Course: Integrated Science ECLPE, Topic: UNIT 9: AUTOTROPHIC NUTRITION

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UNIT 9: AUTOTROPHIC NUTRITION
Key unit competence:
Explain photosynthesis as an energy transfer process, its limiting factors
and adaptations.
Introductory Activity
Make a quick lab
Materials: Large clear plastic cup, sodium bicarbonate solution, elodea
plant, large test tube.
Procedure:
– Fill a large clear plastic cup with sodium bicarbonate solution (source
of CO₂)
– Place an elodea plant in a large test tube with the cut stem at the
bottom. Fill the tube with sodium bicarbonate solution. Caution: hand
the test tube carefully.
– Hold your thumb over the mouth of the test tube. Turn the tube over,
and lower it to the bottom of the cup. Make sure there is no air trapped
in the tube.
– Place the cup in bright light.
– After at least 20 minutes, look closely at the elodea leaves. Record
your observations.
Analyze and conclude:
a) What do you observe on the elodea leaves?
b) What substance accumulated in the leaves? Should the substance
be considered as a waste product? Explain.
c) What plant organelle carries out photosynthesis and produces the
gas?
All organisms require macromolecules like carbohydrates, proteins and
fats for their growth and development. Some organisms produce these
organic compounds from inorganic sources on their own. Such organisms
are called autotrophs or producers and the process of synthesizing complex
compounds from simple inorganic sources is called autotrophic nutrition.
While others including humans are heterotrophs or consumers, which depend
on autotrophs for source of chemical energy. Green plants are autotrophs
and require chlorophyll, sunlight, carbon dioxide, water and minerals for
preparing their own food.
9.1. Types of autotrophic nutrition
Activity 9.1
From what you learnt in previous classes about plant nutrition, differentiate
the types of autotrophic nutrition.
Autotrophic nutrition is a process by which living organisms make their
own food. This process is carried out by photoautotrophs like green plants,
green algae and green bacteria; and chemoautotrophs. Living organisms
which make their own food are called autotrophs, while others, including
humans, which cannot make their own food but depend on autotrophs are
called heterotrophs.

There are two types of autotrophic nutrition such as chemoautotrophic and
photoautotrophic nutrition.

9.1.1. Chemoautotrophic nutrition
It is an autotrophic nutrition where organisms (mainly bacteria) get energy
from oxidation of chemicals, mainly inorganic substances like hydrogen
sulphide and ammonia.
9.1.2. Photoautotrophic nutrition
It is an autotrophic nutrition where organisms get energy from sunlight and
convert it into sugars. Green plants and some bacteria like green Sulphur
bacteria can make their own food from simple inorganic substances by a
process called photosynthesis. Photosynthesis is a process by which,
autotrophs make their own food by using inorganic substances in presence
of light energy and chlorophyll.
(Green Sulphur bacteria).
(Green plants).
Application activity 9.1
1. Define Photosynthesis
2. Differentiate;
a) Autotrophs and heterotrophs
b) Chemoautotrophs and photoautotrophs.
3. Animals’ life depends on plants. Defend this statement by providing
two convincing reasons.
9.2 Structure adaptation and role of chloroplast in the
process of photosynthesis
Activity 9.2
To show that oxygen is produced during photosynthesis
Requirements:
Two large beakers, two funnels (glass), two test tubes, water with sodium
hydrogen carbonate dissolved in it, splints, match box, water weed e.g.
Elodea or Spirogyra
Procedure 1: Prepare two set-ups of apparatus as shown below.
Note:
Set up A placed in a dark cupboard Set up B placed in a bright sunshine
2. Observe the set-up in the dark cupboard.
• What did you notice?
3. Observe the set-up in the bright sunshine.
• What do you notice?
4. Test any gas produced using a glowing splint.
Study questions
a) Explain the necessity of sodium hydrogen carbonate (sodium
bicarbonate) dissolved in the water?
b) What happens to the glowing splint when it is exposed to the gas
in the test tubes?
• What is your conclusion from the observation?
c) What was the role of the setup that was placed in the dark cupboard?
d) Name the plant cell organelle in which photosynthesis takes place.
Plants are autotrophs because they can make their own food by using
energy from the sun, carbon dioxide and water as raw materials to make
food in a process known as photosynthesis. The chlorophyll contain by
plants traps light energy from the sun. In the process, oxygen is given off as
a by-product.
The process of photosynthesis can be summarized as follows:

