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If you want the online textbook—Click on online student edition—full version 9. For Parents and Students Lighten the backpack load; leave some textbooks at school. Find This grade 7 Science worksheet on Life Science aims at upgrading a student? Cancel anytime. Life Science. The symptoms of pellagra progress through the three Ds: Deficiency of niacin causes pellagra. If an enzyme is denatured or dissociated into its subunits. It is a remarkable molecular device that determines the pattern of chemical transformations.
Their catalytic activity depends on the integrity of their native protein conformation. It changes only the rate at which equilibrium is achieved. Alcohol dehydrogenase. It increases the rate of a reaction by lowering the activation energy. With the exception of a small group of catalytic RNA molecules.
Enzymes have several properties that make them unique. Some cofactors are simple metal ions and other cofactors are complex organic groups. A cofactor can be linked to the protein portion of the enzyme either covalently or non- covalently.
The complete. Virtually all cellular reactions or processes are mediated by enzymes. Removal of cofactor from a conjugated enzyme leaves only protein component. Cofactors which are tightly associated with the protein covalently or non-covalently are called prosthetic group. They are highly specialized proteins and have a high degree of specificity for their substrates.
Xanthine oxidase Se Glutathione peroxidase 89 www. Enzymes can be divided into two general classes: Lysyl oxidase. Kinases Transfer phosphate from ATP to a substrate.
The last number is a serial number in the sub-subclass. The first three numbers define major class. EC 2 Transferases Transferases catalyze reactions that involve the transfer of groups from one molecule to another. There are six classes to which different enzymes belong. Oxygenases Directly incorporate oxygen into the substrate. According to this rule.
The enzyme commission has developed a rule for naming enzymes. Common trivial names for the transferases often include the prefix trans. The Enzyme Commission EC has given each enzyme a number with four parts.
These classes are: EC 1 Oxidoreductase Oxidoreductase catalyzes oxidation-reduction reactions. As for example. Because of the confusion that arose from these common names. Many enzymes are named for their substrates and for the reactions that they catalyze. Common names provide little information about the reactions that enzymes catalyze.
EC 3 Hydrolases Hydrolases catalyze reactions in which the cleavage of bonds is accomplished by adding water. Transaminases Transfer amino group from amino acids to keto acids. Examples of such groups include amino. Peroxidases Use H2O2 as an electron acceptor.
Dehydrogenases Use molecules other than oxygen e. Phosphorylases Transfer inorganic phosphate to a substrate. Under constant temperature and pressure. Thermodynamic principles The First law of thermodynamics states that the energy is neither created nor destroyed.
Free Study Material, Sample Questions, Notes on Life Sciences for CSIR NET JRF Exam
The change in the free energy. Chapter 02 Bioenergetics and Metabolism 2. The Second law of thermodynamics states that the total entropy of a system must increase if a process is to occur spontaneously. The chemical reaction has a characteristic standard free energy change and it is constant for a given reaction.
The free energy change which corresponds to this standard state is known as standard free energy change. B is also being converted to A. The concentration of reactants and products at equilibrium define the equilibrium constant.
If the reaction A B is allowed to go to equilibrium at constant temperature and pressure. At constant temperature and pressure. R is the gas constant.
The equilibrium constant Keq depends on the nature of reactants and products. It can be calculated from the equilibrium constant of the reaction under standard conditions i. T is the absolute temperature. In this state. Regulation occurs in following different ways: Allosteric regulation of enzymes by a metabolic intermediate or coenzyme. It consists of hundreds of enzymatic reactions organized into discrete pathways.
These pathways proceed in a stepwise manner. Those in eukaryotic cells occur in specific cellular locations. Metabolism serves two fundamentally different purposes: Its allosteric site will bind to the end product of the pathway which alters its active site so that it cannot mediate the enzymatic reaction. They are irreversible. The basic strategy of catabolic metabolism is to form ATP and reducing power for biosyntheses. Feedback inhibition and feedback repression In feedback inhibition or end product inhibition.
Catabolic pathways are involved in the oxidative breakdown of larger complex molecules and usually exergonic in nature. The feedback inhibition is different from feedback repression. To achieve these. A number of central metabolic pathways are common to most cells and organisms.
