Answers and Explanations to Practice Test 2 - Cracking the AP Biology Exam

Cracking the AP Biology Exam


Answers and Explanations to Practice Test 2

1. E Animal cells have centrioles. (A) and (C), Both animal and plant cells have an endoplasmic reticulum, membrane-bound organelles, and lysosomes. (B) and (D), Only plants have a cell wall made of cellulose and large vacuoles.

2. A Plants were able to conquer land because they had a waxy cuticle to prevent water loss, stomates that allow for gas exchange, roots to anchor them, and vascular tissues to transport food and water. Land plants had to develop spores and gametes with a protective covering in order to reproduce.

3. A There are four genetically distinct kinds of gametes that could be produced, ABCDE, AbCDE, aBCDE, and abCDE. Notice that the only alleles that vary are A and B (22 = 4).

4. D If the contents of the cell separated from the cell wall, then water was moving out of the cell. (B), This would cause the space between the cell wall and the cell membrane to expand. This also means that the concentration of solutes in the extracellular environment would therefore be hypotonic with respect to the cell’s interior. (C), Because the vacuole is within the cell, it contains a solution with a much lower osmotic pressure than that of the sugar solution. (E), If water was moving out of the cell, the sugar solution would pass freely through the cell wall but not the cell membrane. (A), Lastly, because the fluid in the cell was hypertonic to the sugar solution, fluid was moving out of the vacuole and caused it to become smaller.

5. E If bacteria divide every 20 minutes, you would produce 512 bacterial cells. One method would be to use the equation 2x, where x equals the number of 20 minute intervals in three hours; 29 = 512.

6. E This question tests your ability to associate what happens when enzymes are denatured and what would happen in the synaptic cleft. Acetylcholinesterase is an enzyme that degrades acetylcholine in the synaptic cleft. (A), (B) and (C), If acetylcholinesterase is denatured, acetylcholine will still be released from the presynaptic membrane and continue to diffuse across the synaptic cleft and bind to the postsynaptic membrane because acetylcholine is not degraded. (D), The chemical agent will have no affect on acetylcholine.

7. B The ratio of purines to pyrimidines should be constant because purines always bind with pyrimidines, no matter which ones they may be.

8. C Iodine is the major component of thyroid hormones T3 and T4 and can bind to a specific receptor on the cell membrane of a thyroid cell. (A), Iodine does not alter gene expression. (B), Calcitonin and parathyroid hormones regulate plasma calcium levels. (D), Insulin lowers the concentration of glucose in the blood. (E), Iodine is not a hormone.

9. C The extraembryonic membrane that functions in respiration and excretion is the allantois. (A), The amnion is an extraembryonic membrane that protects the embryo. (B), The chorion is the outermost layer that surrounds the embryo. (D), The yolk sac provides nourishment for the embryo. (E), The placenta is the structure through which materials are exchanged between the fetus and mother.

10. D If substance F leads to the inhibition of enzyme 3, then substances D and E and enzymes 3, 4, and 5 will be affected. The activity of enzyme 5 will be decreased, not increased.

11. D The liver does not break down peptides into amino acids. It performs all of the following functions, (A), stores amino acids absorbed in the capillaries, (B), detoxifies harmful substances, (C), makes bile, and (E), stores fatty acids absorbed by the lacteals.

12. C Based on the graph, fetal hemoglobin has a higher affinity for oxygen than maternal fetal hemoglobin. (A), Fetal hemoglobin does not give up oxygen more readily than maternal hemoglobin. (B), The dissociation curve of fetal hemoglobin is to the left of the maternal hemoglobin. (D), Fetal hemoglobin and maternal hemoglobin are different structurally, but you can’t tell this from the graph. (E), Fetal hemoglobin does not convert to maternal hemoglobin.

13. C The set of genotypes that represents a cross that could produce offspring with silver fur from parents that both have brown fur is Bb and Bb. Complete a Punnett square for this question. In order for the offspring to have silver fur, both parents must have the silver allele.

14. A (B), (D), and (E), Hemoglobin’s affinity for O2 decreases as the concentration of H+ increases (or the pH decreases) and as the concentration of CO2 increases (or the concentration of HCO3 increases). (C), Hemoglobin’s affinity for oxygen in tissue muscles does not increase during exercise.

