Cell Organelles PDF

Al-zintan School of Medicine Department of Biochemistry and Molecular Biology Osamah S. Alrawab 1 CELL AND CELL ORGANELLES Osamah S. Alrawab 2  CELL AND CELL ORGANELLES From cells, all living things are created. Cell collections ar...

Al-zintan School of Medicine Department of Biochemistry and Molecular Biology Osamah S. Alrawab 1 CELL AND CELL ORGANELLES Osamah S. Alrawab 2  CELL AND CELL ORGANELLES From cells, all living things are created. Cell collections are the building blocks of all animal tissues, including humans. Thus, the cell is the fundamental unit of life. When cells die, tissues also perish and stop working. Osamah S. Alrawab 3  Cell Theory There are a few main claims that make up modern cell theory 1- Each organism is composed of cells. 2- Every cell starts as a precursor cell (All cells arise from other cells). 3- The genetic code needed to keep cells functioning and make new ones is passed down from one generation to the next. 4- Metabolism, which encompasses both anabolism and catabolism, refers to the chemical events (reactions) occurring inside an organism's cells. Osamah S. Alrawab 4  Types of Cells In general, two types of cells exist in nature. They are: 1. Prokaryotic cells. 2. Eukaryotic cells. Osamah S. Alrawab 5  Prokaryotic Cells Typical prokaryotic cells (Greek: Pro-before and karyon-nucleus) include Bacteria and Cyanobacteria. The most studied prokaryotic cell is Escherichia coli (E. coli). Characteristics ❑ It has a minimum of internal organization and is smaller in size. ❑ It does not have any membrane-bound organelles. ❑ Its genetic material is not enclosed by a nuclear membrane. ❑ Its DNA is not complex with histones. Histones are not found in prokaryotic cells. ❑ Its respiratory system is closely associated with its plasma membrane ❑ Its sexual reproduction does not involve mitosis or meiosis. Osamah S. Alrawab 6  Eukaryotic cells The eukaryotic cells (Greek: Eu-true and karyon-nucleus), include fungi, plants, and animals including humans. Cells are larger compared to prokaryotic cells. Characteristics ❑ It has a considerable degree of internal structure with a large number of distinctive membranes enclosed having specific functions. ❑ The nucleus is the site for informational components collectively called chromatin. ❑ Sexual reproduction involves both mitosis and meiosis. ❑ The respiratory site is the mitochondria. Osamah S. Alrawab 9 Prokaryotic cells Osamah S. Alrawab 10  Cell Organelles Nucleus The nucleus contains more than 95 percent of the cell’s DNA and is the control center of the eukaryotic cell. I. Nuclear envelope: A double membrane structure called the nuclear envelope separates the nucleus from the cytosol. II.Nuclear pore complexes: These are embedded in the nuclear envelope. These complex structures control the movement of proteins and the nucleic acid ribonucleic acids (RNAs) across the nuclear envelope. Osamah S. Alrawab 11  Cell Organelles Nucleus III. Chromatin: DNA in the nucleus is coiled into a dense mass called chromatin, so named because it is stained darkly with certain dyes. IV. Nucleolus: A second dense mass closely associated with the inner nuclear envelope is called nucleolus. V. Nucleoplasm: The Nucleoplasm of the nucleus contains various enzymes such as DNA polymerases, and RNA polymerases, for mRNA and tRNA synthesis. Osamah S. Alrawab 12  Cell Organelles Nucleus Functions: ❖ DNA replication and RNA transcription of DNA occur in the nucleus. Transcription is the first step in the expression of genetic information and is the major metabolic activity of the nucleus. ❖ The nucleolus is non-membranous and contains RNA polymerase, RNAase, ATPase, and other enzymes but no DNA polymerase. The nucleolus is the site of synthesis of ribosomal RNA (rRNA). ❖ The nucleolus is also the major site where ribosome subunits are assembled. Osamah S. Alrawab 13  Cell Organelles Mitochondrion The