Anabolic pathways are involved in the synthesis of compounds and ender- gonic in nature. They are referred to as amphibolic pathways. They are regulated. By extracellular signal such as growth factors and hormones that act from outside the cell in multicellular organisms.
Availability of substrate. Each one has a first committed step. Some pathways can be either anabolic or catabolic. Most of the reactions in living cells fall into one of five general categories: Characteristics of metabolic pathways are: An inhibitory feedback system in which the end product produced in a metabolic pathway acts as a co-repressor and represses the synthesis of an enzyme that is required at an earlier stage of the pathway is called feedback repression.
Metabolic pathways involve several enzyme-catalyzed reactions. Each step of metabolic pathways is catalyzed by a specific enzyme. These pathways. Bioenergetics and Metabolism 2. The first enzyme in the pathway is an allosteric enzyme. Glycolysis takes place in the cytosol of cells in all living organisms. Although carbohydrates. An exergonic reaction proceeds with a net release of free energy. When one mole of glucose g is completely oxidized into CO2 and water. Respiration is an oxidative process.
Cells acquire free energy from the oxidation of organic compounds that are rich in potential energy. Energy is required for the maintenance of highly organized structures. Free energy is released in multiple steps in a controlled manner and used to synthesise ATP. The complete oxidation of substrates occurs in the presence of oxygen.
During oxidation within a cell. As the substrate is never totally oxidized. Table 2. Oxidation of glucose is an exergonic process. Carbohydrates are most commonly used as respiratory substrates. During cellular respiration. ATP acts as the energy currency of the cell. Glycolysis — Cytosol Citric acid cycle — Mitochondrial matrix Oxidative phosphorylation — Inner mitochondrial membrane In prokaryotes. A complete oxidation of respiratory substrates in the presence of oxygen is termed as aerobic respiration.
Glycolysis — Cytosol Citric acid cycle — Cytosol Oxidative phosphorylation — Plasma membrane www. The compounds that are oxidized during the process of respiration are known as respiratory substrates. The oxidative phosphorylation takes place in the inner mitochondrial membrane. For each molecule of glucose degraded to carbon dioxide and water by respiration.
Part of this energy is used for synthesis of ATP. The citric acid cycle takes place within the mitochondrial matrix of eukaryotic cells and in the cytosol of prokaryotic cells. The negative charge of the phosphate prevents the passage of the glucose 6-phosphate through the plasma membrane. Hexokinase is present in all cells of all organisms. It is a unique pathway that occurs in both aerobic as well as anaerobic conditions and does not involve molecular oxygen. Hexokinase and glucokinase are isozymes.
Step 2: Isomerization A readily reversible rearrangement of the chemical structure isomerization moves the carbonyl oxygen from carbon 1 to carbon 2. Phosphorylation Glucose is phosphorylated by ATP to form a glucose 6-phosphate.
Glycolysis occurs in the cytosol of all cells. Glucokinase is present in liver and beta-cells of the pancreas and has a high Km and Vmax as compared to hexokinase. This irreversible reaction is catalyzed by hexokinase. Bioenergetics and Metabolism Solution a. Valinomycin is an ionophore. Voltage gradient membrane potential across the inner mitochondrial membrane with the inside negative and outside positive. A mitochondrion actively involved in aerobic respiration typically has a membrane potential of about mV negative inside matrix and a pH gradient of about 1.
Antimycin A strongly inhibits the oxidation of Q in the respiratory chain. Inhibition of NADH dehydrogenase by rotenone decreases the rate of electron flow through the respiratory chain. Determination of electric potential and pH gradient Because mitochondria are very small.
Because antimycin A blocks all electron flow to oxygen. The electrochemical proton gradient exerts a proton motive force pmf. In a typical cell. In the presence of valinomycin. Bioenergetics and Metabolism Experimental proof of chemiosmotic hypothesis Experimental proof of chemiosmotic hypothesis was provided by Andre Jagendorf and Ernest Uribe in A burst of ATP synthesis accompanied the transmembrane movement of protons driven by the electrochemical proton gradient.
The F0 component is embedded in the inner mitochondrial membrane. The multiprotein ATP synthase or F0F1 complex or complex V catalyzes ATP synthesis as protons flow back through the inner membrane down the electrochemical proton gradient.