15. B Legumes (bean plants) are vehicles for organisms that add nitrates into the soil. An example of a legume is a lima bean.

16. B Because snapdragon plants display intermediate dominance, the heterozygous phenotype is affected by the alleles of both homozygotes. If a homozygous red plant is crossed with a homozygous white plant, all offspring would be heterozygous and pink.

17. B Viruses are made up of nucleic acid surrounded by a protein coat called a capsid. They do not contain a cell wall, proteins, or cell membrane.

18. E All of these organisms are protists—single-celled eukaryotic organisms. (A), These organisms do not undergo alternation of generation. (B), Sporozoans have spores. (C), Flagellates use flagella for motility. (D), Green algae are autotrophic.

19. B The word Drosophila refers to the classification group known as genus. All organisms are given a scientific name consisting of a species name and a genus name. Melanogaster refers to the species name. The order of classification is, kingdom, phylum, class, order, family, genus, and species.

20. B In humans, fertilization normally occurs in the fallopian tube. (A), The ovary is the female gonad that contains the eggs. (C), The uterus is the organ that houses the developing embryo. (D), The placenta is the structure that nourishes the embryo. (E), The vagina is the female organ in which sperm cells are deposited.

21. B The development of an egg without fertilization is known as parthenogenesis. Parthenogenesis is a form of asexual reproduction found in insects and lizards. (A), Meiosis is a form of sexual reproduction that produces gametes. (C), Embryogenesis refers to the early stages of embryo development. (D), Vegetative propagation is a form of asexual reproduction by which plants produce identical offsprings from stem, leaves, or roots. (E), Regeneration is the restoration or new growth of a body part by an organism.

22. D All of the choices are examples of hydrolysis except the conversion of pyruvic acid to glucose. Hydrolysis is the breaking of a covalent bond by adding water. In all of the correct examples, complex compounds are broken down to simpler compounds. The conversion of pyruvic acid to acetyl CoA is an example of decarboxylation—a carboxyl group is removed as carbon dioxide and the 2-carbon fragment is oxidized.

23. A Peroxisomes catalyze reactions that produce hydrogen peroxide, ribosomes are involved in protein synthesis, and mitochondria contain enzymes involved in cellular respiration. (B) and (D), Lysosomes are the sites of degredation; they contain hydrolytic enzymes, but do not produce hydrogen peroxide. (C), The Golgi apparatus sorts and packages substances that are destined to be secreted out of the cell. (E), The smooth endoplasmic reticulum is the site of lipid synthesis. Interestingly, O2 concentration generally doesn’t play an important role in regulating respiration.

24. B Use process of elimination. The respiratory rate in humans will be affected by an increase in the amount of CO2, a drop in pH levels, which is the same as an increase in hydrogen ion levels. Strenuous exercising will also modify the respiratory rate. O2 concentration generally does not play an important role in regulating respiration.

25. B The primary site of glucose reabsorption is the proximal convoluted tubule. (A), The glomerulus is a tuft of capillaries that filters fluid into the Bowman’s capsule. (C), The loop of Henle is the site of salt reabsorption. (D), The collecting duct is the site in which urine is concentrated. (E), The distal convoluted tubule conducts filtrate and is adjacent to the collecting tubule of the nephron.

26. E The carrying capacity is the maximum number of organisms of a given species that can be maintained in a given environment. Once a population reaches its carrying capacity, the number of organisms will fluctuate around it.

27. D During the exponential growth phase of a population, the size doubles during each time interval. This part of the graph looks like a parabola.

28. B Insulin decreases the level of blood glucose by increasing the storage of glycogen in muscles. The other hormones are correctly paired with their respective function.

29. E The failure in oogenesis that could produce this syndrome would occur in anaphase I or anaphase II. Anaphase refers to the stage of meiosis in which chromatids separate from each other. If the chromosomes or chromatids fail to separate during anaphase, one egg cell will contain two X chromosomes, instead of one.

30. A The tendency of climbing vines to twine their tendrils around a trellis is called thigmotropism. Thigmotropism refers to growth stimulated by contact with an object. (B), Hydrotropism refers to growth of a plant toward water. (C), Phototropism refers to growth toward light. (D), Geotropism refers to growth toward or against gravity. (E), Chemotropism refers to growth toward or away from a chemical substance.

