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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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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