CELL
All the living things are composed of cells. A single cell is the smallest unit that has all the characteristics of life.
Cell is defined as the structural and functional unit of the living body.
General Characteristics of Cell 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 survival survival.
5.
6. Shows immediate response to the entry of invaders like bacteria or toxic substances into the body Reproduces by division. There are some exceptions like neuron, which do not reproduce.
TISSUE
This is defined as the group of cells having similar function.
There are many types of tissues in the body.
All the tissues are classified into four major types which are called the primary tissues.
The primary tissues include:
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 defined as the structure that is formed by two or more primary types of tissues, which execute the functions of the organ. Some organs are composed of all the four types of primary tissues.
The organs are of two types, namely tubular or hollow organs and compact or parenchymal organs.
Some of the organs in the body are brain, heart, lungs, stomach, intestine, liver, gallbladder, pancreas, kidneys, endocrine glands, etc.
SYSTEM
The organ system is defined as group of organs that work together to carry out specific functions of the body.
Each system performs a specific function.
Digestive system is concerned with digestion of food particles.
Excretory system eliminates unwanted substances.
Cardiovascular system is responsible for transport of substances between the organs.
Respiratory system is concerned with the supply of oxygen and removal of carbon dioxide.
Reproductive system is involved in the reproduction of species.
Endocrine system is concerned with growth of the body and regulation and maintenance of normal life.
Musculoskeletal system is responsible for stability and move ments of the body.
Nervous system controls the locomotion and other activities including the intellectual functions.
STRUCTURE OF THE CELL
Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane.
Cell body has two parts, namely nucleus and cytoplasm surrounding the nucleus.
Thus, the structure of the cell is studied under three headings:
1. Cell membrane
2. Cytoplasm
3. Nucleus
CELL MEMBRANE
Cell membrane is a protective sheath, enveloping the cell body.
It is also known as plasma membrane or plasmalemma.
This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF).
The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between ECF and ICF. Thickness of the cell membrane varies from 75 to 111Å.
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 elec-tron-dense layers.
The two electron-dense layers are placed one on either side of the central layer.
The central layer is a lipid layer formed by lipid substances.
The other two layers are protein layers formed by proteins.
Cell membrane contains some carbohydrate molecules also.
FUNCTIONS OF CELL MEMBRANE
1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cyto-plasm
2. Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier for other substances
3. Absorptive function: Nutrients are absorbed into the cell through the cell membrane
4. Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane
5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane
6. Maintenance of shape and size of the cell: Cell mem-brane is responsible for the maintenance of shape and size of the cell.
CYTOPLASM
Cytoplasm of the cell is the jellylike material formed by 80% of water. It contains a clear liquid portion called cytosol and various particles of different shape and size.
These particles are proteins, carbohydrates, lipids or electrolytes in nature.
Cytoplasm also contains many organelles with distinct structure and function.
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.
ORGANELLES IN CYTOPLASM
Cytoplasmic organelles are the cellular structures embedded in the cytoplasm. Organelles are considered as small organs of the cell. Some organelles are bound by limiting membrane and others do not have limiting membrane.
Each organelle is having a definite structure and specific functions.
ORGANELLES WITH/WITHOUT LIMITING MEMBRANE
ENDOPLASMIC RETICULUM
Endoplasmic reticulum is a network of tubular and microsomal vesicular structures which are interconnect-ed with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids. The lumen contains a fluid medium called the endoplasmic matrix and it's about 700Angstrons in diameter. The membrane of the endoplasmic reticulum is continuous with that of the nucleus. Thus there is a relative communication between the two by the interconnected membranes.
Types of Endoplasmic
ReticulumEndoplasmic reticulum is of two types, namely;
» Rough endoplasmic reticulum and
» Smooth endoplasmic reti-culum.
Both the types are interconnected and continuous with one another. Depending upon the activities of the cells, the rough endoplasmic reticulum changes to smooth endoplasmic reticulum and vice versa.
Rough Endoplasmic Reticulum
It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum.
Rough endoplasmic reticulum is vesicular or tubular in structure.
Functions of Rough Endoplasmic Reticulum
1. Synthesis of proteins
Rough endoplasmic reticulum is concerned with the synthesis of proteins in the cell. It is involved with the synthesis of mainly those proteins which are secreted from the cells such as insulin from βcells of islets of Langerhans in pancreas and antibodies from B lymphocytes. Ribosomes arrange the amino acids into small units of proteins and transport them into the rough endoplasmic reticulum. Here, the carbohydrates are added to the protein units forming the glycosylated proteins or glycoproteins, which are arranged in the form of reticular vesicles. These vesicles are transported mainly to Golgi apparatus for further modification and processing. Few vesicles are transported to other cyto-plasmic organelles.
