(a) Define the following in (i) elimination (ii) excretion (iii) secretion. (b) Describe the digestion of boiled fish in the gut of humans (c) Make a diagra...

(a) Definitions

• (i) Elimination (egestion): the removal of undigested food materials (faeces) from the alimentary canal through the anus.
• (ii) Excretion: the removal from the body of waste products of metabolism, such as carbon dioxide, urea and excess water and salts.
• (iii) Secretion: the production and release of useful substances (such as enzymes, hormones and mucus) by cells or glands for use in the body.

(b) Digestion of boiled fish (protein) in the human gut

1. Mouth: the fish is chewed and mixed with saliva to form a bolus; no chemical digestion of protein occurs here.
2. Stomach: gastric juice containing hydrochloric acid and the enzyme pepsin acts on the protein. Pepsin digests proteins into peptones and polypeptides. \(\text{Proteins} \rightarrow \text{peptones/polypeptides}\)
3. Duodenum: pancreatic juice adds trypsin, which continues breaking proteins and peptones into smaller peptides.
4. Ileum: intestinal juice contains peptidase (erepsin) which completes digestion, converting peptides into amino acids. \(\text{Peptides} \rightarrow \text{amino acids}\)
5. The soluble amino acids are then absorbed into the bloodstream through the villi.

(c) A labelled longitudinal-section diagram of a villus, 8-10 cm long, is required (showing epithelium, lacteal, blood capillary network, arteriole, venule and goblet cells).