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Human Nutrition


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nutrition&digestion


Summary

The Digestion Process (through the alimentary canal)

1. Mouth
  • Chewing and action of salivary amylase (both chemical and mechanical digestion)
2. Oesophagus
  • Food passes down by peristalsis
3. Stomach
  • Curdling of milk proteins
  • Digestion of proteins by pepsin
4. Small intestine

a. Digestion
  • action of bile
  • action of pancreatic enzymes
  • action of intestinal enzymes
b. Absorption of digested products

5. Colon
  • absorption of water and mineral salts
6. Rectum
  • storage of faeces
7. Anus
  • egestion or defaecation

Peristalsis

  • It is the rhythmic wave-like contractions of the walls of the gut.
  • It is caused by the alternate contractions of the circular and longitudinal muscles (smooth muscles) in the walls. 
  • It helps to push food along the gut.

Digestive juices

 Digestive juice   
 Source Contents
 salivasalivary glands
 salivary amylase and mucin
 gastric juice
gastric glands in stomach
rennin, pepsin, hydrochloric acid
 pancreatic juice
pancreas
 pancreatic amylase, trypsin, lipase
 intestinal juice
intestinal glands in small intestine
 enterokinase, maltase, lactase, sucrase, erepsin, lipase

Digestion of starch, proteins and fats

Starch -------------------------------------------> Maltose --------------------------------------> Glucose
     (amylase in saliva & pancreatic juice)                  (maltase in intestinal juice)

Proteins -------------------------------------> Polypeptides -------------------------------------> amino acids
                    (pepsin in gastric juice)                                (erepsin in intestinal juice)
                   (trypsin in pancreatic juice)

Fats ------------------------------------------------------> fatty acids and glycerol
             (lipase in pancreatic and intestinal juice)

Adaptations of the small intestine for absorption of food substances

  • large surface area
  • richly supplied with blood capillaries and lymphatic capillaries to carry away absorbed food substances - this continual removal maintains diffusion gradient for absorption
  • villi have thin walls (epithelium only one-cell thick)
  • long to provide sufficient time for absorption

MCQ Questions

1. Erepsin converts ________________.
a. proteins to amino acids
b. proteins to peptides
c. peptides to amino acids
d. none of the above

2. An enzyme that acts only in an acidic medium is
a. pepsin
b. trypsin
c. rennin
d. amylase

3. Maximum absorption takes place in the small intestine because of
a. presence of villi
b. length
c. thin walls
d. all of the above

4. In the mouth, food is rolled into
a. chyme
b. chyle
c. bolus
d. pellets

5. Appendix is part of
a. ileum
b. duodenum
c. caecum
d. colon

6. Bile juice is
a. alkaline
b. acidic
c. neutral

7. The enzyme that is secreted in an inactive form is
a. lipase
b. trypsin
c. rennin
d. ptyalin

8. The molecules known as the energy currency of the cell are
a. NAD
b. NADP
c. ATP
d. ADP

9. Which of the following are mainly digested in the stomach?
a. carbohydrates
b. proteins
c. fats
d. lipids

10. Large intestines in man mainly carries out
a. digestion of fats
b. absorption
c. assimilation
d. digestion of carbohydrates

11. The part of the digestive system where no digestion takes place is
a. mouth
b. oesophagus
c. ileum
d. stomach

12. Pyloric valve is present in the
a. heart
b. liver
c. stomach
d. intestine

MCQ Answers

1. c
2. a
3. d
4. c
5. c
6. a
7. b
8. c
9. b
10. b
11. b
12. c

Structured Question Worked Solutions

1. Coeliac disease is a human gastrointestinal disease that is characterised by the destruction of villi in the small intestine.
a. Explain the effect of the destruction of villi in the small intestine on absorption.
b. Suggest and account for signs and symptoms that may arise as a result of coeliac disease.


Solution

a. Function of villi is to absorb nutrients. Villi increase surface area to volume ratio for absorption. If the villi are destroyed, the surface area to volume ratio for absorption will decrease, leading to decreased absorption of digested food into the bloodstream.

b. Villi absorb nutrients. Nutrients are then distributed to the body cells to provide substrates for tissue respiration and new material for growth and repair. When the villi are destroyed, little or no nutrients will be taken in. Body mass will decrease, body functions will become less effective, and energy levels will decrease. Hence a person with coeliac disease will lose weight, be easily fatigued and have insufficient energy and also show dietary deficiency diseases

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