Essentials of Physiology for Dental Students K Sembulingam, Prema Sembulingam
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1General Physiology
Chapters
  • 1. Cell
  • 2. Cell Junctions
  • 3. Transport Through Cell Membrane
  • 4. Homeostasis2

CellChapter 1

  • ◼ INTRODUCTION
    • ◼ CELL
    • ◼ TISSUE
    • ◼ ORGAN
    • ◼ SYSTEM
  • ◼ STRUCTURE OF THE CELL
  • ◼ CELL MEMBRANE
    • ◼ COMPOSITION
    • ◼ STRUCTURE
    • ◼ FUNCTIONS
  • ◼ CYTOPLASM
  • ◼ ORGANELLES IN CYTOPLASM
    • ◼ ORGANELLES WITH LIMITING MEMBRANE
    • ◼ ORGANELLES WITHOUT LIMITING MEMBRANE
  • ◼ NUCLEUS
    • ◼ STRUCTURE
    • ◼ FUNCTIONS
  • ◼ CELL DEATH
    • ◼ APOPTOSIS
    • ◼ NECROSIS
 
INTRODUCTION
 
CELL
Cell is defined as the structural and functional unit of the living body because it has all the characteristics of life.
 
TISSUE
The tissue is defined as the group of cells having similar function. The tissues are classified into four major types which are called the primary tissues. The primary tissues include:4
  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. Some organs are composed of all the four types of primary tissues. The organs may be tubular like intestine or hollow like stomach.
 
SYSTEM
The system is defined as group of organs functioning together to perform a specific function of the body. For example, digestive system is made out of groups of organs like esophagus, stomach, intestine, etc., which is concerned with digestion of food particles.
 
STRUCTURE OF THE CELL
Each cell is formed by a cell body and a cell membrane or plasma membrane that covers the cell body. The important parts of the cell are (Fig. 1.1):
  1. Cell membrane.
  2. Nucleus.
  3. Cytoplasm with organelles.
 
CELL MEMBRANE
The cell membrane is a protective sheath that envelops the cell body. It separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). It is a semipermeable membrane and allows free exchange of certain substances between ECF and ICF (Fig. 1.2).
 
COMPOSITION OF CELL MEMBRANE
The cell membrane is composed of three types of substances:
  1. Proteins (55%).
  2. Lipids (40%).
  3. Carbohydrates (5%).
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FIGURE 1.1: Structure of the cell
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FIGURE 1.2: Diagram of the cell membrane
 
STRUCTURE OF CELL MEMBRANE
The cell membrane is a unit membrane having the ‘fluid mosaic model’, i.e. the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement of different colored pieces of stone, tile, glass or other such materials) lipids and carbohydrates. The electron microscopic study reveals three layers in the cell membrane namely, one electron-lucent lipid layer in the center and two electron-dense layers. The two electron-dense protein layers are placed on either side of the central layer. Carbohydrate molecules are found on the surface of the cell membrane.5
 
Lipid Layer of Cell Membrane
It is a bilayered structure formed by a thin film of lipids. It is fluid in nature and the portions of the membrane along with the dissolved substances move to all areas of the cell membrane. The major lipids are:
  1. Phospholipids.
  2. Cholesterol.
 
1. Phospholipids
The phospholipid molecules are formed by phosphorus and fatty acids. Each phospholipid molecule resembles the headed pin in shape (Fig. 1.3). The outer part of the phospholipid molecule is the head portion which is water soluble (hydrophilic) and the inner part is the tail portion that is not soluble in water (hydrophobic). The hydrophobic tail portions meet in the center of the membrane. The hydrophilic head portions of outer layer face the ECF and those of the inner layer face the cytoplasm.
 
2. Cholesterol
The cholesterol molecules are arranged in between the phospholipid molecules. As phospholipids are soft and oily in nature, cholesterol helps to ‘pack’ the phospholipids in the membrane and maintain the structural integrity of cell membrane.
 
