The Human Cell - Term Paper Example

 The Human Cell INTRODUCTION The phenomenon of life is complex and has never ceased to intrigue our senses. Understanding this complex phenomenon requires the interpretation of processes at the very basic level. Cell is the basic unit of living organisms. It is the simplest and the smallest living unit. It has organelles for various functions. The chemical processes and complex interactions that occur inside the cell form the bridge between the nonliving and the living (Sherwood 2010). Inside the human body, various processes that go on synchronized to provide an efficient healthy functioning are interrelated and coupled for optimum functional output. This requires organization at various levels. In complex living beings like humans, there are many levels of organization, as opposed to single cellular organisms like bacteria. The study about the cell and its structure was revolutionized with the invention of microscope. The discovery of microscope in 16th century encouraged scientists to study these basic building blocks. It was Robert Hooke who discovered the cell in 1965 and named it Cellulae (Latin: tiny rooms) as he observed box like structures in a thin slice of cork under his microscope (Lee et al 2009). He was the first one to describe the fungus Mucor. Later Antony Van Leeuwenhoek improved the microscope and documented some initial findings about protozoa and bacteria (Gest 2004). The contributions of Matthias Schleiden, Theodor Schwann and Rudolf Virchow lead to the development of ‘The Cell Theory’ in 1839. According to the cell theory: 1. All living things are made from one or more cells. 2. The cell is the basic unit of life. 3. All cells come from preexisting cells. Thus the cell is an essential component of the human body and this paper will explore the organization and cellular membrane of the cell. It will also analyze the energy processes of the cells accompanied with the enzymes and the metabolic activities of the cells. ORGANIZATION OF CELLS Advances in microscopy and imaging led to further research into the cell structure. It is now established fact that cells are composed of various organelles and there are complex signaling pathways that control the expression of various molecules. The structure and composition of different cell types vary, but certain characteristics are common to all cells. Cells of similar kind together form the tissues (Marieb 2006). There are four basic types of tissues that form the entire organism, epithelium, supporting or connective tissue, muscle and nerve tissue. Organs are formed by two or more tissue types combined together for a specific function. One tissue type forms the parenchyma, the functioning part of the organ, while other tissues provide the supporting frame work, i.e. the stroma (Crowley 2007). The organization of different organs working together to perform a particular function forms the organ system, like nervous system, digestive system, musculoskeletal system and others. Finally, the different organ systems work together in an individual to maintain a state of homeostasis and optimum health functions (Ganong 2005; Guyton et al 2007). Protoplasm and Cell Membrane: The basic cellular structural components are similar in all cell types. Cells are bounded by a plasma membrane (cell membrane) and contain the fluid called protoplasm. The protoplasm is divided in clear cytoplasm and the central nucleus. The protoplasm is made up of five basic substances, water, carbohydrates, proteins, lipids and electrolytes. Water is the basic fluid medium of cell and forms up to 75- 80% of its concentration. It provides a medium for the suspension of particulate matter and in which chemical reactions can take place. Proteins are the building blocks of cells and form up to 10- 20 % of cell mass. They are of two types, functional and structural. Functional proteins are the enzymes that catalyze the cell reactions. They can be present as part of the membranes or can be mobile and catalyze the reactions as they come in contact with different substances. Structural proteins on the other hand form supporting cytoskeleton of the cell. They are present as long filaments and microtubules. Phospholipids and cholesterol form 2 % of the cell mass. These lipids are important components of cell membrane and other intracellular membranes. Fat cells represent the energy storehouse of the body and contain up to 95% triglycerides. Nutrition is provided by the carbohydrates in the cell. Unlike lipids, carbohydrates are not stored in cells; rather they form average 1% of cell mass in most cell types. They are present in dissolved form in extracellular fluid and cells can always take up glucose and metabolize it to release energy. Small amounts of glycogen (storage form of carbohydrates) are present in cells to provide energy in times of need. The electrolyte balance is maintained through well regulated quantities of ions. Potassium, magnesium, bicarbonate, phosphate and sulphate are the major ions. Smaller quantities of sodium, chloride and calcium are also present. The ions are inorganic components for cellular reactions. Certain cell functions are dependant upon changes in ion concentrations, like conduction of nerve impulses across plasma membranes (Guyton et al 2007). The cell is separated from the extracellular environment and from other cells by the cell membrane