Recent Articles

Monday, December 13, 2010

ATP

Monday, December 13, 2010 - 0 Comments

ATP: (Adenosine Tri-phosphate):

The cell’s Energy Currency:      Major energy currency of all cells is nucleotide called Adenosine Tri-phosphate (ATP). As ATP plays central role as the energy currency in all organisms, it must have appeared early in the history of life.
The ability of ATP to store and release energy is because of the structure of ATP molecule. Each ATP molecule has three subunits.
(1) Adenine an organic molecule composed of two carbon nitrogen rings.
(2) Ribose, fine carbon sugar.
(3) Three phosphate groups in a linear chain.
The covalent bond connecting these phosphates is indicated by “tilde” symbol (–) and is high energy bond. The energy is not localized in the bond itself, it is property of entire molecule and is simply released as phosphate bond breaks. These bonds have low activation energy and breaks easily. The breaking of one bond releases about 7.3 K. Cal: (7300 calories) per mole of ATP.
The energy from ATP is sufficient to derive most of the cells endergonic reactions in a typical energy reaction only the outermost of the two high energy bonds, break. When this happens, ATP becomes ADP (Adenosine di-phosphate). In some cases ADP is further hydrolyzed to AMP (Adenosine mono-phosphate) as follows:
ATP ------------- > ADP + Pi + 7.7 K. Cal:
ADP ------------ > AMP + Pi + 7.3 K. Cal:

Cells contain reservoir of ADP and phosphate (Pi). As long as a cell is living, ATP is constantly being converted into ADP plus phosphate to drive the cells many energy requiring processes enabling the animal to perform biological work. ATP can not be stored for long, as ATP lasts only few seconds before it is used to perform biological work. Thus cells constantly recycle ADP, with the energy derived from food stuffs, stored fats and starches. ADP and phosphate recombine to form ATP, with 7.3 K. Cal: of energy per mole contributed to each newly formed high energy phosphate bond.

Coenzymes

Some enzymes have the ability to act independently while other can be functional in presence of some helpers. These helpers are of different chemical nature and two of these are important. They are cofactors and coenzymes.

Coenzymes:

They are non protein molecules that participate in enzymes catalyzed reaction, often by transporting electrons in the form of hydrogen atoms from one enzyme to another. Many vitamins like niacin and riboflavin function as coenzymes or are used to make coenzymes. The main function of coenzyme is to transport energy in the form of hydrogen atoms from one enzyme to another. One of the most important coenzyme in the cell is hydrogen acceptor micotinamide adenine dinucleotide (NAD+), which is made from vitamin B. Since animals have lost the ability to synthesize the vitamin components of coenzymes, it should be provided along with diet to prevent vitamin deficiency. In the body vitamins are recovered in their original form and are used repeatedly. When NAD+ acquires hydrogen atom from an enzyme, it reduces to NADH. The electron of hydrogen atom contains energy that the NADH molecule then carries. When various foods are oxidized in the cells, the electrons are removed from the food molecules and are transferred to these coenzymes, as NAD+ is reduced to NADH.

Cofactors

Some enzymes have the ability to act independently while other can be functional in presence of some helpers. These helpers are of different chemical nature and two of these are important. They are cofactors and coenzymes.
Cofactors:

Many enzymes require certain metal ions to change non functioning active site to a functioning one. These metal ions that help enzyme in their catalytic activity are known as cofactors. Some of common cofactors are Ca2+, Mg2+, Mn2+, Cu2+ and Zn2+. The attachment of a cofactor with main enzyme (apoenzyme) changes the shape of protein and allows it to combine with substrate. Enzyme carbolic anhydrase, contains zinc the cytochromes contain iron and Troponin (a muscles contraction in enzyme) contains calcium. The cofactors of an enzyme participate in the temporary bonds between the enzyme and it substrate when the enzyme substrate complex forms.

Enzymes and the Factors Affecting Enzyme Activity

Most enzymes are proteins, some are nucleic acids (RNA). Enzymes have enormous catalytic power. They greatly enhance the rate at which specific chemical reactions take place. Enzymes regulate chemical reactions. An enzyme is biological catalyst that can accelerate specific chemical reaction by lowering the required activation energy but it self remains unaltered and can be used again. The substance on which enzyme acts are called reactants or substrates. Enzyme activity depends upon the concentration of enzyme concentration of substrate, temperature and pH. Many digestive enzymes like Pepsin and trypsin, all enzyme names end with the suffix –ase and are named after their substance.
Enzyme structure:

