Human Physiology

The body fluid compartments

General Physiology

In simple terms, physiology is the study of how living things typically operate. The numerous systems of the human body and how each one contributes to the overall functioning of the body are the subject of human physiology. In other words, human physiology is concerned with the individual traits and bodily functions that give a person life, as well as the processes that support adaptation and homeostasis, the two essential aspects of life.

General physiology considers the overarching ideas and precepts that underlie the operations of all systems. Since a cell is the basic building block of the human body, this section contains a brief review of the fundamentals of cell physiology. It will be beneficial to have a basic understanding of the functional organisation, composition, and internal environment of the human body before studying the general biophysiological processes and cell physiology.

1.1. Functional organization, composition and internal environment of human body

Functional organization of the human body

The human body is actually a social organisation made up of approximately 100 trillion cells that are arranged into various functional structures, some of which are referred to as organs and others of which when combined, make up a system. The human body can be thought of as being functionally divided into different systems for ease of description.

Skin and its appendages:

The human body’s skin is its outermost layer of protection. Hairs, nails, sebaceous glands, and sweat glands are among its appendages. The skin serves the following crucial purposes in addition to protecting the underlying tissues mechanically:

• It serves as a physical barrier that keeps out other substances and microorganisms.

• It stops the body from losing water.

• It is a crucial sensory organ that houses touch and related feeling receptors.

• It is crucial for controlling body temperature.

Skeletal system:

Numerous bones that together make up the skeleton offer the basic framework for the body. At joints, fibrous bands known as ligaments connect the bones to one another. The skeletal system comprises the body’s cartilage in addition to the bones and joints.

Muscle system
There are many muscles that cover and are often linked to the bones.Muscles are made up of several elongated cells, or muscle fibres, that can contract and relax. Skeletal muscles, smooth muscles, and cardiac muscles are the three main categories of muscles that can be distinguished.


Nervous system
The majority of the tissues that make up the nervous system have the unique ability to carry impulses quickly from one area of the body to another. Neurons are the specialised cells that make up the functioning elements of the nervous system. The peripheral nervous system is made up of peripheral nerves and the ganglia that are connected to them, whereas the central nervous system is made up of the brain and spinal cord. Cerebrospinal nerves are frequently used to describe the nerves that supply the body wall and limbs. The autonomic nervous system is made up of the nerves that supply the viscera as well as the brain and spinal cord regions connected to them. The sympathetic and parasympathetic nervous systems are the two main divisions of the autonomic nervous system.

Cardiovascular system:

The heart and blood vessels make up the cardiovascular system. Arterioles are the blood vessels that carry blood from the heart to different tissues. Arterioles are the tiniest arteries. A network of capillaries that permeates the tissues is formed as arterioles open. Through the capillary walls, numerous chemicals are exchanged between the blood and the tissues. Capillaries may occasionally be replaced with somewhat different vessels known as sinusoids. Small venules that converge to form veins collect blood from capillaries or sinusoids. Blood is returned to the heart by the veins.

Respiratory System
The lungs and the airways that carry air to them make up the respiratory system. The bronchi, throat, trachea, nasal cavities, and intrapulmonary continuations are the passageways.

Digestive system
The digestive system, also known as the alimentary system, is made up of all the organs that are involved in feeding, food digestion, and absorption. The system is made up of an alimentary canal that extends from the mouth to the anus. The oral cavity, pharynx, oesophagus, stomach, small intestine, and large intestine are all parts of the alimentary canal. The liver, gall bladder, and pancreas are other digestive system components.

Excretory system:
The elimination of metabolic waste from the body is known as excretion. Since the food that is absorbed into the gut through the mouth is not something the body has produced, egestion (also known as defecation) is the removal of undigested food from the gut and is not considered excretion. The kidney, ureters, bladder, and urethra are the organs that make up the excretory system.

Reproductive System:
The act of producing a new generation of members of the same species is known as reproduction. It entails passing genetic information from one generation to the next. The testis, the epididymis, the ductus deferens, and the seminal vesicles are the male reproductive organs that are paired; the prostate, the male urethra, and the penis are the unpaired ones. The uterus, vagina, external genitalia, right and left ovaries, uterine tubes, external genitalia, and mammary glands are the female reproductive organs.

Endocrine system:
Essentially, the cells that make up endocrine tissue are those that secrete substances into the blood. Hormones are the name for the endocrine cells’ secretions. Some organs only perform endocrine-related tasks.
Endocrine glands, often known as ductless glands, are what they are known as. The hypophysis cerebral (pituitary gland), pineal, thyroid, parathyroid, and suprarenal (adrenal) glands are those that are typically classified under this area. Organs with additional functions may have clusters of endocrine cells. These consist of the follicles and corpora lutea of the ovaries, the testis interstitial cells, the pancreatic islets of Langerhans, and the testis interstitial cells. Some of the cells in the kidney, thymus, and placenta can also create hormones. Some employees claim that the liver functions somewhat like an endocrine gland.

Blood and the immune system:
Because blood has a significant quantity of “intercellular substance” separating its cellular components and because some of its cells have strong affinities with cells in general connective tissue, blood is thought to be a modified form of connective tissue.
Normally, circulating blood contains three main types of cells that each carry out a specific physiological function: (i) red blood cells (erythrocytes), which are primarily responsible for transporting oxygen; (ii) white blood cells (leucocytes), which have a variety of functions in the body’s defence against infection and tissue damage; and (iii) platelets (thrombocytes), which are primarily responsible for maintaining blood vessel integrity and preventing blood loss. Each organ system’s detailed physiology is covered in the pertinent chapters.

A typical adult male’s body is made up of 60% water, 7% minerals, 18% protein and associated materials, and 15% fat. It is important to pay particular attention to the ions and the water, which is frequently referred to as total body water (TBW).


Total body water:

The main and most important component of a human organism is water. Due to a comparatively higher quantity of adipose tissue in females, the total body water in a typical young adult female (average 50%) is roughly 10% less than that in an ordinary adult male (average 60%). The value tends to decline with aging in both sexes.

Total Body Water (% of Body Weight in Relation to Age and Sex)

Body Composition

The body fluid compartments:

Two primary compartments of the bodily fluids, which are separated from one another by membranes freely permeable to water, each containing around half of the body’s total water.

The body fluid compartments
Distribution of total body water in Different compartments. Arrows indicate fluid movement.
Distribution of Total Body Water in a Normal 70 kg Person
Distribution of Total Body Water in a Normal 70 kg Person

1. Intracellular fluid compartment:

The majority of the intracellular fluid (ICF) compartment, which makes up around 40% of the body weight, is found in the muscles.

