Chapter 1: Introduction to Biology

What Is Biology?

The word biology comes from the Greek words bios (life) and logos (study). Biology is the scientific study of living things — everything from the tiniest bacteria invisible to the naked eye to the blue whale, the largest animal on Earth at up to 30 meters (98 feet) long and 180,000 kg (400,000 pounds).

Right now, scientists estimate there are roughly 8.7 million species of life on Earth, but we have only formally described about 1.2 million of them. That means the vast majority of life on this planet is still unknown! Biology is the discipline trying to catalogue, understand, and explain all of it.

The Seven Characteristics of Life

1. Made of Cells: Every living thing is made of at least one cell. Your body contains roughly 37 trillion cells.
2. Use Energy (Metabolism): Living things take in energy and use it to carry out life processes. You burn about 1,500–2,500 Calories per day just to stay alive.
3. Grow and Develop: A fertilized human egg weighs about 1 microgram; a newborn weighs about 3.4 kg — that is a 3.4 billion-fold increase in mass!
4. Reproduce: All living things can produce offspring, either sexually or asexually.
5. Respond to Stimuli: Plants turn toward light; you pull your hand away from something hot. These are responses to stimuli.
6. Maintain Homeostasis: Your body keeps your internal temperature near 37°C (98.6°F) even when it is −20°C outside or 40°C outside.
7. Based on DNA: Every living thing contains deoxyribonucleic acid (DNA), which carries the instructions for building and running the organism.

Major Branches of Biology

Why Does Biology Matter?

Biology is not just something you study in school — it directly affects your daily life in profound ways:

• Medicine: Understanding how bacteria and viruses work led to antibiotics (saving over 200 million lives since 1940) and vaccines (eradicating smallpox, which once killed 300 million people in the 20th century alone).
• Food: Agricultural biology has produced crop varieties that yield 10× more food per acre than 100 years ago, allowing us to feed 8 billion people.
• Environment: Understanding ecosystems guides conservation — without biology, we would not know how to protect endangered species or restore damaged habitats.
• Biotechnology: Biologists use bacteria to produce human insulin (used by over 400 million diabetics worldwide) and are developing cancer treatments using the immune system.

The Scientific Method in Biology

Biology is a science, which means it uses a systematic approach to answer questions. The scientific method is not a rigid checklist — it is a cycle of questioning, testing, and revising.

Theories vs. Hypotheses vs. Laws

In everyday language, "theory" means a guess. In science, a theory is a well-tested, widely accepted explanation supported by large amounts of evidence from multiple independent sources. Evolution is a theory. Germ theory (diseases are caused by microbes) is a theory. These are not guesses — they are among the most rigorously tested ideas in all of human knowledge.

A hypothesis is a specific, testable prediction for a single experiment. A scientific law describes a pattern in nature (e.g., "energy cannot be created or destroyed") but does not explain why that pattern exists — that is the job of a theory.

Chapter 2: The Cell — Basic Unit of Life

Why the Cell?

In 1665, Robert Hooke looked at a thin slice of cork through a primitive microscope and saw tiny box-like compartments. He called them cells because they reminded him of monks' rooms (cells) in a monastery. Two centuries later, Matthias Schleiden (1838) and Theodor Schwann (1839) proposed that all living things are made of cells — a cornerstone of biology ever since.

The Cell Theory has three main points:

1. All living things are made of one or more cells.
2. The cell is the basic unit of structure and function in living things.
3. All cells come from pre-existing cells (Rudolf Virchow, 1855).

How Small Is a Cell?

Most cells are between 1 and 100 micrometers (μm) in diameter. A micrometer is one millionth of a meter. To put that in perspective: a human hair is about 70 μm wide. A typical human cheek cell is about 60 μm across. A bacterium like E. coli is only about 2 μm long. The period at the end of this sentence could fit roughly 500 bacteria side by side.

Prokaryotes vs. Eukaryotes

There are two fundamental types of cells, and every living thing belongs to one category or the other:

Animal Cell vs. Plant Cell

Organelles: The Cell's Specialized Structures

Think of a cell like a city. The nucleus is city hall, the mitochondria are the power plants, the ribosomes are the factories, the cell membrane is the city walls with guarded gates, and the Golgi apparatus is the postal service. Each organelle has a specific job.

The Mitochondria: Powerhouse of the Cell

Mitochondria deserve special attention. They have two membranes: a smooth outer membrane and a highly folded inner membrane called cristae. These folds dramatically increase the surface area available for producing ATP. The fluid inside is called the matrix.

Fascinatingly, mitochondria have their own circular DNA, separate from the cell's nuclear DNA. This is strong evidence for the endosymbiotic theory — the idea that mitochondria were once free-living bacteria that were engulfed by a larger cell about 1.5 billion years ago and formed a mutually beneficial partnership. The chloroplast has a similar origin.