When the pH in the thylakoid lumen became 4.
An aspartic acid residue in the second helix lies on the center of the membrane. Rotational motion is imparted to the rotor by the passage of protons. In similar experiments using inside-out preparations of submitochondrial vesicles. In an elegant experiment. Ionophore uncouple electron transfer from oxidative phosphorylation by dissipating the electrochemical gradient across the mitochondrial membrane.
Most of the ATP generated by oxidative phosphorylation in mitochondria is exported to the cytoplasm. Ionophores are lipophilic molecules that bind specific cations and facilitate their transport through the membrane. ADP and Pi very weakly. Calculation of free energy change The standard free energy change for the movement of protons across the membrane along the electrochemical proton gradient can be calculated from the Nernst equation: DNP in an anionic state picks up protons in the inter-mitochondrial space and diffuse readily across mitochondrial membranes.
The free energy released on proton translocation is harnessed to interconvert three states. It decreases the memberane potential component of pmf without a direct effect on the pH gradient and thus ATP synthesis. ADP and Pi: An O state open state that binds ATP. Most common uncoupling agents are 2. Dicoumarol and FCCP act in the same way. After entering the matrix in the protonated form.
DNP is a weak acid that is soluble in lipid bilayer both in their protonated neutral forms and in their anionic states. A specific transport protein.
Bioenergetics and Metabolism and vice versa. NADH synthesized during the glycolytic process finally transfers the electrons to electron transport chain. NADH in the cytosol transfers electrons to oxaloacetate.
Malate is transported across the inner membrane by the help of transporter. A second membrane transport system is the phosphate translocase. In the matrix. This transport process is also powered by the transmembrane proton gradient.
The electrons are carried into the mitochondrial matrix in the form of malate. Malate then enters the mitochondrial matrix. NADH cannot cross the inner mitochondrial membrane. The malate-aspartate shuttle is the principal mechanism for the movement of NADH from the cytoplasm into the mitochondrial matrix.
H2O2, a toxic product of various oxidative processes, reacts with double bonds in the fatty acid residues of the erythrocyte cell membrane to form organic hydroperoxides. These, in turn, result in premature cell lysis. Peroxides are eliminated through the action of glutathione peroxidase, yielding glutathione disulfide GSSG. So, G6PD deficiency results in hemolytic anemia caused by the inability to detoxify oxidizing agents.
This pathway, first reported by Michael Doudoroff and Nathan Entner, occurs only in prokaryotes, mostly in gram-negative bacteria such as Pseudomonas aeruginosa, Azotobacter, Rhizobium.
In this pathway, glucose phosphate is oxidized to 2-ketodeoxyphosphogluconic acid KDPG which is cleaved by 2-ketodeoxyglucose-phosphate aldolase to pyruvate and glyceraldehydephosphate.
The latter is oxidized to pyruvate by glycolytic pathway where in two ATPs are produced by substrate level phosphorylations. Figure 2. The first process is a light dependent one light reactions that requires the direct energy of light to make energy carrier molecules that are used in the second process.
The calvin cycle light independent process occurs when the products of the light reaction are used in the formation of carbohydrate.
On the basis of generation of oxygen during photosynthesis, the photosynthetic organisms may be oxygenic or anoxygenic. Oxygenic photosynthetic organisms include both eukaryotes as well as prokaryotes whereas anoxygenic photosynthetic organisms include only prokaryotes. Oxygenic photosynthetic organisms Eukaryotes — Plants and Photosynthetic protists Prokaryotes — Cyanobacteria.
Anoxygenic photosynthetic organisms Prokaryotes — Green and purple photosynthetic bacteria. In oxygenic photosynthetic organisms, photosynthetic oxygen generation occurs via the light-dependent oxidation of water to molecular oxygen. This can be written as the following simplified chemical reaction:. Different types of pigments, described as photosynthetic pigment, participate in this process. The major photosynthetic pigment is the chlorophyll. Chlorophyll, a light-absorbing green pigment, contains a polycyclic, planar tetrapyrrole ring structure.
Chlorophyll is a lipid soluble pigment. It has the following important features: Chlorophyll has a cyclopentanone ring ring V fused to pyrrole ring III.