mitochondrion is the powerhouse of the cell. I. Number: The number of mitochondria in a cell varies dramatically. Some algae contain only one mitochondrion, whereas the protozoan Chaos contains half a million. A mammalian liver cell contains from 800 to 2500 mitochondria. II.Size: They vary greatly in size. A typical mammalian mitochondrion has a diameter of 0.2 to 0.8 μm. III.Shape: The shape of the mitochondrion is not static. Mitochondria assume many different shapes under different metabolic conditions. Osamah S. Alrawab 15  Cell Organelles Mitochondrion Mitochondrial Membranes: (a) Outer mitochondrial membrane: The outer mitochondrial membrane consists mostly of phospholipids and contains a considerable amount of cholesterol. The outer membrane also contains many copies of the protein called Porin. Osamah S. Alrawab 16  Cell Organelles Mitochondrion Functions of Porin and other Proteins: 1. These proteins form channels that permit substances with molecular weights of less than < 10,000 Dalton (Da) to diffuse freely across the outer mitochondrial membrane. 2. Other proteins in the outer membrane carry out various reactions in fatty acid and phospholipid biosynthesis and are responsible for some oxidation reactions Osamah S. Alrawab 17  Cell Organelles Mitochondrion (b) The inner mitochondrial membrane: - is a membrane that is abundant in proteins and has a lipid-protein ratio of 0.27:1. It also contains a significant amount of phospholipid cardiolipin. - The inner membrane of the mitochondrion is very resistant to the passage of polar and ionic molecules, allowing only specialized transport proteins to facilitate their entry, in contrast to the outer membrane. - Cristae➔ The inner mitochondrial membrane is highly folded. The tightly packed inward folds are called “cristae”. Osamah S. Alrawab 18  Cell Organelles Mitochondrion c) Intermembrane space: The space between the outer and inner membranes is known ( as the intermembrane space. Since the outer membrane is freely permeable to small molecules, the intermembrane space has about the same ionic composition as the cytosol. Osamah S. Alrawab 19  Cell Organelles Mitochondrion d) Mitochondrial matrix: The region enclosed by the inner membrane is known as the ( mitochondrial matrix. Composition of matrix: - The enzymes responsible for the citric acid cycle and fatty acid oxidation are located in the matrix. - The matrix also contains several strands of circular DNA, ribosomes, and enzymes required for the biosynthesis of the proteins coded in the mitochondrial genome. - The mitochondrion is genetically semi-autonomous (partly independent), and the genes encode most mitochondrial proteins. Osamah S. Alrawab 20  Cell Organelles Mitochondrion Luft’s disease - Age-related degenerative disorders such as Parkinson’s disease. - involving mitochondrial energy transduction. - Mitochondrial DNA can be damaged by free radicals. Osamah S. Alrawab 22  Cell Organelles Endoplasmic reticulum (ER) o The endoplasmic reticulum (ER) extends from the cell membrane, coats the nucleus, surrounds the mitochondria, and appears to connect directly to the Golgi apparatus. o These organelles are involved in protein synthesis, transport, modification, storage, and secretion. o Endoplasmic reticulum (ER) dysfunction might have an important part to play in a range of neurological disorders, including cerebral ischemia, sleep apnoea, Alzheimer's disease, and multiple sclerosis. Osamah S. Alrawab 24  Cell Organelles Rough endoplasmic reticulum (rER) - also known as ergastoplasm. - They are coated with ribosomes. Near the nucleus, this type of ER merges with the outer membrane of the nuclear envelope. The function of rough ER: Rough ER synthesizes membrane lipids and secretory proteins. These proteins are inserted through the ER membrane into the lumen of the cisternae where they are modified and transported through the cell. Osamah S. Alrawab 26  Cell Organelles Smooth endoplasmic reticulum (sER) They do not have attached ribosomes. Function of smooth ER: (i) In lipid synthesis. (ii) Modification and transport of proteins synthesized in the rough ER. enzymes are associated with the endoplasmic reticulum of mammalian liver cells: 1- The enzymes responsible for the synthesis of sterol, triacylglycerol (TG), Phospholipids (PL), and the enzymes involved in the detoxification of drugs. 