Heterotrophic nutrition in humans is the process by which humans obtain their organic molecules and energy by consuming other living or dead organisms. It involves several steps, from the ingestion of food to the absorption of nutrients and the elimination of waste. Here’s a more detailed overview of heterotrophic nutrition in humans:

Diagram of the human digestive system

Ingestion: Heterotrophic nutrition begins with the ingestion of food. Food is taken into the body through the mouth.

Digestion:

Mechanical Digestion:	Chemical Digestion:
In the mouth, food is broken down into smaller pieces through the action of teeth and the mixing of food with saliva. Chewing and the formation of a moist bolus make it easier for food to be swallowed.	As the bolus of food moves into the stomach, it encounters gastric juices that contain hydrochloric acid and the enzyme pepsin. These substances work together to break down proteins into smaller peptides.

Food Processing in the Stomach:

Churning and Mixing	Killing Bacteria
The stomach walls contract to churn and mix the food with gastric juices, forming a semi-liquid mixture called chyme	The acidic environment in the stomach helps kill harmful microorganisms that might be present in the ingested food.

Food Processing in the Small Intestine:

• Bile and Pancreatic Enzymes: The chyme is released into the small intestine. Bile from the liver and pancreatic juices containing enzymes enter the small intestine to further digest the chyme.
• Nutrient Absorption: Nutrients such as amino acids, simple sugars, fatty acids, and vitamins are absorbed through the walls of the small intestine and transported into the bloodstream.
• Villi and Microvilli: The small intestine has finger-like projections called villi, which are covered in even smaller projections called microvilli. These increase the surface area for absorption, making nutrient uptake more efficient.

Passage to the Large Intestine: Any undigested food and unabsorbed materials move from the small intestine to the large intestine.

Absorption of Water and Electrolytes: In the large intestine, most of the remaining water, electrolytes, and vitamins produced by gut bacteria are absorbed.

Formation of Feces: The indigestible material forms feces, which is stored in the rectum until it is ready to be eliminated.

Elimination: Fecal matter is expelled from the body through the rectum and anus during the process of defecation.

Role of Enzymes in Digestion

Enzyme	Source	Site of Action	Substrate	Product(s)	Role in Digestion
Amylase	Salivary glands (salivary amylase)	Mouth	Starch and glycogen	Maltose (disaccharide)	Initiates digestion of carbohydrates by breaking them into maltose, a simpler sugar.
Amylase	Pancreas (pancreatic amylase)	Small intestine	Starch and glycogen	Maltose (disaccharide)	Initiates digestion of carbohydrates by breaking them into maltose, a simpler sugar.
Pepsin	Stomach (chief	Stomach (gastric glands)	Proteins	Peptides	Initiates protein digestion by breaking down proteins into smaller peptide fragments.
Trypsin & Chymotrypsin	Pancreas (pancreatic Juice)	Small intestine	Proteins	Peptides	Continues protein digestion by further breaking down peptides into smaller fragments.
Lipase	Pancreas (pancreatic Lipase	Small intestine	Dietary fats (triglycerides)	Fatty acids and glycerol	Breaks down dietary fats into fatty acids and glycerol for absorption in the small intestine.
Maltase, Sucrase, Lactase	Small intestine (intestinal epithelium)	Small intestine (brush border enzymes)	Maltose	Glucose	Hydrolyzes maltose into glucose for absorption.
Nuclease	Pancreas (pancreatic Juice)	Small intestine	Nucleic acids (DNA and RNA)	Nucleotides	Breaks down nucleic acids into nucleotides for absorption in the small intestine.
Peptidase	Small intestine	Small intestine	Peptides	Amino acids	Further breaks down peptides into individual amino acids, which are absorbed into the bloodstream.

Role of Bile Juice In Digestion

Bile juice plays a vital role in digestion, primarily in the digestion and absorption of fats. It is produced by the liver and stored in the gallbladder before being released into the small intestine. Bile juice has several important functions in the digestive process:

1. Emulsification of Fats: One of the primary functions of bile is to emulsify fats. Dietary fats are typically in the form of large, hydrophobic (water-repellent) globules that are difficult for the water-based digestive enzymes to act upon. Bile contains bile salts, which break down the large fat globules into smaller droplets, increasing the surface area for enzymatic action. This process is known as emulsification.
2. Aiding Enzymatic Action: By emulsifying fats, bile facilitates the action of lipase, an enzyme responsible for breaking down fats into fatty acids and glycerol. Lipase can more effectively work on the smaller fat droplets produced by bile, increasing the efficiency of fat digestion.
3. Neutralizing Stomach Acid: Bile also helps neutralize the acidic chyme (partially digested food) that enters the small intestine from the stomach. The alkaline nature of bile helps to create a more favorable pH environment for the action of pancreatic enzymes, such as pancreatic amylase and trypsin, which work optimally at a slightly alkaline pH.
4. Aiding Fat Absorption: Bile’s emulsifying action enhances the absorption of fat-soluble vitamins (A, D, E, and K) and other fat-soluble nutrients in the small intestine. These nutrients are absorbed in association with fat molecules.
5. Facilitating Fat Transport: After fat digestion and absorption, bile also plays a role in the transport of the resulting fatty acids and glycerol. It forms micelles, small clusters of fat molecules surrounded by bile salts, which transport fat-soluble nutrients to the absorptive surfaces of the small intestine for uptake by the intestinal cells.
6. Storage in the Gallbladder: Bile is produced continuously by the liver but is stored and concentrated in the gallbladder between meals. When a fatty meal is consumed, the gallbladder contracts and releases bile into the small intestine to aid in fat digestion.