Textbook of Biochemistry & Biophysics for Nurses Suresh K Sharma
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1Biochemistry

Introduction to Biochemistry and CellCHAPTER 1

 
INTRODUCTION
Nursing is considered as oldest art and youngest science; therefore nursing professionals base their advanced nursing knowledge on the knowledge acquired from the basic sciences such as anatomy, physiology, biochemistry, pathology and so on. Biochemistry is concerned with the composition, characterization and transformation of matter. Thus, biochemistry can be defined as a science which tells us about the chemical constituents of living cell and the reactions and processes that they undergo. Nurses take care of sick patients who cannot care of themselves with varying level of medical conditions. Therefore, nurses need to be equipped with basic knowledge of biochemistry so that they understand the biochemical basis of disease condition and further understand the changes occurred in biochemical constitution of patient's body during the course of illness so that they can inform about the disease progression and recovery of the patient.
 
DEFINITIONS OF BIOCHEMISTRY
  • Biochemistry can also be defined as the science concerned with the chemical basis of life.
  • Biochemistry constitutes the study of chemistry of bio-molecules and metabolic processes which constitute our body and are essential for life.
  • Biochemistry is the study of things such as the structures and physical properties of biological molecules including protein, carbohydrates, lipids and nucleic acids, the mechanism of enzyme action the chemical regulation of metabolism the chemistry of nutrition; the molecular basis of genetics (inherence); the chemistry of vitamins; energy utilization in the cell and the chemistry of the immune response.
 
IMPORTANCE OF BIOCHEMISTRY IN NURSING
Biochemistry is the study of things such as the structure and physical properties of biological molecules, including proteins, carbohydrates, lipids and nucleic acids; the mechanism of enzyme action; the chemical regulation of metabolism; the chemistry of nutrition; the molecular basis of 4genetics (inheritance); the chemistry of vitamins, energy utilization in the cells and the chemistry of the immune response.
Thus the biochemistry deals with chemical basis of living being including human being. Nurses are the largest group in health care organizations who are directly involved in promotive, preventive, curative and rehabilitative services. Therefore, study of biochemistry is significantly important for nurses because it facilitates the nurses to:
  • Understand the biochemical basis of health and illness: With knowledge of biochemistry, nurses will be able to understand that quite a large proportion of disorders having biochemical basis. In addition they will learn about;
    • Normal and abnormal biochemical processes and their regulations such as:
      • * Composition and functions of fluid and electrolytes in human body.
      • * Enzymes and their role in digestion and absorption of carbohydrates, proteins and fats.
      • * Metabolism, storage and catabolism of carbohydrates, proteins and fats.
    • Role of different biochemical processes in maintenance of health or causation of illness.
    • Good or bad biochemical situations for health-illness continuum.
    • Biochemical factors playing role in an individual's health.
    • Biochemical contribution towards different diseases and disorders.
  • Interpret the biochemical diagnostics test reports: The knowledge of clinical biochemistry will help the nurses to understand the needs of different biochemical tests and will also be able to interpret the biochemical reports of the patients with different disease conditions. The ability of nurses to interpret the biochemical diagnostic test reports will help them in early identification and prompt action during serious life threatening situations, which may significantly affect the morbidity outcome among such patients. Therefore, it is essentially important for nurses to study the biochemistry; so that they can become an active resourceful member of the healthcare team. The common biochemical tests nurses generally encounter and may need to interpret in their day-to-day clinical practices such as:
    • Serum electrolytes level
    • Glucose tolerance test
    • Blood sugar levels
    • Plasma protein levels
    • Renal function test like urea, creatinine
    • Serum cholesterol and lipid profile
    • Serum enzyme levels such as amylase, lipase, etc.
    • Serum hormonal analysis such as TSH, T3, T4, and other hormones.
  • Contributes towards health promotion with biochemical aspect: There are several biochemical tests which are carried out for the screening purposes or to identify the risk of the certain life threatening illness such as Cardiovascular Diseases. For example, if a nurse aware that increased level of serum cholesterol and lipid profile has risk of cardiovascular disorders, then she may educate the people about maintaining the serum lipid level within the normal limits through dietary modification, regular exercises and medicines; so that future risk of coronary artery disease may be minimized. Thus through learning the biochemistry, nurses may contribute towards health promotion with biochemical aspect.
  • Carry-out effective assessment and planning for nursing needs: It is the most essential function of a nurse to identify the priority needs of the patients. This may be further facilitated 5by the basic knowledge of the biochemistry. For example, a nurse is assessing a diabetic patient admitted in medical ward, her biochemistry knowledge facilitates her to interpret biochemical test reports such as blood sugar level that will help her in the identification of the nutritional needs of the patient and plan nursing interventions accordingly.
  • Implement the health education based on biochemical rationalization: Health education is also one of the important roles of nurses; with knowledge of biochemistry nurses will be able to understand the biochemical basis of illness and offer health education to client based on biochemical rationalization when and where required, so that their client can be more convinced and clear about their illness, treatment and prevention.
 
