1.17
Energy Flow and Photosynthesis
Light from the sun is the ultimate source of energy for most living things.
However, organisms cannot use the energy of light directly. Instead, they first
capture solar energy and store it as chemical energy in carbohydrate molecules
such as glucose, and then transfer the energy from glucose to ATP, which all
cells use as an immediate source of energy.
The absorption of light energy and the production of glucose occur
through a process called photosynthesis, and the transfer of energy from
glucose to ATP occurs through a process called cellular respiration.
Photosynthesis occurs only in green plants, cyanobacteria, and plantlike
protists (Figure 1). Cellular respiration occurs in all organisms.
(a)
photosynthesis
light energy
glucose (in green plants, cyanobacteria,
and plantlike protists only)
cellular respiration
glucose
(b)
Figure 1
(a) Sunflowers
(b) Anabaena, a cyanobacterium
autotroph an organism (such as a
plant) that obtains energy directly
from light
heterotroph an organism (such as
an animal) that obtains energy by
eating other organisms
ATP (in all organisms, including green plants,
cyanobacteria, and plantlike protists)
Green plants, cyanobacteria, and plantlike protists are the only organisms on
Earth able to carry out both photosynthesis and cellular respiration. This means
that they are self-sufficient for their energy needs. Because of this, they are called
autotrophs (from Greek auto, meaning “self,” and troph, meaning “to feed”). All
other organisms, including humans, are heterotrophs—organisms that rely on
other organisms for energy. In other words, heterotrophs obtain all of their
energy by eating other organisms. Some heterotrophs, such as sheep, horses, and
squirrels, obtain energy by eating only plants, while others, such as humans,
bears, and raccoons, eat plants and animals for energy. Regardless of the type of
food eaten, heterotrophs convert the nutrients they absorb into glucose and,
through cellular respiration, transfer the energy in glucose to ATP. Cellular
respiration is discussed in section 1.18.
Photosynthesis: The Process
Photosynthesis occurs in all green parts of a plant, but the leaves are
particularly specialized for this purpose. You will learn more about leaves in
Unit 4. Photosynthesis occurs in the chloroplasts of green plants (Figure 2).
Chloroplasts contain a double outer membrane (an envelope) and inner
membranes that form an intricate system of compartments called thylakoids.
Thylakoids stack on top of one another to form structures called grana
(singular: granum). A protein-rich fluid called stroma fills the space between
the envelope and the thylakoid membranes. Molecules of the green, lightabsorbing pigment called chlorophyll are embedded within thylakoid
membranes (Figure 2(c) and (d)). Chlorophyll molecules absorb light
energy and begin the process of photosynthesis.
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Section 1.17
(a)
outer
membrane
(d) chlorophyll molecules
embedded in thylakoid
membrane
(b)
thylakoid
thylakoid
membrane
stroma
lamella
granum
inner
membrane
(c)
Figure 2
(a) Chloroplasts within plant cells
(Elodea)
(b) An artist’s representation of a
chloroplast, showing key
components
(c) An electron micrograph of a
chloroplast
(d) Chlorophyll molecules in the
thylakoid membrane
The photosynthetic process occurs in a series of reactions that uses light
energy, carbon dioxide, and water to produce glucose and oxygen (Figure 3).
Photosynthesis
sun
Oxygen is released as a
product of photosynthesis.
Light energy
drives the reaction.
glucose
made in leaf,
C6H12O6
carbon dioxide
from the
atmosphere
Figure 3
Plants use light energy to form
glucose from water and carbon
dioxide.
Water molecules from the soil are used in photosynthesis.
The following equation summarizes the photosynthetic process:
chlorophyll
6CO2(g) 6H2O(l) light energy
carbon dioxide
water
C6H12O6(aq) 6O2(g)
glucose
oxygen
Photosynthesis does not occur in one big step as shown in the overall
equation. Instead, it takes place through many reactions that may be
summarized in two steps: the light reactions and the Calvin cycle.
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Cellular Biology 73
light reactions of photosynthesis
reactions of photosynthesis in which
light energy is absorbed by
chlorophyll and transferred to ATP
Step 1: The Light Reactions of Photosynthesis
Photosynthesis begins when chlorophyll molecules in thylakoid membranes
trap light and transfer its energy to ATP molecules (Figure 4). Because light is
required for these reactions to occur, they are called the light reactions of
photosynthesis.
granum
(stack of
thylakoids)
sun
chloroplast
chlorophyll
light
energy
thylakoid membrane containing
chlorophyll (green dots)
ADP
thylakoid
ATP
1. LIGHT REACTIONS
energy
6CO2
2. CALVIN CYCLE
Figure 4
The light reactions of
photosynthesis and the Calvin cycle
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monosaccharides
such as glucose
polysaccharides such as starch
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Section 1.17
TRY THIS activity
Photosynthesis and Light
Green plants capture sunlight and transfer the energy to
carbohydrates through the process of photosynthesis.
When plants photosynthesize, they absorb carbon dioxide
and produce oxygen. The oxygen produced is released
into the environment. In this activity, you will observe the
production of oxygen in photosynthesizing plant cells.