The propionyl group on a ring IV of chlorophyll is esterified to a long-chain tetraisoprenoid alcohol. In chlorophyll a and b it is phytol.
Pyrrole ring II contains methyl —CH3 group. It absorbs more violet-blue wavelength than red blue wavelength of light.
Carotenoids are long-chain. Chlorophyll is composed of two parts. It is an essential photosynthetic pigment. Anoxygenic photosynthetic organisms contain bacteriochlorophyll molecules. Accessory pigments Besides the major light-absorbing chlorophyll molecules. BChl b. Carotenoids are lipid soluble pigments and can be subdivided into two classes.
In the pure state. Chl c and Chl d. Oxygenic photosynthetic organisms contain different types of chlorophyll molecules like Chl a. They are related to chlorophyll molecules. BChl c. It absorbs more red wavelengths than violet. It is accessory photosynthetic pigment.
These chlorophyll molecules differ by having different substituent groups on the tetrapyrrole ring. The characteristic www. They are generally C40 terpenoid compounds formed by the condensation of eight isoprene units. Different groups of anoxygenic photosynthetic organisms contain different types of bacteriochlorophyll: BChl a. The two types of accessory pigments are carotenoids and phycobilins. BChl d and BChl e.
The tail is a 20 carbon chain that is highly hydrophobic. Chl b. Bacteriochlorophyll molecules absorb light at longer wavelengths as compared to chlorophyll molecules. Many diseases have been characterized that result from an inherited deficiency of the enzyme.
Two main biosynthetic pathways are known.
Bioenergetics and Metabolism Glycogen storage diseases Glycogen storage diseases are caused by a genetic deficiency of one or another of the enzymes of glycogen metabolism. The most important route to triacylglycerol biosynthesis is the sn-glycerolphosphate or Kennedy pathway. Within all cell types. These defects are listed in the table. In animals. Porphyrin biosynthesis involves three distinct processes: In contrast.
The deoxyribose sugar is generated by the reduction of ribose within a fully formed nucleotide. All deoxyribonucleotides are synthesized from the corresponding ribonucleotides. In de novo means anew pathways.. In salvage pathways. Synthesis of a substituted pyrrole compound. Modification of the side chains. Condensation of four porphobilinogen molecules to yield a partly reduced precursor called a porphyrinogen. The framework for a pyrimidine base is assembled first and then attached to ribose.
Orotate couples to ribose. Carbamoylaspartate then cyclizes to form dihydroorotate which is then oxidized to form orotate. The precursor of carbamoyl phosphate is bicarbonate and ammonia. This reaction is catalyzed by cytosolic carbamoyl phosphate synthetase II. The C-2 and N-3 atoms in the pyrimidine ring come from carbamoyl phosphate.
The synthesis of carbamoyl phosphate from bicarbonate and ammonia occurs in a multistep process. Pyrimidine rings are synthesized from carbamoyl phosphate and aspartate. Chapter 03 Cell Structure and Functions 3. Golgi complex. All organisms. The basic structural and functional unit of cellular organisms is the cell.
Hooke published his findings in his famous work.
Cells that have unit membrane bound nuclei are called eukaryotic. Robert Hooke first discovered cells in a piece of cork and also coined the word cell. Viruses are noncellular organisms because they lack cell or cell-like structure. Hooke only observed cell walls because cork cells are dead and without cytoplasmic contents.
The cell theory holds true for all cellular organisms. Non- cellular organisms such as virus do not obey cell theory. Rudolf Virchow proposed an important extension of cell theory that all living cells arise from pre-existing cells omnis cellula e cellula. Besides the nucleus. On the basis of the internal architecture.
The region of the cell lying between the plasma membrane and the nucleus is the cytoplasm. Evolution of the cell The earliest cells probably arose about 3. The prokaryotic cells lack such unit membrane bound organelles. Primitive heterotrophs gradually acquired www. Eukaryotic cells have a much more complex intracellular organization with internal membranes as compared to prokaryotic cells. The word cell is derived from the Latin word cellula.
The modern cell theory includes the following components: In According to this theory all living things are made up of cells and cell is the basic structural and functional unit of life. Anton van Leeuwenhoek was the first person who observed living cells under a microscope and named them animalcules. Over the time. It is an aqueous compartment bound by cell membrane.
