The Cell Theory has three main tenets: (1) All living things are made of one or more cells; (2) The cell is the basic structural and functional unit of life; (3) All cells arise from pre-existing cells (Omnis cellula e cellula). Cells vary enormously in size (most are 1-100 micrometers), shape, and function. Two major cell types exist: prokaryotic (bacteria and archaea, lacking a nucleus) and eukaryotic (plants, animals, fungi, protists, containing membrane-bound organelles including a nucleus). Key cell structures include the cell membrane (controls what enters/exits), cytoplasm (jelly-like substance), ribosomes (protein synthesis), and genetic material (DNA). Eukaryotic cells contain specialized organelles like mitochondria (energy production), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (modification and packaging), and lysosomes (waste breakdown). Understanding cells is essential for medicine, genetics, and biotechnology.

Photosynthesis occurs in chloroplasts, organelles containing chlorophyll (the green pigment that captures light energy). The process has two stages: light-dependent reactions (thylakoid membranes) and the Calvin cycle (stroma). Light-dependent reactions convert light energy to ATP and NADPH, splitting water molecules and releasing oxygen. The Calvin cycle uses ATP and NADPH to fix carbon dioxide into organic molecules (glucose). Photosynthesis is essential for nearly all life on Earth – it produces oxygen for respiration, removes carbon dioxide from the atmosphere, and forms the base of most food chains. C3, C4, and CAM plants have different photosynthetic adaptations to various environments. Factors affecting photosynthesis include light intensity, carbon dioxide concentration, temperature, and water availability. Without photosynthesis, Earth's atmosphere would lack oxygen and complex life could not exist.

Cellular respiration consists of four main stages: glycolysis (cytoplasm, breaks glucose into pyruvate, produces 2 ATP), pyruvate oxidation (mitochondrial matrix, converts pyruvate to acetyl-CoA), the Krebs cycle (mitochondrial matrix, produces NADH, FADH₂, and 2 ATP), and the electron transport chain (inner mitochondrial membrane, produces about 34 ATP). The total theoretical yield is about 36-38 ATP per glucose molecule, though actual yield is around 30-32 ATP. Aerobic respiration requires oxygen as the final electron acceptor, producing water. Without oxygen, cells perform anaerobic respiration (fermentation), producing lactate (animals) or ethanol and CO₂ (yeast) and only 2 ATP per glucose. Cellular respiration is why we breathe – oxygen is consumed and carbon dioxide is produced. Metabolism includes both catabolism (breaking down molecules, releasing energy) and anabolism (building molecules, consuming energy).

The DNA double helix resembles a twisted ladder: sugar-phosphate backbones form the sides, and base pairs form the rungs. Base pairing is specific: A pairs with T (two hydrogen bonds), and C pairs with G (three hydrogen bonds). The sequence of bases along a DNA strand encodes genetic information. DNA is organized into chromosomes (humans have 46 chromosomes, 23 pairs). Genes are specific DNA segments that code for proteins or functional RNA molecules. DNA replication is semi-conservative – each new DNA molecule contains one original strand and one newly synthesized strand. Mutations are changes in DNA sequence that can be caused by errors during replication, radiation, or chemicals. The Human Genome Project (completed in 2003) sequenced all 3 billion base pairs of human DNA. Genetic engineering, CRISPR-Cas9 gene editing, and DNA fingerprinting are practical applications of DNA knowledge.

Darwin's theory of evolution by natural selection was published in "On the Origin of Species" (1859). Key principles include: (1) Variation exists within populations; (2) More offspring are produced than can survive; (3) Competition for limited resources occurs; (4) Individuals with advantageous traits are more likely to survive and reproduce; (5) Over time, favorable traits become more common in the population. Evidence for evolution includes the fossil record (transitional forms like Tiktaalik), comparative anatomy (homologous structures like mammal forelimbs), embryology (similar early development across species), molecular biology (DNA sequence similarities), and direct observation (antibiotic resistance in bacteria, Darwin's finches beak changes). Evolution explains the unity (common ancestry) and diversity of life. Misconceptions include believing evolution is "just a theory" (scientific theories are well-substantiated explanations) or that individuals evolve (populations evolve). Modern synthesis combines Darwinian evolution with Mendelian genetics.

Ecological organization levels: individual → population (same species, same area) → community (all populations in an area) → ecosystem (community + abiotic factors) → biome (large region with similar climate) → biosphere (global sum of all ecosystems). Producers (autotrophs) – plants, algae, cyanobacteria – capture energy through photosynthesis. Consumers (heterotrophs) include herbivores (primary consumers), carnivores (secondary/tertiary consumers), omnivores, and decomposers (bacteria, fungi) which break down dead matter. Food chains show linear energy transfer; food webs show interconnected relationships. Energy pyramids demonstrate that only about 10% of energy transfers between trophic levels – the rest is lost as heat. Key biogeochemical cycles: carbon (photosynthesis, respiration, combustion, fossil fuels), nitrogen (fixation, nitrification, denitrification), water (evaporation, condensation, precipitation, runoff). Biodiversity increases ecosystem stability and resilience. Human activities like deforestation, pollution, and climate change disrupt ecosystems.