Grinds in Leaving Certificate Biology

Celbridge Tutorials offers a comprehensive course in all areas of Biology, with particular emphasis on areas causing difficulty to

enrolled students.  We often have to go through typical examination questions to explain what is required in order to obtain

optimum marks.     Below, we deal with some topics which have proven to be problematic in the past


Mendel's Laws

Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member of that pair to form  part of the genetic content of different gametes.

Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair – thus the assortment of alleles within the resultant gametes is random. and unpredictable.

         (i) State the Law of Independent Assortment.
(ii) In cattle the allele for red coat (R) is dominant to the allele for black coat (r) and the allele for straight coat (S) is dominant to the allele for curly coat (s). When a bull with a red, straight coat was mated with cows with curly, black coats it was found that calves with four different phenotypes resulted. These four phenotypes occurred in equal numbers.
1. State the genotypes of the bull and of the cows.
2. State the phenotypes of the calves.
3. Which phenotypes of the calves suggest that independent assortment has taken place?
(iii)      Would you expect different phenotypes if the genes for coat colour and coat type were located on the same chromosome? Explain your answer

(1) Mendel's Second Law - the law of independent assortment states:   During gamete formation -- when genes are borne on different chromosooomes --  the segregation of the alleles of one gene is independent of the segregation of the alleles of another gene;  thus the combination of alleles within the resultant gametes is random.
(ii :1)     The cows were all homozygous for the recessive trait: only genes with homozygous recessive alleles can give rise to the recessive phenotype.  Even so, the bull had to have been heterozygous for the dominant traits; otherwise he would have given dominant alleles in all cases and the calves would show the dominant  phenotype.  Thus the bull’s genotype was Rr/Ss, and the cows’ genotypes were rr/ss
(ii: 2)     The calves’ phenotypes were: 25% red with straight hair.  25% black with straight hair.  25% red with curly hair.  25% black with curly hair.   This ratio always occurs in di-hybrid crosses, when one  parent  is heterozygous for the dominant traits and the other parent is homozygous tor the recessive traits -- when the genes involved are borne on different chromosomes
(ii: 3)    All the phenotypes, except those with red and straight coats demonstrate that random assortment has occurred in the production of their father’s gametes.   In only 25% of his gametes has the bull’s dominant R/S gametes been passed on together – instead they have been mixtures:  R/S  R/s  r/S  r/s.  This has been due to the random alignment of alleles during metaphase (1)  in meiosis – the process which produces the “haploid” gamete.
(iii)  If the genes for coat colour and coat type had been borne on the same chromosome, this bull’s gametes would have been (RS) and (rs)., while the cows’ gametes would have been (rs)  (rs)  The calves’ phenotypes would have been  50%  red with straight coats, and 50% black with curly coats.  This is always the ratio when two traits are carried by genes on the same chromosome, and when one parent is heterozygous for the dominant traits, and the other is homozygous for the recessive traits.


Natural selection is the way in which a population adapts to changes within its environment.  Adaptation operates on variation  -- and variation is brought about by  gene mutation.
In a population which has attained an equilibrium within its environment, gene combinations will ocur at random, and there will be a thorough mixing of alleles within the gene pool. This equilibrium  can be disturbed by environmental change which will  now begin to select the phenotypes most suited to the new prevailing  conditions.  Those phenotypes with beneficial traits will begin to increase within the population through sexual reproduction and  greater survival of their progeny. The result is an increase  within the population of these alleles that bear an advantage, with a similar decrease in the less beneficial alternatives.   An excellent example of this phenomenon of natural selection is that of the peppered moth Biston betularia.
Biston betularia  is a bright coloured speckled moth whose niche is the bright  lichen  that grows on the bark of birch trees.  When air pollution in England’s urban areas caused destruction of lichen and blackening  of the bark of the birch, the moth became very vulnerable to destruction by predation – for it had now become very visible.  Then, in the Manchester area, in 1845, a variant arose by mutation. This new genotype was dark coloured and quite invisible against the polluted background of soot-stained  bark.   By 1895 the variant had completely overwhelmed the original bright winged phenotype: 99% of the  population of this genus of moth was now the dark variant.  The new variant was given the name  Biston carbonaria.
  Biston betularia never became extinct; for it continued to prevail within the non polluted countryside, and is now making a comeback within urban areas because of decreasing air pollution.



(a) State a function of the cell membrane ?
(b) State one feature that would allow you to identify an eukaryotic cell ?
(c) Name a human cell that is haploid ?
(d) What term is used to describe a cellular reaction in which large molecules are broken down
to smaller ones? .
(e) What term is used to describe an individual’s genetic make up?
(f) Name a scientist responsible for the Theory of Natural Selection ?



