Excretion1

FORM 4 INTEGRATED SCIENCE NOTES 2010/2011
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EXCRETION
Metabolism occurs in the cells of all living organisms because it is necessary for survival. Metabolism
simply refers to all the chemical reactions that take place in the cells. Some of these reactions break down
substances (Catabolism) and others build up large molecules from smaller one (Anabolism). These
processes produce a variety of waste products that must be removed for the organism before the build up
in the cells and become toxic, thus killing the cells.
 Excretion is the removal of waste products that are produced by metabolic reactions such as urea,
carbon dioxide and sweat. Egestion gets rid of the remains of undigested food, egested material is
known as faeces.
WASTE PRODUCT: PRODUCED BY: WHERE
PRODUCED:
WHERE
EXCRETED:
Carbon Dioxide (CO2) Cellular Respiration All living cells From gas exchange
surfaces:Lungs in
mammals, Stomata in
leaves
Water (H2O) Cellular Respiration All living cells From leaves of plants as
water vapour; through
lungs and skin, in urine
and faeces in mammals
Oxygen (O2) Photosynthesis in green
plants
In chloroplasts of plant
cells
Through stomata in
leaves of plants
Nitrogenous
compounds: urea, uric
acid, ammonia
Deamination In the liver From the kidneys in
urine and through the
skin as sweat
Bile pigments Breakdown of
haemoglobin in
mammals
In the liver In the bile, eventually
excreted in faeces
EXCRETION IN HUMANS
 There are three organs responsible for excretion in humans; the lungs, the skin and the kidneys.
Both carbon dioxide and water vapour are excreted from the body during exhalation.
 The skin is the largest organ in the human body and is responsible for excreted waste in the form
of sweat. Sweat contains mostly water, 1% urea, 1%-2% sodium chloride (salt). To aid in this
process, there are many sweat glands on the skin consisting of tubes of sweat ducts which pass
from the blood capillaries beneath the skin up to the surface. A solution of waste products
diffuses from the blood capillary into the base of the sweat gland and the passes out the duct. This
happens because there is a diffusion gradient between the capillaries and the sweat glands.
 The production of sweat is also a means by which to cool the body. This happens by evaporation.
When water evaporates from the skin it draws with it heat energy creating a cooling effect. It
however, leaves behind urea and salt deposits which accumulate on the skin and must be washed
off. If not urea can be broken down by bacterial action which would result in a foul smell.

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The kidneys which are situated at the back of the abdomen are the main excretory organs. Each kidney is
made up of millions on units called Nephrons.
Pressure and Ultra Filtration
 Ultra filtration is the process where a filter separates different size molecules under pressure.
Blood is filtered through the cup of tubule into the Bowman’s capsule. Water, urea, salts and
glucose are forced into the cavity of the tubule. Blood cells and protein molecules are too big to
filter into the cup and remain in the blood transported in the renal artery.
URINE FORMATION STEPS:
1. The arterial that leads to the Bowman’s Capsule has a larger diameter than the blood
vessel that leaves it
2. Pressure builds up inside the glomerulus
3. Smaller parts of the blood are pushed out of the glomerulus through the gaps between the
capillary walls
4. This fluid collects inside the Bowman’s capsule. It is called glomerular filtrate and is
formed by ultra filtration. Glomerular filtrate contains water, salts and urea. Glucose, red
blood cells, white blood cells, plasma proteins, platelets are too big to pass through the
spaces between the capillaries and remain inside the blood.
Selective Re-absorption
 Is where useful substances are re-absorbed from the nephron back into the blood.
 Most re-absorption takes place at the first convoluted tubule. Here 80% of water and 100% of
glucose is re-absorbed. The presence of glucose in urine indicates diabetes.
 After the first convulsion, the fluid in the nephron is called urine and it contains water, urea and
salts.
 The loop of Henle assists in the re-absorption of water. More water is also re-absorbed from the
collecting duct and the second convulsion, so that 170l of glomerular filtrate is produced. Only
1.5l collects in the collecting duct and passes to the ureter, bladder then urethra,then excreted.