A single liver cell can contain up to 2,000 mitochondria, while a red blood cell has none at all. Muscle cells, which need enormous amounts of energy, are packed with them.

The Cell Membrane: Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model (proposed by Singer and Nicolson in 1972). It consists of:

• Phospholipid bilayer: Two layers of phospholipids. Each phospholipid has a hydrophilic ("water-loving") head that faces outward and two hydrophobic ("water-fearing") fatty acid tails that point inward. This arrangement automatically forms a barrier between water-based environments inside and outside the cell.
• Proteins: Some span the entire membrane (integral/transmembrane proteins) and act as channels or pumps. Others sit on the surface (peripheral proteins) and act as receptors or enzymes.
• Cholesterol: Tucked between phospholipids, cholesterol helps maintain the right fluidity — keeping the membrane from becoming too rigid when cold or too loose when warm.
• Carbohydrate chains: Attached to proteins (glycoproteins) or lipids (glycolipids), they act like identification tags — this is how your immune cells recognize "self" vs. "foreign."

Chapter 3: Cell Processes

Cell Membrane Transport

The cell membrane controls traffic into and out of the cell. There are two main categories: passive transport (no energy required) and active transport (energy required).

Diffusion

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration — moving down the concentration gradient. Think of spraying perfume in one corner of a room — eventually the scent molecules spread evenly throughout.

No energy (ATP) is needed because molecules are naturally in random motion. Molecules move "downhill" from crowded areas to less crowded ones. This continues until equilibrium is reached (equal concentration everywhere).

Osmosis

Osmosis is a special type of diffusion — it is the movement of water specifically through a selectively permeable membrane. Water moves from an area of low solute concentration (dilute solution, more water) to an area of high solute concentration (concentrated solution, less water).

• Hypotonic solution (lower solute outside): Water enters the cell → cell swells. In animal cells, this can cause the cell to burst (lysis). In plant cells, the cell wall prevents bursting; the cell becomes firm (turgid) — this is how plants stay upright!
• Hypertonic solution (higher solute outside): Water leaves the cell → cell shrinks. In animal cells, this is called crenation. In plant cells, this is called plasmolysis (the membrane pulls away from the cell wall) — this is why plants wilt when over-salted.
• Isotonic solution (equal solute): No net movement of water → cell stays the same size. Human blood plasma is isotonic to our red blood cells (0.9% salt solution).

Active Transport

Active transport moves molecules against the concentration gradient — from low to high concentration, like pushing a ball uphill. This requires energy in the form of ATP and special carrier proteins called pumps.

A famous example is the sodium-potassium pump (Na⁺/K⁺ pump) in nerve and muscle cells. It pumps 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) in, using 1 ATP molecule per cycle. This creates the electrochemical gradient that allows nerve impulses to fire. At rest, a neuron runs this pump constantly, using about one-third of all the ATP the cell produces.

Other types of active transport include endocytosis (cell engulfs large molecules by wrapping the membrane around them — like phagocytosis in white blood cells eating bacteria) and exocytosis (cell expels substances by fusing a vesicle with the membrane — how neurons release neurotransmitters).

Cellular Respiration

Cellular respiration is how cells release the chemical energy stored in glucose and convert it to usable ATP. It is NOT the same as breathing (though breathing supports it). Every cell in your body — plant, animal, fungus, bacterium — performs some form of cellular respiration.

Step 1: Glycolysis (in the cytoplasm)

One glucose molecule (6 carbons) is split into two molecules of pyruvate (3 carbons each). This produces a net gain of 2 ATP and 2 NADH. No oxygen is needed. This step happens in the cytoplasm, not the mitochondria. Glycolysis is an ancient process — even bacteria that lived before oxygen existed in Earth's atmosphere used glycolysis.

Step 2: Krebs Cycle / Citric Acid Cycle (in mitochondrial matrix)

Each pyruvate is converted to Acetyl-CoA and enters the Krebs Cycle. This 8-step cycle strips electrons from carbon compounds, producing CO₂ as a waste product (this is the CO₂ you exhale!) and generating electron carriers (NADH, FADH₂) plus a small amount of ATP (2 ATP total). Think of the Krebs cycle as stripping hydrogen atoms off of carbon skeletons.

Step 3: Electron Transport Chain (on inner mitochondrial membrane)

This is where most of the ATP is made — about 32–34 ATP. The NADH and FADH₂ from the previous steps donate their electrons to a series of protein complexes embedded in the inner mitochondrial membrane. Electrons pass down the chain like falling water, releasing energy that is used to pump H⁺ ions across the membrane. These ions then flow back through an enzyme called ATP synthase, which spins like a turbine to produce ATP. Oxygen is the final electron acceptor, combining with H⁺ to form water. This is why you need oxygen to survive — without it, the electron transport chain stops and ATP production crashes.