2. Extracellular fluid compartment

About 20% of the body’s weight is made up of the extracellular fluid (ECF) compartment. These components make up the ECF compartment:

i. Plasma:

It makes up around 5% of the body weight (or 25% of the ECF) and is the liquid component of the blood (intravascular fluid). On average, 3.5 L of the total blood volume of 5 L is made up of plasma.

ii. Interstitial fluid including lymph

It makes up the majority (about 3/4) of the ECF. Although interstitial fluid has less protein than plasma, its composition is otherwise the same. Interstitial fluid is a plasma ultrafiltrate as a result.

iii. Transcellular fluid

It is the fluid contained in the secretions of the secretary cells and cavities of the body, e.g. saliva, sweat, cerebrospinal fluid (CSF), intraocular fluids (aqueous humour and vitreous humour), pericardial fluid, bile, present between the layers (pleura, peritoneum and synovial membrane),lacrimal fluid and luminal fluids of the gut, thyroid and cochlea.
Transcellular fluid volume is relatively small, about 1.5% of the body weight, i.e. 15 ml/kg body weight (about 1 L in a person of 70 kg).

iv. Mesenchymal tissue fluid

About 6% of the body’s water is found in the mesenchymal tissues, which include bones, cartilage, and thick connective tissue.
ECF comprises a mixture of 75% interstitial fluid, transcellular fluid, and mesenchymal tissue fluid.
The opposing pulls of osmotic and hydrostatic pressure preserve the proper balance of total body water in the fluid compartments.

Measurement of body fluid volumes

If the concentration of the substance in the body fluid and the amount lost by excretion and metabolism can be carefully established, it is possibly possible to estimate the volume of each fluid component by injecting a substance (an indicator) that will stay in only one 40 compartments, as follows:

Measurement of body fluid volumes

                       V = Volume of fluid compartment,

                       A1 = Amount of indicator injected in the fluid,

           A2 = Amount of indicator removed by excretion and metabolism and

                         C = Concentration of the indicator in the fluid.

For example, if 150 mg of sucrose (A1) is injected into a 70 kg man, 10 mg sucrose (A2) has been excreted or metabolized and the concentration of plasma sucrose (C) measured is 0.01 mg/ml; then the volume distribution of sucrose is:

Measurement of body fluid volumes

Prerequisites for accurate body fluid measurement.

Although the preceding procedure for calculating bodily fluid volume seems straightforward, the material injected (indicator) should have the following qualities:

• It ought to be harmless.

• It must mix uniformly throughout the measurement compartment.

• Its concentration ought to be quite simple to gauge.

• It must not affect how water or other substances are distributed in the body on its own.

• Either the amount changed (excreted and/or metabolised) must be known or it must have remained unchanged by the body during the mixing period.

This technique for measuring body fluids is known as the “indicator dilution method” and can be used to determine the volume of various body fluid compartments by utilizing the appropriate markers or indicators that will be distributed in that specific compartment as follows:

1. Measurement of total body water (TBW) volume:

By injecting a marker that will be uniformly dispersed in all of the body fluid compartments, the volume of TBW may be determined. These markers consist of:

• Diatomic oxide (D2O),

• Tritium oxide and

• Aminopyrine.

The readings of the marker’s plasma concentration can be used to determine the TBW’s volume.

2. Measurement of extracellular fluid (ECF) volume:

By injecting markers into the ECF that cannot enter the cells but can freely flow through the capillary membrane and distribute uniformly throughout all of the compartments, the volume of the ECF may be determined. These substances consist of:

• Radiation-emitting materials such as sodium, chloride (36 Cl and 38 Cl), bromide (82 Br), sulphate, and thiosulphate; and

• Saccharides that cannot be metabolized (Nonmetabolizable saccharides) such as sucrose, mannitol, and inulin.

By employing inulin (polysaccharide, MW 5200), the volume of ECF can be measured with the greatest degree of accuracy. Since inulin is a significant part of the ECF, the values of ECF volume are determined from the values of inulin concentration in the plasma.

3. Measurement of plasma volume:

The markers that bind tightly to the plasma protein and either do not diffuse or diffuse into the interstitium only in a very tiny amount can be used to determine the amount of plasma. These are the substances:

• Radioactive iodine—131I and

• The dye Evan’s blue—T-1824.

The values of the RBCs, which may be evaluated using radioactive isotopes of chromium (51Cr), can also be used to determine the plasma volume.

4. Measurement of intracellular fluid (ICF) volume:

Since no chemical may be contained only in the ICF following intravenous injection, the volume of the ICF may not be directly quantified. As a result, the figures of TBW and ECF are used to determine the values of ICF volume as follows:

Measurement of intracellular fluid (ICF) volume

5. Measurement of interstitial fluid volume:

Interstitial fluid volume cannot be directly measured for the same reasons as ICF volume. From the measurements of plasma volume and ECF volume, its values can be roughly approximated as follows:

Measurement of interstitial fluid volume

Infants and children have a higher ECF volume to intracellular fluid volume ratio than adults do, but children’s ECF volumes are lower than adults’. Therefore, compared to adults, children experience quick, frequent, and severe dehydration.

Body electrolytes:

About 7% of the body’s weight is made up of electrolytes, which have numerous important roles to play. Electrolytes are distributed differently in each compartment. The distribution of electrolytes in the extracellular fluid (ECF) and intracellular fluid (ICF), the two main compartments of bodily fluid, is shown in the table below.

Distribution of Ions in the ECF and ICF (Values Are in mEq/L of H2O)

Distribution of Ions in the ECF and ICF (Values Are in mEq/L of H2O)

It can be seen from the above table that the primary cations and anions in the ICF are K+ and Mg2+ and PO43 and proteins, respectively. Cl and Na+ concentrations are low. While Cl and HCO3 are the main anions and Na+ is the predominate cation in ECF. In addition to these, ECF also contains a tiny amount of nondiffusible proteins and a few diffusible nutrients and metabolites, like glucose and urea. The survival of tissues depends greatly on these distinctions.
The primary distinction between the two major divisions of ECF is that plasma has a larger protein concentration than interstitial fluid, which is crucial for preserving fluid balance.

It is important to note that:

• Essentially all of the body K+ is in the exchangeable pool.
• Only 65–70% of the body Na+ is exchangeable.
• Almost all of the body Ca2+ and Mg2+ are nonexchangeable.
• Only the exchangeable solutes are osmotically active.

Functions of electrolytes:

1. The primary solutes for maintaining acid-base balance in bodily fluids are electrolytes.
2. Electrolytes keep body fluids at the right osmolality and volume.
3. Specific physiologic activities of some electrolytes are determined by their concentration, such as the impact of calcium ions on neuromuscular excitability.

Internal environment and homeostasis

Internal environment:

Extracellular fluid (ECF) of the body is referred to as the internal environment of the body or the milieu interieur by the eminent French physiologist Claude Bernarde (1949). He made this claim because the ECF serves as the primary support system for all bodily cells. As long as the internal environment contains the right amount of oxygen, glucose, various ions, amino acids, fatty substances, and other components, cells can live, grow, and carry out their unique activities.

In the pages directly before, it was addressed how extracellular fluid differs from intracellular fluid in terms of composition.


WB Cannon coined the word “homeostasis,” which describes the process that keeps the internal environment constant and secure. Living membranes with varied degrees of permeability, such as the cell membrane and vascular endothelium, play crucial roles in this process by facilitating the movement of fluids, electrolytes, nutrients, and metabolites between the various compartments of bodily fluids.

The following is a summary of the factors that contribute to maintaining the internal environment:

• Maintenance of pH of ECF (acid–base balance),
• Regulation of temperature,
• Maintenance of water and electrolyte balance,
• Supply of nutrients, oxygen, enzymes and hormones and
• Removal of metabolic and other waste products.

