abiotic (无生命的)
Nonliving materials in the environment—such as elements, sunlight, and soil—that influence and are influenced by living (biotic) entities on the planet.
acetylcholine(乙酰胆素): 运动神经释于一种化学物质,与其接受体结合后，能产生足够的电位变化来引起肌肉运动
A neurotransmitter released from the axons of nerve cells to excite an action potential or trigger a muscle to contract.
active immunity(主动免疫)
Protective immunity to a disease in which the individual produces antibodies as a result of previous exposures to the antigen.
acids(酸)
Hydrogen ion (H+) donors. Acids are very important in the chemical reactions of life because they are highly reactive. Acids have pH values below 7. They are the opposite of bases.
actin (肌动蛋白)
Protein filaments that, along with myosin, allow muscles to contract.
active site(酶活性部分)
The part of an enzyme that interacts with, or binds to, a substrate.
active transport(主动运输)
The movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration. Because active transport involves moving the molecule against the natural flow of the concentration gradient, the process requires energy.
adaptive radiation(适应辐射) ：同起源生物类群演化成多种不同类型的后代，以适应不同环境的现象
The evolutionary process by which ancestral forms of an organism are diversified through adaptation to new environments.
adenosine triphosphate (ATP/腺苷三磷酸)
The energy storage molecule for the cell. ATP consists of an adenosine molecule bonded to three phosphate groups. Each phosphate bond contains energy; by breaking these bonds, the cell can get the energy it needs for chemical reactions. Cells build ATP during cell respiration, using the raw material of glucose.
adrenal glands(肾上腺)
Two glands, the adrenal cortex(肾上腺皮质，分泌可的松【cortisone】，醛固酮【aldosteron，用于调节渗透，控制钠，钾离子的重新吸收】等激素) and adrenal medulla(肾上腺髓质，分泌肾上腺素【adrenaline】), located on the kidney.
aerobic respiration(有氧呼吸)
A form of cell respiration requiring oxygen (as opposed to anaerobic respiration, which does not need oxygen). Aerobic respiration is much more efficient than anaerobic respiration; it produces 36 ATP for every molecule of glucose. Aerobic respiration proceeds in three stages: glycolysis(糖酵解), the Krebs cycle(三羧酸循环), and the electron transport chain(电子传递链).
allantois(尿囊): 胚胎时期积存排泄废物、气体交换的场所
The extraembryonic membrane of birds, reptiles, and mammals that serves as an area of gaseous exchange and as a site for the storage of noxious excretion products.
allele(等位基因):位于一对同源染色体的相同位置上控制某一性状的不同形态的基因
A specific form or possible version of a gene having multiple versions. Alleles may be dominant or recessive.
allelic frequency(等位基因频率)
The frequency with which a particular allele for a certain characteristic appears among all possible alleles for that characteristic in a population.
alternation of generations(世代交替)：一种生物(种)，生殖方式不同的世代，周期的、或不规则的交替(苔藓植物和蕨类植物具有此特征)
The fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages that occur in plants.
alveolus(肺泡)
An air sac in the lung, the site of respiratory exchange.
amino acid(氨基酸)
The monomer of a protein. A central carbon attached to an amino group (–NH2), a carboxyl group (–COOH), and a hydrogen atom (–H). The fourth group is variable and defines the amino acid’s chemical identity.
amnion(羊膜) ：位于最内侧直接覆蓄胚体的膜
The extraembryonic membrane in birds, reptiles, and mammals that surrounds the embryo, forming an amniotic sac.
anaerobic respiration(无氧呼吸)
A form of cell respiration that does not use oxygen (as opposed to aerobic cell respiration). Anaerobic respiration is less efficient than the aerobic variety and produces just 2 ATP per molecule of glucose. Anaerobic respiration has two stages: glycolysis and fermentation(发酵).
analogous trait (相似特征)：来源于相同祖先，与其他生物种功能、形态上相似的结构
A trait that is morphologically and functionally similar to that of a different species but that arose from a distinct, ancestral condition.
anaphase (分裂后期)
The stage of mitosis in which sister chromosomes are separated and pulled to opposite ends of the cell by microtubules; the fourth stage of the first meiotic division (meiosis I), during which maternal and paternal homologous pairs are separated on microtubules; the fourth stage of the second meiotic division (meiosis II), during which either maternal or paternal sister chromatids are separated on microtubules.
androgen(雄性激素)
A male sex hormone. (e.g. testosterone【睾酮】)
Angiosperm(被子植物)
A vascular flowering plant in which seeds are enclosed inside protective ovaries, such as fruit or flowers. Angiosperms can be monocots(单子叶) or dicots(双子叶).
Anther(花粉囊，花药)
Pollen-producing structure at the top of the stamen, the male reproductive organ of flowers.
Anticodon(反密码子) ：位于tRNA上，和mRNA的密码子相反配对
The sequence of three nucleotides on tRNA that pairs with a codon of mRNA at the A site of a ribosome during translation.
Antigen(抗原)
A protein coat on the surface of red blood cells; a red blood cell may have a protein coat of type A, B, or AB. If the cell has no antigens, it is called type O. The presence of a foreign antigen in a body will cause blood to clot.
Aorta(大动脉)
The largest artery in the body; carries oxygenated blood from the left ventricle of the heart.
aphotic zone(无光带)
Literally, zone without light. The aphotic zone is part of the marine pelagic zone and begins 600 feet below the surface of the ocean. Only chemosynthetic organisms, scavengers, and predators are able to survive in this habitat.
Appendage(附属肢体)
A structure that extends from the trunk of an organism and is capable of active movements.
Artery(动脉)
Vessel that carries blood away from the heart and has thick, elastic, muscular walls that can dilate or contract to control blood pressure within the vessels. Blood in arteries is oxygenated, with the exception of the blood in the pulmonary artery(肺动脉，把心脏中的血输送到肺部的动脉).
autonomic nervous system(自主神经系统)
The involuntary half of the peripheral nervous system. The autonomic nervous system is in two antagonistic parts: the sympathetic(交感神经的) and parasympathetic(副交感神经的) nervous systems. Their interactions control smooth and cardiac muscle, glands, and organs and processes such as heartbeat, the movements of the digestive tract, and the contraction of the bladder.
Autosome(正染色体，非性染色体)
Any chromosome that is not a sex chromosome. Humans have 44 autosomes, in 22 homologous pairs. The two sex chromosomes are the twenty-third pair of chromosomes.
Autotroph(自养生物)
An organism that can produce the organic molecules and energy necessary for life through the processes of photosynthesis or chemosynthesis. Autotrophs do not rely on other organisms for food. In a food web, autotrophs are producers.
Auxin(茁长素) ：一种植物激素，刺激细胞伸长
One in a class of plant hormones that stimulates (among other things) cell elongation, secondary tissue growth, and fruit development.
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