STRUCTURE OF A CELL
The cell, discovered by Robert Hooke in 1665, is a basic, living, structural and functional unit of the body. They are often called as the building block of the life. “Cell is the structural and functional unit of all living organism”. Human body cells are of different types, sizes and shapes such as muscle cells, bone and cartilage cells, nerve cells, visual cells in eye, etc. However, a cell consists of three main parts, i.e. cell membrane, the nucleus and the cytoplasm with other additional components.
 
 
Types of Cells
Some organisms are unicellular such as bacteria, while some are multicellular, e.g. human cells are of many types. But basically on the basis of structural differences, they are divided as eukaryotes and prokaryotes (Table 1.1). Prokaryotes are simple and usually independent cells, while eukaryotes are often found in multicultural organisms.
  • Prokaryotic cells: The most primitive cell structure where there is absence of nuclear membrane, nucleolus and membrane bound organelles. The genetic material in them is stored in the form of nucleoid, found freely distributed in the cytoplasm. Generally this genetic material is made-up of one or more folded DNA stands without presence of histone proteins. The bacteria, viruses and some of the fungi are the good examples of prokaryotic cells, which are significant in health sciences.
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    Fig. 1.1: Basic structure and components of a eukaryotic cell
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  • Eukaryotic cells: It is the most advanced cell with well developed nucleus with nucleic membrane, nucleolus and genetic material in the form of chromosomes. These cells are found in the higher organism such as human beings. Structurally and functionally eukaryotic cells are larger in size, advanced, developed and complex (Fig. 1.1).
Table 1.1   Difference between Prokaryotic and Eukaryotic cells
Features
Prokaryotic Cells
Eukaryotic Cells
Cell size
Size of prokaryotic cell varies between 0.1 to 10 microns in diameter
Typically eukaryotic cell is 10-100 micron in diameter
Cell membrane
Prokaryotic cells are covered by a cell envelop, which generally is made-up of muramic acid.
Eukaryotic cells are covered with a complex structure cell wall made-up of phospholipids, glycolipid, cholesterol and proteins.
Cell wall appendages
Simple structured flagella are present on cell envelop, which are not differentiated into axomeme and sheath.
Complex structure flegella, cillia and microvilli are present on cell wall. Flagella if present are differentiated into axomeme and sheath.
Nucleus
Nucleus is not present, no nucleic membrane and nucleolus; simple chromosomes are directly suspended in cytoplasm.
Most of the cells have a nucleus, which covered with double layered nucleic membrane, complex structured chromosomes and nucleolus is present in nucleus.
Genetic material
Simple structure chromosomes without presence of histone proteins.
Complex structure chromosomes are presented associated with histone proteins.
Cytoplasm and cell components components
Cytoplasm is present without major cell components such as mitochondria, endoplasmic reticulum, etc.
Cytoplasm is present and most of the cells contains major cell components such as mitochondria, endoplasmic reticulum, etc.
Ribosomes
Cell contains ribosomes of only one size.
Cell contains two types of ribosomes, one in cytoplasm and smaller type is in mitochondria.
Cell division
Cell division does not involve meiosis; reproduce sexually by the transfer of DNA fragments through conjugation. The are generally fast multiplying cells
Cell division involves mitosis and meiosis and relatively slow multiplying but versatile
 