Photosynthesis

Photosynthesis is the process by which plants, algae, and cyanobacteria capture light energy and store it as chemical energy (glucose). It is the opposite of cellular respiration. All food on Earth ultimately traces back to photosynthesis — every hamburger, salad, piece of fruit, and grain of rice.

Photosynthesis occurs in the chloroplast and has two main stages:

Stage 1: Light Reactions (in the thylakoid membranes)

Chlorophyll and other pigments absorb sunlight (mostly red and blue wavelengths; they reflect green — which is why plants look green!). This energy is used to:

• Split water molecules (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen released here is the oxygen you breathe!
• Produce ATP and NADPH (energy carriers for stage 2).

The light reactions happen on the stacked membrane structures inside the chloroplast called thylakoids (stacks called grana).

Stage 2: Calvin Cycle (in the stroma)

Using the ATP and NADPH from the light reactions, the Calvin Cycle "fixes" CO₂ from the air into organic molecules. In a 3-carbon pathway (the most common — called C3 photosynthesis), CO₂ is attached to a 5-carbon molecule called RuBP, eventually producing G3P (glyceraldehyde-3-phosphate), which is used to build glucose. The Calvin Cycle runs 6 times to produce one glucose molecule, using 18 ATP and 12 NADPH.

Fermentation

Fermentation is an anaerobic process (no oxygen required) that allows glycolysis to continue when oxygen is unavailable. Without fermentation, glycolysis would stop because NAD⁺ (needed to accept electrons during glycolysis) would run out.

• Alcoholic fermentation (yeast and some bacteria): Pyruvate → ethanol + CO₂. This is how bread rises (the CO₂ creates bubbles in dough), how beer and wine are made, and how biofuel ethanol is produced.
• Lactic acid fermentation (animal muscle cells, some bacteria): Pyruvate → lactic acid. When you sprint and your muscles can't get enough oxygen, your cells switch to lactic acid fermentation. The buildup of lactic acid causes the burning sensation in your muscles. Bacteria use this process to make yogurt, cheese, and sauerkraut.

Note: fermentation produces only 2 ATP per glucose (just from glycolysis) compared to ~38 ATP from aerobic respiration. It is far less efficient, which is why you can't sprint at full speed for very long.

Chapter 4: DNA & Genetics

The Discovery of DNA

For centuries, people observed that traits pass from parents to offspring — but no one knew the mechanism. In 1953, James Watson and Francis Crick, building on X-ray crystallography work by Rosalind Franklin and Maurice Wilkins, proposed the double helix structure of DNA — one of the most important discoveries in all of science.

DNA Structure

DNA (deoxyribonucleic acid) is a polymer — a long chain of repeating units called nucleotides. Each nucleotide has three components:

1. A phosphate group (−PO₄)
2. A deoxyribose sugar (5-carbon sugar)
3. A nitrogenous base (one of four: Adenine, Thymine, Guanine, or Cytosine)

Two strands of nucleotides wind around each other forming the double helix — resembling a twisted ladder. The sugar-phosphate groups form the sides of the ladder; the bases pair across the middle as the rungs.

The human genome contains about 3.2 billion base pairs of DNA. If you stretched out all the DNA from a single human cell, it would be about 2 meters (6.5 feet) long! Yet it fits inside a nucleus about 6 micrometers across because it is coiled around proteins called histones, forming structures called nucleosomes, which are further coiled and compacted into chromosomes.

Chromosomes and Genes

Human cells contain 46 chromosomes (23 pairs) in the nucleus of every body cell. One chromosome from each pair came from your mother (via the egg) and one from your father (via the sperm). Chromosomes 1–22 are called autosomes; the 23rd pair determines biological sex: XX = female, XY = male.

A gene is a specific segment of DNA on a chromosome that codes for a particular protein or trait. The human genome contains approximately 20,000–25,000 protein-coding genes. Genes make up only about 1.5–2% of the total genome — the rest was once called "junk DNA" but is now known to have regulatory and structural functions.

DNA Replication

Before a cell divides, it must copy all of its DNA so each daughter cell gets a complete set. DNA replication is semiconservative — each new DNA molecule consists of one original (template) strand and one newly synthesized strand.

Steps of DNA Replication:

1. Unwinding: An enzyme called helicase unzips the double helix by breaking the hydrogen bonds between base pairs, creating a "replication fork."
2. Primer placement: Short RNA primers mark where replication will begin.
3. Synthesis: DNA polymerase reads each template strand (3'→5') and builds the new strand (5'→3') by adding complementary nucleotides. A = T, G = C.
4. Proofreading: DNA polymerase checks its work as it goes, correcting most errors. The error rate is about 1 in 10 billion base pairs after proofreading.
5. Joining: An enzyme called ligase seals gaps between DNA fragments.