Functions of lipid layer
The lipid layer is semipermeable in nature and allows only the fat-soluble substances like oxygen, carbon dioxide and alcohol to pass through it. It does not allow the water-soluble materials like glucose, urea and electrolytes to pass through it.
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FIGURE 1.3: Lipids of the cell membrane
 
Protein Layers of the Cell Membrane
The protein layers of the cell membrane are the electron-dense layers situated on either side of the central lipid layer. The protein substances present in these layers are mostly glycoproteins. These protein molecules are classified into two categories:
  1. Integral proteins.
  2. Peripheral proteins.
 
1. Integral proteins
The integral proteins, also known as transmembrane proteins, are tightly bound with the cell membrane. These protein molecules pass through the entire thickness of the cell membrane from one side to the other side.
 
2. Peripheral proteins
The peripheral proteins also known as peripheral membrane proteins do not penetrate the cell membrane but are embedded partially in the outer and inner surfaces of the cell membrane. These protein molecules are loosely bound with the cell membrane and so dissociate readily from the cell membrane.
 
Functions of protein layers
  1. Integral proteins provide structural integrity of the cell membrane.
  2. Channel proteins provide route for diffusion of water-soluble substances like glucose and electrolytes.
  3. Carrier proteins help in transport of substances across the cell membrane.
  4. Receptor proteins serve as receptor sites for hormones and neurotransmitters.
  5. Enzymes: Some of the protein molecules form the enzymes which control chemical reactions within the cell membrane.
  6. Antigens: Some proteins act as antigens and induce the process of antibody formation.
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Carbohydrates of the Cell Membrane
Carbohydrate molecules form a thin loose covering over the entire surface of the cell membrane called glycocalyx. Some carbohydrate molecules are attached with proteins and form glycoproteins and some are attached with lipids and form glycolipids.
 
Functions of carbohydrates
  1. The carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.
  2. The glycocalyx from the neighboring cells helps in the tight fixation of cells with one another.
  3. Some of the carbohydrate molecules form the receptors for some hormones.
 
FUNCTIONS OF CELL MEMBRANE
  1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm.
  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 membrane is responsible for the maintenance of shape and size of the cell.
 
CYTOPLASM
The cytoplasm is the fluid present inside the cell. It contains a clear liquid portion called cytosol which contains various substances like proteins, carbohydrates, lipids and electrolytes. Apart from these substances, many organelles are also present in cytoplasm. The cytoplasm is distributed as peripheral ectoplasm just beneath the cell membrane and inner endoplasm between the ectoplasm and the nucleus.
 
ORGANELLES IN CYTOPLASM
All the cells in the body contain some common structures called organelles in the cytoplasm. Some organelles are bound by limiting membrane and others do not have limiting membrane (Box 1.1). The organelles carry out the various functions of the cell (Table 1.1).
 
ORGANELLES WITH LIMITING MEMBRANE
 
1. ENDOPLASMIC RETICULUM
Endoplasmic reticulum is made up of tubules and microsomal vesicles.7
TABLE 1.1   Functions of cytoplasmic organelles
Organelles
Functions
Rough endoplasmic reticulum
  1. Synthesis of proteins
  2. Degradation of worn out organelles
Smooth endoplasmic reticulum
  1. Synthesis of lipids and steroids
  2. Role in cellular metabolism
  3. Storage and metabolism of calcium
  4. Catabolism and detoxification of toxic substances
Golgi apparatus
  1. Processing, packaging, labeling and delivery of proteins and lipids
Lysosomes
  1. Degradation of macromolecules
  2. Degradation of worn out organelles
  3. Removal of excess of secretory products
  4. Secretory function
Peroxisomes
  1. Breakdown of excess fatty acids
  2. Detoxification of hydrogen peroxide and other metabolic products
  3. Oxygen utilization
  4. Acceleration of gluconeogenesis
  5. Degradation of purine to uric acid
  6. Role in the formation of myelin
  7. Role in the formation of bile acids
Centrosome
  1. Movement of chromosomes during cell division
Mitochondria
  1. Production of energy
  2. Synthesis of ATP
  3. Initiation of apoptosis
Ribosomes
  1. Synthesis of proteins
Cytoskeleton
  1. Determination of shape of the cell
  2. Stability of cell shape
  3. Cellular movements
Nucleus
  1. Control of all activities of the cell
  2. Synthesis of RNA
  3. Sending genetic instruction to cytoplasm for protein synthesis
  4. Formation of subunits of ribosomes
  5. Control of cell division
  6. Storage of hereditary information in genes (DNA)
These structures form an interconnected network which forms the link between the organelles and cell membrane.
 