31. C Females with Turner’s syndrome lack an X chromosome. If females with this syndrome have a high rate of hemophilia, they must not have the second X to mask the expression of the disease.

32. D This type of mutation is called frameshift mutation. The insertion of DNA leads to a change in the normal reading frame by one base pair. The other answer choices refer to chromosomal aberrations. (A) Duplication is when an extra copy of a chromosome segment is introduced. (B), Translocation is when a segment of a chromosome moves to another chromosome. (C), Inversion is when a segment of a chromosome is inserted in the reverse orientation. (E), It is not known if this would be lethal or not.

33. C The hypothalamus releases GnRH, which stimulates the release of FSH. The key word in this question is “releasing.” The organ that produces releasing or inhibiting factors is the hypothalamus. (A), The anterior pituitary is the master gland that secretes several hormones. (B), The posterior pituitary secretes oxytocin and vasopressin. (D), The pineal gland is a gland at the base of the brain that secretes melatonin and helps regulate circadian rhythms. (E), The ovary secretes estrogen and progesterone.

34. B Eutrophication is the aging process through which a body of water becomes choked by an overabundance of green algae. This causes lakes to become marshes, and then terrestrial communities. (A), Succession is the gradual progression of one community into another. (C), Evolution is the gradual change in a population over time. (D), The greenhouse effect is the gradual warming of the earth’s surface by increasing concentrations of CO2 in the atmosphere. (E), Lake turnover is the vertical movement of water brought on by seasonal temperature changes.

35. C The principle inorganic compound found in living things is water. Water is a necessary component for life.

36. C The loop of Henle is responsible for the concentration of urine. The longer the loop of Henle, the more water would be reabsorbed, which would make the filtrate more concentrated. (D), If the loop of Henle were longer, the collecting ducts may or may not also be longer. (A), If the walls of the nephrons were thicker and impermeable, water would not be conserved. (E), The proximal convoluted tubules are not associated with the concentration of urine.

37. E All of the following are modes of asexual reproduction except meiosis, which is the production of gametes for sexual reproduction. (A), Sporulation is a form of asexual reproduction in which spores are produced. (B), Fission is the equal division of a bacterial cell. (C), Budding is a form of asexual reproduction in yeasts in which small cells grow from a parent cell. (D), Cloning refers to a population of cells descended by mitotic division from one cell.

38. B The moist skin of earthworms, the lenticels of plants, and the spiracles of grasshoppers are all associated with the process of respiration—gas exchange.

39. B Locomotion in annelids is accomplished through the interaction of muscles and paired setae. Setae are bristles that help earthworms grip the soil as they inch forward. (A), (C), and (D), Insects have an exoskeleton and jointed appendages to help them move. (E), Amoeba have pseudopods to help them move.

40. B All of the choices are examples of connective tissue except muscle. Connective tissue connects and supports other tissues. Ligaments, blood, cartilage, and bone are all connective tissues.

41. D In most plants, germination is triggered by the presence of oxygen, water, and temperature. Use Process of Elimination for this question. Soil is not necessary for germination; eliminate answer choice (A). All plants require water, so eliminate answer choice (E). All plants require oxygen; eliminate answer choices (B) and (C).

42. E Each smaller trophic level has less available energy than the previous level. Use Process of Elimination for this question. (A), A net loss occurs as energy is transferred from one organism to another. (B), The total energy in plants is more than that in herbivores. (C), The first tropic level is at the bottom of a pyramid. (D), The total mass of carnivores is less than the total mass of plants.

43. B When a forest of trees is devastated by fire, secondary succession is most likely to occur. Secondary succession refers to ecological succession in a disturbed community. (A), Plants and animals will continue to inhabit the region once the community is reestablished. (C), Tough grasses are not pioneers. (D), It cannot be determined if the number of species will be stabilized. (E), The forest could support living things once new plants took root and grew.

44. B Translation, the synthesis of proteins from mRNA, occurs in the cytoplasm. (I), DNA replication occurs in the nucleus. (II), Transcription, the synthesis of RNA from DNA, occurs in the nucleus.

