CELL:- Cell is defned as the structural and functional unit of the living body.
Each cell in the body:
1. Needs nutrition and oxygen
2.Produces its own energy necessary for its growth, repair and other activities.
3.Eliminates carbon dioxide and other metabolic wastes.
4.Maintains the medium, i.e. the environment for its surviva.
5.Shows immediate response to the entry of invaders like bacteria or toxic substances into the body.
6.Reproduces by division. There are some exceptions like neuron, which do not reproduce.
TISSUE:- Tissue is defned as the group of cells having similar function. Allthe tissues are classifed into four major types which are called the primary tissues.
1.Muscle tissue (skeletal muscle, smooth muscle and cardiac muscle)
2. Nervous tissue (neurons and supporting cells)
3. Epithelial tissue (squamous, columnar and cuboidal epithelial cells)
4. Connective tissue (connective tissue proper, cartilage, bone and blood) .
ORGAN:- An organ is defned as the structure that is formed by two or more primary types of tissues.The organs are of two types, namely tubular or hollow organs and compact or parenchymal organs.
SYSTEM:- The organ system is defned as group of organs that work together to carry out specifc functions of the body.
Thus, the structure of the cell is studied under three headings:
1. Cell membrane
CELL MEMBRANE:- Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasmalemma.
COMPOSITION OF CELL MEMBRANE
Cell membrane is composed of three types of substances:
1. Proteins (55%)
2. Lipids (40%)
3. Carbohydrates (5%).
STRUCTURE OF CELL MEMBRANE
On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane. The electron microscopic study reveals three layers of cell membrane, namely, one central electron-lucent layer and two electron-dense layers.
Lipid Layers of the Cell Membrane
Major lipids are:
Phospholipid:- The outer part of the phospholipid
molecule is called the head portion and the inner portion
is called the tail portion.
Head portion is the polar end and it is soluble in water and has strong affnity for water (hydrophilic). Tail portion is the non-polar end. It is insoluble in water and repelled by water (hydrophobic).
Functions of Lipid Layer in Cell Membrane
Lipid layer of the cell membrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxide and alcohol can pass through this lipid layer. The water-soluble substances such as glucose, urea and electrolytes cannot pass through this layer.
Protein Layers of the Cell Membrane
The protein substances present in these layers are mostly glycoproteins.
Protein molecules are classifed into two categories:
1. Integral proteins or transmembrane proteins.
2. Peripheral proteins or peripheral membrane proteins
Functions of Proteins in Cell Membrane
1. Integral proteins provide the structural integrity of the cell membrane.
2. Channel proteins help in the diffusion of watersoluble substances like glucose and electrolytes.
3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport.
4. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane.
5. Receptor proteins serve as the receptor sites for hormones and neurotransmitters.
6. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane.
7. Antigens: Some proteins act as antigens and induce the process of antibody formation.
8. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbors or to basal lamina.
Carbohydrates of the Cell Membrane
Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane called glycocalyx.
Functions of Carbohydrates in Cell Membrance
1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.
2. Glycocalyx from the neighboring cells helps in the tight fxation of cells with one another3. Some carbohydrate molecules function as the receptors for some hormones.
FUNCTIONS OF CELL MEMBRANE
5.Exchange of gases
6.Maintenance of shape and size of the cell
Cytoplasm of the cell is the jellylike material formed by 80% of water. It contains a clear liquid portion called cytosol.
Cytoplasm is made up of two zones:
1. Ectoplasm: Peripheral part of cytoplasm, situated just beneath the cell membrane.
2. Endoplasm: Inner part of cytoplasm, interposed bet ween the ectoplasm and the nucleus.
Ribosomes aremade up of 35% of proteins and 65% of ribonucleic acid (RNA). RNA present in ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the cell.
Types of Ribosomes
Ribosomes are of two types:
i. Ribosomes that are attached to rough endoplasmic reticulum
ii. Free ribosomes that are distributed in the cytoplasm.