which covers the cell like an envelope. It is composed of 55% proteins, 25% phospholipids, 13% cholesterol, 4% of other lipids and 3% carbohydrates. The plasma membrane is actually a lipid bilayer. The phosphate ends of the phospholipids’ molecules are hydrophilic and face the intracellular and extracellular compartments, while water repelling hydrophobic ends are composed of fatty acid molecule and face each other on the inside of the lipid bilayer. This central lipid content of the membrane is responsible for its impermeability to water and water soluble molecules. The water soluble substances are transported across the membrane through specialized carrier proteins. Fat soluble substances easily pass through the membrane and do not need any carrier proteins. Cholesterol molecules dissolved in the membrane are responsible for determining the degree of permeability. The carrier proteins are mostly composed of glycoprotein and can be either integral or peripheral proteins. Integral proteins span the entire thickness of the membrane while peripheral proteins are attached to one surface of the membrane only. Other integral proteins work as carrier proteins to facilitate active transport against concentration gradient. Cell membrane proteins also serve as receptors for peptide hormones and affect the second messenger signaling pathways (Ganong 2005; Guyton et al 2007). Organelles: Cytoplasm has various organelles each of which is targeted to perform its specialized function. The clear portion of cytoplasm is known as cytosol. It has dissolved proteins, electrolytes and glucose. The organelles are dispersed in cytosol. The organelles include endoplasmic reticulum, Golgi apparatus, ribosome, mitochondria, lysosomes and peroxisomes. Apart from these, it contains secretory vesicles, glycogen granules and fat globules (Guyton et al 2007). The endoplasmic reticulum is composed of series of stacks and tubules and functions to transport various substances within the cell. Granular endoplasmic reticulum has ribosomes attached to its surface and functions in the synthesis of proteins. Agranular (smooth) endoplasmic reticulum is associated with the synthesis lipids for the cell. After being processed in the endoplasmic reticulum, the molecules pinch off and fuse with the Golgi apparatus. Golgi apparatus processes these molecules to form the secretory vesicles and lysosomes. The secretory vesicles store protein proenzymes. When required, these are secreted outside the cell and become activated in their target tissues to release the enzymes. Lysosomes constitute the intracellular digestive system. They contain different types of hydrolase enzymes and engulf damaged cellular structures and foreign particles that have entered the cell including bacteria. The hydrolases split the molecules into two or more parts by using a water molecule. They hydrolyze proteins to amino acids, lipids to fatty acids and carbohydrates into glucose. Peroxisomes are formed by self-replication and contain oxidases. Oxidase along with catalase (another enzyme found in peroxisomes) is responsible for oxidation of substances that are toxic to cell, like alcohol. The mitochondria are known as the ‘powerhouses of the cell’ as they have enzymes needed to metabolize the nutrients to liberate the energy. The number of mitochondria in different cells varies according to the energy requirement. Liver and cardiac muscle have more mitochondria to meet the increased energy demands. The mitochondria are composed of two layers of lipid. Inner layer is thrown into folds that form the shelves, cisternae where oxidative enzymes can be found. The mitochondrial matrix contains enzymes. The matrix enzymes and the oxidative enzymes catalyze the oxidation of nutrients to release the energy in the form of Adenosine Triphosphate (ATP) which is a high energy molecule (Ganong 2005; Guyton et al 2007). Nucleus serves as the control center of the cell. It is bounded by a double nuclear membrane which has several pores. Nucleus contains DNA which is responsible for determining the genetic make up of an individual. The genes are responsible for the expression of various substances within the cell. They also control cell replication and cell division. The genetic code is contained within the chromosomes (Crowley 2007). The chromosomes are composed of DNA and proteins. DNA controls the transcription of all molecules. It synthesizes the RNA, which diffuses out of the nucleus into the cytoplasm. In association with specific proteins, RNA assembles in ribosomes which then synthesize various proteins as directed by the DNA (Watson 2006). Prokaryotic and Eukaryotic Cells: The living beings are fundamentally divided into prokaryotes and eukaryotes on the basis of their cellular organization. The prokaryotic cell does not have true nucleus. Bacteria belong to this division. Prokaryotes have a circular molecule of loosely aggregated DNA which serves as genetic material. It is not bounded by the nuclear membrane and there is no mitotic apparatus. The cell has smaller 70S ribosome and there are no any organelles. They are bounded by rigid external cell wall, which is composed of peptidoglycan (amino acids and sugars). Unlike eukaryotes, the cell membrane does not contain sterols (Levinson 2008). Eukaryotic cells have a true nucleus that is bounded by nuclear membrane and has multiple chromosomes. The mitotic division