Organelles Of Cell (Physiology)

CYTOPLASM

Cytoplasm of the cell is the jelly-like 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, lpids 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 be- tween the ectoplasm and nucleus.

ORGANELLES IN CYTOPLASM

Cytoplasmic organelles are the cellular structures embed- ded in the cytoplasm. Organelles are considered as small organs of the cell. In Latin the word organelle means small organ. 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 LIMITING MEMBRANE

ENDOPLASMIC RETICULUM

Endoplasmic reticulum is a network of tubular and micro- somal vesicular structures which are interconnected with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids. The lumen of endoplasmic reticulum contains a fluid medium called endoplasmic matrix. The diameter of the lumen is about 400 to 700 À. The endoplasmic reticulum forms the link between nucleus and cell membrane by connecting the cell membrane with the nuclear membrane.

Types of Endoplasmic Reticulum

Endoplasmic reticulum is of two types, namely rough en- doplasmic reticulum and smooth endoplasmic reticulum. 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 B-cells 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 endoplas- mic reticulum. Here, the carbohydrates are added to the protein units forming the glycosylated proteins or gly- coproteins, 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 cytoplasmic organelles.

2. Degradation of worn-out organelles

Rough endoplasmic reticulum also plays an important role in the degradation of worn-out cytoplasmic organelles like mitochondria. It wraps itself around the wornout organelles and forms a vacuole which is often called the autophagosome. Autophagosome is digested by lysosomal enzymes (see below for details).

Smooth Endoplasmic Reticulum

It is the endoplasmic reticulum with smooth appearance. Ribosomes are not attached to this. Hence, it is also called agranular endoplasmic 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

catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-oroducing sub- Smooth endoplasmic reticulum is also concerned with stances) 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. Usu- ally, 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. Re- ticular 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, pack- ing, labeling and delivery of proteins and other molecules ike lipids to different parts of the cell.

1. Processing of materials

Vesicles containing glycoproteins and lipids are trans- ported 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 orga- nelles found throughout the cytoplasm. The lysosomes are formed by Golgi apparatus. The enzymes synthe- sized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi ap- paratus. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. Among the organelles of the cytoplasm, the lys- osomes 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

Primary lysosome is pinched off from Golgi appa- ratus. It is inactive in spite of having hydrolytic en- zymes.

2. Secondary lysosome

Secondary lysosome is the active lysosome. It is formed by the fusion of a primary lysosome 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 execute 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 on- gulfed by the coll via endocytosis.

2. Autophagy: Digestion and degradation 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. 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 endo- cytosis are called endosomes. The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome. Secondary lysosome becomes acidic and the lysosomal enzymes are activated. Bacteria and the other macromolecules are digested and degraded by these enzymes. Second- ary lysosome containing these degraded waste products moves through cyloplasm and fuses with cell membrane. Now the waste products are eliminated by exocytosis.

2. Degradation of worn-out organelles

The rough endoplasmic reticulum wraps itself around the worn-out organelles such as mitochondria and form autophagosomes. One primary lysosome fuse with one autophagosome and form the secondary lysosome. 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 secre- tory granules.

4. Secretdry function:

Secretory lysosomes Lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in cells of immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory

Examples of secretory lysosomes:

a. Lysosomes in the cytotoxic T lymphocytes and natu- ral killer (NK) cells secreto 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 sary, of the proteins) and cause the cell death by apoptosis.

b. Secretory lysosomes of melanocytes secrete melanin.

c. Secretory lysosomes of mast cells secrete serotonin, which is a vasoconstrictor substance and inflammatory mediator.

PEROXISOMES

Peroxisomes or microbodies are the membrane limited somes contain some oxidative enzymes such as catalase, urate oxidase and D-amino acid oxidase. inward matrix

Functions of Peroxisomes Peroxisomes:

1. Breakdown the fatty acids by means of a process called beta-oxidation. This is the major function of peroxisomes.

2. 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 per- oxisomes 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.

