Excretion

“Fear is a reaction, courage is a decision.”

HELLO AGAIN, fellow travellers, how are you this week? I don’t know why I am asking, because I know that you are good and on top of the world! After all, what do you have to worry about? OK, OK, do not crucify me! I was just kidding! The entire world is on your shoulder! Guess what, I am here to help you bear the weight as we go through yet another topic from your syllabus; this time the topic is Excretion.

Unfortunately, there is some misconception, even among biology students, about what the word ‘excretion’ means, because there is a tendency to confuse it with the word ‘defecation’. The first thing that we will address is its definition.

DEFINITION

Excretion is the removal of waste products, produced as a result of the cell’s metabolism, from the body.

What makes the distinction? The products must be produced as a result of metabolism. Remember what metabolism is?

Metabolism covers all the chemical reactions that take place within the cell.

Defecation serves to get rid of undigested food from the body. Do not confuse the two!

The waste products that are produced from metabolism are known as excretory products. Some of these products are toxic (poisonous to the body) and if they are allowed to accumulate (increase in amounts) and remain in the body, they can damage and eventually kill body cells. It follows then that these cannot be allowed to accumulate in the body and must be removed as soon as they are produced.

Another misconception is that only animals carry out excretion, but think about it – if excretion results from metabolism and both plants and animals carry out metabolism, then both must carry out excretion. We will look first at the excretory products of animals and then turn our attention to the plants, where we will not only look at, but study, the excretory products of plants.

EXCRETORY PRODUCTS PRODUCED IN ANIMALS

Remember that these are produced from metabolic activity taking place in the cells. We looked at some of these metabolic activities when we did our studies on digestion and respiration, so the products that are produced should be familiar. For those who have not yet completed these topics, here is a recap.

FROM RESPIRATION

The two excretory products that are produced are:

■ Heat

■ Carbon dioxide

FROM RED BLOOD CELLS

Red blood cells do not remain in our bodies for very long. Reminder: This is because they do not have nuclei. A nucleus is needed to govern the activities of the cell, hence, if there is no nucleus to govern its activities, the cell cannot survive for long periods. After about three months, the red blood cells can no longer serve their purpose and are destroyed in the liver or the spleen. Red blood cells contain haemoglobin; haemoglobin is a protein molecule which contains iron. After the cells die, their protein is converted to amino acids, which are used by the cells of the body. The iron is stored to be used again. Only the bile pigments remain, and these are used to form bile which is stored in the gall bladder to be used in digestion and excreted afterwards.

FROM PROTEIN METABOLISM

■ Proteins, when digested, are converted to amino acids. Not all the amino acids are used by the body.

■ Excess amino acids are deaminated in the liver.

■ Deamination removes the amino group (NH2) which is then converted to urea, and after that to urine, an excretory product.

EXCRETORY PRODUCTS FROM PLANTS

Plants do not produce as many excretory products as animals. What they do produce, they remove from the cell in two ways:

1. By releasing them to the surroundings.

2. By depositing them within their cells.

In plants, carbon dioxide, one of the end products of respiration, and oxygen, an end product of photosynthesis, are two products of metabolism. Reminder: The carbon dioxide can be used in photosynthesis but if all of what is produced from respiration is not used in photosynthesis, then the excess is excreted to the external environment of the plant through the stomata. Much more oxygen is produced in photosynthesis than is used in respiration, so the excess is excreted through the stomata.

OTHER EXCRETORY SUBSTANCES PRODUCED BY PLANTS

■ Waste pigments – these are excreted through leaf fall (the leaves age, they change colours prior to being shed from the trees).

■ Insoluble wastes, e.g., calcium oxalate, found as crystals in the leaf, is also excreted through leaf fall.

■ Oils and alkaloids are stored in leaves, bark, flowers, fruits and seeds. Plants such as citrus (rind of the fruits), pimento (fruits/leaves), cinnamon (leaves and bark), and eucalyptus (leaves) are common examples. These stored products are used commercially as dyes, spices and as oils for perfumes and medicines.

■ The milky white substance in the stems of the oleander plant (Nerium oleander) as well as the stem of the allamanda and plants of the family Euphorbiaceae contains alkaloids. Some of these alkaloids are poisonous (Did you know that goats will not eat these plants, as they know instinctively that they are poisonous?) and some are used commercially in medicine, e.g., quinine, codeine and morphine.

■ Some waste products are transported to structures within the plant that are dead, e.g., tannins to the bark, dyes to the xylem. These substances serve as means of preservation and protection, and also provide useful products to man.

Interesting isn’t it? I bet you never thought of some of these things before this! Biology is fun! Enjoy it!

See you next week as we continue our study.