Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy. In the presence of carbon dioxide, such cells are able to convert this solar energy into energy-rich organic molecules, such as glucose.
Yes, you can add a little lemon juice or vinegar to the goat's milk to make it into a buttermilk or kefir. This will lower the pH of the milk, and allow it to release the CO2 stored in the sodium bicarbonate (baking soda), causing the bread to rise. Goat's milk generally has less fat and more vitamins.
It takes 1 gallon of milk to yield 1/2 pint of true buttermilk. Nowadays, most commercial buttermilk is made by adding a lactic acid bacteria culture to pasteurized sweet whole milk or, more commonly skim milk or non-fat milk, and it may or may not have added butter flecks.