Introduction to Cell and Molecular Biology

It studies structure and functions of the cell as basic unit of living organisms.

Robert Hooke in 1665, while observing a thin slice of cork.

Anthony van Leeuwenhoek in 1674, observing protozoa, bacteria, and blood cells.

He showed that the nucleus was a common (constant) component of all cells.

Schleiden (botanist) and Schwann (zoologist).

"Omnis cellula e cellula" (All cells arise from pre-existent cells).

Electron microscopy, which offered higher magnification and resolution.

Electron microscopy and cell fractioning by differential centrifugation.

It is indispensable for understanding fundamental biomedical disciplines (e.g., physiology, histology), connecting fundamental and clinical sciences, and recognizing that diseases manifest at cellular and molecular levels.

George Emil Palade, in 1974, for his fundamental contributions to electron microscopy and cell fractioning, including the discovery of ribosomes.

He discovered characteristic corpuscles in nervous cells of animals and humans deceased due to rabies, known as Babes-Negri corpuscles.

Prokaryotes (e.g., bacteria, green-blue algae) are always unicellular. Eukaryotes (e.g., plants, animals, humans) can be unicellular (protozoa) or pluricellular (metazoa, metaphyta).

Eukaryotes have a proper nucleus surrounded by a nuclear envelope with specific chromosomes. Prokaryotes lack a 'real' nucleus; their nuclear material (a single DNA molecule) is organized in a nucleoid, in direct contact with the cytoplasm.

Prokaryotes multiply by direct division (binary fission), resulting in two identical cells. Eukaryotes have a more complex process of indirect division, with two forms: mitosis and meiosis, during which chromatin condenses into visible chromosomes.

Eukaryotes possess several membrane-separated cytoplasmic organelles (e.g., mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum) that partition the cytoplasm, allowing specific enzymatic processes in defined conditions. Prokaryotes lack these membrane-separated organelles and cytoplasmic partitioning.

Besides the plasma membrane, prokaryotes have a cell wall containing N-acetyl muramic acid, and their plasmalemma can exhibit cytoplasmic extensions called mesosomes. Eukaryotes lack these specific structures, and while plants have cell walls, they are made of cellulose and are structurally different from prokaryotic ones.

Eukaryote cells feature cytoplasmic differentiations (filaments and microtubules) forming the cytoskeleton, responsible for specific cellular movements like amoeboidal locomotion or cytoplasmic streaming. Prokaryotes' locomotion is provided by relatively simple flagella, which differ significantly from eukaryotic flagella (e.g., in human spermatozoa).

Vegetal cells have large cytoplasmic vacuoles (which absorb water, reducing osmotic pressure) and chloroplasts (organelles containing chlorophyll for photosynthesis). Animal cells typically only have cytoplasmic vacuoles under pathological conditions.

Viruses are biological entities considered halfway between alive and dead matter. They have a simple structure: a core of nucleic acid (DNA or RNA) as genetic material, surrounded by protein subunits forming a capsid. Some also have an envelope (e.g., HIV).

Earth formed 4.5-5 billion years ago. Prokaryotes appeared approximately 3.5 billion years ago, eukaryotes 1.5 billion years ago, and man around 1.8 million years ago.

The human body is estimated to have millions of billions (10^15) of cells. Red blood cells are the most numerous (tens of thousands of billions, 10^13). Hepatocytes and neurons are in the hundreds of billions (10^11), while glial cells are 10 times more numerous (10^12).

Cell shape varies depending on age and specific role. Young cells are generally spherical. As they mature, their shape adapts to function: contractile cells are elongated (muscle fibers), conductive cells have prolongations (neurons), red blood cells are biconcave discs for max oxygen transfer, glial cells are star-shaped, and others can be cubic, cylindrical, or polyhedral (e.g., endothelial cells), or even peculiar (e.g., Purkinje cells).

Mean sizes of human cells are 20-30 µm. The smallest are cerebellar neurons (3-6 µm) and lymphocytes (4-5 µm). The largest include giant frontal cortex neurons (125-150 µm, pyramidal) and the ovocyte (~250 µm). An ostrich egg yolk is a single cell of about 10 cm.

The law of volume constancy states that certain cell types from a specific organ have approximately the same volume across various animal species, regardless of body size (e.g., human vs. mouse red blood cells). The dimension of organs is determined by the number of cells, not their individual size.

A eukaryotic cell has three main components: plasmalemma, cytoplasm, and nucleus. Its structure varies with the cell cycle, which includes interphase (G1, S, G2) and cell division.

With an optical microscope, the plasmalemma is not visible. The nucleus, with its nucleoli and chromatin (DNA and histones), can be seen. The cytoplasm appears homogenous, containing particles and granules like organelles, secretion granules, pigments, and lipids. It's divided into an internal, brighter, more fluid endoplasm (cellular metabolism) and an external, more viscous, darker ectoplasm (surface processes, cell-environment changes).

Electron microscopy reveals the cell's ultrastructure: the membrane may have external extensions (microvilli, cilia, flagella) and intercellular fixation structures (desmosomes, cell junctions). Organelles are membrane-bound, compartmentalizing the cell. The nucleus has a double membrane with pores, and the outer nuclear membrane is continuous with the rough endoplasmic reticulum (RER).

RER has ribosomes for protein synthesis. SER lacks ribosomes and is involved in fat metabolism and processing toxic compounds. Their lumen and membranes are continuous.

The Golgi apparatus consists of flat sacks from which secretion vesicles detach. Mitochondria have a double membrane with inner folds called cristae. Lysosomes and peroxisomes are characterized by a single membrane.

Plant cells have a cell wall outside the plasmalemma (composed of polysaccharides), chloroplasts for photosynthesis, and large vacuoles.

The plasmalemma acts as a barrier, regulates exchanges with extracellular space, and mediates intercellular interactions. The nucleus is the genetic center and regulates all cellular processes. The nucleolus synthesizes ribosomal RNA and forms ribosomes. Mitochondria produce energy.