45. C Crossing over permits scientists to determine chromosome mapping. Chromosome mapping is a detailed map of all the genes on a chromosome. The frequency of crossing-over between any two alleles is proportional to the distance between them. The farther apart the two linked alleles are on a chromosome, the more often the chromosome will break between them. Crossing-over does not tell us about the chance of variation in zygotes, the rate of mutations, or whether the traits are dominant, recessive, or masked.

46. E Legume plants assimilate nitrogen through the activity of small microorganisms. Review the nitrogen cycle. Legumes are able to take in nitrogen and convert it to nitrates with the help of nitrogen-fixing bacteria. (A), Transpiration in plants is not part of the nitrogen cycle. (B), Green plants do not take in nitrogen in the form of ammonia. (C), Soil bacteria convert atmospheric nitrogen into nitrates.

47. E The results of the process of cloning are most similar to the results of the process of mitosis. Cloning is the asexual reproduction of genetically identical cells or organisms. The only answer choice that is a form of asexual reproduction is mitosis—new cells produced from a parent cell. (A), Gametogenesis is a series of cell divisions that leads to the production of gametes. (B), Fertilization is the union of egg and sperm to form a zygote. (C), Pollination is the process of transferring pollen grains from the anther to the stigma of a flower. (D), Meiosis is the process of cell division that produces gametes (nonidentical cells).

48. B The most likely explanation for this phenomenon is that these birds have different ecological niches. An ecological niche is the position or function of an organism or population in its environment. (A), We do not know if there is a short supply of resources. (C), We do not know how long the bird species live together. (D), The breeding patterns of the bird species does not explain the lack of competition. (E), A habitat is the physical place or environment in which an organism normally lives. These birds clearly share the same habitat.

49. B This relationship is an example of parasitism. Parasitism is a form of symbiosis in which one organism benefits and the other is harmed. (A), Commensalism is a form of symbiosis in which one organism benefits and the other is unaffected. (C), Mutualism is a form of symbiosis in which both organisms benefit. (D), Gravitropism is the growth of a plant toward or away from gravity. (E), Thigmotropism refers to growth stimulated by contact with an object.

50. C If the nucleotide sequence of a DNA molecule is 5-C-A-T-3, then the transcribed DNA strand (mRNA) would be 3-G-U-A-5. The nucleotide sequence of the tRNA codon would be 5-C-A-U-3.

51. A Viruses are considered an exception to the cell theory because they are not independent organisms. They can only survive by invading a host. (B), Viruses have either DNA or RNA. (C), Not all viruses have a tail. (D), Viruses did not evolve from ancestral protists. (E), Viruses have no nuclei.

52. C All of the listed organs secrete digestive enzymes except the gall bladder. The gall bladder stores and secretes bile produced by the liver, which is an emulsifier, not an enzyme. (A), The mouth secretes salivary amylase. (B), The stomach secretes pepsin. (D), The small intestine secretes protease. (E), The pancreas secretes pancreatic amylase and lipase.

53. C Memory loss would most likely be due to a malfunction of the cerebrum. The cerebrum controls all voluntary activities and receives and interprets sensory information. (A), The medulla controls involuntary actions such as breathing. (B), The cerebellum coordinates muscle activity and controls balance. (D), The pons is a mass of nerve fibers running across the surface of the mammalian brain. (E), The hypothalamus maintains homeostasis.

54. D In nonplacental mammals, the embryo obtains its food from the yolk sac. The yolk sac provides food for the embryo. (A), The ovary is where eggs mature. (B), The uterus is the organ that contains the developing embryo. (C), The oviduct (also known as the fallopian tube) is a tube that carries the egg from the ovary toward the uterus. (E), The allantois is an extraembryonic sac that gets rid of wastes.

55. B The similarity suggests that humans and chimpanzees are more closely related than humans and dogs. Because these two organisms share similar amino acid sequences, they must share more recent common ancestors than with the dog.

56. B The event that had to occur before oxygen filled the atmosphere was that autotrophs, which make their own food and give off oxygen, had to evolve.

