BIOLOGY FREE-RESPONSE QUESTIONS & ANSWERS FOR YEAR 9, 10 & 11 STUDENTS (Am.)

Nー1
An aquarium, with a balanced non-living and living environment, was placed in natural light and then sealed; the living organisms included autotrophs, herbivorous and carnivorous heterotrophs, and saprotrophs.
(a) State and explain how the aquarium would change if all the ingestive heterotrophs were removed and then the aquarium resealed. [5]
(b) State and explain how the aquarium would change if all the plant autotrophs were removed and then the aquarium resealed. [5]
(c) Suggest and explain how the sealed aquarium might change over periods of time. [10]

(a) Initially, the numbers of pond plants would increase because there would be no herbivores to eat them. Then their numbers would stabilize because carbon dioxide would be a limiting factor; i.e., the constant amount of this abiotic source of carbon would determine the amount of chemical energy transduced from light energy via photosynthesis,

               Light / Chlorophylls / Enzymes
 6CO2(g) + 6H2O(l) 覧覧覧覧覧覧覧覧覧覧覧 C6H12O6(aq) + 6O2(g)   +DE 

(b) In the absence of plants producing chemical energy and dioxygen via photosynthesis, there would be a rapid collapse of the food webs within the aquarium; herbivores would die first, then the carnivores, and finally the saprotrophs (after an initial increase in numbers). []

(c) In the short term, the original balanced ecosystem should remain unaltered because the autotrophs provide chemical energy and dioxygen (via photosynthesis), the autotrophs, ingestive heterotrophs, and the saprotrophs excrete carbon dioxide (via respiration),

                        Enzymes
 C6H12O6(aq) + 6O2(g) 覧覧覧覧覧覧 6CO2(g) + 6H2O(l)    -DE (38ATP) 

and the saprotrophs recycle nutrient ions for the autotrophs. In the (very) long term, however, alterations in this ecosystem must occur for three reasons: first, there would be an accumulation of toxic excretion products; second, all the organic compounds would eventually form fossil fuels; and third, favorable mutations in the reproductive cells of one or more of the living organisms would result in the evolution of new species by natural selection.

[ Without competition from plants, a range of chemosynthetic and photosynthetic bacteria might flourish exemplified by Thiobacillus ferrooxidans and Rhodopseudomonas capsulata, respectively.]


Nー2
(a) Explain how the nutrition of green plants differs from fungi. [6]
(b) Explain each of these terms: 'ecosystem'; 'biodiversity'. [14]

(a) Green plants are photosynthetic autotrophs; i.e., they synthesize their chemical energy, from the abiotic components carbon dioxide and water, via the process of photosynthesis. By contrast, fungi are absorptive heterotrophs; i.e., they obtain their chemical energy from biotic components, via the extracellular enzymic digestion of dead organisms and the subsequent absorption of soluble ions and molecules.

(b) An ecosystem is a complete life-supporting environment; it includes the entire community of interacting organisms, their physical and chemical environments, energy flows, and the types, amounts, and cycles of nutrients in the various habitats within the system. Ecosystems range in size from a well-balanced aquarium to the biosphere (i.e., the Earth's land and water surfaces, together with the lower atmosphere).
     Until relatively recently, biodiversity the range of biological species in the biosphere had been determined by natural selection. Now, however, three major artificial selection pressures are occurring: first, Man's unparalleled ability to misuse the environment (e.g., by emission of 'greenhouse gases', by depletion of the 'ozone layer', and by causing 'acid rain'); second, in order to obtain more land suitable for crops, there is steady destruction of the (non-renewable) tropical rainforests, which is leading to the extinction of a countless number of naturally evolved species every year; and third, new organisms are being introduced into the biosphere as a result of the rapid advances in both cross-breeding and recombinant DNA technology. []

[  Thus far, neither politicians nor scientists have directed their attentions to the medium to long term effects of these introductions. In this context, albeit indirectly, it is worth noting the long term environmental effects of CFCs, DDT, and leaded-gasoline: despite their undoubted short term advantages to one species (Man).]


Nー3
Plants synthesize glucose by the process of photosynthesis; apart from its direct use as a respiratory substrate, to release ATP, plants use this carbohydrate to biosynthesize all the other substances required for growth and reproduction.
(a) State the uses of four other carbohydrates in plants. [4]
(b) Summarize and explain two examples of the use of radioactive labeling in investigating plant biosynthesis. [16]

(a) Cellulose is a component of cell walls; fructose is a component of the nectar used to attract insect pollinators; starch is the major energy storage compound; and sucrose, also a component of nectar, is the principal sugar translocated in the phloem.

