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**1. Digestive System Overview:**
Digestion is the breakdown of large food compounds for absorption.
– Mechanical digestion involves physical breakdown through mastication and segmentation contractions.
– Chemical digestion uses enzymes to break down food into small compounds.
– Gastric juice in the stomach initiates protein digestion using hydrochloric acid and pepsin.
– 95% of nutrient absorption occurs in the small intestine, with water and minerals reabsorbed in the colon.
– Ingestion, mechanical and chemical breakdown, absorption, and egestion are the key stages in vertebrate digestion.

**2. Secretion Systems in Bacteria:**
– Bacteria use channel transport systems to transport various chemical species.
– Molecular syringe systems allow bacteria to inject nutrients into host cells.
– Conjugation machinery in some bacteria can transport both DNA and proteins.
– Examples include the VirB complex in Agrobacterium tumefaciens and inter-kingdom conjugation in Rhizobia.
– Release of outer membrane vesicles containing virulence factors is a part of bacterial secretion systems.

**3. External vs. Internal Digestion:**
– External digestion is used by most fungi, while animals have a gastrointestinal tract for internal digestion.
– Some organisms secrete biotoxins and digestive chemicals before ingestion.
– Internal digestion can occur in vesicles, sac-like structures, or specialized organs.
– Animals have evolved specialized organs like beaks, tongues, teeth, crops, and gizzards for efficient digestion.
– Specialized behaviors like regurgitation, stomach eversion, and coprophagia aid in digestion in various animals.

**4. Nutrient Breakdown and Absorption:**
Protein digestion involves enzymes like pepsin, trypsin, and chymotrypsin breaking down proteins into amino acids.
Fat digestion primarily occurs in the small intestine with the help of pancreatic lipase and bile emulsification.
Carbohydrate digestion involves enzymes like amylase, lactase, and sucrase breaking down starches into glucose, galactose, and fructose.
– DNA and RNA are broken down into mononucleotides by DNase and RNase for assimilation.
– Digestive hormones like gastrin, secretin, and cholecystokinin regulate enzyme secretion and bile release.

**5. pH Regulation and Specialized Digestive Processes:**
– pH plays a crucial role in the digestive tract’s functioning.
– Specialized processes like non-destructive digestion of Vitamin B using haptocorrin and bile secretion from the liver are essential.
– Digestive hormones like gastrin, secretin, and motilin play critical roles in regulating gastrointestinal functions.
– Specialized structures like the Gastrovascular cavity in Venus Flytrap and phagosomes in Entamoeba histolytica demonstrate unique digestive adaptations.
– Earthworms exhibit a specific digestive system with mouth, pharynx, esophagus, crop, gizzard, and intestine for efficient digestion.

Digestion (Wikipedia)

Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism that is often divided into two processes based on how food is broken down: mechanical and chemical 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 the mouth through mastication and in the small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small compounds that the body can use.

Digestive system
Latinsystema digestorium
Anatomical terminology

In the human digestive system, food enters 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. The saliva also contains mucus, which lubricates the food; the electrolyte hydrogencarbonate (HCO3), which provides the ideal conditions of pH for amylase to work; and other electrolytes (Na+, K+, Cl). About 30% of starch is hydrolyzed into disaccharide in the oral cavity (mouth). After undergoing mastication and starch 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. They provide a slimy layer that acts as a shield against the damaging effects of chemicals like concentrated hydrochloric acid while also aiding lubrication. Hydrochloric acid provides acidic pH for pepsin. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is 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 the small intestine. Studies suggest that increasing the number of chews per bite increases relevant gut hormones and may decrease self-reported hunger and food intake.

When the pyloric sphincter valve opens, partially digested food (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 ~ 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. Absorption of water, simple sugar and alcohol also takes place in stomach. Waste material (feces) is eliminated from the rectum during defecation.

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