Enzymes are complex three dimensional globular protein molecules or nucleic acids that vary in size from small, simple with a molecular weight of 10,000 to highly complex molecules with molecular weights upto one million. Many enzymes are pure proteins that are delicately folded and interlinked chains of amino acids. Binding of the substrate to the enzyme changes the enzyme’s shape, a phenomenon called induced fit. The active site’s embrace of the substrate brings chemical groups of the active site into positions that enhance their ability to work on the substrate and to catalyze the chemical reaction. When the reaction is complete, the product of catalyzed reaction is released and the enzyme resumes its previous shape and is ready to catalyze another chemical reaction. For convenience the active enzyme is termed as a holoenzyme. It may in turn comprise of major protein part of enzyme the apoenzyme. A number of other chemical substances help the enzyme to be functional. These chemicals are termed as prosthetic group and may be coenzyme, cofactors.
Enzyme function: When a substrate molecule binds to an enzyme’s active site, an enzyme substrate complex (ES) forms. This is the essential first step in enzyme catalysis and can be summarized as
Enzyme (E) + Substrate (S) ------> Enzyme Substrate Complex (ES) --------> Products (P) + Enzymes (E).
                                          <------                                                   <---------
Once the unstable high energy ES forms, amino acid side groups of the enzyme are placed against certain bonds of the substrate. These side groups stress or distort the substrate bonds lowering the activation energy needed to break the bonds. The bonds break releasing the substrate which now reacts to produce the final product and release the enzyme.
Factors affecting Enzyme activity:
Any condition that alters three dimensional shape of an enzyme also affects the enzyme’s activity. Two factors that affect enzyme activity are temperature and pH.
(1)        Temperature:
The shape of prein or nucleic acid is determined largely by hydrogen bonds present in its structure. Temperature changes easily, disrupt hydrogen bonds thus changing the structure of enzyme. In higher vertebrates such as birds and mammals, the body temperature is between 35 and 40°C as such the enzyme present in these animals function best within this narrow temperature range. Below 35°C the bonds that determine protein shapes are not flexible enough to permit the shape change necessary for substrate to fit into a reactive site. Above 40°C, the bonds are too weak to hold the protein in proper position and to maintain its shape. When proper shape is lost, the enzyme is destroyed; this loss of shape is called denaturation.
(2)        pH:
Most enzymes also have optimum pH usually between 6 and 8. With a change in pH, the shape of the enzyme can also be altered. When the pH is too low, H+ ions combine with R groups of the enzyme’s amino acids, reducing their ability to bind with substrate. Acidic environments can also denature enzymes not adapted to such conditions. Some enzymes function at low pH. For example pepsin, the enzyme found in stomach of mammals has an optimal pH of about 2. Pepsin functions at such a low pH because it has an amino acid sequence that maintains it’s ionic and hydrogen bonds, even in the presence of large number of H+ ions (low pH). On the other hand trypsin is active in more basic medium (PH9) found in small intertue of mammals. Slawary any lose acts around 7.5 pH. Generally the pH optimum of an enzyme reflects the pH of the body fluid in which the enzyme is found.

Thursday, November 11, 2010

Energy, its Forms and Laws of Energy Transformation

Thursday, November 11, 2010 - 0 Comments

Energy is the capacity to work. Work is the transfer of energy. Energy can also be transformed. For example a plant can transform solar (radial) energy into chemical energy. Plant can then be burned in a stream generator and the energy transformed into the energy of motion that is the turning of the wheel. Energy has many forms: the heat from furnace, the sound of jet plane, the electric current that lights a bulb, the radioactivity in a heart pacemaker or the pull of magnet.

Forms of Energy:
Energy exists in two states, kinetic energy and potential. Kinetic energy is the energy of motion that is a thundering waterfall. Potential energy is stored energy like a gait boulder poised on a pinnacle. In a living animal, energy in chemical bonds is a form of potential energy. Animals use thin bond energy in organic molecules to accomplish biological work. Much of the work an animal performs, involves the transformation of potential energy to kinetic energy in its cells.

Units of Energy:
Employed unit for measuring heat in an animal is the kilocalorie (K cal :) or nutritional calorie (C). A kilocalorie is the amount of heat necessary to raise the temperature of 1 kg: of water through 1°C and is equal to 1000 calories. A reasonable daily intake of energy for an average person is approximately 2000 to 2500 K. Cal. A calorie (C) is the amount of heat required to raise the temperature of 1g (K. C) of water 1°C from 14.5 to 15.5°C.

The Laws of Energy Transformations:
There are two laws of thermodynamics that control energy transformations namely first law of thermodynamics and second law of thermodynamics.

First Law of Thermodynamics:
It is also known as Law of energy Conservation. It states that ‘energy can neither be created nor destroyed but only transformed. Energy can change from one form to another form like electric energy passes through a hot plate to produce heat energy. It can be transformed from potential to kinetic energy as when a squirrel eats a nut and then uses this energy to climbs a tree, but it can never be lost or created. As the energy is neither created nor destroyed thus the total amount of energy in the universe remains constant.

Second Law of Thermodynamics:
It states that all objects in the universe tend to become more disordered and that the total measure of this degree of disorganization is called entropy. The natural gas burns in a stove, the potential chemical energy stored in bonds of the gas molecules is converted to light in the form of blue flame and heat. Some of heat energy can be used to boil water on the stove and some is dispersed into the kitchen, where it is no longer available to do work. This unusable energy represents increased entropy.
Activation energy: Most chemical reactions require an input of energy to start a reaction is match is lit and the heat energy is used to start wood-burning in a fireplace. At the chemical level, the input energy must break existing chemical bonds before new bonds can form. In thermodynamics this input energy is called activation energy. It is reaction with net release of energy; the reactant contains more energy than the products. In other words, the amount of this excess energy (free energy) released into the environment is greater than the activation energy required to initiate the reaction. These reactions occur spontaneously and are called exergonic. In constant a chemical reaction in which the product contains more energy than the reactants require greater input of energy from the environment than is released. Because these reactions do not occur spontaneously, they are called endergonic. The amount of reactant substance converted to product substance in a given period of time is the reaction rate. The reaction rate of exergonic reaction does not depend on how much energy the reaction releases but on the amount of activation energy required for the reaction to begin. The larger the activation energy of chemical reaction, the more slowly the reaction occurs, because at a given temperature, fewer molecules succeed in overcoming the initial energy hurdle. Activation energies are not fixed. For example when certain chemical bonds are stressed, they may break more easily.