VARIOUS COMPONENTS OF A CELL
The cells are too small to be seen with naked eyes. However, a microscope is used to view a cell and its components after staining with appropriate dyes. Cell and its components absorb the dye and their structure and size may be clearly differentiated using microscope. A typical eukaryotic cell consists of following components:
  • Plasma membrane: Outer, limiting membrane separating the cell's internal components from the extracellular materials and external environment.
  • Cytosol: The thick intracellular fluid is termed as cytosol. The cytosol contains many dissolved proteins and enzymes, nutrients, ions and other small molecules, which all participate in various phases of metabolism. Organelles and inclusions are suspended in the cytosol.
    • The term cytoplasm includes cytosol, all organelles (except nucleus) and inclusions.
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  • Organelles: Highly organized structures with characteristic shapes that are specialized for specific cellular activities. The organelle with the largest volume is the nucleus, which contains most of the cell genetic material. The other major organelles are mitochondria, endoplasmic reticulum, golgi apparatus, lysosomes, ribosomes and vacuoles and proxisomes.
  • Inclusions: These are the temporary structures in the cytoplasm that contain secretions and storage product of the cell.
 
I. PLASMA MEMBRANE
Plasma membrane is the gatekeeper that regulates the passage of substances into and out of the cell. In other words, plasma membrane is a lipid bilayer of phospholipids that separates cytoplasm from environment. Within the phospholipid bilayer, cholesterol, glycolipids, proteins and glycoproteins are interseparated (Fig. 1.2). The structure of plasma membrane can be explained with the help of Fluid Mosaic Model.
The fluid mosaic model of membrane structure describes the molecular arrangement of the plasma membrane and other membranes in living organisms. A mosaic is a pattern of many small pieces fitted together. According to this model, the membrane is a mosaic of proteins floating like icebergs in a sea of lipids.
Plasma membrane like the other membranes is composed of lipids and proteins. These two major components arrange themselves in a manner to form plasma membrane. There are following three types of lipids involved in formation of plasma membrane.
  • Phospholipids: About 75% of the lipids are phospholipids that contain phosphorus.
    • Phospholipids are arranged in two parallel layers forming a phospholipid bilayer. So, this model is also known as phospholipid bilayer model. This arrangement occurs because phospholipids are amphipathic in nature, each molecule consisting of polar and nonpolar regions. The polar part is the phosphate containing “head”, which is hydrophilic and nonpolar parts are two fatty acid “tails”, which are hydrophobic. The head faces outward on either side, towards the watery cytosol and extracellular fluid.
      zoom view
      Fig. 1.2: Basic structure of plasma membrane
  • Glycolipid: About 5% of lipids are gycolipids with attached sugar groups. Glycolipids are also amphipathic in nature. They only appear in the layer that faces extra cellular fluid (ECF).
  • Cholesterol: The remaining 20% of plasma membrane lipids are cholesterol molecules, which are located among the phospholipid in animal cells. However, plant cells lack cholesterol in cell membrane.
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Plasma membrane also contains two types of proteins; which are discussed as follows:
  • Integral proteins: These are extended across the phospholipid bilayer among the fatty acids tails. Most integral proteins are glycoprotreins, which are proteins with attached sugar groups.
  • Peripheral proteins: These are proteins that do not extend across the phospholipid bilayer. They are loosely attached to the inner and outer surfaces of the membrane and are easily separated.
 