3. Form the major site of oxygen utilization in the cells.

4. Accelerate gluconeogenesis from fats.

5. Degrade purine to uric acid.

6. Participate in the formation of myelin

. 7. 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 mem- brane 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, 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 p. 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 glycerophosphate 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 molecules in cristae are collectively known as respiratory chain or electron transport system.

Enzymes and other proteins of respiratory chain

1. Succinic dehydrogenase

2. Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase

3. Cytochrome oxidase

4. Cytochrome C

5. 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 repro- ducing itself. Mitochondrion contains its own deoxyribonu- cleic 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, carbohydrates and lipids by the oxidative enzymes in cristae. During the oxidative process, water and carbon dioxide are produced with release of energy. The re- leased energy is stored in mitochondria and used later for synthesis of ATP.

2. Sýnthesis of ATP

The components of respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules dif- fuse 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 mitochondria are involved in apoptosis (see below).

4. Storage of calcium

Endoplasmic reticulum forms the major site of calcium ions storage in the cell. Mitochondrion also stores calcium in its matrix.

5. Detoxification functions

Mitochondria in liver cells are responsible for detoxification of ammonia.

ORGANELLES WITHOUT LIMITING MEMBRANE

RIBOSOMES

Ribosomes are organelles without limiting membrane. These organelles are granular and small dot-like struc- tures with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA), RNA present in ribosomes is called ribosomal RNA (FRNA). 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 Ribosomos

Ribosomes are called 'protein factories' because of their role in the synthesis of proteins. Messenger RNA (MRNA) carries the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turo arrange the amino acids into small units of proteins, Ribosomes attached to rough endoplasmic reticulum are involved in the synthesis of proteins such as the en- zymatic proteins, hormonal proteins, lysosomal proteins and the proteins of the cell membrane. Free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria,

CYTOSKELETON

Cytoskeleton of cell is a cellular organelle formed by complex network of structures in various sizes present throughout the cytoplasm. It determines the shape of the cell and gives support to the cell. It is also essential for the cellular movements and the response of the cell to external stimuli. Cytoskeleton consists of three major protein com- ponents:

1. Microtubule.

2. Intermediate filaments.

3. Microfilaments.

1. Microtubules

Microtubules are the straight, hollow and tubular struc- tures of the cytoskeleton. These organelles without the limiting membrane are arranged in different bundles. Each tubule has a diameter of 20 to 30 nm. Length of microtubule varies and it may be 1,000 times more than the thickness. Structurally, the microtubules are formed by bundles of globular protein called tubulin. Tubulin has two subunits, namely a(Alpha)-subunit and B(Beta)-subunit.

Functions of microtubules

Microtubules may function alone or join with other pro- teins to form more complex structures like cilia, flagella or centrioles and perform various functions.

i. Determine the shape of the cell.

ii. Give structural strength to the cell.

iii. Act like conveyor belts which allow the movement of granules, vesicles, protein molecules and some organelles like mitochondria to different parts of the cell.

iv. Form the spindle fibers which separate the chromo- somes during mitosis.

v. Are responsible for the movement of centrioles and the complex cellular structures like cilia.

2. Intermediate Filaments

Intermediate filaments are the structures of cytoskeleton that form a network around nucleus and extend to the periphery of the cell. Diameter of each filament is about 10 nm. The intermediate filaments are formed by rope- like polymers, which are made up of fibrous proteins.

Subclasses of intermediate filaments

Intermediate filaments are divided into five subclasses:

i. Keratins (in epithelial cells).

ii. Glial filaments (in astrocytes).

iii. Neurofilaments (in nerve cells).

iv. Vimentin (in many types of cells).

v. Desmin (in muscle fibers).

Functions of intermediate filaments

Intermediate filaments:

i. Help to maintain the shape of the cell.

ii. Connect the adjacent cells through desmosomes.

3. Microfilaments

Microfilaments are long and fine thread-like structures with a diameter of about 3 to 6 nm. These filaments are made up of non-tubular contractile proteins called actin and myosin. Actin is more abundant than myosin. Microfilaments are present throughout the cyto- plasm. The microfilaments present in ectoplasm contain only actin molecules and those present in endoplasm contain both actin and myosin molecules.