In the year Cell theory In Both proteins and lipids are free to move laterally in the plane of the bilayer. The DNA is. Details of the evolutionary path from prokaryotes to eukaryotes cannot be deduced from the fossil record alone. The fossil record shows that earliest eukaryotic cells evolved about 1. Jonathan Singer and Garth Nicolson proposed fluid-mosaic model.
Three major changes must have occurred as prokaryotes gave rise to eukaryotes. Peripheral protein Phospholipid bilayer Integral protein Peripheral protein Figure 3.
Different models were proposed to explain the structure and composition of plasma membranes. The cyanobacteria are the modern descendants of these early photosynthetic O2 producers. A very significant evolutionary event was the development of photosynthetic ability to fix CO2 into more complex organic compounds.
It describes both the mosaic arrangement of proteins embedded throughout the lipid bilayer as well as the fluid movement of lipids and proteins alike. One important landmark along this evolutionary road occurred when there was a transition from small cells with relatively simple internal structures. It acts as a selectively permeable membrane and regulates the molecular traffic across the boundary. The plasma membrane exhibits selective permeability.
Cell Structure and Functions the capability to derive energy from certain compounds in their environment and to use that energy to synthesize more and more of their own precursor molecules. The original electron hydrogen donor for these photosynthetic organisms was probably H2S. Integral proteins float in this lipid bilayer.
In this model. The fatty acyl chains in the lipid bilayer form a fluid. These DNA-protein complexes called chromosomes become especially compact at the time of cell division. Some aerobic bacteria evolved into the mitochondria of modern eukaryotes. The plasma membrane of animal cells contains four major phospholipids. Cell Structure and Functions Chemical constituents of plasma membrane All plasma membranes. Phospholipids Phospholipids are made up of four components: Carbohydrates bound either to proteins as constituents of glycoproteins or to lipids as constituents of glycolipids.
The hydrophilic unit. Glycerophospholipids or phosphoglycerides contain glycerol. Sphingomyelin is the most abundant sphingophospholipid. Rarer phospholipids have a net positive charge.
There are two types of phospholipids: Sphingophospholipids contain an amino alcohol called sphingosine instead of glycerol. Phosphoglycerides are the most numerous phospholipid molecules found in plasma membranes. Phosphoglyceride molecules are classified according to the types of alcohol linked to the phosphate group. The ratio of protein to lipid varies enormously depends on cell types. In sphingophospholipid. Phospholipids derived from glycerol are called glycerophospholipids.
Lipid bilayer The basic structure of the plasma membrane is the lipid bilayer. The primary physical forces for organizing lipid bilayer are hydrophobic interactions. Carbohydrates are especially abundant in the plasma membranes of eukaryotic cells.
At neutral pH. Three classes of lipid molecules present in lipid bilayer. The fatty acid components are hydrophobic. This bilayer is composed of two leaflets of amphipathic lipid molecules. All cells have an electrical potential difference. Electrical potential across cell membranes is a function of the electrolyte concentrations in the intracellular and extracellular solutions and of the selective permeabilities of the ions.
The resulting separation of charge across the membrane constitutes an electric potential. At equilibrium. Cell Structure and Functions 3. Ion concentration gradients and selective movements of ions create a difference in electric potential or voltage across the plasma membrane. In addition to ion pumps. Electrogenic transport affects and can be affected by the membrane potential.
Its electrogenic operation directly contributes to the negative inside membrane potential. This is called membrane potential.
Active transport of ions by ATP-driven ion pumps. How membrane potentials arise? To help explain how an electric potential across the plasma membrane can arise. The channel undergoes through these various conformations as a result of voltage changes that take place during an action potential.
During the depolarizing phase. Leaky channels. Action potentials are the direct consequence of the voltage-gated cation channels. During the repolarizing phase. This process is called repolarization. Ion channels may be either leaky channels or gated channels.
The influx of positive charge depolarizes the membrane further. Movement of ions occurs through ion channels. Gated channels. Following the repolarizing phase there may be an after-hyperpolarizing phase. At resting potential about —70 mV. Cell Structure and Functions Let us now consider the changes in potential during an action potential.