Appendages of Plasma Membrane
Following are the main appendages of a plasma membrane, i.e. cilia, flagellum and microvilli, etc.
  • Cilia: These are the extensions of the plasma membrane containing parallel microtubules, 10 micrometer in length. The main function of the cilia is to move materials over the surface of the cells.
  • Flagellum: These are the extensions of the plasma membrane containing parallel microtubules, 55 micrometer in length. In human cells flagellum are responsible for movement of spermatozoa.
  • Microvilli: These are the extensions of the plasma membrane containing microfilaments. The main functions of the microvilli are to increase the surface area of the plasma membrane for absorption and secretion and modified to form sensory receptors.
 
Functions of Plasma Membrane
Following are the major functions of the plasma membrane of a cell:
  • Semi-permeable: The plasma membrane regulates the entry and exit of materials inside the cell. It permits the passage of certain substances and restricts the passage of others. This property of membrane is known as “selective permeability.”
  • Receptor: The plasma membrane proteins can identify and attach to a specific molecule such as hormones, neurotransmitter or a nutrient, that is important for some cellular function.
  • Communication: The plasma membrane functions in cellular communication. This includes interactions with the other body cells, foreign cells, hormones, neurotransmitters, enzymes, nutrients and antibodies in extracellular fluid.
  • Electrochemical gradient: The membrane maintains an electrical and chemical gradient, called an electrochemical gradient, between the inside and outside the cell. The electrochemical gradient and the resulting membrane potential are important for proper functioning of most the cells.
 
II. CYTOSOL
Chemically, cytosol is 75 to 90% water plus solid components. Proteins, carbohydrates, lipids and inorganic substances comprise most of the solids. Inorganic substances and smaller organic substances, such as simple sugars and amino acids are soluble in water and are present as solutes.
Larger organic compounds, like proteins and polysaccharides, glycogen, are found as colloids particles that remain suspended in the surrounding medium although they are not dissolved. The colloids bear electrical charges that repel each other and thus remain suspended and separated. The cytosol contain enzymes that catalyze catabolic and anabolic reactions, ATP is produced in glycolysis reactions.
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III. ORGANELLES
The main organelles present in the cell are nucleus, endoplasmic reticulum, golgi complex, mitochondria, ribosomes, lysosomes, peroxisomes, centrioles, and spindle fibers, etc.
 
1. Nucleus
It occupies a central position in the cell. It is spherical or oval and much denser than the cytoplasm. The nucleus is the control center of a cell that is generally situated in center of the cell, covered with a double layered membrane called as nuclear membrane and contains chromosomes, nucleoplasm and nucleolus. Chromosomes carry genetic material of the cell and nucleolus serves as site for ribosome synthesis.
Most of the cells contain single nucleus, but few may have more such as some of the liver cells have more then one nucleus and long muscle cells do have hundreds of nuclei. However, red cell during their maturation loose their nucleus.
The main parts of nucleus are (Fig. 1.3):
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Fig. 1.3: Structure of nucleus
  • Nuclear membrane: It is a double membrane enclosing the nucleus. The outer membrane is continuous with the endoplasmic reticulum; nuclear pores extend through the nuclear envelope. The nuclear membrane helps to maintain the shape of the nucleus and assist in regulating the flow of molecule including RNA into and out of the nucleus through nuclear pores.
  • Chromatin: Thin strands of DNA, histones and other proteins condenses to form chromosomes during cell division.
  • Nucleolus: In some cells, a portion of the nucleus may be seen as lighter shaded area. This is called ‘nucleolus’. It consists of ribosomal RNA and proteins and nucleolus serves as site for ribosome synthesis.
 
Functions of Nucleus
Following are the main functions of the nucleus in a cell:
  • It is the seat of all metabolic activities of the cell.
  • Nuclear membrane helps in separating nucleus from cytoplasm and regulates movement of material into and out of nucleus.
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  • All cells in the human body contain nucleus except mature RBC's in circulation.
  • Nucleus contains DNA, the chemical basis of the genes, which governs all the functions of the cell.
  • DNA replication and RNA synthesis are taking place inside the nucleus.
  • Nucleolus is the area for RNA processing and ribosome synthesis.
 