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 There is a ring of muscle at the base of the bladder known as the sphincter. This allows the
bladder to hold in urine so that it is not released uncontrollably. In babies however it is not fully
strengthen and baby can only hold a small amount of urine before their bladder releases it.
Incontinence occurs in older folks, where they lose control of the muscle releasing urine
uncontrollably.
 Kidney stones- Kidney stones result when urine becomes too concentrated and substances in the
urine crystallize to form stones. These substances can create small crystals that become stones.
The block the tubule path lack of re-absorption of water can result in dehydration. They are often
not noticed until they move down the ureter tubes, which is quite painful.
 Diabetes-some people are unable to produce sufficient insulin to regulate their blood glucose
level. After a meal a diabetic person can have too much glucose in their blood. This upsets the
concentration of the blood and damages delicate cells, for e.g. kidney cells and cells in the eye.
Some people are born with this disease others develop it. Glucose in the urine is a sign of
diabetes. Diabetes can be controlled by watching your diet (eat one low in carbohydratess),
regular exercise which uses glucose in respiration, tablets to reduce glucose, and intravenous
injections of insulin. Insulin is injected into the bloodstream. This is done because it is a protein
and would be digested into amino acid if taken orally.
 High blood pressure- can damage the kidneys also other disease which prevents them from
removing waste substances effectively. For this reason many people with kidney failure have to
resort to dialysis to help them filter out the waste substance so they do not build up and kill them.
The person is hooked up to machine containing dialysis fluid; their blood is separated from the
fluid by a semi-permeable membrane which allows only the small particles that are in urine to
pass through by diffusion. Glucose and mineral salts also diffuse in, however because diffusion is
multi-directional, they are able to pass back into the blood and the now clean blood re-enters the
body through the vein. For some people instead of living on dialysis, they are able to get a kidney
transplant.
NB: take note of the tables on page 111 (showing contents of waste products in blood and urine) and page
108 (showing water balance over 24hrs).
THE LIVER AND ITS ROLES:
The liver is the largest internal organ. It is situated in the upper right abdomen. It is described as the
chemical factory of the body; it has over 400 chemical functions.
Functions of the liver:
1. It helps with digestion. It makes bile which emulsifies fat which is then broken down by lipids.

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2. It produces heat as a result of the chemical reactions that take place there. This heat is transported
by the bloodstream to the extremities (fingers, arms, legs, etc). This helps to keep all areas of the
body the correct temperature.
3. Detoxification- liver converts many toxic chemicals into harmless ones. Many of the substances
are produced by products of body metabolism (e.g. alcohol) > Alcoholics suffer liver damage
alcohol becomes concentrated on their lives, kills liver cells and are replaced by scar tissue. This
is called cirrhosis Scar tissue does not do the job of the liver cells so alcoholics can die from liver
failure.
4. Makes Urea- this process is called deamination. Excess proteins are broken down into
carbohydrate and ammonia. Ammonia is extremely poisonous and is quickly converted into urea.
This process is necessary because excess protein cannot be stored by the body. Excess proteins
come from the diet and the breakdown of worn cells. The urea is carried by the blood stream to
the kidneys for excretion.
5. It breaks down worn out blood cells- haemoglobin is converted into bile pigments giving faeces
its characteristic brown colour.
6. It acts as a food store- Carbohydrates are stored as glycogen. Vitamins (e.g Vitamin K) and
Mineral (e.g. Iron) are stored in the liver
7. Control of blood glucose level- after a meal containing carbohydrates, glucose is absorbed in the
ileum and carried to the liver via the hepatic portal vein. The hormone insulin stimulates the
conversion of glucose into glycogen. Insulin is produced by the Islets of Langerhans which are
found on the outside of the pancreas. The insulin is carried to the liver by the bloodstream.
 Liver resection - Liver resection is the surgical removal of a portion of the liver. This is usually
done to remove tumors that are located in the liver. The goal of liver resection is to completely
remove the tumor and the appropriate surrounding liver tissue without leaving any tumor behind.