Nervous and Muscle tissue

Nervous Tissue:
It is composed of several different types of cells but two are important named Neuron and neuroglia. Neurons are impulse conducting cells and consist of three parts, dendrites, cell body and axon. Dendrite receives nerve impulses and pass on to the cell body. The cell body is thickest area that contains muscles and other cell organelles of the cell. Axon is long thread like extension of cell body which conducts nerve impulse away from the cell body. In some peripheral nerves, the axon is enclosed by Schwann cells that act as insulating material. Neurons are specialized to transmit signals called nerve impulses from one neuron to the other, from sense organs to the brain, from brain to effectors that is the muscles and glands. Neuroglia is non nervous cell that insulates the membranes of neurons, provides protection support and nourishment to the neurons. Nervous tissue is located in brain, spinal cord and nerves.

Muscle Tissue:
Muscle tissues are composed of long, excitable cells capable of considerable contraction. In the cytoplasm of muscle cells are present large number of parallel arranged structures called microfilaments that are made up of contractile proteins actin and myosin. Muscle is the most abundant tissue in animals and the contraction of muscle accounts for much of the energy consuming cellular working in an active animal. There are three types of tissues called Smooth muscle, skeletal muscle and Cardiac muscle.

(1) Smooth Muscle Tissue:
Smooth muscles lack cross striations and are spindle shaped containing single nucleus. These are involuntary in action and are thus named as involuntary muscles. The contract slowly than the skeletal muscles but for longer periods of time than skeletal muscles. These cells are arranged closely to form sheets and are mostly located in the walls of hollow organs like gut, blood vessels, and ducts of reproductive and excretory systems. It helps in the movement of substances like food, urine.




(2) Skeletal Muscle Tissue:

Skeletal fibre is composed of striated muscle fibres that are long, cylindrical and contains many peripheral nuclei. They are voluntary in nature and are attached with bones by tendons. The contraction helps in the bone to move and hence it helps in locomotion of animal.

(3) Cardiac Muscle Tissue:



It consists of striated branched spindle shaped cells that are shorter than the skeletal cells. Each cell contains single nucleus and specialized cell junctions called inter calated dises that allow ions to move quickly from one cell to the other. These are located in wall of the heart and it has involuntary control. As the walls of the heart contract cardiac muscle tissue pumps blood into circulation.

Friday, October 22, 2010

Kinds of Connective Tissue

Friday, October 22, 2010 - 7 Comments

Connective tissues are diverse group of tissues that serve various binding and supportive functions. Connective tissue is composed of smaller cells, large number of fibres suspended in extra cellular ground substance. Cells that are present in connective tissue are fibroblasts and macrophages. Fibroblasts produce protein ingredients of extra cellular fibres. Macrophages are amoeboid cells that move around in the network of fibres and engulf bacteria and debris of dead cells by phagocytosis.
Ground substance is matrix that is secreted by cells of connective tissue and may be liquid, jelly like or solid in nature. The nature and chemical composition of matrix determine the functional properties of various connective tissues. Fibrous network of connective tissue consists of three types of fibres namely collagenous fibre, elastic fibre and reticular fibre. All these fibres are proteins in nature strong flexible collagenous fibre consists of protein collagen that has great tensile strength and is produced by fibroblast. Elastic fibres are long threads made up of protein elastin. These fibres are elastic in nature and resist stretching. Reticular fibres are very thin and branched and are also made up of collagen. All three fibres are tightly interwoven and form a fabric like structure that joins connective tissue with adjacent tissue. Connective tissues may be classified on basis of loose and dense arrangement of these fibres. Major types of connective tissue in vertebrates are loose connective tissue, fibrous connective tissue, adipose tissue, cartilage, bone and blood.

(1)        Loose Connective Tissue:
In loose connective tissue strong, flexible fibres of protein collagen that are interwoven with fine, elastic and reticular fibres. Collagen fibres are made up of protein collagen that has great tensile strength and is produced by fibroblast. All these compounds give loose connective tissue its elastic consistency and make it excellent binding tissue. It is widely distributed under the epithelia of human body and binds the skin of underlying muscle tissue. Loose connective tissue also wraps different organs and act as cushions for these organs.


(2)        Fibrous Connective Tissue:
In fibrous connective tissue the collagen fibres are densely packed and may lie parallel to one another creating very strong cords. Number of fibroblast in fibrous connective tissue is higher than loose connective tissue. It is located in the dermis of skin, subnucosa of digestive tract, fibrous capsules of organs and joints where it provides structural strength. Tendons and ligaments are two important forms of fibrous connective tissue. The tendons connect muscles to bones or to other muscles while ligaments not only lie between two bones but also help in joint formation.

(3)        Adipose Tissue:
Adipose tissue is specialized form of loose connective tissue that consists of large sized cells known as adipocytes or adipose cells distributed throughout matrix. Each adipose cell contains large fat droplets that push the nucleus and cytoplasm towards one side closer to plasma membrane. Adipose cells swell when fat is stored and shrinks when it is used as fuel by the body. These cells accumulate in large number to form material commonly known as fat. It is located in the dermis of skin, in bones, breasts and mesenteries in abdomen and around the kidney and heart. It provides reverse fuel in the form of lipids. It acts as insulating pad and helps in prevention of heat loss. They also support and protect organs.

