DIGESTION
All organisms (regardless of size and complexity) must have a way of obtaining essential nutrients. They are the basic, raw materials needed for organisms to make their own structures, perform life functions, and obtain energy for survival.
For example, proteins and their building blocks, amino acids, are needed for building structures like muscles and enzymes.
Carbohydrates (sugars and starches) are needed for energy.
Lipids (fats) are needed for insulation and cell membranes.
Fibre, water, and minerals are needed for regulating body processes.
In heterotrophic organisms, the digestive system is used to break down food into its basic nutrients, which are absorbed by the circulatory system and transported around the body.
INTRACELLULAR vs. EXTRACELLULAR DIGESTION
Some organisms digest food right inside the cell. The cell engulfs the food and it is broken down into its nutrients right in the cell. Intracellular digestion is performed by single­celled organisms (paramecium, amoeba).
In most other animals, nutrients are absorbed into the bloodstream after they have been broken down in a different place. Waste is excreted at the other end of a digestive tract. This is known as extracellular digestion.
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THE STAGES OF PROCESSING FOOD
All organisms need to process food in the same manner. This include 4 major steps:
taking in food
breaking down food into nutrients
taking nutrients into cells
removing and expelling waste
MECHANICAL DIGESTION
The breakdown of food happens in two distinct processes. Mechanical digestion is the physical breakdown of large food pieces into smaller food pieces. This is achieved through chewing, mashing, and chopping food. This process increases the surface area of food particles to allow for more enzymes to come in contact with them for further breakdown.
THE DIGESTIVE TRACT
It takes approximately 24­33 hours for food to completely pass through your digestive system. It is a complex process with many structures associated with it.
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THE DIGESTIVE TRACT
The human digestive tract begins in the mouth (oral cavity). Food is physically (mechanically) broken down into smaller pieces by teeth. This process is called mastication.
One of the strongest muscles in your body, your tongue, has regions to detect things that are bitter, sweet, salty, and sour.
It pushes food around into a ball known as a bolus, which passes into the pharynx. Food is also coated with saliva in the mouth (but that's tomorrow).
As we swallow, the epiglottis folds over the trachea, preventing us from choking. This ensures that the bolus enters the esophagus.
The esophagus is the tube that lies directly behind the trachea and is ~ 24 cm long.
It is lined with both circular and longitudinal muscles to help push food along the digestive tract.
Food moves through our digestive system by peristalsis. The esophagus has both circular and longitudinal muscles that contract and relax in a series of waves, pushing the food mass through.
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THE DIGESTIVE TRACT CON'D
The bolus must now pass through the lower esophageal sphincter, a tight muscle that keeps stomach acid out of the esophagus.
The stomach is a big, muscular pouch that churns the bolus (mechanical). It has large folds called rugae and contains gastric juice, a mixture of stomach acid and enzymes (chemical digestion).
The stomach contains hydrochloric acid (HCl) which kills bacteria and viruses, and has developed a thick lining of mucus to protect it from the acid (pH ~ 1).
Some absorption occurs here. After the bolus is mixed with gastic juice, it is now called chyme and leaves through the pyloric sphincter.
THE INTESTINES, ETC.
The chyme passes into the small intestine, where most of the absorption will occur. It has three sections:
The duodenum (~ 25 cm) is important because it contains the common bile duct, where the pancreas and liver secrete enzymes to aid in digestion.
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THE INTESTINES CON'D
The jejunum (~ 2.5 m) is where the majority of absorption takes place. It has tiny finger­like projections on the inside wall, called villi (villus), which greatly increase the surface area for nutrient absorption.
Each villus itself even has its own tiny projections, called microvilli, which further increase the surface area for absorption.
The final section of the small intestine is called the ileum (~ 3 m). It has fewer villi than the jejunum, and does absorb nutrients, but it basically compacts the leftovers of the food and passes it through the caecum, which is the first area of the large intestine (also the location of the useless appendix). Appendicitis... not for the faint of heart...
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THE LARGE INTESTINE
The large intestine (colon) has the final few jobs in digestion. Now that the nutrients are all absorbed, the colon will absorb the remaining water and minerals that are undigested.
It is much larger in diameter, but smaller in length (~ 1.5 m). Leftover waste is compacted and stored at the end of the colon, known as the rectum. Once the rectum is full, the anal sphincter loosens and allows the final waste product (now called feces) to be excreted through the anus. 6