 Liver transplant - Liver transplantation is the only option for those with irreversible liver failure.
Most transplants are done for chronic liver diseases leading to cirrhosis, such as chronic hepatitis
C, alcoholism, autoimmune hepatitis, and many others.
EXCRETION IN PLANTS
 Plants produce few excretory products. This is because:
1. Their metabolism is slower than in animals
2. They make their own proteins. They only produce as much as they require, therefore
do not need to excrete nitrogenous waste. As plants produce little waste, they usually
store it inside the organism.
WASTE SUBSTANCES PRODUCED BY PLANTS
1. The ones stated in the table above as well as:
2. Tannins- dark compounds found in the woody tissues of trees which are no longer used for
transport of useful substances, here they do not interfere with metabolic processes. Some trees
shed their barks getting rid of it one time. E.g. maple tree

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3. Alkaloids- poisonous compounds stored in the leaf. An advantage of this is that it makes it
difficult for herbivores to eat its leaf (defence mechanism).
4. Oxalate- crystals of this substance are nearly always found in the cytoplasm of all plants cells.
Waste products such as 2, 3 and 4 are gotten rid of when the plant loses it leaves.
OSMOREGULATION
Is the maintenance of a constant concentration of the tissue/bodily fluids (primarily water concentration).
It is maintained by a part of the brain known as the Pituitary gland by a feedback mechanism. Compare
temperature and re-absorption.
IN HUMANS:
Salty food eaten or water lost by sweating (low
water concentration, higher salt concentration)
Concentration of blood increases; this is
detected by the hypothalamus
Hypothalamus causes Pituitary gland to secrete
ADH (anti diuretic hormone) into blood
Excess liquid drunk
Concentration of blood decreases; hypothalamus
detects this
Hypothalamus tells the Pituitary gland to stop
producing ADH
ADH causes kidney to re-absorb more water
from urine
No ADH- little water is re-absorbed by kidneys
Small quantities of concentrated urine produced Large volumes of dilute urine produced
IN PLANTS:
Plants can be put into groups based on the environmental conditions they are in because it can affect the
way it obtain, store and prevent water loss. They each have certain adaptation to deal with osmoregulation
based on the conditions they are in. The groups are:
1. Hydrophytes- plants that live in fresh water. They have a constant supply of water, therefore they
do not need special structures for osmoregulation. e.g. water lilies and pondweeds

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2. Mesophytes- mostly your regular flowering plants that live on land and have enough water for
their needs. They have waxy cuticles, protected or small stomata to prevent severe water loss
through transpiration. E.g. hibiscus, sunflower.
3. Halophytes- Plants that are adapted to living in salty condition, some are able to excrete salts
through glands in their leaves. E.g. mangroves
4. Xerophytes- plants that live in dry areas e.g. cacti and oleanders. They have special adaptations
such as; waxy cuticles, fewer and sunken stomata, on curled leaves provided a moist micro –
environment which closed in the day and open at night, fines hairs on the plant surface, small
leaves or spines that reduce surface area for water loss; all of the previous features reduce the rate
of transpiration. They also have underground tubers, succulent leaves and thick stems, all of
which are fleshy which allows the plant to store water for times of shortage. They also have
shallow, widespread, very deep roots; some even have dual root systems, all of which increase
water uptake.
IN AMOEBA:
An amoeba is a single cell organism which lives in fresh water. Its cytoplasm is more concentrated than
its surrounding. Water constantly enters the amoeba by osmosis. It gets rid of excess water using a
contractile vacuole. The vacuole fills up with water,moves to the outside and the water is released.
IN FISHES:
1. For salt water fishes, they are covered in scales and slime which reduces osmosis. The gills
however are exposed. The environment is more concentrated then the tissues of the fish.
Therefore the fish loses water by osmosis and in urine and gained by drinking
2. Fresh water fishes. Water is gained by osmosis because the concentration in the fish tissues isles
than the environment. Water lost in urine, more so than salt water fish.
Drinksalotof water
Special cellsingills
secrete salt
Water lossbyosmosis
Small quantitiesof
concentratedurine produced
Large amountsof dilute
urine produced
Cellsactivelyuptakesalts
fromenvironment
Foodonly,doesnotdrink
Water gainby
osmosis

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