CELL, TISSUE AND CELL DIVISION

                                    CELL, TISSUE AND CELL DIVISION

Physiology is the science that seeks to explain the physical and chemical mechanism that is responsible for the origin, development, and progression of life.

Human physiology attempts to explain the specific characteristics and mechanism of the human body that make it a living thing.

Cell is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". Human has more than 10 trillion(1013) cells.

System is a group of organs working together to perform a certain function of the body.

 The systems of the body are-

1. Circulatory or Cardiovascular system: Carries oxygen & nourishment and removes waste materials.

2. Respiratory system: Allows the exchange of gases between body & environment.

3. Digestive system: Concerned with digestion & absorption.

4. Urinary system: Deals with excretion of water-soluble waste products from the body.

5. Endocrine system: Produces hormones, which control varieties of functions.

6. Reproductive system: Responsible to give birth of the same kind.

7. Nervous system: Governs & coordinates the activities of some kind.

8. Musculoskeletal system: Responsible for movement.

9. Integumentary System: Skin, Hair& Nail

                                                  Cell

The cell is the structural and functional unit of the living organism. Every human contains more than 100 trillion cells. A typical cell has two major parts, the nucleus, and cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane and the cytoplasm is separated from the surrounding fluid by the plasma membrane. 

# Physical structure of cell
 a. Cell membrane
 b. Protoplasm

 i.Nucleus 

1. Nuclear membrane 

2. Nucleolus 

3. Nucleoplasm 

4. Chromatin  

                           ii.Cytoplasm 

1. Organelles 

a. Membrane organelles 

i. Mitochondria 

ii. Endoplasmic reticulum 

iii. Golgi apparatus 

iv.Lysosome 
v. Peroxisome 

b. Non-membrane organelles 

i. Ribosome 

ii. Centrioles 

iii. Filaments 

iv.Microtubules  

2. Inclusion 

a. Secretory granules 

b. Pigment granules 

c. Lipid and glycones

d. Crystal  

# Cell membrane

The cell membrane (also known as the plasma membrane or cytoplasmic membrane) is a biological membrane that separates the interior of all cells from the outside environment. The basic function of the cell membrane is to protect the cell from its surroundings.  

It consists of the lipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes like attachment surfaces for several extracellular structures.

Structure 

The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the location in the body, lipids can make up from 20 to 80 percent of the membrane.  

1. Cell membrane lipid (42%) 

a. Phospholipid (25%) 

i. Phospholipids are a major component of cell membranes.  

ii. They form a lipid bilayer in which their hydrophilic head areas are facing the aqueous cytosol and the extracellular fluid. 

iii. The hydrophobic tail areas are facing away from the cytosol and extracellular fluid.  

iv. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane. 

b. Cholesterol (13%) 

i. Cholesterol is another lipid component of animal cell membranes.  

ii. Cholesterol molecules are dispersed between membrane phospholipids.  iii. This helps to keep cell membranes from becoming stiff.  

  iv. Cholesterol is not found in the membranes of plant cells. 

c. Glycolipid (4%) 

i. Glycolipids are located on cell membrane surfaces. 

                         ii. They have a carbohydrate sugar chain attached to the surface. 

2. Cell membrane protein (55%) 

i. The cell membrane contains two types of proteins: (peripheral protein, integral protein). 

ii. Peripheral membrane proteins are exterior to the membrane. 

iii. Integral membrane proteins are inserted into the membrane. 

Functions

i. The membrane acts as a barrier for certain molecules and ions. 

ii. Some small molecules, ions such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by passive diffusion.  

iii. Sugars or amino acids like nutrients enter the cell through transmembrane protein of the cell membrane. 

iv. The plasma membrane creates inside a small deformation, in which they engulf substances. 

v. The plasma membrane takes part in the exocytosis process where they remove the undigested materials. 

# Protoplasm 

The different substances that make up the cell are collectively called protoplasm. It is composed of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates.

