Cells and Genes - Essay Example

a) Describe, using d examples and annotated diagrams where appropriate, membrane-bound nucleus and organelles within eukaryotic cells; The Prokaryotic cells are the most primitive life-form known capable of thriving under the pre-biotic conditions and differ markedly from Eukaryotic cells The Eukaryotic cell is perfectly designed to execute efficiently all the tasks a life form needs to survive in a hostile environment. A group of such cells is called a tissue and similar functioning tissues make an organ while one or more organs constitute an organ system. Now the survival of the entire organism is dependent on the working of each cell (Pavelk. Roth, 2010). A cell is composed of many sub-cellular parts or organelles, each with a specific role to fulfill, so no single cell is over-burdened. Energy demand is met along with a significant conservation of energy as well. In biological system this energy currency is ATP, which is a chemical link between catabolism and anabolism. This compartmentalization is prevalent in prokaryotic as well as a eukaryotic cell. The dominant type i.e. the eukaryotic cell consists of single and double membrane bound organelles in two types of cells, Animal cells and Plant cells, both having only minute differences yet accounting for the great diversity of life. The roles performed by each organelle are essential to the life of the entire organism. The organelles are as follows: CELL WALL: This is freely permeable, rigid and firm and gives proper shape to the cell. It is composed of cellulose and other conjugated molecules. Animal cells lack cell walls and so do not have a properly contained shape. Substances that help keeping the cell wall rigid are chains of pectin, glycan and cellulose. All are conjugated compounds or polysaccharides with beta 1, 4 linkages among the consecutive glucose residues, in case of cellulose. (Rose, 2003) CELL MEMBRANE: This is selectively permeable and helps control the traffic into and out of the cell. It is comprised of a lipid bi-layer in which protein molecules are embedded like a mosaic. It stops the cytoplasm from flowing out and provides a barrier from the outer-cell environment. Present in both plants and animal cells. Figure 2 . (Papa. Tager, 1995) Fluid Mosaic model of the cell membrane The presence of maintained levels of cholesterol helps to maintain fluidity of the membrane. It should be noted that this membrane is not static but is fluid, Hydrophobic ends of the phopholipid subunits are embedded inside the membrane while the hydrophilic ends protrude outwards. NUCLEUS: The double membrane bound nucleus is the control centre of the cell. It commands and controls the working of the entire cell via mRNAs (messenger Ribonucleic Acids). The nucleus is further composed of sub units. Figure 3 Nucleus (Mackintosh, Al- Khalili, Jonson, Pena. 2012) Nucleolus: Ribosomes are synthesized in the nucleolus by the combination of rRNA ribosomal RNA with proteins. It is therefore composed of ribonucleic acids and protein. Nucleoplasm: It is the protoplasm of the nucleus and helps to submerge the nucleolus in the nucleus. It is composed mainly of water along with dissolved mineral ions and is also known as Karyoplasm. Moreover the nucleus also consists of the following: Nuclear pores: the nuclear membrane is discontinuous at certain points forming an entrance or exit for metabolites and RNA to enter or leave the cell, into the cytoplasm. Usually it is found in continuity with the endoplasmic reticulum. Nuclear pores can be as many as three thousand in a particular nucleus. Figure 4 Animal Cell DNA: This is present in the chromosomes, the Deoxy ribonucleic acid. DNA basically transcribes information present in its allelic pairs of genes in the form of base pairs and nucleotides composed of nitrogenous bases larger double ringed bases adenine, guanine, and smaller single ringed bases cytosine and thymine and uracil in case of RNA. Adenine combines with Thyamine by Double bonds while Guanine combines with Cytosine by Triple bonds, so the amount of Thymine equals Adenine while that of Guanine equals Cytosine, moreover it contains a deoxy ribose sugar which differs from ribose sugar present in RNA at C-5 and a phosphate group. These nucleotides then form a base pair with other nucleotides via phosphodiester bond. The DNA has evolved to perform major functions for the efficient survival of itself and the organism with the properties of self replication and repair. The evolution of DNA from RNA took place at the hands of a process called phosphorylation. The systematic procedure comprising of transcription, translation and protein synthesis synthesizes enzymes and various other proteins. Proteins hydrolyze into amino acids with the help of various hydrolazes and hydrolytic enzymes and can later be incorporated in the carbohydrate metabolic pathway at one or more positions to form either glucose or pyruvate. Figure 5 (McCabe, 2008) Ribosomes: These are single-membrane bound entities that play a major role in protein synthesis. They are often found in the nucleolus, freely dispersed in the cytoplasmic soup or attached to the rough endoplasmic reticulum. mRNA and tRNA operate with the help of these ribosomes, incorporating required amino acids to form a protein. The basic structure of a ribosome consists of two subunits one larger and the other smaller, distinguished on the basis of centrifugal unit Svedberg S. Figure 6 (Spedding, 1990) Cytoplasm: This is composed of a ground substance where all the waste metabolites and storage materials are present and a viscous sol or gel containing the