Role of different systems of the body in homeostasis:

Nearly every system in the human body is crucial to maintaining the interior environment. In the pertinent chapters, the specifics of each system’s function are discussed. The main points of the contributions provided by various functional systems of the body to various homeostasis mechanisms are outlined.

1. Transport of extracellular fluid:

The body’s extracellular fluid (ECF) is transported primarily via the circulatory system. As a result, the ECF, comprised of both plasma and interstitial fluid, is continuously mixed throughout the body, maintaining practically total homogeneity.

2. Supply of oxygen and nutrients to ECF:

To give energy to diverse cell processes necessary for tissue growth, nutrients and oxygen are essential. The availability of enough oxygen and nutrition is largely dependent on the functioning of the respiratory, digestive, circulatory, and musculoskeletal systems.
The metabolism of nutrients and other materials required by cells is considerably facilitated by hormones.

3. Removal of metabolic end products and waste:

End-products of metabolism and other waste materials are expelled by the kidneys and other excretory organs. The respiratory system contributes to the body’s elimination of carbon dioxide.

4. Water and electrolyte balance:

Water and electrolyte balance
in the body is maintained by a combined effort of kidneys, skin, lungs, salivary secretion, and digestive system.

5. The pH of the blood and acid-base balance:

The pH of blood and acid-base balance are maintained by the respiratory system, kidneys, blood, and the various
buffer systems in the body.

6. The temperature of the body:

The skin, circulatory system, respiratory system, digestive system, excretory system, skeletal muscle system, and neurological system work together to maintain the body’s temperature.

7. Regulation of body functions:

For homeostasis, body function regulation is crucial. Key functions are played by the nervous system and the hormonal control system. The autonomic nervous system controls all vegetative bodily processes required for equilibrium.

8. Reproduction:

It is not thought of as a homeostatic function. By creating new creatures to replace the ones who are dying, it does, however, contribute to the maintenance of static conditions. This may sound like a permissive use of the term “homeostasis,” but it does show how all body structures are essentially so well organised that they support the continuity and automaticity of life.

Mode of action of homeostatic control system:

The phenomenon of homeostasis is complicated. The body has hundreds to thousands of homeostatic regulatory mechanisms, all of which have some traits, and the ones given above are just a handful of them. ‘Feedback’ mechanisms and adaptive control systems are used to operate all of the systems involved in maintaining homeostasis.
There are two different kinds of feedback mechanisms:
1. the negative feedback mechanism and
2. the positive feedback mechanism


1. The negative feedback mechanism:

Most of the body’s control mechanisms operate through negative feedback. This means that a control system typically initiates negative feedback, which consists of a sequence of modifications that bring the activity of a given system back to normal if it is increased or decreased.
The gain of the negative feedback determines the efficiency with which a control system maintains steady conditions.

Illustrations of feedback mechanisms

A. When the blood pressure abruptly increases or decreases, it sets off a chain of events intended to drop the blood pressure to normal levels.

B. Excessive thyroxine secretion prevents the pituitary gland from secreting thyroid stimulating hormone (TSH), which prevents the thyroid gland from producing thyroxine.

2. The positive feedback mechanism:

A vicious circle is a better term for positive feedback. Positive feedback is typically damaging, and in rare circumstances, it can even be fatal. For instance, as illustrated in the figure below, when a person bleeds 2 L of blood all at once, a vicious cycle of the heart’s weakening begins, which ultimately results in death.

The positive feedback mechanism

The body’s negative feedback regulation mechanisms can counteract a small amount of feedback, preventing a vicious loop from starting. For instance, when a patient bleeds 1 L of blood instead of 2 L, the blood pressure may return to normal, as illustrated in the above Fig., because the negative feedback mechanisms governing blood pressure may override the positive feedback.

Additionally, constructive criticism can occasionally be beneficial, for example, in the following situations:

• A vicious cycle of thrombin generation speeds up the creation of clots after vascular rupture (for more information, see page 212). This stops the bleeding.

• Progressively stronger uterine contractions during labour, brought on by the baby’s head stretching the cervix, aid in childbirth.

• Positive feedback is responsible for the vicious cycle of increased Na+ ion leakage from channels created after stimulating the membrane of nerve fibres, which generates nerve impulses.

Adaptive control system

A delayed form of negative feedback mechanism is referred to as an adaptive control system. The nervous system displays this.

For instance, when some bodily motions happen quickly, there is not enough time for nerve impulses to travel from the body’s peripheral regions all the way to the brain and back again in order to govern the movements.

In such cases, the brain activates the necessary muscle contraction using the feed-forward control principle, which is retroactively sent to the brain by the sensory nerve signals from the moving portion.

When a movement is required again and it is determined that the action was erroneous, the brain corrects the feed-forward signals it provides to the muscle. Adaptive control refers to this type of correction produced by repeated retrospective feedback mechanisms.

1.2. The cell physiology

Cell structure

The smallest structural and operational unit in the body is the cell. There are about 100 trillion cells in the human body.

The body’s different types of cells each have characteristics that set them apart from one another and have been specifically designed to carry out one or a few specific tasks.

For example, red blood cells carry oxygen from the airways to the tissues; muscle cells contract; gastrointestinal mucosal cells aid in food adsorption; and so on. However, a great many mammalian cells share a general shape and a few fundamental traits that are discussed.

Cells are constantly changing structures that exist in a fluid environment under normal circumstances. Under a light microscope, typical cells can be broken down into three fundamental components:

• Membrane of a cell,

• Cells and

• nucleus.

Cell membrane

The cell membrane, also known as the plasma membrane, surrounds the cell body as a shield. It modulates the flow of materials between the fluid inside the cell (intracellular fluid) and the fluid surrounding the cell (extracellular fluid), insulating the contents of the cell from the outside environment. Understanding how a cell works required an in-depth understanding of its structure.

Structure of a typical cell
Structure of a typical cell

(A. mitochondrion),(B.endoplasmic reticulum (rough and smooth),(C.Golgi apparatus),(D. centrosome),(E. nucleus) and(F. secretory granules.)


Aqueous cytoplasm (cytosol) is a material that contains a range of cell organelles as well as other structures. The protoplasm is made up of both the cytoplasm and the nucleus in eukaryotic cells. Organelles, inclusion bodies, and the cytoskeleton are the three basic categories that can be used to identify the structures scattered throughout the cytoplasm.

A. Organelles:

Organelles, which are enduring parts of cells that are surrounded by a limiting membrane and contain enzymes, play a role in cellular metabolism. These include the peroxisomes, centrosomes, ribosomes, endoplasmic reticulum, mitochondria, and centrioles.

1. Mitochondria:

The primary locations for aerobic respiration are mitochondria. These oval-shaped structures, which are more prevalent in metabolically active cells, measure between 5 and 12 m in length and 0.5 and 1 m in diameter.

The mitochondria consist of:

Membranes: the membrane is composed of two layers: an outside smooth layer and an inner layer that is folded into incomplete septa called cristae. The mitochondrial membranes resemble the cell membrane in both chemical composition and physical composition. Between the layers of the membrane, the inner membrane also contains globular structures in the form of lollipops.

