IGCSE Biology Paper-4: Specimen Questions with Answers 153 - 155 of 279


Alcohol can be made by the microorganism yeast.

Question 153 (1 of 2 Based on Passage)


Write in Short

Short Answer▾

State the balanced chemical equation to produce alcohol by yeast.


In anaerobic respiration of carbohydrate by glycolysis apparently 2 ATP molecules are formed per glucose molecule. Therefore, efficiency of anaerobic respiration will be:

Question 154 (2 of 2 Based on Passage)


Write in Short

Short Answer▾

Name the organ that breaks down alcohol in the human body.


Most of the ethanol in the body is broken down in the liver by an enzyme called alcohol dehydrogenase (ADH) , which transforms ethanol into a toxic compound called acetaldehyde (CH3CHO) , a known carcinogen.

Question 155


Describe in Detail


Enzymes, such as proteases, are important in digestion.

Describe in detail how enzymes function, using other digestive enzymes as examples.


Digestion of Protein

The digestion of proteins begins in the stomach and is completed in the small intestine. The enzymes that act on proteins are known as proteases or peptidases.

In the Stomach

  • The semi-digested food rests in stomach for about 4 - 5 hours, then gets mixed with acidic gastric juices by churning movement of muscular wall and is called chyme at this stage. Chyme is the semifluid mass into which food is converted by gastric secretion and which passes from the stomach into the small intestine.
  • Gastric juice contains HCl, pepsinogen, rennin, and lipase.
  • Hydrochloric acid dissolves the bits of food and creates an acidic medium so that pepsinogen is converted into pepsin.

Pepsin is a protein- digesting enzyme. It is secreted in its inactive form called pepsinogen, which then gets activated by hydrochloric acid. HCl provides optimum pH of for pepsin.

The activated pepsin then converts proteins into proteases and peptides

Rennin is a proteolytic enzyme, released in an inactive form called prorennin. Prorennin is activated by HCl in acidic medium to rennin. Rennin plays an important role in the coagulation of milk

The food in the stomach is thick, acidic, and semi-digested and is called chyme.

In the Small Intestine

  • The food from the stomach (chyme) is acted upon by three enzymes present in the small intestine – pancreatic juice , intestinal juice (known as succus entericus- ) , and bile juice .
  • Pancreatic juice and bile reach the duodenum through hepatopancreatic duct.

Action of Pancreatic Juice

Pancreatic juice contains a variety of inactive enzymes such as trypsinogen, chymotrypsinogen, and carboxypeptidases.

These enzymes play an important role in the digestion of proteins. Trypsinogen is present in an inactive form in the pancreatic juice. The enzyme Enterokinin (enteropeptidase) secreted by the intestinal mucosa – activates trypsinogen into trypsin

The activated trypsin then further hydrolyses the remaining trypsinogen and activates other pancreatic enzymes such as chymotrypsinogen and carboxypeptidase

The activated chymotrypsin plays an important role in the further breakdown of the partially-hydrolyzed proteins

Trypsin also helps in breaking down proteins into peptides

Carboxypeptidases act on the carboxyl end of the peptide chain and help in releasing the last amino acids

Thus, in short, we can say that the partially-hydrolyzed proteins present in the chyme are acted upon by various proteolytic enzymes of the pancreatic juice for their complete digestion. Proteins, peptones Trypsin/Chymotrypsin.

Action of Intestinal Juice

Intestinal juice (also known as Succus entericus) contains enterokinase (enteropeptidase) , aminopeptidases and dipeptidases.

Enterokinase converts trypsinogen of pancreatic juice into trypsin

The proteases (Aminopeptidase and dipeptidases) hydrolyze peptides into dipeptides and finally into amino acids.

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