Protein Synthesis: From DNA to Protein

DNA contains the instructions for making proteins. Proteins do almost everything in the body — they are enzymes, structural components (muscle, hair, nails), transporters (hemoglobin), hormones (insulin), and antibodies. The process of reading DNA to make protein has two stages: transcription and translation.

Transcription (DNA → RNA, in the nucleus)

The gene's DNA is used as a template to make a complementary strand of mRNA (messenger RNA). RNA is similar to DNA but: it is single-stranded, uses ribose sugar instead of deoxyribose, and uses Uracil (U) instead of Thymine (T) — so A pairs with U in RNA.

The enzyme RNA polymerase reads the DNA template strand and builds the mRNA. The mRNA then leaves the nucleus through nuclear pores and travels to the ribosome.

Translation (RNA → Protein, at the ribosome)

The ribosome reads the mRNA sequence in groups of three bases called codons. Each codon specifies one amino acid (or a start/stop signal). The genetic code is a table of 64 codons (4³ = 64 possible combinations of 4 bases taken 3 at a time) that code for 20 amino acids. The code is nearly universal — the same codons mean the same amino acids in bacteria, plants, and humans.

tRNA (transfer RNA) molecules bring the correct amino acids to the ribosome, matching their anticodon to the mRNA codon. The ribosome links amino acids together with peptide bonds, building a polypeptide chain that will fold into a functional protein.

Mendelian Genetics

Gregor Mendel (1822–1884), an Austrian monk, experimented with pea plants for 8 years, crossing thousands of plants and recording results. He discovered the fundamental rules of inheritance — decades before anyone knew DNA existed.

Key Terms

• Allele: Different versions of a gene (e.g., tall T vs. short t)
• Dominant allele: Expressed whenever present (capital letter, e.g., T)
• Recessive allele: Only expressed when two copies are present (lowercase, e.g., t)
• Genotype: The genetic makeup (e.g., TT, Tt, or tt)
• Phenotype: The observable trait (e.g., tall or short)
• Homozygous: Two identical alleles (TT or tt)
• Heterozygous: Two different alleles (Tt)

Punnett Squares: Worked Examples

Example 1 — Monohybrid Cross: Cross a heterozygous tall pea plant (Tt) with a short pea plant (tt).

Result: 50% Tt (tall), 50% tt (short). Genotypic ratio = 1 Tt : 1 tt. Phenotypic ratio = 1 tall : 1 short.

Example 2 — F2 Cross: Cross two heterozygous tall plants (Tt × Tt).

Result: 25% TT, 50% Tt, 25% tt. Phenotypic ratio = 3 tall : 1 short. This is the famous "3:1 ratio" Mendel observed in his pea plants!

Mutations

A mutation is a change in the DNA sequence. Mutations can be caused by errors during DNA replication, radiation (UV light, X-rays), chemicals (mutagens), or viruses.

Not all mutations are harmful. Many are neutral (no effect on the protein). Some are beneficial — mutations in bacteria can create antibiotic resistance; mutations over evolutionary time gave our ancestors adaptations like lactose tolerance, disease resistance, and larger brains.

Chapter 5: Evolution & Natural Selection

What Is Evolution?

Evolution is the change in the inherited characteristics of populations over successive generations. It is not about individuals changing — it is about the frequency of alleles in a population changing over time. Evolution is the central unifying theory of all biology.

It is worth clarifying: in everyday speech, "evolve" might mean anything that changes. In biology, evolution specifically refers to genetic change in populations across generations due to natural selection, genetic drift, gene flow, or mutation.

Charles Darwin and the Voyage of the Beagle

Charles Darwin (1809–1882) spent 5 years (1831–1836) sailing around the world on HMS Beagle. He collected thousands of specimens and made observations that would change science forever. Key observations:

• Galápagos finches: Darwin found 13–14 species of finches on different islands, all resembling a common mainland finch but with very different beak shapes suited to different foods (seeds, insects, cacti). Same ancestor, different adaptations.
• Fossil record: Ancient organisms resembled but were different from living ones. Extinct giant sloths resembled living sloths, suggesting a relationship over time.
• Geographic distribution: Similar habitats on different continents had different but functionally similar species (Australian marsupials vs. African placental mammals).

Darwin published On the Origin of Species in 1859, presenting his theory of evolution by natural selection.

