BIOLOGY TOPICS GRADE 10

1. Environmental
studies
Energy flow and nutrient cycles

2. The sun is the source of all natural energy on earth
Plants are able to absorb the energy from the sun
by using a special pigment called chlorophyll.
The chlorophyll is found in an organelle called
chloroplast.
The plant uses some of this energy for its own activities
like growth, production of organic compounds and for chemical
reactions.
The rest of this energy is converted to chemical potential energy and
stored in the plant.
Energy Flow

3. Plants are able to trap enough energy from the sun for their own
needs and for the needs of the animals.
This is possible because there are more plants than animals.
When herbivores feed on the plants this stored energy is transferred
to the animal’s body.
The animals uses some of the energy for their own activities like
growth, movement, respiration.
The rest of the energy is stored in the animals tissues.
Animals will use more energy than plants because they are more
active.
Energy flow:

4. The energy is transferred from animal to animal as the
animals feed on each other.
The energy is transferred from the herbivores and or
omnivores to the carnivores.
When the plants and animals die the energy still trapped
in their bodies are released by the decomposers.
The decomposers release the energy when they break
down the dead plant and animal matter.
This energy can be used by other plants and animal.
This flow of energy from one living organism to the next is
called energy flow.
Energy flow

5.  A food chain shows the transfer of energy from one organism to the
next: grass worm spider  frog
In reality a food chain never exists alone. This is because a
single organism may provide food to many different organisms.
Therefore in nature there is usually a number of interwoven food
chains: called a food web.
Food chain:

6.  Try to complete this food chain
Something for you to do

7. A trophic level may also be called a feeding level.
All the plants and animals in any habitat can be grouped into
trophic levels.
There are generally four trophic levels.
Tophic level 1 is made up of all the green plants because they
are able to make their own food by photosynthesis. These
plants also have the highest total energy.
Trophic level 2 is made up of all the organisms that feed on the
green plants. They have a slightly lower energy than the green
plants. These organisms are the herbivores .
Trophic levels

8. Trophic level 3 is made up of all the organisms that feed on the herbivores
in trophic level 2.
The organism that make up this trophic level are carnivores. This trophic
level may also be made up of omnivores.
In this trophic level organisms have less energy than the herbivores and
of trophic level 2.
Trophic level 4 is made up of organisms that feed on the carnivores in
trophic level 3.
The carnivores found in trophic level 4 are called top carnivores. They
have less energy than the carnivores of trophic level 3.
This trophic level can be made up of carnivores and omnivores.
Trophic levels

9. Trophic level 1: Producers
Trophic level 2: Primary
consumers
Trophic level 3: Secondary
consumers
Trophic level 4: Tertiary
consumers

10. Can you name the organisms found in the different
trophic levels?
To which trophic level does the producer belong?
Name all the trophic levels that the omnivores can be a
part of.
Something to do

11. The decomposers can occur at any trophic level
depending on which organism they are breaking down.
In a food chain trophic levels occurring in the following
order:
Trophic level 1 trophic level 2 trophic level 3 trophic level 4
Where do the decomposers fit?

12. 1. Which food chain correctly describes the flow of
energy in an ecosystem?
A. grass-->cow-->human
B. caterpillar-->leaf-->human
C. cow-->grass-->human
D. leaf-->bird-->caterpillar
Something for you to do

13. 2. Rabbits eat grass and other plants to survive, but
they do not eat animals. What kind of animal are
rabbits?
A. decomposers
B. carnivores
C. producers
D. herbivores

14. 3. How do decomposers help other organisms in
an ecosystem?
A. They are food for consumers.
B. Through photosynthesis they can make
their own food.
C. They return nutrients back to the soil.
D. They eat plants

15. 4. In what order do a hawk, grass, and rabbit form
a food chain in a meadow?
A. hawk-->grass-->rabbit
B. grass-->hawk-->rabbit
C. rabbit-->grass-->hawk
D. grass-->rabbit-->hawk

