Which is the first enzyme to mix with food in the digestive tract?

$(a)$ Why does a piece of bread taste sweet when chewed for some time?
$(b)$ Cellulose acts as roughage in humans but serves as a source of nutrients in cows. Justify the statement.

Why does absorption of digested food occur mainly in the small intestine?

In each of the following situations,what happens to the rate of photosynthesis?
$(a)$ Cloudy days
$(b)$ No rainfall in the area
$(c)$ Good manuring in the area
$(d)$ Stomata get blocked due to dust

Explain the three pathways of breakdown of glucose in living organisms.

What is the main toxic waste that the kidneys filter from the blood?