The chlorophyll arefound in chloroplasts.
Chloroplast is an example of a plastid. It is the organelle in a plant cell where
photosynthesis takes place. Chloroplasts are found in the cytoplasm of the
cells found in either palisade cells mesophyll, spongy mesophyll and guard
cells in a leaf. Cells that have chloroplasts are called photosynthetic
cells. To find out whether the leaf is the site for photosynthesis, we test for
the presence of starch in the leaf.
9.2.1. Structure of the chloroplast
In eukaryotes photosynthesis takes place in chloroplasts which is one of
plant cell organelles. A chloroplast contains many sets of disc like sacs called
thylakoids, which are arranged in stacks known as grana. Each granum
looks like a stack of coins where each coin being a thylakoid. In the thylakoid,
proteins are organized with the chlorophyll and other pigments into clusters
known as photosystems. The photosystems are the light-collecting units of
the chloroplast.
The function of thylakoids is to hold the chlorophyll molecules in a suitable
position for trapping the maximum amount of light. A typical chloroplast
contains approximatively 60 grana, each consisting of about 50 thylakoids.
The space outside the thylakoid membranes are made by watery matrix
called stroma. The stroma contains enzymes responsible for photosynthesis.

Note: Photosynthetic prokaryotes have no chloroplasts, but thylakoids often
occur as extensions of the plasma membrane and are arranged around the
periphery of the prokaryotic cell.
9.2.2. Adaptations for photosynthesis
Activity 9.2.2
Sample a green leaf and analyze its structure. Observe again the illustration
showing the internal structure of a leaf to describe the adaptations of the
plants and leaf for photosynthesis.
By considering both external and internal structures of the leaf, we can
recognize several adaptations for photosynthesis.
a) Adaptation of leaf for photosynthesis considering to its internal
structure

Note: when stomata are opened, the rate of photosynthesis may be 10
to 20 times as fast as the maximum rate of respiration. If the stomata are
closed, photosynthesis still can continue, using CO₂produced during cell
respiration. The equilibrium can be reached between photosynthesis and
cell respiration.
Photosynthesis uses CO₂ from respiration, and respiration uses Oxygen
from photosynthesis. However, the rate of photosynthesis under these
circumstances will be much slower than when an external source of CO₂ is
available. The stomata cannot remain closed indefinitely, they have to be
open in order to maintain transpiration of the plant.
b) Adaptation of leaf for photosynthesis considering its external
structure

– Leaves are thin and flat, this facilitate absorption of the maximum
amount of light.
– The cuticle is transparent to allow absorption of light into tissues.
– Presence of a waxy substance on the cuticle to prevent excessive
water loss from photosynthetic tissues.
– Presence of the midrib and veins containing vascular tissues like: the
Xylem which brings water and minerals from soil to photosynthetic
tissues, and Phloem which carry away manufactured organic food from
photosynthetic tissues to other parts (translocation).
– Having the leaf stalk which holds the lamina in a good position to
receive the maximum amount of the light.
9.2.3. Absorption and action spectra
In addition to water and CO₂, photosynthesis requires light and chlorophyll.
The chlorophyll pigment is found in the chloroplasts. The light that our eyes
perceive as white light is a mixture of different wavelengths. Most of them
are visible to our eyes and make up the visible spectrum. Our eyes see
different wavelengths of visible spectrum as different colours (violet, blue,
green, yellow, orange and red) except indigo which is not visible to our
eyes. Plants absorb the light energy by using molecules called pigments
such as: chlorophyll a, chlorophyll b, carotene (orange), xanthophyll
(yellow) and phaeophytin (grey) but chlorophyll a is the principle pigment
in photosynthesis.
The chlorophyll absorbs light very well in blue-violet and red regions of
visible spectrum. However, chlorophyll does not absorb well the green
light, instead it allows the green light to be reflected. That is why young
leaves and other parts of the plants containing large amount of chlorophyll
appear green.