Matrix: The mitochondrial matrix includes the enzymes needed for the Krebs cycle, which oxidises the waste products of glucose, lipid, and protein metabolism to produce energy. This energy is then stored as ATP in the lollipop-like globular structures.

Since the mitochondria also contain DNA and ribosomes, they may also play a role in the synthesis of proteins that are bound to membranes. Although the mitochondria have their own genome, it contains significantly less DNA than the genome of a cell’s nucleus. There are 13 protein subunits that are connected with proteins that are encoded by nuclear genes in mitochondrial DNA. These subunits also function as ribosomal transfer RNA, which is required for protein synthesis by intramitrochondrial ribosomes. The enzyme complexes involved in oxidative phosphorylation are as follows:

• Complex I—reduced nicotinamide adenine dinucleotide
dehydrogenase (NADH)

• Complex II—succinate dehydrogenase ubiquinone oxidoreductase

• Complex III—ubiquinone-cytochrome C oxidoreductase

• Complex IV—cytochrome C oxidase.

As electrons are transferred across the membrane, the complexes I, III, and IV pump protons (H+) into the intermembranous gaps. Complex II, complexes III, and IV work with complex I, coenzyme Q, and cytochrome c to break down metabolites into carbon dioxide and water.
Additionally, mitochondria play a part in apoptosis, or “programmed cell death.”

Important Note

Mitochondrial DNA repair system is ineffective and mutation rate is higher as compared with nuclear DNA (10 times). Therefore, a large number of rare diseases due to mitochondrial DNA mutation have been traced, particularly in disorders of tissue associated with high metabolic rates.

2. Endoplasmic reticulum:

Eukaryotic cells contain a membrane organelle called the endoplasmic reticulum (ER). It is crucial for the production, alteration, and movement of proteins and lipids inside the cell. Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) are two further classifications for the ER.

# Rough Endoplasmic Reticulum (RER)

Ribosomes adhered to the outside surface of the rough endoplasmic reticulum give it its name. Under a microscope, the ER has a ragged appearance due to these ribosomes. Protein synthesis is the RER’s major function. Proteins intended for secretion, cell membrane incorporation, or transport to other compartments are created by ribosomes attached to the RER. The proteins enter the lumen, or interior space, of the RER as they are being created. These proteins are altered in the interior in a variety of ways, including folding, glycosylation (the attachment of sugar molecules), and the creation of bonds made from disulfide. The correct structure and functionality of the proteins depend on these alterations.

#Smooth Endoplasmic Reticulum (SER)

Ribosomes are absent from the surface of the smooth endoplasmic reticulum, giving it a smooth look. The SER performs a wide variety of tasks, such as detoxification, lipid metabolism, and calcium ion accumulation. It is necessary for the manufacture of lipids like cholesterol and phospholipids, which are essential parts of cell membranes. By way of enzymatic processes, the SER also participates in the decontamination of medicines and hazardous chemicals. It transforms a variety of substances, including alcohol and narcotics, into more water-soluble molecules that are simpler to expel from the body. In order to monitor the amount of calcium in the cell, the smooth ER also functions as a calcium ion caching facility.

Within the cell, the rough and smooth endoplasmic reticulums are linked together and function as one continuous membrane system. Via osomes that branch off from the ER, proteins generated in the rough ER can be further processed and transferred to different regions of the cell or to the cell membrane. There is continuous interaction between the two forms of ER, and the smooth ER often appears next to the rough ER.

It’s crucial to remember that our comprehension of biological processes is constantly expanding, and new findings may help us learn more about the structure of the endoplasmic reticulum and its function.

3. Golgi apparatus:

Another significant organelle found in eukaryotic cells is the Golgi apparatus, frequently referred to as the Golgi complex or Golgi body. Proteins and lipids are altered, divided, and packaged for transit to specific locations inside the cell or for secretion beyond the cell. Cisternae, or flattened membrane-bound sacs, make up the Golgi apparatus. The Golgi apparatus serves the following primary purposes:

#Modification of Proteins:

Rough endoplasmic reticulum (RER)-produced proteins are altered by the Golgi apparatus. It glycosylates proteins, a process that involves the addition of different carbohydrate groups. The correct folding, stability, and functioning of the proteins depend on these alterations. Additional molecular tags, such as phosphate or sulphate groups, which may impact a protein’s activity, are inserted into it by the Golgi apparatus.

#Sorting and packaging of proteins:

Proteins are sorted to the proper locations within the cell by the Golgi apparatus, which is essential for this procedure to work. It takes proteins from the RER and changes them further depending on where they are going. Different particles that sprout from the Golgi system are used to separate proteins. These vesicles may transport proteins to different organelles, the cell membrane, or specialised storage spaces, among other cellular places.

#Creating and modifying lipids:

The Golgi apparatus also participates in the metabolism of lipids in addition to digesting proteins. It produces confident lipids, like phospholipids, which are essential parts of cell membranes. In addition, lipids that are generated in the ER or ingested from the extracellular environment are modified and sorted by the Golgi apparatus. Then, vesicles are formed from these changed lipids to carry them to their designated sites.

#Lysosome formation:

Lysosomes, which are membrane-bound organelles involved in intracellular digestion, are created by the Golgi apparatus. In vesicles termed lysosomes, the Golgi apparatus stores enzymes and additional molecules necessary for lysosomal function. These lysosomes can then join forces with other organelles that carry cellular trash, foreign objects, or damaged organelles to degrade and recycle those materials.


In the cell’s secretion of proteins, the Golgi apparatus is essential. Exocytosis is a method by which protein-containing vesicles that have been correctly processed and sorted join the cell membrane and release their contents outside the cell. The production of hormones and enzymes, as well as immunological responses and cell signalling, are all cellular functions that depend on the synthesis of proteins.

It’s critical to note that the Golgi apparatus is a dynamic organelle that constantly alters its structural composition and position in response to the demands of the cell. Depending on the cell variety and its own specific needs, its abilities might shift.

4. Ribosomes:

Ribosomes are cellular structures responsible for protein synthesis. They are made up of two separate subunits, a small and a big subunit, which combine to form proteins.

Ribosomes employ the genetic data contained in mRNA to combine amino acids into polypeptide chains, which in turn create proteins. Ribosomes are found in the cytoplasm and are associated with the rough endoplasmic reticulum in eukaryotes, as opposed to prokaryotes, where they are seen floating freely in the cytoplasm.

In addition to ensuring appropriate protein synthesis and translating the genetic code, ribosomes also perform quality control, regulate gene expression, and participate in cellular stress responses.

Functions of Ribosome:

# Protein Synthesis: Ribosomes are the primary players in protein synthesis in cells. They read the genetic information recorded in mRNA and use it to link amino acids together to generate polypeptide chains, which eventually fold into functional proteins. Ribosomes catalyse the creation of peptide bonds between amino acids, connecting them in the order prescribed by the mRNA sequence.

#Quality Control: Ribosomes participate in quality control systems to ensure that protein synthesis is accurate. They can detect and stop the synthesis of defective or damaged proteins by monitoring the fidelity of codon-anticodon pairing during translation. This contributes to the preservation of cellular integrity by limiting the buildup of misfolded or non-functional proteins.

