Answer: for humans, food supplies the body with necessary molecules that make the body function. in the case of energy, humans get their glucose from consuming photosynthetic organisms such as plants. the body then breaks down the glucose in a process calledcellular respiration. This process produces ATP, which is the molecule that animal cells use for energy.
Answer: when they say fat yields this much energy or carbs give that much energy they are talking about calories. and more specifically they are talking about Kilocalories. 1 Kilocalorie is the amount of heat it takes to increase the temperature of 1 kilogram of water 1 degree (i believe fahrenheit) at one atmosphere (basically means sea level ish). heat is the energy. carbohydrates yield 4 calories/gram proteins yield 4 calories/gram fats yield 11 calories/gram alcohol is 9 calories/gram (just threw that one in there) ^^