Sample Material of Our Online Coaching Programme
Subject: General Science
Topic: The Excretory System
Cells produce water and carbon dioxide as by-products of
metabolic breakdown of sugars, fats, and proteins. Chemical groups such as
nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to
which they were formerly attached, as part of preparing themfor energy
conversion. The continuous production of metabolic wastes establishes a steep
concentration gradient across the plasma membrane, causing wastes to diffuse out
of cells and into the extracellular fluid.
Single-celled organisms have most of their wastes diffuse out
into the outside environment. Multicellular organisms, and animals in
particular,must have a specialized organ system to concentrate and remove wastes
from the interstinal fluid into the blood capillaries and eventually deposit
thatmaterial at a collection point for removal entirely from the body.
EXCRETORY SYSTEMS IN VARIOUS ANIMALS
Excretory systems regulate the chemical composition of body
fluids by removing metabolic wastes and retaining the proper amounts of water,
salts, and nutrients. Components of this systemin vertebrates include the
kidneys, liver, lungs, and skin.
Not all animals use the same routes or excrete their wastes the same way as
humans do. Excretion applies to metabolic waste products that cross a plasma
membrane. Elimination is the removal of feces.
A. NITROGEN WASTES
Nitrogen wastes are by product of protein metabolism. Amino
groups are removed fromamino acids prior to energy conversion. The NH2 (amino
group) combines with a hydrogen ion (proton) to form ammonia (NH3). Ammonia is
very toxic and usually is excreted directly by marine animals. Terrestrial
animals usually need to conserve water. Ammonia is converted to urea, a compound
the body can tolerate at higher concentrations than ammonia.
Birds and insects secrete uric acid that they make through
large energy expenditure but little water loss. Amphibians and mammals secrete
urea that they form in their liver. Amino groups are turned into ammonia, which
in turn is converted to urea, dumped into the blood and concentrated by the
(B) WATER AND SALT BALANCE
The excretory system is responsible for regulating water
balance in various body fluids. Osmoregulation refers to the state aquatic
animals are in: they are surrounded by freshwater and must constantly deal with
the influx of water. Animals, such as crabs, have an internal salt concentration
very similar to that of the surrounding ocean. Such animals are known as
osmoconformers, as there is little water transport between the inside of the
animal and the isotonic outside environment. Marincompositione vertebrates,
however, have internal concentrations of salt that are about one-third of the
surrounding seawater. They are said to be osmoregulators.
Osmoregu-lators face two problems: prevention of water loss
from the body and prevention of salts diffusing into the body. Fish deal with
this by passing water out of their tissues through their gills by osmosis and
salt through their gills by active transport.
Cartilaginous fish have a greater salt concentration than
seawater, causing water to move into the shark by osmosis; this water is used
for excretion. Freshwater fishmust prevent water gain and salt loss. They do not
drink water, and have their skin covered by a thin mucus. Water enters and
leaves through the gills and the fish excretory system produces large amounts of
dilute urine. Terrestrial animals use a variety of methods to reduce water loss:
living in moist environments, developing impermeable body coverings, production
of more concentrated urine. Water loss can be considerable: a person in a 100
degree F temperature loses 1 liter of water per hour.