results in allocation of equal number of chromosomes to the daughter cells. The cells contain organelles including mitochondria and lysosomes. The Ribosome is larger 80S. The cell membrane has sterols in case of fungi and phospholipids in higher organisms. Human cells, cells of fungi, protozoa and helminthes are eukaryotes (Levinson 2008). Storing and processing of information: Maintenance of cellular processes for homeostasis and replication of cells depends upon the complex interaction between the DNA, chromosomes and the expression of various proteins. The deoxyribonucleic acid (DNA) along with proteins forms the chromosomes. The unit of DNA is nucleotide, which is composed of a phosphate group linked to a 5- carbon sugar moiety and a nitrogen base. The nitrogen bases can be purines with double ring structure or pyrimidines which have a single ring structure. Purine bases are adenine and guanine while pyrimidine bases are cytosine and thymine. These nucleotides, bonded with sugar- phosphate form the length of the DNA molecule while bases form the rungs of the ladder like structure on DNA. The DNA molecule is a double helix joined with weak bonds. The base pairing in these two strands is fixed, i.e. adenine always pairs with thymine and guanine always pairs with cytosine. These bases are said to be complementary base pairs. For the process of replication, two strands of DNA separate and duplicates are formed for each strand. Thus two daughters DNA are formed, each having one original strand from the parent DNA. The DNA has instructions for all cell processes, but it is the RNA (Ribonucleic acid) that executes these functions by forming a communication between the DNA and other cell machinery present in the cytoplasm. Messenger RNA (mRNA) is single stranded and carries the genetic information coded on the DNA through the pores in the nuclear membrane to the cytoplasm. Transfer RNA (tRNA) transfers this genetic information to the ribosomes which assemble the amino acids into proteins according to the code determined by the mRNA (Crowley 2007). The Endomembrane System: Eukaryotic cells and their organelles are bounded by membranes. These membranes not only function to protect the cell and maintain its shape, but also contain various enzymes and protein molecules necessary to maintain normal signaling and homeostasis. The endomembrane system consists of the plasma membrane, nuclear envelope; endoplasmic reticulum (smooth and rough), Golgi apparatus and various membranes bound vesicles derived from them (Guyton et al 2007). The endomembrane system is associated with the formation of proteins specified by the DNA as well as their packaging and transport. As directed by the mRNA, polypeptides are synthesized on the surface of rough endoplasmic reticulum. Moving through the membranes of rough endoplasmic reticulum, these polypeptide chains are modified and released as vesicles. These vesicles enter the Golgi apparatus and release their contents. Golgi apparatus further processes these proteins and packs them as vesicles and releases them for their delivery to target destinations. These vesicles can either perform intracellular functions (lysosomes) or they can fuse with the plasma membrane and release their contents to the exterior (exocytotic vesicles). Similarly, when contents from extracellular environment enter the cell through endocytosis, they take up a plasma membrane envelope and form endocytotic vesicle. This process is responsible for balanced exchange of proteins and lipids between the plasma membrane and endomembrane system (Sherwood 2010). Processing Energy: Humans consume various types of food materials rich in carbohydrates, protein, fats and other nutrients. To derive energy from these nutrients various processes take place at the cellular level. As mentioned earlier, mitochondria are the powerhouses of the cell. Mitochondria extract the energy out of these nutrients and supply it to the body for various functions in the form of Adenosine Triphosphate (ATP), which is the energy currency of the body (Sherwood 2010). It has a high energy phosphate bond; it is this bond that breaks to release enormous amounts of energy as ATP is converted into ADP (adenosine diphosphate). To derive energy from these nutrients, they are first converted into their smaller units. Carbohydrates are converted into glucose, proteins into amino acids and fats into fatty acids. Within the mitochondrial matrix, these moieties are converted into Acetyl coenzyme A (acetyl-coA). Series of chemical reactions take place, which split acetyl-coA into hydrogen and carbon dioxide. Carbon dioxide is released as a byproduct of the process while hydrogen being highly reactive combines with Oxygen. This releases enormous amounts of energy and leads to the formation of ATP. This reaction is catalyzed by the enzyme ATP synthetase present on the shelves of mitochondrial membrane. The ATP thus formed diffuses out of the cell to circulate in the body (Guyton et al 2007). This cycle of reactions is known as citric acid cycle or Krebs cycle. The ATP provides energy for all cellular functions, including the energy required for transport of substances across membranes, syntheses of chemical compounds as well as other mechanical work (Ganong 2005; Guyton et al 2007). Cytoskeleton- micro filaments, intermediate filaments, and microtubules: The cell and its organelles are