• Fuel (Chemical Energy): for all cellular work in body
• Organic raw materials animals use in biosynthesis(carbon skeletons to make their own molecules)
• essential nutrients/ substances such as vitamins that the animal cannot make for itself from any raw material.

• Many species of aquatic animals: sift small food particles from the water.
• Cilia sweep the food particles in a film of mucus to the mouth.

• animals that live i or on their food source, eating their way through the food.
• some borrow into animal carcasses

• suck nutrient rich fluid from a living host (mosquito)
• some benefit hosts (hummingbirds and bees move pollen)

• eat relatively large pieces of food.

• When fewer calories are taken in that are expended
• fuel is taken out of storage depots and oxidized
• the human body expends liver glycogen first and then draws on muscle glycogen and fat.

• if the diet of a human or other animal is chronically deficient in calories.
• the stores of glycogen and fat are used up, the body begins breaking down its own proteins for fuel, muscles begin to decrease in size, and the brain can become protein deficient..
• common where drought, war, or other crisis.
• anorexia nervosa (starve themselves to lose weight)

• (obesity) body hoards fat, it tends to store any excess fat molecules obtained from food instead of using them for fuel.

1. when blood glucose level rises, the pancreas secretes insulin into the blood.
2. insulin enhances transport of glucose into body cells and stimulates the liver and muscle cells to store glucose as glycogen. blood glucose level drops.
3. When blood glucose level drops, the pancreas secretes the hormone glucagon, which opposes th effect of insulin.
4. Glucagon promotes the breakdown of glycogen in the liver and the release of glucose into the blood, increasing blood glucose level.

• Leptin
• PYY
• Ghrelin
• Insulin

• Produced by adipose (fat) tissue.
• suppresses appetite as its level increases.
• body fat decreases, leptin levels fall,appetite decreases

• secreted by the small intestine after meals.
• acts as an appetite suppressant tat counters the appetite stimulant ghrelin.

Ghrelin:

• secreted b the stomach wall
• one of the signals that triggers feeling of hunger as mealtimes approach.
• hard to stay on a diet because ghrelin levels increase.

• suppresses appetite by acting on the brain
• rise in blood sugar level after a meal stimulates the pancreas to secrete insulin.

• Materials that must be obtained in preassembled form because the animal's cells cannot make them from any raw material.