Integrated Science SME

Key Unit competence: Describe the process of photosynthesis
and explain the various environmental
factors that influence the rate of
photosynthesis

Introductory Activity 11

Read the passage below and then answer questions that follow.
Vital process that makes up our life
The life is all about autotrophic nutrition. There is a number of reactions
that take place during the autotrophic nutrition but all are simpled
summarised by the saying inorganic matters are transformed into organic
matters. And as said in physics, the energy is neither created nor lost but
only transformed from one form to another, the process of autotrophic
nutrition transforms light energy into chemical energy. But, opposite to
most of metabolic reactions which produce harmful waste products, the
waste product from photosynthesis is useful for life. So, no autotrophic
nutrition, no life either.
1. Suggest a name to autotrophic nutrition
2. Which waste product from autotrophic nutrition is useful for life?
3. Do you agree that no autotrophic nutrition, no life either? Why?
4. The overall equation of autotrophic nutrition should be written as:

5. What requirements are missing on the above equation?
6. What factors can affect the rate of autotrophic nutrition.

11.1. Types of autotrophic nutrition

Activity 11.1

Do a research on different types of autotrophic nutrition and present them
on manila paper.

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.

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.

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.

Application activity 11.1

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.

11.2. Structure of the chloroplast and Adaptations for
photosynthesis

Activity 11.2

With a compound microscope and well prepared slides of plant cell
• Observe the slides under the compound microscopes, and draw the
structure of chloroplast.
• Search and relate the structure of chloroplast with the process of
photosynthesis.

In eukaryotes photosynthesis takes place in chloroplasts. 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.

The structure of chlorophyll
The chlorophyll molecule is made of atoms of Carbon and Nitrogen joined
in a complex porphyrin ring containing an atom of Magnesium in the center
of the ring. The chlorophyll also has long hydrophobic carbon tail of 20
carbon atoms (phytol) which hold it in the thylakoid membrane. In short, the
chlorophyll consists of a porphyrin ring and a phytol tail.
The chlorophyll a differs from the chlorophyll b in that: the porphyrin of the
chlorophyll a has the methyl group as a functional group, which is
replaced by an aldehyde group (-CHO) for chlorophyll b.

The difference between the chlorophyll a and the chlorophyll b shifts the
wavelength of light absorbed and reflected by chlorophyll b, so that the
chlorophyll b is yellow-green, whereas the chlorophyll a is bright-green.

Adaptations for photosynthesis
By considering both external and internal structures of the leaf, we can
recognize several adaptations for photosynthesis.
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 CO2 produced during cell
respiration. The equilibrium can be reached between photosynthesis and
cell respiration.
Photosynthesis uses 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

is available. The stomata cannot remain closed indefinitely, they have to be
open in order to maintain transpiration of the plant.
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.

Application activity 11.2

1. Describe the structure of a chloroplast.
2. Explain adaptations of both thylakoid and stroma for their functions.
3. Relate the internal structure of the leaf with the process of
photosynthesis

11.3. Absorption, action spectra and other carbon dioxide
fixation pathways

Activity 11.3

1. Is it possible for a plant to grow without solar light? Justify your answer.
2. Why do plants have different colours? Justify each color of plant
observed in environment.

In addition to water and 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 colors (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
colours 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.
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.

a) 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:
Step 1: 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).

Step 2: 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   (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.

Step 3: The fixation of Pi to ADP+ to form ATP is called photophosphorylation.
Photophosphorylation can be done into two processes: cyclic
photophosphorylation, and non-cyclic photophosphorylation.
i) Cyclic photophosphorylation
It involves only photosystem I and not photosystem II. There is no
production of NADPH and no release of Oxygen. When the light hits
the chlorophyll in PSI, the light-excited electron leaves the molecule.
This light-excited electron is taken up by an electron acceptor which
passes it along an electron transfer chain (a series of electron carriers)
until it returns to the chlorophyll molecule that it left (cyclic process).
As an excited electron moves along an electron transfer chain, it loses
energy which will be used for the synthesis of ATP from ADP+ and
inorganic phosphate in the process called chemiosmosis. Electron
carriers can vary, but the principle include the cytochromes.
ii) Non-cyclic photophosphorylation
It is the main route of ATP synthesis. It is done in the following steps:
• When the photosystem II (in chlorophyll) absorbs light, an electron is
excited to a higher energy level and captured by the primary electron
acceptor.
• Enzymes extract electrons from a water molecule replacing each
electron that the chlorophyll molecule lost when absorbed light
energy. This reaction dissociates a water molecule into hydrogen
ions (2H+) and Oxygen which is released for animals’ respiration.
• Excited electron moves from the primary electron acceptor of
photosystem II to photosystem I, via an electron transport chain.
• When excited electron moves from the primary electron acceptor of
photosystem II to photosystem I, via an electron transport chain its
energy level lowers. The energy removed is used to synthesize ATP
from ADP and in a process called: Non-cyclic phosphorylation.

