IB MYP Biology- Digestion Study Notes

IB myp 4-5 Biology – Study Notes- All Topics

Topic :Metabolism-Digestion

Topic :Metabolism– Weightage : 21 % 

All Questions for Topic : Nutrition,Digestion,Biochemistry and enzymes,Movement and transport,Diffusion,Osmosis,Gas exchange,Circulation,Transpiration and Translocation,Homeostasis

Digestion

Nutrition is a series of processes that enable us to use and transform the substances we need to stay alive. It converts food molecules to a useful form.

Multicellular organisms have a number of organs, grouped into systems, which prepare nutrients, distribute them to all the cells and expel the waste products from cellular activity. This involves four systems:
           
Digestive system: prepares food so that the cells can use it. 

Respiratory system: supplies oxygen to the blood and eliminates carbon dioxide released during cell metabolism.

Circulatory system: takes oxygen and nutrients to all the cells, and takes away waste products.

Excretory system: expels waste products released during cell metabolism and transported via the circulatory system.

Digestion  is the breakdown of large, insoluble food molecules into small, soluble food molecules do that they can be absorbed into the blood stream.

The processes which convert these biomolecules into useful forms take place in the alimentary canal, gut or digestive system.

The digestive system is made up of the digestive tract and accessory glands.

The digestive tract is about 9 meters long. It is narrow in some parts and quite wide in others. It starts at the mouth and ends at the anus. (oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine)

Accessory glands  are organs that release secretions into the digestive tract. These include the salivary glands (in the mouth), the gastric glands (in the stomach) and the intestinal glands (in the intestine). The liver and the pancreas are lager glands which release digestive secretions into the digestive tract

Processes in Digestion of Food

The digestive system has to do with the process of consumption and breaking down of food and nutrition in our bodies. Digestion is both chemical and mechanical. Its phases include:

Ingestion: Food intake

Mechanical digestion: Chewing and grinding in the mouth, as well as movement of food by peristalsis along the digestive tract.

Chemical digestion: Enzymes breaking down large molecules.

Absorption: The entry of small food molecules from the gut to the blood.

Assimilation: The entry of those small food molecules from the blood into the cell. Now these nutrients can be used by cells for energy, growth and repair.

Egestion: Indigestible substances make their way to exit the body.

Defecation: Indigestible substances are expelled from the body.

 The GI tract (Gastrointestinal Tract) is the main pathway in the digestive system. It comprises of:

  • Mouth
    • Teeth
    • Epiglottis (flap-like structure that prevent food from going down the windpipe).
  • Esophagus
  • stomach
  • Intestines
    • Small
    • Large
  •  Rectum
  • The Esophagus is a $10^{\text {” long }}$ tube that secretes mucus, moves food from the throat using wave-like motion (peristalsis):

  • In the mouth, teeth mechanically break down food into smaller pieces. The tongue mixes saliva with the food. The amylase in the saliva helps break down starch. The epiglottis prevents food from going down the windpipe, after that, it goes to the stomach.
  • The stomach is a J-shaped – muscular sac that has digestive juices (such as Hydrochloric Avid) with enzymes that help break down protein/Polypeptides. The stomach has a pH of 1.5 . The enzymes that break down proteins a re called proteases. the food stays in the stomach for 4 hours. Fast food stays longer. The pyloric sphincter covers the mouth of the stomach, and the churning food does not come out.
  •  In the mouth, teeth mechanically break down food into smaller pieces. The tongue mixes saliva with the food. The amylase in the saliva helps break down starch. The epiglottis prevents food from going down the windpipe after that, it goes to the stomach.
  • The stomach is a J-shaped muscular sac that has digestive juices (such as Hydrochloric Avid) with enzymes that help break down Proteins/Polypeptides. The stomach has a pH of 1.5 . The enzymes that break down proteins a re called proteases. the food stays in the stomach for 4 hours. Fast food stays longer. The pyloric sphincter covers the mouth of the stomach, and the churning food does not come out.
  • After 4 hours in the stomach, the food (which most of has been digested anyway), the food moves to the small intestine. This is the longest single organ of the body. The food is digested using pancreatic amylase. It has small, finger like projections (villi), which themselves have smaller versions of themselves (microvili:). They aid in increasing the surface Area to volume Ratio. Within the small intestine, lacteal (lymphatic vessels) absorb fats. The small intestine absorbs:
    • $.80 \%$ ingested water
    •  Carbohydrates
    •  Proteins Polypeptides
    •  Fats/Lipids
    •  Vitamins
    • Minerals
  • The large intestine accepts what the small does not. The bacteria in the large intestine foment carbohydrates and break down proteins. The large intestine continues to the rectum, where food is excreted in the form of faeces.

 Accessory organs are organs which play indirect roles in an organ system.