2. Endoplasmic Reticulum
It is a system of membrane which has enclosed channels of varying shapes called cisterns. The endoplasmic reticulum is continuous with the nuclear envelope. It is of two types (Fig. 1.4).
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Fig. 1.4: Structure of endoplasmic reticulum
  • Rough endoplasmic reticulum: It is also known as granular endoplasmic reticulum. These are studded with ribosomes on its outer surface. The rough endoplasmic reticulum is involved in protein synthesis, protein sorting and initial post-translational modification such as glycosylation, sulfation, phosphorylation and hydroxylation of proteins and also the synthesis of phospholipids. Protein sorting involves several factors and a sequence of events.
  • Smooth endoplasmic reticulum: It is also known as agranular endoplasmic reticulum. These are not studded with ribosomes on its outer surface. Smooth endoplasmic reticulum is found abundant in cells. The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. It serves as transitional area for vesicles that transport ER products to various destinations. In liver cells the smooth ER produces enzymes that help to detoxify certain compounds and glycogen metabolism. In muscles cells it stores calcium that assists in the contraction of muscle cells. In adrenal cortex and gonad cells it synthesizes steroid hormones. It is also involved in lipid metabolism and membrane formation.
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Functions of Endoplasmic Reticulum
Following are the main functions of endoplasmic reticulum:
  • Ribosomes associated with the rough ER synthesize proteins destined for insertion into the plasma or export from the cell.
  • Rough ER serves as a temporary storage area for newly synthesized molecules and may add sugar groups to certain proteins, thus forming glycoprotiens.
  • Smooth ER is the site of fatty acid, phospholipid and steroid synthesis.
  • Also within certain cells, enzymes within the smooth ER can inactivate or detoxify a variety of chemicals, including alcohol, pesticides and carcinogens.
  • Endoplasmic reticulum is also considered as cell skeletal system and helps to keep cell organelle in position.
  • Endoplasmic reticulum also helps in formation of vacuoles.
 
3. Golgi Complex
It is an organelle located near the nucleus. It consists of flattened sacs called cisterns. Associated with the cisterns are small golgi visicles, which cluster along the expanded edges of cisterns (Fig. 1.5).
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Fig. 1.5: Structure of golgi complex
 
Functions of Golgi Complex
  • Golgi complex processes, sorts, packages and delivers proteins and lipids to the plasma membrane and forms lysosomes and secretory vesicles.
  • As mentioned above the primary function of golgi complex is the formation of secretary vesicles and lysosomes.
  • Golgi complex helps the endocrine cells in the secretion of hormones.
  • It is also involved in formation of acrosomes, during the maturation of sperms.
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  • Golgi complex also activates mitochondria to produce ATP.
  • It also helps in modification of proteins by adding certain prosthetic groups.
  • Golgi complex also helps in the regulation of cellular fluid balance by expelling extra fluid from cell.
 
4. Mitochondria
It is known as the “power house” of the cell. Mitochondria consist of two membranes, each of which is similar in structure to plasma membrane: (a) outer mitochondrial membrane which is smooth and (b) inner mitochondrial membrane which is arranged in series of folds called cristae. The central cavity of mitochondria, enclosed by the inner membrane and cristae is called the matrix. The mitochondrial membranes divides the mitochondria into two chambers (Fig. 1.6):
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Fig. 1.6: Structure of mitochondria
  • Outer chamber: The outer chamber is formed between outer smooth membrane and inner mitochondrial membrane, which is arranged in series of folds.
  • Inner chamber: The inner chamber is formed by the inner membrane, which is arranged in series of folds named as matrix. This inner chamber contains semi fluid matrix. The matrix has protein particles, ribosomes, RNA, DNA, enzymes of kreb cycle, amino acid synthesis and fatty acid metabolism, etc.
 
