P4 Science (Digestive Systems): The human digestive system is responsible for the movement and breaking down the food that we eat into nutrient parts small enough to be absorbed and used by the body.
The process starts from the mouth, where food consumed is broken down into smaller pieces by chewing. Saliva is also produced, moistening the food pieces thus facilitating a smooth passage down the esophagus into the stomach, as well as breaking down starches in the food for absorption later on down the tract.
At the stomach, digestive juices and enzymes are secreted and mixed with the food boluses to further break them down into a soupy liquid paste called chyme, which is eventually moved into the small intestines where further digestion and absorption of nutrients take place.
Here is a simulation video providing a zoom-in into the on-goings in the stomach upon the arrival of boluses from the esophagus. Watch it here.
S3/4 Food and Nutrition (Topic 1: Nutrition and Health) / Biology (Nutrition in Humans)
When food enters the mouth, the amylase inside of saliva helps to break down the carbohydrates (primarily polysaccharides like starch) into simple sugars like galactose and fructose. Saliva is 99.5% water, and the rest are mucus, proteins & enzymes, electrolytes and lingual lipase (to break down fat molecules into glycerides / cholesterol) to facilitate processes to make the food slippery and easy to swallow, anti-bacterial etc.
Food that enters the stomach are greeted by gastric enzymes (pepsin, rennin etc.) the pepsinogen from saliva becomes activated in the acidic environment, becoming pepsin. These enzymes help to break down proteins into amino acids which would then be absorbed by the small intestine later. (There's also gastric lipase to help break down fat molecules further)
The pancreas releases pancreatic juices (primarily alkaline to neutralise the acidic environment in the stomach) which contains useful enzymes like elastase, trypsinogen, carboxypeptidase, proelastase etc. These help to break down resilient proteins into the simplest amino acids for the small intestine can absorb easily.
The liver produces bile (water, bile salts, bilirubin, inorganic salts etc.) which emulsifies the fat droplets (think of egg white making oil and water mix together evenly) so they can be broken down by lipase much faster than before (larger surface area = more parts of fat exposed to lipase) Food is allowed to digest further through the duodenum.
The food is finally absorbed by the small intestine (at the ileum), which assimilates the simple sugars, cholesterol and glycerides, amino acids, vitamins and minerals into the bloodstream. Water and most of the leftover minerals are absorbed in the large intestine before the undigested portions (faeces) are expelled from the body through the anus.