photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Lecture 6a. Photosynthesis


For Reporter # 15


Photosynthesis

– Process that feeds the biosphere

– Process that converts solar energy into chemical energy


• Photosynthesis

– Occurs in plants, algae, certain protists, and some prokaryotes

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 µm

40 µm(d) Cyanobacteria

1.5 µm(e) Pruple sulfurbacteria


Classification of Organisms

1) Photoautotrophs

– Use the energy of sunlight to make organic molecules from water and carbon dioxide

– Are the producers of the biosphere


2. Heterotrophs

– Obtain their organic material from other organisms

– Are the consumers of the biosphere


• The leaves of plants

– Are the major sites of photosynthesis

Vein

Leaf cross section

Mesophyll

CO2 O2Stomata

Where Photosynthesis Happens?


Chloroplasts

– Are the organelles in which photosynthesis occurs

– Contain thylakoids and grana

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

Chloroplasts: The Sites of Photosynthesis in Plants


• Photosynthesis is summarized as

6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2 O


The Splitting of Water

• Chloroplasts split water into

– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O66 H2O 6 O2


Photosynthesis as a Redox Process

• Photosynthesis is a redox process

– Water is oxidized, carbon dioxide is reduced

Redox reactions - Transfer electrons from one reactant to another by oxidation and reduction

• In oxidation - A substance loses electrons, or is oxidized

• In reduction - A substance gains electrons, or is reduced


• Examples of redox reactions

Na + Cl Na+ + Cl–

becomes oxidized(loses electron)

becomes reduced(gains electron)


For Reporter # 16


The Two Stages of Photosynthesis: A Preview

1) The Light Reactions

– Occur in the grana

– Split water, release oxygen, produce ATP, and form NADPH

2) The Calvin Cycle

– Occurs in the stroma

– Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power


Photosynthesis

Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere

Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions

O2

CO2H2O

Light

Light reaction Calvin cycle

NADP+

ADP

ATP

NADPH

+ P 1

RuBP 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

Figure 10.21


The Nature of Sunlight

• Light

– Is a form of electromagnetic energy, which travels in waves

• Wavelength

– Is the distance between the crests of waves

– Determines the type of electromagnetic energy


• The electromagnetic spectrum

– Is the entire range of electromagnetic energy, or radiation

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energyFigure 10.6


• The visible light spectrum

– Includes the colors of light we can see

– Includes the wavelengths that drive photosynthesis

• Pigments

– Are substances that absorb visible light

Photosynthetic Pigments – light receptors


– Reflect light, which include the colors we see

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight


Types of Pigments

• Chlorophyll a

– Is the main photosynthetic pigment

• Chlorophyll b

– Is an accessory pigment

• Other accessory pigments

– Absorb different wavelengths of light and pass the energy to chlorophyll a


Organic compounds produced by photosynthesis

– Provide the energy and building material for ecosystems


Sources:

Lectures for Biology 7th Ed. (N. Campbell and J. Reece) by Chris Romero

• Starr, C. 2003. Biology: Concepts and Applications, 5th ed. Brooks/Cole, USA. 799 pp.

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