During an action potential. The x-axis for time is the same in both graphs. The refractory period limit the number of action potentials that can be produced by an excitable membrane in a given period of time. During the absolute refractory period.
The top graph depicts an action potential. The relative refractory period is the time period during which a second action potential can be initiated. Gated Na and K channels closed Time millisecond Figure 3. It can be absolute or relative. The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the refractory period.
It may be a constitutive secretory pathway carried out by all cells or regulated secretory pathway carried out by specialized cells. An example of transcytosis is the movement of maternal antibodies across the intestinal epithelial cells of the newborn rat. Vesicle containing soluble proteins for constitutive secretion Constitutive secretory pathway Trans-Golgi network Extracellular space Regulated secretory pathway Secretory Golgi complex vesicle containing secretory proteins Plasma membrane Figure 3.
Cell Structure and Functions plasma membrane at the opposite side. Many soluble proteins are continually secreted from the cell by this pathway. The lumen of the gut is acidic. The two pathways diverge in the trans Golgi network. On exposure to the neutral pH of the extracellular fluid that bathes the basolateral surface of the cells.
Specialized secretory cells also have a regulated secretory pathway. The constitutive secretory pathway operates in all cells.
Examples of proteins released by such constitutive or continuous secretion include collagen by fibroblasts. The complexes remain intact and are retrieved in transport vesicles that bud from the early endosome and subsequently fuse with the basolateral domain of the plasma membrane. The receptor-antibody complexes are internalized via clathrin coated vesicles and are delivered to early endosomes. This pathway also supplies the plasma membrane with newly synthesized lipids and proteins.
The regulated secretion of small molecules. Ribosomes consist of rRNA and r-proteins. The signal that directs secretory proteins into such vesicles is not known.
Proteins destined for secretion called secretory proteins are packaged into appropriate secretory vesicles in the trans Golgi network. The r-proteins are termed as L or S depending on whether the protein is from the large or small subunit.
The ribosome is approximately globular structure. The functional ribosomes consist of two subunits of unequal size. In this secretory pathway. The secreted product can be either a small molecule such as histamine or a protein such as a hormone or digestive enzyme. There are generally more copies of the 5S genes than of the rRNA genes. The human genome contains about copies of rRNA genes per haploid set.
In all eukaryotes studied so far. Cell Structure and Functions The regulated secretory pathway is found mainly in cells specialized for secreting products rapidly on demand such as hormones. Table 3. Many other species. The sedimentation coefficient has units of second. It is the ratio of a velocity to the centrifugal acceleration.
Within the cell. All proteins synthesized by membrane free ribosomes are translocated post-translationally. Microsomes lacking attached ribosomes are called smooth microsome. Microsomes derived from RER are studded with ribosomes on the outer surface and are called rough microsomes. Transmembrane transport: In transmembrane transport. The transport of selected proteins from the cytosol into the ER lumen or into mitochondria is an example of transmembrane transport.
Protein translocation describes the movement of a protein across a membrane. The enclosed compartment is called the ER lumen. When cells are disrupted by homogenization. Gated transport: The protein translocation between the cytosol and nucleus occurs through the nuclear pore complexes. This process is called gated transport because the nuclear pore complexes function as selective gates that can actively transport specific macromolecules.
The transfer of proteins from the endoplasmic reticulum to the Golgi apparatus. It is an extensive network of closed and flattened membrane-bound structure. Proteins synthesized by membrane bound ribosomes are translocated co-translationally. ER membranes are physiologically active. Protein translocation may occur co-translationally or post-translationally.
Vesicular transport: In vesicular transport. Figure 3. The cisternal space or lumen remains continuous with the perinuclear space. Proteins synthesized by ribosomes associated with the membrane of RER enter into the lumen and membrane of RER by the process of co-translational translocation. In the lumen of the RER, five principal modifications of proteins occur before they reach their final destinations: The SER acts as the site of lipid biosynthesis, detoxification and calcium regulation.
N-linked glycosylation is the attachment of a sugar molecule to a nitrogen atom in an amino acid residue in a protein. In the RER, this process involves the addition of a large preformed oligosaccharide precursor to a protein. This precursor oligosaccharide is linked by a pyrophosphoryl residue to dolichol, a long-chain 75—95 carbon atoms polyisoprenoid lipid that is firmly embedded in the RER membrane and acts as a carrier for the oligosaccharide.
