How can I write an essay on the process of digestion?

My recommendation for this particular essay is that you essentially write about a journey through the digestive system. You can be analytical about this—or if your teacher permits it, you could have some fun with it and write about the journey of "Sammy the Cheese Sandwich" as he travels through the digestive tract all while having the absorption of nutrients happen around him. By setting up your essay in this format, you can hit and highlight just about every term and question that the prompt requires.
Start with the mouth. I know that this is probably obvious, but you can knock off some terms right away. The mouth is going to allow you discuss what mechanical digestion is, and it will allow you discuss chemical digestion. The mouth is loaded with salivary glands that secrete amylase. You can discuss that enzymes are proteins that catalyze and speed up chemical reactions. Because you introduced your reader to the word "protein," this will allow your essay to teach that proteins are long strings of amino acids. As you discuss the enzyme amylase, be sure to mention that amylase breaks down sugars. This is where you can discuss the different types of sugars and how amylase helps break down polysaccharides into disaccharides and disaccharides into monosaccharides.
After the mouth comes the esophagus, which leads to the stomach. Don't just move food quickly through the esophagus, because you can mention how the epiglottis ensures that food doesn't go down the trachea. The esophagus is also a good place to discuss the wave contractions of the digestive system known as peristalsis. Finally, at the base of the esophagus is the cardiac sphincter which prevents stomach contents from coming back up the esophagus.
Once in the stomach, the essay can highlight the chemical bath that hits food there. Specifically mention pepsin and HCl. After several hours in the stomach, the food (now known as chyme) passes through the pyloric sphincter into the small intestine. The first section is the duodenum, and many more enzymes will attack the food here that have been produced in the pancreas. Be sure to discuss bile at this point as well as where it is produced (the liver), stored (the gallbladder), and what it helps digest. The two remaining sections of the small intestine are the jejunum with its villi for nutrient absorption and the ileum. You could explain how the villi work, but at the very least, mention how they increase surface area as a result of their shape/structure.
Next, move the essay into discussing the sections of the large intestine: ascending colon, transverse colon, and descending colon. Emphasize to your reader that this is where a lot of water reabsorption happens, and you can even discuss what happens if this fails.
There are few terms from the list that I haven't incorporated into this guide, but before moving the remaining materials into the rectum and out of the anus, be sure to discuss those terms.
https://www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works