Types of Endoplasmic Reticulum
The endoplasmic reticulum is of two types namely, rough endoplasmic reticulum and smooth endoplasmic reticulum.8
 
Rough Endoplasmic Reticulum
Rough endoplasmic reticulum is the one to which the granular ribosome is attached. This gives the rough appearance and so, it is called the rough endoplasmic reticulum. Attachment of the granular ribosome also gives the beaded or granular appearance and so, it is also called granular endoplasmic reticulum (Fig. 1.4).
 
Functions of rough endoplasmic reticulum
It is concerned with the protein synthesis in the cell especially those secreted from the cell such as insulin from ‘β’ cells of islets of Langerhans in pancreas and antibodies in leukocytes.
It also plays an important role in degradation of worn out cytoplasmic organelles like mitochondria. It wraps itself around the worn out organelles and forms a vacuole which is often called the autophagosome. It is digested by lysosomal enzymes.
 
Smooth Endoplasmic Reticulum
Smooth endoplasmic reticulum is also called as agranular endoplasmic reticulum because of its smooth appearance without the attachment of ribosome. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum.
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FIGURE 1.4: Endoplasmic reticulum
 
Functions of smooth endoplasmic reticulum
  1. It is responsible for synthesis of cholesterol and steroid.
  2. It is concerned with various metabolic processes of the cell because of the presence of many enzymes on the outer surface.
  3. It is concerned with the storage and metabolism of calcium.
  4. It is also concerned with catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer producing substances) in liver.
Rough endoplasmic reticulum and smooth endoplasmic reticulum 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.
 
2. GOLGI APPARATUS
Golgi apparatus (Golgi body or Golgi complex) is present in all the cells except red blood cells. It consists of 5 to 8 flattened membranous sacs called cisternae (Fig. 1.5).
The 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.
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FIGURE 1.5: Golgi apparatus
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The 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
  1. It is concerned with the processing and delivery of substances like proteins and lipids to different parts of the cell.
  2. It functions like a post office because, it packs the processed materials into the secretory granules, secretory vesicles, and lysosomes and dispatch them either out of the cell or to another part of the cell.
  3. It also functions like a shipping department of the cell because it sorts out and labels the materials for distribution to their proper destinations.
 
3. LYSOSOMES
These are small globular structures filled with enzymes. These enzymes are synthesized in rough endoplasmic reticulum and transported to the Golgi apparatus. Here, these are processed and packed in the form of small vesicles. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. There are small granules containing the hydrolytic enzymes in the cytoplasm of the lysosome.
 
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 the hydrolytic enzymes.
  2. Secondary lysosome which is active lysosome formed by the fusion of a primary lysosome with phagosome or endosome.
 
Functions of Lysosomes
 
i. Digestion of unwanted substances
With the help of hydrolytic enzymes like proteases, lipases, amylases and nucleases, lysosome digests and removes the unwanted substances.
 
ii. Removal of excess secretory products in the cells
Lysosomes in the cells of the secretory glands play an important role in the removal of excess secretory products by degrading the secretory granules.
 
iii. 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.
Examples of secretory lysosomes:
  1. In cytotoxic T lymphocytes and natural killer (NK) cells, lysosomes secrete perforin and granzymes which destroy both virus infected cells and tumor cells.
  2. In melanocytes, secretory lysosomes secrete melanin.
  3. In mast cells, secretory lysosomes secrete serotonin which is an inflammatory mediator.
 