Materials: living green plants with leaves, baking soda,
liquid soap or detergent, medicine dropper, water,
drinking straw, 35-mm film canister with lid, 5-mL syringe
1. Add enough baking soda to barely cover the bottom
of a film canister. Fill the canister with water (almost
to the top), replace the lid, and shake to dissolve the
baking soda.
2. Remove the lid, add one small drop of liquid soap,
replace the lid, and gently swirl the contents to
dissolve the soap. Do not shake. The soap will help
prevent static electricity.
3. Use a new straw like a cookie-cutter to cut four leaf
discs from a plant leaf. The leaf discs will accumulate
inside the straw.
5. Draw 4 mL of baking-soda solution (prepared in
6.
7.
8.
9.
(a)
(b)
(c)
Never share straws with others. Always use a new
straw. When finished using the straw, discard it
according to your teacher’s instructions.
4. If the syringe you are using has a cap on the tip,
remove the cap. Pull the plunger out of the syringe.
Blow the leaf discs out of the straw and into the
syringe. Replace the plunger.
(d)
steps 1 and 2) into the syringe. Invert the syringe
so that the tip end is pointing up. Gently push the
plunger to remove the air near the tip.
Put your finger over the syringe tip and pull the
plunger. This will create a vacuum, which will pull air
and oxygen from the leaf discs.
Tip the end of the syringe down so that the leaf discs
are in the solution. Release the plunger and remove
your finger. Turn the syringe back up and tap the side
repeatedly until all (or most) of the discs sink.
Place the syringe, open end up, in bright sunlight.
As the leaf discs photosynthesize, they will float to
the top.
(i) What causes the leaf discs to float to the top
while they’re in sunlight?
(ii) Would the discs float to the top if the syringes
were kept in the dark? Explain.
Why is baking soda added to the solution in the
syringe?
Did the leaf discs all float to the top at the same
time? Explain why or why not.
How could this procedure be used to investigate
whether or not different colours of light cause plants
to photosynthesize equally well? Design a
procedure for such an experiment (include a list of
materials).
Step 2: The Calvin Cycle
You can see from the overall equation of photosynthesis that the carbon
atoms in glucose come directly from the carbon atoms in carbon dioxide.
The chemical reactions that produce carbohydrates from carbon dioxide are
called the Calvin cycle (Figure 4, on the previous page). This process is
named after Melvin Calvin, the biochemist who discovered the reactions. The
energy of ATP molecules produced in the light reactions is required for the
Calvin cycle to occur. This energy (originally from the sun) is transferred to
the carbohydrate molecules that are produced.
Calvin cycle reactions of
photosynthesis in which carbon
dioxide molecules are used to
produce carbohydrates such as
glucose
Accessory Pigments
Chlorophyll is not the only light-absorbing molecule in chloroplasts. A
number of accessory pigments, including orange carotenoids and yellow
xanthophylls, help chlorophyll absorb a broad spectrum of light energy. In
spring and summer, leaves appear green because of the high concentration
of chlorophyll in chloroplast membranes. When temperatures cool off in the
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accessory pigment a
multicoloured pigment in chloroplast
membranes that assists chlorophyll
in absorbing light energy
Cellular Biology 75
autumn, leaves stop producing chlorophyll and break apart the molecules of
chlorophyll already there. This causes the bright yellow and orange colours of
the accessory pigments to show, giving rise to some of the characteristic
colours of leaves in the fall (Figure 5). The bright red colour in autumn leaves
is caused by the production of a red pigment called anthocyanin.
Figure 5
(a) Summer leaves contain
chlorophyll and accessory
pigments, but the green colour
of chlorophyll overwhelms the
colour of the accessory
pigments.
(b) Autumn leaves contain less
chlorophyll, causing the colours
of the accessory and other
pigments to become visible.
(a)
(b)
Section 1.17 Questions
Understanding Concepts
1. Name three large groups of organisms that carry out
photosynthesis.
2. Distinguish between autotrophs and heterotrophs.
Provide one example of each.
3. (a) What is the name of the pigment molecule in
green plants that begins the reactions of
photosynthesis?
(b) What are accessory pigments? What is their role
in photosynthesis?
(c) Name two accessory pigments and their
associated colours.
4. (a) What is the name of the organelle illustrated in
Figure 6?
(b) Name the labelled parts of the organelle in
Figure 6.
(c) In which part of the organelle do the reactions of
photosynthesis begin?
C
D
A
Figure 6
B
(b) What is the source of carbon dioxide for land
plants? water plants?
(c) What happens to the oxygen molecules formed
during photosynthesis?
6. Why are the leaves of deciduous trees, such as
maple trees, green in the summer and yellow and red
in the fall?
Making Connections
7. On a sheet of blank paper, draw a labelled diagram
with a single caption that you would use to teach the
process of photosynthesis to a grade 4 student who
has never heard of the process.
8. Several biotechnology companies are experimenting
with the possibility of producing a “green”
(environmentally friendly) plastic from plants. One
procedure converts sugar from corn to polylactide
(PLA), a plastic similar to the plastic polyethylene
terephthalate (PET), which is used to make pop
bottles and a variety of synthetic fabrics. Conduct
research to complete the following tasks:
(a) Describe one or two other “green” plastics and
their potential uses.
(b) Describe some of the costs and benefits of
producing green plastics on a large scale.
GO
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5. (a) Write the overall equation for photosynthesis.
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