57. E The structure in an earthworm that has a function similar to that of the alveoli of a human is the skin, which is where gas exchange takes place in an earthworm, and from which moisture is lost. (A), The excretory organs in insects are called malpighian tubules. (C), The chitinous exoskeleton is the protective layer of insects. (D), Gills are the respiratory organs in aquatic animals. (B) Nephridia are the excretory organs of invertebrates.

58. E Parathyroid hormone raises calcium and lowers phosphate levels in the blood. Calcium phosphate is a component of bone.

59. A Cortisol is a hormone (produced by the adrenal cortex) that increases blood sugar concentration.

60. C Progesterone is a sex hormone responsible for preparing the body for pregnancy.

61. D The hypothalamus secretes hormones that stimulate or inhibit the actions of the anterior pituitary.

62. D Classical conditioning—also known as Pavlovian conditioning—is when a certain stimulus is repeatedly coupled with a stimulus triggering a behavioral response.

63. C Imprinting is the brief period during which goslings associate any moving object with their mother.

64. B Operant conditioning is learning based on trial and error.

65. E A circadian rhythm is an internal cycle (biological clock) that repeats approximately every 24 hours. Plants placed in continued darkness continue to open and close their stomates on a 24-hour cycle.

66. B The ectoderm gives rise to the skin and nervous system.

67. C The endoderm gives rise to the inner linings of the digestive tract and associated organs.

68. A The mesoderm gives rise to the skeletal, muscular, and connective tissues.

69. E The tropical rain forest is the biome that has soil that’s depleted of nutrients.

70. B The tundra is a biome that’s characterized by a short growing season because it’s seasonally covered by snow.

71. A Taiga is a biome characterized by black bears, moose, and wolves.

72. C The embryo develops in the ovary of the plant.

73. D The anther has a similar function to that of the human testes, in that they both produce the male gametes.

74. A The stigma is the sticky portion of the flowering plant that traps pollen grains.

75. D Meiosis in flowering plants occurs within the ovary and the anthers—microspores are produced by the male reproductive organs, the anthers.

76. E A nucleotide is a building block of nucleic acids.

77. C Fatty acids are a component of phospholipids.

78. A Monosaccharides (like glucose) are a major energy source for the body. (C) is also an energy source but not a major one.

79. D Amino acids are the building blocks of proteins.

80. D The graph shows the average number of offspring per female per day. Because there are 100 females in the liter, the total number of offspring on the twentieth day would be 100 females times 2 offspring per day, which equals 200 offspring.

81. D The fact that lines A and B decrease suggests that the reproductive rate is related to the population density. After several weeks, as the number of offspring in both jars increase, the reproductive rate decreases.

82. B Based on the graph, the number of Daphnia offspring in jar B is half that of jar A and the reproductive rate is half. Therefore, the reproductive rate in jar B is the same as the reproductive rate in jar A.

83. B The largest amount of energy is available to producers. Population B is most likely composed of producers, because they have the largest biomass.

84. B An increase in the number of organisms in population C would most likely lead to a decrease in the biomass of B because population B is the food source for population C. Make a pyramid based on the biomasses given. If population C increases, population B will decrease. (A) and (C), We cannot necessarily predict what will happen to the biomass of populations that are above population C. (D), The food source available to population C would most likely decrease, not increase. (E), We do not know if an increase in the number of organisms will lead to intense competition for the food resources. It depends on how large the population increase is. If the population increase is small, the organisms would probably not have to compete for food.

85. C A 90 percent reduction of productivity in the grass is a reduction of 9,000 pounds. That means the grass should be able to support 1,000 pounds of crickets.

86. B Chromosomes replicate during interphase, the S phase. (A) and (C), During G1 and G2, the cell makes protein and performs other metabolic duties. (E), Cytokinesis refers to cytoplasmic division of the cell.

87. A One of the first signs of prophase in mammalian cells is the appearance of chromosomes.

88. C Mitosis occurs in all of the following type of cells except mature red blood cells. Mature red blood cells are short-lived and do not divide.

89. B Because neurons are not capable of dividing, it is reasonable to conclude that these cells will not complete the G1 phase. This is a reading comprehension question. The passage states that cells that do not divide are arrested at the G1 phase. (A), These cells will not be committed to go through cell division. (C) and (D), These cells will not enter the G2 or S-phase. (E), The duration of the cell cycle would be brief, not long.