4. PEROXISOMES
Peroxisomes are otherwise called as microbodies. These are pinched off from endoplasmic reticulum. Peroxisomes contain some oxidative enzymes such as catalase, urate oxidase and D-amino acid oxidase.10
 
Functions of Peroxisomes
Peroxisomes:
  1. Degrade the toxic substances like hydrogen peroxide and other metabolic products by means of detoxification.
  2. Form the major site of oxygen utilization in the cells.
  3. Breakdown the excess fatty acids.
  4. Accelerate gluconeogenesis from fats.
  5. Degrade purine to uric acid.
  6. Participate in the formation of myelin and bile acids.
 
5. CENTROSOME AND CENTRIOLES
The centrosome is situated near the center of the cell close to the nucleus. It consists of two cylindrical structures called centrioles which are responsible for the movement of chromosomes during cell division.
 
6. SECRETORY VESICLES
The secretory vesicles are globular structures, formed in the endoplasmic reticulum, and processed and packed in Golgi apparatus. When necessary, the secretory vesicles rupture and release the secretory substances into the cytoplasm.
 
7. MITOCHONDRION
The mitochondrion (plural ‘mitochondria’) is a rod or oval-shaped structure with a diameter. It is covered by a double layered membrane (Fig. 1.6). The outer membrane is smooth and encloses the contents of mitochondrion. It contains various enzymes such as acetyl-CoA synthetase and glycerophosphate acetyltransferase.
The inner membrane forms many folds called cristae and covers the inner matrix space. The cristae also contain many enzymes and other protein molecules which are involved in respiration and ATP synthesis. Because of these functions, the enzymes and other protein molecules in cristae are collectively known as respiratory chain or electron transport system.
The mitochondria move freely in the cytoplasm of the cell and are capable of reproducing themselves. The mitochondria contain their own DNA which is responsible for many enzymatic actions.
 
Functions of Mitochondrion
 
i. Production of energy
The mitochondrion is called the ‘power house of the cell’ because it produces the energy required for the cellular functions. The energy is produced by oxidation of the food substances like proteins, carbohydrates and lipids by the oxidative enzymes in cristae. During oxidation, water and carbon dioxide are produced with release of energy. The released energy is stored in mitochondria and used later for synthesis of ATP.
 
ii. Synthesis of ATP
The components of respiratory chain in the mitochondrion are responsible for the synthesis of ATP by utilizing the energy through oxidative phosphorylation. The ATP molecules defuse throughout the cell from mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down.
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FIGURE 1.6: Structure of mitochondrion
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iii. Apoptosis
Mitochondria are involved in apoptosis (see below) also.
 
ORGANELLES WITHOUT LIMITING MEMBRANE
 
1. RIBOSOMES
The ribosomes are small granular structures with a diameter of 15 nm. Some ribosomes are attached to rough endoplasmic reticulum while others are present as free ribosomes in the cytoplasm. The ribosomes are made up of proteins (35%) and RNA (65%). The RNA present in ribosomes is called ribosomal RNA (rRNA).
 
Functions of Ribosomes
Ribosomes are called protein factories because of their role in the synthesis of proteins. Messenger RNA passes the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turn arrange the amino acids into small units of proteins. The ribosomes attached with endoplasmic reticulum are involved in the synthesis of proteins like the enzymatic proteins, hormonal proteins, lysosomal proteins and the proteins of the cell membrane.
The free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria.
 
2. CYTOSKELETON
The cytoskeleton of the cell is a complex network that gives shape, support and stability to the cell. It is also essential for the cellular movements and the response of the cell to external stimuli. The cytoskeleton consists of three major protein components, viz.:
  1. Microtubules.
  2. Intermediate filaments.
  3. Microfilaments.
 
Microtubules
Microtubules are straight and hollow tubular structures formed by bundles of globular protein called α- and β-tubulin (Fig. 1.7A).
 
Functions of microtubules
Microtubules:
  1. Determine the shape of the cell.
  2. Give structural strength to the cell.
  3. Act like conveyor belts which allow the movement of granules, vesicles, protein molecules and some organelles like mitochondria to different parts of the cell.
  4. Form the spindle fibers, which separate the chromosomes during mitosis.
  5. Responsible for the movements of centrioles and the complex cellular structures like cilia.
 