(4)        Cartilage Tissue:
It is hard but flexible tissue comprising of few cells, a ground substance and large number of fibres. In the cartilaginous tissue there are present large numbers of small spaces called lacunae. The rubbery matrix the chondrion which is secreted by chondroblasts surrounds the lacunae. Chondrion is protein carbohydrate complex and it along with collagen fibres gives cartilage its strength and elasticity. Cartilage lacks blood supply; all nutrients and waste material must diffuse through the ground substance from the surrounding tissue.
Fishes like sharks, rays and skates have cartilaginous skeleton other vertebrates also have the cartilaginous skeleton during embryonic development which is replaced by bony skeleton. Cartilagenous skeleton is located as flexible support in outer ear, nose and rings supporting wind pipe, dises that act as cushion between vertebrate and caps on ends of some bones.

(5)        Hyaline Cartilage:
In Hyaline cartilage cells are located in lacunae surrounded by inter cellular material containing fine collagen fibres. Hyaline cartilage forms embryonic skeleton, covers the ends of long bones and form cartilage of nose, trachea and larynx. It also maintains the shape of these structures and also provides support.




(6)        Elastic Cartilage:
It consists of fine collagenous fibres and many elastic fibres in its inter cellular material. It is located in external ear and epiglottis where it maintains the shape of these structures also provides great flexibility.






(7)        Fibro Cartilage:
Fibro cartilage contains many large collagenous fibres in it inter cellular material. Fibroblasts are present in fibro-cartilage. They are present in inter verbal dise, Public symphysis and discs of knee joint. It is responsible for absorbing compression shock.







(8)        Bone Tissue:
It is the strongest of vertebrate connective tissue containing mineralized collagen fibres. Bone cells also known as osteocytes are located within lacunae but the matrix around them is heavily impregnated with calcium phosphates? Making this kind of tissue hard and ideally suited for its functions of support and protection.
Bone matrix is deposited in concentric layers around central canal, the osteonic canal, located in bones and support, protects and provides lower system for muscles to act on, stores calcium and fat from blood cells.


(9)        Blood:
Blood is also a kind of connective tissue having the fluid matrix in the form of plasma, suspends specialized red blood cells, white blood cells and platelets. Matrix plasma consists of water, salts and variety of dissolved proteins. Suspended in the plasma are red blood cells or erythrocytes which carry oxygen to different parts of the body. Other type of blood cells is called white blood cells or leucocytes that fight against viruses, bacteria and other invaders. Platelets are the cell fragments and are involved in blood clothing. It is located within blood vessels in higher organisms. It transports substances like oxygen, carbon dioxide, nutrients, wastes, hormones, minerals, vitamins etc throughout the body of animal.

Kinds of Epithelial Tissues

Epithelial tissue exists in many structural forms and mostly either covers or lines an organ or a duct. These cells typically consist of renewable sheets of cells that have surface specialization adapted for their specific roles. All epithelial cells are supported by underlying basement membrane which is condensed form of connective tissue. Epithelial tissues are classified on the basis of shape and number of layers present. Epithelium can be simple, stratified. Individual epithelial cells can be flat, cube shaped or column like. Epithelial tissues absorb, transport, excrete, protect and contain nerve cells for sensory reception. The size, shape and arrangement of the epithelial cells are related to these specific functions.
(1)        Simple squamous epithelium:

It consists of single layer of tightly packed, flattered cells with disc shaped central nucleus. They are located in air sacs of the lungs, kidney glomeruli, lining of heart, blood vessels and lymphatic vessels. It allows passive diffusion of gases and tissue fluids into and out of cavities.
(2)        Simple Cuboidal epithelium:

It consists of single layer of tightly packed. These cells are smaller in size cube or box with almost central nucleus and have pentagonal or hexagonal outline. They are located in kidney tubules, small glands and surface of the ovary. These cells are responsible in secretions and absorption.
(3)        Simple columnar epithelium:

It consists of single layer of elongated cells with an elongated nucleus located near the basal end of the cell. These cells often bear minute finger like projection called microvilli that increase its absorptive surface area. In some cases they also develop cilia and become ciliated cells as in the lining of female reproductive tract. Some are specialized as the global cells that secrete mucus. They are located in the lining of digestive tract, gall bladder, female reproductive tract and excretory ducts of some glands. They are highly absorptive in nature and as such are present along the intestinal tract of most animals. They are also responsible for absorption of different material and enzyme secretion.
(4)        Pseudostratified ciliated columnar epithelium:

It consists of columnar cells with a tuft of cilia at the top except the goblet cells. The cells of psendostratified ciliated columnar epithelium appear stratified or layered as each cell has two or more nuclei. They are located in the lining of branch, uterine, tubes and some regions of uterus. These cells help the reproductive cells and mucous to move by ciliary action.
(5)        Stratified squamous epithelium:

It consists of many layers of cells. They form the lining of oral cavity, oesophagus, digestive canal and vagina. Some keratinized cells also line the surface of the skin. They protect the underlying tissues against abrasion. The basal layers of cell undergo division, pushing the cells towards the surface where they are sloughed off and replaced by new cells form beneath.

Saturday, October 16, 2010

Structure and Function of Cytoplasmic Organelles of Cell

Saturday, October 16, 2010 - 0 Comments

Cytoplasm of cell consists of Endoplasmic reticulum, Golgi apparatus, Golgi bodies, Mitochondria, Ribosomes, Vacuoles and Vesicles, etc.

(1)        Ribosomes:
They are non membrane bound structures that are the sites for protein synthesis. They contain almost equal amount of protein and rRNA. Some ribosomes attach to endoplasmic reticulum and some float freely in cytoplasm. Clusters of ribosomes connected in strand of mRNA are called polysomes.







