Saprophytes(腐生植物)
See decomposers.
Segregation(隔离)
Mendel’s conclusion that individuals have two copies of each gene, and that these copies separate randomly during gamete formation, one copy per gamete. This law is true except for genes on sex chromosomes in males, who have only one copy of each such gene. It is often called Mendel’s first law.
semicircular canal(半规管)
Fluid-filled structure within the ear that can detect balance.
Sepal(萼片)
Green, leaflike structure that encloses and protects the unopened flower bud.
sex cells(配子细胞)
See gamete.
sex chromosome(性染色体)
Refers to a chromosome involved in defining the sex of an individual. Humans have two sex chromosomes and 44 autosomes. In females, both sex chromosomes are X chromosomes. Males have one X chromosome and one Y chromosome.
sex-linked(与性染色体有关的)
Refers to a gene located on a sex chromosome or to a trait defined by such a gene. Such traits will appear with different frequencies in males and females, and males and females will differ in their ability to transmit the trait to their offspring. In order for a recessive sex-linked trait such as hemophilia or color blindness to manifest in the phenotype of a woman, it must be inherited from both mother and father; such diseases will be present in the phenotype of a man if he inherits it just from his mother.
somatic cell(体细胞)
Any plant or animal cell that is not a germ cell, meaning it is not passed down to offspring. The class of cell formed during mitosis.
somatic nervous system(体神经系统)
One half of the motor system of the peripheral nervous system. Responsible for voluntary, or conscious, movement. Neurons in this system target skeletal muscles and release the neurotransmitter acetylcholine.
Speciation(物种形成)
The development of a species through evolution. A species forms when its members become reproductively isolated from all other organisms. Speciation can occur through geographic separation that eliminates gene flow or through adaptive radiation.
Species(种)
A group of organisms defined by their ability to interbreed with only each other.
Sperm(精子)
The male gamete in sexual reproduction.
Spermatogenesis(精子发生)
The process in which haploid sperm cells form through meiotic division.
spinal cord(脊髓神经)
A long cylinder of nervous tissue that extends along the vertebral column from the head to the lower back. It controls some autonomic responses and connects the brain to the peripheral nervous system.
Spores(孢子)
Usually unicellular and microscopic, spores are produced by protist molds, fungi, and plants and are able to develop into new individuals. Spores are able to survive without food or water for long periods. Most fungi spend part of their life cycle as hyphae and part as spores.
Sporophyte(孢子体)
A diploid plant or plant structure that produces haploid spores through meiosis.
stabilizing selection(稳定选择)
When selection pressures favor the average form of a trait.
Stamen(雄蕊)
The male reproductive organ of the flower, consisting of an anther and filament.
Stigma(柱头)
The top part of the pistil, where pollen grains are received.
Stomata(气孔)
Small epidermal pores, surrounded by two guard cells, through which gases diffuse and water transpires in and out of a leaf.
stop codon(终止密码子)
A codon on mRNA that signals the termination of DNA translation. There are three stop codons: UAA, UAG, or UGA.
Style(花粉管)
The shaft of the pistil that leads from the stigma down into the ovary.
Substrate(基质)
The starting material that will undergo chemical change in a chemical reaction facilitated by an enzyme.
Symbiosis(共生)
A type of interaction within a community that falls into one of three categories: a parasitic relationship benefits one organism and hurts the other; a commensal relationship benefits one and does not affect the other; a mutualistic relationship benefits both organisms.
Synapse(突触)
The gap between two neurons, spanning the space between the axon of one and the dendrites of the other. In order to pass an impulse across a synapse, neurons must release neurotransmitters.
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