The RER synthesizes proteins. The SER is involved in fat metabolism and detoxification. The Golgi apparatus processes secretion products, traffic vesicles, and sorts/concentrates substances. Lysosomes perform intracellular digestion. Peroxisomes contain oxidative enzymes for detoxification. The cytosol, with its cytoskeleton, maintains cell shape and enables cellular movements.

Cytoplasmic compartments formed by organelles separate biochemical reactions, preventing interference and allowing inconsistent reactions to occur simultaneously. Organelle membranes also increase the surface area available for membrane-bound enzymes.

From the total cellular volume, the cytosol occupies the main part, followed by mitochondria, and then the endoplasmic reticulum.

Living matter is made up of over 60 chemical elements, mainly 'light' ones, as heavy elements are chemically inert and insoluble in water. They are grouped into macroelements, microelements, and oligoelements.

Macroelements are major chemical elements, each present in amounts of 2-60% of the cell's composition. They form the core of cell structures. The main macroelements are Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

Carbon is a tetravalent element, forming simple, double, or triple bonds with other atoms (H, N, O). Its valence lines point towards the tips of a regular tetrahedron. This allows for the formation of long linear, ramified, or cyclic chains, leading to an enormous variety of macromolecules. Its ability to form double/triple bonds also results in unsaturated, highly reactive compounds, vital for metabolic processes.

Microelements are less abundant (about 0.02-0.1% each). They include non-metal elements like Phosphorus (P), Sulfur (S), and Chlorine (Cl), and metals like Sodium (Na), Potassium (K), Calcium (Ca), and Magnesium (Mg). Notably, Na is more abundant outside the cell, while K is more abundant inside the cell.

Macroelements and microelements are collectively known as plastic elements because they are the fundamental components from which all biological structures are built.

Oligoelements (trace elements) are present in very small amounts (less than 0.02% each) but are crucial for life and medicine. They can be part of protein structures (e.g., Iron (Fe) in hemoglobin and myoglobin for oxygen binding/transport; Iodine (I) in thyroid hormones). They also act as enzymatic activators/inhibitors and influence vital systems (e.g., cardiovascular, gamete formation, embryonic development, neuropsychic development in children).

Water is considered the essence of life and is the primordial and most abundant molecule in all living cells. It is the main constituent of every cell.

In the organism, water is present in two main compartments: intracellular water (about 55%) and extracellular water (about 45%). Extracellular water is found in plasma, lymph, interstitial liquids, digestive secretions, cerebrospinal fluid, and serous cavities (pleura, peritoneum). Younger cells, with higher metabolic activity, have a higher water content (up to 95%) compared to older ones (around 60%).

Water is the unique solvent of living matter. All other compounds are diluted or suspended in it, and virtually all chemical reactions essential for life occur in aquatic solutions.

The arrangement of oxygen and hydrogen atoms creates an electric dipole in the water molecule (negative pole at oxygen, positive at hydrogens). This polar nature gives water a high dielectric constant (80 times higher than vacuum), significantly attenuating electrical interactions. Consequently, it acts as an electrical shield , protecting living structures from intense electrical fields. Its polarity also makes it an excellent solvent for substances with ionic bonds (which dissociate) and polar substances with covalent bonds.

Water molecules dissociate into protons (H+) and hydroxyl ions (OH-). A proton can attach to another water molecule, forming a hydronium ion (H3O+). Thus, water participates in chemical reactions through this equilibrium: 2H2O ↔ H3O+ + OH- .

One water molecule can bind to 1 to 4 other water molecules via hydrogen bonds . These bonds are stable in ice but dynamic in liquid water, where molecular 'packages' continuously form and break. This dynamic bonding confers water a high caloric capacity , making it an excellent cooler and thermal shield (absorbing heat from biochemical processes). It also gives water a high value of vaporization heat , important for cooling organisms through evaporation ( thermoregulatory property ).

In the aquatic phase within cells:

• Free water (95%) acts as a solvent or dispersion medium for organic and inorganic substances.
• Bound water (5%) consists of water molecules bound by hydrogen bonds to other structures, primarily proteins.

Additionally, living cells contain non-aquatic phases where water is excluded, such as the inner space of macromolecules (proteins, nucleic acids) or inside cell membranes.

Water transport through membranes is accomplished by specialized proteins called water-channel proteins or aquaporins . The first such protein, Aquaporin 1 (AQP1), was discovered by Prof. Gheorghe Benga's team in human red blood cell membranes in 1985. Peter Agre, who rediscovered AQP1 years later, was awarded the 2003 Nobel Prize for Chemistry for this discovery.

• AQP1 : Found in red blood cell membranes, epithelial cells of renal proximal tubules (important for urine concentration), capillary endothelium, and choroid plexus.
• AQP2 : Located in renal collecting tubules, playing a role in urine concentration alongside antidiuretic hormone (ADH).
• AQP3 : Found in renal collecting tubules, lungs, and brain.
• AQP0 : Present in the crystalline lens, helping maintain its transparency.

Aquaporins regulate all rapid water transport processes in organisms.

Dysfunctions of aquaporins are implicated in various pathologies, including diabetes insipidus (impaired water reabsorption in kidneys), edematous cardiac insufficiency (fluid retention due to heart failure), and certain diseases of the nervous system .

Mineral salts exist as ions or in combinations bound to macromolecules (e.g., proteins, nucleic acids).

Key cations include: Na+ (main extracellular), K+ (main intracellular), Ca2+ , and Mg2+ .

Key anions include: phosphates (PO43-, HPO42-, H2PO4-), sulphate (SO42-), carbonates (HCO3-, CO32-), and nitrate (NO3-).

Ions are vital as they influence:

• Enzyme activity and various cellular processes.
• Permeability of cell membranes .
• Excitability, conductibility , and contractility .
• Cytoplasmic viscosity and cellular division .
• Osmotic pressure and the acido-basic equilibrium (intra- and extracellular pH).

Small variations in their concentration can lead to major functional alterations (e.g., cardiac arrhythmias) or even sudden death.

The ionic concentration is relatively constant in all living bodies, typically around 1% of total weight . This constancy serves as another piece of evidence for the material unity of the biosphere.