(b) Investigations with radioactive-labeled hydrogen have shown that the reactant water used in photosynthesis comes from the roots via the xylem, and not by diffusion through the stomata: i.e.,

                     Light / Chlorophylls / Enzymes
           (via roots) 覧覧覧覧覧覧覧覧覧覧覧 C63H12O6(aq) + 6O2(g)
 6CO2(g) + 63H2O(l)
           (via stomata) 覧覧覧覧//覧覧覧覧 C63H12O6(aq) + 6O2(g) 
                     Light / Chlorophylls / Enzymes

Investigations with radioactive-labeled carbon have shown that, in the absence of limiting factors, the initial photosynthetic product is rapidly condensed to insoluble starch (so raising the cell's water potential); this starch is then hydrolyzed back to glucose (e.g., during periods of low light intensity); this glucose is condensed to sucrose before translocation via the phloem; and finally, the sucrose is condensed to starch in a storage organ: i.e.,

                  Light / Chlorophylls / Enzymes
 614CO2(g) + 6H2O(l) 覧覧覧覧覧覧覧覧覧覧覧覧 14C6H12O6(aq) + 6O2(g) 
-nH2O / Enzymes +nH2O / Enzymes n14C6H12O6(aq) 覧覧覧覧覧覧覧覧覧覧覧覧覧覧覧 (-14C6H10O5)n
-nH2O / Enzymes -nH2O / Enzymes n14C12H22O11(aq) 覧覧覧覧覧覧覧覧覧覧覧覧覧覧 (-14C6H10O5)n sucrose starch

Nー4
(a) Briefly describe the common function in the following pairs: cornea and pinna; retina and cochlea. [4]
(b) Compare the nervous and endocrine systems of a mammal. [10]
(c) Compare plant auxins and mammalian hormones. [6]

(a) The cornea and the pinna both focus external stimuli: the cornea focuses light onto the retina, whereas the pinna focuses sound towards the ear drum (where it is transduced to mechanical energy).
     The retina and cochlea are both energy transducers: the retina contains rod and cone cells which transduce light (via chemical) into electrical energy, whereas the cochlea contains sensory neurones which transduce mechanical into electrical energy.

(b) The nervous and endocrine systems both have a direct role in sensitivity; that is, the characteristic of all living organisms to detect changes in their internal and external environment, and then to respond appropriately to these stimuli.
     The basic mechanism of sensitivity is common to both systems: thus, stimulus (a change in the internal or external environment) 覧 receptor (a cell or tissue which is sensitive to a particular stimulus) 覧 coordinator (which organizes a possible response to the mammal's overall needs) 覧 effector (which usually produces a response appropriate to the stimulus).
     Functionally, the nervous system is capable of rapid responses, because of the speed of electrical nerve impulses. By contrast, the endocrine system produces slower and longer term responses, because hormones are transported to the target organ(s) by the blood circulatory system.

(c) Auxins and hormones are chemicals which are both produced in response to external stimuli and both result in changes in metabolic activity. However, whereas the production of auxins invariably induce permanent tropic responses in regions of mitotic cell division, the secretion of hormones usually result in temporary responses though noteworthy exceptions include estrogen, testosterone, and thyroxine.


Nー5
Write an account of the environmental importance of each of the following: 'greenhouse gases'; ozone (i.e., trioxygen). [20]

     Certain gases in the atmosphere including carbon dioxide, chlorofluorocarbons, and methane are termed 'greenhouse gases' because they absorb outgoing infrared radiation but do not significantly affect incoming visible radiation (i.e., they act like the panes of glass in a greenhouse). These gases trap heat radiated from the Earth, and so cause the so-called 'greenhouse effect'; furthermore, the higher their concentration in the atmosphere, the more radiation returned, and so the higher the temperature of the Earth's surfaces. Massively increased industrialization and land cultivation during the twentieth century has led to massive increases in the emission of various greenhouse gases, whose accumulation has led to the phenomenon of 'global warming' (i.e., the gradual increase in the temperature of the lower atmosphere). The ecological effects of global warming are expected to include either the disruption or the destruction of various balanced ecosystems.
     The Earth is protected from harmful ultraviolet radiation emitted from the Sun by a layer of ozone in the upper atmosphere. During the twentieth century, this 'ozone layer' has become increasingly depleted because of the increased emissions of various gases which decompose ozone (e.g., chlorofluorocarbons and nitrogen oxides). This depletion is known to have at least two important biological effects: an increase in mutation rates (which will increase the rate of genetic change if these mutations occur in reproductive cells), and a steady decrease in the major producers of the oceans (i.e., the phytoplankton).
     Ozone, when produced by a complex series of photochemical reactions involving nitrogen oxides and unburnt hydrocarbons emitted from vehicle exhausts, is a component of photochemical smog. In sharp contrast to its protective role in the upper atmosphere, 'low-level' ozone causes the destruction of a diverse range of naturally occurring molecules (including enzymes, hormones, and nucleic acids).