1. Water 

a. It is the principal fluid medium of the cell. 

b. It has a concentration between 70-85% of the cell c. Many cellular components are dissolved and/or suspended in water

2. Ions 

a. The most important electrolytes in the cell are K+, Mg++, PO4_3, SO4-2, HCO3-, Na+, Ca++, 

Cl-. 

b. They mainly provide ions for the necessary chemical reactions c. They also play a vital role in cellular mechanisms. 

3. Proteins a. The constituents about 10-20%v of total cell mass. 

b. There are two types of protein: structural and functional. 

c. Structural proteins are the proteins that are present as structural filaments in the cell. 

d. Functional proteins are present as the enzymes in the cell.

4. Lipids 

a. Lipids are the substances that are grouped together to act different functions b. The most important lipids are phospholipids and cholesterol. 

c. They constitute the primary cell membranes.

5. Carbohydrates

a. They play a major role in the nutrition of the cell. 

b. Glycogen, a stored form of carbohydrates used rapidly to supply the cell energy. 

c. It is about 1% of the total cell mass.

# Mitochondria

i. Mitochondria are rod-shaped organelles that can be considered the power generators of the cell.

 ii. It converts oxygen and nutrients into adenosine triphosphate (ATP).  

iii. The total number per cell varies in size and shape, from few hundred nm to 1 µm. For example; red blood cells have no mitochondria, whereas liver cells can have more than 2000. 

iv. They are made of the lipid bilayer membranes. 

v. The outer membrane covers the organelle and contains it like skin.  

vi. The inner membrane folds over many times and creates layered structures called cristae. 

 vii. The fluid contained in the mitochondria is called the matrix. 

viii. Mitochondria are special because they have their own ribosomes and DNA floating in that matrix. 

Functions 

i. The most important function of mitochondria is to produce energy.  

ii. Mitochondria help the cells to maintain the proper concentration of calcium ions within the compartments of the cell.  

iii. The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen.  

iv. The liver cells' mitochondria have enzymes that detoxify ammonia.  

v. The mitochondria also play important role in the process of apoptosis or programmed cell death.  

# Ribosomes 

1. Ribosomes are the particle that is present in large numbers in all living cells and serves as the site of protein synthesis.  

2. They occur both as free particles in prokaryotic and eukaryotic cells and as particles attached to the membranes of the endoplasmic reticulum in eukaryotic cells. 

3. They are minute granules having 20 nm in diameter. 

4. Ribosomes are made up of ribosomal proteins (40-80 different types) and ribosomal RNA (rRNA) (3 or 4 types).  

5. Each ribosome is composed of two subunits, a larger one and a smaller one. 

6. The small and large subunits of eukaryotes are designated the 40S and 60S. 

Functions 

1. Synthesize cytoplasmic protein; hemoglobin 

2. Synthesis protein is necessary for peroxisome and mitochondria. 

3. Synthesis all transmembrane protein Synthesis proteins for stored in Golgi body, 

lysosome and endosomes

# Endoplasmic reticulum  

1. These are the network of tubular and vesicular structures, presents in cytoplasm. 

2. The tubular and vesicular structures are interconnected. 

3. The walls of ER are made of lipid-bilayer, containing large amount of protein. 

4. Their surface area can be varying from organ to organ. 

5. Their internal space is filled with a different fluid, called the endoplasmic matrix. 

6. They are two types: Granular ER and agranular ER. 

7. In granular ER, their outer surface is covered with ribosomes that synthesized the protein. 

8. In agranular ER, their outer surface doesn’t have any ribosome, and they synthesize lipid.

Structure of the Smooth & Rough Endoplasmic Reticulum (SER & RER)

The main difference in the structure of RER and SER is that RER contains ribosomes on its surface, which give it a rough appearance. SER does not contain this cell organelle, and hence the name smooth endoplasmic reticulum. SER looks like a group of smooth tubules and is more tubular in shape than RER. Another type of endoplasmic reticulum is Sarcoplasmic Reticulum, which is a type of specialized SER that can be found in smooth, as well as striated muscles.