(2)        Endoplasmic reticulum:
It is complex membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The ER is continuous from the nuclear envelope to plasma membrane. They are series of channels that help various materials to circulate throughout the cytoplasm. ER with attached ribosomes is rough ER and ER without attached ribosomes is smooth ER. Smooth ER is site for lipid production, detoxification of wide variety of organic molecules and storage of calcium ions in muscle cells.




(3)        Golgi apparatus:
It is composed of flattered stacks of membrane bound disternae. Golgi apparatus sorts, packages and secrets proteins and lipids. Proteins that ribosomes synthesize are sealed off in little packets called transfer vesicles. Transfer vesicles pass from ER to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified. Proteins are packaged into secretary vesicles which are released into cytoplasm close to plasma membrane. When the vesicles reach plasma membrane they fuse with it and release their contents to the outside of cell by exocytosis.
Golgi apparatus are most abundant in cells that secrete chemical substances. Golgi apparatus also produces lysosomes.

(4)        Lysosomes:
They are membrane bound spherical organelles that contain enzymes called acid hydrolases which are capable of digesting organic molecules (lipids, proteins, nucleic acids and polysaccharides) under acidic conditions. Enzymes are synthesized in ER, transported to Golgi apparatus for processing and then secreted by Golgi apparatus in the form of lysosomes or as vesicles that fuse with lysosomes. Lysosomes fuse with phagocytic vesicles, thus exposing the vesicle’s contents to lysosomal enzymes.


(5)        Mitochondria (Power generators):
Mitochondria are double membrane bound organelles that are spherical to elongate in shape. Small space separates outer membrane form inner membrane. Inner membrane folds and doubles in on itself to form incomplete partitions called cristae. The cristae increase the surface area available for chemical reaction that trop usable energy for the cell. The space between cristae is the matrix. The matrix contains ribosomes, circular DNA and other material. Because they convert energy to usable form, mitochondria are called, ‘Power generators’ or ‘Power house’ of the cell. Mitochondria usually multiply when a cell needs to produce more energy.

(6)        Centrioles and Microtubule organizing centres:
The specialized non membranous regions of cytoplasm near nucleus are microtubule organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. For example one type of centre gives rise to Centrioles that lie at right angles to each other. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. The centroils are duplicated preceding cell division are involved with chromosomes movement and help to organize they cytoskeleton.




(7)        Vacuoles:
They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. For example some protozoa and sponges have contractile vacuoles that collect water and pump it outside to maintain the organism’s internal environment. Other protozoa and sponges have vacuole for storing food.

Structure and Function of Nucleus in the Cell

In the centre of eukaryotic cell, very important cell organelle is located which is named as nucleus. The nucleus is differentiated from the cytoplasm due to the present of a membranous structure called nuclear membrane or nuclear envelope. In prokaryotic cells the nuclear envelope is absent, thus no distinct nucleus is present. The shape of the nucleus is generally spherical but it may slightly irregular. Nucleus contains DNA and is the control and information centre for eukaryotic cell. It has two major functions. The nucleus directs chemical reactions in cells by transcribing genetic information from DNA to RNA, which then translates this specific information into proteins like enzymes that determine the cell’s specific activities. Nucleus also stores genetic information and transfers it during cell division from one cell to the next and from one generation of organisms to the next. Nucleus comprises of nuclear envelope, chromosomes and nucleolus.
Structure of Cell:

Nuclear envelope: It is gateway to nucleus. The nuclear envelope is a structure that separates the nucleus from the cytoplasm that is continuous with endoplasmic reticulum at number of points. It acts as a barrier between the contents of the nucleus and cytoplasm. The nuclear envelope is made up of two layers, other nuclear membrane and inner nuclear membrane. The structure and chemical composition of these membranes is the same as that of the cell membrane. There is present space between two nuclear envelopes layers the cisternae. Outer layer may be continuous with endoplasmic reticulum, cytoplasm and adjacent cells or exterior.
Nuclear pore:
Nuclear membrane has at places small pores called nuclear pores that are formed by the fusion of two layers of nuclear envelope. In addition to fusion of two layers of nuclear envelope, the pore is composed of an ordered array of globular and filamentous granules forming nuclear pore complex. These granules are made up of protein. These nuclear pores control the transport of different molecules into and out of the nucleus.
Size of pore is also important as it allows specific sized molecules to pass through. Generally it presents the movement of DNA but permits RNA to be moved out. These pores also provide direct contact of nucleus to the cytoplasm, endoplasmic reticulum or event to the exterior through endoplasmic reticulum. Number of pores present in the nucleus is variable and depends upon specific function of that particular cell. The nucleus of undifferentiated cells like eggs may have over thirty thousand pores while the differentiated cell like eukaryocytes may have only three or four pores in single nucleus. In majority of cells nuclear pores may exceed over three thousand in single nuclear envelope.
Chromosomes:
Genetic Containers:
Inside the nuclear envelope there is present a fluid material called nucleoplasm. In non dividing cells the nucleoplasm contains nucleoli, chromosomes and enzymes for the synthesis of DNA and RNA. In addition it also performs a number of other functions as well. Genetic material is in the form of network of threads called chromatic or chromatin network. Chromatin consists of uncoiled, tangled mass of chromosomes that are coloured bodies containing hereditary information in segments of DNA called genes chromosomes are self duplicating and carry the hereditary instructions. During cell division each chromosome coils tightly which makes the chromosome visible when viewed through light microscope. Chromosomes are made up of bead like structure, the nucleosomes. Nucleosomes are connected with one another by means of a strand of DNA called the linker DNA or linker that consists of 50 nucleotides. A nucleosome is made up of an octamere of histones surrounded by two turns of DNA ribbon that consists of about 200 nucleotides. The octamere is formed by eight different types of histones called H2A, H2B, H3 and H4. Another histone H1 fixes DNA helix over histone octamere and parents from uncoiling. The number of chromosomes in all individuals of the same species remain constant generation after generation e.g. in man each cell has 46 chromosomes, frog cell has 26, chimpanzee has 48 and fruit fly has 8 chromosomes.
Nucleolus:
It is permeable point for ribosomes. Nucleus contains one or two discrete non membrane bound structure called nucleolus in the nucleoplasm of non-dividing cells. Sometimes the number of nucleoli may be two or more, even in thousands in case of amphibian egg. They can be readily stained with basophilic dyes; chemically the nucleoli are composed of nucleic acids, especially the ribnucleic acid (RNA) and some proteins. Nucleolus is pre assembly points for ribosomes in many stages of synthesis and assembly. Assembly of ribosome is completed after they leave the nucleus through pores of nuclear envelope into cytoplasm where they are helpful in protein synthesis.