16. 5. Which of the following lists only consumers?
A. hawks, lizards, chipmunks
B. acorns, squirrels, rabbits
C. grass, chipmunks, eagles
D. mice, squirrels, grass

17. 1. A
2. D
3. C
4. D
5. A
Solution:

18.  An ecological pyramid is a graphical representation of
the energy or matter contained within each trophic
level.
Ecological pyramids

19. From the diagram on the previous slide it is clear that
there are 3 different types of ecological pyramids.
1. Pyramid of energy
2. Pyramid of numbers
3. Pyramid of biomass.
Ecological pyramids

20. Pyramid of energy
This pyramid shows the
amount of available energy
at each trophic level

21. The available energy gets less and less as we move up the
pyramid.
This means that the producers have the most available
energy while the tertiary consumer has the least.
Therefore the amount of energy available in each trophic
level is less than the previous trophic level.
The decreasing available energy results in the pyramid
shape.
Pyramid of energy

22. There are 3 reasons for the decrease in available energy:
The organisms use some of the energy for their own activities.
Some energy is lost because the organism does not consume
the entire resource.
Some energy is lost as faeces and urine.
Why does the energy decrease at
each trophic level?

23. Pyramid of
numbers This pyramid shows
the number of
organisms in each
trophic level.
Pyramid shape exists
because the number
of organisms
decreases in each
trophic level from
the bottom to the
top.

24. Inverted
pyramid of
numbers
Sometimes the
pyramid of numbers
can be inverted.
This can be because a
single plant
( e.g. fruit seeds) can
provide food for many
organisms.
Similarly many
organisms can provide
food for a single
organism.
Rose bush
aphids
bird

25. Pyramid of biomass
This pyramid
shows the total
mass of
organisms in
each trophic
level.
This pyramid can
also become
inverted, if the
ecosystem
becomes
imbalanced.

26. Study the following paragraph and answer the questions
that follow:
A baboon was seen lifting stones and looking for food in a patch of
grass. The baboon found some beetles and ate them. Every so
often, the baboon would scratch himself, because he had a large
number of fleas on his body
1. Draw a food chain which clearly illustrates the feeding relationships
described above. [4]
Assessment task

27. 2.Draw a pyramid of numbers to illustrate the
description above. [4]
3.What is a pyramid of numbers? Comment on the
shape of this pyramid of numbers in your
answer. [3]
4.Draw a food web which illustrates a feeding
relationship between at least ten organisms.
Include the four organisms in the food chain you
drew in number 1. [10]

28. 5.Choose one organism in the food web you drew in
number 4 and draw a circle around this organism.
Imagine that this organism was eliminated (removed)
completely from the feeding relationship. What
impact would this have on the rest of the food web?
Explain fully. [4]
[25 marks]

29. 1. Grass  Beetle  Baboon  Flea
2. pyramid
Solutions

30. 3. A pyramid of numbers shows the relative numbers of
organisms  in a particular food chain;  usually a pyramid has
the smallest portion on top, this one does not. 
4. Learner responses will differ. Mark according to the web they
produce. Check that arrows flow in the correct direction (in the
direction of energy flow) and check that links are realistic. One
mark per organism correctly inserted in the web.
5. Learners will vary. However they should indicate that the
ecosystem will become imbalanced. They should indicate how
trophic level before and after the chosen one is altered.
Solutions

31. Nutrient recycling is the process that ensures that
nutrients become available for reuse.
Some examples of these important nutrients are carbon
dioxide, water vapour, oxygen, minerals and energy.
Nutrient recycling occurs in the following general way.
Nutrient recycling

32. During photosynthesis green plants use sunlight, carbon
dioxide and water to make food in the form of glucose.
When the primary consumers (herbivores and omnivores)
feed on the green plants the food is transferred to their
bodies.
Remember this food contains carbon dioxide, water, and
energy.
When the secondary consumer (carnivores) feed on the
primary consumer the food is transferred to their bodies
from the primary consumer.
Nutrient recycling occurs in the
following general way.