The four main categories of organic substances in cells are:

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

Four common small chemical groups that appear repeatedly in biological molecules are:

• Methyl (-CH3)
• Hydroxyl (-OH)
• Carboxyl (-COOH)
• Amino (-NH2)

These groups possess distinct chemical and physical properties that significantly influence the behavior of the molecules containing them.

Glucids (sugars/carbohydrates) serve two main roles in cells:

• Plastic role : Certain monosaccharides, like ribose or deoxyribose, are structural components (e.g., in nucleic acids).
• Energetic role : Glucose is the primary monosaccharide used as cellular 'fuel' to provide energy.

Glucose is particularly well-suited as a cellular fuel because:

• It is highly soluble in water , facilitating easy absorption from the digestive tract and transport throughout the organism via blood.
• It is an extremely stable hexose , more stable than pentoses or heptoses.
• Breaking its covalent bonds releases an extremely high amount of energy , providing significant energetic resources.
• It is easily metabolized (via glycolysis and other pathways) to produce energy for ATP synthesis.

Glycogen is the main storage form of glucose in human and animal organisms. In plants, this role is fulfilled by starch.

Glycogen is the main storage form of glucose in human and animal organisms. In plants, this role is fulfilled by starch.

Glycogen is a polysaccharide made of glucose residues linked by:

• α-1-4 glycosidic bonds , forming long chains.
• α-1-6 glycosidic bonds , creating ramifications (branches).

The branched structure of glycogen offers two main advantages:

• It allows for the storage of thousands of glucose molecules within a single glycogen molecule, significantly reducing osmotic pressure in the cell (osmotic pressure depends on particle number, not size).
• The numerous ramifications provide many ends for enzymes to work on simultaneously, enabling very quick metabolic processes for both glycogenesis (glycogen formation) and glycogenolysis (glucose release).

In the human body, glycogen is present in high amounts in liver and muscle cells .

During hyperglycemia (after sugary meals), glucose is quickly stored in the liver and muscles, returning blood glucose levels to normal.

During hypoglycemia (between meals or intense physical efforts), glucose molecules are rapidly released from hepatic glycogen for energy. Muscular glycogen is primarily used for muscle activity and is restored over a longer period, consumed only after hepatic glycogen is depleted.

The nervous system, particularly neurons, is a major consumer of glucose , as it is their primary 'fuel'. However, the nervous system does not have its own glycogen storage system . Consequently, it is highly sensitive to hypoglycemia (low blood glucose), as it cannot readily access stored glucose.

Mucopolysaccharides are polysaccharides where the monomer units contain amino derivatives of monosaccharides (e.g., glucosamine, galactosamine), forming long, fibrous molecules. When these mucopolysaccharides attach to smaller polypeptidic chains, they form proteoglycans .

Mucopolysaccharides are polysaccharides where the monomer units contain amino derivatives of monosaccharides (e.g., glucosamine, galactosamine), forming long, fibrous molecules. When these mucopolysaccharides attach to smaller polypeptidic chains, they form proteoglycans .

Mucopolysaccharides are important components of the extracellular matrix and form the fundamental substance of the connective tissue . Due to their high hydration (forming gels with high viscosity), they primarily perform mechanical functions (support, shock absorption, lubrication) in connective tissue. They also actively participate in tissue metabolism.

Examples of mucopolysaccharides include: hyaluronic acid, chondroitin-4-sulfate, chondroitin-6-sulfate, keratan-sulfate, dermatan-sulfate, heparan-sulphate , and heparine .

Lipids (fats) play diverse and crucial roles in the cell:

• Plastic role : They are integral components of cell membranes.
• Energetic role : They constitute the 'fuel' with the highest energetic value, providing more energy than equivalent quantities of carbohydrates or proteins.
• Regulatory role : Accomplished by various lipid-derived molecules such as steroid hormones, lipidic vitamins, and prostaglandins.

Free fatty acids are simple lipids utilized for energy gain through oxidative processes or for the synthesis of other lipid types. Cells primarily use fatty acids with chains of 16 and 18 carbon atoms . Fatty acids with less than 14 C atoms act as detergents, destroying cellular membranes, while those with more than 20 C atoms are highly insoluble and metabolically useless.

Free fatty acids are simple lipids utilized for energy gain through oxidative processes or for the synthesis of other lipid types. Cells primarily use fatty acids with chains of 16 and 18 carbon atoms . Fatty acids with less than 14 C atoms act as detergents, destroying cellular membranes, while those with more than 20 C atoms are highly insoluble and metabolically useless.

Triglycerides (also called neutral fats) are the storage form of lipids. They are primarily stored in specialized cells known as adipose cells or adipocytes , where the nucleus is pushed to the periphery by a large lipidic vacuole. In pathological conditions, triglycerides can also accumulate in the cytosol of other cells, such as in hepatic steatosis (fatty liver) in obese or alcoholic patients.

Triglycerides (also called neutral fats) are the storage form of lipids. They are primarily stored in specialized cells known as adipose cells or adipocytes , where the nucleus is pushed to the periphery by a large lipidic vacuole. In pathological conditions, triglycerides can also accumulate in the cytosol of other cells, such as in hepatic steatosis (fatty liver) in obese or alcoholic patients.

Lipids present in the cytosol can be observed using specific staining methods:

• With "Sudan Black" , they stain black .
• With "Sudan III" , they stain orange .

Complex lipids are found in cellular membranes, primarily as phospholipids and glycolipids .

Complex lipids are found in cellular membranes, primarily as phospholipids and glycolipids .

Glycerin-phosphatides are phospholipids based on a glycerol molecule. Two carbon atoms are esterified with 16-18 C fatty acids (which can have up to 6 double bonds if unsaturated). The third carbon atom is attached to a polar group containing a phosphoric acid radical and various other substances. Examples include phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine , and phosphatidylinositol . Another important phospholipid is sphingomyelin .