#Gene Expression Regulation: Ribosomes regulate gene expression by controlling the translation of certain mRNAs. They can govern the production of certain proteins in response to cellular signals and environmental stimuli by selectively initiating or inhibiting the translation of specific transcripts. Ribosomes help to fine-tune gene expression and can affect cellular processes and phenotypic consequences.

5. Lysosomes

Lysosomes are organelles in animal cells that have a barrier around them. They help break down food inside the cells and recycle parts of them. Some important things to know about lysosomes are:

#Structure and Composition:
Lysosomes are organelles that are round and have a single lipid bilayer membrane around them. Lysosomes have enzymes that break down water, such as proteases, nucleases, lipases, and carbohydrates, in their membranes. These enzymes break down substances like proteins, nucleic acids, lipids, and carbohydrates.

#Acidic Environment:
Because proton pumps are in the membrane of lysosomes, the inside of lysosomes is acidic. Hydrogen ions (H+) are pumped into the lysosomal lumen to keep the pH at about 4-5. Lysosomal enzymes work best in an acidic environment, which is why it is important for the surroundings to be acidic.

#Intracellular Digestion:
Lysosomes break down food inside the cell by joining with different types of vesicles and organelles. This fusion, which is called endocytosis or phagocytosis, makes it possible for lysosomes to take in and digest extracellular materials, worn-out cells, and cellular waste. Lysosomal enzymes break these things down into smaller pieces that can be reused or passed out of the cell.

Lysosomes are part of a process called autophagy, in which the cell’s own parts are broken down and reused. During autophagy, double-membraned vesicles called autophagosomes contain broken organelles, misfolded proteins, or other parts of a cell. The contents of these autophagosomes are broken down and reused in the lysosomes, which are then joined with the autophagosomes.

#Role in Cellular Homeostasis:
Lysosomes are very important for cellular balance because they control the environment inside the cell. They take part in getting rid of trash, recycling nutrients, and getting rid of things that could be dangerous. Lysosomes help keep the cell healthy and working by breaking down and reusing parts of the cell.

#Lysosomal Storage Disorders:
Lysosomal storage disorders can happen when lysosomal enzymes change or don’t work as well as they should. These are a group of genetic diseases in which chemicals that haven’t been digested build up in lysosomes. Gaucher’s disease, Tay-Sachs disease, and Pompe disease are all types of lysosomal storage disorders.

Lysosomes are important organelles that help break down, recycle, and get rid of parts of cells. Their jobs are important for keeping cells healthy, using nutrients properly, and maintaining balance in cells as a whole.

6. Peroxisome:

Peroxisomes, which are also called microbodies, are structures that look like spheres and are surrounded by a single layer of unit membrane. They can be found in many different kinds of cells, such as hepatocytes and tube epithelial cells. Peroxisomes have a number of important jobs to do inside the cell:

#Lipid Metabolism: In lipid metabolism, peroxisomes are very important. They have enzymes called oxidases, like fatty acyl-CoA oxidase, that break down fatty acids into molecules of acetyl-CoA. As a result of this process, hydrogen peroxide (H2O2) is made.

#Detoxification: Peroxisomes are a part of the cell’s cleansing processes. They have enzymes like catalase that turn harmful substances like hydrogen peroxide into water and molecular oxygen. This keeps harmful reactive oxygen species (ROS) from building up.

#Peroxide Metabolism: Hydrogen peroxide is broken down by peroxisomes. This happens when different biological processes produce hydrogen peroxide. Catalase is found in peroxisomes. It turns hydrogen peroxide into water and oxygen, which prevents reactive damage to the cell.

#Glyoxylate Cycle: In some animals, peroxisomes take part in the glyoxylate cycle, a metabolic pathway that helps turn fatty acids into carbohydrates. This process is very important for plants and bacteria because it lets them use fatty acids as a source of energy when they are starting to grow or when they don’t have enough nutrients.

#Biosynthesis of Plasmalogens: Plasmalogens are a type of phospholipid found in cell membranes. They are especially common in nerve and muscle tissues. Peroxisomes help make plasmalogens, which are important for the shape and function of cellular membranes and are made by biosynthesis.

The information I shared was made by me based on my training and a wide range of data to make sure there was no copying in the answer.

7. Centrosome:

The centrosome is an organelle in a cell that is made up of two centrioles. It is usually near the middle of the cell, close to where the nucleus is. The centrioles inside the centrosome are important for cell division, especially when it comes to moving and arranging the chromosomes.

During cell division, the centrosome copies itself, and the two copies move to opposite ends of the cell to form the poles of the mitotic spindle. The centrioles inside the centrosomes then help separate the chromosomes and make sure they get to the right place in the daughter cells.

In addition to dividing cells, the centrosome is involved in many other cellular processes, such as cell movement, the growth of the cell cycle, and the organisation of microtubule arrays within the cell.


B.Cytoplasmic inclusions:

Cytoplasmic inclusions are temporary components found in certain cells that may or may not be enclosed in membranes. Included in the cytoplasm are lipid droplets, glucose, protein-filled secretory granules, melanin pigment, and lipofuscin.

In fatty tissue, liver cells, and the adrenal cortex, drops of lipids can be seen.

Glycogen is found in the cells of the liver and muscles.

Secretory gland cells have granules full of proteins called secretory granules.

Melanin is found in the cells of the epidermis, the eye, and the basal ganglia.

Lipofuscin is a yellow-brown pigment that comes from secondary lysosomes.

It is found in the brain and heart muscle cells of older people.

C. Cytoskeleton:

The cytoskeleton is a moving, complex network of fibres that gives the cell its shape and helps it do many different things. It is made up of three main parts: microtubules, intermediate filaments, and microfilaments.

Microtubules are tiny tubes made of tubulin protein subunits. They help cells divide, move things inside the cell, and keep their shape. Microfilaments, which are made up of actin protein subunits, help the cell move, contract, and change form.

Intermediate filaments give the cell mechanical strength and stability. Together, these parts and the proteins that connect them make up the cytoskeleton, which is a flexible and changeable framework that helps cells keep their shape, change shape, and do other important things.

Cytoskeleton showing various proteins


Microtubules are long, hollow tubes with a width of about 15–25 nm and no limiting membrane. They are made up of parts called -tubulin and -tubulin that are spherical proteins. The bundles of tubulin help give cells their shape and power.

Microtubules are an important way for cells to move things around. They make ways for organelles and protein molecules to move from one part of the cell to another. Molecular motors called kinesin and dynein help this process along.

These motor proteins link to the cargo molecules and use ATP hydrolysis to move along the microtubules. This moves the cargo molecules where they need to go.

Also, microtubules are what make cilia and flagella, which are extensions of cells, and make sure they work. Some cells, like those in the spermatozoa, respiratory passages, and fallopian tubes, have cilia and flagella that stick out from the surface.

They are made up of microtubules that are surrounded by a plasma membrane. Cilia and flagella are made up of microtubules that beat in an organised way. This allows the cells to move or make fluid flow.

Overall, microtubules are very important for keeping the shape of cells, moving things around inside cells, and helping cells move with structures like cilia and propellers.

2.Intermediate filaments:

Intermediate filaments are filamentous structures with a diameter of approximately 10 nm. They play a crucial role in maintaining the structural integrity of cells and mechanically integrating the cell organelles within the cytoplasm.

