Topic 6.1: Digestion

Assessment Statements 6.1.1: Explain why digestion of large food molecules is essential 6.1.2: Explain the need for enzymes in digestion 6.1.3: State the source, substrate, products and optimum pH conditions for one amylase, one protease, and one lipase 6.1.4: Draw and label a diagram of the digestive system 6.1.5: Outline the function of the stomach, small intestine and large intestine 6.1.6: Distinguish between absorption and assimilation 6.1.7: 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? When you eat a snack or a meal, you begin a set of events that leads to your body cells being provided with needed nutrients Ingestion Digestion Absorption Transport

Digestion Digestion solves a problem of molecular size Food is too large to pass through cell membranes Food must be chemically digested to a suitable size Molecule type Molecular form ingested Molecular form after digestion Protein Amino acids Lipids Triglycerides Glycerol and fatty acids Carbohydrate Polysaccarides, disaccharides, and monosaccharides Monosaccharides Nucleic acids DNA, RNA Nucelotides

Digestion Digestion allows you to turn molecules into ‘your own’ All the food that you eat is composed of plant or animal cells, thus containing molecules characteristics of a living organisms that is not a human being Each type of living organism has its own set of proteins, nucleic acids, and carboydrates

Digestion When we digest food molecules, we break them down (hydrolyze them) into smallest components. The components can be reassembled into larger molecules that are useful to you!!

Digestion As food moves through your alimentary canal, many digestive enzymes are added along the way Each digestive enzyme is specific for a specific food type Examples: Lipase: is an enzyme is specific for lipid molecules Amylase: is specific for amylose (starch)

Digestion The real function of enzyme is to lower the activation energy of reactions that they catalyze Digestive enzymes all help to catalyze hydrolysis reactions

Examples of digestion enzymes Salivary Amylase Pepsin (a Protease) Pancreatic Lipase Source Salivary glands Stomach cells Pancreas cells Substrate Amylose (starch) Proteins (polypeptides) Lipids Products Maltose and glucose Amino acids Glycerol and fatty acids Optimum Neutral (pH 7) Acidic (pH 3)

Human Digestive System Much of the human digestive system is a tube called the alimentary canal Alimentary canal consists of: Mouth Esophagus Stomach Small Intestine Large Intestine (colon) Rectum Any foods that you ingest must either be digested and absorbed for use by the body or remain undigested and be eliminated as solid waste

Stomach Food is brought to your stomach by a muscular tube called the esophagus When you swallow, the food is forced down to your stomach by a sequential series of smooth muscle contractions called peristalsis

Stomach Once in the stomach, the food is held for a period of time in order to mix it with a variety of secretions collectively known as gastric juice

Stomach Gastric juice is a mixture of three secretions form the cells of the stomach inner lining: Pepsin- a protease enzyme most active in acidic pH Hydrochloric acid- helps degrade and break down foods and creates the acidic pH necessary for pepsin to be active Mucus- lines the inside the stomach wall to prevent stomach damage from the hydrochloric acids

Stomach The muscular wall of the stomach creates a churning motion in order to mix food with the gastric juice After a period of time, a valve at the lower end of the stomach opens and the food enters the small intestine

Small Intestine The first portion of the small intestine is called the duodenum Three different accessory organs secrete juices into the small intestine in order to continue the digestive process These secretions include: Bile from the liver and gall bladder Trypsin (a protease), lipase, amylase, and bicarbonate from the pancreas

Small Intestine As the digestive process continues in the small intestine, molecules are produced that are small enough to be absorbed The inner wall of the small intestine is made up of thousand of finger-like extensions called villi

Small Intestine Each villus contains a capillary bed and a lacteal Lacteal is a small vessel of your lymphatic system If the inner lining of your small intestine were smooth, you would have a fairly limited membrane surface area for absorption The function of the villi is to greatly increase the surface area for absorption of molecules such as glucose, amino acids, and fatty acids

Small Intestine Most molecules absorbed are taken into the capillary bed within each villus Except fatty acids which are more efficiently absorbed into the lacteal All absorbed molecules are taken to a wide variety of body cells by the circulatory system

Large Intestine The vast majority of useful nutrients are absorbed while food is still inside the small intestine Much of the water that we drink or that is naturally contained in many food is also present

Large Intestine The primary function of the large intestine is water absorption Leaving water in the alimentary canal as long as possible is beneficial because it keeps the moving food in a fluid environment

Large Intestine The large intestine is also home to a very large number of naturally occurring bacteria including Escherichia coli.

Large Intestine These bacteria are examples of mutualistic organisms within us. We provide nutrients, water, and a warm environment for them while they synthesize vitamin K and maintain a healthy overall environment for us in our large intestine Any food undigested by us or the bacteria is eliminated from the body as solid waste or feces