Glycerin-phosphatides are phospholipids based on a glycerol molecule. Two carbon atoms are esterified with 16-18 C fatty acids (which can have up to 6 double bonds if unsaturated). The third carbon atom is attached to a polar group containing a phosphoric acid radical and various other substances. Examples include phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine , and phosphatidylinositol . Another important phospholipid is sphingomyelin .

Sphingomyelin is a phospholipid based on sphingosine (an amino-alcohol with a long carbon chain) esterified with a fatty acid, and its polar group is phosphorylcholine (similar to lecithin).

Glycolipids are also based on sphingosine , with a fatty acid bound to it, but their polar groups consist of one or several glucidic radicals (e.g., in cerebrosides or gangliosides).

Sphingomyelin is a phospholipid based on sphingosine (an amino-alcohol with a long carbon chain) esterified with a fatty acid, and its polar group is phosphorylcholine (similar to lecithin).

Glycolipids are also based on sphingosine , with a fatty acid bound to it, but their polar groups consist of one or several glucidic radicals (e.g., in cerebrosides or gangliosides).

Molecules like phospholipids and glycolipids are called amphiphiles because they possess both a hydrophilic (water-loving) head (formed by the polar group) and a hydrophobic (water-fearing) tail (formed by the nonpolar fatty acid chains).

When placed in water, these molecules spontaneously rearrange to form stable structures such as micelles or, most importantly, lipidic bilayers , which represent the fundamental structural basis of biological membranes.

Cholesterol is a major lipid in eukaryotic cell membranes. Its ratio is typically higher in membranes with a predominant barrier function, such as the plasmalemma and myelin sheath . Conversely, its concentration is lower in intracellular membranes.

Cholesterol is a major lipid in eukaryotic cell membranes. Its ratio is typically higher in membranes with a predominant barrier function, such as the plasmalemma and myelin sheath . Conversely, its concentration is lower in intracellular membranes.

The four major phospholipids found in all cell membranes are: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine , and sphingomyelin .

Cardiolipin is a marker lipid specifically found in the inner mitochondrial membrane .

Fluidity refers to the ability of the lipid bilayer to transition from a gel (crystalline) state to a fluid (liquid crystal) state at a specific phase transition temperature .

• Fatty acid chain length and saturation : Shorter and more unsaturated fatty acids lead to higher bilayer fluidity and a lower phase transition temperature.
• Cholesterol : In artificial bilayers, cholesterol increases fluidity with saturated lipids (lowers transition temp) and decreases fluidity with unsaturated lipids (increases transition temp). At physiological temperatures, cell membranes maintain a fluid state, which is essential for their functions.

Fluidity refers to the ability of the lipid bilayer to transition from a gel (crystalline) state to a fluid (liquid crystal) state at a specific phase transition temperature .

• Fatty acid chain length and saturation : Shorter and more unsaturated fatty acids lead to higher bilayer fluidity and a lower phase transition temperature.
• Cholesterol : In artificial bilayers, cholesterol increases fluidity with saturated lipids (lowers transition temp) and decreases fluidity with unsaturated lipids (increases transition temp). At physiological temperatures, cell membranes maintain a fluid state, which is essential for their functions.

Phospholipid molecules within a membrane exhibit several types of movements:

• Movements inside the molecule : Flexion movements of C atoms in methylene groups (more mobile towards the bilayer middle, more rigid near the polar group) and movements of atoms in the polar group.
• Movements of the entire molecule : Lateral diffusion (very rapid, covering ~2 µm/sec), rotation movement around the longitudinal axis (also rapid), and transversal diffusion (flip-flop) from one layer to another (very slow spontaneously, but accelerated by flipases ).

While lipids primarily provide the barrier function of membranes, membrane proteins confer the functionality . They are responsible for:

• Transport of particles across membranes.
• Performing enzymatic functions .
• Acting as receptors for signals.

Therefore, membranes with more functions typically have a higher protein-to-lipid ratio.

Membrane proteins are typically separated, identified, and characterized using polyacrylamide gel electrophoresis in the presence of sodium-dodecylsulphate (SDS-PAGE) .

Proteins have numerous and diverse important roles at the cellular level, including:

• Plastic role : Components of all cellular structures, including membranes and chromatin.
• Transport and storage of substances (e.g., iron, oxygen).
• Involved in muscle contraction and all other cell movements.
• Provide resistance and elasticity to tissues.
• Regulatory roles : Control cell growth, development, multiplication, and cellular division.
• Catalytic role : Almost all enzymes, crucial for metabolic processes, are proteins.
• Involved in immune defensive reactions .
• Receptor role (e.g., synaptic receptors).
• Nutrition (e.g., ovalbumin).
• Maintaining osmotic pressure and pH at constant levels.

Proteins exhibit such diverse roles primarily due to their enormous structural diversity and their high specificity .

Proteins are macromolecules formed by the polycondensation of amino acids through peptide bonds (-CO-NH-). This process results in polypeptide chains of highly variable length, ranging from very short to thousands of amino acids.

Proteins are composed of 21 different amino acids . The last amino acid discovered is Selenocysteine (Sec) .

The heterogeneity in amino acid size (from small to very voluminous) and polarity (hydrophilic, polar, or neutral) is directly reflected in the diversity of proteins. The vast availability of amino acids for a large variety of combinations leads to an enormous number of possible sequences, contributing to significant protein heterogeneity.

The primary structure of a protein is its specific sequence of amino acids. This sequence largely determines the spatial arrangement of the polypeptide chain, leading to its secondary (bi-dimensional, like alpha-helices or beta-sheets) and tertiary (three-dimensional) structures. The protein adopts the most thermodynamically stable conformation based on its primary sequence.

The three-dimensional (spatial) structure of a protein, which is specific for each family of proteins, is essential for its biological activity. Different amino acid sequences result in extremely varied spatial structures and, consequently, diverse biological functions.

