Comparing Photosynthesis and
Cellular Respiration
3.6
Photosynthesis and cellular respiration are closely related to one another. In plants and
other autotrophs, both processes may occur within individual cells. This is possible
because plants contain mitochondria and chloroplasts. Animals and other heterotrophs
undergo cellular respiration, but not photosynthesis. Nevertheless, heterotrophs require
the products of photosynthesis to carry out cellular respiration. Photosynthesis uses the
products of cellular respiration, and cellular respiration uses the products of photosynthesis (Figure 1).
photosystem
II
O2
photosystem
I
H2O
NADP+
ADP
ATP
NAD+
NADPH
Calvin
cycle
CO2
glucose
pyruvate
chloroplast
ATP
electron
transport
system
ATP
NADH
Krebs
cycle
ATP
mitochondrion
Figure 1
The food/energy cycle.
Photosynthesis uses the products
of cellular respiration and cellular
respiration uses the products of
photosynthesis.
Photosynthesis and cellular respiration are related in several other ways:
• The Calvin cycle includes reactions that are similar to reactions in cellular
respiration but that occur in reverse.
• The proteins, quinones and cytochromes of the electron transport chains in photosynthetic membranes and in respiratory membranes are similar in structure
and, in some cases, are exactly the same.
• Both processes use chemiosmosis to transform energy from one form to another.
In mitochondria, H⫹ ions are pumped from the matrix into the intermembrane
space, with ATP synthesis occurring in the matrix. In chloroplasts, H⫹ ions are
pumped from the stroma into the thylakoid lumen, with ATP synthesis occurring
in the stroma (Figure 2, page 180).
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Photosynthesis
179
H+ H+
ATP
H+
H+
ATPase complexes
H+
H+
H+
H+
H+
ADP
+
Pi
H+
H+
H+
ADP
+
Pi
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+ H+ H+
H+
+
H + H+ H
H+
ATP
Cellular Respiration
(mitochondria)
(a)
ATP
H+
H+
H+
H+
H+
ADP + Pi
ADP + Pi
H+
H+
ATP
H+
H+
ATP
H+
H+
H+
H+
H+
ADP
+
H+ Pi H+
H+
H+
ADP + Pi
ATP
H+
H+ H+
H+ H+
H+
H+
H+
H+
Photosynthesis
(chloroplasts)
(b)
Figure 2
Chemiosmosis in chloroplasts and mitochondria
(a) During cellular respiration in mitochondria, H⫹ ions move from the matrix into the intermembrane space. ATP is synthesized in the matrix as H⫹ ions move through the ATPase
complexes embedded in the inner mitochondrial membrane.
(b) During the light reactions of photosynthesis, H⫹ ions move from the stroma into the thylakoid lumen. ATP synthesis occurs in the stroma as the H⫹ ions move through ATPase
complexes embedded in the thylakoid membranes.
The relationships that exist between photosynthesis and cellular respiration reveal a
dependency between autotrophs and heterotrophs. Heterotrophs produce the majority
of the CO2 in the environment that autotrophs use in photosynthesis, and autotrophs
produce the majority of the O2 in the environment that heterotrophs and autotrophs use
in cellular respiration.
EXPLORE an issue
Take a Stand: Tropical Rain-Forest
Depletion: Is There Cause for
Concern?
The destruction of the tropical rain forests, and the resulting
extinction of species, is one of the greatest global threats. Many
scientists, environmentalists, and concerned citizens believe that
the social and economic benefits of preserving the rain forests
far exceed all current benefits of destroying them for their commercial resources. Those in favour of rain-forest preservation
claim that deforestation causes loss of biodiversity, destruction of
forest-based societies, and disruption in climate. The rain forests
are often referred to as “the lungs of Earth.” However, this is not
exactly true. Rain forests are a climax vegetation, where no more
carbon can be fixed. If the rain forests were destroyed, however,
the fixed carbon would be converted into carbon dioxide and
contribute significantly to the greenhouse effect and global
warming. The tropical rain forests contain an amount of carbon
equal to almost half of the carbon in the atmosphere. Individuals
and groups fighting to preserve the forests claim that action
must be taken quickly or the rain forests of the world will be
destroyed by 2025.