The hydrogen ions (2H+) produced from dissociation of water
molecule combines with NADP+ to form NADPH₂.

• Both ATP and NADPH2 will be used in the light-independent reactions
(Calvin cycle) for synthesis of sugars.

The significance of the cyclic phosphorylation
Non-cyclic photophosphorylation produces ATP and NADPH in equal
quantities, but the Calvin cycle consumes more ATP than NADPH. The
concentration of NADPH in a chloroplast may determine which pathway
(cyclic versus non-cyclic) electrons pass through.
If a chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate
as the cycle slows down. The rise of NADPH may stimulate a shift from non-
cyclic (which produces ATP only) to cyclic electron pathway until ATP supply
catches with the demand.

b) 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.

Step 1: Carbon fixation (Carboxylation) in form of CO2
The Calvin cycle begins with a 5-Carbon sugar phosphate called
Riburose-1, 5 biphosphate (RuBP) which fixes the CO₂ from air. This
reaction is catalyzed by an enzyme called RuBPcarboxylase-oxygenase
(RUBISCO), which makes up about 30% of the total protein of the leaf, so
it is probably one of the most common proteins on the Earth.
The combination of RuBP and CO2 results in a theoretic 6-carbon
compound which is highly unstable. It immediately splits into two
molecules of 3-carbon known as phosphoglyceric acid (PGA) or glycerate
3-phosphate, or 3-phosphoglycelate.

Step 2: 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.

Step 3: 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:

Other carbon dioxide fixation pathways (C4 CAM)
The most common pathway combines one molecule of CO₂ with a 5-carbon
sugar called ribulose biphosphate (RuBP). The enzyme which catalyzes
this reaction (nicknamed “Rubisco”) is the most abundant enzyme on earth!
The resulting 6-carbon molecule is unstable, so it immediately splits into
two 3-carbon molecules. The 3 carbon compound which is the first stable
molecule of this pathway gives this largest group of plants the name “C-3
plants”.

Dry air, hot temperatures, and bright sunlight slow the C-3 pathway for
carbon fixation. This is because stomata, which normally allow CO₂ to enter
and O₂ to leave, must close to prevent loss of water vapor. Closed stomata
lead to a shortage of CO₂. Two alternative pathways for carbon fixation
demonstrate biochemical adaptations to differing environments. Plants such
as corn solve the problem by using a separate compartment to fix CO₂.

Here CO₂ combines with a 3-carbon molecule, resulting in a 4-carbon
molecule. Because the first stable organic molecule has four carbons, this
adaptation has the name C-4. Shuttled away from the initial fixation site, the

4-carbon molecule is actually broken back down into CO2, and when enough
accumulates, Rubisco fixes it a second time!

In some temperate plants such as wheat, rice, potato and bean only Calvin
cycle occurs. Such plants are called C-3 plants. While in some other plants
dual carboxylation takes place: (1) carboxylation of phosphoenol pyruvate
(PEP) and (2) carboxylation of RuBP. Such plants are called C-4 plants e.g.
maize, sugar cane and sorghum. In these, the first product formed during
carbon dioxide fixation is a four carbon compound oxalo acetic acid (OAA).
C-4 plants have special type of leaf anatomy called Kranz Anatomy. They
have special large cells around vascular bundles called bundle sheath cells.
These are characterized by having large number of chloroplasts, thick walls
and no intercellular spaces. The shape, size and arrangement of thylakoids
in chloroplasts are also different in bundle sheath cell as compared to
mesophyll cell chloroplasts.

The pathway followed by C-4 plants is called C-4 cycle or Hatch and Slack
pathway. This was discovered by Hatch and Slack in sugar cane. The
primary CO₂ acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP).
The reaction is catalyzed by PEP carboxylase or PEP case in mesophyll cell
chloroplast. It forms 4-carbon compounds like OAA, malic acid or aspartate,
which are transported to the bundle sheath cells. In bundle sheath cells,
these acids are broken down to release CO₂ and 3-carbon molecule. The
3-carbon molecule is transported back to mesophyll cells and converted to
PEP again, while CO₂ enters into C-3 cycle to form sugars. C-4 plants are
more efficient than C-3 plants as in C-4 plants, photosynthesis can occur at
low concentration CO₂ and photorespiration is negligible or absent.

