Excretion is the process by which organisms eliminate waste products of metabolism. [1] In single-celled organisms, waste is discharged directly through the cell surface. Multicellular organisms use more complex excretory methods like organs. [2] Plants eliminate gases through stomata and pores, and excess water by transpiration. They also excrete some wastes into the soil. Aquatic animals excrete ammonia directly into water, while land animals convert ammonia into other less toxic materials. Birds excrete nitrogenous waste as uric acid, which is more efficient for water retention. Insects use Malpighian tubules to transport waste to intestines.

1. Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from
an organism. This is primarily carried out by the lungs, kidneys and skin.[1]This is in contrast with secretion, where the
substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life.
In single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize
more complex excretory methods. Higher plants eliminate gases through the stomata, or pores, on the surface of leaves.
Animals have special excretory organs.
Non-mammalian
Chemical structure of uric acid.
In plants, breakdown of substances is much slower than in animals. Hence accumulation of waste is much slower and there are
no special organs of excretion. Green plants in darkness or plants that do not contain chlorophyll produce carbon dioxide and
water as respiratory waste products. Carbon dioxide released during respiration gets utilized during photosynthesis. Oxygen
itself can be thought of as a waste product generated duringphotosynthesis, and exits through stomata, root cell walls, and
other routes. Plants can get rid of excess water by transpiration and guttation. Waste products may be stored in leaves that fall
off. Other waste materials that are exuded by some plants — resins, saps, latexes, etc. are forced from the interior of the plant
by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These processes do not need added energy,
they act passively.[2] Plants also excrete some waste substances into the soil around them. [3]
Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there
is ample water available for dilution. In terrestrial animals ammonia-like compounds are converted into other nitrogenous
materials as there is less water in the environment and ammonia itself is toxic.
White cast of uric acid defecated with the dark feces from a lizard. Insects, birds and some other reptiles also undergo a similar mechanism.
Birds excrete their nitrogenous wastes as uric acid in the form of a paste. This is metabolically more expensive, but allows more
efficient water retention and it can be stored more easily in the egg. Many avian species, especially seabirds, can also excrete
salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak In insects, a system
involving Malpighian tubules is utilized to excrete metabolic waste. Metabolic waste diffuses or is actively transported into the

2. tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal
matter.