33.  When any of these organisms die (producer, primary,
secondary or tertiary consumer) their bodies are broken
down by decomposers.
 Decomposers release nutrients so that they can be
reused.
 The decomposers releases carbon dioxide and energy in
the form of heat into the atmosphere and water vapour
and mineral salts into the soil.
 Without nutrient recycling important nutrients will
become exhausted.
Role of decomposers in nutrient recycling

34. Water cycle

35.  Water cycle ensures that water supply is never
exhausted.
The water cycle occurs as follows:
 Water vapor from the atmosphere falls to the ground.
 This water vapors can fall to the earth as rain, dew,
frost, mist and hail: precipitation.
 Some of this water reaches the soil and then runs off
into the rives and streams
 Some of this water will also reach the dams, lakes and
oceans.
Water cycle

36.  The ground water may soak into the soil.
One of two things may happen to this water:
 It may remain in the soil as capillary water around the soil particles or
it may reach the water table.
 Plants absorb the water from the soil using their roots.
 Animals obtain their water from the rivers, lakes and streams or from
the food they eat.
 This water is then used by plants and animals for various functions.
 Plants lose some of the water as water vapour during the process of
transpiration.
 Animals also lose some water as water vapour during the process of
respiration.
 Animals also lose water during sweating and breathing.
Water cycle

37.  When the ground, air or water surface becomes heated
the water changes from a liquid into a gas.
 This gas is called water vapour.
 The process by which water changes from a liquid to a
gas is evaporation.
 The water vapour then rises into the atmosphere with
the warm air that rises.
 As the water vapour goes to higher altitudes cooling of
this water occurs.
Evaporation

38.  As the water vapour goes to higher altitudes cooling of this water occurs.
 When the water vapour cools it changes from a gaseous state to a liquid
state: condensation.
 This results in the formations of clouds.
 The air becomes saturated when more and more water collects at this
level, clouds will form.
 The clouds then become heavy ( the clouds are holding the maximum
amount of water in a specific volume at a specific temperature.)
 Then the water begins to fall as precipitation.
 The cycle is completed and begins all over again.
Condensation

39.  Water is an inorganic compound made up of hydrogen
and oxygen.
 The elements hydrogen : oxygen occur in the following
ratio 2: 1.
 Two thirds of the earth’s surface is made up of water.
 About 70 to 80% of the protoplasm of both plant and
animal cells is made up of water.
 Water is also regarded as a universal solvent.
 It has many important uses in organisms and in nature.
 It is the medium of most chemical reactions.
Importance of water:

40.  It is used to transport nutrients, gases and waste
products in both plants and animals.
 It is also a habitat for plants and animals.
 It is a reactant in photosynthesis.
 It is a by-product during the process of cellular
respiration in both plants and animals.
 Water is used to cool the body down.
 Water is also used during digestion of food.
Importance of water:

41.  The concentration of carbon in the air 0,03%.
 The carbon in the form of carbon dioxide is used by
green plants to manufacture food by photosynthesis.
 Carbon dioxide is released by both plants and animals
during the process of cellular respiration.
 While photosynthesis uses up carbon dioxide, cellular
respiration releases it.
 In this way the levels of carbon dioxide is maintained.
Carbon cycle

42. Carbon cycle

43.  Carbon dioxide is absorbed by green plants from the
atmosphere and water to be used during
photosynthesis.
 The carbon is used to make glucose and then other
organic material.
 When the consumers (herbivores, omnivores and
carnivores) feed on the plant that organic material is
transferred to the to these consumers.
 The carbon is now present in the bodies of both plants
and animals.
Carbon cycle

44. The carbon occurs in organic compounds such has carbohydrates,
proteins, lipids, hormones, pigments, enzymes etc.
REMEMBER: organic compounds contains carbon.
 During cellular respiration all living organisms release carbon
dioxide.
 The carbon dioxide is released into the water or atmosphere.
 When plants and animals die decomposers break down their
bodies and release carbon dioxide into the atmosphere or soil.
Carbon cycle