2. Degradation of worn-out organelles
Rough endoplasmic reticulum also plays an important role in the degradation of worn-out cytoplasmic orga-nelles like mitochondria. It wraps itself around the worn-out organelles and forms a vacuole which is often called the autophagosome. Autophagosome is digested by lysosomal enzymes.
Smooth Endoplasmic Reticulum
It is the endoplasmic reticulum with smooth appearance. It is also called agranular reticulum. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum.
Functions of Smooth Endoplasmic Reticulum
1. Synthesis of non-protein substance Smooth endoplasmic reticulum is responsible for synthesis of non-protein substances such as cholesterol and steroid.
This type of endoplasmic reticulum is abundant in cells that are involved in the synthesis of lipids, phospholipids, lipoprotein substances, steroid hormones, sebum, etc. In most of the other cells, smooth endoplasmic reticulum is less extensive than the rough endoplasmic reticulum.
2. Role in cellular metabolism
Outer surface of smooth endoplasmic reticulum contains many enzymes which are involved in various metabolic processes of the cell.
3. Storage and metabolism of calcium
Smooth endoplasmic reticulum is the major site of storage and metabolism of calcium. In skeletal muscle fibers, it releases calcium which is necessary to trigger the muscle contraction.
4.Catabolism and detoxification
Smooth endoplasmic reticulum is also concerned with catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-producing substances) in the liver.
GOLGI APPARATUS
Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the processing of proteins.
It is present in all the cells except red blood cells. It is named after the discoverer Camillo Golgi.
Usually, each cell has one Golgi apparatus. Some of the cells may have more than one Golgi apparatus.
Each Golgi apparatus consists of 5 to 8 flattened membranous sacs called the cisternae.
Golgi apparatus is situated near the nucleus.
It has two ends or faces, namely;
» cis face and
» trans face.
The cis face is positioned near the endoplasmic reticulum.
Reticular vesicles from endoplasmic reticulum enter the Golgi apparatus through cis face.
The trans face is situated near the cell membrane. The processed substances make their exit from Golgi apparatus through trans face.
Functions of Golgi Apparatus
Major functions of Golgi apparatus are processing, packing, labeling and delivery of proteins and other molecules like lipids to different parts of the cell.
1. Processing of materials Vesicles containing glycoproteins and lipids are transported into Golgi apparatus. Here, the glycoproteins and lipids are modified and processed.
2. Packaging of materials
All the processed materials are packed in the form of secretory granules, secretory vesicles and lysosomes, which are transported either out of the cell or to another part of the cell.
Because of this, Golgi apparatus is called the ‘post office of the cell’.
3. Labeling and delivery of materials
Finally, the Golgi apparatus sorts out the processed and packed materials and labels them (such as phosphate group), depending upon the chemical content for delivery (distribution) to their proper destinations.
Hence, the Golgi apparatus is called ‘shipping department of the cell’.
LYSOSOMES
Lysosomes are the membrane-bound vesicular organelles found throughout the cytoplasm. The lyso-somes are formed by Golgi apparatus. The enzymes synthesized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The membrane is formed by a bilayered lipid material. It has many small granules which contain hydrolytic enzymes.
Types of Lysosomes
Lysosomes are of two types:
1.Primary lysosome, which is pinched off from Golgi apparatus. It is inactive in spite of having hydrolytic enzymes
2.Secondary lysosome, which is the active lyso some. It is formed by the fusion of a primary lyso some with phagosome or endosome (see below).
Functions of Lysosomes
Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes, through which lysosomes exe cute their functions.Important lysosomal enzymes
1.Proteases, which hydrolyze the proteins into amino acids
2.Lipases, which hydrolyze the lipids into fatty acids and glycerides
3.Amylases, which hydrolyze the polysaccharides into glucose
4.Nucleases, which hydrolyze the nucleic acids into mononucleotides.
Mechanism of lysosomal function
Lysosomal functions involve two mechanisms:
1.Heterophagy: Digestion of extracellular materials engulfed by the cell via endocytosis
2.Autophagy: Digestion of intracellular materials such as worn-out cytoplasmic organelles.
Specific functions of lysosomes
1. Degradation of macromolecules Macromolecules are engulfed by the cell by means of endocytosis (phagocytosis, pinocytosis or receptor-mediated endocytosis.
The macromolecules such as bacteria, engulfed by the cell via phagocytosis are called phagosomes or vacuoles. The other macromolecules taken inside via pinocytosis or receptor-mediated endocytosis are called endosomes.
The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome. The pH in the secondary lysosome becomes acidic and the lysosomal enzymes are activated. The bacteria and the other macromolecules are digested and degraded by these enzymes.
