SAT词汇：以H开头的生物专业词汇

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　　haploid number(单倍目)

　　The number of homologous pairs in a cell. Equal to half the diploid number. Gametes, cells that are passed on to offspring, contain the haploid number of chromosomes. In humans, the haploid number is 23.

　　Heart(心)

　　The muscular organ that pumps blood through the circulatory system. Mammals and birds have a four-chambered heart, with a left atrium and ventricle and a right atrium and ventricle. The right half of the heart receives deoxygenated blood from the body and pumps it to the lungs, while the left half receives oxygenated blood from the lungs and pumps it to the body.

　　Heredity(遗传)

　　The genetic transmission of traits from parents to offspring, so that offspring resemble their parents. Traits transmitted this way are called hereditary traits.

　　Heterotrophs(异养生物)

　　Organisms that can only get the organic molecules and energy necessary for life through the consumption of other organic matter. In the food web, all consumers and decomposers are heterotrophs. Heterotrophs can be herbivores, carnivores, or omnivores.

　　Heterozygous(杂合体)

　　A situation in which an individual (heterozygote) possesses two dissimilar alleles for the same gene. The opposite is homozygous.

　　homologous chromosomes(同源染色体)

　　Chromosomes containing the same series of genes; they may or may not carry the same alleles. Humans receive one set of 23 paternal chromosomes from their male parent and another set of 23 maternal chromosomes from their female parent. Each set matches up to the other for a total of 23 different pairs of homologous chromosomes. During meiosis, homologous pairs line up and are separated. In males, the X and Y chromosomes act as a homologous pair, although they are only partially homologous.

　　homologous trait(同源)

　　A trait found in different species that are morphologically and functionally similar and that comes from the same ancestral condition. A whale’s fin and a human’s arm are homologous structures.

　　Homozygous(纯合体)

　　A situation in which an individual (homozygote) has the exact same allele on both homologous chromosomes. Mating of two individuals with the same homozygous genotype will produce only offspring with that same identical genotype. The two identical alleles may be dominant or recessive (e.g., RR or rr). The opposite of homozygous is heterozygous.

　　Hormones(激素)

　　A chemical messenger that can be made of either peptides or lipids. Secreted by glands in one part of the body, hormones affect glands or organs in another part.

　　Hybrid(杂合物)

　　A genetic mixture; the offspring of two genetically different parents. Hybrids are usually heterozygous for a variety of genes.

　　hydrogen bond(氢键)

　　A weak bond between hydrogen and a set of other elements, including oxygen. Hydrogen bonds are a subset of dipole-dipole interactions.

　　Hydrolysis(水解)

　　A common biochemical reaction in which the bond between two molecules is split by the addition of a water molecule. Hydrolysis is the process that breaks down polymers and dimers. The reverse is dehydration synthesis.

　　Hydrophilic(亲水的)

　　Having an affinity for water; usually polar molecules. For the SAT II Biology, this is principally important in relation to the phospholipid bilayer.

　　Hydrophobic(恐水的)

　　Having a reluctance to mix with water; usually nonpolar molecules. The fatty acids that form the interior pocket between the two layers of the cell membrane are hydrophobic.

　　hydrostatic skeleton(流体静力学性骨骼)

　　A fluid skeleton in many soft-bodied invertebrates, including annelids, that allows an organism to change shape but not volume.

　　Hypertonic(高渗的)

　　A situation in which the concentration of solutes in a solution is higher than what it contains. For example, a sodium solution of 10 percent would be hypertonic to an animal cell (with a sodium concentration of about 0.9 percent), causing water to leave the cell by osmosis.

　　Hypothalamus(下丘脑)

　　Part of the brain responsible for temperature regulation, controlling hunger and thirst, and managing water balance. It also helps generate emotion.

　　Hypotonic(低渗的)

　　A situation in which the concentration of solutes in a solution is lower than what an organism contains. An example is a paramecium in pond water: the organism has more solutes than its environment, so water flows into the cell by osmosis. Paramecia have evolved contractile vacuoles to keep from exploding.

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