Objective 5.1- distinguish between egestion and excretion.

Excretion is most commonly defined as the removal of metabolic waste (see explanation below) from the body. Meanwhile, egestion refers to the removal of undigested food/ material in the form of faeces.

The syllabus provides the example of the removal of undigested material (egestion) versus the removal bilirubin in faeces, and urea in urine (excretion).

To clarify, bilirubin is a metabolic waste excreted through faeces, even though faeces are themselves egested, not excreted.

It may also be useful to understand what is metabolism, and hence what metabolic waste refers to:

Metabolism is the sum of all the chemical reactions which take place in the cells of the body, while metabolic waste refers to the waste products produced from these reactions. These waste products will be examined further on in this note.

Objective 5.2- discuss the importance of excretion in living organisms

As previously mentioned, excretion is the removal of metabolic waste. If this waste was allowed to remain in the body, its accumulation would be toxic; it would harm the body. Thus, to prevent this build-up of toxicity, this metabolic waste must be removed.

Some metabolic waste products are listed below. Failure to have them excreted would result in a toxic build up, ultimately harming the organism.

In humans:

• Carbon dioxide, a waste product from the process of respiration.
• Heat, produced from metabolism/ metabolic activity.
• Urea, produced from the removal of ammonia from leftover amino acids in the body (referred to as the process of deamination).
• Water, from the process of respiration, excess water is excreted.

In plants:

• Water (Excess water produced by respiration or taken up by the roots is excreted)
• Oxygen (Produced as a waste product from photosynthesis)
• Carbon dioxide (Produced as a waste product during respiration in plants)
• Calcium oxalate (Plant product)
• Tannins (Plant product)

Objective 5.3- state how metabolic wastes are excreted from plants and animals

In animals, excretory organs/ organ systems are involved in the removal of excretory products, as shown below:

Lungs- Excretion of Carbon Dioxide and Water (in the form of water vapour from respiration), through the process of exhalation.

Skin- Water & Salts (in the form of sweat), Excretion of Heat, Excretion of Urea.

Kidneys- Excretion of water & urea (in the form of urine), Excretion of calcium oxalate in humans.

Liver- Excretion of Bilirubin (in the form of urine and faeces, which contain bilirubin).

N.B. Bilirubin can be considered a waste product, as it is formed by the breakdown of haemoglobin from red blood cells. This breakdown occurs in the liver, and it should be noted that bilirubin is the main pigment found in bile.

In plants, excretion may occur through leaf fall, loss of bark and storage in plants. While not mentioned in this objective of the current CSEC Biology Syllabus, excretion of gases also importantly occurs, as will be mentioned below:

1. Leaf fall- when leaves fall, the excretory products stored in them are removed. Calcium oxalate is an excretory product removed by this method.
2. Loss of bark- Plant wastes such as tannins can be stored in bark, which eventually sheds off, thus removing these products.
3. Storage in plants- In plants, some products are rendered harmless such as resins. These are simply stored in the plant as solid bodies.
   
   N.B. Resins may either be stored in bark and lost, or permanently stored in the plant.

Not directly mentioned in this objective of the syllabus, but also important:

4. Transpiration- Excess water can be excreted through the process of transpiration in plants, where water molecules are evaporated from the stomata.
5. Removal through stomatal pores- Both carbon dioxide and oxygen can be removed through the stomatal pores on the leaves of plants.