What is Digestion: Definition and 24 Discussions

Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery 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 mouth through mastication and in small intestine through segmentation contractions. In chemical digestion, enzymes break down food into the small molecules the body can use.
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, and hydrogen carbonate, which provides the ideal conditions of pH (alkaline) for amylase to work. 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. 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. 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. Studies suggest that increasing the number of chews per bite increases relevant gut hormones and may decrease self-reported hunger and food intake.After some time (typically 1–2 hours in humans, 4–6 hours in dogs, 3–4 hours in house cats), the resulting thick liquid is called 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 ~ 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.

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  1. fresh_42

    How invariant is digestion of a certain substance?

    In around 40% of people, ##C_4H_6O_2S_2## is enzymatically broken down into sulfur-containing compounds such as methanethiol, dimethyl sulfide, or dimethyl disulfide. Is it basically possible to belong to these 40% and later in life to the other 60%? How stable is "enzymatically" and what are...
  2. nomadreid

    Calories not used for digestion while fasting

    A friend, whose knowledge of biology comes primarily from pseudoscientific works, considers occasional fasting beneficial -- on this I will not take a stand, because as far as I know, the jury is still out on this. However, his justifications are annoying, (To paraphrase a notorious quotation...
  3. ORF

    Medical Why doesn't the intestine digest itself?

    Hello I have read this article about why doesn't the stomach digest itself https://www.scientificamerican.com/article/why-dont-our-digestive-ac/ but also during the digestion along the small intestine, there are enzymes. How does the small intestine defend against these enzymes? ( I suppose...
  4. G

    Medical Energy & Digestion: How Age Impacts Our Ability to Get Energy

    Hi Friends! Our body makes glucose from food we eat in around 10 hours time .If we eat glucose we get instant energy. In young age and old age by eating same amount of food the young has more energy because he digests food almost whole and an old person gets less energy because he cannot digest...
  5. harini07

    What enzyme is responsible for the initial step in milk digestion in humans?

    Homework Statement The intial step in the digestion of milk in Humans is carried out by which of the following enzymes? A) lipase B) trypsin C) rennin (No, not renin) D) pepsin Homework Equations Casein is converted to Paracasein by rennin and then paracasein along with calcium is acted upon...
  6. mktsgm

    Medical Are there any other Macronutrients?

    I have a few questions on energy from foods. Do we have any other macronutrients that contribute energy to the body other than carbohydrates, proteins and fats in our body? What is the role of Hydrochloric acid (HCL) in our stomach? Is the HCL involved in all three above macronutrient digestion?
  7. M

    How can grass be turned into fertilizer after digestion?

    I don't understand how grass can enter an animal like a cow, have nutrients extracted by the cow and then be excremented as fertilizer? My point is that if i got the same grass, soaked in water and added it to plants without removing any nutrients it would be a useless fertilizer. How come you...
  8. caters

    Writing: Input Wanted Is it useful to have 2 stomachs?

    Here I will talk about the digestive system of my Kepler Bb humanoids. Mouth The teeth are much stronger than bone(Strong enough that they can eat bones no problem). Other than that and all time tooth formation(so a loose adult tooth is no problem for them since another will grow in its...
  9. M

    Digestion of Pb-alloy for ICP-OES analysis

    Hello everybody, I am looking for a disgestion procedure of Pb-alloy, so that we can measure the elements with ICP-OES. Pb should be removed, that it doesn't contaminate the system. So far we used HCl for digestion, but the results weren't very good. Elements of interest are: Sn Sb Bi Cu Cd As...
  10. gracy

    Paramecium ingestion and digestion process

    The structure of the Paramecium is rather complicated. Its plasma membrane is specially equipped with thickenings, so the membrane can be strengthened. Then, this membrane is called the pellicle. A paramecium has one big nucleus and two smaller nuclei. The bigger nucleus is about nutrition, and...
  11. A

    What is the difference between digestion and aging of a precipitate?

    Is digestion related to heating and aging related to allowing the precipitate to sit?
  12. J

    What Actually Happens During Digestion?

    What actually happens when cells break down food and combines it with oxygen? Does the sugar release a particle or energy in the form of light? If a particle, what is happening? Does the release of energy transform two different particles (one is the sugar, the other being the one that receives...
  13. A

    Medical Does digestion really begin in the mouth?

    Most textbooks say it begins in the mouth. In fact, THAT is a quote from a text. Does it? Define digestion, specifically. So where does it begin?
  14. A

    How to Calculate the Biogas Volume of an Anaerobic Digester?

    Hey guys Does anyone know how to calculate the biogas volume of an anaerobic digester depending on the type of biomass fed in. Are there any good sources on this specific question? Thanks
  15. C

    Heat Exchanger for Anaerobic Digestion Process

    I have a problem whereby sludge of quantity 1000 kg/day is required to fill an anaerobic digester tank. The sludge enters at 20deg and is heated to 35deg. With refernece to this link http://books.google.co.uk/books?id=D...obic+digestion it can be seen that if I have a quantity of 1000kg/day...
  16. C

    Heat exchanger for anaerobic digestion

    I have a problem whereby sludge of quantity 1000 kg/day is required to fill an anaerobic digester tank. The sludge enters at 20deg and is heated to 35deg. With refernece to this link http://books.google.co.uk/books?id=DwMWvTki7h8C&pg=PA197&dq=heat+required+to+heat+sludge+anaerobic+digestion"...
  17. B

    Digestion Qs: Humans, Plants, Stomach & Intestine

    Hi, 1) Humans cannot digest cellulose. Plant cell walls are made of cellulose. How can we get any nutrition fro plants if we can't break it's outermost structure? 2) When we take medicine by mouth, what prevents it from being broken down in the acidic stomach, or the enzymes on the small...
  18. F

    Plants, cell communication, digestion, and circulation

    Homework Statement I would like to have some of my homework problems checked. I looked in the book, and answered these the best I could. My answers are in bold. Thanks in advance for the help! Homework Equations N/A. The Attempt at a Solution 1. Which functional plant cells lack a...
  19. P

    Is Multi-Tasking While Eating Affecting Our Digestion?

    For humans, is it best to do nothing while eating? So that the digestive system can function with full vigour without resources directed to other activities during eating? From experience doing other physical activity is definitely not good but what about doing mental activities while eating...
  20. B

    Understanding Nutrient Digestion: Pathways and Differences

    I'm looking for information about how different nutrients are digested, absorbed, and metabolized during each stage of digestion. So basically how food is digested, absorbed, metabolized and how it differs from nutrient to nutrient. This would mean the different pathway which the sugar...
  21. A

    Quiz: Do the changing seasons affect digestion in cattle?

    I'm an artist, currently researching the four stomachs of a cow for my sculpture project. As part of my finished piece I wish to incorperate the seasons of the year. Four seasons and four stomachs would fit together nicely. If there happens to be some kind of real connection it would benifit...
  22. wolram

    Do Fish Break Wind? Investigating Digestion in Aquariums

    Do fish---- Break wind, when looking at them in aquariums i have never seen bubbles coming from that end, so may be their digestive system works differently.
  23. A

    Protein digestion happens alot in the stomach

    Okay so protein digestion happens a lot in the stomach...how come the stomach doesn't just digest itself??
  24. wasteofo2

    Saturated/Hydrogenated fats and their digestion.

    Is there a difference between saturated and hydrogenated fats, or are hydrogenated fats simply unsaturated fats which have been saturated? How readily are saturated fats (and hydrogenated, if they're a different thing) digested/stored/used for energy? I've heard claims by supposed health...