sub cellular organelles. It is sticky and keeps the components in place. Moreover the cytoskeleton of the cytoplasm is an extremely complicated network of protein fibers. Cytoskeleton serves different and highly significant functions for the cell, it is not adamant but undergoes construction or destruction in part almost continuously, and it helps in the cytosolic movement of the cell components and metabolites. Moreover it provides an effective resistance against deformities and keeps the cell rigid. During cell division it efficiently distributes the cytoplasm between the daughter cells and helps in the protrusion of flagella, pilli and fimbrae. Flagella being the locomotory organs while pilli and fimbrae are used for reproduction via conjugation alongwith locomotion. Vacuoles: These are the storehouse of essential metabolites and storage products and can vary from one to many in different eukaryotic cells. Plant cells are mono vacuolar in which usually a very large central vacuole is present which pushes the remaining cell contents to the boundaries. While animals have many vacuoles freely dispersed in the cytoplasm. Mitochondria: These are viewed as the power house of the cell. Mitochondria are double membrane bound and generate the cell’s energy currency: ATP. Mitochondria and chloroplasts have their own independent DNA. The structure of Mitochondria consists of an outer membrane; inter membrane space, inner membrane, cristae and matrix. Cristae consist of embedded enzymes which play an important role in the electrochemical chains generating energy and controlling trans-membrane transfer of protons, thus establishing a gradient. Figure 7 Mitochondria (Bock, Knoop . 2012) Chloroplasts: These are unique to plant cells and double membrane bound with an independent DNA. They consists of stroma which is a jelly-like substance. Immersed in the stroma are grana and inter-granna. The stacking grana form the thylakoid membranesacs, containing chlorophyll, a pigment, which carries out photosynthesis in the lumen. Figure 8 (Bock, Knoop. 2012) Endoplasmic reticulum: Two types of endoplasmic reticulum (ER) are present: the rough endoplasmic reticulum and the smooth endoplasmic reticulum. Rough ER has embedded ribosomes which give it a rough texture. It is involved in the formation of protein substances for example in hepatic liver cells. The smooth ER on the other hands works in lipid metabolism, detoxification and carbohydrate metabolism. The endoplasmic reticulum is continuous with the nuclear membrane at one end and the plasma membrane at the other end. Golgi Apparatus: This apparatus sorts and packages the products of the endoplasmic reticulum into finished products and gets them ready for exocytosis or secretion outside the cell. These secretions might either work as stimulants, lymphocytes or substrates for other cells either in the blood stream or the interstitial space. It also carries out the following functions: Figure 9 (Pavelka, Roth. 2010) Intra-cell transport of lipids Formation of conjugated compounds: it effectively glycosylates proteins to form proteoglycans and glycosaminoglycans which are the components of membranes, skin, joints, bones, cartilage, tendons and ligaments. It is also involved in phophorylation, addition of phosphate groups to compounds but this process requires energy which is supplemented in the form of ATP. Phagocytosis: Golgi complex role is to synthesize from the golgi vesicles, lysosomes, which are hydrolytic in nature and function in recycling. They engulf the worn out and used parts of the cells, making the raw material available for new incorporation and assimilation. These lysosomes are abundant in hydrogen peroxide. Evolutionary advantages of intracellular compartmentalization: Previously compartmentalization was believed to be unique to the eukaryotic cell but it is now known not to be the case. In fact it differs in both composition of the protein and conjugated membrane systems. As far as the advantages are concerned, compartmentalization aided by specific check points helps in coordinating the function of the cell without creating a meshwork of substrates, metabolites and organelles. Separating these structures are membranes, composed of proteins, carbohydrates or lipids in the forms of agar, pectin, lignin, cutin, cellulose and dextrins. If these membranes were not developed, the molecules would swim in the cytoplasmic soup, get bumped and the probability of an undesired chemical reaction would mount up too. Moreover the proteins or substrates required by each compartment are transferred into it after recognition by regulatory sites. REFERENCES: Gary Spedding. Ribosomes and Protein Synthesis: A practical approach. Life Sciences. Biochemistry. IRL Press at Oxford University Press. 1990. Print. Jocelyn K. C. Rose. The Plant Cell Wall. Life Sciences, Biochemistry. CRC Press. 2003. Print. Linda L. McCabe, Edward R. B. McCabe. DNA: Promise and Peril. Science, life sciences, genetics and genomics. University of California Press. 2008. Print Margit Pavelka, Jurgen Roth. Functional Ultrastructure: Atlas of Tissue Biology and Pathology. Life sciences, Biology. Springer Science & BusinessMedia. 2010. Print Ralph Bock, Volker Knoop. Genomics of Chloroplasts and Mitochondria. Life sciences, Botany. Springer Science & Business Media. 2012. Print Ray Mackintosh, Jim Al-Khalili. Bojorn Jonson, Teresa Pena. Nucleus: A Trip into The Heart Of Matter. Johns Hopkins University Press. 2012. Print Sergio Papa, J.M. Tager. Biochemistry of Cell Membranes. A Compendium of Selected Topics. Springer Science & Business Media. 1995. Print Read More