Protein specificity means proteins can specifically bind with high affinity to particular substances, even if those substances are in very low concentrations. This specific bonding is a consequence of the structure-function relationship and underlies various molecular interactions in living systems, such as:

• Antigen-antibody reactions
• Enzyme-substrate interactions
• Receptor-ligand reactions

Protein specificity means proteins can specifically bind with high affinity to particular substances, even if those substances are in very low concentrations. This specific bonding is a consequence of the structure-function relationship and underlies various molecular interactions in living systems, such as:

• Antigen-antibody reactions
• Enzyme-substrate interactions
• Receptor-ligand reactions

Proteins can be studied using:

• Cytochemical reactions : Amino- or sulfhydryl- groups in polypeptide chains can be visualized after reactions that produce colored precipitates (visible by light microscopy) or electron-dense precipitates (visible by electron microscopy).
• Enzymatic reactions : For enzymes, stained precipitates can be observed on tissue sections.
• Specific antibodies : Antibodies tagged with fluorescent groups can be seen with light microscopes (in UV light), while electron-dense metals (like gold or iron) linked to antibodies (e.g., colloidal gold, ferritin) allow identification under electron microscopy.

Nucleic acids are macromolecules formed by the polycondensation of nucleotides . A nucleotide consists of three main components:

• A nitrogenous base : either a purine (adenine, guanine) or a pyrimidine (thymine, cytosine, uracil).
• A pentose sugar : either ribose (in RNA) or deoxyribose (in DNA).
• A phosphoric ester (phosphate radical).

Nucleic acids are macromolecules formed by the polycondensation of nucleotides . A nucleotide consists of three main components:

• A nitrogenous base : either a purine (adenine, guanine) or a pyrimidine (thymine, cytosine, uracil).
• A pentose sugar : either ribose (in RNA) or deoxyribose (in DNA).
• A phosphoric ester (phosphate radical).

In any nucleic acid molecule, four nitrogenous bases are found: two purines ( adenine, guanine ) and two pyrimidines ( thymine, cytosine in DNA; uracil, cytosine in RNA). These constitute four different types of nucleotides: ribonucleotides (in RNA) and deoxyribonucleotides (in DNA).

Nucleic acids are the heredity molecules . They represent the genetic material and serve as the storage system for genetic information , which is encoded in their nucleotide sequences.

Heredity is the particular feature of living organisms to produce offspring similar to the parents, under specific environmental conditions. It involves the transfer of hereditary characteristics, which are contained in genes . Genes are formed from hundreds to thousands of nucleotides and determine all biochemical, morphological, and behavioral traits of an organism.

Heredity is the particular feature of living organisms to produce offspring similar to the parents, under specific environmental conditions. It involves the transfer of hereditary characteristics, which are contained in genes . Genes are formed from hundreds to thousands of nucleotides and determine all biochemical, morphological, and behavioral traits of an organism.

The DNA macromolecule typically has a double-helix (duplex) structure , where two polynucleotide chains are coiled to form a huge spiral. This is a universal structure. In the case of RNA, universal types of spatial structures are observed in transfer RNA ( tRNA ), messenger RNA ( mRNA ), and ribosomal RNA ( rRNA ).

Nucleic acids in cells can be visualized using specific cytochemical reactions with methyl green and pyronine (combined staining). Methyl green stains DNA a green-blue color, while pyronine turns RNA pink . Generally, cells show a green-blue nucleus (due to DNA excess) and a pink nucleolus and cytoplasm (due to large amounts of rRNA).

The unitary principles concerning the chemical and biological organization of the cell include:

• Cells are composed of identical chemical elements and substances in similar percentages.
• The ionic composition of body fluids is constant across various animal species.
• Biochemical processes in the biosphere follow a unitary scheme , with main metabolic processes catalyzed by similar enzymes.
• Some molecules are universally distributed within living cells.
• ATP is the universal currency in energetic exchanges at the cellular level.
• DNA replication and protein biosynthesis follow the same mechanisms in all living cells.
• Control mechanisms for cellular processes are common across the biosphere.

Examples of molecules universally distributed within living cells include: D-glucose , fatty acids with 16 to 18 carbon atoms, all 21 amino acids , sterols , and the purine and pyrimidine bases . Additionally, co-factors such as NAD, NADP, CoA, coenzyme Q, and various vitamins are omnipresent.

There are three basic systems for ATP generation found in all cells:

• Glycolysis
• Oxidative phosphorylation in mitochondria (oxidative process coupled with ATP synthesis)
• Photosynthesis (which is specific for plants)

The cytoplasmic matrix is the part of the cell occupying the space between the plasma membrane, nucleus, and cytoplasmic organelles. It is also known as "hyaloplasm" (classical cytology) or "cytosol" (after cell fractioning).

The cytoplasmic matrix is a "homogenous system" primarily consisting of ions and molecules dissolved in water. However, the presence of macromolecules (10-5 – 10-7 cm) dispersed in water also leads to colloidal phenomena like diffusion, Brownian movement, sedimentation, Tyndall phenomenon (light scattering), and sol-gel transitions.

Sol and gel states are characterized by viscosity: the "sol state" has low viscosity, allowing particles to move independently. The "gel state" has higher viscosity, as particles are entrapped in a network. These "sol-gel transitions" occur permanently in living matter, reflecting its dynamic nature. In contrast, dead cells have cytoplasm permanently in either a sol or gel state.

The cytoplasmic matrix contains various cytoplasmic differentiations, including: "Myosin filaments" "Actin filaments" "Intermediate filaments" "Microtubules" Each type is characterized by its constituent proteins, polymerization/depolymerization conditions, structure, location, functions, and medical applications.

Myosin filaments are primarily made of the protein "myosin" , with "myosin II" being the prototype (muscle myosin). Each myosin II molecule consists of "six polypeptide chains" : two heavy chains (each with a globular "head" and a fibrous "tail") and two pairs of light chains (four total). The heads contain an "ATP-binding site" (hydrolyzing ATP for mechanical energy) and an "actin-binding site" , enabling myosin to step on actin filaments for movement.

Myosin filaments assemble through the interconnection of the heavy chain "tails," both parallel and anti-parallel. This arrangement results in a central "heads-free central area (bare region)" of the filament, while the two peripheral zones display the globular heads.

