Digestion

Introduction to Digestion

Human cell and body as a whole requires energy and various nutrients to make structures and to perform various physiological functions. The alimentary tract provides the body with a continual supply of water, electrolytes, vitamins, and nutrients. This material and energy is provided by food which we take as diet. However the structure of various molecules is not compatible with our own requirements so there is need to change various molecules which can be easily assimilated into or body. This process is called digestion.

Functions of digestive system

Each part of human digestive system is adapted to its specific functions, some to simple passage of food, such as the esophagus, others to temporary storage of food, such as the stomach, and others to digestion and absorption, such as the small intestine.

The digestive system performs following general functions

·        Ingestion-Ingestion of food into the body by mouth through eating and drinking

·        Movements- Movement of food through the alimentary tract by producing movements in alimentary canal

·        Secretion- Secretion of digestive juices for digestion of the food

·        Digestion- to change the structure of ingested food to make it fit for absorption

·        Absorption- Absorption of water, various electrolytes, vitamins, and digestive products into blood

·        Assimilation- Utilization of absorbed molecules

·     Blood circulation- Circulation of blood through the gastrointestinal organs to carry away the absorbed substances

·        Egestion- Removal of waste products of digestion from the body

·        Control- Control of all these functions by local, nervous, and hormonal systems

Definition

      Digestion is the breakdown of large insoluble molecules of food  into small water soluble molecules with the help of specific digestive enzymes to make them fit for absorption into the blood plasma to be assimilated in and through the liver.

Types of digestion

·        Extracellular- These are digestive processes that occur in lumen of alimentary canal

·        Intracellular- These are digestive processes that occur inside cells of body

Extracellular digestion processes

Digestion is a form of catabolism that is often divided into two processes based on how food is broken down-

1. Mechanical Digestion

The term mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. Mechanical digestion takes place in mouth through mastication and in small intestine through segmentation contractions.

2. Chemical Digestion

In chemical digestion, enzymes break down large molecules of food into the smaller molecules so that the body can absorb, assimilate and use. This is actual process of digestion.

Summary of digestion

The human gastrointestinal tract is around 9 meters long. The digestion physiology varies between individuals and dependent upon other factors such as the characteristics of the food and size of the meal, and the process of digestion which normally takes between 24 and 72 hours.

In the human digestive system, food enters through the mouth and mechanical digestion of the food starts by the action of mastication (chewing), a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food and salivary lipase which digests fats in mouth and in stomach.

The saliva also contains mucus, which lubricates the food, and hydrogen carbonate, which provides the ideal conditions of pH (alkaline) for amylase to work, and electrolytes (Na⁺, K⁺, Cl^-, HCO^-₃). About 30% of starch is hydrolyzed into disaccharide in oral cavity (mouth). After undergoing mastication and starch and some fat digestion, the food will be in the form of a small, round slurry mass called a bolus.

It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. In infants and toddlers gastric juice also contains rennin to digest milk proteins.

As the first two chemicals may damage the stomach wall, mucus  and bicarbonates are secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals like concentrated hydrochloric acid and mucus also helps in lubrication. Hydrochloric acid also provides acidic pH for pepsin action.

At the same time when digestion is occurring in stomach, mechanical mixing occurs by peristalsis in the form of waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.

Pepsin breaks down proteins into peptides or proteoses, which is further broken down into dipeptides and amino acids by enzymes in small intestine. Absorption of water, simple sugar and alcohol also takes place in stomach.

After 1–2 hours in humans, the resulting thick liquid is converted into chyme. When the pyloric sphincter valve opens, chyme enters the  duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and then passes through the small intestine, in which digestion continues.

When the chyme is fully digested, it is absorbed into the blood. 95% of nutrient absorption occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 to 6.9.

Some vitamins, such as biotin  and vitamin K  (K₂MK7) produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from the rectum  during defecation.

Non Destructive Digestion

Some nutrients are complex molecules for example vitamin B₁₂ which would be destroyed if they were broken down into their functional groups. To digest vitamin B₁₂ non-destructively, haptocorrin, also known as transcobalamin-1 or cobalophilin in saliva strongly binds and protects the B₁₂ molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.

After the B₁₂-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B₁₂ molecules which rebind to intrinsic factor (IF). These B₁₂-IF complexes travel to the ileum portion of the small intestine where cubilin receptors enable assimilation and circulation of B₁₂-IF complexes in the blood.

Phases of digestion

There are some distinct and different phases of digestion as follows-

1. Cephalic phase- The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal cell (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.

2. Gastric phase- The gastric phase takes 3 to 4 hours. It is stimulated by distension of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the pH of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin which in turn stimulates parietal cells to secrete gastric acid. This acid release can also be triggered by   acetylcholine and histamine.

3. Intestinal phase- The intestinal phase has two parts-

·        Excitatory part

·        Inhibitory part

Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

Intracellular digestion- It is the breakdown of substances within the cytoplasm of a cell. Following phagocytosis, the ingested particle, now called phagosome fuses with a lysosome containing hydrolytic enzymes to form a phagolysosome; the pathogens or food particles within the phagosome are then digested by the lysosome's enzymes.

Intracellular digestion is divided into two types-

·        Heterophagic digestion  

·        Autophagic digestion 

Both types take place in the lysosome.

Heterophagic intracellular digestion is to break down all molecules that are brought into a cell by endocytosis or phagocytosis. The degraded molecules are delivered to the cytoplasm after the molecules are hydrolyzed in the lysosome. Then these are assimilated in the cell.

Autophagic intracellular digestion (autophagy) meaning self devouring is processed in the cell, which means it digests the internal molecules. Autophagy includes three processes which are- 

·        Macroautophagy 

·        Microautophagy 

·        Chaperone-Mediated Autophagy

Macro autophagy- 

It is the main pathway, used primarily to eradicate damaged cell organelles or unused proteins. At the start the phagophore engulfs the material that needs to be degraded, which forms a double membrane known as an autophagosome, around the organelle marked for destruction. The autophagosome then travels through the cytoplasm of the cell to a lysosome in mammals, or vacuoles in yeast and plants, and the two organelles fuse. Within the lysosome/vacuole, the contents of the autophagosome are degraded via acidic lysosomal hydrolase.

Micro autophagy-

It is the direct engulfment of cytoplasmic material into the lysosome. This occurs by invagination, meaning the inward folding of the lysosomal membrane, or cellular protrusion.

Chaperone-mediated autophagy (CMA)

It is a very complex and specific pathway, which involves the recognition by the hsc70-containing complex. This means that a protein must contain the recognition site for this complex which will allow it to bind to this chaperone, forming the CMA- substrate/chaperone complex. This complex then moves to the lysosomal membrane-bound protein that will recognize and bind with the CMA receptor. Upon recognition, the substrate protein gets unfolded and it is translocated across the lysosome membrane with the assistance of the lysosomal hsc70 chaperone. CMA is significantly different from other types of autophagy because it translocates protein material in a one by one manner, and it is extremely selective about the material that crosses the lysosomal barrier