Excretory Products

Carbon Dioxide

This is a by-product of respiration of both plants and animals. If it is allowed to accumulate in the cells and associated tissue fluid; the pH will be lowered. Since enzymes are pH sensitive, the Carbon dioxide must be removed.

Carbon dioxide is excreted through the pores of the stomata in plants (some of the carbon dioxide produced by respiration is used in photosynthesis). In man, carbon dioxide is transported by the blood from the cells, where it is produced. The blood then travels to the lungs where it enters the alveolus (air sac). Here carbon dioxide diffuses from the blood into the air sacs and leaves the body through exhalation.

Water

The concentration of water in cells must be kept within narrow limits. Too little or too much water can have a negative effect on the osmotic condition in and around the cell. Therefore, it has to be regulated. Plant cells are protected from bursting by their cell walls. Animals do not have cell walls, and will burst if they have too much water. Excess water is lost from the surface of gaseous exchange in both plants and animals. In mammals, water is also lost through sweat and through osmo-regulation controlled by the kidneys.

Urea

This is a compound produced in mammals from the breakdown of excess amino acids. Amino acids cannot be stored because their accumulation is toxic. They are therefore converted into a less toxic substance. This process occurs in the liver and is called de-aminiation.

De-amination results in the amino acid being broken down into 2 parts. One is a carbohydrate or fat where it is used for respiration or stored for later energy use. The other part of the broken amino acid contains ammonia and is highly toxic. Ammonia is converted to urea by the liver.  Urea is transported by blood to the kidneys where they are excreted.

The kidneys are also used to remove uric acid, water, excess salts, excess hormones and bile pigments.

Calcium oxalate

This is a waste material produced by plants and is stored as an insoluble crystalline structure in the cells. Calcium oxalate is stored in aging leaves, stems and roots, flowers or fruits.

Oxygen

Through the process of photosynthesis, oxygen is produced as a by-product. Some of the oxygen is used for respiration, and the remainder is excreted through the stomata of the leaves.

In plants, some waste substances are stored in parts of the plant that are dead. Examples of this are the tannin in the bark of trees such as mangroves and the dyes in the heartwood of trees such as logwood. The purpose of the storage of waste material ranges from protection to a decreased risk of being consumed.