Natural Selection: The Four Requirements

Natural selection occurs when these four conditions are met:

1. Variation: Individuals in a population differ from one another (different colors, sizes, speeds, disease resistance, etc.). This variation comes from mutations and sexual reproduction mixing alleles.
2. Heritability: Variations must be heritable — passed from parent to offspring through DNA. Traits acquired during life (like a bodybuilder's muscles) are NOT heritable.
3. Differential Survival/Reproduction (Selection): Some variations give individuals a survival or reproductive advantage in their specific environment. Those individuals survive longer and/or produce more offspring.
4. Time: Over many generations, advantageous traits become more common in the population; disadvantageous traits become rarer or disappear.

Evidence for Evolution

1. The Fossil Record

Fossils are preserved remains or traces of ancient organisms found in sedimentary rock layers. Deeper layers are older. The fossil record shows a clear pattern: simpler organisms appear in older (deeper) layers, and more complex organisms appear in younger (shallower) layers. Importantly, we find transitional fossils — organisms with features intermediate between ancestral and descendant forms.

Tiktaalik (discovered 2004 in Canada) is a 375-million-year-old fish with fins that have a wrist-like joint — a transitional form between fish and the first land vertebrates. It was found exactly where evolutionary theory predicted it would be found, at exactly the age predicted.

2. Homologous Structures

Homologous structures are body parts that share the same basic structure and evolutionary origin, even if they now serve different functions. The human arm, bat wing, whale flipper, and horse leg all contain the same arrangement of bones: humerus, radius, ulna, carpals, metacarpals, and phalanges. This makes no sense from a design standpoint — why would a completely independent design use the same bones? It makes perfect sense if these limbs all evolved from a common ancestor.

3. Vestigial Structures

Vestigial structures are reduced, non-functional (or minimally functional) structures that were functional in ancestors. Humans have a vestigial tailbone (coccyx) — evidence that our ancestors had tails. Whales have vestigial hip and leg bones embedded in their bodies — evidence that whale ancestors walked on land. Blind cave fish have vestigial eye sockets.

4. Molecular Evidence (DNA and Proteins)

The most powerful modern evidence comes from comparing DNA sequences. Species that share a recent common ancestor have more similar DNA. Humans and chimpanzees share 98.7% of their DNA. Humans and mice share ~85%. Humans and yeast share about 31%. The phylogenetic trees built from DNA data match almost exactly the trees built independently from fossil evidence and anatomy — powerful convergent evidence.

5. Direct Observation

We observe evolution happening in real time in bacteria and viruses. The evolution of antibiotic resistance in bacteria is evolution by natural selection observed directly. When we use too many antibiotics, we kill the sensitive bacteria and leave the rare resistant ones to reproduce — within months or years, the whole population can be resistant.

Speciation

Speciation is the formation of a new species. A species is traditionally defined as a group of organisms that can interbreed and produce fertile offspring. Two populations become separate species when they become reproductively isolated — separated so completely that they can no longer exchange genes.

• Allopatric speciation: Geographic separation (a mountain range, ocean, river) prevents gene flow. The two isolated populations evolve independently in different directions until they can no longer interbreed. This is the most common mechanism. Example: Darwin's finches on different Galápagos islands.
• Sympatric speciation: New species form without geographic separation — often through polyploidy (chromosome number doubling) in plants. About 70% of all flowering plant species are ancient polyploids.

Chapter 6: Classification of Life

Why Do We Classify Life?

With ~8.7 million known and estimated species, we need a system to organize life so scientists worldwide can communicate clearly about organisms. If a German scientist, a Japanese scientist, and a Brazilian scientist are all studying the same beetle but using different local names, they will never be able to collaborate. Classification (taxonomy) solves this problem.

Taxonomy is the science of naming, describing, and classifying organisms. The modern system was largely created by Swedish botanist Carl Linnaeus (1707–1778), who introduced the hierarchical system and binomial nomenclature still used today.

Binomial Nomenclature

Every species has a two-part Latin scientific name: the genus (first word, capitalized) and the species epithet (second word, lowercase). Together they are written in italics (or underlined when handwritten).

The Taxonomic Hierarchy

Classification uses a hierarchy of groups, from broadest to most specific. A memory aid: Dear King Philip Came Over For Good Soup (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species).

The Three Domains

The most fundamental division of life is into three domains, established by Carl Woese in the 1970s based on ribosomal RNA sequences:

• Bacteria: Prokaryotes without a nucleus. Found everywhere — soil, water, your gut, deep-sea vents. Most are harmless or beneficial; some cause disease.
• Archaea: Prokaryotes that look like bacteria but are biochemically more similar to eukaryotes. Often found in extreme environments (volcanic hot springs, salt lakes, deep-sea pressure vents) — called extremophiles. Also abundant in ocean plankton and animal digestive tracts.
• Eukarya: All organisms with true membrane-bound nuclei — this includes all four eukaryotic kingdoms.