2- Cytochrome P450 which participates in drug hydroxylation resides in the ER. Osamah S. Alrawab 28  Cell Organelles Golgi complexes o also known as Dictyosomes, are unique stacks of smooth compartments in eukaryotic cells. o The ER is often associated with these complexes, which contain flattened, fluid-filled Golgi sacs (Golgi cisternae). o The Golgi complex has a Proximal compartment, a Medial compartment, and a Distal compartment. o Recent evidence suggests the complex serves as a unique sorting device for newly synthesized proteins containing signal or transit peptides (transport of a protein) from the ER. o Proteins without signal or transit Osamah peptide S. Alrawab regions are rejected by the Golgi 29  Cell Organelles Golgi complexes Functions: (i) The proximal or cis side: ▪ The Golgi complexes receive the newly synthesized proteins by ER via transfer vesicles. (ii) the Golgi lumen (median part): ▪ The post-translational modifications take place in the lumen, where the carbohydrates and lipid precursors are added to proteins to form glycoproteins and lipoproteins respectively. Osamah S. Alrawab 32  Cell Organelles Golgi complexes (iii) On the distal or trans side: ▪ They release proteins via modified membranes called secretory vesicles. ▪ These secretory vesicles move to and fuse with the plasma membrane where the contents may be expelled by a process called exocytosis. Osamah S. Alrawab 33  Cell Organelles Golgi complexes Mutations in genes encoding Golgi proteins cause genetic diseases: 1- results in membrane trafficking defects (e.g., Wilson disease caused by mutations in the copper transporter ATP7B). 2- Golgi stacks dispersion is observed in cancer, infectious, and neurodegenerative diseases, including Amyotrophic Lateral Sclerosis, Alzheimer’s, and Parkinson’s. Osamah S. Alrawab 34  Cell Organelles Lysosomes ✓ Lysosomes are cellular organelles that house packets of enzymes. ✓ They were first identified and characterized by Belgian Biochemist de Duve in 1955. ✓ The word lysosome is derived from the Greek words lysis (loosening, breaking) and soma (body) and literally means ‘digestive body’. ✓ Lysosomes are found in all animal cells, except erythrocytes, in varying numbers and types. ✓ pH inside the lysosomes is lower than that of cytosol. The lysosomal enzymes have an optimal pH of around 5. Acid phosphatase is used as a marker enzyme for this organelle. Osamah S. Alrawab 36  Cell Organelles Lysosomal Enzymes 1- Proteolytic enzymes 4- Carbohydrate splitting 3- Lipid hydrolyzing enzymes enzymes 1- Cathepsins (Proteinase) 1- α-glucosidase 1- Lipases 2- Collagenase 2- Phospholipases 2- β-galactosidase 3- Elastase 3-Fatty acyl esterase 3- HyaIuronidase 2- Nucleic acid hydrolyzing enzymes 4- Aryl sulphatase 1- Ribonucleases (Rnase) 1-Acid phosphatase 4- Other enzymes 2-Deoxyribonucleases (Dnase) 2- Catalase Osamah S. Alrawab 38 Cell Organelles Lysosomes Osamah S. Alrawab 39  Cell Organelles Lysosomes ❖ In autophagic processes, cellular organelles such as mitochondria and the endoplasmic reticulum undergo digestion within the lysosome. The enzymes are active at postmortem autolysis. ❖ In the death of a cell, lysosomal bodies disintegrate, releasing hydrolytic enzymes into the cytoplasm with the result that the cell undergoes autolysis. ❖ The acrosome, located at the head of the spermatozoa, is a specialized lysosome and is probably involved in some way in the penetration of the ovum by the sperm. Osamah S. Alrawab 40 Cell Organelles Lysosomes Osamah S. Alrawab 41  Cell Organelles Lysosomes Lysosomal dysfunction: 1. Allergic responses and arthritic conditions: Released enzymes from the ruptured lysosomal membrane can hydrolyze external biopolymers (substrates) leading to tissue damage in many types of allergic responses and arthritic conditions. 