  • The Liver affects digestion in the following wags.
    • Emulsifies fats. The liver breaks down the fats into many smaller pieces. This increased surface area allows for sped up digestion.
    • Filters toxins, that leave the body as urea (Particle that make urine yellow)
    • secretes bile (made of worm out Red Blood Cells) that helps in  digestion of fats.
  • The Gall Bladder stores this bile, and releases it into the small intestine when necessary.
    •  Too many fatty diets can cause gallstones.
  •  The pancreas produces digestive enzymes (biological catalysts) that help break down proteins and lipids. It also produces insulin, which regulates blood sugar levels.
  • The Digestive System is closely tied with the excretory System. This has to do with releasing waste, from the body.
ENZYMES

During digestion, food molecules are broken down by chemical reactions (hydrolysis). Our bodies need special and specific molecules for these reactions to happen. We call these molecules enzymes.

Enzymes are proteins that control and speed up biochemical reactions in living organisms without being consumed during the reaction. Enzymes are very specific, for example, proteases break down proteins, but do not affect carbohydrates or lipids, and lipases break down lipids but do not affect proteins or carbohydrates and so on. An enzyme’s activity is affected by temperature and pH an it drastically decreases outside its optimal range

Anatomy Of Digestive Systems

Oral Cavity (MOUTH)

Teeth 
Food enters the mouth and digestion begins with the teeth breaking down the food into smaller pieces. This serves 2 purposes:
  • Makes the food easier to swallow
  • Gives food a large surface area for enzymes to work on

Salivary glands 
The salivary glands secrete saliva, which mixes with the chewed food. Saliva has two functions:

  • Saliva contains the enzyme amylase which is a carbohydrase and breaks down starch into sugar. The pH in our mouth is around 7.0, where amylase works best.
  • Saliva also contains mucus mucus, which lubricates the food (forming into a ball called the bolus) and helps it pass down the esophagus.

Esophagus and Pharynx (SWALLOWING)

Swallowing is a reflex reaction and happens without us thinking about it. Before swallowing the tongue rolls the food into the bolus and pushes it to the back of the mouth. The food pushes the soft palate upwards which blocks the upper pharynx and stops food going into the nasal cavity. Voluntary muscles in the face, neck and tongue push the food through the pharynx. As the food is swallowed it passes over the epiglottis which covers the opening of the respiratory system and prevents food entering it. Food passes the epiglottis and into the oesophagus which connects the pharynx to the stomach.

The oesophagus has circular muscles in the wall. These muscles contract behind the bolus to push it along and the muscles in front of the food relax. This way food passes along the oesophagus to the stomach. This movement is known as peristalsis

STOMACH

The bolus enters the stomach. The stomach cells (found in glands in its lining) make gastric juices which mix with the food.
  • The gastric juices contain a protease enzyme called pepsin which breaks down proteins into amino acids.
  • The juices also contain hydrochloric acid (HCl), this is because pepsin works best in an acidic environment of pH 2.

The acid in the juices also kills any germs. The muscular walls of the stomach churn the food and mix it well with the secretions. After 2-3 hours of churning the food is a thick liquid called chyme that can be released in spurts into the duodenum, the first part of the small intestine.

SMALL INTESTINE

The small intestine is about 6 or 7 meters long.  Chyme leaves the stomach via the pyloric sphincter and enters the small intestine.  The small intestine consists of three parts, the duodenum, jejunum and ileum. The first part of the small intestine is mainly for digestion and the remaining sections is where absorption takes place.

In the small intestine 3 important digestive juices are added to the food secreted by the liver and pancreas.
 
Bile: The liver produces bile which is stored in the gall bladder and enters the small intestine via the bile duct.  Bile has 2 important functions:
  • Bile is alkaline and neutralises the acid which was added to the food in the stomach.  This provides a basic pH, optimal conditions for the enzymes in the small intestine to work. 
  • Bile emulsifies fats; it breaks large molecules of fat into smaller droplets which increases the surface area of fats for the enzyme lipase to work on.

Pancreatic juice: The pancreas produces pancreatic juices which contain carbohydrases, proteases and lipases.  These enzymes empty into the duodenum to further continue digesting the food.

Intestinal juices:  The glands in the wall of the small intestine produce intestinal juice.  This also contains carbohydrases, proteases and lipases.  These enzymes complete the digestion of the food.

Pancreatic amylase, will keep digesting carbohydrates into its glucose components. Trysin will further break down proteins into its amino acids. And lipase will break down fats into fatty acids and glycerol.

As mentioned above, In addition to digestion the small intestine has another important job and this is ABSORPTION. The small intestine is ideal for absorption because it has:

  • A thin lining
  • Plentiful blood supply
  • Very large surface area

The surface area of the small intestine is around 9 square meters! This is possible because the small intestine is very long (around 6 or 7 meters) and it is lined with tiny finger like projections called villi. Each villus in turn is covered with even smaller microvilli, there are millions of villi which provide a massive surface area to maximise the rate of absorption. The villi have very thin walls and a good blood supply, as each one is surrounded by capillaries, which means that the digested food (nutrients) can be easily absorbed from the gut into the blood, by diffusion.

LARGE INTESTINE

Any indigestible or non absorbed chyme passes into the large intestine. Excess water and salts are absorbed and the remaining chyme is converted into faeces. Feaces are stored in the rectum until excreted through the anal canal.

Intestinal bacteria

The intestines are home to more than 500 species of bacteria, which they need to function effectively. Some of these bacteria protect the intestines from disease. Certain bacteria in the large intestine make vitamin K, which the body needs for blood clotting

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