Functions 

1. Synthesis of protein in RER.  Rough ER looks like sheets. The RER is attached to the nuclear membrane. The Golgi apparatus tends to be on the other side of the RER.

2. Smooth Endoplasmic Reticulum (SER) is mainly concerned with the synthesis of carbohydrate and lipids, and sometimes, with their metabolism. Steroid hormones are produced in SER present in the adrenal and endocrine glands. SER also produces cholesterol and membrane phospholipids, which are used for membrane formation.

3. Folding and transport of various proteins, specifically carrying them to the Golgi apparatus. 

4. It provides the enzymes that control glycogen breakdown when needed. 

5. It provides enzymes for the detoxification of substances. 

# Golgi apparatus 

1. Golgi apparatus is present in eukaryotic cells as one or more groups of flattened, membrane-bounded compartments or sacs.  

2. They are located very near the rough endoplasmic reticulum and hence near the nucleus. 

3. Their membranes are made of lipid bilayer membranes having a large amount of proteins. 

4. They are made up of a series of flattened, stacked pouches called cisternae. 

5. In general, the Golgi apparatus is made up of approximately four to eight cisternae. 

6. The cisternae are held together by matrix proteins. 

  

Functions 

1. The Golgi apparatus is a major collection and dispatch station of protein products received from the endoplasmic reticulum (ER).  

2. Proteins synthesized in the ER are packaged into vesicles, which then fuse with the Golgi apparatus.  

3. The Golgi apparatus is also involved in lipid transport and lysosome formation.

 4. It is associated with the transmission of secretory products 

5. It has the capacity of synthesizing certain carbohydrates. 

# Lysosomes 

1. Lysosome is the organelle that is found in all types of eukaryotic cells. 

2. They are responsible for the digestion of macromolecules, old cell parts, and microorganisms. 

3. Each lysosome is surrounded by an acidic membrane. 

4. Lysosomes contain a wide variety of hydrolytic enzymes (acid hydrolases) that break down macromolecules such as nucleic acids, proteins, and polysaccharides.  

5. They are usually 250-750 nm in diameter. 

6. Lysosomes originate by budding off from the membrane of the Golgi network. 

7. They contain the following enzymes: 

i. Ribonuclease

ii. Deoxyribonuclease

iii. Phosphatase iv. Glycosidase 

v.Arylsulphatases

 vi. Collagenase 

Functions 

i)  Release enzymes outside of the cell, which destroy the materials around the cell. 

ii. Provide an intracellular digestive system for the cell to digest cellular substances. iii. Removal of damaged cells from the body. 

iv. Kills the bacteria harmful for the body.

# Peroxisomes 

i. They are small vesicles found around the cell.  

ii. They have a single membrane that contains digestive enzymes for breaking down toxic materials in the cell.  

iii. They differ from lysosomes in the type of enzyme they hold. 

iv. Peroxisomes hold on oxidative enzymes. 

v. They developed from the budding off smooth ER.

Functions 

i. They are involved in the catabolism of very long-chain fatty acids, branched-chain fatty acids, amino acids, and polyamines. 

ii. Involved in the biosynthesis of plasmalogens, (ether phospholipids critical for the normal function of mammalian brains and lungs). 

iii. They also play a role in cholesterol synthesis and the digestion of amino acids. 

Main Function Lysosomes break down biological polymers like proteins and polysaccharides. Peroxisomes oxidize organic compounds, breaking down metabolic hydrogen peroxides. 

Composition Lysosomes consists of degradative enzymes. Peroxisomes consist of oxidative enzymes. 

Function Lysosomes are responsible for the digestion in the cell. Peroxisomes are responsible for the protection of the cell against metabolic hydrogen peroxide. 

Presence Lysosomes are only found in animals. Peroxisomes are found in all eukaryotes. 

Origin Lysosomes are derived from either the Golgi apparatus or endoplasmic reticulum. Peroxisomes are derived from the smooth endoplasmic reticulum. 

Size  Lysosomes are comparatively large in size. Peroxisomes are small. 

Energy Generation Degradative reactions in the lysosomes do not generate energy. Oxidative reactions in peroxisomes generate ATP energy. 