The secondary lysosome containing these degraded waste products moves through cytoplasm andfuses with cell membrane. Now the waste products are eliminated by exocytosis.
2. Degradation of worn-out organellesThe rough endoplasmic reticulum wraps itself around the worn-out organelles like mitochondria and form the vacuoles called autophagosomes.
One primary lysosome fuses with one autophagosome to form the secondary lysosome. The enzymes in the secondary lysosome are activated. Now, these enzymes digest the contents of autophagosome.
3. Removal of excess secretory products in the cells
Lysosomes in the cells of the secretory glands remove the excess secretory products by degrading the secretory granules.
4. Secretory function – secretory lysosomes
Recently, lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in the cells of immune system.
The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory product of the cell).
Examples of secretory lysosomes:
i. Lysosomes in the cytotoxic T lymphocytes and natural killer (NK) cells secrete perforin and granzymes, which destroy both viral-infected cells and tumor cells. Perforin is a pore-forming protein that initiates cell death.
Granzymes belong to the family of serine proteases (enzymes that dislodge the peptide bonds of the proteins) and cause the cell death by apoptosis
ii. Secretory lysosomes of melanocytes secrete melanin
iii.Secretory lysosomes of mast cells secrete serotonin, which is a vasoconstrictor substance and inflammatory mediator.
PEROXISOMES
Peroxisomes or microbodies are the membrane limited vesicles like the lysosomes. Unlike lysosomes, peroxisomes are pinched off from endoplasmic reticulum and not from the Golgi apparatus.
Peroxisomes contain some oxidative enzymes such as catalase, urate oxidase and Damino acid oxidase.
Functions of Peroxisomes
i. Breakdown the fatty acids by means of a process called betaoxidation: This is the major function of peroxisomes
ii. Degrade the toxic substances such as hydrogen peroxide and other metabolic products by means of detoxification.
A large number of peroxisomes are present in the cells of liver, which is the major organ for detoxification. Hydrogen peroxide is formed from poisons or alcohol, which enter the cell. Whenever hydrogen peroxide is produced in the cell, the peroxisomes are ruptured and the oxidative enzymes are released. These oxidases destroy hydrogen peroxide and the enzymes which are necessary for the production of hydrogen peroxide
iii. Form the major site of oxygen utilization in the cells
iv. Accelerate gluconeogenesis from fats
v. Degrade purine to uric acid
vi. Participate in the formation of myelin
vii. Play a role in the formation of bile acids.
CENTROSOME AND CENTRIOLES
Centrosome is the membrane-bound cellular organelle situated almost in the center of cell, close to nucleus. It consists of two cylindrical structures called centrioles which are made up of proteins. Centrioles are responsible for the movement of chromosomes during cell division.
SECRETORY VESICLES
Secretory vesicles are the organelles with limiting membrane and contain the secretory substances. These vesicles are formed in the endoplasmic reticulum and are processed and packed in Golgi apparatus. Secretory vesicles are present throughout the cytoplasm. When necessary, these vesicles are ruptured and secretory substances are released into the cytoplasm.
MITOCHONDRION
Mitochondrion (plural = mitochondria) is a membrane-bound cytoplasmic organelle concerned with production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1 μ. It is covered by a bilayered membrane.
The outer membrane is smooth and encloses the contents of mitochondrion. This membrane contains various enzymes such as acetyl-CoA synthetase and glycerolphosphate acetyltransferase. The inner membrane is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space.
Cristae contain many enzymes and other protein molecules which are involved in respiration and synthesis of adenosine triphosphate (ATP). Because of these functions, the enzymes and other protein moleculesn cristae are collectively known as respiratory chain or electron transport system.
Enzymes and other proteins of respiratory chain
i.Succinic dehydrogenase
ii. Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase
iii. Cytochrome oxidase
iv. Cytochrome C
v. ATP synthase. Inner cavity of mitochondrion is filled with matrix which contains many enzymes. Mitochondrion moves freely in the cytoplasm of the cell. It is capable of reproducing itself. Mitochondrion contains its own deoxyribonucleic acid (DNA), which is responsible for many enzymatic actions. In fact, mitochondrion is the only organelle other than nucleus, which has its own DNA.
Functions of Mitochondrion
1. Production of energy
Mitochondrion is called the ‘power house’ or ‘power plant’ of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbo-hydrates and lipids by the oxidative enzymes in cristae. During the oxidative process, water and carbon dioxide are produced with release of energy. The released ener-gy is stored in mitochondria and used later for synthesis of ATP.
2. Synthesis of ATPThe components of respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules diffuse throughout the cell from mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down.
3. Apoptosis
Cytochrome C and second mitochondria-derived activator of caspases (SMAC)/diablo secreted in mito chondria are involved in apoptosis.
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