90. A According to the graph, the resting membrane potential of the muscle fiber is close to –90mV.

91. E Refer to the second graph about the membrane permeability of ions during a muscle contraction. During depolarization, the membrane is permeable to Na+. (A), The voltage-gated K+ channels do not open until after depolarization. (B), The concentration of Ca2+ does not become more negative. (C), The membrane potential changes from –90 mV to +20 mV.

92. D Refer to both graphs in the passage. In cardiac fibers, the duration of an action potential—a neuronal impulse—is approximately 0.3 seconds.

93. C In cardiac muscle fibers, depolarization is prolonged compared to that in skeletal muscle fibers. (A), The membrane is permeable to both Na+ and K+. (B), In cardiac muscle fibers, voltage-gated K+ channels open during repolarization. (D), The refractory period is longer in cardiac muscle fibers compared to skeletal muscle fibers. (E), The speed of a contraction is faster in skeletal muscle fibers.

94. C The body plan associated with nematodes is Organism 3. Nematodes—roundworms—are pseudocoelomates. They have a body cavity, but the cavity is only partially lined with mesodermally-derived tissue.

95. B The body plan associated with flatworms is organism 2. Flatworms have no body cavity. They are acoelomates.

96. E Organism 5 should be placed in the phylum chordata. Coelomates have a body cavity that forms distinct cell layers (ectoderm, mesoderm, and endoderm). The only answer choice with complex processes, coelmates deuterostomes, and bilateral symmetry is chordata.

97. A All of the listed organisms exhibit bilateral symmetry except the hydra. Organisms with bilateral symmetry have two similar halves on either side of a central plane. Animals with radial symmetry have body parts arranged around a central line. Animals that exhibit radial symmetry are hydra, jellyfish, and sea anemones.

98. B When the level of CTP is low in a cell, the metabolic traffic down the pathway increases. This pathway is controlled by feedback inhibition. The final product of the pathway inhibits the activity of the first enzyme. When the supply of CTP is low, the pathway will continue to produce CTP.

99. B This enzymatic phenomenon is an example of feedback inhibition. Feedback inhibition is the metabolic regulation in which high levels of an enzymatic pathway’s final product inhibit the activity of its rate-limiting enzyme. (A), Transcription is the production of RNA from DNA. (C), Dehydration synthesis is the formation of a covalent bond by the removal of water. (D), In photosynthesis, radiant energy is converted to chemical energy. (E), Hydrolysis is the breaking of a covalent bond by adding water.

100. C The biosynthesis of cytidine 5’-triphate requires a nitrogenous base, three phosphates, and the sugar ribose. Pyrimidines are a class of nitrogenous bases with a single ring structure. The sugar they contain is ribose (shown in the pathway).


Chlorophyll is the principle light-harnessing pigment found in the thylakoid membrane of chloroplasts. It is a molecule that’s made up of a large ring structure composed of smaller rings (pyrroles). In the center of the large porphyrin ring is a magnesium atom surrounded by four nitrogen atoms. The small rings consist of many alternating single and double bonds. These alternating bonds allow electrons in the porphyrin ring and magnesium atoms to move around freely. Alternating double and single bonds are commonly found in molecules that strongly absorb visible light. Photosynthesis occurs when sunlight activates chlorophyll by exciting their electrons. The chlorophyll molecule has a long hydrophobic hydrocarbon tail. This shape allows many chlorophyll molecules to be grouped together like a stack of saucers.

The following experiment could determine the influence of sunlight on chlorophyll. First, extract chlorophyll from the leaf by boiling it gently in a dissolved solution. Then pass light of a known wavelength through the solution and measure it, using a spectrophotometer. The wavelength of the entering light can be varied to see which wavelength is most absorbed by the solution. You could then plot the data on a graph to form an absorption spectrum. The absorption spectrum will show two peaks, which represent the type of light absorbed by the pigment. The valley would represent the light that’s reflected.

Based on this experiment, it would be inferred that certain wavelengths, such as orange-red and violet-blue, are strongly absorbed by chlorophyll, whereas green and yellow hues are least absorbed, and are reflected. Chlorophyll gives plants their characteristic green color because it reflects yellow-green light.


