Study Guide: Biology I

Biology I: Introduction to Biological Knowledge

Biology is a relatively recent science, although the observation of living beings has occurred for many centuries. Since humans appeared on the planet, they have observed the organisms around them, initially describing those that provided a benefit or caused harm, and later grouping them based on differences and similarities. Formal study began in Greece with the work of philosophers and naturalists whose knowledge remained valid for nearly 2000 years, despite a lack of rigorous methodology to guarantee veracity. In the 4th century B.C., Aristotle performed the first classification of living beings. By the 2nd century B.C., Herophilus of Chalcedon and Galen (130–200 A.D.) made early contributions to human anatomy and physiology; Galen, the last great representative of ancient medicine, served as a physician to gladiators and emperors, and his scientific system persisted well into the Middle Ages. In the 1st century B.C., Pliny the Elder wrote "Historia Naturalis," an encyclopedic treatise describing the plants and animals of ancient Greece.

Following a period of scientific stagnation in Europe during the Middle Ages (centuries I to XVI), significant advancements resumed in 1590 with Galileo Galilei and Zacharias Janssen, who contributed to the invention of the microscope. In 1628, William Harvey observed blood circulation, noting that blood moves from arteries to veins to return to the heart, which pumps it via heartbeats. In the mid-17th century, Anthony Van Leeuwenhoek constructed a microscope capable of magnifying images up to $270 \times$, revealing the microscopic world. The 18th century saw Carl von Linnaeus find universally valid principles for classifying life; his binomial nomenclature of genus and species in Latin remains the basis for biological naming today.

In the 19th century, the Cell Theory was postulated by Matthias Schleiden, Theodor Schwann, and Rudolf Virchow, stating that the cell is a living being that is born, feeds, reproduces, and dies. Jean-Baptiste Lamarck proposed that species transform over time due to varying environmental demands. In 1859, Charles Darwin published "On the Origin of Species by Means of Natural Selection," arguing that evolution occurs through natural selection. By the mid-19th century, Louis Pasteur, the father of microbiology, debunked the theory of spontaneous generation and developed a rabies vaccine. In 1865, Gregor Mendel established the laws of inheritance through pea plant cultivation. The 20th century featured Alexander Ivanovich Oparin’s biochemical theory on the origin of life (1921), Thomas H. Morgan’s discovery of the mutagenic power of X-rays (1927), and James Watson and Francis Crick’s 1953 discovery of the double-helix structure of DNA. Recent milestones include the 1997 cloning of Dolly the sheep and the 2000 deciphering of the human genome.

Branches and Auxiliary Sciences of Biology

Biology is divided into specialized branches to study the vast diversity of life from different perspectives. Anatomy examines the structure of living beings, while Botany focuses specifically on plants. Cytology is dedicated to the structure and function of cells, and Histology studies tissues. Genetics investigates the principles regulating the transmission of hereditary traits. Ecology studies the interactions between living beings and their environment. Evolution tracks the origin and transformation of living matter over time. Paleontology studies life from past eras through fossils. Taxonomies order and classify living beings based on evolutionary kinship. Physiology investigates the functions of living beings, Zoology focuses on animals, and Microbiology studies microorganisms.

Biology also relies on auxiliary sciences. Physics studies the state and movement of matter; Chemistry examines the molecular composition and structure of living matter; Geography studies physical phenomena on the Earth's surface; History provides temporal context for biological discoveries; and Mathematics quantifies biological phenomena. These interactions have given rise to hybrid disciplines: Biophysics (applying physics to living systems), Biochemistry (studying chemical components and metabolic changes), and Biogeography (explaining the geographic distribution of organisms).

