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.
Buttermilk has a pH of 4.5, which is somewhat acidic. Goat milk has a pH of 6, which is a lot less acid. So you can substitute goat milk for the buttermilk, but you would need to add some extra acid, like a small amount of lemon juice or vinegar, to make the recipe come out right. The baking soda in the recipe reacts with the acid to make bubbles of carbon dioxide, which is what makes the bubbles inside a fluffy bread.