45.  Sometimes carbon dioxide becomes trapped in the
bodies of dead plants and animals when these
organisms do not decompose.
 These dead bodies become compressed and form
fossil fuels such as coal, oil and gas.
 When these fossil fuels are burnt the carbon dioxide
trapped in them is released.
The cycle then starts over.
Carbon cycle

46.  Oxygen is required by both plants and animals for
cellular respiration.
 Oxygen is found in the atmosphere and dissolved in the
water.
 The concentration of oxygen in the atmosphere is 21%.
Oxygen cycle

47. Oxygen in the
water and
atmosphere is
Used by plants
and animals
during cellular
respiration to
Release
carbon
dioxide
which
enters the
water and
atmosphere
Carbon dioxide is
used by plants
during
photosynthesis
To release
oxygen
Oxygen cycle
Combustion of
Fossil fuels

48.  Oxygen found in both the atmosphere and water is used by both
plants and animals during cellular respiration to release the energy
trapped in food.
 Oxygen is also used during the combustion of fossil fuels.
 The levels of oxygen decreases.
 Both combustion and cellular respiration releases carbon dioxide .
 This carbon dioxide is used by plants during photosynthesis to make
organic compounds.
 During photosynthesis oxygen is released and the levels of oxygen
return to normal.
The cycle begins once more.
Oxygen cycle

49.  About 79% of the atmosphere is made up of nitrogen.
 Nitrogen is required to manufacture proteins by both plants
and animals.
 Plants cannot absorb nitrogen as it occurs in the atmosphere.
 The nitrogen must first be converted to nitrates before the
plants absorbs it.
 Animals obtain their nitrogen when they feed on plants or
feed on animals that feed on plants.
Nitrogen cycle

50. Nitrogen cycle

51.  Since plants can only absorb nitrogen in the form of nitrates,
atmospheric nitrogen must first be converted into nitrates before it is
absorbed by the plants.
 This conversion can take place in 3 ways:
1. Lightening can convert the nitrogen into nitrate.
2. The root nodule bacteria found in the roots of some plants may
convert nitrogen gas into nitrates which is absorbed by the plant. The
plant in turn provides the bacteria with food.
3. Nitrogen fixing bacteria found in the soil converts the nitrogen in the
soil into nitrate which then can absorbed by the plants.
Nitrogen cycle

52.  Plant then uses the nitrogen to manufacture proteins.
 Nitrogen is transferred to animals when they feed on plants or
animals that have fed on plants.
 When the plants and animals die the decomposer breaks
down the dead and decaying matter to release ammonia.
 The ammonia is converted into nitrites by nitrifying bacteria.
 The nitrifying bacteria than converts the nitrates into nitrates.
Nitrogen cycle

53. One of two things may happed to the nitrates:
 It maybe absorbed by plants to manufacture
proteins.
 Denitrifying bacteria may convert the nitrates
back into nitrogen and the cycle continues.
Nitrogen cycle

54.  This flow of energy from one living organism to the next is
called energy flow.
 A food chain shows the transfer of energy from one
organism to the next organism.
 A trophic level can also be called a feeding level.
 An ecological pyramid is a graphical representation of the
energy or matter contained within each trophic level.
 Pyramid of energy shows the amount of available energy at
each trophic level
TERMINOLOGY

55.  Pyramid of numbers shows the number of organisms in each
trophic level.
 Pyramid of biomass shows the total mass of organisms in
each trophic level.
 Nutrient recycling is the process that ensures that nutrients
become available for reuse.
 Precipitation is when water vapors can fall to the earth as
rain, dew, frost, mist and hail.
 The process by which water changes from a liquid to a gas is
called evaporation.
 Condensation is when the water vapour cools it changes
from a gaseous state to a liquid state.
Terminology

56.  Water is an inorganic compound made up of hydrogen and
oxygen.
 Nitrogen fixing bacteria converts the nitrogen in the soil
into nitrate which then can absorbed by the plants.
 The nitrifying bacteria converts the nitrates into nitrates.
 Denitrifying bacteria converts the nitrates back into
nitrogen and the cycle continues.
Terminology