WAEC Biology 2013 Theory — Question 6

Model answer

(a)(i) Excretion is the removal of waste products of metabolism from the body of an organism. (ii) Excretory organ — waste excreted: Skin — water, salt and urea. Liver — bile salt, water and urea. Lungs — carbon dioxide and water vapour. Kidneys — urea, water and salt. (b) Functioning of the kidney as excretory: blood from the renal artery enters the Bowman's capsule; through the afferent arteriole; ultrafiltration occurs in the Bowman's capsule; high pressure in the glomerulus causes the filtration of small molecules/amino acids/glucose/mineral salts/water/urea out of the blood plasma into the cavity of the capsule; the filtrate/glomerular filtrate passes through the nephron, where selective reabsorption of useful metabolites/glucose/amino acids takes place in the descending/first proximal convoluted tubule; the reabsorption of salts and water is regulated in the loop of Henle, and the ascending/second distal convoluted tubule; large waste molecules like creatine are secreted at the loop of Henle; depending on blood concentration, the filtrate continues into the main collecting duct/tube where further reabsorption of water and salt takes place; the remaining fluid in the duct becomes urine. (c)(i) The excretory organ in insects is the malpighian tubule. (ii) The excretory organ in earthworms is the nephridium (plural: nephridia).