2. In Gout: Urate crystals are deposited around joints. These crystals when phagocytosed cause physical damage to lysosomes and release of enzymes producing inflammation and arthritis. Osamah S. Alrawab 42  Cell Organelles Lysosomes 3. Inherited disorders: Many hereditary diseases involving the abnormal accumulation of complex lipids or polysaccharides in cells of the afflicted individual have now been traced to the absence of key acid hydrolases in the lysosomes of these individuals. Osamah S. Alrawab 43  Cell Organelles Lysosomes 4. I-Cell disease: o I-cell disease is a rare condition in which lysosomes lack all of the normal lysosomal enzymes. o The disease is characterized by severe progressive psychomotor retardation and a variety of physical signs, with death often occurring in the first decade. o I-cell disease was found to lack almost all of the normal lysosomal enzymes. The lysosomes thus accumulate many different types of undegraded molecules forming inclusion bodies. Osamah S. Alrawab 44 Cell Organelles Lysosomes Osamah S. Alrawab 45  Cell Organelles Lysosomes o I-cell disease was found to be deficient in the enzyme GlcNAc phosphotransferase (N- acetylglucosamine-1-phosphate transferase), leading to a lack of normal transfer of GlcNAc-1-P in specific mannose residues of certain lysosomal enzymes. Hence these enzymes lack Mannose-6-P (the marker) and are secreted into plasma leading to high plasma levels. They are not targeted to lysosomes Osamah S. Alrawab 46  Cell Organelles Peroxisomes o Peroxisomes are small organelles also called Microbodies, present in eukaryotic cells. o The particles are approximately 0.5 μ in diameter. o These organelles have no energy-coupled electron transport systems and are probably formed by budding from smooth endoplasmic reticulum (ER). Osamah S. Alrawab 47  Cell Organelles Peroxisomes Functions: (i) They carry out oxidation reactions in which toxic hydrogen peroxide (H2O2) is produced, which is destroyed by the catalase enzyme. (ii) Recently it has been shown that liver peroxisomes have an unusually active β- oxidative system capable of oxidizing long-chain fatty acids (C 16 to 18 or > C 18). (iii)Peroxisomes may be absent in the inherited disorder Zellweger’s syndrome. Osamah S. Alrawab 48  Cell Organelles Cytoplasm (Cytosol) o This is the simplest structure of the cell. o Organelles-free sap is called cytosol. o Many metabolic reactions take place in cytosol where substrates and cofactors interact with various enzymes. o It has a high protein content and ribosomal RNA (rRNA). o Cytosol also contains free ribosomes often in the polysome form. Osamah S. Alrawab 49  BIOLOGICAL MEMBRANES STRUCTURE AND FUNCTION Osamah S. Alrouwab 50 ❑ The plasma membrane, a prototype cell membrane, studied extensively. ❑ It separates the cell contents from the outer environment. ❑ Such a membrane barrier that separates cellular contents from the environment is an absolute necessity for life. ❑ Plasma membranes have selective permeability that mediate the flow of molecules and ions into and out of the cell. ❑ They also contain molecules at their surfaces that provide for cellular recognition and communication. ❑ Eukaryotic cells contain many internal membrane systems that surround the cell organelles. ❑ Each internal membrane system is specialized to assist in the function of the organelle it surrounds. Osamah S. Alrouwab 51  The Chemical Composition of the Membranes ❑ Membranes are composed of lipids, proteins, and carbohydrates. ❑ The relative content of these components varies widely from one type of membrane to