Intermediate Filaments
The intermediate filaments form a network around the nucleus and extend to the periphery of the cell. These are formed by fibrous proteins (Fig. 1.7B) and help to maintain the shape of the cell. The adjacent cells are connected by intermediate filaments by desmosomes.
 
Functions or intermediate filaments
Intermediate filaments help to maintain the shape of the cell. These filaments also connect the adjacent cells through desmosomes.
 
Microfilaments
Microfilaments are long and fine thread-like structures, which are made up of non-tubular contractile proteins called actin and myosin (Fig. 1.7C). Actin is more abundant than myosin.12
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FIGURE 1.7: A. Microtubules; B. Intermediate filament; C. Microfilament of ectoplasm.
 
Functions or microfilaments
Microfilaments:
  1. Give structural strength to the cell.
  2. Provide resistance to the cell against the pulling forces.
  3. Responsible for cellular movements like contraction, gliding and cytokinesis (partition of cytoplasm during cell division).
 
NUCLEUS
Nucleus is present in those cells which divide and produce enzymes. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes (e.g. red blood cells). Prokaryotes do not divide or synthesize the enzymes.
Most of the cells have only one nucleus (uninucleated). Few types of cells like skeletal muscle cells have many nuclei (multinucleated). Generally the nucleus is located near the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in different cells.
 
STRUCTURE OF NUCLEUS
 
Nuclear Membrane
The nucleus is covered by a double layered membrane called nuclear membrane. It encloses the fluid called nucleoplasm. Nuclear membrane is porous and permeable in nature and it allows nucleoplasm to communicate with the cytoplasm.
 
Nucleoplasm
It is a gel-like ground substance and contains large quantities of the genetic material in the form of DNA. The DNA is made up of chromatin threads. These chromatin threads become the rod-shaped chromosomes just before the cell division.
 
Nucleoli
One or more nucleoli are present in each nucleus. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. The RNA is synthesized by chromosomes and stored in the nucleolus.
 
FUNCTIONS OF NUCLEUS
Nucleus:
  1. Controls all the activities of the cell.
  2. Synthesizes RNA.
  3. Forms subunits of ribosomes.
  4. Sends genetic instruction to the cytoplasm for protein synthesis through mRNA.
  5. Controls the cell division through genes.
  6. Stores the hereditary information (in genes) and transforms this information from one generation of the species to the next.
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CELL DEATH
The cell death occurs by two distinct processes:
  1. Necrosis.
  2. Apoptosis.
 
APOPTOSIS
Apoptosis is defined as the programmed cell death under genetic control. Originally apoptosis (means ‘falling leaves’ in Greek) refers to the process by which the leaves fall from trees in autumn. It is also called ‘cell suicide’ since the genes of the cell play a major role in the death.
This type of programmed cell death is a normal phenomenon and it is essential for normal development of the body.
 
Functional Significance of Apoptosis
The main function of apoptosis is to remove unwanted cells without causing any stress or damage to the neighboring cells. The functional significance of apoptosis:
  1. Plays a vital role in cellular homeostasis. About 10 million cells are produced every day in human body by mitosis. An equal number of cells die by apoptosis. This helps in cellular homeostasis.
  2. Useful for removal of a cell that is damaged by a virus or a toxin beyond repair.
  3. An essential event during the development and in adult stage. For example, 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.
 
NECROSIS
Necrosis (means ‘dead’ in Greek) is the uncontrolled and unprogrammed death of cells due to unexpected and accidental damage. It is also called ‘cell murder’ because the cell is killed by extracellular or external events. After necrosis, the harmful chemical substances released from the dead cells cause damage and inflammation of neighboring tissues.
 
Causes for Necrosis
Common causes of necrosis are injury, infection, inflammation, infarction and cancer. Necrosis is induced by both physical and chemical events such as heat, radiation, trauma, hypoxia due to lack of blood flow, and exposure to toxins.