In "muscle cells" , myosin filaments are called "thick filaments" (composed of ~500 myosin molecules) and are permanent structures, surrounded by six thin (actin) filaments. They are crucial for "muscular contraction" . In "non-muscle cells" , myosin filaments are "labile structures" (10-20 myosin molecules) that can polymerize and depolymerize based on cellular needs. Their functions include "amoeboid movement, microvilli movements, cytoplasmic currents" , and forming the "actin-myosin contractile ring during cell division (cytokinesis)" .

Actin molecules represent about "15%" of all cytosolic proteins in muscle cells and "1-5%" in non-muscle cells, making actin the "most abundant cytosolic protein" in all cells.

The actin monomer is a small, globular protein called "G-actin" . Each G-actin has a gap containing an "ATP-Mg2+ complex" . The ATP molecule provides the energy for the polymerization process. G-actins assemble head-to-tail to form a tight, right-handed helix, creating a filamentous structure known as "F-actin" . This head-to-tail assembly makes the filament asymmetrical, with distinct polymerization/depolymerization speeds at its two ends: a "faster growing '+' end" and a "slower-growing '-' end" .

Actin is the main factor determining cytoplasm viscosity: when actin is "depolymerized" , the cytoplasm is in a "sol state" (less viscous). When it "polymerizes" , the cytoplasm turns into a "gel state" (more viscous).

In "non-muscle cells" , repeated actin monomers form "microfilaments" (~5 nm diameter), which interconnect at the cytoplasm periphery to form a "cortical network" (stress fibres) that confers mechanical strength. They are also found in the "contractile ring" during cell division and in "apical rings" in intestinal epithelial cells. In microvilli, actin bundles facilitate movement. In "muscle cells" , actin filaments are called "thin filaments" , composed of F-actin, "tropomyosins" (covering actin monomers), and the "troponin complex" (binding Ca2+ to trigger contraction by uncovering actin for myosin interaction, leading to sliding filaments).

Several chemical substances influence actin polymerization: "Cytochalasins" (fungal metabolites) inhibit polymerization by binding to the '+' end of actin filaments, causing toxic effects. "Phalloidins" (from Amanita phalloides mushroom) bind tightly along actin filaments and stabilize them, inhibiting depolymerization and preventing gel-sol transition, leading to serious medical conditions like chronic renal insufficiency or death.

Intermediate filaments have diameters between microfilaments (5-7 nm) and microtubules (25 nm). They are notably prominent in cells subjected to "mechanical stress" . Unlike other cytoplasmic differentiations, intermediate filaments often consist of "2-10 co-polymerized fibrous proteins" . Their polymerization involves "head-to-head and lateral interactions" to form dimers, tetramers, and eventually 8 parallel protofilaments. A key feature is their "high stability" , preserving their shape once polymerized, unlike labile actin and microtubules.

There are two main categories of intermediate filaments: "Nuclear intermediate filaments" : Common to all cells, they form "nuclear lamins (A, B, and C)" that line the inner membrane of the nuclear envelope, providing anchorage sites for chromosomes. "Cytoplasmic intermediate filaments" : Vary by cell type: "Keratin filaments (cytokeratin)" : Characteristic of epithelial cells, imparting mechanical strength. "Desmin filaments" : Found in muscle cells (except smooth muscles in blood vessels). "Vimentin filaments" : Characteristic of mesenchymal cells (e.g., fibroblasts, macrophages, endothelial cells, smooth muscle cells of blood vessels). "Neurofilaments" : High concentrations along neuron axons. "Glial filaments" : Found in glial cells.

Intermediate filaments have significant medical applications: "Cancer detection" : Monoclonal antibodies against specific intermediate filament proteins are used to detect the origin of malignant tumors (e.g., keratin for carcinomas, vimentin for non-muscle sarcomas, desmin for muscle sarcomas). "Amyotrophic Lateral Sclerosis (ALS)" : Associated with abnormal accumulation and assembly of neurofilaments in motor neurons, interfering with axonal transport, and neurofilament levels serve as a disease progression biomarker. "Hutchinson-Gilford Syndrome (Progeria)" : A premature aging genetic disease caused by a point mutation in the "LMNA gene" (lamin A), leading to very short telomeres and affecting nuclear mechanical stability and chromatin organization. "Epidermolysis Bullosa Simplex" : A genetic skin blistering disease caused by defective expression of keratin filaments in the epidermis basal layer.

Microtubules are cytoplasmic differentiations made of globular proteins called "tubulins" . The basic subunit is a "hetero-dimer" formed from one "α-tubulin" and one "ß-tubulin" . Each tubulin molecule has a "GTP molecule attached" ; one GTP is converted to GDP and Pi during dimer formation, providing energy. These dimers polymerize unidirectionally to form longitudinal rows called "protofilaments" . A microtubule is typically formed from "13 such protofilaments" arranged as a hollow cylindrical structure, approximately "25 nm in diameter" . All tubulin subunits point in the same direction, making the microtubule an "asymmetrical dynamic structure" with distinct '+' (growing) and '-' (shortening) ends.

Microtubules exist as both "permanent assemblies" and a "labile network" in the cytoplasm. "Permanent microtubules" are found in the "axoneme of cilia and flagella" , as well as in their "basal bodies" and in "centrioles" . "Free microtubules" in the cytoplasm are highly labile and form a network around the "cell center (centrosome)" during interphase. The centrosome, acting as a "microtubule organizing center (MTOC)" , consists of two perpendicular centrioles surrounded by dense pericentriolar material (containing pericentrin and γ-tubulin) and coordinates tubulin polymerization. During cell division, centrioles replicate to form the mitotic spindle poles.

Microtubules have two major roles: "Structural function" : They provide structural support for the cytoplasm, are responsible for cell shape and permanent extensions (cilia, flagella, neuronal processes), establish spatial geometry of cells (holding organelles), organize the cytoskeleton, and influence intermediate filament distribution. They also form temporary scaffolds for cell components (e.g., during spermatogenesis). "Cellular movements" : They play a crucial role in cellular movements based on motor proteins (like kinesins and dyneins) stepping along them.

