All matter can be broadly divided into two major groups “Pure” and “Impure”. The term ‘purity’ has quite a different meaning in chemistry than in our day-to-day life. Normally when we refer to pure water, pure milk, etc., what is implied is that the water, milk etc., are free from harmful substances such as bacteria, fungi, viruses, etc. ‘Purity’ in chemistry is entirely of a different nature. When we say a substance is pure, it means that the substance is made of only one type of constituent particles.

Example: In chemical terms, pure water means that it is made of only one type of molecules i.e., H2O.

As mentioned above matter can be divided into pure and impure substances. The pure substances can be further divided into “Elements” and “Compounds”. The impure substances, commonly called “Mixtures” can also be divided further into ‘Homogeneous’ and ‘Heterogeneous’ mixtures.

John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.
Main assumptions or postulates of Dalton

* All matter consists of indivisible particles called atoms.

* Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.

* Atoms cannot be created or destroyed.

* Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.

* Atoms of same element can combine in more than one ratio to form two or more compounds.

* Atoms are the smallest unit of matter that can take part in a chemical reaction.

Drawbacks of Dalton’s atomic theory of matter

* The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.

* According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.

* Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.

* According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.

* The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.

Merits of Dalton’s atomic theory

* It has enabled us to explain the laws of chemical combination.

* Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).

Free Radicals Defined:

Free radicals are a byproduct of normal cell function. When cells create energy, they also produce unstable oxygen molecules. These molecules, called free radicals, have a free electron. This electron makes the molecule highly unstable. The free radical bonds to other molecules in the body – causing proteins and other essential molecules to not function as they should. Luckily, antioxidants can minimize free radical damage.
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Although scientists as far back in history as Aristotle recognized that the features of one generation are passed on to the next (…like begets like…) it was not until the 1860′s that the fundamental principles of genetic inheritance were described by Gregor Mendel. Mendel’s work with common garden peas, pisum sativum, led him to hypothesize that phenotypic traits (physical characteristics) are the result of the interaction of discrete particles, which we now call genes, and that both parents provide particles which make up the characteristics of the offspring. His theories were, however, widely disregarded by scientists of the time. In the last quarter of the 19th century, however, microscopists and cytologists, interested in the process of cell division, developed both the equipment and the methods needed to visualize chromosomes and their division in the processes of mitosis (A. Schneider, 1873) and of meiosis (E. Beneden, 1883).

As the 20th century began many scientists noticed similarities in the theoretical behavior of Mendel’s particles, and the visible behavior of the newly discovered chromosomes. It wasn’t long before most scientists were convinced that the hereditary material responsible for giving living things their characteristic traits, and chromosomes must be one in the same. Yet, questions still remained. Chemical analysis of chromosomes showed them to be composed of both protein and DNA. Which substance carried the hereditary information? For many years most scientists favored the hypothesis that protein was the responsible molecule because of its comparative complexity when compared with DNA. After all, DNA is composed of a mere 4 subunits while protein is composed of 20, and DNA molecules are linear while proteins range from linear to multiply branched to globular. It appeared clear that the relatively simple structure of a DNA molecule could not carry all of the genetic information needed to account for the richly varied life in the world around us!
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(Nutrition Comes Alive, The Nutrient Connection Developed by the Division of Nutritional Sciences Extension Service, Cornell University, 1986)

All foods are ready-to-eat. Values taken from Nutritive Values of Foods, USDA Bulletin.
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