Functions of Mitochondria
Following are the main functions of the mitochondria:
  • It is the main site for the generation of ATP.
  • The elaborated folds of the cristae provides an enormous surface area for a series of chemical reactions, known as “cellular respiration”, which provides most of a cell's ATP. Enzymes that catalyze these reactions are located in the matrix and on the cristae.
  • Active cells such as muscles, liver and renal tubule, have large number of mitochondria and use ATP at high rate.
  • Mitochondria self-replicate, they divide to increase in number. Their replication is controlled by the genes within the mitochondria. Self-replication usually occurs in response to increased cellular need for ATP and at the time of cell division.
  • Mitochondrial matrix contains various enzymes for synthesis of fatty acids.
 
5. Ribosomes
Ribosomes are tiny granules that contain ribosomal RNA (rRNA) and many ribosomal proteins. Structurally, ribosomes consists of two subunits, one about half the size of the other (Fig. 1.7). They 13are made-up of ribonucleic acid and protein, so called ribonucleo-protein particles. Ribosomes are popularly known as ‘protein factories’. Ribosomes have four sites that is helps in protein synthesis, i.e. (a) mRNA binding site, (b) A (Aminoacyl) site, (c) P (Peptidyl) site (d) E (Exit) site.
Some ribosomes called ‘Free Ribosomes’ float in the cytosol, they have no attachments to other organelles. The free ribosomes occur singly or in clusters. Other ribosomes attach to a cellular structure called the endoplasmic reticulum. Further ribosomes may occur singly as monosomes or in groups named polyribosomes.
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Fig. 1.7: Structure of ribosome
 
Types of Ribosomes
Based on the size and sedimentation coefficient, ribosomes are classified into two main types:
  1. 70S Ribosomes: These are smaller in size and found in prokaryotic cells, mitochondria and chloroplast of eukaryotic cells. 70S ribosome consists of large (50S) and smaller (30S) subunits.
  2. 80S Ribosomes: These ribosomes are larger in size and found in eukaryotic cells. They consists of larger (60S) and smaller (40S) subunits. A tunnel occurs between the larger and smaller subunit for passage of mRNA during protein synthesis.
 
Functions of Ribosome
  • The main function of the ribosomes is the sites of protein synthesis.
  • The ribosomes contain rRNAs for providing attaching points to mRNA and tRNAs.
 
6. Lysosomes
They are membrane enclosed visicles, that form in the golgi complex. They are tiny sac-like granules bounded by a single membrane containing hydrolytic enzymes in it. They are also known as ‘Suicide Bags’ because of number of digestive enzymes in them. Lysosomes contain as many as 40 kinds of powerful digestive enzymes capable of breaking down a wide variety of molecules. The structure and functions of lysosome is depicted in Fig. 1.8.
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Functional Polymorphism of lysososome
On functional basis lysosomes at different stages presents with polymorphism and these functional stages of polymorphism are of three types, i.e. primary lysosomes, secondary lysosomes and residual bodies.
  • The primary lysosome: They are small size tiny sac-like bodies bounded by a single membrane containing many hydrolytic enzymes and popularly known as storage granules. They are present in silent form but ready to act whenever need of auto-digestion within cell is desired.
  • The secondary lysosomes: They are active digestive vacuoles containing the foreign material and enzymes within the membrane for the auto-digestion. When a liquid foreign material is ingested in these vacuoles is for auto-digestion process of cell, they are known as pinosomes and process is called as pinocystosis or cell drinking. On other hand when a solid foreign material is ingested in these vacuoles is for auto-digestion process of cell, they are known as phagosome and process is called as phagocytosis or cell eating.
  • The residual body: After the process of pinocytosis or phagocytosis, an undigested material is left in the cell. A residual body of lysososme is formed which attach to the plasma membrane and are thrown out of the cell by ephagy.
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Fig. 1.8: Illustration depicting structure and functions of lysosome
 
