Biology High School MCAS 2026 Student Study Guide Vocabulary

Comprehensive vocabulary list covering Biomolecules, Cell Biology, Metabolism, Genetics, Ecology, and Evolution for the HS Biology MCAS exam.

Organic Molecule

A carbon-based molecule; all four classes of biological macromolecules (carbohydrates, lipids, proteins, nucleic acids) are organic.

Monomer

A small repeating subunit that serves as the building block of a polymer (e.g., glucose is the monomer of starch).

Polymer

A large molecule made of many monomers bonded together into a chain.

Dehydration Synthesis (Condensation)

A chemical reaction that joins monomers together to build polymers by removing a water molecule.

Hydrolysis

A chemical reaction that breaks apart polymers into monomers by the addition of water.

Carbohydrates

Macromolecules composed of monosaccharide monomers that provide a quick energy source and structural support (e.g., starch, cellulose, glycogen).

Lipids

Non-polymer organic molecules made of glycerol and fatty acids; functions include long-term energy storage, insulation, and forming cell membranes.

Proteins

Polymers made of amino acid monomers that perform various functions including catalysis (enzymes), support (keratin), and transport (hemoglobin).

Nucleic Acids

Polymers made of nucleotide monomers (DNA and RNA) that store and transmit genetic information.

Enzyme

A protein catalyst that speeds up biochemical reactions by lowering the activation energy required.

Substrate

The specific reactant molecule that an enzyme acts upon by binding to its active site.

Denaturation

The loss of a protein’s (or enzyme's) specific 3D shape and function due to extreme heat, pH changes, or chemicals.

Prokaryotic Cell

A cell that lacks a nucleus and membrane-bound organelles; DNA floats freely in the cytoplasm (e.g., bacteria).

Eukaryotic Cell

A cell that contains a nucleus and membrane-bound organelles like mitochondria and endoplasmic reticulum (e.g., plants, animals, fungi).

Mitochondria

The eukaryotic organelle that serves as the site of cellular respiration and ATP production.

Chloroplast

An organelle found in plants and algae that performs photosynthesis using the green pigment chlorophyll.

Ribosome

The site of protein synthesis (translation) where amino acids are assembled into polypeptide chains.

Selectively Permeable

A property of the cell membrane that allows some substances to pass through while blocking others.

Diffusion

The passive movement of molecules from an area of high concentration to an area of low concentration.

Osmosis

The diffusion of water across a selectively permeable membrane from high water potential to low water potential.

Hypertonic

A solution with a higher solute concentration than the inside of the cell, causing water to leave and the cell to shrink.

Hypotonic

A solution with a lower solute concentration than the inside of the cell, causing water to enter and the cell to potentially swell or burst.

Active Transport

The movement of substances against a concentration gradient (low to high) using carrier proteins and energy in the form of $ATP$.

Mitosis

A type of nuclear division that produces two genetically identical diploid ($2n$) daughter cells for growth and repair.

Meiosis

A two-stage cell division that produces four genetically unique haploid ($n$) gametes (sperm or egg cells).

Crossing Over

The exchange of DNA segments between homologous chromosomes during Prophase I of meiosis, creating genetic variation.

ATP (Adenosine Triphosphate)

The primary energy currency molecule used by all living cells to perform work.

Autotroph (Producer)

An organism that produces its own food from inorganic sources, usually through photosynthesis or chemosynthesis.

Heterotroph (Consumer)

An organism that obtains energy by consuming other organisms for food.

Glycolysis

The first stage of cellular respiration occurring in the cytoplasm; it splits glucose into two pyruvate molecules and nets $2$ $ATP$.

Krebs Cycle

The second stage of aerobic respiration occurring in the mitochondrial matrix where pyruvate is broken down and $CO_2$ is released.

Electron Transport Chain (ETC)

The final stage of aerobic respiration in the inner mitochondrial membrane that uses oxygen as the final electron acceptor to produce approximately $32$ to $34$ $ATP$.