# Nucleus 

i. The cell nucleus is a membrane-bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. 

ii. Eukaryotes usually have a single nucleus, but a few cells like red blood cells have no nuclei. 

iii. Cell nuclei contain multiple long linear DNA molecules in complex with histones protein, to form chromosomes. 

iv. The cell nucleus is bound by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm. 

v. The nuclear envelope consists of phospholipids bilayer. 

vi. The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into and out of the nucleus through nuclear pores. 

vii. Chromosomes, containing DNA are located within the nucleus.  

viii. When a cell is not dividing, the chromosomes are organized into long structures called chromatin. 

ix. A dense structure composed of RNA and proteins called the nucleolus is located in the nucleus. 

x. The nucleolus helps to synthesize ribosomes. 

Functions 

1. It is the dynamic center of life as it controls the nutritive and respiratory activities of the cell. 

2. It influences growth and initiative divisions of the cell. 

3. The DNA acts as a regulator of the synthesis of enzyme protein. 

4. The DNA also inherit the character of the offspring through gene. 

# Chromosomes 

1. In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes.  

2. Each chromosome is made up of tightly coiled DNA (40%) and around histone proteins (60%). 

3. Each chromosome has a central point called the centromere, which divides the chromosome into two sections, or “arms” or chromatids.  

4. The short arm of the chromosome is labeled the “parm” And the long arm of the chromosome is labeled the “q arm.” 

5. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46.  

6. Twenty-two of these pairs, called autosomes, look the same in both males and females.  

7. The 23rd pair, the sex chromosomes, differs between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome. 

# Gene

A specific sequence of nucleotides in DNA or RNA that is located usually on a chromosome is called a gene. It is the functional unit of inheritance controlling the transmission and expression of one or more traits. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary from a few hundred DNA bases to more than 2 million bases. It is estimated that humans have between 20,000 and 25,000 genes (26,564). 

Introns and exons are parts of genes. Exons code for proteins, whereas introns do not. Exons are parts of DNA that are converted into mature messenger RNA (mRNA). This mRNA is used to synthesize proteins. Introns are parts of genes that do not directly code for proteins. On average, there are 8.8 exons and 7.8 introns per gene. 

DNA vs. RNA – A Comparison Chart

Comparison of DNA RNA

Full Name Deoxyribonucleic Acid Ribonucleic Acid

Function DNA replicates and stores genetic information. It is a blueprint for all genetic information contained within an organism RNA converts the genetic information contained within DNA to a format used to build proteins, and then moves it to ribosomal protein factories. 

Structure DNA consists of two strands, arranged in a double helix. These strands are made up of subunits called nucleotides. Each nucleotide contains a phosphate, a 5-carbon sugar molecule, and a nitrogenous base. RNA only has one strand, but like DNA, is made up of nucleotides. RNA strands are shorter than DNA strands. RNA sometimes forms a secondary double helix structure, but only intermittently. 

Length DNA is a much longer polymer than RNA. A chromosome, for example, is a single, long DNA molecule, which would be several centimeters in length when unraveled. RNA molecules are variable in length, but much shorter than long DNA polymers. A large RNA molecule might only be a few thousand base pairs long. 

Sugar The sugar in DNA is deoxyribose, which contains one less hydroxyl group than RNA’s ribose. RNA contains ribose sugar molecules, without the hydroxyl modifications of deoxyribose.

Bases The bases in DNA are Adenine (‘A’), Thymine (‘T’), Guanine (‘G’) and Cytosine (‘C’). RNA shares Adenine (‘A’), Guanine (‘G’) and Cytosine (‘C’) with DNA, but contains Uracil (‘U’) rather than Thymine.

Base Pairs Adenine and Thymine pair (A-T)

Cytosine and Guanine pair (C-G)  Adenine and Uracil pair (A-U)

Cytosine and Guanine pair (C-G)        

Location DNA is found in the nucleus, with a small amount of DNA also present in mitochondria. RNA forms in the nucleolus, and then move to specialized regions of the cytoplasm depending on the type of RNA formed. 