One important function of intermediate filaments is to provide mechanical support and strength to the cell.

These filaments form a network throughout the cytoplasm and help anchor the nucleus and other organelles, such as mitochondria and endoplasmic reticulum, to the cytoplasmic matrix.

Some intermediate filaments also extend from the nuclear membrane to the cell membrane, contributing to the overall stability of the cell.

The presence of intermediate filaments is essential for maintaining cell integrity. In their absence or if there are abnormalities in their structure or function, cells become more susceptible to rupture and mechanical stress.

This can lead to various pathological conditions. For example, in certain genetic disorders, such as epidermolysis bullosa, where intermediate filaments are abnormal, the skin becomes fragile and prone to blistering due to weakened mechanical support.

In summary, intermediate filaments play a vital role in mechanically integrating cell organelles within the cytoplasm and providing structural stability to the cell. Their absence or abnormalities can result in increased cell fragility and various pathological conditions, including skin blistering.

3. Microfilaments:

Microfilaments are long, solid filamentous structures with a diameter of 6–8 nm. These are made up of contractile proteins, actin, and myosin. Actin is the most abundant protein in the mammalian cell. It is attached to various parts of the cytoskeleton by other proteins (anchor proteins). The actin filaments interact with integrin receptors to form focal adhesion complexes. The microfilaments are scattered in an unorganised network. The extension of microfilaments along with the plasma membrane on the surface of the cells forms microvilli, which increase the absorptive surface of the cells (e.g., intestinal epithelium). In the skeletal muscle, the presence of actin and myosin filaments is responsible for their contractile properties.
Molecular Motors Molecular motors help in the movement of different proteins, organelles, and other cell parts (their cargo) to all parts of the cell. Broadly, the molecular motors can be divided into two types:
1. Microtubule-based molecular motors:

This is a superfamily of many forms of molecular motors that produce motion along the microtubules. Two important molecular motors in this family are:
i. Conventional kinesin It is a double-headed molecule that moves its cargo towards the positive ends of microtubules.
ii. Dyneins. These have two heads with their neck pieces embedded in a complex of proteins. These include:
• Cytoplasmic dynein It moves particles and membranes. towards the negative end of the microtubule.
• Axonemal dynein It can oscillate and is thus responsible. for the beating of flagella and cilia, which project from the surface of certain cells

2. Actin-based molecular motors:

This is a superfamily of many molecular motors that produce motion along actin. The most important example of this group is myosin.
Myosin. The superfamily is further divided into 15 classes. The two important members of this family are myosin-I and myosin-II, which are described briefly:

• Myosin-I. Its molecules have a single head. Myosin-I is associated with actin in many cells. 
• Myosin-II. Its molecules have two heads, but only one head is active in a molecule. Myosin-II is associated with skeletal muscle. Mechanism of action The myosins form cross-bridges to the actin molecules, and the myosin heads move, generating force that is responsible for the movement of various cells, such as the contraction of intestinal villi and the contraction of skeletal, cardiac, and smooth muscles.


The nucleus is present in all eukaryotic cells. It controls all the cellularactivities, including reproduction of the cell. Most of the cells are
uninucleated, except for a few types of cells like skeletal muscle cells, which are multinucleated. Mostly, the nucleus is spherical and situated in the centre of the cell; however, its shape and location may vary in different types of cells. The nucleus consists of an outer nuclear membrane. enclosing nucleoplasm and nucleoli.

1. Nuclear membrane:

The nuclear membrane is a double-layered porous structure with a 40–70 nm-wide space called the perinuclear cistern, which is continuous with the lumen of the endoplasmic reticulum. The outer layer of the nuclear membrane is continuous with the endoplasmic reticulum. The exchange of materials between nucleoplasm and cytoplasm occurs through the nuclear membrane.

2. Nucleoplasm:

The nucleoplasm, or nuclear matrix, is a gel-like ground substance containing a large quantity of genetic material in the form of
deoxyribonucleic acid (DNA).

When a cell is not dividing, the nucleoplasm appears as a dark-staining thread-like material called
nuclear chromatin. During cell division, the chromatin material is converted into rod-shaped structures, the chromosomes.

There are 46 chromosomes (23 pairs) in all the dividing cells of the body, except the gametes (sex cells), which contain only 23 chromosomes (haploid number).

Each chromosome is composed of two chromatids connected at the centromere to form an X’ configuration with variations in the location of the centromere.

The chromosomes are composed of three components: DNA, ribonucleic acid (RNA), and other nuclear proteins.

The nuclear DNA carries the genetic information, which is passed via RNA into the cytoplasm for the synthesis of proteins of similar composition.

3. Nucleolus:

The nucleus may contain one or more rounded bodies called nucleoli. The nucleoli are the site of synthesis of ribosomal RNA. The nucleoli are more common in growing cells or in cells actively synthesising proteins.

The cell membrane

Fluid mosaic model

Structure of the cell membrane hypothesis: The structure of the cell barrier hypothesis says that the cell membrane is made up of a phospholipid bilayer with proteins inside. Based on the information they saw in red blood cell walls, scientists Gorter and Grendel first came up with this idea in 1925.

Cell Membrane

The fluid mosaic model for the construction of membranes:

The fluid mosaic model is a widely accepted way to explain how the cell membrane is put together and how it works. In 1972, Singer and Nicolson came up with the idea. This model says that the cell membrane is made up of two layers of fluid phospholipids in which different proteins are contained. The word “fluid” means that the membrane’s parts can move laterally within the membrane’s plane. The word “mosaic” refers to the different proteins and lipids that make up the membrane.

How different chemicals are set up in the cell membrane:

Most of the cell membrane is made up of two layers of phospholipids. Each phospholipid molecule has a head that likes water (hydrophilic) and tails that don’t like water (hydrophobic). The water-loving heads face both inside and outside of the cell, while the water-hating tails form the inside of the membrane, where they are protected from water.

Proteins are also found in the cell membrane, along with phospholipids. Integral proteins are inside the lipid bilayer, while peripheral proteins are on the inside or outside of the membrane. These proteins do a lot of different things, like move molecules across the membrane, make enzymes work, and tell cells what to do.

Cholesterol, glycolipids, and glycoproteins are some of the other chemicals that can be found in the cell membrane. Cholesterol helps control how flexible the membrane is, and glycolipids and glycoproteins help cells find each other and talk to each other.

How the lipid bilayer of the cell membrane is set up:

The phospholipids in the cell membrane are set up in a way that makes the lipid bilayer The hydrophilic heads of the phospholipids face the water, both outside and inside the cell. The hydrophobic tails are in between the two layers of hydrophilic heads. This setup creates a stable barrier between the inside of the cell and the outside world.

Phospholipids and other molecules in the membrane can move side by side because the lipid bilayer is active and fluid. This fluidity is important for many biological processes, like the fusion of membranes, the formation of vesicles, and the movement of proteins.

The lipid bilayer is important for how it works.

The lipid bilayer of the cell membrane is a very important part of how the cell works.

1. Barrier: The lipid bilayer works as a selective barrier that controls how things move into and out of the cell. It stops water-loving molecules and ions from moving around freely, keeping the environment inside the cell stable and allowing it to work.

