Biology 12 - Solar Energy Converters - Section 7-2
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Transcript of Biology 12 - Solar Energy Converters - Section 7-2
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
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?
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.
Energy Absorption of Pigments
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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-green range.
• Photosynthesis begins when light
is absorbed.
Figure 7.5 Photosynthetic pigments and
photosynthesis.
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
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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UNIT A Chapter 7: Photosynthesis Section 7.2
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Noncyclic Electron PathwayFigure 7.6
Noncyclic electron
pathway: Electrons
move from water to
NADP+.
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.
Organization of the Thylakoid Membrane
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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
Organization of the Thylakoid Membrane
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Figure 7.8
Organization of a
thylakoid.
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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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.
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UNIT A Chapter 7: Photosynthesis Section 7.2
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