Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in chloroplasts and involves two stages - the light-dependent reactions where ATP and NADPH are produced, and the light-independent Calvin cycle where glucose is formed. Photosynthesis is essential as it feeds the biosphere and converts solar energy into chemical energy that supports life.

1. Lecture 6a. Photosynthesis
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2. For Reporter # 15
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3. Photosynthesis
– Process that feeds the biosphere
– Process that converts solar energy into
chemical energy
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4. • Photosynthesis
– Occurs in plants, algae, certain protists, and
some prokaryotes
(a) Plants
(b) Multicellular algae
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(c) Unicellular protist 10 mm
(d) Cyanobacteria 40 mm
(e) Pruple sulfur 1.5 mm
bacteria

5. 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
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6. 2. Heterotrophs
– Obtain their organic material from other
organisms
– Are the consumers of the biosphere
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7. Where Photosynthesis Happens?
• The leaves of plants
– Are the major sites of photosynthesis
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Vein
Leaf cross section
Mesophyll
CO Stomata 2 O2

8. Chloroplasts: The Sites of Photosynthesis in Plants
Chloroplasts
– Are the organelles in which photosynthesis occurs
– Contain thylakoids and grana
Chloroplast
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Mesophyll
5 μm
Outer
membrane
Intermembrane
space
Inner
membrane
Thylakoid
space
Thylakoid
Stroma Granum
1 μm

9. • Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy ® C6H12O6 + 6 O2 + 6 H2 O
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10. The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the
electrons of hydrogen into sugar molecules
Reactants: 6 CO2 12 H2O
Products: C6H12O6 6 H2O 6 O2
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11. 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
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12. • Examples of redox reactions
becomes oxidized
(loses electron)
Na + Cl Na+ + Cl–
becomes reduced
(gains electron)
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13. For Reporter # 16
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14. 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
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15. Photosynthesis
CO2 H2O
+ P 1
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
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Starch
(storage)
G3P
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
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
RuBP 3-Phosphoglycerate
Amino acids
Fatty acids
Sucrose (export)
Photosystem II
Electron transport chain
Photosystem I
Chloroplast
Figure 10.21

16. 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
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17. • The electromagnetic spectrum
– Is the entire range of electromagnetic energy, or
radiation
Gamma
rays X-rays UV Infrared
Visible light
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Micro-waves
Radio
waves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm
1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Shorter wavelength
Higher energy
Longer wavelength
Lower energy
Figure 10.6

18. • 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
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19. – Reflect light, which include the colors we see
Light
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Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light

20. 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
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21. Organic compounds produced by photosynthesis
– Provide the energy and building material for
ecosystems
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22. 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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