The chlorophyll a as a principle and abundant pigment, it is directly
involved in light reactions of photosynthesis. Other pigments (chlorophyll
b, carotene, xanthophyll and phaeophytin) are accessory pigments. They
absorb light colors that chlorophyll a cannot absorb, and this enables plants
to capture more energy from light.
The amount of energy that the pigment can absorb from the light, depends
on its intensity and its wavelengths. So, the greater the intensity of light,
the greater amount of energy will be absorbed by the pigment in a given
time.
9.2.4. Calvin cycle and the process of photosynthesis in C3
plants
a) Stages and sites of photosynthesis in a chloroplast
The process of photosynthesis occurs through two main stages such as:
– The light-dependent reactions: which take place in thylakoids, and
– The light-independent reactions (Calvin cycle): which take place in
stroma.
Table 9.4: Comparison between light-dependent reactions and The light independent
reactions (Calvin cycle)

i. The light-dependent reactions
They require light energy and occur in thylakoids. They produce Oxygen
gas and convert ADP and NADP+ into ATP and NADPH.
The light-dependent reactions involve the following steps:
• Photosynthesis begins when the chlorophyll a in photosystem II
absorbs light at different wavelengths of light.
– When the light energy hits the chlorophyll a, the light energy is absorbed
by its electrons, by raising their energy level.
– These electrons with high potential energy (electrons with sufficient
quantum energy) are passed to the electron-transport chain.
– Excited electrons are taken up by an electron acceptor (NADP+:
oxidized Nicotinamide Adenine Dinucleotide Phosphate), and pass
along electron transfer chain from photosystem II to the photosystem
I. (Note: The photosystems are the light-collecting units of the
chloroplast).
• Enzymes in thylakoids and light absorbed by photosystem II are
used to break down a water molecule into energized electrons,
hydrogen ions H+, and Oxygen.

– Oxygen produced is released to be used by living things in respiration.
– Electrons and H⁺ from photolysis of water are used to reduce NADP+
to NADPH (Reduced Nicotinamide Adenine Dinucleotide Phosphate).
– The light-dependent reactions also allow generation of ATP (Adenosine
Triphosphate) by adding inorganic phosphate to ADP+ (Adenosine
Diphosphate):

Generally, the light-dependent reactions use light energy, ADP, Pi, NADP+
and water to produce ATP, NADPH and Oxygen. Or simply:
Both ATP and NADPH are energy carriers which provide energy to sugars
(energy containing sugars) in Light-independent reactions.
ii. The light-independent reactions (Calvin cycle)
The light-independent reactions occur in stroma, and consist of reducing
CO₂ into sugars by using ATP and NADPH both coming from light-dependent
reactions in thylakoids. The Calvin cycle involves three main stages such as:
– Carbon fixation in form of CO₂.
– Carbon reduction from CO₂ to glucose.
– Regeneration of RuBP.
• Carbon fixation (Carboxylation) in form of CO₂
Carboxylation: is the process of fixation of carbon in stable organic
intermediate, phosphoglyceric acid.
The Calvin cycle begins with a 5-Carbon sugar phosphate called Riburose-1,
5 biphosphate (RuBP) which fixes the CO₂ from airThis reaction is
catalyzed by called RuBPcarboxylase-oxygenase (RUBISCO). Rubiscobis-
phospahte (RuBP) is the initial acceptor or substrate for dark reaction.
• Carbon reduction from CO2 to glucose
With energy from ATP and reducing power from NADPH, the phosphoglyceric
acid is reduced into 3carbon molecules known as glyceraldehyde-3-
phosphate or phosphoglyceraldehyde (PGAL).
Each molecule of PGA receives an additional phosphate group from ATP,
becoming 1, 3-biphosphoglycerate, and a pair of electrons and H⁺ from
NADPH reduces the carboxyl group of 3-phosphoglycerate to the aldehyde
group of PGAL which stores more potential energy.
ATP gives one phosphate group becoming ADP+, and NADPH gives H+ and
electrons to become NADP+. Both ADP+, and NADP+ will be used again in
light-dependent reactions.
With 6 turns of Calvin cycle, the plant cell fixes 6CO2 molecules which are
used to synthesize 2 molecules of PGAL which leave the cycle and combine
to make one molecule of glucose or fructose. This glucose can be converted
into:
– Sucrose: when Oxygen combined with fructose. It is a form by which
carbohydrates are transported in plants.
– Polysaccharides like starch for energy storage, and cellulose for
structural support.
– Amino acids when combined with nitrates,
– Nucleic acids when Oxygen combined with phosphates, and
– Lipids.

