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TEACHERS’ GUIDE
SUBJECT
TOPIC
SUB-TOPIC
CLASS
NO OF STUDENTS
TIME REQUIRED
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BIOLOGY
Excretion and Homeostasis
Excretion in plants
Senior Three
60 Students
Minimum: 120 – 160 minutes (i.e. 3-4 periods)
Brief description of the Unit
This unit explores excretion in plants by discussing the wastes of plants and
how these are got rid of. In addition it shows how these plant waste products
can be used in different ways to make useful products.
Main content and concepts to emphasize
 Plant waste products: carbon dioxide (Co2), Oxygen (O2), water, resins,
tannins, latex.
 Special methods of getting rid of waste products by plants.
 Useful plant waste products.
 Harmful waste products.
Objectives
By the end of this unit the learners should be able to;
(i) Name plant waste products
(ii) Explain the role of stomata in getting rid of water vapour and carbon
dioxide.
(iii)
Describe how plants get rid of other waste products.
(iv)
Explain how some waste products of plants are useful to humans.
(v) Name some waste products which are harmful to humans and other
animals.
Materials required
(i) Apparatus to show plants given off oxygen as by-product of
photosynthesis.
(ii) Charts with list of other waste products and their uses.
(iii)
Manila sheets for use by learners to write uses of waste products,
to humans.
Job related life skills
1.
Personal attributes – self confidence, time management,
creativity/imaginativeness, recording skills
2.
Communication – observation and listening skills,
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3.
4.
5.
Team work – task- oriented, leadership skills
Problem solving - information seeking, environmental protection and
conservation
Application of number - numeracy (as they compare crop yields in
treated and untreated plots)
Introduction
Plants excrete oxygen, carbon dioxide and water vapour. These gaseous waste
products are got rid of by diffusion through the stomata and lenticels. The
oxygen is a waste product of photosynthesis while carbon dioxide is produced in
the process of respiration.
Other waste products of plants include: Tannins, alkaloids, resins, latexes,
excess ions, nicotine, quinine, caffeine, morphine and gums.
The gums, resins and latexes once collected, have a wide range of industrial
applications. From these plant waste products we get useful products such as
turpentine, paints, varnishes, soaps, cosmetics, surgical goods, golf balls,
bubble gum and rubber.
Excretion of plant wastes
 Some ions go into ageing leaves and flowers. These ions are excreted
when ageing leaves fall off the plant.
 Some ions are stored as crystals in the cells’ cytoplasm e.g. Oxalates the
leaves of Oxalis.
 Other waste products are excreted into the roots, stems, bark, flowers
and even fruits, but in a non-toxic form.
However, there are some waste products which are harmful e.g. there is a
bitter variety of cassava, which contains toxic substances in its outer cover of
the tubers. Tobacco plants contain Nicotine, a very dangerous drug. (Find
details about useful and harmful waste products in the teacher’s notes).
Plants do not have complex excretory organs because of the following reasons:
(i)
The gaseous wastes are lost by diffusion through the stomata.
(ii)
Some of the wastes are utilised e.g. oxygen is used for respiration
and carbon dioxide for photosynthesis.
(iii) Plants have a low rate of metabolism; hence the waste products are
produced at a low rate.
(iv) Some wastes are stored in a non-toxic form e.g. resins in barks of
some trees.
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Learners’ Activities
Activity One
To show that plants give off oxygen (O2) as a by-product of photosynthesis
Apparatus required
Beaker
Rain or tap water
Pond weed (Elodea) or spirogyra
Sodium hydrogen carbonate
Glass filter funnel
Plasticine / wooden block
Test tubes
Wooden splint
Apparatus set up
Procedure
Students form groups (5-10)
The experiment is set up as shown in the diagram above. The following
procedure should be followed.
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1.
2.
3.
4.
5.
6.
7.
8.
To a beaker of rain water, (or tap water that has been standing to
eliminate chlorine) add a pinch of sodium hydrogen carbonate to
provide enough carbon dioxide. Place pond weed e.g. Elodea or
Spirogyra into the water.
Cover the pond weed to the brim with an inverted funnel, resting it
on wooden blocks or plasticine for support.
Fill a test tube to the brim with water, place a thumb over the
mouth of the test tube under the water surface in the beaker and
then remove your thumb making sure that little or no air enters the
tube during the operation.
Move the test tube over the funnel stem and lower it gently to rest
on the funnel as shown in the figure above.
Place the apparatus near a window so that it receives enough
sunlight. Record your observations.
Leave the experiment undisturbed until the test tube is half full of
the gas.
Carefully remove the test tube and ensure that the gas does not
escape.
Insert a glowing wooden splint into the gas collected. Note what
happens.
Activity Two
In groups of 5, students should find out other waste products by plants. They
should visit the library and where possible visit botanical gardens of Makerere
University and UWEC at Entebbe to particularly see useful plants. Text books
are another good source of information.