Cacti and succulent (water-storing) plants such as the jade plant avoid water
loss by fixing CO₂ only at night. These plants close their stomata during the
day and open them only in the cooler and more humid nighttime hours. Leaf
structure differs slightly from that of C-4 plants, but the fixation pathways are
similar. The family of plants in which this pathway was discovered gives the
pathway its name, Crassulacean Acid Metabolism, or CAM. All carbon
fixation pathways lead to the Calvin cycle to build sugar.

The CAM pathway is similar to the C4 pathway in that carbon dioxide is
first incorporated into organic intermediates before it enters the Calvin cycle.
The difference is that in C4 plants, the initial steps of carbon fixation are
separated structurally from the Calvin cycle whereas in CAM plants, the two
steps occur at separate times.

The CAM pathway and the C4 pathway compared:

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 unlike photosynthesis,
photorespiration generates no food. In fact, photorespiration decreases
photosynthetic output by using material from the Calvin cycle.

Application activity 11.3

1. Why are light and chlorophyll needed for photosynthesis?
2. Describe the relationship between the chlorophyll and the color of
plants.
3. How well would a plant grow under pure yellow light? Explain your
answer.
4. Appreciate the presence of accessory pigments in leaves for the
process of photosynthesis.
5. differentiate the light-dependent stage and light-independent stage of
photosynthesis.
6. Distinguish the cyclic and non-cyclic photophosphorylation.
7. Explain the stages of the Calvin cycle.
8. Appreciate the significance of the cyclic phosphorylation.
9. In a tabular form, compare the process of photosynthesis in C₃ and
C₄ plants.
10. What do you understand by photorespiration?

11.4. Rate of photosynthesis: limiting factors of
photosynthesis

Activity 11.4

By using prior knowledge, what do you think should influence the
photosynthesis process in plants?

The photosynthesis rate varies with the species but also varies within
individuals for a same species; this varies under the influence of certain
external factors which are: the temperature, CO2 concentration in the
atmosphere, light intensity and soil humidity.

Photosynthesis is low at 0˚C, certain alpine plants do photosynthesis at has
less -15˚C.

Photosynthesis presents an optimum towards 35 to 40 ˚C, then it decreases
and is cancelled at around 50˚C.

b. CO2 concentration in the atmosphere

The photosynthetic rate is zero in place lacking CO₂, it increases with the
increase concentration of CO₂ in the atmosphere and reaches an optimum
ranging between 5 and 8%CO₂ concentration.

The photosynthesis rate is low during night, it increases when the light
intensity increases but the optimum varies according to the plants.

11.4.4. Availability of water for the plant
The photosynthesis rate is low when the soil is dry, it increases when the
content of water increases for the terrestrial plants, and for the aquatic plants
it remains constant as long as they are fixed in water.

Note: The limiting factors work together to influence the rate of photosynthesis

Application activity 11.4
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. A student who studies Biology talked to his Biology group members
that:
a) “In Rwanda, the rate of photosynthesis is generally lower at 5:30
AM than it is at 12:30 PM, during a sunny day”. Defend him by
providing two convincing reasons.
He also said that:
b) “The rate of photosynthesis is generally higher in Rwanda during
the sunny day than in Sahara Desert”. Defend him with a convincing
reason.

11.5. Importance of autotrophic nutrition and Tests for
starch in terrestrial plants and for oxygen in aquatic
plants

Activity 11.5

Describe the following topic: Animals depend on Plants, rather than
plants depend on animals

A. Importance of autotrophic nutrition
Autotrophic nutrition is a process by which living organisms (autotrophs:
photoautotrophs and chemoautotrophs) make their own food. The
autotrophism 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:

• 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.

• 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.

• 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.

• Cleaning the atmosphere
Photoautotrophs absorb carbon dioxide from surrounding air, and release
Oxygen (produced by photosynthesis) in atmosphere.

• 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.
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.

B. Tests for starch in terrestrial plants
The process of photosynthesis results in production of Oxygen and Organic
substances like simple sugars, double sugars starch, amino acids among
other. It is evident that, if someone need to be sure that photosynthesis has
occurred, he can simple test for the presence of some of those substances
produced by photosynthesis.

Materials and substances needed to test for the presence of starch in
terrestrial plants
Bunsen burner, tripod stand, wire gauze,forceps, the petri dish, test tube
holder, 250 cm3 beaker, dropping pipette, boiling tube, a water bath, leaf
to be tested (hibiscus leaves are excellent),90% ethanol,cold water, iodine/
potassium iodide solution.