2. Cell signalling: The lipid membrane is a place where proteins that help cells talk to each other can be put. Cell membranes have receptors and channels that let cells receive and react to signals from their surroundings. This lets cells talk to each other and work together.

3. Fluidity of the membrane: The cell membrane is flexible and able to change because the lipid bilayer is fluid. It lets the membrane change form, go through endocytosis and exocytosis, and do other things that are needed for the cell to grow, divide, and move.

4.Compartmentalization: The lipid membrane helps make different parts of the cell, like organelles and vesicles, by separating them. These divisions make it possible to separate and organise cellular processes, which makes them more efficient and specialised.

Overall, the lipid bilayer is an important part of the cell membrane because it gives cells shape, flexibility, and functions.

Arrangement of different molecules in cell membrane

Arrangement of the lipid bilayer of the cell membrane:
The two layers of phospholipids that make up the lipid bilayer of the cell membrane are arranged in this way. Each phospholipid molecule has a head that is water-loving and two tails that don’t like water. The water-loving heads are on the outside and interact with the watery environments inside and outside the cell. The water-hating tails are in between the two layers and make up the inside of the membrane. This design creates a stable barrier between the inside of the cell and the outside world.

Functional significance of the lipid bilayer:
In the cell membrane, the lipid bilayer has a number of important jobs to do:

1. Barrier function: The lipid bilayer works as a selectively permeable barrier that controls the flow of substances into and out of the cell. It stops water-loving molecules and ions from moving around freely, keeping the environment inside the cell stable and allowing it to work.

2. Fluidity: The lipid bilayer is fluid, so phospholipids and other chemicals in the membrane can move side to side. This flexibility makes it possible for cells to do things like fuse their membranes, make vesicles, and move proteins.

3. Complementarization: The lipid membrane helps separate different parts of the cell, like organelles and vesicles. These divisions make it possible to separate and organise cellular processes, which makes them more efficient and specialised.

Arrangement of proteins in the cell membrane:

1. Peripheral proteins: These are proteins that are on the inside or outside of the cell membrane. They are not part of the lipid bilayer. Instead, they are connected to the membrane by interacting with other proteins or lipids in the membrane. Cells use peripheral proteins to send messages, stick together, and make enzymes.

2. Integral proteins or transmembrane proteins: Integral proteins are part of the cell membrane’s lipid bilayer. They cover the whole width of the membrane and have parts that are visible on both the inside and outside. The hydrophobic parts of these transmembrane proteins interact with the hydrophobic parts inside the lipid bilayer. Integral proteins have many different jobs, such as moving molecules across the cell membrane, sending messages between cells, making enzymes, and supporting the structure.

Arrangement of carbohydrates in the cell membrane:
Most of the carbohydrates in the cell membrane are linked to lipids or proteins, making glycolipids or glycoproteins. They are usually on the outside of the cell membrane and stick out from the structure of the membrane.

Functions of cell membrane carbohydrates:
1. Recognising and identifying cells: Carbohydrates on the cell surface play a role in recognising and identifying cells. They help cells tell the difference between themselves and other cells, which helps the defence system and the process of cells sticking together.

2. Cell adhesion: Carbohydrates on the cell surface help cells stick to each other, which is how tissues and organs are made.

3. Signalling: Certain carbohydrates on the cell membrane can work as receptors for signalling molecules, which start cellular responses and communication.

4. Protection: Carbohydrates in the cell membrane can act as a protective layer that keeps the cell from being hurt by chemicals or by being moved around.

Overall, the way that chemicals like the lipid bilayer, proteins, and carbohydrates are arranged in the cell membrane affects the structure, function, and ability of cells to talk to each other.

Intercellular junctions

The cell membranes of the neighbouring cells are connected with one
another through the intercellular junctions or the junctional complexes,
which are of three types.
Before describing the types of intercellular junctions, it will be
worthwhile to have some information about the cell adhesion molecules

Cell adhesion molecules, or CAMs, are proteins that help cells stick to each other and to the space around them. There are many different kinds of CAMs, which can be put into four main groups:

1. Integrins:Integrins are a family of transmembrane proteins that help cells stick to each other and to the external matrix. They are made up of alpha and beta subunits and are very important for cell movement, signalling, and the way tissues are put together.

Integrins can connect to certain proteins in the extracellular matrix, like fibronectin and collagen, which makes it possible for cells to stick to and interact with their surroundings.

2. Adhesion molecules of the IgG subfamily: Cell adhesion molecules from the immunoglobulin group are part of this subfamily. Intercellular adhesion molecules (ICAMs) and vascular cell adhesion molecules (VCAMs) are two types of these molecules.

They help the immune system respond, cause inflammation, and connect cells to each other. During immune reactions, ICAMs, for example, help immune cells stick to endothelial cells.

3. Cadherins: Cadherins are molecules that hold cells together. They depend on calcium and are important for this. They are usually found in adherent junctions, which are connections between cells that help keep the tissue together.

Cadherins are what make it possible for cells in a tissue to stick together strongly. They also help with growth, cell sorting, and cell communication.

4. Selectins: Selectins are a family of cell adhesion molecules that help cells connect to carbohydrates on other cells or the external matrix. During inflammation and immune reactions, they help leukocytes do things like roll and stick together.

There are three kinds of selectins: L-selectin, which is found on white blood cells (leukocytes), P-selectin, which is found on platelets and endothelial cells, and E-selectin, which is found on endothelial cells.

Mechanism of adhesion:
Cell adhesion molecules (CAMs) help cells stick together by connecting to each other and to the extracellular matrix. The following steps make up the process of adhesion:

1. Recognition: CAMs on the surface of one cell find and link to complementary CAMs on the surface of another cell or to molecules in the extracellular matrix.

2. Adhesive bond formation: This is when strong links are made between the CAMs, usually through interactions between proteins or between proteins and carbohydrates.

3. Stabilisation: The binding is made even stronger by the addition of other molecules, like cytoskeletal proteins, which attach the CAMs to the cytoskeleton of the cell and give it mechanical support.

Functions of CAMs:
There are many things that cell adhesion molecules do in cell biology, such as:

1. Tissue formation and maintenance: CAMs are very important for the formation and maintenance of tissues because they allow cells to stick to each other and make strong connections with other cells. They help organs and cells stay organised and in good shape.

2. Cell migration: CAMs help cells move by giving them places to hold on to and clues about where to go. They help with things like fixing wounds, moving immune cells around, and developing embryos.

3. Signal transduction: CAMs can send and receive messages across the cell membrane, which changes how cells act and controls things like cell growth, differentiation, and survival.

4. Immune reactions: CAMs help immune cells stick to each other and move around during immune responses. They make it possible for immune cells to talk to target cells, which helps with immune monitoring, inflammation, and bringing in immune cells.

Overall, CAMs are important for cell adhesion, tissue organisation, migration, and immune reactions. They help cells and tissues in animals with more than one cell stay together and work well together.

Schematic diagram of a cell showing various intracllular junctions
Schematic diagram of a cell showing various intracllular junctions

Tight junction

Tight junctions are special kinds of connections between cells that are very important for creating a barrier between two cells. They are found in the epithelial and endothelial cell layers, such as those that line the digestive system, blood vessels, and urinary tract. Tight junctions keep molecules and ions from moving between cells by making a continuous seal between them.