Thursday, September 30, 2010

Vacuoles

Thursday, September 30, 2010 - 0 Comments

They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. In animals like protozoa and sponges, contractile vacuoles are present that collect water from the cytoplasm and pump it outside to maintain organism’s interval environment. Other protozoa and sponges have vacuoles for storing food.

Centrioles and Microtubule Organizing Centres:

The specialized non membranous regions of cytoplasm near the nucleus are micro tubular organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. One such type of centre gives rise to centrioles that lie at right angles to each other. Centrioles are small hollow cylinders that occur in pairs in most animal and lower plant cells in distinctly staining region of the cytoplasm known as centrosome or centrosphere. They are located near the nuclear envelope and are double structures that lie at right angles to each other. Each centriole is about 300 – 500nm long and 200nm in diameter. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. Adjacent triplets are attached to each other by fibrils. Centrioles are also present at the bases of cilia and flagella where they are known as basal bodies or kinetosomes. At the beginning of nuclear division, the centriole replicate and two new pairs migrate to opposite poles of the spindle, the structure on which the chromosomes become aligned. Centrioles help in the formation of poles during cell division mitosis and meiosis. Centrioles form the base of the cilia and flagella.

Friday, September 24, 2010

Cilia and Flagella (Short Note)

Friday, September 24, 2010 - 0 Comments

Cilia and flagella are elongated appendages on the surface of some cells by which the cells including many unicellular organisms move. In non motile cells, if cilia and flagella are present they move material over the cell’s surface. Flagella are 5 to 20 times as long as cilia and move some what differently. Cilia and flagella have similar structure and consist of membrane bound cylinders that enclose the matrix. In matrix is an axonene or axial filament which consists of nine pairs of microtubules arranged in circle around two central tubules. This is called 9+2 pattern of microtubules. Each microtubule pair, a doublet also has pairs of dynein arms made up of proteins, projecting toward neighbouring doublet and spokes extending toward the central pair of microtubules. Cilia and flagella move as a result of microtubule doublets shading along one another.
In the cytoplasm at the base of each cilium or flagellum lies a short of cylindrical basal body which is also made up of microtubules and is structurally identical to the centriole. The basal body controls the growth of microtubules in cilia and flagella. Microtubules in the basal body form 9+0 pattern: nine sets of three with none in the middle.
Flagella ad cilia have fundamental similarity of ultra structure. They are made up of two central fibres surrounded by nine double peripheral fibres arranged in a circle. This bundle of fibres is called axonene. Each peripheral fibre is composed of protein tubulin and consists of A and B microtubule. Each a microtubule has pairs of arms at regular intervals along its length. Arms are composed of another protein dynein that is capable of hydrolysing ATP because it is an enzyme ATP ASE. The central fibres are connected to a microtubule of peripheral fibres by radical spokes. These fibres arise from basal granule or kinetosome that is identical in structure to axonene but has 9+0 structure and is derived from a centriole.

Microtubules (Short Note)

Majority of eukaryotic cells contain unbranched hollow, cylindrical organelles called microtubules. They are very fine tubules made up of globular sub units of protein called tubulin. They may extend for several micrometers in length. At intervals cross bridges or arms sometimes project from their walls linked with other microtubules. They help in movement of organelles such as secretary vesicles. They are involved in the movement of chromosomes during division of the cell nucleus. Microtubules are involved in the over all shape changes that cells undergo during period of specialization.

Cytoskeleton (Short Note)


In most cells a flexible cellular framework known as cytoskeleton is present which is extended throughout the cytoplasm connecting various organelles and cellular components. The skeleton of the cell is present in the form of microtubules, intermediate filaments and microfilaments. Chemically cytoskeleton comprises of contractile proteins like tubulin, actin, myosin, troponin and tropomyosin.

Monday, September 20, 2010

Mitochondria or Chondriosomes as Power House of Cell

Monday, September 20, 2010 - 0 Comments

Mitochondrion is rod like organelle in the cytoplasm of all eukaryotic cells. The number of mitochondria in a cell is variable and ranges from one to ten thousand depending upon the cells’ function. It converts energy to a usable form, so the mitochondrion is considered to be the power house or power generator of the cell.
This shape of mitochondrion may be spherical, elongated or cylindrical in most of the animal and plant cells. The size varies depending upon the physiological conditions of the cells. Mitochondrion is double membrane bound organelle. Outer membrane is smooth while the inner membrane folds and doubles in on itself to form incomplete partitions called crystal. A small space known as inter-membranous space separates the outer membrane from the inner membrane. The crystal increases the surface area available for chemical reactions that trap usable energy for the cell. Inner membrane contains a gel like fluid the mitochondrial matrix where the inner membrane folding the crustae extends.