Fermentation

An anaerobic process that allows glycolysis to continue making $2$ $ATP$ when oxygen is absent by regenerating $NAD^+$.

Calvin Cycle

The light-independent reactions of photosynthesis occurring in the stroma; uses $CO_2$, $ATP$, and $NADPH$ to produce sugar.

Transcription

The process of making an $mRNA$ copy from a $DNA$ template strand in the nucleus using the enzyme $RNA$ polymerase.

Translation

The process of reading $mRNA$ codons at a ribosome to assemble a specific sequence of amino acids into a protein.

Codon

A three-nucleotide sequence on $mRNA$ that codes for a specific amino acid or a start/stop signal.

Mutation

A change in the $DNA$ sequence; can be point mutations (silent, missense, nonsense) or frameshift mutations.

Allele

An alternative version or form of a gene (e.g., T for tall vs t for short).

Genotype

The actual combination of alleles an organism carries for a specific trait (e.g., $Tt$).

Phenotype

The observable physical characteristic or trait expressed by an organism (e.g., tall).

Incomplete Dominance

An inheritance pattern where the heterozygote shows a blended phenotype (e.g., red and white flowers producing pink offspring).

Codominance

An inheritance pattern where both alleles are fully and simultaneously expressed in the heterozygote (e.g., $AB$ blood type).

Sex-linked Trait

A trait encoded by a gene located on a sex chromosome, usually the X; these traits appear more frequently in males.

Polygenic Trait

A trait controlled by two or more genes, resulting in a continuous range of phenotypes (e.g., human height).

Gel Electrophoresis

A biotechnology technique that separates $DNA$ fragments by size using an electric field; smaller fragments travel farther.

Genetic Engineering

The process of inserting a gene from one organism into the genome of another organism to produce desired traits.

Trophic Level

A feeding position in a food chain or energy pyramid (e.g., producer, primary consumer, etc.).

10% Energy Rule

The ecological principle stating that only about $10$% of stored energy is transferred from one trophic level to the next; $90$% is lost as heat.

Carrying Capacity (K)

The maximum population size that an environment can sustainably support based on available resources.

Density-dependent Factor

A limiting factor whose impact changes based on the population's density (e.g., competition, disease, predation).

Mutualism

A symbiotic relationship in which both participating species benefit ($+/+$).

Commensalism

A relationship in which one species benefits while the other is neither helped nor harmed ($+/0$).

Parasitism

A relationship in which one organism benefits at the expense of its host ($+/-$).

Keystone Species

A species that has an exceptionally large impact on the structure of its ecological community relative to its abundance.

Invasive Species

A non-native species introduced to an area that lacks natural predators and disrupts the ecosystem balance.

Ecological Succession

The gradual and predictable process of change in the species composition of a community over time following a disturbance.

Bioaccumulation

The process where non-biodegradable toxins become increasingly concentrated in the tissues of organisms at higher trophic levels.

Nitrogen Fixation

The process by which specialized bacteria convert atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$) that plants can use.

Natural Selection

The process where individuals with favorable heritable traits survive and reproduce at higher rates, causing adaptations over generations.

Fitness

A measure of an organism's ability to survive and successfully produce fertile offspring in its environment.

Adaptation

A heritable trait that increases an organism's fitness and chance of survival in a specific environment.

Homologous Structures

Anatomical features in different species that share a common underlying structure inherited from a common ancestor, even if their functions differ.

Analogous Structures

Structures that have similar functions but different evolutionary origins due to convergent evolution (e.g., bird wing vs. insect wing).

Vestigial Structure

A reduced or nonfunctional remnant of a structure that served a purpose in an organism's ancestors.

Genetic Drift

Random changes in allele frequencies within a population, typically having a more significant impact on small populations.

Allopatric Speciation

The formation of new species resulting from the geographic isolation of a population.

Cladogram

A branching diagram used to show evolutionary relationships based on shared derived characteristics or $DNA$ similarity.