cell noun The basic structural and functional unit of living organisms. We examined an onion cell under the microscope. + organism noun An individual living thing. A single bacterium is an organism. + nucleus noun The central part of an atom or cell that contains most of its mass or genetic material. The cell's nucleus holds the DNA that directs its growth and function. + membrane noun A thin, flexible layer of tissue that covers, lines, or separates parts of an organism. The cell membrane controls which substances can enter or leave the cell. + photosynthesis noun The process in which plants, algae, and some bacteria use light energy to make sugars. The leaves use light for photosynthesis. + respiration noun The process by which living organisms take in oxygen and release carbon dioxide to produce energy. Respiration in humans occurs mainly through the lungs, where oxygen enters the bloodstream. + chromosome noun A thread-like structure of tightly coiled DNA that carries genes and passes hereditary information to offspring. Humans typically have forty-six chromosomes arranged in twenty-three pairs within each cell. + gene noun A unit of hereditary material, made of DNA, that determines a particular trait in an organism. A single gene can determine whether a pea plant produces smooth or wrinkled seeds. + DNA abbreviation The molecule inside cells that carries the genetic instructions used to build and maintain living organisms. Scientists compared DNA samples to confirm the identity of the remains. + mitosis noun The process by which a cell divides its nucleus to produce two genetically identical daughter cells. Mitosis allows a single fertilized egg to develop into billions of cells with the same DNA. + meiosis noun A type of cell division that produces reproductive cells containing half the usual number of chromosomes. The biology class studied how meiosis creates genetic variation among offspring. + enzyme noun A protein produced by living cells that speeds up a specific chemical reaction in the body. The enzyme lactase helps the body break down sugar found in milk. + protein noun A nutrient made of amino acids that builds and repairs body tissue and supports muscle growth. Eggs, chicken, and beans are all good sources of protein for a balanced diet. + ecosystem noun The living things in an area together with their nonliving surroundings and the ways they interact. The pond ecosystem includes insects, plants, water, and sediment. + habitat noun The place and conditions in which an organism normally lives. The hedgerow provides habitat for nesting birds. + species noun A group of living organisms that share common characteristics and can interbreed to produce fertile offspring. Scientists discovered a new species of frog deep in the rainforest. + evolution noun The gradual process by which species of organisms change over successive generations. Darwin's theory of evolution explains how species adapt to their environments over time. + adaptation noun The process of changing to become better suited to new conditions, or a modified version of an original work. The lizard's coloring is an adaptation that helps it hide from predators. + natural selection noun The process by which organisms better suited to their environment tend to survive and pass on their traits. Darwin used natural selection to explain how species gradually change over generations. + reproduction noun The biological process by which living organisms produce offspring, or a copy of an original work. The textbook chapter explained sexual and asexual reproduction in plants. + metabolism noun The chemical processes in a living organism that convert food into energy and maintain life. Regular exercise can help boost your metabolism significantly over time. + homeostasis noun The tendency of a living organism to maintain stable internal conditions despite changes outside it. Sweating helps the body maintain homeostasis by cooling it down in hot weather. + organelle noun A specialized structure within a cell that performs a specific function, such as a mitochondrion. The mitochondrion is an organelle that converts nutrients into usable energy for the cell. + tissue noun A soft, disposable paper used for wiping or blowing the nose, or a group of similar living cells. She pulled a tissue from the box to wipe her watery eyes. + taxonomy noun A system for classifying things, especially organisms, into ordered groups based on shared characteristics. Scientists updated the taxonomy of the beetle family after new genetic evidence emerged. + biodiversity noun The variety of plant and animal life found in a particular habitat or the world. Rainforests contain an extraordinary level of biodiversity compared to most other ecosystems. + mutation noun A permanent change in an organism's genetic material that can be passed on to offspring. A random mutation gave the beetle population a slightly darker shell color. + heredity noun The passing of physical or mental traits from parents to their offspring through genes. Heredity explains why the twins share the same curly hair and dimples. + osmosis noun The gradual movement of a solvent through a membrane from a less concentrated to a more concentrated solution. Water enters plant roots by osmosis, moving from the soil into cells with higher salt concentration. + phylum noun A major taxonomic rank in biological classification, grouping organisms that share a basic body plan, below kingdom and above class. Humans belong to the phylum Chordata, along with all other animals possessing a backbone. +