supported by a framework of proteins forming the ‘cell cytoskeleton’. It serves as scaffolding which supports, organizes and allows the movement of the cell. It also supports the cell membrane and maintains the shape of the cell. The proteins of the cytoskeleton are mainly fibrillar proteins and are organized in three types: microtubules, microfilaments and intermediate filaments. Microtubules are the largest with the diameter of 22nm. They maintain the asymmetric cell shapes owing to different structures of various organelles. They also facilitate movement within the cell including that of secretory vesicles, cell projections like cilia and flagella, and movement of chromosomes during cell division. Microfilaments are the smallest with the diameter of 6nm. They function as contractile proteins for the cell (Actin and Myosin). Apart from that, the microfilaments also mechanically strengthen various cellular projections like microvilli. Intermediate filaments have an intermediate diameter of 7- 11 nm. They appear as threadlike fibers and are meant to provide structural stability to withstand mechanical stress. Intermediate filaments vary in different types of cells. In nerve cell axons, they form neurofilaments whereas in skin, intermediate filaments are present in the form of Keratin (Ganong 2005; Guyton et al 2007). Extracellular Structures: Eukaryotes do not have significant extracellular structures but prokaryotic cells have important extracellular structures. These include the cell wall, capsule, flagella, pili and glycocalyx. The cell wall is a structural component of mainly bacterial species which lies outside the cell membrane. The cell wall has two further layers which include the peptidoglycan which lies on the inner side and it has a membrane on the exterior side. The main purpose of the cell wall is to hold the structure of the cell and at the same time uphold its proper shape. The capsule is another extracellular component which lines the outer side of the cell wall and is made up of gelatin. It has a function in bacterium to save the bacterium from being attacked by the immune cells of the human body. The flagellum is an essential component for a few cells which assist organisms in movement to the desired sites. Pili are structures that resemble hair and they assist the bacterial cells to attach to specific receptors of choice. The glycocalyx is a coat that is used by the bacterium to stick to different locations (Levinson 2008). CELLULAR MEMBRANE Composition and structure:  The cell membrane is an essential structure of the cell which is important for the maintenance of the integrity of the cells. The cellular composition is based upon varying amounts of macromolecules which include proteins, phospholipids, cholesterol, other lipids and carbohydrates in the concentration of 55%, 25%, 13%, 4% and 3% respectively. The cellular membrane of the cell is a bilayer of lipids with two ends: the hydrophobic and the hydrophilic end. The charged ends of the phospholipids molecules which are formed due to the presence of phosphates are hydrophilic and hence are directed towards the extracellular side and towards the cytoplasm. The hydrophobic ends are uncharged and are made up of the fatty acid molecule and face each other in the centre. The centre of the membrane is composed of lipids which are not permeable to water as well as water soluble molecules. The cholesterol is essential for the maintenance of the permeability of the cell membrane (Ganong 2005; Guyton et al 2007; Murray et al 2006). The proteins are also an essential component of the cell membranes with the highest concentration within the cells. Fifty five percent of the cell membrane is constituted by proteins. That is for approximately fifty molecules of lipids, there is one protein molecule present. The proteins play a part in the maintenance of the shape of the cell membrane which is essential for functioning of the cell as well. The proteins within the cell membrane are mainly of two types. Proteins that span the entire cell membrane are referred to as integral proteins or transmembrane. On the other hand another class of membrane proteins is the peripheral proteins. These proteins as the name indicates are only found on either the inner side of the membrane or the outer side of the membrane but do not span the entire length of the membrane as do the integral proteins. The integral proteins serve to function as a path for the exchange of substances between the fluids that lie within the cell and the fluids outside the cell membrane. The cell membrane consists of lipids as well and substances which are water soluble and certain other molecules are unable to cross this layer of lipids. The integral proteins then play their role of transporters and act as paths for these substances. They also have the capability of allowing transport not only from a region of higher concentration to a lower concentration but also in the other direction by a process referred to as active transport. Hormones which are not lipid soluble use the integral proteins as receptors. The integral proteins undergo changes in their structure after the binding of the hormones. This results in changes in the integral proteins on the intracellular side and there is transmission of signals from outside the cell to the internal side and a cascade of reactions is initiated within the cell. Hence integral