Reactivity Due to its deoxyribose sugar, which contains one less oxygen-containing hydroxyl group, DNA is a more stable molecule than RNA, which is useful for a molecule which has the task of keeping genetic information safe. RNA, containing a ribose sugar, is more reactive than DNA and is not stable in alkaline conditions. RNA’s larger helical grooves mean it is more easily subject to attack by enzymes.

Ultraviolet (UV) Sensitivity DNA is vulnerable to damage by ultraviolet light. RNA is more resistant to damage from UV light than DNA.

 # Membrane protein 

Membrane proteins are proteins that interact with or are part of biological membranes. They include integral membrane proteins and peripheral membrane proteins. Integral proteins are permanently part of the membrane and peripheral membrane proteins that are only temporarily attached to the lipid bilayer. 

Functions: 

1. They act as pumps, active transportation of ions across the membranes. 

2. They act as the transportation of molecules against concentration gradients. 

3. They function as receptors that binds with neurotransmitter and hormones. 4. They function as enzymes and catalyzing reactions 

5. Sometimes act as an antibody to protect the body. 

6. They act as junctions, to serve to connect and join two cells together 

7. They act as anchorage to attach cytoskeleton and extracellular matrix 

# Glycocalyx

1. The glycocalyx is a carbohydrate enriched coating that covers the outside of many eukaryotic cells, particularly bacteria.  

2. On bacterial cells, the glycocalyx provides a protective coat from host factors.  

3. This coating consists of several carbohydrate parts of membrane glycolipids and glycoproteins. 

4. Generally, the carbohydrate portion of the glycolipids found on the surface of plasma membranes. 

5. There are two prominent functions of the glycocalyx. 

6. The first function is to enable bacteria to become protective of the immune cells. 

7. The second function of a bacterial glycocalyx is to promote the adhesion of the bacteria to living and inert surfaces. 

# Glycoprotein 

1. A glycoprotein is a type of protein molecule that has a carbohydrate attached to it.  

2. The carbohydrate is an oligosaccharide chain (glycan) that is covalently bonded to the polypeptide side chains of the protein.  

3. This glycoprotein is more attracted to water than proteins.  

4. Glycoproteins are frequently present at the surface of cells where they function as membrane proteins or as part of the extracellular matrix. 

5. There are three types of glycoproteins based: N-linked glycoproteins, O-linked glycoproteins, and nonenzymatic glycosylated glycoproteins. 

6. They act in cell-cell recognition and binding of other molecules.  

7. Cell surface glycoproteins are also important to add strength and stability to a tissue. 

8. Glycoproteins in plant cells are what allows plants to stand upright against the force of gravity. 

# Cytokines 

1. The term "cytokine" is derived from a combination of two Greek words - "cyto" meaning cell and "kinos" meaning movement.  

2. Cytokines are cell signaling molecules that aid cell to cell communication in immune responses. 

3. It also stimulates the movement of cells towards sites of inflammation and infection. 

4. Cytokines exist in peptide, protein, and glycoprotein (proteins with a sugar attached) forms. 

5. Cytokines are produced by a broad range of cells, including macrophages, B lymphocytes, T lymphocytes and mast cells.  

6. Examples of cytokines include interleukin and interferon. 

# Tissue

A tissue is an ensemble of similar cells from the same origin that together perform a specific function. Biological tissue is a collection of interconnected cells that perform a similar function within an organism.  

# Classification 

The human body is composed of four basic types of tissues; epithelium, connective, muscular, and nervous tissues. 

