Science / Biology Glossary
Salivary Amylase: An enzyme secreted by the salivary glands that begins the breakdown of complex sugars and starches.
Salivary Glands: Glands that secrete salvia into the mouth.
Saprophytes: Organisms that obtain their nutrients from decaying plants and animals. Saprophytes are important in recycling organic material.
Sapwood: Layers of secondary xylem that are still functional in older woody plants; visible as the outer lighter areas in the cross section of a tree trunk.
Sarcomeres: The functional units of skeletal muscle; consist of filaments of myosin and actin.
Saturated Fat: A fat with single covalent bonds between the carbons of its fatty acids.
Schwann Cells: Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axons of some neurons, thus forming the myelin sheath.
Scientific Method: Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluation that forms the basis for modern science.
Sclereids: Plant cells with thick secondary walls that provide the gritty textures in pears.
Sclerenchyma: One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardened with lignin; provide support for the plant. Sclerenchyma cells include fibers and sclere . . . View Full Definition
Scrotum: In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides proper temperature for the testes.
Second Law Of Thermodynamics (Entropy): The energy available after a chemical reaction is less than that at the beginning of a reaction; energy conversions are not 100% efficient.
Second Messenger: The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the cells plasma membrane activating a molecule&emdash;the second messenger&emdash;that activ . . . View Full Definition
Secondary (Lateral) Meristems: Plant meristems that produce secondary growth from a cambium.
Secondary Cell Wall: In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulose and lignin.
Secondary Compounds: Plant products that are not important in metabolism but serve other purposes, such as attracting animals for pollination or killing parasites.
Secondary Extinction: The death of one population due to the extinction of another, often a food species.
Secondary Growth: Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth . . . View Full Definition
Secondary Immunity: Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells produced during the first exposure to the antigen, the second response is faster and more . . . View Full Definition
Secondary Macronutrients: Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.
Secondary Phloem: Phloem produced by the vascular cambium in a woody plant stem or root.
Secondary Structure: The structure of a protein created by the formation of hydrogen bonds between different amino acids; can be a pleated sheet, alpha helix, or random coil. Shape of a protein caused by attract . . . View Full Definition
Secondary Xylem: Xylem produced by the vascular cambium in a woody plant stem or root; wood.
Secretin: A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits gastric emptying.
Secretion: The release of a substance in response to the presence of food or specific neural or hormonal stimulation.
Word of the Day:
Income Tax: Common stock with a high dividend yield and few profitable investment opportunities.