proteins also perform the role of carrying of information from the extracellular compartment to the intracellular one. These proteins also serve certain enzymatic functions (Ganong 2005; Guyton et al 2007; Murray et al 2006). Peripheral proteins on the other hand exist mainly attached to the integral proteins. They have two significant roles which include working as enzymes. The cell membrane consists of pores as well. The peripheral proteins act as regulators of the substances that make way through these pores (Ganong 2005; Guyton et al 2007). Cell Adhesion: The adhesion of the cells is an essential part of the functionality of the cells. The importance of the adhesion of the cells can be analyzed from the fact that it is vital during the gestational period for the fetal growth, the creation of important bodily systems as well as in the process of tissue repair and the spread of cancers from their sites. Furthermore, in pathological conditions that alters the cell adhesion, the cells move towards the process of cell death. Cell adhesion molecules (CAMs) are the substances that perform the role of cell adhesion and work towards connecting the cell with the basal lamina and to other cells. The cell adhesion molecules may attach to the skeletal infrastructure of cells or the laminins in the extracellular compartment. They may attach to similar molecules present in other cells or they may attach to different molecules in the other cells. They tend to transfer signals across different cells as well as between the cell and the extracellular compartments. Integrins, cadherins and selectins are some of the types of cell adhesion molecules (Ganong 2005; Guyton et al 2007; Murray et al 2006). Passive Process Membrane Transport:  The cell is surrounded by extracellular fluid and it needs nutrients as well as other substances for its normal functioning. These substances enter the cell via different methods. Passive processes of membrane transport constitute important methods of membrane transportation. Diffusion is one of the methods of passive membrane transport. It is basically the movement of substances from a region where they are in increased concentration to a region where they are in decreased concentration. This process does not require energy and is determined by various factors. If the difference in concentration is very high across the membrane and if the substance size is small, the speed of the process will be increased. Substances that are soluble in solids also have a higher diffusion capacity. Facilitated diffusion is a modified form of diffusion which is the movement of substances across the cell membrane with the assistance of a carrier molecule. Osmosis is also a passive process of membrane transport. It is basically the movement of only liquids and in particular water from a region where it is in increased concentration to a region where it is in decreased concentration. Osmotic pressure is the main factor which determines the process of osmosis (Anatomy and Physiology 2002; Ganong 2005; Guyton et al 2007; Murray et al 2006). Active transport  The cell may also need substances for its normal functioning which cannot be transferred through the passive processes of transport within the cell. In this, another active transport mechanism sets into action which is referred to as active transport. Active transport is basically the movement of substances across the cell membrane in a direction opposite to the passive processes. That is, it is the movement of substances from regions of decreased concentrations to areas of higher concentration. The process is referred to as active owing to the requirement of energy to drive this process. ATP is required as the energy source for this process. The sodium potassium pump in the human cells provides an example of active transport. These pumps work on the principle of active transport and use ATP for their energy (Solomon 2004; Ganong 2005). Endocytosis and Exocytosis Endocytosis and exocytosis are also processes of transport. Endocytosis is a process whereby the cell membrane folds around a particular substance and leads to the formation of a vesicle which is released within the cell. The process is meant for the intake of substances within the cell which have large sizes and cannot move by means of other methods. Phagocytosis, pinocytosis and receptor mediated endocytosis are the different types of this process. Phagocytosis is performed by the white blood cells to move the infective organisms within the cell and destroy them. Pinocytosis works towards the movement of liquids and substances within the cell. Receptor mediated endocytosis is a process whereby the receptor along with the substance attached to the molecule is moved within the cell. Exocyotsis is the opposite of endocytosis whereby substances from the cells are moved out via forming vesicles that bud off from the cell membrane (Newsholme et al 2009; Rowland 1992). ENERGY, ENZYMES and METABOLISM  ATP- The Transferring of Energy  The major source of energy for the human body is Adenosine Triphosphate (ATP) which is the form of energy utilized by the different bodily cells for the performance of major functions. It has been defined as the “energy currency of the cell” and was discovered in the year 1926. It is an important part of the metabolic reactions that take place in the body and many enzymatic pathways utilized in the