The digestive system consists of the mouth, esophagus, stomach, small intestine and large intestine. Food passes through these organs where muscles squeeze and break up food while digestive juices containing enzymes break down molecules. Cellulose, a type of dietary fiber, is not broken down as humans lack the enzyme to digest it. Hormones and nerves control the secretion of digestive juices from exocrine glands like the stomach and pancreas in response to food. Digestive enzymes remain attached to cells lining the small intestine so products of digestion can be immediately absorbed. Pepsinogen and trypsinogen are secreted as inactive enzymes and activated once in the stomach and small intestine, respectively.

1. Digestion Summary:
Food digested as passes along ________________________.
-> consists of the mouth, pharynx, esophagus, stomach, small intestine, and large
intestine.
___________________and _____________ muscle fibers in its wall contract and relax,
squeezing food and breaking up large solid lumps. Digestive juices (secreted by
________________________, _______________________________and ____________
=_______________________) with enzymes mix with food to break down the molecules
(proteins, nucleic acids, starch etc.) into smaller soluble forms.
Indigestible macromolecules: e.g. _____________(part of dietary fiber). Humans lack
________________(enzyme which digests cellulose). Cellulose moves food through the
digestive tract quickly and efficiently, though.
Exocrine glands
- secrete their products into ducts which lead directly to external environment
- digestive juice released from cells by exocytosis => discharged from gland by traveling
along ducts

2. Exocrine glands and their cells:
Digestive juice secretion:
- controlled by ___________ and _______________
- before food reaches stomach, sight or smell of food stimulate brain to send nerve
impulses to _____________________in stomach wall to produce _________________.
- More gastric juice secreted when food enters stomach and comes into contact with
__________________________ and ______________________ in the stomach lining
and ________________________in the stomach wall.
- Receptors send nerve impulses to brain -> sends impulses to ____________ gland cells
- Receptors send nerve impulses to ___________ gland cells in stomach lining -> secrete
hormone: ___________ -> carried to exocrine gland cells to stimulate
_______________________________ => causes ph of food in stomach to fall to about
pH 3.0

3. Membrane-bound digestive enzymes:
- some enzymes, produced by the wall of the small intestine, don’t mix with the food but
remain in the ________________________ of cells on the surface of the villi
(epithelium cells)
- the ___________________ of the enzymes are exposed to the food in the small intestine
-> digest substrates and the products of digestion can be immediately absorbed.
- After abrasion of tips of villi, the enzymes mix with food in small intestine and continue
work
Sources of digestive enzymes:
- pepsin & trypsin harmful to exocrine gland cells which secrete them.
-> secreted as ______________________________ (a substance from which another
substance is formed) called pepsinogen and trypsinogen.
Pepsinogen: first secreted by cells in ____________________ and then activated by
_______________________(also secreted by cells in stomach wall), which converts it
into pepsin
Trypsinogen: first secreted by _____________ and enters ________________________.
Then activated by enzyme called __________________ (secreted by lining of small
intestine)