Each of these statements was once regarded as a biological fact but is now under
ID: 51375 • Letter: E
Question
Each of these statements was once regarded as a biological fact but is now understood
to be untrue. In each case, indicate why the statement was once thought to be true and why it is no longer considered a fact.
(a) Plant and animal tissues are constructed quite differently because animal tissues do not have conspicuous boundaries that divide them into cells.
(b) Living organisms are not governed by the laws of chemistry and physics, as is nonliving matter, but are subject to a vital force that is responsible for the formation of organic compounds.
(c) Genes most likely consist of proteins because the only other likely candidate, DNA, is a relatively uninteresting molecule consisting of only four kinds of monomers (nucleotides) arranged in a relatively invariant, repeating tetranucleotide sequence.
(d) The fermentation of sugar to alcohol can take place only if living yeast cells are present.
Explanation / Answer
(a) In 1838,Schleiden concluded that plant and animal tissues are constructed quite differently because animal tissues do not have conspicuous boundaries that divide them into cells.He concluded that all plant tissues are composed of cells and that an embryonic plant always arises from a single cell.
Theodor Schwann examined animal cartilage cells and became convinced of the fundamental similarity between animal and plant cells because animal cartilage cells have well defined boundaries that are well defined by thick deposits of collagen fibres.
(b)Early in the 19th century, according to the theory of Vitalism,the chemical reactions occuring within the living matter are directed by a vital force and do not follow the laws of Physics and Chemistry.
In 1828, Friedrich Wohler,a French scientist demonstrated that the biological compound,urea could be synthesized from an inorganic compound,ammonium cyanate in the laboratory thereby disapproving the theory of Vitalism.
(c) Till the middle of twentieth century,it was widely considered that genes most likely consist of proteins because proteins were the only nuclear components that seemed to account for the obvious diversity of the genes.
In 1944 ,Avery,MacLeod and McCarty through their landmark experiment clearly showed DNA to be the genetic material and not proteins.Their work mainly focused on the genetic transformation of bacteria and the proved that the factor responsible for the transformation is DNA.
(d) Louis Pasteur linked the activity of living organisms to specific processes by showing that living cells were needed to carry out the fermentation of sugar into alcohol.
In 1897,Eduard and Hans Buchner showed that fermentation could also take place with extract from yeast cells called ferments and intact living cells are not required.
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