Digestion

Explain why digestion of large food molecules is essential Explain the need for enzymes in digestion State the source, substrate, products and optimum pH conditions for one amylase, one protease, and one lipase Draw and label a diagram of the digestive system Outline the function of the stomach, small intestine and large intestine Distinguish between absorption and assimilation Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion

Why do we digest food? Ingestion – you eat the food To start with, this is a breakdown of basic processes: Ingestion – you eat the food Digestion – series of chemical reactions that convert food into smaller and smaller molecular forms Absorption – food molecules absorbed by cells and passed to blood vessels Transport – you circulatory system moves food molecules throughout your body

Many of the foods that we eat are composed of very large molecules and these molecules may be too big to cross the cell membranes and enter your bloodstream Molecule Type Ingested Form After Digestion Protein Amino acids Lipids Triglycerides Glycerol/fatty acids Carbohydrates Poly, di, monosaccharides monosaccharides Nucleic acids DNA, RNA nucleotides

Proteins to amino acids via hydrolysis reactions

Triglycerides into glycerol and fatty acids via hydrolysis reactions

Polysaccharides into Monosaccharides via hydrolysis reactions

Using enzymes specific for certain tasks, your body is able to breakdown the foods you eat via hydrolysis reactions (your body can then reassemble the monomers into polymers as needed) Remember, enzymes are protein molecules which act as catalysts for reactions; they don’t cause the reactions, they just lower the activation energy (heat) required to allow the reaction to take place (temperature, pH, substrate levels help determine enzyme effectiveness)

Salivary Amylase Pepsin (Protease) Pancreatic Lipase Source Salivary glands Stomach cells Pancreas cells Substrate Carbohydrates Proteins Lipids Products maltose and glucose Amino acids Glycerol and fatty acids Optimum pH Neutral (pH 7) Acidic (pH 3) Neutral (pH 7)

Parts of the Digestive System Much of the digestive system is a tube called the alimentary canal which consists of: Mouth Esophagus Stomach Small Intestine Large Intestine Rectum

The Stomach Food is forced down the esophagus to the stomach via muscle contractions called peristalsis Food is held in the stomach in order for it to mix with secretions known as gastric juice Gastric juice is: - pepsin – a protease enzyme most active in acidic pH - hydrochloric acid – makes the pH necessary for pepsin to activate and helps break down foods - mucus – lines the stomach wall to prevent stomach damage from hydrochloric acid

The Small Intestine 2-6 hours after a meal, the valve at the bottom of the stomach will start slowly releasing the food/enzyme/acid mixture into the small intestine The beginning of the small intestine (named the duodenum) is where accessory organs begin secreting juices into the small intestine for the purposes of continuing digestion What is added includes: - bile from liver and gall bladder - trypsin (a protease), lipase, amylase and biocarbonate from the pancreas Duodenum is ~25 cm of 6 m of small intestine (most digestion compete by the end of duodenum)

Absorption in Small Intestine In the small intestine food molecules are small enough to be absorbed into the bloodstream/lymphatic system This can happen b/c inner wall of small intestine is made up of thousands of finger-like extensions called villi and each villi contains a capillary bed and lacteal (this increases surface area for maximum absorption of monomers) Most molecules go into bloodstream via capillaries but fatty acids are absorbed into lacteal (remember diffusion/active transport from earlier this year) because they are too large to pass through the membranes of capillaries

Assimilation of Molecules After molecules are absorbed into bloodstream via capillaries or into lymphatic system via lacteals it is transported around the body by the circulatory system Once a molecule has reached a body cell where it is needed, it will leave the bloodstream the same way it entered (diffusion or active transport across membranes) into the cell The molecule may be used for energy (e.g. glucose) or it may be used to build a polymer inside the cell (e.g. amino acids used to make proteins) The process of bringing a molecule to a body cell and then using it to make larger molecules (polymers) is called assimilation

The Large Intestine Most useful nutrients are absorbed in the small intestine but the H2O we drink/eat is still present The primary function is water absorption (this makes sense b/c having our undigested foods in an aqueous environment for as long as possible aids in movement) contains many bacteria including E. coli (these are mutualistic organisms – we provide food/H2O,warm enviro and they help us synthsize vitamin K and maintain healthy intestines) Any undigested food by us or the bacteria (e.g. cellulose) is eliminated as solid waste