The Six Kingdoms

How to Read a Dichotomous Key

A dichotomous key is a tool used to identify unknown organisms. It presents a series of paired statements (dichotomous = two choices), and you follow the path based on the organism's features until you reach an identification.

Real dichotomous keys for professional use can have hundreds of steps and use precise measurements and microscopic features.

Chapter 7: Ecosystems & Ecology

What Is Ecology?

Ecology is the study of how organisms interact with each other and with their physical environment. Ecologists study individual organisms, populations of one species, communities of multiple species, and entire ecosystems.

Biotic vs. Abiotic Factors

Both types shape where organisms can live. A cactus tolerates high temperature and low water (abiotic) but struggles if surrounded by tall shade trees (biotic). The combination of all biotic and abiotic factors an organism can tolerate defines its fundamental niche.

Food Chains and Food Webs

A food chain shows a single linear pathway of energy transfer from one organism to the next. Arrows point in the direction of energy flow (from eaten to eater).

A food web is a more realistic representation — it shows all the feeding relationships in a community, forming an interconnected web. Real ecosystems have hundreds of overlapping food chains.

Trophic Levels

Energy Pyramids and the 10% Rule

Energy is lost at each trophic level because organisms use energy for their own metabolism, movement, and heat generation. On average, only about 10% of the energy from one trophic level is transferred to the next (the other 90% is used by the organisms themselves or lost as heat).

Nutrient Cycles

While energy flows through ecosystems in one direction (and is lost as heat), matter cycles. Atoms are recycled over and over.

The Carbon Cycle

• Photosynthesis removes CO₂ from the atmosphere, converting it to organic molecules (glucose) in plants.
• Cellular respiration (by all organisms) releases CO₂ back to the atmosphere.
• Decomposition releases CO₂ from dead organic matter.
• Combustion (burning) of fossil fuels (coal, oil, natural gas — ancient compressed organic matter) rapidly releases CO₂ that was stored for millions of years. This is the primary cause of rising atmospheric CO₂ (from ~280 ppm pre-industrial to ~420 ppm today) and climate change.
• Ocean absorption: Oceans absorb about 25% of human CO₂ emissions, becoming more acidic in the process (ocean acidification, damaging coral reefs).

The Nitrogen Cycle

All living things need nitrogen to build proteins and DNA. The atmosphere is 78% nitrogen gas (N₂), but most organisms cannot use N₂ directly — the bond is too strong to break without help.

• Nitrogen fixation: Special bacteria (like Rhizobium in root nodules of legumes, and free-living Azotobacter) convert N₂ into ammonia (NH₃) — a form plants can use. Lightning also fixes small amounts of nitrogen.
• Nitrification: Soil bacteria convert ammonia to nitrites, then to nitrates (NO₃⁻), which plants absorb through roots.
• Assimilation: Plants build nitrogen into amino acids and proteins. Animals eat plants and assimilate nitrogen.
• Decomposition/Ammonification: Dead organisms and waste are broken down by bacteria, releasing ammonia.
• Denitrification: Some bacteria convert nitrates back to N₂ gas, completing the cycle.

Major Biomes

Population Ecology

A population is all individuals of one species in an area. Population size changes based on four factors:

• Births (natality) — add individuals
• Deaths (mortality) — remove individuals
• Immigration — add individuals from elsewhere
• Emigration — remove individuals that leave

Carrying capacity (K) is the maximum population size an environment can sustain. When a population exceeds K, resource competition intensifies, death rates rise, and the population typically declines back toward K.

Predator-Prey Relationships

The classic example is the lynx and snowshoe hare in Canada, studied over 90 years using Hudson's Bay Company fur records. When hare populations are high, lynx have plenty of food and their populations increase. The increased number of lynx eat more hares, causing hare populations to crash. With less food, lynx populations also crash. With fewer predators, hares recover — and the cycle repeats every 9–11 years.

Chapter 8: Human Body Systems

Overview of Body Systems

The human body has 11 organ systems, each consisting of multiple organs working together to perform specific functions. No system works in isolation — they all communicate and depend on each other.

The Digestive System: Step by Step

Digestion converts large food molecules into small molecules that can be absorbed into the blood. The entire digestive tract is essentially a 9-meter (30-foot) long tube running from mouth to anus.