The Carbon Cycle
Carbon is the backbone of all organic molecules and is the most prevalent element in cellular (organic) material. Autotrophs, which include plants, and algae are  photosynthetic.  They  use CO2 as a sole source of carbon for growth, Heterotrophs require organic carbon for growth; they get this from autotrophs  and ultimately convert it back to CO2.
The relationship between autotrophs and heterotrophs is: autotrophs fix carbon needed by heterotrophs, and heterotrophs produce CO2 used by the autotrophs.
CO2 + H2O-----------------> CH2O (organic material)   autotrophy
CH2O + O2-----------------> CO2 + H2O heterotrophy
CO2 is the most prevalent greenhouse gas in the atmosphere.  Thus  When these two equations get out of balance (i.e. heterotrophy predominating over autotrophy, as when rain forests are destroyed and replaced with cattle), it adds to the process of “greenhouse warming.”
The methanogens  are archaea  that are inhabitants of virtually all anaerobic environments . They use CO2 in their metabolism in two distinct ways. About 5 percent of CO2 taken up is reduced to cell material during autotrophic growth; the remaining 95 percent is reduced to CH4 (methane gas), methane accumulates in rocks as fossil fuel ("natural gas"), in the rumen of cows and guts of termites, in sediments, swamps, landfills and sewage digesters.  Methane is the second most prominent gas in the atmosphere and is also a major factor in “greenhouse warming”.
Biodegradation is the decomposition of organic material (CH2O) back to CO2 + H2O and H2. In soil habitats, the fungi play a significant role in biodegradation, but the procaryotes are equally important. Overall Process of Biodegradation (Decomposition)

 In the Carbon Cycle. Organic matter (CH2O) derived from photosynthesis (plants, algae and cyanobacteria) provides nutrition for heterotrophs (e.g.  animals and associated bacteria), which convert it back to CO2. Organic wastes, as well as dead organic matter in the soil and water, are ultimately broken down to CO2 by microbial processes of biodegradation.



(1)Lightening makes Nitrogen from the air and carries it to the soil where it becomes available to plants

(2)Bacteria fix nitrogen in air for assimilation to proteins in certain plants
Other plants obtain Nitrogen through their roots from the soil

(3)Herbivorous animals receive their nitrogen as proteins  from digestion of plants.  Carnivorous animals receive Nitrogen from eating herbivorous animals.

(4)Decomposition of dead plants and animals return Nitrogen to the soil, where it can again become available to plants.   However, some denitrifying bacteria can return Nitrogen to the air.




The nitrogen cycle describes the path of the element nitrogen through nature. Nitrogen is essential for life. It is found in amino acids, proteins, and genetic material. – such as DNA.  Nitrogen is the most abundant element in the atmosphere (~78%). However, gaseous nitrogen N2 must be changed into another form so that it can be used by living organisms: this is called “Nitrogen Fixing”

There are two main ways through which nitrogen is 'fixed':  (1) Fixing by Lightning and (2) Biological Fixation
FIXING BY LIGHTNING   The energy from lightning causes nitrogen in the air N2 to combine with Oxygen O2 to form Nitrogen oxides which are carried to the earth by precipitation.  Here these are converted to Nitrates NO3 which  can be assimilated into amino acids and proteins by plants.

About 90% of nitrogen fixation is done by bacteria which possess the enzyme nitrogenase that can convert N2 to N03  Azotobacter  is such a Nitrogen Fixer: it is free-living – living within the soil.  It is heterotrophic : requiring carbohydrate compounds within its environment in order to obtain its energy.

Bacteria that are symbiotic depend on a mutually beneficial relationship with plants: the plants supply the bacteria with carbohydrates and energy, while the bacteria supply the Nitrogen to the plants.  These plants include legumes – like clovers and peas;  which have mutually symbiotic bacteria living within special root swellings, called “root-nodule.“  Rhizobium is such a mutually symbiotic bacterium – possessing nitrogenase and assimilating N2 into organic compounds within its host.

Ammonia NH3 can be taken up directly by plants; however, most of the ammonia produced by decay is converted into nitrates – which can be more easily absorbed by plant roots and thereafter assimilated into organic compounds.
• Bacteria of the genus Nitrosomonas oxidize  Ammonia NH3 to nitrites (NO2−). This bacterium obtains its energy from  this  chemical reaction alone: thus it is both aerobic and  a chemotrophic autotroph.
• Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates NO3− These are also aerobic and are chemotrophic autotrophs.
• Nitrification occurs by the following reactions:
2 NH3 + 3 O2 - > 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 -> 2 NO3-

When plants and animals die, bacteria convert nutrients – such as amino acids  R-CH(NH2)COOH-- into ammonia NH3. This conversion process is called ammonification    Clostridium is such a bacterium.  It is an anaerobic heterotroph – obtaining its energy from the breakdown of organic matter.

Denitrification returns nitrogen to the atmosphere, and thus it is undesirable in agriculture.. In this process the heterotrophic bacteria  Pseudomonas  and  Serratia convert soil nitrates NO3 to gaseous Nitrogen  N2





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