Nー6
(a) Five divisions within the Plant kingdom are bryophytes, ferns, conifers, and monocotyledonous and dicotyledonous flowering plants. Use your knowledge of the characteristics of these divisions to construct a simple key which would place a plant into its correct division. [6]
(b) Living organisms are adapted to live in certain environments. State and explain carefully:
why bryophytes are restricted to damp habitats;
[6]
four adaptions of cacti to arid environments. [8]

(a) Use of the following dichotomous key should allow a particular plant to be placed into its correct division.
     1a Sexual reproduction involves spores ... go to 2
     1b Sexual reproduction involves seeds ... go to 3
     2a True vascular tissue absent ... Bryophyte
     2b True vascular tissue present ... Fern
     3a Seed enclosed in cone ... Conifer
     3b Seed enclosed in fruit ... Flowering plant; go to 4
     4a Embryo contains one seed-leaf ... Monocotyledon
     4b Embryo contains two seed-leaves ... Dicotyledon

(b) Bryophytes are restricted to growing in damp conditions for three reasons. First, they do not have either true root or vascular systems, so absorption of water and nutrient ions must occur over the whole of their lower surfaces. Second, they do not have a cuticle to control water loss. And third, they are dependent on water for the transfer of swimming sperm to the ova.
     Four adaptions of cacti to arid environments are as follows. First, they have small surface area to volume ratios, so less heat is gained by radiation. Second, they have reduced leaves (or spines), so there is less surface area for water loss by evapotranspiration. Third, their stems contain stomata which open at night, to allow diffusion of the carbon dioxide required for photosynthesis, but close during the day, to reduce water loss. And fourth, they have extensive fibrous root systems, to ensure the maximum absorption of available water.


Nー7
(a) By means of concise statements, distinguish between these terms: gene and allele; homozygous and heterozygous; dominance, recessive, and codominance; genotype and phenotype. [9]
(b) Human blood groups are controlled by three alleles (A, B, and o) of gene I; A and B are codominant, whereas o is recessive. Show how two parents, who are heterozygous for blood groups A and B, could produce four children with different phenotypes. [7]
(c) Explain, in detail, one reason why it is important that blood of a pregnant mother does not transfer to the fetus.[4]

(a) The basic unit of inheritance for a given characteristic is a gene, and an allele is an alternate form of a particular gene. In the diploid condition, homozygous means that both alleles are identical, whereas heterozygous means that the alleles are different. A dominant allele is one which influences the appearance of the phenotype in either the homozygous or heterozygous condition; a recessive allele is one that influences the appearance of the phenotype only when homozygous; and codominant alleles are ones that are both fully expressed in the phenotype. The genotype is the genetic make-up in terms of alleles, whereas the phenotype is the physical or chemical expression of the genotype.

(b) Two parents, heterozygous for blood groups A and B, could produce four children with different phenotypes, as follows:

Genetic diagram for offspring of parents (heterozygous for blood groups A and B)

(c) If the pregnant mother and fetus have different blood groups, then mixing would induce a homeostatic immune response. Thus, a fetus would produce antibodies to react with the antigens of the mother's blood; as a result, clumping would occur, and so the dioxygen-carrying capacity of fetal blood would be severely reduced: in turn, this would probably lead to brain damage of the fetus because dioxygen is required for the aerobic respiration of all cells.


Nー8
Apart from viruses [], bacteria are considered to be the simplest type of organism.
(a) Explain the role of each of the following structures or substances in a typical bacterium: cell membrane; mitochondria; enzymes; DNA. [8]
(b) Recombinant DNA technology, popularly known as genetic engineering, is the transfer of genes from one organism to another. Describe the biotechnological processes used to manufacture insulin, one of which involves recombinant DNA technology. [12]

(a) The cell membrane controls movement of materials in to and out of the cell, and also promotes homeostatic concentrations of energy, ions, and molecules. Mitochondria biosynthesize the cell's immediate source of chemical energy (ATP), which is required for endergonic processes. Enzymes are used to control the speed of all the biochemical reactions. And, DNA provides the genetic code required both for the bacterium to reproduce and to biosynthesize proteins from amino acids.

