IGCSE Biology - Excretion

(I) Excretion (by Almaaz Salie)

Excretion is the removal of toxic materials, the waste products of metabolism and substances in excess to requirements in an organism.

2.70 waste products of plants

In plants, carbon dioxide and oxygen are the waste products of metabolism and they diffuse out of the leaf through the stomata. In plants, carbon dioxide is excreted through respiration; 

Glucose + Oxygen → Carbon dioxide + water

C6H12O6 + 6O2 → 6CO2 + 6H2O (aerobic respiration)

And Oxygen is excreted through Photosynthesis;

Carbon dioxide + water → Glucose + Oxygen

6CO2 + 6H2O → C6H12O6 + 6O2 (photosynthesis)

2.71 Excretory products of organs in Humans

Excretion in humans enables the body to prevent build up of dangerous concentrations. The major excretory products are carbon dioxide, excess water & salts and nitrogenous compounds (amino acids, urea, uric acid, etc.).   

• Carbon dioxide and water are produced in the process of tissue respiration. 

• Nitrogenous compounds are produced by the breakdown of proteins and amino acids. 

The main excretory organs are;

Lungs - remove carbon dioxide and excess water (in vapour).

Skin - provides a surface for small amounts of excess water, salts and urea to move out of the body.

Kidneys - removes substances such as urea and excess water and salts.

2.73B the structure of the urinary system (biology only in bold)

• The inner part of the kidney is called the ​medulla ​and the outer part is called the ​cortex​.

• The ​ureter ​carries urine from the kidneys to the ​bladder ​to be excreted out of the body.

• The kidney is supplied by the​ renal artery​ and a​ renal vein ​takes blood away.

• Each kidney contains millions of kidney tubules or ​nephrons

2.74B the structure of a nephron    

• 1)  The​ renal artery​ transports oxygenated blood to the ​Bowman’s capsule​ under high pressure

• 2)  In the ​glomerulus​ (ball of capillaries within the capsule) the pressure increases even further as the capillaries leaving (​efferent​ arteriole = exiting to renal vein) is narrower than the capillary entering(​afferent arteriole)

• 3)  This pressure leads to ​ultrafiltration - water, salts, glucose and urea pass out of the capillary and into the Bowman’s capsule and into the tubules.

• 4)  Proteins and blood cells are too big to leave the capillaries and so stay within the blood

• 5)  All ​glucose​ is reabsorbed at the​ proximal convoluted tubule​ by ​active transport,​ whilst the rest of the filtrate continues through the tubules. There are many ​mitochondria​ here to provide the energy for active transport.

• 6)  Salts are reabsorbed by the​ loop of Henle

• 7)  Water is reabsorbed into the blood at the​ collecting ducts,​ depending on the water levels in the body (and therefore ADH produced)

• 8)  The remaining filtrate (water, salts and urea) in the collecting duct will form ​urine

• 9)  This is transported through the ​ureters​ to be stored in the bladder and then through the urethra​ to leave the body

2.72B how the kidney carries out its roles of excretion and osmoregulation

Excretion

Excretion is filtering the bad…