Various factors affect microtubule polymerization/depolymerization, with important medical applications: "Temperature" : Polymerization is spontaneous at 37 ºC; depolymerization occurs at 0 ºC. "Divalent cations (Ca2+, Mg2+)" : Stimulate polymerization at μM concentrations and depolymerization at mM concentrations. "Cytosolic proteins (MAPs, Tau proteins)" : Associate with tubulins to stabilize microtubules. "Colchicine" (from autumn crocus): Inhibits microtubule polymerization. Used in "gout treatment" (by blocking lysosomal enzyme release from phagocytes) and for "karyotype determination" (arresting mitosis at metaphase). "Vinblastine and Vincristine" : Similar to colchicine, these cytostatic drugs inhibit tubulin polymerization in the mitotic apparatus, preventing cancer cell division. "Taxol and heavy water (D2O)" : Stabilize microtubules (inhibit depolymerization), also preventing cellular division. Taxol is used as a cytostatic in ovarian cancer treatment.

The cytoskeleton is a "complex tridimensional network of protein filaments" (actin filaments, microtubules, and intermediate filaments) that extends throughout the cytoplasm in all directions. It is specific to eukaryotic cells and can be observed using fluorescent-labeled antibodies.

The cytoskeleton consists of "stable components" (intermediate filaments) and "labile components" (actin filaments and microtubules). Its dynamic structure, vital for cell functions, is maintained by the continuous polymerization and depolymerization of its labile elements.

The dynamics of the cytoskeleton are controlled by: "Divalent cations" (Ca2+ and Mg2+), with Ca2+ binding to "calmodulin" , which then regulates cytoskeleton changes. "Macroergic molecules" : ATP stimulates actin polymerization, GTP stimulates tubulin polymerization, and cyclic nucleotides (cAMP and cGMP) stimulate both. "Cytosolic proteins" : These have diverse effects, such as linking monomers (tropomyosin, tropomodulin), cutting filaments (gelsolin), linking microfilaments into networks (spectrin, α-actinin) or parallel bundles (fimbrin), and organizing microtubules (MAPs).

The cytoskeleton performs several major functions: "Provides cell shape" and internal spatial organization of the cytoplasm, and is responsible for most shape changes. "Participates in all cellular movements" . "Interacts with the plasmalemma" through the membrane skeleton (a network under the plasmalemma). "Interacts with cell organelles" and provides their movements (e.g., mitochondria, lysosomes).

Cellular movements are characteristic of life and are achieved by special proteins called "motor proteins" . These are molecular machines that convert the energy of "ATP hydrolysis" into mechanical force, leading to changes in the protein's spatial structure and movement.

Cellular movements are classified based on their underlying molecular mechanism: "I. Movements based on actin-myosin mechanism" : This includes muscle contraction, amoeboid locomotion, movements from microvilli, and some cytoplasmic currents (cyclosis). "II. Movements based on microtubule and other motor proteins" : This includes axoplasmic transport and movements of cilia and flagella.

Muscle contraction is based on interactions between "thin filaments (actin)" and the "globular heads of myosins (thick filaments) ".

The contractile unit of a myofibril is called a "sarcomere" . Each sarcomere is formed from an ordered array of parallel thin (actin) and thick (myosin) filaments. Thin filaments attach at their '+' ends to a "Z disc" at each sarcomere end and extend towards the middle, overlapping with thick filaments.

Sarcomere shortening (muscle contraction) is caused by the "myosin filaments sliding past the actin filaments" . In the presence of "Ca2+" (attached to troponins), tropomyosin molecules change conformation, allowing myosin globular heads to link with actin monomers. Myosin's ATPase activity provides energy for conformational changes, enabling each globular head to step on the adjacent actin filament. Links are disrupted and remade, leading to gradual shortening. During relaxation, all these links are concomitantly disrupted.

Amoeboid movement was first described in "Amoeba proteus" . In animals and humans, "neutrophil leukocytes" (for diapedesis and infection site migration) and "fibroblasts" (for wound healing and collagen deposition) use this mechanism.

Amoeboid movement is based on the "actin-myosin system" and "sol-gel and gel-sol transitions" of cytoplasm. Through these transitions, the cell emits extensions (pseudopodia in Amoeba/neutrophils, lamellipodia in fibroblasts) in the direction of movement. The cytoplasm then flows into the extension, pulling the cell forward.

Amoeboid locomotion involves three distinctive activities: 1. "Protrusion of a leading edge" (production of extension): Cell membrane is pushed out due to actin polymerization. 2. "Attachment to the substratum" : Protrusion adheres to the surface; myosin II connects actin filaments to the substratum via integrin-mediated adhesions. 3. "Traction" : The entire cell body is drawn forward due to gel-sol transition of the cytoplasm.

Amoeboid locomotion is guided by "physical and chemical factors" . For example, fibroblasts prefer metal over glass surfaces. Leukocytes are attracted by "chemotactic substances" (chemical signals) produced by bacteria, a movement called "chemotaxis" .

Microvilli are "permanent, short extensions" found on the surface of various cell types, such as "enterocytes, hepatocytes" , and epithelial cells from proximal renal convoluted tubules. Their primary role is in "absorption processes" .

Absorption via microvilli is enhanced by two mechanisms: 1. "Passive mechanism" : Microvilli increase the apical surface area (e.g., 20 times in enterocytes), providing a larger area for absorption. 2. "Active mechanism" : Involves the interaction between actin microfilaments and myosin molecules. Actin bundles within microvilli are anchored to the plasmalemma and laterally by myosin I arms. The actin-myosin interaction causes the microvillus to shorten, pushing absorbed material into the enterocyte cytoplasm.

Cytoplasmic currents are "intracellular movements" that transport organelles within the cell, such as the jumping movements of mitochondria or "cyclosis" (continuous movement of cytoplasm and chloroplasts in vegetal cells). They ensure contact between organelles and mix the cytoplasm. They are influenced by light, pH, and temperature.

Axoplasmic transport is a specific type of cytoplasmic current responsible for transport along the "axons of neurons" . It occurs in two directions: "anterograde" (forward, from perikaryon to axon end) and "retrograde" (backward, from axon end to perikaryon).

