Absolute Configuration

This is a way of denoting the absolute structure of an optical isomer. Two conventions are in use. The D-L convention relates the structure of the molecule to some reference molecule in the case of sugars and similar compounds, the dextrorotatory form of glyceraldehydes (HOCH2CH(OH)CHO). 2.3 dihdroxyproppanal) was used. The rule is as follows. Write the structure of this molecule down with asymmetric carbon in the centre, the CHO group at the top the OH on the right, the CH2OH at the bottom and the H on the left.

Now Imagine that the central carbon atom is at the centre of a tetrahedron with the four groups at the corners and that the H and the OH come out of the paper and the CHO and CH2OH groups go into the paper. The resulting three-dimensional structure was taken to be that of d-glyceraldehyde and was called D-glyceraldehyde.

Any compound that contains an asymmetric carbon atom having this configuration belongs to the L-series. It is important to note that the prefixes D- and L- do not stand for dextrorotary .

Agriculture

This is the study and practise of cultivating land for the growing of crops and the rearing of livestock. The increasing demands for food production during the second half of the 20th century have seen many developments in agricultural technology and practises that have greatly increased crop and livestock production. However these advances in modern intensive farming techniques have had their impact on the environment, particularly use of *fertilizers and *pesticides. The now widespread practise of crop monoculture (in which one crop is grown densely over an extensive area) has required an increase in the used of *pesticides, as monoculture provides an ideal opportunity for crop pests.

Monoculture also requires vast areas of land, which has meant that natural habitats have been destroyed. *Deforestation has resulted from the clearing of forests for crop production and cattle rearing. Advances in technology have included ploughing machine with hydraulic devices that can control the depth to which the soil is ploughed and seed drills that automatically implant seeds in the soil so that ploughing is not necessary.

Food supply in many less-developed countries relies on subsistence farming, in which the crops and livestock produced are used solely to feed the farmer and his family.