1. Epithelium- lines and covers surfaces 

2. Connective tissue- protect, support, and bind together 

3. Muscular tissue- produces movement 

4. Nervous tissue- receive stimuli and conduct impulses 

1. Epithelium tissue 

i. The epithelial tissues are formed by cells that cover the organ surfaces such as skin, the airways, the reproductive tract, and the inner lining of the digestive tract.  

ii. The cells consist of a semipermeable epithelial layer that provides a barrier between the external environment and the organ.  

iii. Also, epithelial tissue also functions in secretion, excretion, and absorption.  

iv.   Epithelial tissue helps to protect organs from microorganisms, injury, and fluid loss. 

v. Some common kinds of epithelium are listed below: 

a. Simple squamous epithelium b. Stratified squamous epithelium c. Simple cuboidal epithelium d. Pseudostratified columnar epithelium e. Columnar epithelium f. Glandular epithelium g. Ciliated columnar epithelium 

2. Connective tissue 

i. Connective tissues are fibrous tissues.  

ii. They are made up of cells separated by extracellular matrix. This matrix can be liquid or rigid.

iii. Connective tissue gives shape to organs and holds them in place.  

iv. Blood, bone, tendon, ligament and adipose tissues are examples of connective tissues.  

v. Connective tissues are divided into three types: fibrous connective tissue, skeletal connective tissue, and fluid connective tissue. 

vi. Connective tissues tend to be very vascular (have a rich blood supply). 

3. Muscular tissue i. Muscle tissue is a soft tissue that composes muscles in animal bodies. 

ii. It is formed during embryonic development. 

iii. Muscle cells form the active contractile tissue of the body known as muscle tissue or muscular tissue.  

iv. Its functions are to produce force and cause movement within internal organs.  

v. Muscle tissue is separated into three distinct categories: visceral or smooth muscle, found in the inner linings of organs; skeletal muscle, attached to bones; and cardiac muscle, found in the heart. 

4. Nervous tissue 

i. Cells comprising the central nervous system and peripheral nervous system are classified as nervous (or neural) tissue.  

ii. In the central nervous system, neural tissues form the brain and spinal cord.  iii. In the peripheral nervous system, neural tissues form the cranial nerves and spinal nerves. 

iv. Nervous tissue is made up of different types of nerve cells, all of which have an axon, and dendrites. 

v. Functions of the nervous system are sensory input, integration, control of muscles and glands, homeostasis, and mental activity. 

# Mutation 

A gene mutation is a permanent alteration in the DNA sequence that makes up a gene. Mutations range in size; they can affect anywhere from a single base pair to a large segment of a base pair. Mutations result from errors during DNA replication or other types of damage to DNA. Mutations may also result from the insertion or deletion of segments of DNA due to other factors.  

Gene mutations can be classified in two major ways: 

1. Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body.  

2. Acquired mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body. These changes can be caused by environmental factors such as ultraviolet radiation from the sun. 

# Neoplasm 

A neoplasm is an abnormal new growth of cells. The cells in a neoplasm usually grow more rapidly than normal cells and will continue to grow if not treated. As they grow, neoplasms can impinge upon and damage adjacent structures. The term neoplasm can refer to benign or malignant growths. 

# Tumor

A tumor is a commonly used, but non-specific, term for a neoplasm. The word tumor simply refers to a mass. This is a general term that can refer to benign (harmless) or malignant (cancerous) growths. 

# Benign tumor 

Benign tumors are a non-cancerous tumors. A benign tumor is usually localized and does not spread to other parts of the body. Most benign tumors respond well to treatment. However, if left untreated, some benign tumors can grow large and lead to serious diseases because of their size.  

# Malignant tumor 

Malignant tumors are cancerous growths. They are often resistant to treatment, may spread to other parts of the body and they sometimes recur after they were removed.  

# Cancer 

Cancer is a group of diseases involving abnormal cell growth with the potential to spread to other parts of the body. Over 100 types of cancers affect humans. The term cancer specifically refers to a new growth that can invade surrounding tissues, metastasize (spread to other organs) and which may eventually lead to the patient's death if untreated. It is also called a malignant tumors. 

# Metastasis

It is the spread of cancer cells from the place where they first formed to another part of the body. In metastasis, cancer cells break away from the original (primary) tumor, travel through the blood or lymph system, and form a new tumor in other organs or tissues of the body. The new, metastatic tumor is the same type of cancer as the primary tumor. 