(b) Essentially, two biotechnological processes are used to manufacture human insulin. The first uses recombinant DNA technology to obtain a bacterium which can synthesize insulin, as follows: a bacterial plasmid is cleaved using a specific restriction endonuclease; human DNA, containing the gene for insulin, is cleaved using the same endonuclease enzyme; this DNA fragment is inserted into the opened bacterial plasmid using ligase enzyme; and finally, this recombinant bacterial plasmid is placed into a host bacterial cell (e.g., Escherichia coli). This new bacterium, 'reprogrammed' to synthesize insulin, is used in a second biotechnological process, which involves allowing this organism to reproduce by binary fission, in the absence of limiting factors, in a closed bioreactor; from the resulting clones, insulin is extracted and purified by standard methods.

[  Viruses, which are invariably parasitic and pathogenic, are viewed as non-living organisms because they do not show the characteristics of living organisms; i.e., cellular organization, homeostasis, excretion, movement, respiration, reproduction, irritability, nutrition, and growth (... ォChemrringサ?).]


Nー9
(a) Some cellular exergonic reactions produce hydrogen peroxide as a toxic metabolite. Hydrogen peroxide is decomposed to water and dioxygen by the action of the enzyme catalase; convenient sources of this enzyme include liver and potato. Controlled experiments focus on 'variables'. Explain, with suitable examples involving this enzyme, each of the following terms: 'qualitative variable'; 'quantitative discontinuous variable'; 'quantitative continuous variable'; 'constants'.[16]
(b) Enzyme activity is a chemical process, whereas osmosis is a physical process. Nevertheless, when potatoes are boiled, catalase activity and endo-osmosis are both reduced. Explain both of these observations. [4]

(a) A qualitative variable is one that is non-numerical or purely descriptive; e.g., "as the type of metal ion changes, [Cu(II); Fe(II); Pb(II); Sn(II); and Zn(II)], so does the rate of decomposition of hydrogen peroxide" (hypothesis 1).
     A quantitative continuous variable is one where the data are obtained by measuring, and so can take any value within a continuous range; e.g., "as the concentration of Pb(II) ions increases, [0.0; 0.25; 0.50; 0.75; and 1.00 mol dm-ウ], the rate of decomposition of hydrogen peroxide decreases" (hypothesis 2).
     A quantitative discontinuous variable is one where the data are obtained by counting, and so are whole numbers; e.g., the number of (dioxygen) bubbles evolved in 60 seconds, [0, 1, 10, 14, 20, 21, 19, 20, 19, ...], for a reaction mixture of hydrogen peroxide and potato.
     In order to 'isolate the (chosen) independent variable', it is essential that the values of all other independent variables are both kept constant and measured; e.g., in investigating hypothesis 1, the minimum set of constants would be the absence of adventitious catalysts (e.g., metal ions), the concentration, pH, and temperature of reaction mixture, and the mass, surface area, and type of potato.

(b) Both observations can be explained by proteins being denatured by heat; i.e., destruction of their three-dimensional structures. Thus, the active site of the catalase is no longer optimal for the substrate hydrogen peroxide; similarly, the structure of the glycoprotein of the cell membrane is sufficiently altered to decrease its permeability to water.


Nー10
(a) Construct a dichotomous key which would place an organism into its correct kingdom. [8]
(b) Compare and contrast the transport systems of a mammal with those of a vascular plant. [12]

(a) Use of the following dichotomous key should allow a particular organism to be placed into its correct kingdom.
     1a Membrane-bound organelles present ... go to 2
     1b Membrane-bound organelles absent ... Bacteria (Monera)
     2a Hyphae present ... Fungi
     2b Hyphae absent ... go to 3
     3a Unicellular ... Protoctista
     3b Multicellular ... go to 4
     4a Autotrophic nutrition ... Plant
     4b Heterotrophic nutrition ... Animal

(b) Each living cell of a large multicellular organism has the same basic needs as an independent, free-living aerobic cell. However, whereas the transport of substances in an independent cell can occur by simple diffusion, multicellular organisms such as vascular plants and mammals require transport systems because they have small surface area to volume ratios.
     A mammal's systemic circulatory system has a similar function to that of a plant's vascular tissues; i.e., the transport of amino acids, hormones, nutrient ions, sugars, and water. However, because a plant exchange gases with its environment by passive diffusion through the stomata of leaves and the epidermis of the stem and roots, it has no equivalent to a mammal's pulmonary circulatory system.
     Mammals generate large amounts of toxic nitrogenous metabolites; the removal of these excretion products requires coordinated activity between circulatory and excretory systems. By contrast, plants generate smaller amounts of such metabolites; what there are of these are either recycled or stored in an insoluble form (e.g., the tannin in leaves).


