Profeta Bio parcial #1 Comprehensive University Study Guide

Fundamental Classification of Bioelements and Chemical Bonds

Biological systems are composed of specific elements classified based on their abundance and necessity. Sodium ($Na$) is categorized as a secondary bioelement, whereas iron ($Fe$) is considered an indispensable bioelement required for vital physiological functions. At the most fundamental level of the organization of matter, the subatomic level represents the simplest stage. The interactions between these elements and molecules are governed by various chemical bonds. Covalent bonds occur when atoms share electrons to achieve stability. In contrast, ionic bonds are formed through the electrostatic attraction between oppositely charged ions. Weaker intermolecular forces also play a crucial role, such as Van der Waals forces, which are characterized as bonds forming between transient dipoles.

Structural Characteristics and Isomerism of Carbohydrates

Carbohydrates, or saccharides, exhibit specific structural arrangements and linkages that define their biological identity. The simplest form of a monosaccharide is glyceraldehyde ($C_3H_6O_3$). Within the hexoses, structural differences are defined by the orientation of groups around specific carbon atoms. Glucose is differentiated from mannose at the level of the second carbon ($C_2$), while glucose is distinguished from galactose at the level of the fourth carbon ($C_4$). In the context of pentoses, ribose is differentiated from deoxyribose by the chemical composition at the second carbon ($C_2$).

Disaccharides and polysaccharides are formed through glycosidic linkages. Lactose is a disaccharide composed of the monosaccharides galactosa and glucose. Polysaccharides are classified by their complexity and bond types; for instance, peptidoglycan (also known as peptidoglucano) is identified as a heteropolysaccharide. The storage polysaccharide starch is characterized by alpha ($\alpha$) type bonds, whereas the structural polysaccharide cellulose utilizes beta ($\beta$) type bonds.

Lipid Classification, Signaling, and Odorous Principles

Lipids serve diverse roles from structural components to signaling molecules. Saponifiable lipids include substances such as waxes. Fatty acids are classified by their saturation; saturated fatty acids are typically solid at room temperature ($25\,^{\circ}C$). In naturally occurring lipids, double bonds in fatty acids generally adopt a cis ($cis$) conformation. Omega fatty acids are categorized by the position of their last double bond. Essential Omega-6 fatty acids include linoleic acid and arachidonic acid; notably, linoleic acid is not an Omega-3 fatty acid.

Complex lipids include sphingolipids, which are characterized by the absence of a glycerol backbone. Within this group, a ceramida combined with an oligosaccharide forms a ganglioside. Sphingolipids also serve as the chemical basis for the antigens found on red blood cells. Glycolipids include examples such as cerebrosides and gangliosides, while phosphatidylcholine (fosfatidilcolina) is a major representative of the phospholipid class.

Biological signaling is often mediated by lipid derivatives. Leukotrienes are responsible for attracting leukocytes to damaged tissue, while thromboxane stimulates platelet aggregation. Eicosanoid metabolism is a key target for pharmaceuticals; Non-Steroidal Anti-Inflammatory Drugs ($AINES$) function by inhibiting the cyclooxygenase ($COX$) enzyme. Glucocorticoids exert their effects by inhibiting the Phospholipase 2 ($PLA_2$) enzyme. Additionally, steroid hormones such as testosterone and progesterone are derived from lipid precursors. Terpenes form the chemical base of odorous principles in plants, an example of which is timol, the characteristic scent found in mandarins.

Protein Selection, Structure, and Nucleic Acid Foundations

Proteins are polymers constructed from amino acid monomers. Most amino acids possess an asymmetric carbon; however, glycine ($Gly$) is the notable exception as it lacks an asymmetric carbon. Specific amino acids like cysteine ($Cys$) are capable of forming stabilizing disulfide bonds. The importance of primary structure is illustrated by sickle cell anemia, a condition where glutamic acid is replaced by valine ($Val$) in the hemoglobin chain.

Nucleic acids consist of nucleotide monomers. A nucleoside specifically consists of a nitrogenous base and a sugar. Adenine ($A$) and guanine ($G$) serve as examples of purines. In DNA, adenine pairs with thymine ($T$) through the formation of two ($2$) hydrogen bonds. The physical structure of DNA varies; for instance, Z-DNA is characterized by a diameter of approximately $1.8\,nm$. Nucleotides within a single DNA strand are linked via phosphodiester bonds.

Microbiology: Prokaryotic Diversity and the Viral Domain

Microorganisms are categorized by their cellular structures and metabolic processes. Prokaryotic cells, despite their simplicity, possess ribosomes for protein synthesis. Certain prokaryotes like Mycoplasma are unique because they lack a cell wall, whereas Gram-positive bacteria are characterized by a thick peptidoglycan wall. Cyanobacteria are notable for performing oxygenic photosynthesis and producing geosmin (geosmina). Archaea are distinguished by their branched membrane fatty acids and the fact that their protein synthesis is not inhibited by the antibiotic chloramphenicol ($Cloranfenicol$). Additionally, in Archaea, the molecule pseudo-purine replaces thymine ($T$). Eukarya are defined by the possession of cytoplasmic organelles dedicated to energy production.

Viruses are non-cellular entities that exhibit only a few cellular properties: reproduction, inheritance, and mutation. They can follow different life cycles, such as the lysogenic cycle where viral DNA integrates into the host chromosome. Viruses are classified by their structure and genome; examples include adenovirus (a naked DNA virus), smallpox or viruela (an enveloped DNA virus), and polio (a naked RNA virus with a polyhedral structure). A potential fourth domain of life is represented by the Mimivirus.

Dynamics and Permeability of the Plasma Membrane

The plasma membrane is a semi-permeable lipid bilayer whose fluidity and permeability are strictly regulated. The relationship between molecular size and permeability is inversely proportional: a larger molecular size correlates with lower permeability ($\text{size} \propto \frac{1}{\text{permeability}}$). Carbon dioxide ($CO_2$) is permeable to the bilayer, whereas water ($H_2O$) and urea are mildly or slightly permeable. Conversely, sodium ($Na$), glucose, and proteins are impermeable. The physical response of an erythrocyte to a hypotonic medium involves the cell exploding (lysis) rather than crenating.

Membrane fluidity is impacted by various components; both cholesterol and calcium ($Ca^{2+}$) ions act to decrease the fluidity of the membrane. Cholesterol is unique in that it is found on both the inner and outer leaflets of the membrane. Other lipids are distributed asymmetrically: phosphatidylserine (fosfatidilserina) is located on the inner face, while phosphatidylcholine (fosfatidilcolina) is located on the outer face. Lipid rafts, specifically flat rafts, are characterized by the presence of the protein flotillin. Amphipathic molecules with a cylindrical cross-section typically possess two chains, and a bilayer of these molecules can form structures known as liposomes.

Membrane Proteins and Intercellular Junctions

Membrane proteins are categorized by their association with the lipid bilayer. Cadherins are integral membrane proteins that engage in homophilic interactions and are essential for various cell junctions. In adherent junctions, cadherins are found on the exterior of the cell, and they also facilitate the intracellular union within desmosomes. Tight junctions (uniones estrechas) involve the alignment between claudin and occludin. Peripheral proteins also serve structural roles; an example is the protein Banda 4.1 found in the erythrocyte membrane.