Explain both the physical and chemical breakdown of food in any one (1) of the four main stages of the digestive system

Explanation:

Table 1. Functions of the Digestive Organs

OrganMajor functionsOther functions

Mouth  

Ingests food

Chews and mixes food

Begins chemical breakdown of carbohydrates

Moves food into the pharynx

Begins breakdown of lipids via lingual lipase

Moistens and dissolves food, allowing you to taste it

Cleans and lubricates the teeth and oral cavity

Has some antimicrobial activity

Pharynx  

Propels food from the oral cavity to the esophagus

Lubricates food and passageways

Esophagus  

Propels food to the stomach

Lubricates food and passageways

Stomach  

Mixes and churns food with gastric juices to form chyme

Begins chemical breakdown of proteins

Releases food into the duodenum as chyme

Absorbs some fat-soluble substances (for example, alcohol, aspirin)

Possesses antimicrobial functions

Stimulates protein-digesting enzymes

Secretes intrinsic factor required for vitamin B12 absorption in small intestine

Small intestine  

Mixes chyme with digestive juices

Propels food at a rate slow enough for digestion and absorption

Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water

Performs physical digestion via segmentation

Provides optimal medium for enzymatic activity

Accessory organs  

Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption

Gallbladder: stores, concentrates, and releases bile

Pancreas: produces digestive enzymes and bicarbonate

Bicarbonate-rich pancreatic juices help neutralize acidic chyme and provide optimal environment for enzymatic activity

Large intestine  

Further breaks down food residues

Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria

Propels feces toward rectum

Eliminates feces

Food residue is concentrated and temporarily stored prior to defecation

Mucus eases passage of feces through colon

Digestive Processes

The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.

The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.

This image shows the peristaltic movement of food. In the left image, the food bolus is towards the top of the esophagus and arrows pointing downward show the direction of movement of the peristaltic wave. In the center image, the food bolus and the wave movement are closer to the center of the esophagus and in the right image, the bolus and the wave are close to the bottom end of the esophagus.

Figure 1. Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.

Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (Figure 1). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.

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