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  Biology fundamentals — cell biology, genetics, evolution, ecology, physiology, and molecular biology basics.




  Sources: OpenStax textbooks (CC-BY), Khan Academy, MIT OCW, NOAA, IPCC reports.
---

# Biology Fundamentals

## Cell biology

**Cell theory:** All living things are made of cells; cells come from pre-existing cells; cells are the basic unit of life.
**Prokaryotic vs. eukaryotic:**
Prokaryotic (bacteria, archaea): No nucleus. Circular DNA. Smaller (~1-10 µm). Faster reproduction.
Eukaryotic (plants, animals, fungi, protists): Membrane-bound nucleus. Linear DNA. Larger (~10-100 µm).

**Key organelles:**
Nucleus: Contains DNA. Command center.
Mitochondria: ATP production through cellular respiration. Have own DNA.
Ribosomes: Protein synthesis from mRNA instructions.
ER (Endoplasmic Reticulum): Rough ER = protein processing; Smooth ER = lipid synthesis.
Golgi apparatus: Package/modify/ship proteins.
Chloroplasts (plants): Photosynthesis. Have own DNA.
Lysosomes: Cellular digestion. Break down waste and foreign material.

## DNA and genetics

**DNA structure:** Double helix. Deoxyribose sugar + phosphate backbone + nitrogenous bases.
Base pairing: A-T, G-C (antiparallel strands).
**Central dogma:** DNA → RNA → Protein (transcription then translation).
**Gene:** Sequence of DNA that codes for a functional product (usually protein).
**Codon:** 3-nucleotide sequence that codes for an amino acid (64 codons, 20 amino acids, genetic code is redundant).

**Mendelian genetics:**
Dominant allele: Expressed when present (capital letter, e.g., B).
Recessive allele: Expressed only when homozygous (lowercase, e.g., b).
Homozygous: Two copies of same allele (BB or bb).
Heterozygous: One copy of each (Bb) — dominant expressed.
Punnett square: Predicts offspring probabilities.

## Evolution

**Natural selection:** Variation exists → some variations help survival/reproduction → heritable variations passed to offspring → population changes over generations.
**Evidence:** Fossil record, comparative anatomy, DNA sequence similarities, direct observation (bacterial antibiotic resistance, beak changes in Galápagos finches).
**Speciation:** Formation of new species. Geographic isolation → reproductive isolation → genetic divergence.

## Ecology

**Population ecology:** How populations change over time. Birth, death, immigration, emigration.
Exponential growth (J-curve): Unlimited resources. dN/dt = rN.
Logistic growth (S-curve): Carrying capacity limits growth. Levels off at K.
**Community ecology:** Interactions between species. Competition, predation, mutualism, commensalism, parasitism.
**Ecosystem ecology:** Energy flow and nutrient cycling. Primary productivity, trophic levels.
Only ~10% of energy transfers between trophic levels (10% rule).
**Biogeochemical cycles:** Carbon, nitrogen, water, phosphorus. Essential elements cycle through living and non-living components.

## Molecular biology basics

**PCR (Polymerase Chain Reaction):** Amplify specific DNA sequences. Foundation of genetic testing, forensics, COVID testing.
**CRISPR-Cas9:** Precise DNA editing. Cut specific sequence → repair can insert or delete. Revolutionary for medicine and research.
**Proteins:** Polymers of amino acids. Function determined by 3D shape. Shape determined by sequence.
Denaturation: Loss of 3D structure (heat, pH extremes) → loss of function.

Sources: OpenStax Biology 2e (CC-BY), Khan Academy Biology, Campbell Biology (principles), MIT OCW 7.012 Biology