Students should visit research stations for example to see cinchona trees for
quinine e.g. Kawanda, Namulonge, Serere or appropriate places within the
community.
Students can go to tobacco or cigarette making factories
Students can visit factories or hardware shops of industrial implements
made from rubber.
They should record their findings while in the field.
Back to the classroom, they should compile their findings and give feed
back to the rest of class
You should display the results on the notice board.
Activity three
(This is an alternative activity for school settings within Kampala)
You may take the students to visit “Sefa organic” on Carol house, Bombo road
to learn more about plants with medicinal value. If this is not convenient, then
a resource person could be invited to address the class.
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Encourage students to discover about harmful plants in their community. They
should specifically observe dogs, goats, cows (i.e. domestic animals) and the
plants they eat or avoid eating whenever they are sick.
Students should write reports after about two weeks of observation within and
around their communities.
Exercise
Multiple Choice questions
Select the best answer to each question:
1. Which one of the following is not an excretory product?
A. Carbon dioxide
B. Water
C. Faeces
D. Nitrogenous waste
2. Which of the following parts of a plant do not store excretory products?
A. Buds
B. Roots
C. Stems
D. Bark
3. A number of waste products of plants may be found in the following
A. Fruits, leaves and roots
B. Bark, flowers and Guard cells
C. Ovary, pollen tubes and ovules
D. Embryo sac, endosperm and ovary
4. Plants do not have specialised excretory organs like those of animals
because:
A. Much of their waste passes out in solution into the soil
B. Much of their waste passes out in gaseous and non-toxic forms
C. Plants do not take in many chemicals which they do not need
D. Plants do not excrete waste products
5. The part of the flowering plants which carries out the main function of
excretion of gaseous waste products is the:
A. Shoot system
B. Root system
C. Leaves
D. Flowers
6.
Which of the following is a waste product of plants?
A.
Urea
B.
Carbon dioxide
C.
Uric acid
D.
Ammonia
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7.
Which of the following waste products from plants is dangerous to
humans?
A.
Rubber
B.
Colchicines
C.
Nicotine
D.
Quinine
8.
Which of the following waste products may be considered very useful to
humans?
A.
Nicotine
B.
Papain
C.
Cocaine
D.
Cannabis
Short answer questions
6 a)
What is excretion?
b) Name the products excreted by
i)
flowering plants
ii)
mammals
c) Name at least 4 useful waste products and 4 harmful / dangerous waste
products of plants.
Essay question
7a)
Describe an experiment to show that plants give off carbon dioxide
despite the fact that carbon dioxide is utilised by plants in the process of
photosynthesis.
Why don’t plants possess special excretory organs like the animals?
ANSWERS
Multiple Choice Questions
1.C
2.A 3.A
4.B
5.C
6.B
7.C
8.B
Short answer questions
6 a) Excretion is the removal of waste products of metabolism from the cells
of living organisms.
b) (i)
Oxygen, water vapour and carbon dioxide
(ii)
Carbon dioxide, nitrogenous waste e.g. urea) Quinine,
colchicines, rubber (latex), oxygen and papain are useful waste products.
However, Nicotine, cannabis, miraa, cocaine and caffeine are harmful wastes.
Essay
7a)
An experiment to show that plants give off carbon dioxide:
Requirements
Bromothymol blue indicator
Boiling tubes (2)
Aluminium foil paper
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Cork
Elodea plant or spirogyra
Procedure
Obtain elodea plants or filaments of spirogyra from a pond.
Put bromothymol blue in a boiling tube which has been covered with aluminium
foil. Place the plant into the indicator
Set up control, i.e. the indicator in a boiling tube, but without a plant.
Keep the apparatus on the side bench for about 3 hours.
Note and explain the observations made.
Apparatus
The experiment is set up as shown in the diagram above. It is important to set
up a control as well. Hydrogen carbonate indicator gives equally good results if
bromothymol blue is not available.
7b)
Plants do not possess special excretory organs like the animals because:
Most of plant wastes are gaseous and are lost by diffusion through stomata.
The waste product, oxygen, is utilised as a fuel in respiration while carbon
dioxide is used as a raw material in photosynthesis.
There is very little accumulation of toxic wastes e.g. nitrogenous wastes.
The main waste products are formed slowly from the breakdown of
carbohydrates, hence a low rate at which wastes are formed.
A number of wastes are stored in a non-toxic form.
TEACHER’S NOTES
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1.
Useful and harmful excretory products of plants and their economic
importance:
Rubber
Rubber comes from the rubber tree Hevea brasiliensis of the family of
flowering plants, Eurphorbiaceas. Most of which produce latex, a white fluid.
About a million metric tonnes of natural rubber are extracted from rubber
trees every year, mainly in South East Asia. Two thirds of this natural rubber is
used for manufacturing tyres. Aircraft and bull dozer tyres consist almost
entirely of natural rubber.
Other rubber products include water proof clothes, gloves, wire insulation,
stoppers, elastic bands, fan belts, shoe soles, balls and condoms. Rubber is the
only non-conducting compound which absorbs vibration and is water proof,
flexible and elastic.