1. Mouth (Oral Cavity): Mechanical digestion by teeth grinds food into smaller pieces (increasing surface area). Salivary glands produce saliva containing amylase, an enzyme that begins breaking down starch into sugars. The tongue forms food into a bolus (ball).
2. Esophagus: A 25 cm muscular tube connecting mouth to stomach. Food moves down via peristalsis — wave-like muscular contractions. Takes about 8 seconds.
3. Stomach: A muscular bag that churns food mechanically and secretes gastric acid (HCl, pH ~2 — as acidic as battery acid!) and pepsin, an enzyme that digests proteins. Produces mucus to protect itself from the acid. Food becomes a thick liquid called chyme. Takes 2–4 hours.
4. Small Intestine (6–7 meters long): Where most digestion and almost all absorption occurs. The liver sends bile (emulsifies fats) and the pancreas sends pancreatic enzymes (lipase, protease, amylase) into the small intestine. The intestinal lining has millions of tiny finger-like projections called villi, and each villus is covered in even tinier microvilli (brush border), creating an enormous surface area — about the size of a tennis court — for absorption. Takes 3–5 hours.
5. Large Intestine (1.5 meters long): Absorbs water and electrolytes from undigested material. Houses ~100 trillion bacteria (your gut microbiome) that produce some vitamins (K, B12) and further ferment undigested materials. Takes 12–24 hours.
6. Rectum and Anus: Stores and eliminates solid waste (feces).

The Circulatory System: The Double Loop

The circulatory system is a closed, double-circuit system. Blood makes two loops:

• Pulmonary circulation: Right side of heart → lungs (picks up O₂, drops CO₂) → left side of heart
• Systemic circulation: Left side of heart → body (delivers O₂, picks up CO₂) → right side of heart

The Heart

The heart is a muscular pump about the size of your fist, beating 60–100 times per minute (100,000 times per day, 2.5 billion times in a lifetime). It has four chambers:

Blood Types

Blood type is determined by protein markers (antigens) on the surface of red blood cells. The ABO system and Rh factor are most important:

The Respiratory System: Gas Exchange

Air enters through the nose/mouth → trachea → bronchi → smaller bronchioles → alveoli (tiny air sacs). The lungs contain about 480 million alveoli, creating a total surface area of about 70 m² — roughly the size of a studio apartment floor.

Gas exchange occurs across the thin (one cell thick!) alveolar walls by diffusion:

• O₂ diffuses from the alveoli (high O₂ concentration) into the surrounding capillaries (low O₂) and binds to hemoglobin in red blood cells.
• CO₂ diffuses from the capillaries (high CO₂) into the alveoli (low CO₂) and is exhaled.

The Nervous System

The nervous system processes information and controls responses. It is divided into the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS) — all the nerves branching out to the body.

Neurons

The basic units of the nervous system are neurons — specialized cells that transmit electrical signals. A typical neuron has:

• Cell body (soma): Contains the nucleus and most cellular machinery.
• Dendrites: Short, branched projections that receive signals from other neurons.
• Axon: Long projection that transmits signals away from the cell body. Can be up to 1 meter long (the sciatic nerve)!
• Myelin sheath: Fatty coating on many axons that speeds signal transmission up to 100 m/s (224 mph). In multiple sclerosis, myelin is destroyed, slowing nerve signals.
• Synaptic terminals: Release neurotransmitters into the synapse to communicate with the next cell.

The Reflex Arc

A reflex is an automatic, fast response that bypasses the brain. The reflex arc pathway: Stimulus → Receptor → Sensory neuron → Interneuron (in spinal cord) → Motor neuron → Effector (muscle/gland) → Response. The whole process takes about 50 milliseconds — your brain gets notified after you have already reacted.

Brain Regions

The Immune System

The immune system has two layers of defense:

Innate (Non-Specific) Immunity — First and Second Line

• First line: Physical barriers — skin (prevents entry), mucus (traps pathogens), cilia (sweeps them out), stomach acid (kills bacteria).
• Second line: When pathogens breach barriers — inflammation (increased blood flow; redness, warmth, swelling), fever (high temperature kills many pathogens and speeds immune response), phagocytes (neutrophils, macrophages — engulf and destroy pathogens), natural killer cells (destroy virus-infected and cancer cells), and the complement system (proteins that mark and destroy pathogens).

Adaptive (Specific) Immunity — Third Line

Takes days to weeks to mount, but is highly specific and creates immunological memory. Key players:

• B cells: Produce antibodies — Y-shaped proteins that bind specifically to one antigen (like a lock-and-key). Antibodies flag pathogens for destruction and prevent them from entering cells.
• T cells: Helper T cells coordinate the immune response; cytotoxic (killer) T cells directly kill infected cells.
• Memory cells: Long-lived B and T cells that "remember" a pathogen. If the same pathogen is encountered again, the response is faster (days instead of weeks) and stronger — this is immunological memory.

Chapter 9: Plants

Why Plants Matter

Plants cover about 80% of Earth's land surface and account for over 80% of the total biomass of all living things on Earth. They produce virtually all the oxygen in our atmosphere, form the base of nearly every food chain, regulate climate by absorbing CO₂, prevent soil erosion with their roots, and provide food, medicine, building materials, and fiber for human civilization. Without plants, complex animal life on land would be impossible.