Function of Lysosomes
  • Lysosomal enzymes digest bacteria and other substances that enter the cell in phagocytic vesicles during phagocytosis, pinocytic vesicles during pinocytosis.
  • Lysosomes take part in natural defense of the body.
  • They also help in breaking down of the aging cells and dead cells. Therefore, they are involved in programmed cell death.
  • Lysosomes also help in intracellular scavenging by removing old or useless organelles.
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7. Peroxisomes
They are group of organelles similar in structure to lysosomes but smaller in size. They are so named because they usually contain one or more enzymes that use molecular oxygen to oxidize (remove hydrogen atoms from) various organic substances. Perxisomes are minute, somewhat spherical and single unit membrane particles with diameter varying from 0.5 to 1.0 µm (Fig. 1.9). The shape and size is variable from cell to cell. They are present in abundant numbers in parenchyma of liver and kidney.
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Fig. 1.9: Structure of peroxisomes
 
Functions of Peroxisomes
  • They detoxify harmful molecules such as ethanol, phenols, formic acid, etc.
  • They contain oxidase and catalase that helps in oxidation and removal of hydrogen peroxide.
  • The chief function is to help in the metabolism of glycolate during photosynthesis.
  • The long chain and branched fatty acids are catabolised by peroxisomes.
  • The substances which cannot be catabolised by normal enzymes are generally catabolised by peroxisomes.
 
8. Centrioles
These are the pair of cylindrical organelles in the centrosome, consisting of triplets of parallel microtubules (Fig. 1.10)
 
Function of Centrioles
Following are the main functions of the centrioles:
  • It is the center for microtubule formation
  • It determines cell polarity during cell division.
  • It forms the basal bodies of cilia and flagella.
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Fig. 1.10: Structure of centrioles
 
9. Spindle Fibers
These are the microtubules extending between the two centrosome, where chromosomes are arranged at middle of the cell during a cell division, which are called as spindle fibers (Fig. 1.11). Spindle fibers only appear during cell division; therefore, they cannot be seen during time other then cell division.
 
Function of Spindle Fibers
These assist in the separation of chromosomes during cell division.
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Fig. 1.11: A cell division depicting appearance of spindle fibers
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IV. CELL INCLUSIONS
Inclusions are considered to be nonliving components of the cell that do not possess metabolic activity and are not bound by membranes. The most common inclusions are glycogen, lipid droplets, crystals and pigments.
  • Glycogen: Glycogen is the most common form of glucose in animals and is especially abundant in cells of muscles, and liver. It appears in electron micrograph as clusters, or rosette of beta particles that resemble ribosomes, located near the smooth endoplasmic reticulum. Glycogen is an important energy source of the cell. Therefore, it will be available on demand. The enzymes responsible for glycogenolysis degrade glycogen into individual molecules of glucose and can be utilized by multiple organs of the body.
  • Lipids: Triglycerides are the storage form of lipids, it is not only are stored in specialized cells (adipocytes) but also are located as individual droplets in various cell type especially hepatocytes. These are fluid at body temperature and appear in living cells as refractile spherical droplets. Lipid yields more than twice as many calories per gram as does carbohydrate. On demand, they serve as a local store of energy and a potential source of short carbon chains that are used by the cell in its synthesis of membranes and other lipid containing structural components or secretory products.
  • Crystals: Crystalline inclusions have long been recognized as normal constituents of certain cell types such as sertolli cells and leydig cells of the human testis, and occasionally in macrophages. It is believed that these structures are crystalline forms of certain proteins which is located everywhere in the cell such as in nucleus, mitochondria, endoplasmic reticulum, golgi body, and free in cytoplasmic matrix.
  • Pigments: The most common pigment in the body, besides hemoglobin of red blood cells is melanin, manufactured by melanocytes of the skin and hair, pigments cells of the retina and specialized nerve cells in the substantia nigra of the brain. These pigments have protective functions in skin and aid in the sense of sight in the retina but their functions in neurons is not understood completely. Furthermore, cardiac tissue and central nervous system neurons shows yellow to brown pigment called lipofuscin, some believed that they have lysosomal activity.
 
QUESTION
Q.1: Explain the structure and functions of ‘Golgi apparatus’ in a cell.
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