The structure of N-linked oligosaccharide is the same in plants, animals and single-celled eukaryotes - a branched oligosaccharide, containing three glucose Glc , nine mannose Man and two N-acetylglucosamine GlcNAc molecules which is written as Glc3 Man9 GlcNAc2. Biosynthesis of oligosaccharide begins on the cytosolic face of the ER membrane with the transfer of N-acetyl glucosamine to dolichol phosphate.
Two N-acetylglucosamine GlcNAc and five mannose residues are added one at a time to a dolichol phosphate on the cytosolic face of the ER membrane.
The first sugar, N-acetyl glucosamine, is linked to dolichol by a pyrophosphate bridge. This high-energy bond activates the oligosaccharide for its transfer from the dolichol to an asparagine side chain of a nascent polypeptide on the luminal side of the rough ER.
Tunicamycin, an antibiotic, blocks the first step in this pathway and thus inhibits the synthesis of oligosaccharide. After the seven-residue dolichol pyrophosphoryl intermediate is flipped to the luminal face. The remaining four mannose and all three glucose residues are added one at a time in the luminal side.
The sugar molecules participate. ER-resident proteins often are retrieved from the Cis-Golgi As we have mentioned in the previous section that proteins entering into the lumen of the ER are of two types- resident proteins and export proteins. How, then, are resident proteins retained in the ER lumen to carry out their work? The answer lies in a specific C-terminal sequence present in resident ER proteins. Several experiments demonstrated that the KDEL sequence which acts as sorting signal, is both necessary and sufficient for retention in the ER.
If this ER retention signal is removed from BiP, for example, the protein is secreted from the cell; and if the signal is transferred to a protein that is normally secreted, the protein is now retained in the ER. The finding that most KDEL receptors are localized to the membranes of small transport vesicles shuttling between the ER and the cis-Golgi also supports this concept. The retention of transmembrane proteins in the ER is carried out by short C-terminal sequences that contain two lysine residues KKXX sequences.
How can the affinity of the KDEL receptor change depending on the compartment in which it resides?
The answer may be related to the differences in pH. Clearly, the transport of newly synthesized proteins from the RER to the Golgi cisternae is a highly selective and regulated process.
The selective entry of proteins into membrane-bound transport vesicles is an important feature of protein targeting as we will encounter them several times in our study of the subsequent stages in the maturation of secretory and membrane proteins. The Golgi complex, also termed as Golgi body or Golgi apparatus, is a single membrane bound organelle and part of endomembrane system.
It consists of five to eight flattened membrane-bound sacs called the cisternae. Each stack of cisternae is termed as Golgi stack or dictyosome. The cisternae in Golgi stack vary in number, shape and organization in different cell types.
The typical diagrammatic representation of three major cisternae cis, medial and trans as shown in the figure 3. In some unicellular flagellates, however, as many as 60 cisternae may combine to make up the Golgi stack. The number of Golgi complexes in a cell varies according to its function. A mammalian cell typically contains 40 to stacks. In mammalian cells, multiple Golgi stacks are linked together at their edges.
Each Golgi stack has two distinct faces: Both cis and trans faces are closely associated with special compartments: Further modifications of N-linked oligosaccharide in the Golgi apparatus gives two broad classes of N-linked oligosaccharides. The vesicles fuse with the Golgi membranes and release their internally stored molecules into the organelle. Proteins and lipids from the smooth and rough endoplasmic reticulum bud off in tiny bubble-like vesicles that move through the cytoplasm until they reach the Golgi apparatus.
High-mannose www. Both networks are thought to be important for protein sorting. It modifies proteins and lipids that have been built in the endoplasmic reticulum and prepares them for export outside of the cell or for transport to other locations in the cell.
Glycosylation of proteins N-linked oligosaccharide chains on proteins are altered as the proteins pass through the Golgi cisternae en route from the ER.
The Golgi apparatus is especially prominent in cells that are specialized for secretion. When completed. ER - lysosome. Proteins and lipids enter the cis Golgi network in vesicular tubular clusters arriving from the ER and exit from the trans Golgi network. Substances from ER enter into the cis face of a Golgi stack for processing and exit from trans face. The modifications to molecules that take place in the Golgi apparatus occur in an orderly fashion.
