The Digestive System and Nutrition

Nutrition photosynthesis Autotroph Herbivores Carnivores Heterotroph Autotrophs – makes their own food Heterotroph –need to obtain food from environment Omnivores

Nutrition Balanced diet includes all 7 components Energy content of food measured in kilocalories The average teen-ager needs Approximately 2200 calories for girls 2800 calories for boys Carbohydrate- Protein – Fats- 4.5 calories/g 9 calories/g Obesity New Food Pyramid 2005 - emphasise importance of controlling weight and physical activity Carbohydrate, protein, fat, vitamins, minerals, fibre and water Activity level, gender, health Obesity – BMI (Body Mass Index) – body mass (kg) versus height (cm) – over 25 indicates obese Around 50% overweight and 13% obese - dietary fats – limit saturated fats 20-35% or less of energy should come from fats, healthiest are monounsaturated and polyunsaturated -limit sugar intake -stress benefits of wholegrains

Alimentary canal Mouth Trachea - windpipe 2 main functions: Digesting and absorbing nutrients Protecting from invasion Mouth -mechanical digestion (mastication) = teeth, tongue -chemical digestion = saliva (amylase, lysozyme) Uvula – prevents food entering the nose Trachea - windpipe Tongue – strongest muscle in body Saliva : Amylase =starch into maltose. Lysozyme = damages bacterial cell wall. An average person produces 25,000l in a lifetime (enough to fill 2 swimming pools) An average person swallows 295 times while eating dinner Incidentally, chewing is hard work and expends a lot of energy. Scientists have calculated that chewing gum increases energy consumption by 20%, by chewing calorie-free gum during waking hours and not changing any other aspect of energy balance, a person should lose about 5 kg of fat per year. Alexander Fleming discovered lysozyme during a deliberate search for medical antibiotics. Over a period of years, he added everything that he could think of to bacterial cultures, looking for anything that would slow their growth. He discovered lysozyme by chance. One day, when he had a cold, he added a drop of mucus to the culture and, much to his surprise, it killed the bacteria. He had discovered one of our own natural defenses against infection. Unfortunately, lysozyme is a large molecule that is not particularly useful as a drug. It can be applied topically, but cannot rid the entire body of disease, because it is too large to travel between cells. Fortunately, Fleming continued his search, finding a true antibiotic drug five years later: penicillin. Epiglottis – safety hatch. A flap of cartilage prevents food from entering the trachea

-transfers food to stomach by peristalsis Oesophagus -transfers food to stomach by peristalsis Cardiac sphincter -opens to allow food oesophagus stomach -heartburn –acid escapes stomach oesaphagus http://library.thinkquest.org/11226/main/c03txt.htm Peristalsis - A series of normal coordinated, rhythmic muscle contractions that occurs automatically to move food through the digestive Tract (oesophagus, small intestine, large intestine) , urine from the kidneys through the ureters into the bladder, and bile from the gall bladder into the duodenum.

Stomach Short term storage reservoir (1L for up to 4h) Digestion = chemical (HCl and enzymes) - proteins = mechanical - liquefication of food Slowly releases food into intestine chyme Cardiac sphincter Pyloric sphincter What causes our stomach to growl? Stomach growling occurs when the stomach receives signals from your brain to begin digestion but the stomach is empty. Your brain might sense you're running low on energy (glucose) or even seeing or smelling something you want to eat can get things going. The motion of the stomach muscles begins, but the organ is hollow. The movement of the muscles mixing the acids of the stomach in the hollow space of the stomach produces vibrations we hear as growling, or rumbling, or gurgling. http://35.9.122.184/images/41-AnimalNutrition/41-16-Duodenum-L.gif