Functions of Ribosomes
Ribosomes are called ‘protein factories’Messenger RNA (mRNA) carries the genetic code for protein synthesis from nucleus to the ribosomesRibosomes attached to rough endoplasmic reticulum are involved in the synthesis of proteinsFree ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria.
Cytoskeleton is the cellular organelle present throughout the cytoplasm. Cytoskeleton consists of three major protein components:
2. Intermediate flaments
Microtubules are the straight, hollow and tubular structures of the cytoskeleton.
Functions of microtubulesi-
2.Intermediate Filaments– Intermediate flaments are divided into five subclasses:
i. Keratins (in epithelial cells)
ii. Glial flaments (in astrocytes)
iii. Neuroflaments (in nerve cells)
iv. Vimentin (in many types of cells)
v. Desmin (in muscle fbers)
Functions of intermediate flaments
Intermediate flaments help to maintain the shape of the cell. These flaments also connect the adjacent cells through desmosomes.
3. Microflaments –Microflaments are long and fne threadlike structures with a diameter of about 3 to 6 nm.
Functions of microflaments
i. Give structural strength to the cell
ii. Provide resistance to the cell against the pulling forces
iii. Are responsible for cellular movements like contraction, gliding and cytokinesis (partition of cytoplasm during cell division).
Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 µ to 22 µ and occupies about 10% of total volume of the cell.
STRUCTURE OF NUCLEUS
Nucleus is covered by a membrane called nuclear membrane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus.
Nuclear Membrane –Nuclear membrane is double layered and porous in nature.
Nucleoplasm is a highly viscous ﬂuid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus.
Chromatin is a thread-like material made up of large molecules of DNA. The DNA molecules are compactly packed with the help of a specialized basic protein called histone. So, chromatin is referred as DNA-histone complex. It forms the major bulk of nuclear material.
All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from mother and one from father. The cells with 23 pairs of chromosomes are called diploid cells. The reproductive cells called gametes or sex cells contain only 23 single chromosomes. These cells are called haploid cells.
Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes.
FUNCTIONS OF NUCLEUS
Major functions of nucleus are the control of cellular activities and storage of hereditary material. Several processes are involved in the nuclear functions.
Functions of nucleus:
1. Control of all the cell activities that include metabolism, protein synthesis, growth and reproduction (cell division)
2. Synthesis of RNA
3. Formation of subunits of ribosomes
4. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA)
5. Control of the cell division through genes
6. Storage of hereditary information (in genes) and transformation of this information from one generation of the species to the next.
Deoxyribonucleic acid (DNA) is a nucleic acid that carries the genetic information to the offspring of an organism. DNA is present in the nucleus (chromosome) and mitochondria of the cell. The DNA present in the nucleus is responsible for the formation of RNA. RNA regulates the synthesis of proteins by ribosomes. DNA in mitochondria is called non-chromosomal DNA.
STRUCTURE OF DNA
Each chain of DNA molecule consists of many
nucleotides. Each nucleotide is formed by:
1. Deoxyribose – sugar
3. One of the following organic (nitrogenous) bases:
Purines – Adenine (A)
– Guanine (G)
Pyrimidines – Thymine (T)
– Cytosine (C) The hereditary informa tion that is encoded in DNA is called genome.
Gene is a portion of DNA molecule that contains the message or code for the synthesis of a specifc proteinfrom amino acids.
Causes of Gene Disorders
Genetic disorders occur due to two causes:
1. Genetic variation: Presence of a different form of gene
2. Genetic mutation: Generally, mutation means an alteration or a change in nature, form, or quality. Genetic mutation refers to change of the DNA sequence within a gene or chromosome of an organism, which results in the creation of a new character
Classifcation of Genetic Disorders
Genetic disorders are classifed into four types:
1. Single gene disorders
2. Multifactorial genetic disorders
3. Chromosomal disorders
4. Mitochondrial DNA disorders.
Chromosomal disorder is a genetic disorder caused by abnormalities in chromosome. It is also called chromosomal abnormality, anomaly or aberration. It often results in genetic disorders which involve physical or mental abnormalities. Chromosomal disorder is caused by numerical abnormality or structural abnormality.
