The Digestive System
2. The digestion and metabolism of food influence the absorption and use of nutrients
2.1 How are food digested and nutrients absorbed in the human body?
- Explore the digestion of food and how this process enables nutrients to be absorbed
Mouth: food is taken into the mouth (ingested). Teeth physically breakdown the food, while
enzymes in your saliva begin to chemically breakdown food into smaller molecules.
Salivary Glands secrete saliva into the mouth, softening food an moistening the mouth for
swallowing. Saliva contains the enzyme, amylase, which begins to break long chains of starch
(polysaccharides) into maltose (disaccharide).
Amylase:
Starch into maltose
Pharynx: connects the mouth to the oesophagus.
Oesophagus: When you swallow the food is pushed down due to muscles in the oesophagus.
The contraction of muscles is called peristalsis.
Stomach: The stomach wall is thick and muscular. The muscles physically move food around and
break it down into smaller pieces. It is covered in small openings called gastric glands. These
release gastric juices, which contain a mixture of hydrochloric acid and enzymes. This acid helps
to kill any germs and lowers the pH, creating the optimum conditions for the enzymes to work.
The enzyme found in the stomach is called pepsin, which helps to break down protein. The
semi-digested food (chyme) moves to the duodenum.
Liver: Glucose, which is absorbed by the small intestine passes into the liver. Any excess glucose
is converted into glycogen (polysaccharide) and stored in the liver (or muscles). The glycogen
can be turned back into glucose when the body needs it.
Also produces bile which helps to emulsify fats (aiding lipase)
Gall Bladder: stores and releases bile into the duodenum
Gastric Juices release –
 Hydrochloric Acid
(lowers pH and kills germs)
 Pepsin:
Protein into polypeptides
Storage of glucose as glycogen
Makes bile
Stores and releases bile
Pancreas: Secretes insulin (hormone) when blood sugar is too high, causing it to be stored as
glycogen in the liver (or glucagon which breaks glycogen down into glucose).
Produces pancreatic juice containing enzymes which are secreted into the duodenum to digest
protein, starch and fat. The pancreatic secretions also contain sodium bicarbonate, an alkali,
which neutralises the acidic chyme.
Duodenum: links the stomach to the small intestine.
Small Intestine: Is made up of two parts; jejunum and ileum. Despite its name it is
actually quite a long section of the digestive tract, (6m in length).
Small intestine secretes digestive enzymes.
The structure of the wall of the small intestine is very muscular and contains
thousands of finger-like projections called villi (singular: villus), creating greater
surface area for the absorption of nutrients. Within the villi there are numerous
blood capillaries for taking up the absorbed food.
Large Intestine (colon): Some water and any material which is not absorbed (waste) in the small
intestine travels through into the large intestine. As material passes along the colon, water and
salts are absorbed. The solid mass (faeces) is then stored in the rectum. The faeces are then
excreted (egested) through the anus by contractions of the rectum wall, and the opening of the
anal sphincter. This called defecation
Appendix: Connected to the large intestine the appendix has no function in humans.
In grass-eating mammals such as rabbits their appendix contains large numbers of bacteria,
which can digest cellulose and break it down into glucose.
Watch: https://clickv.ie/w/Xa4h
Receives pancreatic juices:
 Amylase:
starch into maltose
 Trypsin:
protein into polypeptides
 Lipase:
lipids into fatty acids & glycerol
Intestinal enzymes:
 Maltase:
maltose into glucose
 Sucrase:
sucrose into glucose & fructose
 Peptidase:
polypeptide into amino acids
 Lactase:
lactose into glucose & galactase
Bacteria completely digest soluble
fibre and some insoluble fibre.
Bacteria produce beneficial
substances.
Dead bacteria add bulk to faeces =
soft faeces.
The breakdown of macronutrients;
location
Carbohydrates
𝑎𝑚𝑦𝑙𝑎𝑠𝑒
mouth
𝑠𝑡𝑎𝑟𝑐ℎ →
Protein
Lipids
𝑚𝑎𝑙𝑡𝑜𝑠𝑒
𝑝𝑒𝑝𝑠𝑖𝑛
Stomach
𝑝𝑟𝑜𝑡𝑒𝑖𝑛 →
𝑝𝑜𝑙𝑦𝑝𝑒𝑝𝑡𝑖𝑑𝑒𝑠
bile breaks down fats into smaller droplets
𝑎𝑚𝑦𝑙𝑎𝑠𝑒
Duodenum
𝑠𝑡𝑎𝑟𝑐ℎ →
𝑚𝑎𝑙𝑡𝑜𝑠𝑒
𝑡𝑟𝑦𝑝𝑠𝑖𝑛
𝑝𝑟𝑜𝑡𝑒𝑖𝑛 →
𝑝𝑜𝑙𝑦𝑝𝑒𝑝𝑡𝑖𝑑𝑒𝑠
𝑙𝑖𝑝𝑎𝑠𝑒
𝑙𝑖𝑝𝑖𝑑𝑠 →
𝑚𝑎𝑙𝑡𝑎𝑠𝑒
𝑚𝑎𝑙𝑡𝑜𝑠𝑒 →
Small intestine
𝑠𝑢𝑐𝑟𝑎𝑠𝑒
𝑠𝑢𝑐𝑟𝑜𝑠𝑒 →
𝑙𝑎𝑐𝑡𝑎𝑠𝑒
𝑙𝑎𝑐𝑡𝑜𝑠𝑒 →
Carbohydrates:
Proteins:
Lipids:
𝑔𝑙𝑢𝑐𝑜𝑠𝑒 + 𝑔𝑙𝑢𝑐𝑜𝑠𝑒
𝑔𝑙𝑢𝑐𝑜𝑠𝑒 + 𝑓𝑟𝑢𝑐𝑡𝑜𝑠𝑒
𝑔𝑙𝑢𝑐𝑜𝑠𝑒 + 𝑔𝑎𝑙𝑎𝑐𝑡𝑜𝑠𝑒
𝑝𝑒𝑝𝑡𝑖𝑑𝑎𝑠𝑒
𝑝𝑜𝑙𝑦𝑝𝑒𝑝𝑡𝑖𝑑𝑒𝑠 →
𝑎𝑚𝑖𝑛𝑜 𝑎𝑐𝑖𝑑𝑠
𝑓𝑎𝑡𝑡𝑦 𝑎𝑐𝑖𝑑𝑠 + 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙
ClickView: Body Systems (Ch: The Digestive System)