another, but typically it contains, 40 percent of the dry weight is lipids, about 60 percent proteins, and 1 to 10 percent carbohydrates. ❑ All membrane carbohydrate is covalently attached to proteins or lipids. Osamah S. Alrouwab 52 The Chemical Composition of the Membranes Osamah S. Alrouwab 53  The Chemical Composition of the Membranes A. Lipids: ❖ Lipids are the basic structural components of cell membranes. ❖ Lipid molecules have a ‘polar’ or ionic head hence hydrophilic and the other end (tail) is a ‘nonpolar’ and hydrophobic tail. Hence, they are amphipathic. Types of Lipids Present in Biomembranes: 1. Fatty acids. 2. Glycerophospholipids. 3. Sphingolipids. 4. Cholesterol. Osamah S. Alrouwab 54  The Chemical Composition of the Membranes 1. Fatty acids: o They are major components of most membrane lipids. o The nonpolar tails of most membrane lipids are long-chain fatty acids attached to polar head groups, such as glycerol-3-P. o About 50 % of the fatty acid groups are saturated, i.e. they contain no double bond o The most common saturated fatty acid groups in membrane lipids in animals contain 16 to 18 carbon atoms. o The other half of fatty acid molecules contain one or more double bonds, i.e. unsaturated or polyunsaturated fatty acids. Oleic acid is the most abundant unsaturated fatty acid in animal membrane lipids; others are arachidonic acid, linoleic and linolenic acids. The degree of unsaturation determines the fluidity of the membranes. Osamah S. Alrouwab 55  The Chemical Composition of the Membranes 1. Fatty acids: o They are major components of most membrane lipids. o The nonpolar tails of most membrane lipids are long-chain fatty acids attached to polar head groups, such as glycerol-3-P. o About 50 % of the fatty acid groups are saturated, i.e. they contain no double bond o The most common saturated fatty acid groups in membrane lipids in animals contain 16 to 18 carbon atoms. o The other half of fatty acid molecules contain one or more double bonds, i.e. unsaturated or polyunsaturated fatty acids. Oleic acid is the most abundant unsaturated fatty acid in animal membrane lipids; others are arachidonic acid, linoleic and linolenic acids. The degree of unsaturation determines the fluidity of the membranes. Osamah S. Alrouwab 56  The Chemical Composition of the Membranes 2. Glycerophospholipids: ❑ Phosphatidyl ethanol amine (cephalin), phosphatidylcholine (Lecithin), and phosphatidylserine are among the most common glycerophospholipids. 3. Sphingolipids: ❑ They comprise another group of lipids found in biological membranes, especially in the tissues of the nervous system. ❑ There are three types of sphingolipids sphingomyelin, cerebrosides, and gangliosides. About 6 % of the membrane lipids of grey matter cells in the brain are gangliosides. Osamah S. Alrouwab 57  The Chemical Composition of the Membranes 4. Cholesterol: ❑ Cholesterol is another common component of the biomembranes of animals but not of plants and prokaryotes. ❑ It is oriented with its hydrophilic polar heads exposed to water and its hydrophobic fused ring system and attached hydrocarbon groups buried in the interior. ❑ Cholesterol helps to regulate the fluidity of animal membranes. Osamah S. Alrouwab 58  The Chemical Composition of the Membranes B. Proteins: The main types of membrane proteins are: Integral membrane proteins (also called intrinsic membrane proteins): These proteins are deeply embedded in the membrane. Thus, portions of these proteins are in Van der Waals contact with the hydrophobic region of the membrane. Peripheral membrane proteins (also called extrinsic proteins): These may be weakly bound to the surface of the membrane by ionic interactions or by hydrogen bonds that form between the proteins and the ‘polar’ heads of the membrane lipids. They may also interact with integral membrane proteins. They can be removed without disrupting the membrane. Osamah S. Alrouwab 59