The matrix contains ribosomes, circular DNA, enzymes and coenzymes in addition to other substances.
Mitochondria are self replicating organelle due to the presence of its own DNA and ribosomes. It replicates when a cell needs to produce more energy.
Mitochondria are seen to be in constant motion in living cells. Mitochondria are the centre of aerobic respiration. Interior of Mitochondrion contains fluid like organic matrix with a number of chemical compounds in it.
On the cristae are located enzymes and coenzymes by means of which carbohydrates (starch), fatty acids, lipids and amino acids (proteins) are metabolized to CO2 and H2O. Energy in the form of ATP is release in this process which is stored within mitochondria. Adenosine triphosphate (ATP) is energy rich compound and it provides energy to the cells of organs for various activities. Hence mitochondria are known as “Power house of cell” where energy is stored and released wherever and whenever required by a living body. Mitochondria have semiautonomous existence in the cell. They have their own DNA that directs production of some of their component proteins and they can divide in half and thus reproduce independently of cells normal cell division cycle.
Mitochondria are passed to an animal only by mother. Since mitochondria are present in eggs but not in the part of the sperm that enters the egg. Thus people can trace their mitochondria back to their mothers and grand mothers.
Functions: Mitochondrion is the centre of cells respiratory and metabolic activity where food is oxidized to carbon dioxide and water through kerb’s cycle and Electron transport chain. During these processes, production of energy in the form of ATP (adenosine triphosphate) also results. Therefore they are called “Power house of the cell”.

Lysosomes

Laso = dissolving + some = body, are membrane bound spherical organelles containing lysomosal enzymes, that are hydrolytic in nature. Important enzymes present in the lysosome are acid hydrolases, proteases, lipases and acid phosphatases. These enzymes are capable of digesting organic molecules like lipids, proteins nucleic acids and polysaccharides under acidic conditions. De dive first described their in 1949. Almost all cells contain lysosomes but their number if much increased in the cells involved in the process of phygocytosis except erythrocytes. There are two main types of lysosomes i.e. primary lysosomes and secondary lysosomes.

Formation of primary lysosomes: Enzymes present in the lysosomes are synthesized in rough endoplasmic reticulum and are then transported through the cytoplasm by transport vesicles into Golgi apparatus through Cis Golgi or forming face. These enzymes are further processed in Golgi apparatus and then budded off from Trans – Golgi or maturing face of Golgi apparatus in the form of primary lysosome.
Enzymes are sometimes synthesized by ribosomes, transported through endoplasmic reticulum into Golgi apparatus and from trans Golgi, they are punched of as vesicles, the primary lysosomes.
Secondary lysosomes: They are formed when primary lysosomes fuses with phagocytic vesicles, thus exposing the vesicles contents to lysosomal enzymes. These enzymes present in the primary lysosomes digest the food present and the soluble substances are diffused into the cytoplasm of the cell. Undigested material containing vacuole known as residual body is expelled out by exocytosis.
Phagosomes: When the primary lysosome fuses with a specialized white blood cell, the pliagocyte, an activated phaogsome or phagocytic vesicles formed. They fight against pathogen by engulfing them very rapidly than the ordinary phogocytes.
Autophagic vacuoles: During starvation or after the destruction of cell components especially the liver cells and cells destroyed during metamorphosis fuse with the primary lysosome to form autophagic vacuoles or cytolysosomes.
Function of lysosomes:

Functions are as under:
(1)        Phagocytosis: Any foreign object (pathogens) that gains entry into the cell is immediately engulfed by lysosome and is completely broken into simple digestible pieces. This process is called phygocytosis.
(2)        Intracellular digestion: They are involved in intracellular digestion since they have enzymes to digest the phagocytosed food particles present in food vacuoles.
(3)        Extra cellular digestion: They also help in extra cellular digestion by releasing enzymes.
(4)        Exo-cytosis: Sometimes enzymes of primary lysosome are released from the cell. This occurs during the replacement of cartilage by bone during development. Similarly the matrix of bone may be broken down during the remodelling of bone that can occur in response to injury, new stresses and so on.
(5)        Autophagy: (Self Eating): It is the process by which unwanted structures like damaged mitochondria etc within the cell are removed. Unwanted structures are first enclosed by single membrane, usually derived from smooth endoplasmic reticulum. Then this structure fuses with primary lysosome to form secondary lysosome called autophagic vacuole in which unwanted material is digested.
(6)        Autolysis: It is the self destruction of a cell by release of the contents of lysosomes within the cell. It is normal event in some differentiation processes and may occur throughout a tissue, as when a tadpole tail is reabsorbed during metamorphosis. It also occurs after cells die. Sometimes it occurs as a result of certain lysosomal diseases or after cell damage.
(7)        Recycling of important constituents: As a result of phago cytosis and digestion of different components, the lysosomes help in the recycling of important components of the cytoplasm.