• Regeneration of RuBP
The remaining ten 3-carbon molecules (PGAL) are converted back into six
5-carbon molecules, ready to fix other CO₂ molecules for the next cycle. The
light-independent reactions can be summarized as:
Photorespiration
In most plants, initial fixation of carbon occurs via Rubisco, the Calvin cycle
enzyme that adds CO₂ to ribulose biphosphate. Such plants are called C₃
plants because the first organic product is a three carbon organic compound,
PGA. These plants produce less food when their stomata close on hot and
dry days.
The declining level of CO2 in the leaf starves the Calvin cycle. Making matter
worse, Rubisco can accept O2 in place of CO2. As O2 concentration overtakes
CO2 concentration within the air space, Rubisco adds O2 instead of CO2.
The product splits and one piece, a two-carbon compound is exported from
the chloroplast. Mitochondria then break the two-carbon molecule into CO2.
The process is called photorespiration because it occurs in presence of
light (photo) and consumes O2(respiration). However, unlike normal cellular
respiration, photorespiration generates no ATP, and unlikephotosynthesis,
photorespiration generates no food. In fact, photorespiration decreases
photosynthetic output by using material from the Calvin cycle.
Application activity 9.2
1. Describe the structure of a chloroplast.
2. What may happen to the rate of photosynthesis in a photosynthetic
cell if the thylakoids in chloroplast are damaged completely?
3. Explain adaptations of both thylakoid and stroma for their functions.
4. Relate the internal structure of the leaf with the process of
photosynthesis
5. Explain the involvement of the plant parts bellow in the process of
photosynthesis a) Stomata b) Lamina c) Leaf stalk d)
Leaf cuticle e) Xylem f) Phloem
6. Why are light and chlorophyll needed for photosynthesis?
7. Describe the relationship between the chlorophyll and the color of
plants.
8. How well would a plant grow under pure yellow light? Explain your
answer.
9. Appreciate the presence of accessory pigments in leaves for the
process of photosynthesis.
10. Differentiate the light-dependent stage and light-independent
stage of photosynthesis.
11. Relate the structure of the thylakoid with its function.
12. Explain the stages of the Calvin cycle.
9.3 Rate of photosynthesis: limiting factors of photosynthesis
and importance of autotrophic nutrition.
9.3.1. External factors that affect photosynthesis
Activity 9.3.1.a
Aim: To show effect of carbon dioxide on the rate of photosynthesis.
Materials Required: Elodea, beaker, NaHCO₃, lamp.
Procedure: Place a pond weed Elodea upside in a test tube containing
water at 25°C. Place the tube in a beaker of fresh water. Place excess
sodium bicarbonate (NaHCO₃) in the water to give a constant saturated
solution of CO₂.
Place the lamp at a fixed distance from the plant. Maintain the room
temperature at 20°C. Count the number of oxygen bubbles given off by the
plant in a one minute period.
Observation: The bubbles are formed of oxygen.
Discussion: Discuss why was NaHCO₃added to water.
1. CO₂ concentration: Carbon dioxide is the inorganic substrate
for photosynthesis. Increase in concentration up to 0.05% in
atmosphere can cause an increase in CO₂ fixation. Carbon dioxide
is the major limiting factor, especially in C-3 plants; C-4 plants are
more productive even at low concentration of CO₂. Nevertheless,
both C-3 and C-4 plants show increase in rate of photosynthesis at
high CO₂ concentration and high light intensities. The fact that C-3
plants respond to higher CO₂ concentration by showing increased
rates of photosynthesis leading to higher productivity has been used
for some green house crops such as tomatoes and bell pepper.
They are allowed to grow in carbon dioxide enriched atmosphere as
in glasshouses leading to higher yields.
2. Light: Light is an important factor to carry out photosynthesis. It is
rarely a limiting factor in nature as photosynthesis can occur even at
low light intensities. There is a direct relation between light and CO₂
fixation. With increase in light intensity the rate of photosynthesis
increases. However, at higher light intensities, rate does not increase
linearly but light saturation occurs. At very high light intensity, there
is breakdown of chlorophyll molecules called photo-oxidation and
the rate of photosynthesis decreases. The quality of light and time
of exposure also governs photosynthesis. Green plants show high
rate of photosynthesis at red and blue light.
Light intensity