Hormones are chemical messengers that travel through the blood and act on target cells in the body. Hormone secretion is regulated by a negative feedback system, which is the basis of hormonal regulation. In part, negative feedback loops mean that an excess of a hormone in the bloodstream temporarily shuts down the production of that hormone.

The anterior pituitary gland secretes a hormone called thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to secrete its hormone; thyroxine. Thyroxine regulates the basal metabolic rate in most body tissues. If the blood level of thyroxine rises above normal, it will suppress the secretion of TSH which, in turn, will lower the blood level of thyroxine. Both responses are negative feedback mechanisms.

Two hormones play a critical role in the regulation of calcium, parathyroid hormone and calcitonin. Parathyroid hormone, which is secreted from the parathyroid glands, increases blood calcium levels. It triggers the bones to release the calcium stored inside them. Calcitonin acts as an antagonist of parathyroid hormone. It decreases blood calcium levels. When blood calcium levels are above normal, the thyroid gland secretes calcitonin. This mechanism helps to maintain homeostasis.

ACTH is another hormone that’s released by the anterior pituitary. It stimulates the adrenal cortex to secrete a number of steroid hormones, including cortisol. Cortisol increases the blood’s concentration of glucose and helps the body adapt to stress. When cortisol levels reach a peak level in the bloodstream, the anterior pituitary is temporarily prevented from producing ACTH. This, in turn, shuts down the production of cortisol. Once the level of cortisol falls below normal, the adrenal cortex resumes production of cortisol.


A gene is a heredity unit located at a specific locus along a chromosome. Genes are made up of DNA, and DNA is made up of repeating subunits of nucleotides. A nucleotide consists of three parts, a five-carbon sugar, a phosphate group, and a nitrogenous base.

When a chromosome replicates, the two DNA strands unwind and the hydrogen bonds between them are broken. Each strand serves as a template for the synthesis of a complementary strand. Once the process is initiated (with an RNA primer), DNA polymerase adds nucleotides to each growing strand. One strand serves as the leading strand (it is made continuously), and the other serves as the lagging strand (it is made discontinuously). Each base matches the appropriate bases in the template strand; they are complementary. Once the complementary strands are formed, hydrogen bonds form between the new base pairs, leaving two identical copies of the original DNA molecule.

A gene mutation is a change in the sequence of base pairs in a DNA. It results from defects in the sequence of bases. Mutations that involve a single base change in the DNA sequence are called point mutations. These are base substitutions involving a single DNA nucleotide being replaced by another nucleotide. Another type of mutation is a frameshift mutation, caused by an insertion or deletion of bases in the DNA sequence. This causes a shift in the reading frame of DNA.


Because the amount of intracellular water is critical, one major challenge that various organisms faced was how they would regulate body water and get rid of wastes. Waste products include carbon dioxide, salts, and nitrogenous wastes. This issue is especially important because nitrogenous wastes are highly toxic to the body. Over the course of evolution, organisms developed various structural adaptations to deal with this problem.

Insects developed excretory organs called Malphigian tubules. These long, slender tubules take up water and salt, concentrate the waste, and empty it into the intestine. The waste product, uric acid, is excreted as a dry pellet through the anus. Insects also have a hard, dry cuticle that has a waxy outer layer; this aids in preventing water loss.

Organisms that live in the sea, such as marine fishes, had to cope with the gradual increase in the saltiness of the water. Seawater is hypertonic relative to their body fluids, and marine fish had to protect their body cells from water loss because without a specific mechanism for this, they would become dehydrated even though they’re surrounded by water. They excrete concentrated urine that is isotonic with their body fluids. They also eliminate excess salt by actively pumping it out through their gills.

Because humans are terrestrial organisms, they must conserve plenty of water. Humans also must get rid of nitrogenous wastes, and their major excretory organ is the kidney. Each kidney is made up of millions of function units called nephrons. As the filtrate travels along the nephron, glucose, amino acids, and salts are retained by the body, and the rest of the fluid is concentrated into urine. The skin is another important organ that gets rid of wastes. It contains sweat glands that help to maintain an optimal salt balance in the body.