Chromosomal disorder is classifed into two types:
i. Structural abnormality (alteration) of chromosomes which leads to disorders like chromosome instability syndromes (group of inherited diseases which cause malignancies)
ii. Numerical abnormality of chromosomes which is of two types:
a. Monosomy- due to absence of one chromosome from normal diploid number. Example is Turner’s syndrome, which is characterized by physical disabilities.
b. Trisomy- due to the presence of one extra chromosome along with normal pair of chromosomes in the cells. Example is Down syndrome, which is characterized by physical disabilities and mental retardation.
Ribonucleic acid (RNA) is a nucleic acid that contains a long chain of nucleotide units. RNA is formed from DNA.
STRUCTURE OF RNA
Each RNA molecule consists of a single strand of
polynucleotide unlike the doublestranded DNA. Each
nucleotide in RNA is formed by:
1. Ribose – sugar.
3. One of the following organic bases:
Purines – Adenine (A)
– Guanine (G)
Pyrimidines – Uracil (U)
– Cytosine (C).
Uracil replaces the thymine of DNA and it has similar
structure of thymine
1.Messenger RNA (mRNA)
Messenger RNA carries the genetic code of the amino acid sequence for synthesis of protein from the DNA to the cytoplasm.
2. Transfer RNA (tRNA)
Transfer RNA is responsible for decoding the genetic message present in mRNA.
3. Ribosomal RNA (rRNA)
It is responsible for the assembly of protein from amino acids in the ribosome.
Gene expression involves two steps:
Cell death occurs by two distinct processes:
Apoptosis is defned as the natural or programed death of the cell under genetic control. It is also called ‘cell suicide’ since the genes of the cell play a major role in the death. In contrast to necrosis, apoptosis usually does not produce inﬂammatory reactions in the neighboring tissues.
The functional signifcance of apoptosis:
1. Plays a vital role in cellular homeostasis. About 10 million cells are produced everyday in human body by mitosis.
2. Useful for removal of a cell that is damaged beyond repair by a virus or a toxin
3. An essential event during the development and in adult stage.
1.A large number of neurons are produced during the development of central nervous system. But up to 50% of the neurons are removed by apoptosis during the formation of synapses between neurons.
2. Apoptosis is responsible for the removal of tissues of webs between fngers and toes during developmental stage in fetus.
Apoptosis is activated by either withdrawal of positive signals (survival factors) or arrival of negative signals.
Withdrawal of positive signals
Positive signals are the signals which are necessary for the long-time survival of most of the cells Best examples of chemical stimulants are:
i. Nerve growth factors (for neurons)
ii. Interleukin-2 (for cells like lymphocytes).
The absence or withdrawal of the positive signals activates apoptosis.
Arrival of negative signals
Negative signals are the external or internal stimuli which initiate apoptosis. The negative signals are produced during various events like:
1. Normal developmental procedures
2. Cellular stress
3. Increase in the concentration of intracellular oxidants
4. Viral infection
5. Damage of DNA
6. Exposure to agents like chemotherapeutic drugs,
X-rays, ultraviolet rays and the death-receptor ligands.
Cell shows sequence of characteristic morphological changes during apoptosis, viz.:
1. Activated caspases digest the proteins of cytoskeleton and the cell shrinks and becomes round.
2. Because of shrinkage, the cell losses the contact with neighboring cells or surrounding matrix.
3. Chromatin in the nucleus undergoes degradation and condensation4. Nuclear membrane becomes discontinuous and the DNA inside nucleus is cleaved into small fragments.
5. Following the degradation of DNA, the nucleus breaks into many discrete nucleosomal units, which are also called chromatin bodies.
