoplasmic reticulum is a network of sacs, which play a vital role in manufacturing, processing and transporting different types of chemical compounds for use of both inside and outside of the cell. ▪ It is connected to the double-layered nuclear envelope, which provides the pipeline between the nucleus and the cytoplasm of a cell. ▪ In animal cells, the endoplasmic reticulum is a multifunctional organelle, which synthesis the membrane lipids, proteins and also regulates the intracellular calcium. ◤ Endoplasmic Reticulum Structure ▪ 1. Endoplasmic reticulum is an extensive membrane network of cisternae (sac-like structures), which are held together by the cytoskeleton. The phospholipid membrane encloses a space, the lumen from the cytosol, which is continuous with the Perinuclear space. ▪ 2. The surface of the rough endoplasmic reticulum is studded with the protein manufacturing ribosome, which gives it a rough appearance. Hence it is referred as a rough endoplasmic reticulum ◤ ▪ 3. The smooth endoplasmic reticulum consists of tubules, which are located near the cell periphery. This network increases the surface area for the storage of key enzymes and the products of these enzymes. ▪ 4. Rough endoplasmic reticulum synthesizes proteins, while smooth endoplasmic reticulum synthesizes lipids and steroids. It also metabolizes carbohydrates and regulates calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins. ▪ 5. Endoplasmic reticulum varies extensive extending from the cell membrane through the cytoplasm and forming a continuous connection with the nuclear envelope. ◤ The Major Functions of Endoplasmic reticulum ▪ (1)Common to both Endoplasmic Reticulum: ▪ (i) Forms the skeletal framework. ▪ (ii) Active transport of cellular materials. ▪ (iii) Metabolic activities due to presence of different enzymes. ▪ (iv) Provides increased surface area for cellular reactions. ▪ (v) Formation of nuclear membrane during cell division. ◤ ▪ (2) Function of Smooth Endoplasmic Reticulum: ▪ (i) Lipid synthesis. ▪ (ii) Glycogen synthesis. ▪ (iii) Steroid synthesis like cholesterol, progesterone, testosterone etc. ▪ (iv) Metabolism of carbohydrates. ▪ (v) Detoxification function. ▪ (vi) Major storage and released site of inter cellular calcium ions. ◤ ▪ (3) Function of Rough Endoplasmic Reticulum: ▪ (i) It provides site for protein synthesis. ▪ (ii) Protein translocation, folding and transport of protein. ▪ (iii) Glycosylation (this is the relation of a saccharides group with a hydroxyl or amino functional group to form a glucoside). ▪ (iv) Disulfide bond formation (disulfide bonds stabilize the tertiary and quaternary structures of many proteins). ▪ (v) Membrane synthesis.