3. Temperature: The dark reactions are dependent on temperature
as they are enzymatic. Rate of photosynthesis is best at optimum
temperature. Different plants have different temperature optima that
also depend on their habitats.

1. Water: Only about 1% of water absorbed by plants is used in
photosynthesis. It is an important factor for various metabolic
processes in plant. Water may not have direct affect on photosynthesis
even though it is one of the reactants in light reaction. In water stress
plants wilt and their stomata close. Thus reducing availability of
carbon dioxide and decreasing the rate of photosynthesis. Water
stress will also alter the hydration of enzymatic proteins, affecting
their activities.
2. Oxygen concentration: Atmospheric oxygen content affects
photosynthesis directly or indirectly. The decrease in rate of
respiration at high oxygen concentration was first observed by O.
Warburg in 1920 in Chlorella. The phenomenon is called Warburg
effect.
3. Chemical pollutants: Plant growth has been adversely affected by
accumulation of various undesirable chemicals. Heavy metals such
as lead, mercury, cadmium seem to be affecting photosynthesis
through stomata closure. Air pollutants like SO2, NO2 and O3 are
also known to affect photosynthesis at higher concentrations.
9.3.2. Internal Factors
1. Adaptation of leaf: Leaves are arranged on plants to minimize
overlapping. The shape, size, age and orientation of leaf influences
the absorption of light and thus effects photosynthesis. Most leaves are
broad for more absorption of light. The anatomy of leaf is also highly
specialized for absorption of light. The epidermis is transparent and also
acts as convex lens to focus and intensify light reaching mesophyll cells
for maximum absorption.
Application activity 9.3.a
1. Use the graphs to explain how the limiting factors below may
influence the rate of photosynthesis:
a) Temperature
b) Light intensity
c) Concentration of CO₂ in air.
2. Student-teacher talked to his Biology group members that:
a) “In Rwanda, the rate of photosynthesis is generally lower at 5:30
AM that it is at 12:30 PM, during a sunny day”. Defend him by
providing two convincing reasons.
b) “The rate of photosynthesis is generally higher in Rwanda during
the sunny day than in Sahara desert”. Defend him with a convincing
reason.
9.3.3. Importance of autotrophic nutrition
a) Autotrophic nutrition is a process by which living organisms
(autotrophs: photoautotrophs and chemoautotrophs) make their own
food. The aututrophism is very essential as it allows production of
Oxygen and food for not only themselves but also for heterotrophs.
The roles of autotrophic nutrition include:
b) Independence of green plants from other living organisms to
the nutrition point of view.
This importance relates to their capacity for synthesizing organic
molecules from glucose produced by CO₂ and water, this completely
make them independents of the other living organisms to the nutrition
point of view.
c) Synthesis of the organic substances: food for the heterotrophs
(animal and mushrooms): The organic substances produced by
photosynthesis are the food for the heterotrophs which are unable to
synthesize these substances by their own means.
d) Energy storage
The autotrophs like green plants, by the process of photosynthesis
synthesize certain substances like: the cellulose, starch… which are
variables sources of energy.
e) Production of O₂ for the living organisms’ respiration
The oxygen produced by the photosynthesis is necessary for the
living organisms’ respiration. Thus without photosynthesis, no
oxygen; without oxygen no respiration; without respiration no life on