The process of testing for starch in a leaf
1. Prepare a water bath; heat the water to boiling point and keep it boiling.
2. Detach a leaf from a dicotyledonous plant that has been exposed to
sunlight for at least three hours.

3. With forceps, hold the leaf in boiling water for about 10 seconds. This will
kill the cells, stop all chemical reactions and allow alcohol and iodine to
penetrate the leaf more easily, and will removes the waxy cuticle which
prevents entry of iodine/potassium iodide solution, will also denatures
enzymes, particularly those which convert starch to glucose e.g. diastase.
Boiling arrests all chemical reactions, since enzymes which catalyse the
reactions are denatured. Denatured enzymes have altered or destroyed
active sites due to heat, pH, and ionic concentration.
4. Extinguish the Bunsen burner.
5. Half fill a test tube with ethanol (ethyl alcohol).
6. With forceps, place the leaf in the alcohol.
7. Place the test tube containing ethanol and the leaf into the hot water.
The ethanol will boil and dissolve out the chlorophyll in the leaf. When
the leaf is nearly colorless, remove it from the ethanol with forceps and
wash it in cold water. This will soften the leaf by replacing water removed
by the ethanol.
8. Place the softened leaf in a Petri dish and, using a dropping pipette,
cover it with iodine solution. Leave for 3 minutes.
9. If the colour of the leave turns blue-black, it means that it contains starch,
and therefore photosynthesis has occurred in the leave. If no color
change (Iodine Solution remains brown), starch is not present.

We can also measure the process of photosynthesis via the production of
oxygen, or by using the Audus apparatus to measure the amount of gas
evolved over a period of time. Oxygen can be measured by counting bubbles
evolved from pondweed. If you illuminate a plant (Presume you are using an
aquatic/water plant) it will produce oxygen. You can put the plant under a

funnel and collect the bubbles in an upturned test tube. If the gas is collected
fresh, it will relight a glowing splint. You can also use Elodea, but we find
Cabomba more reliable. Put the weed in a solution of NaHCO₃ solution. You
can then investigate the amount of gas produced at different distances from
a lamp.

Application activity 11.5
1. Which products of photosynthesis may be present but not revealed
by the iodine test?
2. Write down the steps to follow during testing for starch in terrestrial
plant.
3. Appreciate the necessity of boiling the leaf to be used in testing for
starch in plants.
4. Summarize the process of testing for Oxygen in aquatic plants.
5. Explain why the leaf which is nearly colorless, should first be washed
in cold water.
6. Summarize the importances of photosynthesis to the living organisms.

Skills lab 11

Evidence of photosynthesis

Materials: Large clear plastic cup, sodium bicarbonate solution (baking
soda), aquatic filamentous green algae, large test tube, retor stand.

Procedure:
• Fill three large clear plastic cups with water and 3g or 6g of sodium
bicarbonate as shown below:

• Place aquatic green algae in three large test tubes. Fill the tube with
water or sodium bicarbonate solution in the cup. Caution: handle 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.
• Fix each setup in a stand

• Place your setups in bright light.
• After at least 20 minutes, record your observations and draw a
conclusion.

End unit assessment 11
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. CAM cycle includes triple carboxylation.
4. A pigment is a material that changes colour of reflected or transmitted
light.
5. Within leaves, chloroplasts are responsible for respiration.

II. Multiple Choice Questions
1. Green plants does not require which of the following for photosynthesis?
(a) Sunlight   (b) CO₂   (c) O₂   (d) Water

2. C-4 cycle occurs in
(a) Wheat (b) Rice (c) Sugar cane (d) All of the above
3. 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.
2. Relate the structure of a chloroplast with its function and structure of
leaf in photosynthesis.
3. State the pigments involved in light absorption.
4. Outline the three main stages of Calvin cycle.
5. Compare anatomy of C4 and CAM plants.
6. Differentiate between C4, CAM and C3 plants during carbon dioxide
fixation.
7. Investigate the effect of light intensity on the rate of photosynthesis.
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).

8. The diagram below summarizes the movement of materials into and
out of chloroplast. Identify the substances moved, indicated by labels
A-D.

9. The diagram below represents the CALVIN’S Cycle also known as
the light independent reactions of the photosynthesis.

a). Name substances X, Y and Z.
b). (i) State any two steps of the Calvin cycle which are endothermic.
(ii) State the source of the energy consumed in these two steps.
c). Why is it said that the reduction is the most important step of the
Calvin cycle?
d). Name any two products represented by A, B, C, D or E.