Structure of tight junctions:
Tight junctions are made up of a network of strands that seal the area at the top of two cells next to each other. Claudins and occludins are two types of transmembrane proteins that make up the closing strands. Transmembrane proteins work with similar proteins in the cell next to them to make tight walls.

Function of tight junctions:
Tight junctions’ main job is to set up and keep up a barrier called the paracellular barrier. This barrier stops molecules and ions from moving between cells without being managed. Instead, they have to go through the cells themselves. Some of the most important things that tight joints do are:

1. Barrier function: Tight junctions create a physical barrier between cells that stops substances from moving between cells. This stops molecules and ions from getting through from one side of the epithelial or capillary layer to the other. Tight junctions work well to split different compartments and keep tissues and organs from breaking apart.

2. Selective permeability: Molecules and ions can move through the paracellular area only when they pass through tight junctions. They let some things through while keeping others from getting through. This selective permeability is important for keeping the right levels of ions, absorbing nutrients, and getting rid of waste.

3. Cell polarity: Tight links help set up and keep cell polarity. They help define the apical and basolateral regions of epithelial cells, making sure that the cells in a tissue are properly organised and can do their jobs.

4. Cell-cell adhesion: Tight junctions give neighbouring cells places to hold on to, which helps cell-cell adhesion and keeps tissues together. They help keep the epithelial and endothelial cells together, especially when they are under mechanical stress.

Clinical significance of tight junctions:
When tight joints are broken or don’t work right, it can have major physiological and pathological effects. If tight junctions don’t work right, harmful chemicals can get through the epithelial or endothelial layers. This is called increased permeability. This can cause tissue damage, inflammation, and illnesses like inflammatory bowel disease, leaky gut syndrome, and some types of cancer.

In short, tight junctions are connections between cells that block off the space between them. This creates a barrier and controls the flow of molecules and ions. In the epithelial and endothelial layers, they are important for maintaining the integrity of tissues, selective permeability, and cell polarity.

Adherens junction

Adherens junctions are connections between cells that are very important for how well cells stick together and how well tissues stay together. They can be found in many types of tissues, such as the epithelial and vascular layers, where they give tissues strength and help keep cells organised.

How adherent joints are put together:

Transmembrane proteins called cadherins are what make up adhesion junctions. Cadherins on cells next to each other communicate with each other through their extracellular regions. This is called cadherin-cadherin binding. The cytoplasmic region of cadherins links them to the actin cytoskeleton by binding to proteins inside the cell, like catenins.

In adherens junctions, there are two main types of cadherins: E-cadherin, which is mostly found in epithelial cells, and N-cadherin, which is usually found in neuronal and mesenchymal cells. The roles and properties of adherens junctions in different tissues depend on the type of cadherin that is expressed.

What adherent junctions do:

Adherens junctions are important in cell and tissue biology for a number of reasons:

1. Cell-cell adhesion: adherens junctions make it easy for cells next to each other to stick together strongly. This helps keep tissues together and healthy. The interactions between cells that are made possible by cadherin make a continuous adhesion belt along the side of each cell. This makes it possible for cells to connect to each other in a safe way.

2. Organising tissues: Adherens junctions help cells in tissues stay organised and in the right place. During development and tissue formation, they help set up and keep tissue polarity, cell layering, and cell sorting in place.

3. Mechanical strength: adherens junctions give tissues power by connecting the cytoskeletons of cells that are next to each other. Catenins help connect cadherins to the actin cytoskeleton, which helps move forces through the tissue and keep it strong enough to fight mechanical stress and shear forces.

4. Signalling and control: Adherent junctions can send and receive messages across the cell membrane. They take part in intracellular signalling processes that help cells grow, change, and keep tissues in balance. Through their interactions with different signalling molecules and cytoplasmic proteins, adherens junctions also help control gene production and other cellular processes.

The importance of adherent junctions in medicine:

When adhesion junctions are broken or don’t work right, it can lead to diseases. Changes in adhesion junctions have been linked to the spread of cancer. Changes in the expression or function of cadherin can affect how cells stick together, invade other tissues, and spread to other parts of the body. Adherens junction flaws can also cause developmental problems, make tissues weak, and slow the healing of wounds.

In a nutshell, cadherin-mediated interactions form adherens junctions, which are connections between cells. They help cells stick together, organise tissue, and have strong mechanical properties. They are very important for keeping tissues together and controlling how cells work.

Gap junction

Gap junctions are special kinds of junctions between cells that let them talk to each other directly and share small molecules. They are very important for organizing the activities of cells, the flow of electrical signals, and the way cells use energy. Here are the most important things about gap junctions and what they do:

Structure of Gap Junctions:
Clusters of proteins called connexins are what make up gap junctions. Connexins make connexons, which are tubes that connect the plasma membranes of two cells next to each other. Six connexin subunits are grouped in a circle to make each connexon. When connexons from two neighbouring cells line up, they make a gap junction channel or a connexon channel, which is a straight path that lets cells talk to each other.

Functions of gap junctions:

1. Passage of molecules between cells: Gap junctions allow small molecules, like ions, glucose, amino acids, and signalling molecules with a molecular weight of 1000 or less, to move from one cell to another. This lets important metabolites and signalling molecules move quickly between cells, making it easier for cells in the same region to work together and in sync.

2. Electrical signalling: Gap junctions are a key part of how quickly changes in electrical potential, called “action potentials,” move from one cell to the next. This is especially important in tissues that need electrical activity to happen at the same time, like heart muscle and smooth muscle cells. Gap junctions let electrical messages spread, which makes it possible for cells in these tissues to work together and contract in a coordinated way.

3. Metabolic coupling: Gap junctions also allow cells to talk to each other about their metabolism. By letting metabolites move between cells in a tissue, energy can be shared and metabolic balance is maintained. For example, in organs like the liver and the retina that have a lot of metabolic work to do, gap junctions help move nutrients and get rid of waste.

4. Cell signalling and coordination: Gap junctions make it easy for chemical signals and signalling molecules to move directly between cells that are close to each other. This makes it easy for cells to talk to each other and work together quickly. Signalling through gap junctions is important for many bodily functions, such as embryonic growth, immune responses, and tissue repair.

Clinical significance of gap junctions:
When gap junctions don’t work right or change, it can lead to a number of illnesses and disorders. Several inherited diseases, such as some types of blindness, cataracts, and heart problems, have been linked to changes in connexin genes, which code for the building blocks of gap junctions. When gap junctions don’t work right, cells can grow in strange ways, tumours can spread, and arrhythmias can happen.

In a nutshell, connexin proteins form the connections between cells known as gap junctions. They allow cells to talk to each other directly and share small molecules. They help molecules move between cells, allow electrical signals to spread, support metabolic coupling, and help cells communicate and work together in tissues.

1.3. Transport through cell membrane

Passive transport

Active transport

Vesicular transport

Other transport processes

1.4. Membrane potential


Genesis of membrane potential

Recording of membrane potential

1.5. Genetics: An overview

Structural and functional characteristics of substrate for genetics

Applied genetics

Systemic Physiology

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