Nー11
Soil and leaf litter contain a variety of animals, including annelids [e.g., common earthworm (Lumbricus terrestris)] and arthropods [e.g., garden woodlouse (Oniscus asellus)].
(a) With respect to soil fertility, explain one role that earthworms and woodlice have in common. [4]
(b) Explain why annelids and arthropods show behavioral responses to certain named stimuli. [4]
(c) Explain two disadvantages of incomplete (or gradual) metamorphosis and two advantages of complete metamorphosis in the life-cycles of named insects. [8]
(d) Explain one reason why insects are able to exploit a wider variety of habitats than either annelids or fish. [4]

(a) Earthworms and woodlice partially digest ingested leaf litter; their egested wastes, with increased surface area, together with their excretory products, are suitable for conversion by saprotrophic and nitrifying bacteria to the nutrient ions required by the autotrophs.

(b) Both annelids and arthropods, in common with all living organisms, need to maintain the constancy of their internal environment, and so they move away from unfavourable stimuli (e.g., extremes of pH or of light intensity). In addition, as heterotrophs, they need to acquire their chemical energy from biotic components, and so they usually need to move towards sources of food.

(c) In the incomplete metamorphosis of a locust (Locusta migratoria), the growth of nymph to adult involves ecdysis; this periodic loss of exoskeleton makes the insect vulnerable to dehydration and predation.
     In the complete metamorphosis of a monarch butterfly (Danaus plexippus), the distinct growing stages of egg 覧 larva 覧 pupa 覧 adult allow the insect to exploit different habitats and food sources, and so it has a greater chance of surviving to reproductive maturity.

(d) Gaseous exchange surfaces must be moist to facilitate diffusion. In annelids and fish, their epithelial and gill surfaces are directly exposed to the environment. In insects, by contrast, their tracheal surfaces are connected to the environment by closable vents (known as spiracles), and, furthermore, evaporation of internal water is reduced by waterproof chitinous exoskeletons; accordingly, these organisms are not restricted to either dry or moist habitats.


Nー12
(a) Briefly compare and contrast locomotion in annelids, arthropods, and aquatic vertebrates. [10]
(b) Explain the biological basis for three methods of preserving foods; these may include annelids, arthropods, and aquatic vertebrates [exemplified by the edible snail (Helix pomantia), edible crab (Cancer pagurus), and yellowfin tuna (Thunnus albacares), respectively]. [10]

(a) Locomotion, which involves both propulsion and support, is brought about by the contraction of antagonistic muscles in conjunction with a skeleton. An annelid has a hydrostatic skeleton, which is a body cavity containing a fluid under pressure, and which acts as a lever for the alternate contractions of circular and longitudinal muscles; the resulting peristaltic waves cause successive portions of the body to elongate and to press against the ground, and so propel the animal forward. Propulsion in arthropods and vertebrates similarly involves alternate contractions, with two sets of antagonistic muscles for each skeletal part: flexors, which bend a limb joint, and extensors, which straighten the joint. The hydrostatic skeletons of annelids and the exoskeletons of arthropods provide sufficient support for the medium in which these organisms move (i.e., air): by contrast, some aquatic vertebrates such as bony fish require, in addition to the support provided by an endoskeleton, the buoyant effect of a swim bladder.

(b) All methods of preserving food attempt, directly or indirectly, to inhibit the growth and reproduction of saprotrophs by inhibiting their enzymes. Methods of direct inhibition include 'pickling' in an acidic medium and refrigeration; aqueous hydrogen ions, which diffuse into cells across semi-permeable membranes, denature a variety of enzymes, whereas low temperatures result in the substrate and enzyme particles having less kinetic energy, and so fewer have the required activation energy for successful collisions. Methods of indirect inhibition, which focus on the removal of the water required for diffusion of substances in to and out of cells, include immersion in a preservative with a low water potential (e.g., brine); here, water is removed from bacterial and fungal cells by exo-osmosis across semi-permeable membranes.


Dr. R. Peters: Home page; Weihnachtstag-epigenom multidisciplinary resource.