Functions of microfilaments Microfilaments:

i. Give structural strength to the cell.

ii. Provide resistance to the cell against the pulling forces.

iii. Are responsible for cellular movements like contrac- tion, gliding and cytokinesis (partition of cytoplasm during cell division).

NUCLEUS

Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 to 22 p and occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division.

Most cells have only one nucleus (uninucleated cells). Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells.

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. This allows the nucleoplasm to communicate with the cytoplasm. The outer layer of nuclear membrane is continuous with the membrane of endoplasmic reticulum. is continuous with the lumen of endoplasmic reticulum.

Pores of the nuclear membrane are guarded (lined) by protein molecules. Diameter of the pores is about 80 to 100 nm. It is decreased to about 7 to 9 nm because of the attachment of protein molecules to the periphery of pores, Exchange of materials between nucleoplasm and cytoplasm occurs through these pores.

Nucleoplasm

Nucleoplasm is a highly viscous fluid that forms the ground substance of nucleus. It is similar to cytoplasm present outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus. It contains dense fibrillar network of proteins called the nuclear matrix and many substances such as nucleotides and enzymes. The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of nucleoplasm is known as nuclear hyaloplasm.

Chromatin

Chromatin is a thread-like material made up of large molecules of deoxyribonucleic acid (DNA). The DNA molecules are compactly packed with the help of a spe- cialized basic protein called histone. So, chromatin is referred as DNA-histone complex. It forms the major bulk of nuclear material. DNA is a double helix which wraps around central core of eight histone molecules to form the fundamental packing unit of chromatin called nucleosome. Nucle- osomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form chromosome.

Chromosomes

Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary charac- teristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes. Normally, the chromosomes are not visible in the nucleus under microscope. Only during cell division, the chromosomes are visible under microscope. This is because DNA becomes more tightly packed just before cell division, which makes the chromosome visible during cell division.

Diploid cells and haploid cells

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 . sex cells contain only 23 single chromosomes, These cells are called haploid cells.

Sex chromosomes and autosomes

Among the 23 pairs of chromosomes, one pair is con. cerned with determination of sex of the person. Theso chromosomes are called sex chromosomes. Remaining 22 pair of chromosomes that are not concerned with sey determination are named as autosomes. Among the sex chromosomes, one is called X chro- mosome and another one is called Y chromosomes. The cells of females have two X chromosomes and cells of males have one X chromosome and one Y chromosome.

Nucleolus

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. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes. All the subunits formed in the nucleolus are transported to cytoplasm through the pores of nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins.

FUNCTIONS OF NUCLEUS

Nucleus is considered as brain of the cells. Major func- tions of nucleus are the control of cellular activities and storage of hereditary material. So, it is also called control center. Several processes are involved in the nuclear functions.

DEOXYRIBONUCLEIC ACID: DNA

Deoxyribonucleic acid (DNA) is a nucleic acid that carries the genetic information to the offspring of an organism. DNA forms the chemical basis of hereditary characters. It contains the instruction for the synthesis of proteins in the ribosomes. Gene is a part of a DNA molecule. 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

DNA is a double-stranded complex nucleic acid. It is formed by deoxyribose, phosphoric acid and four types of bases. Each DNA molecule consists of two polynu- cleotide chains, which are twisted around one another in the form of a double helix. The two chains are formed by the sugar deoxyribose and phosphate. These two substances form the backbone of DNA molecule. Both chains of DNA are connected with each other by some organic bases..

Each chain of DNA molecule consists of many nucleotides. Each nucleotide is formed by:

1. Deoxyribose: Sugar.

2. Phosphate.

3. One of the following organic (nitrogenous) bases:

Purines:

i. Adenine (A)

ii. Guanine (G).

Pyrimidines:

i. Thymine (T)

ii. Cytosine (C).

Strands of DNA are arranged in such a way that both are bound by specific pairs of bases. The adenine of one strand binds specifically with thymine of opposite strand. Similarly, the cytosine of one strand binds with guanine of the other strand. DNA forms the component of chromosomes, which carries the hereditary information. The hereditary infor- mation that is encoded in DNA is called genome. Each DNA molecule is divided into discrete units called genes.

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