Methodology and Characteristics of Living Beings

The scientific method is a series of ordered steps used to solve biological problems. It begins with the "Statement of the Problem," a clear question resulting from observation. This is followed by the "Theoretical Framework," which involves a bibliographic review. A "Hypothesis" is then formulated as a testable possible answer. "Testing the Hypothesis" involves four stages: Experimental Design (planning the test), Experimentation (carrying out the work), Results (analyzing data, often statistically), and Conclusions (verifying if the hypothesis was correct). If repeatedly verified, a hypothesis may become a Theory, and if a theory is verifiable and expressed through a mathematical model, it may become a Law.

Living beings share specific characteristics that distinguish them from non-living matter. These include "Specific Organization" (all are composed of cells, either unicellular or multicellular), "Metabolism" (the continuous transformation of matter and energy for functions like nutrition and respiration), "Movement" (locomotion or internal fluid displacement), and "Irritability" (the capacity to respond to physical or chemical environmental stimuli, such as plants growing against gravity). Other traits include "Growth" (increasing cell number or body proportions), "Reproduction" (sexual or asexual perpetuation of the species), and "Adaptation" (structural, physiological, or behavioral traits that allow survival in specific environments, such as cactus spines).

Unit II: Life and its Environment

Ecology, defined by Ernest Haeckel in 1870 from the Greek "oikos" (house) and "logos" (study), examines the relationships between organisms and their environment. An organism's “habitat” is where it lives, while its “niche” is the functional role it plays (e.g., plants as producers). A “population” is a group of the same species, and a “community” comprises all populations in an area. An “ecosystem” integrates these biotic and abiotic factors.

Biotic factors are categorized into producers (autotrophs like plants and algae that convert light into chemical energy via photosynthesis), consumers (heterotrophs like herbivores, carnivores, and scavengers), and decomposers (bacteria and fungi that break down dead organic matter). Abiotic factors determine species survival and are classified as Chemical (water, air, soil) or Physical. Key abiotic factors include: Latitude (distance from the equator), Altitude (height above sea level; oxygen decreases at high altitudes), Luminosity (light intensity), Temperature (typically restricted to $-10$ to $40\,^{\circ}\text{C}$), Water (essential for life), and Air (composed of $78\% \text{ Nitrogen}$, $21\% \text{ Oxygen}$, and $0.033\% \text{ Carbon Dioxide}$). Atmospheric Pressure is also vital, as living beings maintain an internal pressure balanced with the external environment.

Energy flows through food chains (theoretical models of feeding relationships) and food webs (interconnected chains within an ecosystem). Producers start the chain, followed by primary consumers (herbivores like deer), secondary consumers (small carnivores like frogs), and tertiary consumers (large predators like tigers). Decomposers close the cycle by returning minerals to the soil.

Biogeochemical Cycles and Photosynthesis

Nutrient cycles describe the movement of substances between living beings and the environment. Carbon is the base of organic compounds, found in the atmosphere as $CO_2$ ($0.033\%$ proportion). Producers fix $CO_2$ during photosynthesis to create glucose ($C_6H_{12}O_6$), which is then used by heterotrophs and released back via respiration, decomposition, or combustion. The Nitrogen Cycle involves "Biological Fixation," where bacteria transform atmospheric nitrogen into nitrates and ammonium for plants. De-nitrifying bacteria return nitrogen to the atmosphere, while "Atmospheric Fixation" occurs via electrical discharges during storms. The Water Cycle involves evaporation, condensation into clouds, and precipitation (rain, snow, dew). Plants contribute through transpiration.

Photosynthesis is the process by which light energy, water, and $CO_2$ are converted into glucose. It occurs in chloroplasts containing chlorophyll. The chemical equation is:

$6H_2O + 6CO_2 \xrightarrow[\text{Chlorophyll}]{\text{Light Energy}} C_6H_{12}O_6 + 6O_2$

It consists of two stages: the "Light Phase" (chlorophyll absorbs light, ATP is produced, and water molecules are split to release oxygen) and the "Dark Phase" (combination of $CO_2$ and hydrogen to form glucose, which later becomes starch, proteins, and lipids).

