- Introduction
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All creatures, from single cell organisms to the very complex multi-celled organisms such as animals and humans, have two processes that are basic and common to all. They all must ingest food and excrete waste. This section will focus on the process of ingesting food. The three stages that will be covered are digestion, absorption, and metabolism.
Two Types of Nutrients
The purpose of digestion is to assimilate nutrients.
As you learned earlier , major nutrients break down into two categories.
Macronutrients: These consist of carbohydrates, fats, and proteins. Macronutrients are sources of energy — they provide calories. Macronutrients break down into smaller components during the digestive process. These smaller parts are called micronutrients.
Micronutrients: These are vitamins and minerals. Micronutrients are not sources of energy. Vitamins and minerals must be provided to the body from the food we eat or, in modern times, by taking nutritional supplements. In other words, vitamins and minerals are sub-components of the macronutrients. While vitamins and minerals do not contain calories themselves, they act as important co-factors or co-enzymes to assist in release of energy from the macronutrients as well as in transportation and other unique and important metabolic functions.
Digestion
The definition of digestion is the conversion of food in the mouth, stomach and intestines into soluble and diffusible material, capable of being absorbed by the blood. So, digestion is the process of masticating food, swallowing it, and breaking it down into smaller particles in the stomach and small intestines.
Absorption in living organisms can be defined as the process by which the building block materials of growth and nutrition are absorbed and conveyed to the tissues and organs. Once the digestive process has delivered the broken down smaller particles to the small intestines, absorption can take place through the intestinal wall into the blood stream. In other words, absorption in living organisms is the process of delivering the digested nutrients, such as vitamins and minerals, to the bloodstream for delivery throughout the body.
The Digestive System
The digestive system consists of two primary components:
- The gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine, and large intestine.
- The accessory organs of digestion are the pancreas, liver, and gall bladder.
The two types of digestion are mechanical and chemical.
- Mechanical — occurs primarily in the mouth, consisting of tearing, cutting, and grinding actions.
- Chemical — starts in the mouth and continues throughout the digestive system. Enzymes are the primary substance used in chemical digestion
The Pathway of Digestion
- After food is taken into the mouth, mastication and the beginning stages of enzymatic chemical digestion occurs. It is then swallowed, entering the esophagus.
- Once the swallowed food has entered the gastrointestinal tract, it is called a bolus. The esophagus's job is to transport the bolus to the stomach. The bolus travels through the esophagus and enters the stomach, where the second stage of chemical digestion happens.
- In the stomach, hydrochloric acid is secreted by parietal cells in the stomach lining. Other digestive enzymes are secreted from the stomach lining and also dumped into the stomach. At this point, the bonds that form the proteins, carbohydrates, and fats begin to break down. This forms a semi-fluid that can now enter the small intestine.
- Next, the bolus travels in to the upper part of the small intestine called the duodenum. Here the liver secretes enzymes, the gall bladder secretes bile to break down fats, and the pancreas secretes a variety of pancreatic enzymes to further break down (primarily) proteins and fats.
- The bolus continues to travel through the small intestine. Movement through the small intestine allows nutrients to be absorbed through the small intestine wall into the blood stream.
- What is not assimilated into the bloodstream through the small intestine moves into the large intestine.
- Waste material is processed into stool, while water and usable chemicals are absorbed into the bloodstream to preserve the body's fluid balance.
- Waste material is excreted through the anus after being prepared for excretion in the rectum.
Your Experience of Eating
Even at the early stage of the first thought of food, digestive processes begin within the body. The smell and sight of food then increase these activities further. When you sit down to begin eating, the digestive processes are standing by, ready for action. When you begin eating your food — chewing and tasting, then swallowing — the process kicks into high gear.
Digestive Catalysts Make Life Possible
Enzymes are catalysts. They assist in practically all bodily functions, speeding up specific chemical reactions that would proceed very slowly, if at all. Digestive enzymes break down food particles for storage in the liver or muscles. These reactions are the key to breaking down the macronutrients into smaller particles. This stored energy is later converted, by other enzymes, for use by the body when necessary. There are two classifications of enzymes: digestive enzymes and metabolic enzymes.
For the purposes of this course we will focus on digestive enzymes. There are many known enzymes and sub enzymes.
Enzymes
The human body makes approximately 22 different digestive enzymes, capable of digesting protein, carbohydrates, sugars, and fats. We digest food in stages and specific enzymes break down different types of food. A protein-digesting enzyme cannot act on a carbohydrate and vice-versa. Enzymes are turned on and off by the environmental pH level. This is how particular enzymes are activated on at the appropriate stage during digestion.
There are specific digestive enzymes for each macronutrient.
Chew Your Food Well
Enzymes are at work in your body, even before eating. As soon as you think of food, amylase begins to be secreted by the mucous membranes in your mouth. As you smell and see the food, the secretion increases. When you take your first bite, amylase secretion is at maximum production. This is why chewing your food is so important. The longer you chew your food, the more amylase is mixed with it and and the more digested it will be before it enters the stomach. The more it is broken down, the easier the rest of the digestion process will be.
From Digestion to Absorption to Metabolism
From Digestion to Absorption: Once your food has been digested and broken down into smaller particles, it goes to the next stage: absorption. This process is dependent upon the health of the small intestinal lining. A healthy digestive tract has a semi-permeable lining that lets properly digested nutrients through and keeps bad things out. An unhealthy digestive tract becomes very porous, letting undigested food, bacteria and toxins into the blood.
From Absorption to Metabolism: Now that the nutrients have been absorbed through the intestinal lining, they can be transported by the blood to the cells throughout the body. It is within the cells that metabolism happens.
Metabolism can be defined as the process by which living tissues or cells take up and convert the nutritive material into energy. The final product in this process is a molecule called adenosine triphosphate (ATP). Cells use ATP as an energy unit to perform their specialized functions. Metabolism, then, is the final step in the three-stage process we are learning about.
The individual cells receive the nutrients —primarily vitamins, minerals, and other micronutrients not covered in this course — from the blood, and are ingested through the cell membrane and make use of them to produce ATP via cellular metabolism. From there, the energy in ATP can be used for various physiological functions including muscle contraction, neuron firing, oxygen intake, and many other specialized functions within the body. Metabolism occurs within the cells and requires this basic energy unit, ATP.