Stomach Epithelium Mucous – goblet cells Prevents self-digestion Enzymes (pepsinogen) – chief cells Activated to pepsin Converts proteins peptides Acid (HCl) – parietal cells pH 1-2 Kills bacteria Loosens fibrous foods Activates pepsinogen Denatures salivary amylase Hormone (gastrin) – G cells Controls gastric motility and acid secretion Ulcers (stomach, duodenum) – peptic ulcers. Most commonly caused by H. pylori Mucous – lubricates food. Prevents stomach self-digesting. Prevents attachment of bacteria HCl – stomach pH = 1-2. Kills bacteria, helps breakdown food, activales pepsinogen, deactivates amylase Pepsinogen (pro-enzyme) – activated by HCl – pepsin (this prevents the enzyme from digesting cells that produce it). Breaks proteins down into peptides Stomach epithelial cells are some of the fastest growing cells in the body, typically replacing themselves about every 3 days

Small Intestine Around 6m in an adult Food takes 1-6 h to pass through 2 main tasks = digestion, absorption 3 parts Duodenum Jejenum Ileum NaHCO3 = neutralises acid Once trypsinogen and chymotrypsinogen are released into the lumen of the small intestine, they must be converted into their active forms in order to digest proteins. Trypsinogen is activated by the enzyme enterokinase, which is embedded in the intestinal mucosa. Once trypsin is formed it activates chymotrypsinogen, as well as additional molecules of trypsinogen. The net result is a rather explosive appearance of active protease once the pancreatic secretions reach the small intestine. Trypsin and chymotrypsin digest proteins into peptides and peptides into smaller peptides, but they cannot digest proteins and peptides to single amino acids. Insulin Glucagon Emulsification = breaks fats and oils into tiny droplets, increasing surface area to aid digestion Salad dressing

Duodenum = digestion = 25cm long Pancreas –pancreatic juice= NaHCO3, enzymes (insulin, glucagon) pH of duodenum = 7-8 Amylase, lipase, trypsinogen, chymotrypsinogen Liver – bile made in liver, stored in gall bladder = Water, salts, bile salts Neutralise HCl Digestion and absorption of fats and fat soluble vitamins (emulsification) Waste products eliminated by secretion into bile and elimination in feces (e.g. bilirubin, biliverdin)

Liver Weighs about 1.5kg Holds about 13% of total blood Liver cell = hepatocyte Unique ability to regenerate – average life = 150 days Right lobe Left lobe The liver performs over 500 jobs. Some of these are: Makes bile (600mls/day) Detoxifies body (alcohol, drugs etc) Breaks down excess amino acids urea (deamination) kidney Converts glucose glycogen for storage (source of quick energy) Converts excess carbohydrates fat Stores vitamins - A, D, E and K Stores minerals – Fe, Cu, Zn Makes plasma proteins e.g. fibrinogen – blood clotting Makes cholesterol – needed to form many hormones Produces heat to warm blood Clears blood of particles, including bacteria Fights infections –half the body’s macrophages -destroy bacteria Produces hormones, including the sex hormones Blood rich in food from ileum Aas can’t be stored – become toxic and hence aas not needed are broken down by liver in a process known as deamination http://www.britishlivertrust.org.uk/content/liver/about.asp

Small Intestine cont. Jejenum – digestion/ absorption. 2.5m long Ileum – absorption. 4m long Walls only one cell thick Villi, microvilli – increase surface area for absorption Rich blood supply – capillaries absorb water and soluble nutrients (glucose, amino acids, vitamins, minerals) and the blood carries the nutrients to the liver, which stores nutrients and releases them as required Lacteal – contains lymph. Fatty acids and glycerol are absorbed by the epithelial cells where they reform into fats. They become coated in protein (chylomicrons) and pass into the lymph in the lacteals. It takes around 18h for lymph to rejoin the blood, the protein coat dissolves and fats are absorbed into cells The ileum is 4m long. It produces enzymes that are responsible for the final stages of digestion. The main job of the ileum is absorption and the inside surface of the ileum is also covered in villi, which increase the surface area which can absorb nutrients, such as vitamins, amino acids and simple sugars. These nutrients are absorbed into the bloodstream and distributed around the body. By the time the food leaves the ileum, most of the useful nutrients have been absorbed.

Large Intestine 1.5m long, 6cm diameter Food stays 10h to a few days Caecum Appendix Function unknown – in herbivores they contain bacteria that help digest cellulose Colon Reabsorbs water – so waste is converted to semi-solid = faeces – egested Diarrhoea, constipation (fibre helps stimulate peristalsis)