Ecosystem Diversity and Environmental Problems

Ecosystems are divided into aquatic and terrestrial. Aquatic environments include saltwater (oceans/seas) and freshwater (rivers/lakes). Marine organisms are classified as: Pelagic (wandering in open sea), Plankton (floating with little movement), Nekton (active swimmers like whales/turtles), and Benthos (living on the seafloor like corals). Abyssal organisms inhabit great depths and often emit light. Major issues include oil pollution and irrational fishing.

Mexican terrestrial ecosystems are varied:

• Tropical Forest (Selva): Hot, humid, year-round rain. Features jaguars, monkeys, and mahogany. Problems: logging and illegal species trafficking.
• Coniferous Forest: Cold and humid. Features pines, firs, wolves, and owls. Problems: logging and fires.
• Mixed/Temperate Forest: Diverse trees (oak, pine). Features foxes and woodcocks.
• Savanna: Hot with long droughts. Features grasses, coyotes, and hares. Problem: overgrazing.
• Desert: Hot days, cold nights, scarce rain. Features xerophytic plants (cacti), roadrunners, and rattlesnakes. Problem: illegal trafficking.

Unit III: Diversity of Life and Taxonomy

To manage biological diversity, scientists categorize organisms by nutrition (Autotrophs vs. Heterotrophs), size (Microorganisms vs. Macroorganisms), cell count (Unicellular vs. Pluricellular), habitat (Terrestrial vs. Aquatic), respiration (Aerobic vs. Anaerobic), and cell type (Prokaryotic vs. Eukaryotic). Taxonomy uses a hierarchical system: Kingdom (broadest), Phylum/Division, Class, Order, Family, Genus, and Species (unit of classification). Members share a phylogenetic history.

Linnaeus's Binomial Nomenclature rules require: 1) The first name is the Genus (capitalized); 2) The second is the Species (lowercase); 3) Names must be in Latin and italicized or underlined. Example: Canis familiaris (dog).

The Five Kingdoms of Life

1. Monera: Prokaryotic, unicellular. Includes bacteria (cocci, bacilli, spirilla, vibrios) and cyanobacteria (photosynthetic).
2. Protista (Protoctista): Eukaryotic, mostly aquatic. Includes algae (green, brown, red, diatoms) and protozoans like amoebas. Some move via cilia, flagella, or pseudopods.
3. Fungi: Eukaryotic, heterotrophic by absorption (parasites or saprophytes). Includes microscopic yeasts/molds and macroscopic mushrooms. Lack chlorophyll.
4. Plantae: Eukaryotic, multicellular, autotrophic. Includes Bryophytes (mosses), Pteridophytes (ferns), Gymnosperms (conifers), and Angiosperms (flowering plants).
5. Animalia: Eukaryotic, multicellular, heterotrophic by ingestion. Includes Poriferans (sponges), Coelenterates (jellyfish), Mollusks, Annelids, Arthropods, Echinoderms, and Vertebrates (Fish, Amphibians, Reptiles, Birds, Mammals).

Mexico is a "megadiverse" country, home to $10\%$ of the world’s species due to its privileged geography crossing the temperate and intertropical zones. It ranks first in reptiles and fourth in mammals. Many species are endemic (exclusive to Mexico). Endangered Mexican species include the Ajolote (Ambystoma mexicanus), Bighorn Sheep (Ovis canadensis), Peregrine Falcon (Falco peregrinus), Jaguar (Felis onca), Mexican Wolf (Canis lupus baileyi), and Ocelot (Felis pardalis).

Unit IV: Evolution

Evolution is the fact that species change over time and share common ancestors. Early theories included Catastrophism (George Cuvier), positing that disappearances resulted from natural catastrophes followed by divine creation. Lamarck proposed the first scientific theory: use and disuse of organs and the inheritance of acquired characters. This was rejected due to lack of evidence.

Charles Darwin’s theory (Natural Selection) posits that evolution is gradual and continuous, organisms are related, and selection acts on variability. Darwin's