Thursday, September 16, 2010

Golgi Apparatus

Thursday, September 16, 2010 - 0 Comments

Camillo Golgi in 1898 while studying the eukaryotic cells observed a system of tightly packed smooth surfaced vesicles lying near the nucleus. This structure was named as Golgi apparatus, Golgi body, and Golgi complex in animals and in plants as dictyosome. Golgi complex is an organelle with diverse shape and number. In most of animals’ cells there is present only one Golgi apparatus but are abundantly found in cells that secrete chemical substances like pancreatic cells which secrete digestive enzymes and neve cells that produce neurotransmitters. In certain plant cells the number may be in hundreds. Golgi apparatus is formed flattered sacs or cisternae but some tubules and vesicles may also participate in the formation of Golgi complex. Number of fluid filled flattered sacs may range from 3 – 7 in most of animals out lower organisms have up to 30 flattered cells. These flattered cells are arranged in a concentric fashion, the convex sacs lie closer to the nuclear membrane and are termed as cis Golgi or forming face. Farthest concave sacs are named as trans-Golgi or maturing face. Proteins or material enter Golgi body through forming face and after modification are released from maturing face.
Golgi body is chemically made up of lipoprotein and a number of other organic molecules that are present or transported through it. Golgi complex is continuous with endoplasmic reticulum canals on one side and to secretary vesicles leading the cell membrane on the other.

Mechanism of secretion of Golgi complex: Following six steps of secretion are involved in pancreas and other zymogen secreting glands.
(1)        Ribosome stage: It explains synthesis of protein molecules protein molecules by ribosomes attached with ribosomal endoplasmic reticulum.
(2)        Cistarnae stage: This stage involves the flow of protein (forward form ribosomes) through ER tubes called cisternae towards dictyosomes.
(3)        Intracellular transport: Secreted proteins are pinched off as transitional vesicles and tubules from ER and they flow in the cytoplasm towards dictyosomes where they fuse to form large condensing vacuole at the forming face of Golgi.
(4)        Concentration of Secretion: By process of concentration the condensing vacuole is converted into zymogen granules.
(5)        Intracellular stage: Zymogen granules are now changed by Golgi into secretary granules and are stored in the cell and are released in response to proper stimulus (a hormone or neurotransmitter) that acts on the cell.
(6)        Exocytosis: The discharge of secretary granules is effected by exocytosis.
Function of Golgi bodies:
(1)        Secretion: It secrets many secretary granules like lysosomes, peroxisomes.
(2)        Exocytosis: Proteins packed in secretary vesicles are released into cytoplasm close to the plasma membrane when the vesicles reach the plasma membrane, they fuse with it and release their contents to the outside of cell by exocytosis.
(3)        Storage of proteins: Proteins synthesized by Ribosomes are sealed off in little packets called transfer vesicles which pass from the endoplasmic reticulum to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified and can be used with cell or can be exported out of the cell.
(4)        Formation of Glycolipid and Glycoprotein: Carbohydrates, lipids and proteins synthesized by endoplasmic reticulum are modified as glycolipid and glycoprotein in within Golgi complex.
(5)        Cell wall formation: Golgi bodies are also involved in the formation of new plant cell wall.

Endoplasmic Reticulum of the Cell

Endoplasmic reticulum (ER) is a complex, membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The number and shape of endoplasmic reticulum may vary from one cell type to another, also in different physiological and developmental stages of the same cell type. The enclosed shapes called cisternae contain certain chemical substances that may vary from cell to cell. Endoplasmic reticulum has communication with the exterior, with the nuclear envelope as well as to the Golgi apparatus. Endoplasmic reticulum is continuous from the nuclear envelope to the plasma membrane in the form of a series of channels that help various material to circulate throughout the cytoplasm. It is also a storage unit for enzymes and other proteins and a point of attachment for ribosomes.
Types of endoplasmic reticulum: On the basis of appearances, endoplasmic reticulum is classified as:
(1) Rough or granular endoplasmic reticulum.
(2) Smooth or agranular endoplasmic reticulum.

(1)        Rough or granular endoplasmic reticulum:
This is the type of endoplasmic reticulum that bears on its cytoplasmic face large number of small granular structures, the ribosomes. Due to the presence of these ribosomes endoplasmic reticulum gives rough appearance so named as rough or granular endoplasmic reticulum. Ribosomes present on ER help in protein synthesis that is then transported to different parts of the cell including Golgi bodies through cisternae.









(2)        Smooth or agranular endoplasmic reticulum:
This is the type of endoplasmic reticulum without ribosomes. They have their own enzyme system and perform certain important functions.
Most cells contain both types of endoplasmic reticulum although relative proportion varies among cells.










Functions of Endoplasmic reticulum:

It plays an important role in the activity of cell. The functions are:
(1) Mechanical support: Due to flexible nature of plasma membrane and ability to extend into the cytoplasm, it has connections with nuclear envelope and Golgi apparatus which help to provide mechanical support to the cell.
(2) Transportation of material: As the endoplasmic reticulum has direct or indirect convection with the important organelles of the cell as well as with the cytoplasm and exterior, it acts as a transporter for the transportation of different material within the cell and from surrounding cells.
(3) Synthesis and transportation of proteins: The rough endoplasmic reticulum is involved in the synthesis and transportation of cellular proteins.
(4) Detoxificaiton of harmful substances: Smooth endoplasmic reticulum due to its own enzyme system metabolizes or destroys the toxic substances like steroids, carcinogens and toxins.
(5) Synthesis of lipids: Smooth endoplasmic reticulum synthesis different types of lipids that are used for the formation of plasma membrane and steroid hormones like testosterone and estrogens. Glycogen and glycolipids are also synthesized here.
(6) Site of new membranes: They are site for synthesis of proteins and lipids and are also considered to be primary site of new membranes.
(7) Storage of calcium ions: Smooth endoplasmic reticulum sores calcium ions in muscle cells which are required for muscle contraction.

© 2013 Notes for Pakistan. All rights reserved.
Designed by SpicyTricks