Earth.
f) Cleaning the atmosphere
Photoautotrophs absorb carbon dioxide from surrounding air, and
release Oxygen (produced by photosynthesis) in atmosphere.
g) Formation of Ozone layer
Ozone layer is a thick layer in the atmosphere which is formed
Ozone molecule (O₃).Oxygen atoms which make ozone molecule
are produced by photosynthesis. Ozone layer protects the Earth
from high solar radiations, and this allows the existence of the life on
the Earth.
Application activity 9.3.b
1. Without autotrophs, the life is impossible on the Earth. By providing
possible reasons, defend or disagree with this statement.
End unit assessment 9
Do all these exercises in your exercise book.
I. Choose whether the given statements are True (T) or False (F)
1. Organisms that are heterotrophic can make their own food.
2. Photosynthesis has two stages—light reaction and dark reaction.
3. Environmental factors improve crop yield.
4. Pigment is a material that changes color of reflected or transmitted
light.
5. Within leaves, chloroplasts are responsible for respiration.
II. Multiple Choice Questions
1. Green plants require which of the following for photosynthesis?
a) Sunlight (b) CO₂
b) O₂
c) Water
2. What is true about action spectrum?
a) It can be carried out in isolated pigments
b) It gives the function of pigments
c) It is used to identify pigments
d) It does not involve light
3. By looking at which internal structure, you can tell whether a plant
is C-3 or C-4?
a) Mesophyll cell (b) Bundle sheath cells
b) Vascular bundles (d) epidermal cells
4. How many ATP are required to produce 2 molecules of glucose?
a) 12 (b) 24
b) 18 (d) 36
5. Autotrophs are commonly called producers because they
a) Produce young plants
b) Produce CO2 from light energy
c) Produce sugars from chemical energy
d) Produce water from light energy
III. Long Answer Type Questions
1. State and explain the types of autotrophic nutrition. Also explain
the role of light in autotrophic nutrition.
2. Analyse and appreciate the importance of photosynthesis as an
energy transfer process.
3. State the role of chloroplast and structure of leaf in photosynthesis.
Giving illustrative diagrams, explain your answer.
4. State the pigments involved in light absorption. Throw light on
absorption and action spectra of chloroplast pigments.
5. Outline the three main stages of Calvin cycle. State the uses of
Calvin cycle intermediaries in plant cell.
6. Summarize the limiting factors affecting photosynthesis. Also state
how this can help yield crop production.
7. Investigate the effect of light intensity or light wavelength on the
rate of photosynthesis.
8. Describe the relationship between the structure and function in the
chloroplast, using diagrams and electron micrographs.
9. Acknowledge the importance of autotrophic nutrition in
sustaining the balance of life on Earth. Also state the ways to
keep the environment sustained. Predict various facts related to
photosynthesis that state the importance of nutrition for all living
beings.
10. The chart below shows the sequence of events that takes place in
the light dependent reactions.
a) Identify the point A and B
b) What process is taking place at C?
c) What are the products of the light dependent reaction? (They are
indicated by? on the diagram).
11. The diagram below summarizes the movement of materials into
and out of chloroplast. Identify the substances moved, indicated
by labels A-D.

Integrated Science ECLPE

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UNIT 9: AUTOTROPHIC NUTRITION

UNIT 9: AUTOTROPHIC NUTRITION

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