Excretion in plants and animals: waste products and removal

Learning objectives

By the end of this lesson, you should be able to:

• define excretion and distinguish it from egestion
• identify metabolic waste products in plants and animals
• explain how plants remove or store waste substances
• describe the main organs involved in human excretion
• explain why waste removal is essential for homeostasis

Meaning and importance of excretion

Excretion is the removal from an organism of metabolic waste products and substances present in excess of requirements. Metabolic wastes are produced by chemical reactions in cells. If they accumulate, they may become toxic or disturb the internal conditions needed for normal cell activity.

Excretion is different from egestion. Egestion removes undigested food from the alimentary canal as faeces. Excretion removes wastes produced by metabolism, such as carbon dioxide and urea.

Excretory products in animals

Carbon dioxide is produced during aerobic respiration and is removed mainly through the lungs in mammals. Urea is formed in the liver when excess amino acids are deaminated. The amino group is converted into ammonia, which is toxic, and then into urea, which is less toxic and can be transported safely in blood to the kidneys.

Excess water and mineral salts must also be regulated. Some drugs, hormones and other substances are removed through urine. The skin removes small amounts of water, salts and urea in sweat, although its main role is temperature regulation rather than nitrogenous waste removal.

Excretion in plants

Plants produce waste substances during metabolism but do not have specialised excretory organs like mammalian kidneys. Gases such as oxygen from photosynthesis and carbon dioxide from respiration can diffuse out through stomata and other openings. Water vapour is lost mainly through stomata by transpiration; this is important for water movement and temperature regulation, although it is not the same as excretion of all metabolic wastes.

Some waste products are stored in vacuoles or in leaves, bark and old tissues. When leaves or bark are shed, stored wastes are removed from the plant. Certain wastes may be deposited as crystals, resins, gums or latex. Some substances are released into the surrounding soil through roots.

Plants can also reuse or transform some metabolic products. Oxygen from photosynthesis is not simply a useless waste: it can diffuse out, but it is also used by plant cells in respiration.

Main human excretory organs

The lungs remove carbon dioxide and water vapour. The kidneys remove urea, excess water and mineral salts in urine. The liver forms urea from excess amino acids and also detoxifies or modifies various substances. The skin's sweat glands remove water, salts and small quantities of urea.

These organs work together to maintain a stable internal environment. The regulation of water, salts, temperature and chemical conditions is part of homeostasis.

WASSCE examination focus

Define excretion in terms of metabolic waste. Do not list faeces as an excretory product: it is removed by egestion. When discussing plants, state that they lack specialised excretory organs but remove, store or shed waste products through several routes.

Quick check

1. Define excretion.
2. What is the difference between excretion and egestion?
3. Name two metabolic waste products in plants and how they may leave or be stored.
4. Which organ forms urea and which organ removes most urea from blood?

Answers: 1. Removal of metabolic waste and excess substances from an organism. 2. Excretion removes metabolic wastes; egestion removes undigested food. 3. Oxygen can diffuse out through stomata; waste substances may be stored in vacuoles or old leaves and removed when tissues are shed. 4. The liver forms urea; the kidneys remove most of it from blood.