The Digestive System

• PURPOSE:

  • digestion
  • food is to be broken down into smaller pieces so that it can be absorbed into the body
  • 2 types:
  • Mechanical - breaks food down into smaller chunks
  • Chemical - breaks biomacromolecules into their subunits
  • provide enough absorptive surface area so that food can be absorbed
• MECHANICAL DIGESTION

  • When food is mechanically digested it is broken into smaller pieces of food.
  • greatly increases the SA:V ratio, so that it can be chemically digested more efficiently
  • this does not alter the chemical nature of the food
  • These movements include chewing, muscle contractions, and the stirring of food and digestive juices by muscle movements.

  
  

  EXAMPLES: chewing, food churning in the stomach
• CHEMICAL DIGESTION

  • changes what the food is made of, due to a hydrolysis reaction (a catabolic chemical reaction).
  • involves digestion by enzymes (a protein that speeds up chemical reactions)
  • takes molecules and breaks them down through enzymes and stomach acid, producing smaller molecules that are capable of being absorbed into the bloodstream.
  • enzymes are very sensitive to pH and temperature. each enzyme has an optimal pH, in which it works its fastest
  • The three major types of digestive enzymes are amylases (which act on carbohydrates), proteases (proteins), and lipases (lipids).
• CHEMICAL DIGESTION OF LIPIDS:

  • Lipids -fats and oils
  • Enzymes called lipases break down lipids into fatty acids and glycerol (the monomers of what the lipids are made up of)
  • This happens inside the small intestine
• CHEMICAL DIGESTION OF COMPLEX CARBS:

  • Complex carbohydrates - aka polysaccharides
  • Enzymes called amylases break complex carbs into simple sugars, (monosaccharides). E.g glucose
  • Carbs are polymers (consisting of large molecules). salivary amylase breaks down these carbs in the mouth and small intestine. This breaks it down into glucose (a monomer, a building block that makes up polymers)
  • Starts in the mouth but mostly happens in the small intestine
• CHEMICAL DIGESTION OF PROTEINS:

  • proteins are broken down into amino acids by enzymes called proteases
  • polymer - a large molecule made of a chain of similar smaller molecules called monomers.
  • e.g protein is a large polymer, made up of chains of amino acids. amino acids are the monomers.
  • This happens inside the stomach
• Organs that make up the digestive system:

  • Can be divided into 2 groups
  • The gastrointestinal tract (GI tract)
  • Mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, and anus
  • The GI tract performs all digestive functions
  • Accessory organs
  • Tongue, salivary glands, pancreas, liver, and gall bladder
  • Accessory organs assist in digestion (ie through the secretions of enzymes)
• GI tract

  • mouth
  • pharynx
  • oesophagus
  • stomach
  • small intestine
  • large intestine - incl rectum and anus
• Accessory organs

  • tongue
  • salivary glands
  • pancreas
  • liver
  • gall bladder

  these organs help in mechanical or chemical digestion. (tongue helps with chewing. pancreas secrete enzymes. food does not 'pass' through them.)
• Organs of the digestive system in order

  • Mouth (incl tongue [a], teeth [a], and salivary glands)
  • Pharynx
  • Oesophagus
  • Stomach
  • Liver [a]
  • Gall bladder [a]
  • Pancreas [a]
  • Small intestine
  • Large intestine (incl rectum then anus)
• THE MOUTH:

  • largely involved in the mechanical digestion of food by mastication (chewing) by teeth and churning of food by the tongue
  • the mouth also mixes the food with saliva from salivary glands. Saliva contains salivary amylase which digests starch. this is an example of chemical digestion.
  • also, saliva contains water which moistens food and mucus which makes it slippery.
  • Involved in the process of ingestion.
  • food that is ready to be swallowed is called a bolus.
• THE OESOPHAGUS

  • the bolus leaving the mouth enters the oesophagus (by the pharynx).
  • through a process of peristalsis, the food is pushed down a smooth tube to the stomach, by peristaltic waves of smooth muscle that surround the epithelial lining.
• THE STOMACH:

  • connects the oesophagus to the duodenum (first part of the small intestine)
  • Functions
  • store and regulate the release of food into the duodenum
  • acts as a mixing area of acid, pepsin, and food to form chyme (a semi-liquid slurry) through muscular contractions. This is an example of mechanical (churning) and chemical (breaking down)
  • pepsin has a low optimal pH (1.5), which is acidic. the stomach secretes hydrochloric acid to make pepsin work at its optimal condition, and mucus to protect the stomach wall from this acid
  • the entry point of the stomach and duodenum is the pylorus
• THE SMALL INTESTINE:

  • important because it's the main organ when food is absorbed into the body.
  • produces intestinal juice containing amylase, protease, lipase, and mucus. continues the [c]digestion of carbohydrates and proteins. initiates the [c]digestion of lipids. pancreas enzymes and bile aid in chyme breakdown
  • important in digestion and main function in absorption.
  • reason for length. the food has more time to be absorbed.
  • surface covered in villi, which is then covered in microvilli. absorbs nutrients, and certain waste products of digestion and delivers that to the circulatory system
• THE LIVER

  • stores excess glucose in the form of glycogen, which can be converted back to glucose when needed for energy.
  • is the site of bile production. Bile is important in the physical breakdown of fats – smaller fat particles are then more readily broken down by lipase enzymes.
  • bile is stored in the gall bladder before being ejected into the duodenum
  • has important roles in regulating metabolism, toxin removal, and processing nutrients. blood filtration before it is passed to the rest of the body
• THE GALL BLADDER

  • concentrates the stored bile secreted by the liver and dispenses into the duodenum.
  • bile is like detergent/surfactant, which emulsifies fats and makes them turn into smaller pieces, increasing the SA:V, and increasing absorption.
  • without bile, ingested lipids would pass out in faeces
  • as food comes out of the stomach, the gall bladder empties itself, and the bile mixes with the food, at the first loop of the small intestine (the duodenum)
• PANCREAS:

  • it's just under the stomach and has a duct that leads to the very start of the small intestine.
  • the pancreas secretes pancreatic juice (digestive enzymes) into the duodenum (first curve of the small intestine) when food reaches there. Pancreatic juice contains amylase, lipase and protease. it also contains bicarbonate ions, which neutralise the pH of the food (chyme) leaving the stomach.
  • also regulates blood sugar levels
  • chyme is food mixed with gastric juice, whilst bolus is food mixed with saliva
• LARGE INTESTINE:

  • food leaves the small intestine and enters the large intestine.
  • main purpose is to absorb water, vitamins, and minerals and store the remainder of the waste material, which is stored in the rectum for defecation. water gets absorbed and food becomes solid and compact.
  • also contains around 700 species of bacteria and archaea (gut flora), which perform diverse important functions like producing vitamins B and K, and metabolising bile acids and other chemicals.
  • made up of the cecum, colon, and rectum. The junction between the small and large intestines is called the cecum.
• Rectum

  • Final area of the large intestine that stores faeces for elimination.
• Anus

  • The end of the digestive tract where faeces are expelled from the body.
• Movement of food through the digestive system

  • Ingestion
  • Digestion
  • Absorption
  • Elimination/egestion
• Ingestion

  • the first step of the process is when food is taken into the body via the oral cavity, aka mouth.
  • teeth, saliva, and tongue play an important part in humans' ingestion
  • teeth physically break down food, salivary enzymes chemically break down food
• Digestion

  • occurs alongside the digestive tract, where the chemically broken down food (bolus) travels from the mouth into the body
  • continue to be broken down physically and chemically by a variety of organs
• Absorption

  • once the food macromolecules are broken down into smaller molecules, they are absorbed across plasma membranes of cells in the digestive tract into the bloodstream
  • it is here when the energy from food is ready to be used by the body
• Elimination/egestion

  • the final step is the elimination of undigested food content that has travelled along the digestive tract but has not been absorbed
  • it is eliminated from the body as faeces
• Animals are heterotrophs (gain energy from eating other organisms) 


  Plants are autotrophs (make their own energy