Others disagree. They do not consider deforestation to be a
serious problem. They believe that flooding, soil erosion, and the
loss of biodiversity are not symptoms of a problem, but natural
180 Chapter 3
Decision-Making Skills
Define the Issue
Defend the Position
Analyze the Issue
Identify Alternatives
Research
Evaluate
events occurring in the ongoing evolution of life on Earth. For
some, there exists a powerful incentive to clear wood to make
room for agricultural land and industrialization. They feel that
developing countries have the same right to change their land
now as developed countries had in the past. They view interference by developed countries as an infringement on the fundamental rights of sovereign nations to self-determination.
Statement
The governments of developing countries have the right to
make room for agriculture and industrialization by clearing
tropical rain forests.
• Add your own ideas to the above-mentioned points.
• Find information to help you learn more about the issue.
GO
www.science.nelson.com
• In a group, discuss the ideas.
(a) Communicate your opinion and arguments through a
position paper, an organized small-group discussion, or
a formal debate.
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Section 3.6
Comparing Photosynthesis and
Cellular Respiration
SUMMARY
•
The comparison of photosynthesis and cellular respiration is summarized in
Tables 1 to 5.
Table 1 Comparison of the Overall Reactions
Respiration
Photosynthesis
reactants
organic molecules (e.g., glucose)
CO2 ⫹ H2O
products
CO2 ⫹ H2O
organic molecules (e.g., glucose)
energy
released
stored
Table 2
Electrons
Respiration
Photosynthesis
source
organic molecules (e.g., glucose)
water
carrier(s)
NAD⫹,
NADP⫹
FAD
Table 3 Electron Transport System
Respiration
Photosynthesis
Energy
Energy
energy profile
Time
Time
electron source
NADH and FADH2
water
electron sink
oxygen
NADPH
products
ATP
ATP and NADPH
Table 4 ATP Synthesis
Respiration
Photosynthesis
H⫹ ions pumped by ETC
yes
yes
ATP synthesis by chemiosmosis
yes
yes
membrane-embedded ATPase complex
yes
yes
Table 5 Organelle Structure and Function
Mitochondrion
Chloroplast
(cristae)
(thylakoid)
inner membrane functions
electron transport
H⫹ ion transport
ATP synthesis
electron transport
H⫹ ion transport
ATP synthesis
contains DNA, ribosomes,
etc. for replication
yes
yes
location of H⫹ reservoir
intermembrane space
thylakoid lumen
location of ATP synthesis
matrix
stroma
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Photosynthesis
181
Section 3.6 Questions
Understanding Concepts
1. Are photosynthesis and respiration exact opposites? Explain.
2. Describe the potential energy change in an electron as it is transported from water
to NADPH in light reactions.
3. (a) What would happen to humans and most other living organisms on Earth if
photosynthesis stopped?
(b) Why would this happen?
4. (a) Draw a labelled diagram of a chloroplast next to a mitochondrion within the
cytoplasm of a plant cell.
(b) On your diagram, Indicate the part of the organelle in which the following
activities occur:
(i) glycolysis
(ii) Krebs cycle
(iii) H⫹ reservoir
(iv) ATP synthesis
(v) electron transport (vi) light reactions
(vii) Calvin cycle
(viii) pyruvate oxidation
5. Explain why the energy profile for photosynthesis is a zig-zag line while the line for
respiration is straight.
6. How does a tropical fish aquarium containing fish and plants demonstrate the rela-
tionships between photosynthesis and cellular respiration?
Making Connections
7. The diagram on the Unit 1 opening pages is part of a much larger metabolic path-
ways chart that outlines almost all known metabolic reactions. Conduct Internet
research to see the complete chart and to answer the following questions.
GO
www.science.nelson.com
(a) Identify all of the major metabolic pathways you have studied in this unit, and
note the interactions among them.
(b) Why do scientists produce such charts? How are they useful?
182 Chapter 3
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