Science / Biology Glossary

Thorns: Stems modified to protect the plant.

Thoroughfare Channels: Shortcuts within the capillary network that allow blood to bypass a capillary bed.

Thylakoids: The specialized membrane structures in which photosynthesis takes place. Internal membranes in the chloroplast where the light reaction chemicals are embedded. Collections of thylakoids form . . . View Full Definition

Thymine: One of the pyrimidine bases in DNA, thymine is replaced by uracil in RNA.

Thyroid-Stimulating Hormone: A hormone produced by the anterior pituitary that stimulates the production and release of thyroid hormones.

Tight Junctions: Junctions between the plasma membranes of adjacent cells in animals that form a barrier, preventing materials from passing between the cells.

Tissues: Groups of similar cells organized to carry out one or more specific functions. Groups of cells performing a function in a multicellular organism.

Toxins: Term applied to poisons in living systems.

Trace Fossil: Any indication of prehistoric organic activity, such as tracks, trails, burrows, or nests.

Trachea: In insects and spiders, a series of tubes that carry air directly to cells for gas exchange; in humans, the air-conducting duct that leads from the pharynx to the lungs.

Tracheids: Long, tapered cells with pitted walls that form a system of tubes in the xylem and carry water and solutes from the roots to the rest of the plant. One type of xylem cells. Tracheids are lon . . . View Full Definition

Transcription: The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule.

Transfer Rnas (TRNAS): Small, single-stranded RNA molecules that bind to amino acids and deliver them to the proper codon on messenger RNA. The trucks of protein synthesis that carry the specified amino acid to th . . . View Full Definition

Transformation: In Griffiths experiments with strains of pneumonia bacterium, the process by which hereditary information passed from dead cells of one strain into cells of another strain, causing them to t . . . View Full Definition

Transforming Factor: Griffiths name for the unknown material leading to transformation; later found to be DNA.

Transition Reaction: Biochemical process of converting 3-carbon pyruvate into 2-carbon acetyl and attaching it to coenzyme A (CoA) so it can enter Krebs cycle. Carbon dioxide is also released and NADH is formed . . . View Full Definition

Translation: The synthesis of protein on a template of messenger RNA; consists of three steps: initiation, elongation, and termination. Making of a polypeptide sequence by translating the genetic code of . . . View Full Definition

Translocation: 1) The movement of a segment from one chromosome to another without altering the number of chromosomes. 2) the movement of fiuids through the phloem from one part of a plant to another, with . . . View Full Definition

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