# Homeostasis: As animals have evolved, specialization of body structures has increased. For cells to function efficiently and interact properly, internal body conditions must be relatively constant.

Homeostasis is the state of steady internal conditions maintained by living things. 

-The dynamic constancy of the internal environment is called homeostasis.

This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid balance, being kept within certain pre-set limits (homeostatic range). 

-It is essential for life. 

Cellular differentiation is the process where a cell changes from one cell type to another.[2][3] Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. 

# Stem cell 

Stem cells are cells with the potential to develop into many different types of cells in the body. They serve as a repair system for the body. There are two main types of stem cells: embryonic stem cells and adult stem cells. 

Adult or somatic stem cells exist throughout the body after embryonic development and are found inside of different types of tissue, like the brain, bone marrow, blood, blood vessels, skeletal muscles, skin, and the liver. 

The intracellular fluid (ICF)  is the fluid that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood.

#Body compartment fluid 

1. Synovial fluid 

i. Synovial fluid is an egg-white like viscous fluid found in the cavities of synovial joints.  

ii. The principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.  

iii. The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity.  

iv. Synovial fluid is an ultrafiltrate from plasma, and contains proteins derived from the blood plasma.  

v. This fluid forms a thin layer (roughly 50 μm) at the surface of the cartilage.  

vi. It contains two cell types (type A and type B). Type A is derived from blood monocytes, and it removes the debris from the synovial fluid. Type B produces hyaluronan, that lubricates the joints.  

vii. The functions of the synovial fluid include: 

a. It keeps the bones slightly apart, protecting their cartilage from frictions. 

b. It absorbs shocks and protects the cartilage c. It lubricates the joint and helping to work freely 

2. Pericardial fluid 

i. Pericardial fluid is the fluid secreted by the serous layer of the pericardium into the pericardial cavity.  

ii. The pericardium consists of two layers, an outer fibrous layer, and the inner serous layer.  

iii. This serous layer has two membranes that enclose the pericardial cavity into which is secreted the pericardial fluid.  

iv. The fluid is made up of a high concentration of lactate dehydrogenase (LDH), protein and lymphocytes.  

v. In a healthy adult, there are up to 50 ml of clear, straw-colored fluid. vi. The fluid serves to cushion and allow some movement of the organ.  

3. Intraocular fluid 

i. Intraocular fluid or aqueous humor is a transparent, watery fluid, containing low protein concentrations.  

ii. It is secreted from the ciliary body. iii. It fills both the anterior and the posterior chambers of the eye. iv. It contains Amino acids, water, electrolytes, Ascorbic acid, and Immunoglobulins. 

v. The function of the intraocular fluid is to maintain intraocular pressure. 

                                

4. Cerebrospinal fluid i. Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills and surrounds the brain and the spinal cord ii. It formed primarily in the ventricles of the brain. iii. CSF is slightly alkaline and is about 99 percent water. iv. It contains 15 to 45 mg/dl protein and 50-80 mg/dl glucose.  

v. There are about 100 to 150 ml of CSF in the normal adult human body. 

vi. It provides a mechanical barrier against shock.  

vii. It also provides lubrication between surrounding bones and the brain and spinal cord. viii. It protects the brain tissue from injury when hit. 

                                              # CELL DIVISION

      What is mitosis?

Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. In this process DNA of the cell's nucleus is split into two equal sets of chromosomes. The great majority of the cell divisions that happen in your body involve mitosis. During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. 

   Stages of mitosis: Prophase, prometaphase, metaphase, anaphase, telophase. 

  Cytokinesis is the process of dividing the cell contents to make two new cells - starts in anaphase or telophase.

1. Prophase

The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears. In early prophase, the cell starts to break down some structures and build others up, setting the stage for a division of the chromosomes.

The chromosomes start to condense (making them easier to pull apart later on).

 It is the first stage of mitosis cell division

 Condensation of chromatin into chromosomes. 

 spindle fibers appear

 the nuclear envelope and nucleoli disappear

2. Prometaphase (late prophase)

  The nuclear envelope breaks down and the chromosomes are fully condensed.