In such cells. Secretory vesicles form from the trans Golgi network. The chemical make-up of each face is different and the enzymes contained in the cisternae between the faces are distinctive. As we have seen. Once inside. The majority of eukaryotic cells are diploid. Each cell is programmed to respond to specific extracellular signal molecules. This is accomplished by a variety of signal molecules that are secreted or expressed on the surface of one cell and bind to receptors expressed by other cells.
The number of chromosomes in a species has no specific significance nor does it indicate any relationship between two species which may have the same chromosome number. One chromosome contains multiple origin of replication. It consists of a long array of short.
Binding of the signal by a specific receptor leading to its activation. Depending on the eukaryotic organism. Cell Structure and Functions termed as heterochromatin. The centromeres serve both as the sites of association of sister chromatids and as the attachment sites for microtubules of the mitotic spindle.
Transport of the signal to the target cell. Every cell maintains a characteristic number of chromosomes. Centromere The constricted region of linear chromosomes is known as the centromere.
Because of its condensed state. Although this constriction is called the centromere. Origin of replication The origin of replication also called the replication origin is a particular sequence in a chromosome at which replication is initiated.
Telomere Telomeres are specialized structures. Chromosome number All eukaryotic cells have multiple linear chromosomes. Species Haploid number of chromosome Saccharomyces cerevisiae budding yeast 16 Schizosaccharomyces pombe fission yeast 03 Caenorhabditis elegans 06 Arabidopsis thaliana 05 Drosophila melanogaster 04 Tetrahymena thermophilus Micronucleus 5. Initiation of signal-transduction pathways. Synthesis and release of the signaling molecule by the signaling cell.
Extracellular signaling usually involves the following steps: One important example of such is the response of cells of the vertebrate immune system to foreign antigens. In paracrine signaling. Membrane bound signal molecules remain bound to the surface of the cells and mediate contact dependent signaling. These molecules are divided into two categories — membrane bound and secretory signal molecules.
Notch signalling and classical cadherin signalling are examples of juxtacrine signaling. It is a long-range signaling in which signal molecule is transported by the blood stream. Cell Structure and Functions In animals. Secreted extracellular signal molecules are further divided into three general categories based on the distance over which signals are transmitted: In most cases. In juxtacrine signaling. In autocrine signaling. An example of this is the action of neurotransmitters in carrying signals between nerve cells at a synapse.
In endocrine signaling. Certain types of T- lymphocytes respond to antigenic stimulation by synthesizing a growth factor that drives their own proliferation.
Unlike other modes of cell signaling. Bcl2 was the first protein shown to cause an inhibition of apoptosis. In the presence of an apoptotic stimulus. The pro-apoptotic Bcl2 proteins consist of two subfamilies. Most cancers are initiated by genetic changes and majority of them are caused by changes in somatic cells and therefore are not transmitted to the next generation. In the absence of an apoptotic stimulus.
Mammalian Bcl2 family of proteins regulate the intrinsic pathway of apoptosis mainly by controlling the release of cytochrome c and other intermembrane mitochondrial proteins into the cytosol. When an apoptotic stimulus triggers the intrinsic pathway. BH3-only proteins are activated and bind to the anti-apoptotic Bcl2 proteins so that they can no longer inhibit the BH proteins.It consists of five to eight flattened membrane-bound sacs called the cisternae.
When no further additions are made, the resulting compound is phosphatidic acid, the simplest phosphoglyceride. The two molecules are mirror images and cannot be superimposed on one another. The rod shaped E. Such a donor bacterial cell is called an Hfr strain for high frequency of recombination because it exhibits a very high efficiency of chromosomal gene transfer in comparison with F-cells. Biomolecules and Catalysis Table 1.
For example, trypsin, a proteolytic enzyme, is secreted by the pancreas. In the presence of an apoptotic stimulus, BH3-only proteins are activated and bind to the anti-apoptotic Bcl2 proteins so that they can no longer inhibit the BH proteins. Phosphorylation Glucose is phosphorylated by ATP to form a glucose 6-phosphate. The most widely used is cyanogen bromide CNBr.
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