Interesting question, there is some conversion of sugar from saturated to unsaturated chains (those which have C=C bonds in them), called a Maillard reaction, which is catalysed by amino acids from the egg and flour. This is what makes a cake have that lovely brownish sheen.
Baking powder (sodium bicarbonate with [usually] tartaric acid) releases carbon dioxide when mixed into liquid and heated up, making the bubbles that cause the cake to rise.
Gluten in the flour is activated by working the batter, and provides the polymeric structure that makes a cake doughy, with fat making it crumbly. http://nzic.org.nz/ChemProcesses/food/6D.pdf