The mitotic spindle begins to capture and organize the chromosomes. The chromosomes finish condensing, so they are very compact. 

The nuclear envelope breaks down, releasing the chromosomes. The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.

Once the nuclear envelope is gone, some of the spindle microtubules attach to chromosomes, throwing them into an agitated motion

3. Metaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles.

The stage when chromosomes line up in a straight line midway between the centrioles. 

Before proceeding to anaphase, the cell will check to make sure that all the chromosomes are at the metaphase plate with their kinetochores correctly attached to microtubules. 

4. Anaphase. The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles.

       In anaphase, the sister chromatids separate from each other and are pulled towards opposite ends of the cell.

5. Telophase: The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense.

In telophase, the cell is nearly done dividing, and it starts to re-establish its normal structures as cytokinesis (a division of the cell contents) takes place.

The mitotic spindle is broken down into its building blocks. Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear. The chromosomes begin to decondense and return to their “stringy” form.

• much like prophase in reverse

• a nuclear envelope and nucleoli reappear, and

• the chromosomes unwind, forming thread-like chromatin.

Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis. It may start in either anaphase or telophase, depending on the cell, and finishes shortly after telophase.

G1: Cell growth phase

S: DNA Synthesis phase

G2: More cell growth phase

                                                                              MEIOSIS

                 Meiosis is a cell division in which four haploid cells are formed from a single diploid cell.

It usually occurs in the reproductive organs or gonads of the organisms.

Meiosis is also known as reductional cell division because four daughter cells produced contain half the number of chromosomes than that of their parent cell

1. Meiosis-I (Reductional  division)

2. Meiosis-II (Equational  division)

Meiosis-I has four different phases or stages:

1. Prophase-I

2. Metaphase-I

3. Anaphase-I

4. Telophase-I

Meiosis-II is exactly similar to mitosis, so it is also known as meiotic mitosis

1. Prophase-II

2. Metaphase-II

3. Anaphase-II

4. Telophase-I

Comparison  MeiosisVs mitosis

To understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis.

Meiosis Mitosis

The end result Normally four cells, each with half the number of chromosomes as the parent Two cells, having the same number of chromosomes as the parent

Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle Cellular reproduction, growth, repair, asexual reproduction

Where does it happen? Almost all eukaryotes (animals, plants, fungi, and protists)

In gonads, before gametes (in diplontic life cycles); 

After zygotes (in haplontic); 

Before spores (in haplodiplontic) All proliferating cells in all eukaryotes

Steps Prophase I, Metaphase I, Anaphase I, Telophase I, 

Prophase II, Metaphase II, Anaphase II, Telophase II Prophase, Prometaphase, Metaphase, Anaphase, Telophase

Genetically same as parents? No Yes

Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely

The pairing of homologous chromosomes? Yes No

Cytokinesis occurs in Telophase I and Telophase II Occurs in Telophase

The cell cycle is a series of events containing cell growth and cell division that produces two new daughter cells. The cell cycle has two major phases: interphase and the mitotic phase. During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell divides.

1.Interphase

During interphase, the cell undergoes normal growth processes while also preparing for cell division. Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during interphase. The three stages of interphase are called G1, S, and G2.  

a. G1 Phase (first Gap) 

The first stage of interphase is called the G1 phase or the first gap. During the G1 stage, the cell is accumulating the chromosomal DNA and associated proteins. In this phase, the cell increases its supply of proteins, increases the number of organelles (such as mitochondria, ribosomes), and grows in size. 

b. S Phase (Synthesis of DNA) 

S phase starts when DNA synthesis commences. In the end, all of the chromosomes have been replicated. Thus, during this phase, the amount of 

DNA in the cell has effectively doubled. Rates of  

RNA transcription and protein synthesis are very low during this phase. But histone production is accelerated. 

c.G2 phase 

During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. G2 phase occurs after DNA replication and is a period of protein synthesis and rapid cell growth to prepare the cell for mitosis.

2.Mitotic Phase 

Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells, called mitosis cell division. 

# Mitosis 

Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus.