Biology 12 - Solar Energy Converters - Section 7-2

2. UNIT A: Cell Biology
Chapter 2: The Molecules of Cells
Chapter 3: Cell Structure and Function
Chapter 4: DNA Structure and Gene
Expression
Chapter 5: Metabolism: Energy and
Enzymes
Chapter 6: Cellular Respiration
Chapter 7: Photosynthesis: Section 7.2

3. UNIT A Chapter 7: Photosynthesis
In this chapter you will learn how certain pigments, like the ones
that give leaves their particular colours, trap energy from the Sun
and use it for photosynthesis.
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Chapter 7: Photosynthesis
Which pigments provide the
maximum efficiency for a plant as
it conducts photosynthesis?
Why do leaves appear green in
the spring and summer and then
turn to red or yellow in the fall?

4. UNIT A Chapter 7: Photosynthesis Section 7.2
7.2 Plants as Solar Energy Converters
In the light-dependent reactions, various pigments absorb solar
energy.
• Energy can be described in terms
of wavelength and energy content.
• Gamma rays have the shortest
wavelength and radio waves the
longest.
• Visible (white) light is only a
portion of the spectrum.
• Different colours (wavelengths) of
visible light range from violet to red.
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Figure 7.4 The electromagnetic spectrum.

5. UNIT A Chapter 7: Photosynthesis Section 7.2
Energy Absorption of Pigments
Pigments found in photosynthesizing cells absorb certain
wavelengths of light.
• Chlorophylls a and b absorb violet,
indigo, blue, and red light best.
Leaves look green because green
light is mostly reflected, not
absorbed.
• Carotenoids absorb in the violet-blue-
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green range.
• Photosynthesis begins when light
is absorbed.
Figure 7.5 Photosynthetic pigments and
photosynthesis.

6. UNIT A Chapter 7: Photosynthesis Section 7.2
Light Reactions
The light reactions occur in the thylakoid membrane. They
consist of two electron pathways:
• Noncyclic electron pathway
produces ATP and NADPH
• Cyclic electron pathway
produces ATP
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From Figures 7.6 and 7.7

7. UNIT A Chapter 7: Photosynthesis Section 7.2
Noncyclic Electron Pathway
In the noncyclic electron pathway, the e− flow is from H2O
to NADP+.
•Uses two photosystems (PS I and PS II), each consisting of
a pigment complex and an e− acceptor
•PS II absorbs solar energy, and e− in the reaction centre are
passed to an acceptor
•e− pass through an electron transport chain, resulting in
production of ATP by chemiosmosis
•Replacement e− for PS II are from H2O, releasing O2
•PS I absorbs energy and e− goes to an acceptor; NADP+
accepts two e− and H+, forming NADPH
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8. UNIT A Chapter 7: Photosynthesis Section 7.2
Noncyclic Electron Pathway
Figure 7.6
Noncyclic electron
pathway: Electrons
move from water to
NADP+.
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9. UNIT A Chapter 7: Photosynthesis Section 7.2
Cyclic Electron Pathway
The cyclic electron
pathway occurs under
conditions such as high O2
levels.
•PS I absorbs energy, and e−
move to electron acceptors
•e− are passed along an
electron transport chain,
which produces ATP, and
returned to PS I
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Figure 7.7 Cyclic electron pathway: Electrons
leave and return to photosystem I.

10. UNIT A Chapter 7: Photosynthesis Section 7.2
Organization of the Thylakoid Membrane
The following complexes are in the thylakoid membrane:
• Photosystem II consists of a pigment complex and electron
acceptor molecule and receives electrons from water
• The electron transport chain carries electrons from
photosystem II to photosystem I and pumps H+ from the
stroma to the thylakoid space
• Photosystem I consists of a pigment complex and electron
acceptor molecule and is adjacent to the enzyme that
reduces NADP+ to NADPH
• The ATP synthase complex spans the thylakoid membrane
and catalyzes formation of ATP
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11. UNIT A Chapter 7: Photosynthesis Section 7.2
Organization of the Thylakoid Membrane
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Figure 7.8
Organization of a
thylakoid.

12. UNIT A Chapter 7: Photosynthesis Section 7.2
ATP Production
The thylakoid space is a reservoir for H+.
• Each time O2 is removed from water, two H+ remain in the
space
• As electrons move through the electron transport chain,
electrons give up energy that is used to pump H+ from the
stroma into the space
An electrochemical gradient across the membrane forms.
• Electrons flow from the thylakoid space to the stroma
through the ATP synthase complex. This provides energy
for the enzyme complex to produce ATP from ADP and
phosphate (chemiosmosis).
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13. UNIT A Chapter 7: Photosynthesis Section 7.2
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Check Your Progress
1. Explain why leaves appear green.
2. Compare the production of NADPH to ATP in
noncyclic photosynthesis.
3. Identify which part of a thylakoid will contain the
photosystems, electron transport chain, and the ATP
synthase complex.
4. Explain why the H+ gradient across a thylakoid
membrane is referred to as a storage of energy.

14. UNIT A Chapter 7: Photosynthesis Section 7.2
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15. UNIT A Chapter 7: Photosynthesis Section 7.2
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