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					Capturing Solar Energy: Photosynthesis

(a)

(b) internal leaf structure

mesophyll cells (c) chloroplast in mesophyll cell outer membrane inner membrane thylakoid stroma vein
stoma chloroplasts

channel interconnecting thylakoids

Photosynthesis
Complex series of chemical reactions involving a transition in forms of energy



Photosynthesis


Uses light energy to make food and is a process by which some organisms can make organic compounds from simple inorganic compounds using energy from the sun

Photosynthesis


Light energy captured and stored as chemical potential energy in the covalent bonds of carbohydrate molecules

6 CO2 + 6 H2O + light  C6H12O6 + 6 O2

Life depends on photosynthesis


A. Foundation of energy for most ecosystems B. Source of oxygen C. Key component of the carbon cycle

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The mechanism of photosynthesis
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Solar energy and light
The electromagnetic spectrum
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

Pigment molecules absorb some wavelengths of light and reflect others
•

Chlorophyll—a green photosynthetic pigment associated with the thylakoid membranes of chloroplasts

(a)

(b)
chlorophyll carotenoids

phycocyanin

(c)

The mechanism of photosynthesis


Chloroplasts are the sites of photosynthesis
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Have a membrane system within internal space (stroma) Arranged in disk-shaped sacks (thylakoids)
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The thylakoids contain light-harvesting photosynthetic pigments & enzymes Internal membranes define space (lumen) that is separate from the rest of the stroma



chloroplast thylakoids

reaction center

electron transport system light- harvesting complex

The mechanism of photosynthesis
Photosynthesis occurs in two steps
1. Light-dependent reactions
•

a. Provides the energy necessary to fix carbon
b. Occurs in the thylakoid membranes

•

•

c. Generates ATP
d. Photolysis—light, electrons and water

•

7 8

3

electron transport system 5

6

reaction center 2 4 energy to drive

1

synthesis reaction center

photosystem I

9
photosystem II

Light-Dependent Reactions


What happens during light reactions?


Chlorophyll in plants (Photosystem I [PS I])
Absorbs blue & red light and reflects back the green light


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Blue & red light energy boosts electrons to higher energy levels Electrons are passed to electron transport molecule in the thylakoid



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Can only be passed when highly energized

Light-Dependent Reactions


What happens during light reactions?
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During transport of electrons from PS II to PS I
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Some energy is harnessed to produce ATP



Eventually, chlorophyll from PS II is oxidized


Gets replacement electrons from water

Light-Dependent Reactions


Energy of light has thus been captured in two forms:


The synthesis of NADPH from NADP+
Proton gradient across the thylakoid membrane
 

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Cannot be used directly to make food Must first be converted to ATP by chloroplast ATP synthase

The mechanism of photosynthesis
Energy carriers ATP and NADPH transport energy from the lightdependent reactions to the lightindependent reactions

energy from sunlight

Light-dependent reactions occur in thylakoids.
Light-independent reactions (C3cycle) occur in stroma.

The mechanism of photosynthesis
2. Light-independent reactions
a. Uses energy of the light-dependent reaction to make sugar from CO2 b. Occurs in the stroma

Light-Independent Reactions
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Steps in Light-Independent Reactions:
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CO2 joins with RuBP forming an unstable 6-C molecule

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Breaks into two 3-C PGA molecules
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This first step in Calvin-Benson cycle is catalyzed by enzyme Called ribulose biphosphate carboxylase (Rubisco)

1

Carbon fixation combines CO2 with RuBP.

3

RuBP regeneration uses energy and 10 G3P.

2

G3P synthesis uses energy.

2 G3P available for synthesis of organic molecules.

C4 plants utilize an alternate pathway to make sugars in dry environments


Closing stomata to conserve water results in photorespiration in C3 plants

(a)

C3 plants use the C3 pathway

mesophyll cell in C3 plant

Much photorespiration occurs under hot, dry conditions.

In a C3 plant, most chloroplasts are in mesophyll cells. (b)
C4 plants use the C4 pathway mesophyll cell in C4 plant

CO2 is captured with a highly specific enzyme.

bundlesheath cells

In a C4 plant, both mesophyll and bundle-sheath cells contain chloroplasts.

Much glucose synthesis occurs.

Almost no photorespiration occurs in hot, dry conditions.

bundle-sheath cell in C4 plant


				
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