Biology Cheat Sheet

The core ideas of Biology distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Cell Theory

Cell theory states that all living organisms are composed of one or more cells, that the cell is the basic unit of structure and function in life, and that all cells arise from pre-existing cells. This foundational principle unifies all of biology and was established through the combined work of Schleiden, Schwann, and Virchow in the 19th century.

Natural Selection

Natural selection is the process by which organisms with heritable traits better suited to their environment tend to survive and reproduce at higher rates than organisms without those traits. Over many generations, this differential reproductive success leads to changes in the genetic composition of populations, driving adaptation and evolution.

DNA and the Genetic Code

Deoxyribonucleic acid (DNA) is the molecule that carries the hereditary instructions for building and maintaining an organism. DNA is organized into genes, which are transcribed into RNA and translated into proteins according to the genetic code, a set of rules mapping three-nucleotide codons to specific amino acids.

Homeostasis

Homeostasis is the ability of a biological system to maintain relatively stable internal conditions despite changes in the external environment. This dynamic equilibrium is achieved through feedback mechanisms, most commonly negative feedback loops, that detect deviations from a set point and trigger corrective responses.

Photosynthesis

Photosynthesis is the biochemical process by which plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water as raw materials and releasing oxygen as a byproduct. The process occurs in two main stages: the light-dependent reactions in the thylakoid membranes and the Calvin cycle in the stroma of chloroplasts.

Cellular Respiration

Cellular respiration is the metabolic process by which cells break down glucose and other organic molecules to produce adenosine triphosphate (ATP), the primary energy currency of the cell. The process consists of glycolysis, the citric acid cycle, and oxidative phosphorylation, and in aerobic organisms it consumes oxygen and produces carbon dioxide and water.

Mendelian Genetics

Mendelian genetics describes the patterns of inheritance first characterized by Gregor Mendel through his experiments with pea plants in the 1860s. Mendel's laws of segregation and independent assortment explain how alleles of genes are transmitted from parents to offspring, producing predictable ratios of phenotypes in subsequent generations.

Ecosystem Dynamics

Ecosystem dynamics refers to the complex interactions among living organisms and their physical environment, including the flow of energy through trophic levels and the cycling of nutrients such as carbon, nitrogen, and phosphorus. These interactions determine the structure, biodiversity, and resilience of ecosystems.

Mitosis and Meiosis

Mitosis and meiosis are the two forms of cell division in eukaryotic organisms. Mitosis produces two genetically identical daughter cells for growth and tissue repair, while meiosis produces four genetically unique haploid gametes through two successive divisions, enabling sexual reproduction and genetic diversity.

Protein Structure and Function

Proteins are large, complex macromolecules made of amino acid chains that fold into specific three-dimensional shapes, and their structure directly determines their function. Proteins serve as enzymes, structural components, transport molecules, hormones, and immune defenders, making them essential to virtually every biological process.

Key Terms at a Glance

Adaptation:A heritable trait or characteristic that has evolved through natural selection because it enhances an organism's fitness in its particular environment.

Allele:One of two or more alternative forms of a gene that occupy the same locus on a chromosome and can produce different phenotypic effects.

Amino Acid:An organic molecule containing an amino group and a carboxyl group that serves as the building block of proteins, with 20 standard types encoded by the genetic code.

Biogeochemical Cycle:The pathway through which a chemical element or molecule moves through the biotic and abiotic compartments of Earth, such as the carbon, nitrogen, and water cycles.

Biome:A large-scale ecological community defined by its dominant vegetation type and climate, such as tropical rainforest, desert, tundra, or temperate grassland.

Cell:The fundamental structural and functional unit of all living organisms, bounded by a membrane and capable of carrying out the basic processes of life.

Chromosome:A thread-like structure of tightly coiled DNA and histone proteins found in the nucleus of eukaryotic cells that carries genetic information in the form of genes.

Codon:A sequence of three consecutive nucleotides in messenger RNA that specifies a particular amino acid or a stop signal during protein synthesis.

Cytoplasm:The gel-like substance enclosed within the cell membrane, excluding the nucleus, that contains organelles, enzymes, and various molecules necessary for cellular processes.

Diffusion:The passive movement of molecules or ions from a region of higher concentration to a region of lower concentration, driven by the kinetic energy of the particles.

Dominant Allele:An allele that expresses its phenotypic effect even when present in a heterozygous combination with a different allele at the same genetic locus.

Endoplasmic Reticulum:A network of membranous tubules and sacs within eukaryotic cells that functions in protein processing (rough ER) and lipid synthesis (smooth ER).

Evolution:The change in the inherited characteristics of biological populations over successive generations, driven by mechanisms including natural selection, genetic drift, mutation, and gene flow.

Food Web:A complex network of interconnected food chains within an ecosystem that illustrates the multiple feeding relationships among producers, consumers, and decomposers.

Gamete:A mature reproductive cell (sperm or egg) that is haploid, containing half the number of chromosomes of the organism, and fuses with another gamete during fertilization.

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