In "anterograde transport" , materials like those for axonal growth, neurotransmitter vesicles, mitochondria, lipid/protein vesicles for membrane maintenance, and soluble cytosolic proteins (actin, tubulin, MAPs) are transported. "Retrograde transport" brings old organelles and neurotransmitter vesicles back to the perikaryon for recycling.

Axoplasmic transport occurs at varying speeds: "Fast transport" : 3 µm/sec (e.g., synaptic vesicles). "Intermediate transport" : For mitochondria. "Slow transport" : Less than 1 mm/day (e.g., cytoskeleton molecules).

"Kinesin" is the main motor protein for "anterograde transport" . It is a dimer with two heavy chains (each with a globular head and coiled-coil region) and two light chains (attached to the tail). The heads interact with microtubules, hydrolyze ATP, and step towards the '+' end of the microtubule, transporting cargo attached to the light chains. When kinesin reaches the microtubule's '+' end, transport continues on peripheral actin microfilaments by myosin molecules. "Dynein" molecules perform "retrograde transport" . They move from the '+' end to the '-' end of microtubules, composed of heavy, intermediate, and light chains, with globular heads that have ATPase activity and interact with microtubules.

"Kinesin" is the main motor protein for "anterograde transport" . It is a dimer with two heavy chains (each with a globular head and coiled-coil region) and two light chains (attached to the tail). The heads interact with microtubules, hydrolyze ATP, and step towards the '+' end of the microtubule, transporting cargo attached to the light chains. When kinesin reaches the microtubule's '+' end, transport continues on peripheral actin microfilaments by myosin molecules. "Dynein" molecules perform "retrograde transport" . They move from the '+' end to the '-' end of microtubules, composed of heavy, intermediate, and light chains, with globular heads that have ATPase activity and interact with microtubules.

Axonal transport is crucial for delivering macromolecules and organelles to synapses (anterograde) and for retrograde delivery of signaling endosomes and autophagosomes for degradation. "Impairments of axonal transport" occur early in many "neurodegenerative diseases" (e.g., Alzheimer's, Parkinson's) and play a fundamental role in axonal degeneration.

Cilia and flagella are varieties of the same cellular structure, with an internal structure called an "axoneme" . They are permanent extensions, longer and larger than microvilli. In humans, "ciliated cells" are found in the respiratory tract, oviduct, and inner ear. "Sperm cells" are the only human cells with a flagellum.

Cilia movement has two steps: an "active step" (power stroke), where the cilium is straight and beats like a whip, and a "passive step" (recovery stroke), where it bends. All cilia move in coordinated, successive waves to displace mucus and foreign materials, cleaning epithelia and preventing infections. A flagellum exhibits a continuous "helical movement" .

A cilium consists of three parts: a "free part" (containing the axoneme), "roots" , and a "basal body" . The axoneme, surrounded by cytoplasm and plasmalemma, has a "9 peripheral doublets of microtubules" surrounding "2 central microtubules" (the 9+2 arrangement). Each peripheral doublet has a complete A subfiber (13 protofilaments) and an incomplete B subfiber (11 protofilaments). "Dynein arms" (protein arms) are linked to the A subfiber, with globular heads interacting with the adjacent doublet in a clockwise direction. Peripheral doublets are linked by "nexin" , and central microtubules are connected to peripheral doublets by "radial spokes" .

The axoneme in a flagellum has an identical 9+2 structure, but it is surrounded by "mitochondria" arranged spirally. These mitochondria provide the increased energy necessary for flagellar movement, as ATP molecules cannot diffuse from the cytoplasm over the flagellum's much longer length.

Cilia and flagella move through the formation and disruption of "lateral bridges" between the "dynein arms" of one doublet and the tubulins of an adjacent doublet. When dynein is activated by "ATP" , it attempts to walk along the adjacent microtubule doublet, forcing sliding. However, the presence of "nexin and radial spokes" between microtubule doublets prevents complete sliding, converting the dynein force into a "bending movement" .

If "axonemal dynein is absent or deficient" , cilia and flagella become immotile, leading to "repeated respiratory infections" and "sterility in males" (due to immotile sperm). This condition is known as "immotile cilia syndrome" (or primary ciliary dyskinesia). Sometimes, this syndrome is associated with "situs inversus" (inversed internal organs), forming "Kartagener syndrome" . This abnormal symmetry is due to disrupted or fluid flow in the developing embryo, as coordinated ciliary movement is essential for proper organ placement during development.

"Centrioles" and "basal bodies" are identical structures in terms of molecular organization and can interchange their roles. The "centrosome" (or cell center) in most animal cells contains two L-shaped centrioles surrounded by dense pericentriolar material (containing pericentrin and γ-tubulin).

"Centrioles" and "basal bodies" are identical structures in terms of molecular organization and can interchange their roles. The "centrosome" (or cell center) in most animal cells contains two L-shaped centrioles surrounded by dense pericentriolar material (containing pericentrin and γ-tubulin).

Centrioles are cylindrical structures made of "9 triplets of microtubules" (one complete and two incomplete in each triplet) with an empty central region. In interphase, the centrosome coordinates the dynamic network of microtubules. During cellular division, the two centrioles replicate to form the two poles of the mitotic spindle, with microtubules having their '-' end in the pericentriolar material and their '+' end distal.

The "basal body" is continuous with the axoneme, where the axonemal doublet becomes a triplet, and central microtubules stop. Like cytoplasmic microtubules, axonemal microtubules have their '-' end in the basal body and '+' end distal. Basal bodies coordinate the movements of cilia and flagella and the polymerization of tubulin within the axoneme. Nexin, spokes, and the internal sheath also contribute to movement coordination.

Basal bodies can take on the functions of centrioles. For example, in the unicellular alga "Chlamydomonas" , when mitosis begins, its two flagella retract, and their basal bodies migrate near the nucleus to become centrioles. This demonstrates their functional interconversion and role in coordinating tubulin polymerization/depolymerization processes.