Tannins
Tannins are organic substances of diverse composition with pronounced
astringent properties that hasten the healing of wounds and inflamed mucous
membranes.
Externally, the bark of oak, Lady’s mantle, Agrimony and Garden sage, are
used for treating varicose ulcers, haemorrhoids and burns, as mouthwashes for
treating inflammation and periodontal disease. Internally, they are used to
treat diarrhoea and biliousness.
Nicotine
Nicotine is obtained from tobacco. The three constituents of tobacco smoke
which do most harm are Nicotine, Carbon monoxide and Tar.
Nicotine is a very powerful drug which affects nearly every organ in the body.
It is addictive and quite poisonous. If the nicotine content of a single cigarette
was injected into a person intravenously, it would be fatal. It is quickly
absorbed into the blood, reaching the brain in 20 seconds. It causes the
platelets to become sticky, leading to clotting of blood. It stimulates
production of adrenaline, leading to increased heart rate and raised blood
pressure, which puts an extra strain on the heart. It may interfere with passage
of impulse across the synapse and at times blocking sensations of pain. If it
goes across the placenta it could damage the foetus. This is because it alters
the heart rate and breathing pattern of the foetus and constricts blood vessels
in the placenta hence leads to slower growth. As such, it leads to an increased
risk of spontaneous abortion, still birth and mental or physical retardation in
later childhood.
Cannabis
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It is obtained from fruits, flowers and leaves of Indian hemp, also known as
marijuana, hashish, bhang, etc
Cannabis will interfere and disturb transmission of impulses across the synapse.
This drug may cause growth retardation if taken by a pregnant woman.
Cocaine
It is obtained from cola leaves.It is another dangerous narcotic drug. It will
cause synapse transmitter e.g. noradrenalin to linger in the synapse causing
excessive activation of neurons and extra information being transmitted around
the brain.
A user of cocaine will feel highly aroused (“Feels high”). Such a person may
also suffer damage to organs including the heart.
Quinine
For over 300 years it has been widely known that the drug quinine obtained
from the bark of the cinchona tree can cure malaria. The bark of this tree is
used locally to cure malaria.
Quinine is one of the oldest anti-malarial drugs. Unfortunately, it is effective
only against the parasites in the red blood cells, and not those in liver cells.
However it is still useful.
Alkaloids
These are found in a number of medicinal plants. They are very efficient and
therapeutically significant plant substances. They are organic nitrogen
compounds which are generally, extremely toxic and for external application
only.
They are used as basic medicinal agents all over the world for their analgesic,
antispasmodic and bacterial effects.
Colchicines
This is an alkaloid substance extracted from the Crocus colchicum. Polyploidy
can be induced experimentally by heat or cold shock or by various chemical
agents such as colchicines. If applied in the correct amounts colchicine
prevents spindle formation during mitosis and so doubles the chromosome
number. The chromosome replicate in the usual way, but the absence of a
spindle means that anaphase fails and when the nuclear membrane reforms,
cells often result with twice the normal number of chromosomes.
Caffeine
This is a drug found in coffee, cocoa and tea leaves. It is a stimulant, but if
taken in small quantities may not be dangerous.
Miraa (khat)
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This is more concentrated in young leaves which are chewed as stimulants. It is
commonly used by long distance drivers to keep awake for long hours. Miraa is
grown in Kenya and some parts of Uganda.
Papain
Commercial papain(proteolytic enzyme) is used as a meat tenderiser.
However, for home use unripe pawpaw is cut into pieces and cooked with tough
meat to soften it.
Allergic reactions occur when using papain for a long time.
2.
UWEC (Uganda Wildlife Education Centre)
Uganda Wildlife education centre is at Entebbe. It remains the most visited
fauna and flora park by domestic tourists. It is a “must see” for any one coming
to Uganda for the first time. It has medicinal plant gardens with over 500 herbs
plus information about the diseases they cure.
3.
GLOSSARY
Resin – a thick sticky fluid that comes out of some trees.
Tannin (also tannic acid) – a reddish acid used in preparing leather; it is also
used in making ink, etc.
Latex – a thick whitish liquid produced by some plants, especially the rubber
tree.
Waste –( waste materials) substances which are unwanted because the good
part of them has been removed. Examples are chiefly water, carbon dioxide
and nitrogenous compounds.
Excretion – it is the removal of the waste products of metabolism of cells from
the body. Excretion is a vital life process.
Homeostasis – the physiological maintenance of the chemical and physical
properties of the body at a constant state.
Morphine – a powerful and addictive drug used for stopping pain and making
people calmer.
Turpentine – a type of oil used for making paint more liquid or removing it from
clothes, brushes, etc.
Gum – a sticky substance found in the stems of some trees.
Lenticels – pores, found on stems, for gaseous exchange.
Metabolism – the chemical activity in your body that uses food to produce the
energy you need to work and grow.
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