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Chapter 8: Photosynthesis

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Radiant Energy

Photosynthesis converts solar energy into the chemical energy of a carbohydrate :

Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2

Carbohydrate (glucose)

OxidizedReduced

Electrons from H2O are energized by the sun.

The oxygen given off comes from water – 18O experiments.

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Photosynthetic organisms

These organisms are called producers; they synthesize organic molecules from raw materials.

Plant Algae Cyanobacteria

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Nearly all life is dependent on solar energy because photosynthetic organisms:

1) use solar energy to produce organic nutrients.

2) provide food (producers) for other organisms (consumers).

And,

3) The bodies of plants became the coal or other fossil fuels used today.

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Visible Light Fig 8.2

Pigments (chlorophylls and carotenoids) found within photosynthesizing cells, are capable of absorbing various portions of visible light.Both chlorophyll a and chlorophyll b absorb violet, blue, and red light best and green least.3

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Mesophyll cells of a leaf contain chloroplasts

Water reaches mesophyll cells from vessels that extend to the roots.

Pores called stomata allow CO2 and O2 to enter the leaf.

Leaf Structure Fig 8.3

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Chloroplast structure

Double membrane

Chlorophyll and other pigments are found in the thylakoids

Thylakoids are interconnected

Fig 8.3

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Two Sets of Reactions for Photosynthesis

Light Dependant Reactions ‘Photo-’Cyclic and Noncyclic electron pathways

-Captures energy from the sun

Light Independent Reactions ‘-Synthesis’Calvin Cycle

-Produces carbohydrate

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The light-dependent reactions: the cyclic electron pathway

Fig. 8.4

-Produces only ATP!

Solar Energy

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The light-dependent reactions: the noncyclic electron pathway

Calvin Cycle

Fig. 8.5

-Splits water (yielding H+, e-, and O2)

-Produces ATP

-Reduces NADP+ to NADPH

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Light-Dependant ATP Production and the Electron Transport System

Hydrogen builds up concentration gradient in the thylakoid space.

The flow of H+ through an ATP synthase complex back into the stroma drives the chemiosmotic production of ATP.

Fig. 8.6

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Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2

Light-Dependant Reactions of Photosynthesis produce O2, ATP, and NADPH – not glucose.

ADP + P ATP

NADP + H NADPH

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Carbohydrate Synthesis

Light-independent reactions

-NADPH and ATP are used to reduce CO2 to carbohydrate.

-This occurs in the stroma of a chloroplast by a series of reactions called the Calvin cycle.

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Stages of the Calvin Cycle

The Calvin cycle can be divided into:

1) Fixation of CO2;

2) Reduction of CO2; and

3) Regeneration of RuBP (ribulase biphosphate).

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The Calvin cycle (simplified)

ATPADP

NADPHNADP

C6H12O6 RuBp

3CO2 + 3RuBp (C5)

6(C6)

6PGA (C3)

6PGAL (C3)

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The light-independent reactions of the Calvin cycle (detailed)

From light-dependant reactions

Fig. 8.8

From light-dependant reactions

Fixation of CO2

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Photosynthesis Efficiency

Ideal laboratory conditions: 25% of solar energy transferred.

Under natural conditions: the efficiency ranges from less than 1% to a maximum of 8%.

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CO2 H2O

Calvin Cycle Electron Pathways

Solar Energy

RuBP PGAL ATP O2

Glucose

Aerobic Cellular Respiration in Mitochondria makes new ATP

Light Independent Reactions Light Dependant Reactions

Photosynthesis in Chloroplast

NADPH

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Photosynthetic organisms carry on both photosynthesis and cellular respiration and rely on solar energy.

Solar Energy + CO2 + H20 C6H12O6 + O2

C6H12O6 + O2 CO2 + H20 + chemical energy(ATP)

Non-photosynthetic organisms only carry on cellular respiration and must rely on an external source of energy.

C6H12O6 + O2 CO2 + H20 + chemical energy(ATP)

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Figure 6.10 from Chapter 6

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Solar Energy

O2 and GlucoseCO2

CO2

Plant Cell Animal Cell

Mitochondria (Respiration)

Chloroplast (Photosynthesis)

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Other Types of Photosynthesis

Plants are able to live under many environmental conditions in part because various modes of photosynthesis have evolved.

The photosynthesis considered so far is called C3 photosynthesis because a C3 molecule is formed immediately following CO2 fixation.

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C3 Versus C4 Photosynthesis

C4 plants fix CO2 by forming a C4 molecule prior to the the Calvin cycle.

In C4 plants, CO2 is taken up in mesophyll cells, and then a C4 molecule (oxaloacetate) is pumped into bundle sheath cells where it releases CO2 to the Calvin cycle.

Thus, O2 cannot accumulate when stomata close.

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In C4 plants, bundle sheath cells also contain chloroplasts, and mesophyll cells are arranged concentrically around bundle sheath cells:

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CAM Photosynthesis

CAM plants also fix CO2 by forming a C4 molecule, but this occurs at night when stomata can open without water loss.

CAM stands for crassulacean-acid metabolism after the Crassulaceae, a family of succulent plants from arid regions.

C4 represents a partitioning in space, whereas CAM is a partitioning in time.

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Photosynthesis Versus Cellular Respiration

Both plant and animal cells carry on cellular respiration in mitochondria; photosynthesis occurs in plant chloroplasts.

Photosynthesis is the building up of glucose, while cellular respiration is the breaking down of glucose.