Plant Cells vs. Animal Cells (Review)

As noted in Chapter 2, plant cells have three major features animal cells lack:

1. Cell wall (made of cellulose) — provides rigid structure and protection
2. Chloroplasts — site of photosynthesis; contain green pigment chlorophyll
3. Central vacuole — large water-storage organelle maintaining turgor pressure (what keeps non-woody plants upright)

Vascular vs. Nonvascular Plants

Plant Parts and Functions

Roots

Roots anchor the plant and absorb water and minerals from the soil. The root hairs — tiny extensions from root epidermal cells — massively increase surface area for absorption. A single rye plant was found to have 14 billion root hairs with a total length of 10,620 km — longer than the distance from New York to London and back! Roots also store carbohydrates (carrots and beets are swollen storage roots).

Stems

Stems provide structural support and contain the vascular tissue (xylem and phloem) that connects roots to leaves. In woody plants, the outer layers of dead xylem become wood; bark is the outer protective layer. In non-woody (herbaceous) plants, turgor pressure in cells filled with water maintains rigidity — this is why plants wilt when dehydrated.

Leaves

Leaves are the primary site of photosynthesis. Their flat blade (lamina) maximizes light absorption. Leaves contain:

• Epidermis: Outer protective layer coated with a waxy cuticle that reduces water loss.
• Mesophyll: Middle layer with chloroplast-packed cells. Palisade mesophyll (upper layer, densely packed) does most photosynthesis; spongy mesophyll (lower, loosely packed) has air spaces for gas exchange.
• Veins: Vascular bundles containing xylem and phloem.
• Stomata: Tiny pores (usually on the underside) flanked by guard cells that open and close the pore. When open, CO₂ enters and O₂ and water vapor exit. When closed (e.g., during drought), gas exchange stops but water loss is minimized.

Transpiration

Transpiration is the evaporation of water from plant leaves through stomata. A large oak tree can transpire up to 400 liters (105 gallons) of water per day! This water loss creates a negative pressure (tension) in the xylem that pulls water up from the roots — called the cohesion-tension theory. Water molecules stick to each other (cohesion) and to xylem walls (adhesion), forming an unbroken column that can be pulled to heights of 115+ meters.

Transpiration rates increase with heat, low humidity, wind, and light. They decrease when stomata close (triggered by the hormone abscisic acid during drought).

Tropisms

Plants respond to environmental stimuli by growing toward or away from them — these responses are called tropisms.

Phototropism is controlled by the hormone auxin. When light hits one side of a stem, auxin migrates to the shaded side, causing cells there to elongate more — bending the stem toward the light.

Plant Reproduction

Asexual Reproduction

Plants can reproduce without fertilization in several ways:

• Vegetative propagation: Runners (strawberries), rhizomes (grass), tubers (potatoes), bulbs (tulips). All offspring are genetically identical clones of the parent.
• Fragmentation: A broken piece of plant can regenerate a whole new plant (willows, pothos).
• Spore production: Ferns, mosses, and fungi produce spores (haploid cells) that can germinate without fertilization.

Sexual Reproduction in Flowering Plants (Angiosperms)

The flower is the reproductive structure of angiosperms (the most diverse plant group — 300,000+ species). Key flower parts:

• Stamen (male): Consists of the anther (produces pollen containing sperm cells) and the filament (stalk).
• Pistil/Carpel (female): Consists of the stigma (sticky surface that catches pollen), style (tube), and ovary (contains ovules with egg cells).
• Petals: Attract pollinators (bees, butterflies, birds, bats, wind).
• Sepals: Protect the bud before it opens.

Pollination is the transfer of pollen from anther to stigma. After pollination, the pollen grain grows a tube down the style to the ovary, allowing sperm to fertilize the egg. The fertilized egg (zygote) becomes a seed; the ovary becomes the fruit (which can be a berry, nut, pod, or any structure protecting seeds and aiding dispersal).

Gymnosperms: Seeds Without Fruits

Gymnosperms (conifers, cycads, ginkgo) produce "naked" seeds — seeds not enclosed in a fruit. Conifers produce seeds inside cones. Pine pollen grains have little air sacs that allow them to float on wind — they travel up to 1,600 km from the parent tree! Gymnosperms dominated Earth's forests for 200 million years before angiosperms diversified, and today's conifers include the oldest (bristlecone pines, 5,000+ years old), tallest (coast redwoods, 115 m), and most massive (giant sequoias, 1,400 tonnes) living things on Earth.

Chapter 10: Practice & Review

Practice Questions

Read each question carefully, work it out, then type your key answer below. You need 7 out of 10 correct to pass and unlock the full solutions — and your score will be turned in to Google Classroom automatically.

Key Vocabulary List