The knowledge of the energy content of food can help diabetics make smart diet choices because diabetic patients need the foods that have high carbohydrate and foods with rich fiber content in them. If the diabetic patient is not aware about the energy contents of a food, they can make a wrong choice and ultimately fall ill. Most diabetic patients require foods that have high energy content and low sugar content. Diabetic patients need full care about the food they intake. Taking the right kind of foods rich in protein and carbohydrates can immensely help a diabetic patient control the amount of sugar in the blood.
From the description you make, it's likely that the structure with letter A is the chloroplast. It is round and small and it's free because of the pigment needed for photosynthesis (chlorophyll). It is located inside the cell, suspended in the cytoplasm.
Directly counting blood cells or tissue cells by using a hemocytometer can determine the concentration of a known volume. Counting the number of colonies that arise on a pour plate can calculate the concentration by multiplying the count by the volume spread on the pour plate.