,
Diet
Balanced diet
A balanced human diet contains all essential ingredients in the correct proportions
There are certain factors that affect diet such as:
- Age – Children require more protein per kg of body weight than adults
- Gender – Males generally use more energy than females
- Activity – Higher levels of physical activity will increase demand for nutrients
- Pregnancy – Higher demands for nutrients in order to supply fetus with energy for development
- Breast feeding mother – Higher requirements for vitamin and water
Malnutrition
Malnutrition is the lack of a balanced diet such as deficiencies, excesses or imbalances in a person’s intake of nutrients.
Malnutrition can lead to consequences such as:
- Obesity (excessive nutrients)
- Starvation (insufficient nutrients)
- Coronary heart disease (excessive saturated fat and cholesterol)
- Kwashiorkor (too much carbohydrates, too little protein)
- Constipation (lack of fibre)
Essential nutrients
Alimentary canal
Useful definitions
- Ingestion – Taking in of substances e.g. food and drink into the body through the mouth
- Egestion – Passing out of food (as feces) that has not been digested or absorbed via the anus
- Mechanical digestion – Break down of food into smaller pieces without chemical change to food molecules
- Chemical digestion – Break down of large insoluble molecules into small, soluble molecules
- Absorption – Movement of chemically digested food molecules through the small intestine walls into the blood
- Assimilation – Movement of digested food molecules into the cells of the body where they are used and become a part of the cells
Structure and function
Mouth
This is the location of ingestion. Mechanical digestion of food occurs as we chew on it with our teeth. Chemical digestion occurs due to amylase enzymes in our saliva which break down starch into maltose.
Esophagus
Round clumps of food (boluses) are passed down the esophagus via peristalsis from the mouth to the stomach.
Peristalsis is the contraction and relaxation of the esophagus wall muscles which creates a wave-like motion that pushes the food down the canal.
Stomach
Mechanical digestion occurs as the stomach walls squeeze the food to liquefy it.
Gastric juices contain pepsin (a protease) which chemically digests proteins. It also contains hydrochloric acid which kill bacteria, but also maintains an optimum acidic pH for pepsin.
Pancreas
The pancreas secretes pancreatic juice into the duodenum.
The juice contains a large variety of different enzymes involved in the chemical digestion of food.
- Proteases break down proteins
- Lipases break down lipids
- Amylases break down carbohydrates
Most enzymes in the pancreatic juice have an optimum pH of around 7. The pH of pancreatic juice is slightly alkaline, and this is to neutralize the acidity of the food coming from the stomach.
Duodenum
This is the first part of the small intestine. It receives pancreatic juice which contains enzymes for the chemical digestion of food.
Ileum
This is the second part of the small intestine. The inner walls have finger-like extensions called villi which massively increases the surface area for nutrient absorption.
Liver
The liver produces a substance called bile which is stored in the gal bladder.
Bile has the function of emulsifying fat into droplets to increase the surface area for lipases to come and digest them.
Bile is also basic which assists in neutralizing the acidity of the food coming from the stomach.
Colon
This is the second part of the large intestine. The main function of the colon is to reabsorb water from undigested food and also bile salts to return back to the liver.
Rectum
The rectum stores feces until it is egested
Anus
Muscles control egestion of feces
Cholera infection
Cholera bacteria releases toxins which causes chloride ions to be secreted into the small intestine. This causes the osmotic movement of water into the gut, and leads to diarrhea.
NOTE: Diarrhea is defined as the loss of watery feces
Diarrhea can be treated using oral rehydration therapy. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium.
Mechanical digestion
Types of human teeth
Mechanical digestion involves physically breaking food into smaller pieces, and our teeth serve exactly that function!
There are four types of teeth:
- Molar
- Located at back of mouth
- 4 or 5 cusps
- 2 or 3 roots
- Used for chewing and grinding food
- Used for chewing and grinding food
- Pre-molar
- Behind canines
- 2 cusps
- 1 or 2 roots
- Used to tear and grind food
- Canine
- On either side of incisors
- More pointy than incisors
- Used to bite pieces of food
- Incisor
- In front of the mouth
- Chisel shaped
- Used to bite off food pieces
Structure of human teeth
Dental decay
- Dental decay is caused by bacteria which is present on the surface of our teeth.
- The bacteria and food deposits form a layer called plaque.
- Bacteria in plaque feed on sugars which produce acid that dissolves the enamel, resulting in a hole.
- As the hole deepens it may eventually reach the nerves which result in pain i.e. tooth ache.
Proper dental care
- Avoid sugary food so that bacteria cannot make acid
- Regular cleaning to remove plaque
- Use floss to remove trapped food
- Visit the dentist regularly for treating early decay and removal of thick plaque
Chemical digestion
Chemical digestion involves breaking down large, insoluble food into smaller soluble nutrients that can be absorbed and used by the cells.
Chemical digestion is carried out by enzymes. A lot about chemical digestion has already been covered above in regards to the alimentary canal, and what enzymes are found where.
Summary of enzymes that you should be aware of:
Remember, the acidic pH of the stomach is maintained by hydrochloric acid which also kills bacteria via denaturing their enzymes.
The alkaline conditions of the small intestine is maintained by pancreatic juice and bile. The bile also has the function of emulsifying fats into droplets to increase the surface area for digestion
Absorption
After large food molecules get digested by enzymes, the small soluble nutrients diffuse into the small intestine walls and then into the blood.
Sometimes the concentration of nutrients i.e. glucose may be higher in the blood than in the small intestine. In such cases diffusion cannot be relied upon. Instead, active transport is used for absorption.
Absorption is defined as the movement of small food molecules and ions through the wall of the intestine into the blood. It can either be done through diffusion or active transport.
The inner walls of the small intestine have finger-like structures called villi which greatly increases the surface area of absorption.
Structure of a villus
- Blood vessels/capillaries
- Absorbs glucose and amino acids by diffusion
- Lacteal
- Absorbs fatty acids and glycerol
- Epithelial lining
- One cell thick to increase diffusion rate
- Microvilli increase surface area even further