6. Cell membrane breaks and shows bubbled appearance.
7. Finally, the cell breaks into several fragments containing intracellular materials including chromatin bodies and organelles of the cell. Such cellular fragments are called vesicles or apoptotic bodies.
8. Apoptotic bodies are engulfed by phagocytes and dendritic cells.
Necrosis (means ‘dead’ in Greek) is the uncontrolled and unprogramed death of cells due to unexpected and accidental damage.
Causes for Necrosis
Common causes of necrosis are injury, infection, inﬂammation, infarction and cancer.
Necrosis results in lethal disruption of cell structure and activity. The cell undergoes a series of characteristic changes during necrotic process, viz.
1. Cell swells causing damage of the cell membrane and appearance of many holes in the membrane.
2. Intracellular contents leak out into the surrounding environment.
3. Intracellular environment is altered.
4. Simultaneously, large amount of calcium ions are released by the damaged mitochondria and other organelles.
5. Presence of calcium ions drastically affects the organization and activities of proteins in the intracellular components.
6. Calcium ions also induce release of toxic materials that activate the lysosomal enzymes.
7. Lysosomal enzymes cause degradation of cellular components and the cell is totally disassembled resulting in death.
8. Products broken down from the disassembled cell are ingested by neighboring cells.
Cell adaptation refers to the changes taking place in a cell in response to environmental changes. Cellular adaptation occurs by any of the following mechanisms.
Atrophy means decrease in size of a cell.
Causes of Atrophy
Atrophy is due to one or more number of causes such as:
i. Poor nourishment
ii. Decreased blood supply
iii. Lack of workload or exercise
iv. Loss of control by nerves or hormones
v. Intrinsic disease of the tissue or organ.
Hypertrophy is the increase in the size of a cell.
1.Muscular hypertrophy: Increase in bulk of skeletal muscles that occurs in response to strength training exercise.
2. Ventricular hypertrophy: Increase in size of ventricular muscles of the heart which is advantageous only if it occurs in response to exercise.
Hyperplasia is the increase in number of cells due to increased cell division (mitosis).
Physiological hyperplasia is the momentary adaptive response to routine physiological changes in the body. For example, during the proliferative phase of each menstrual cycle, the endometrial cells in uterus increase in number.
Dysplasia is the condition characterized by the abnormal change in size, shape and organization of the cell.
Metaplasia is the condition that involves replacement of one type of cell with another type of cell.
Replacement of cells in normal conditions is called physiological metaplasia. Examples are transformation of cartilage into bone and transformation of monocytes into macrophages.
Degeneration may result in functional impairment or deterioration of a tissue or an organ. It is common in metabolically active organ like liver, heart and kidney. Degenerative changes are reversible in most of the cells.
Causes for Cell Degeneration
Common causes for cell degeneration:
1. Atrophy, hypertrophy, hyperplasia and/or dysplasia of cell
2. Fluid accumulation in the cell
3. Fat infltration into the cell
4. Calcifcation of cellular organelles.
Stem cells are the primary cells capable of reforming themselves through mitotic division and differentiating into specialized cells.
TYPES OF STEM CELLS
Stem cells are of two types:
1. Embryonic stem cells derived from embryo
2. Adult stem cells derived from adults.
1.Embryonic Stem Cells
Embryonic stem cells have two important qualities:
i. Self-renewal capacity
ii. Pluripotent nature, i.e. these cells are capable of differentiating into all types of cells in ectodermal, endodermal and mesodermal layers.
2.Adult Stem Cells
Adult stem cells are the undifferentiated multipotent progenitor cells found in growing children and adults. These are also known as somatic stem cells and are found everywhere in the body.
Two types of stem cells are present in bone marrow:
i. Hemopoietic stem cells, which give rise to blood cells.
ii. Bone marrow stromal cells, which can differentiate into cardiac and skeletal muscle cells.