1. The Digestive System
and Nutrition

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

3. Nutrition Balanced diet includes all 7 components
Energy content of food measured in
kilocalories
The average teen-ager needs
Approximately 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

4. 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

5. -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
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.

6. 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.

7. 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

8. 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

9. 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)

10. 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

11. 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.

12. 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)