metabolism of carbohydrates, proteins and lipids. It is also required for the activity of the muscles of the human body and contraction utilizes a large number of ATP molecules. The energy that a person takes from food is stored within the human body mainly in the form of ATP. The highest energy is released from the molecules when the second and the third bond are broken and these bonds are referred to as bond containing levels of “high energy” (Murray et al 2006; Vasudevan et al 2007). Enzymes- Biological Catalysts : Enzymes are biological catalysts that are present in the human body and serve the function of carrying out essential bodily metabolic reactions within the cells. The processes that take place in the different compartments of cell are possible only due to the presence of different enzymes to accomplish the activity. The enzyme functions on substances which are referred to as substrates and the resulting substance that is produced due to the reaction is referred to as product. The ratio of enzyme to substrate can be as high as one is to thousand. Most of the enzymes in the human body are proteins and they can dissolve in water and are sensitive to heat. They are classified into six groups which include oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases (Murray et al 2006; Vasudevan et al 2007). Metabolism And Regulation Of Enzymes : Different metabolic processes occur in the human body. The major metabolic processes include carbohydrate, protein and lipid metabolism. The reactions of these metabolisms take place with the assistance of different enzymes. These reactions result in the formation of ATP which is then processed through the citric acid cycle and the electron transport chain. The activities of enzymes are regulated to ensure the fact that the right reactions take place at the right time and the homeostasis of the body is maintained. The amount of enzyme as well as the amount of substrate and product constitutes essential factors in the enzyme pathways. The temperature is also an important factor. At the optimum temperature, the enzymes work in the best possible manner. Different substances tend to activate enzymes whereas others tend to deactivate them. Feedback regulation of enzymes also occurs. Protein phosphorylation is another method of regulating enzyme activity. It tends to activate a few enzymes whereas deactivate others (Murray et al 2006; Vasudevan et al 2007). CONCLUSION The human cell is the fundamental part of the human body upon which the existence of human beings is based. The cell was discovered by Robert Hooke in the year 1965. The cell has been correctly defined as the basic unit of life. The cell functions like a machinery with the working of different parts. Many cells combine together to form tissues and the constellation of these tissues results in the formation of organs. Thus human survival is dependent upon the proper functioning of the human cell. There are different types of cells which include the eukaryotic cells and the prokaryotic cells which have varying features. The human cell is a eukaryotic cell which comprises of different structural components which are enveloped by a cellular membrane. The cell derives its energy by different processes and enzymes function towards the carrying out of the metabolic activities of the cell. The cell consists of cytoplasm within which lie different organelles. These include the nucleus, endoplasmic reticulum, Golgi apparatus, ribosome, mitochondria, lysosomes and peroxisomes. These organelles are in turn surrounded by their own membranous systems. The information of the cells is mainly stored in the form of DNA which is then processed into RNA and then into proteins. The structure of the cell is supported by virtue of micro filaments, intermediate filaments, and microtubules. The cell membrane is a double layer of lipids which is selectively permeable. Different processes occur along the cell membrane for the transportation of substances. These include passive processes like diffusion, facilitated diffusion and active transport. Active processes include active transport. ATP is the basic form of energy stored and used by the cell. The cell also utilizes enzymes for carrying out its activities. The enzymes serve as biological catalysts and they are regulated by different means. Thus, to conclude the cell is itself a complex structure which is the basic unit of the human life. References Top of Form 1. Anatomy & physiology. (2002). Philadelphia: Lippincott Williams & Wilkins. Bottom of For2. Crowley, L. V. (2007). 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Stamford, Conn: Appleton & Lange. Top of Form10.Newsholme, E. A., Leech, A. R., & Board, M. (2009).Functional biochemistry in health and disease. Hoboken, N.J: Wiley. Bottom of FoTop of Form11. Rowland, M. (1992). Biology. Walton-on-Thames, Surrey: Nelson. Bottom of FoBottom of ForBottom of Form12.Sherwood, L. (2010). Human physiology: From cells to systems. Australia: Brooks/Cole. Top of Form 13.Solomon, E. P., Berg, L. R., & Martin, D. W. (2004).Biology. Pacific Grove, Calif: Brooks/Cole. Top of Form 14.Vasudevan, D. M., & Sreekumari, S. (2007). Textbook of biochemistry for medical students. New Delhi: Jaypee Brothers Medical Publishers. Bottom of Form Bottom of Form 15. Watson, J. D. B. (2006). Molecular biology of the gene. San Francisco: Pearson/Benjamin Cummings Read More