Chapter 6
Photosynthesis
Obtaining Energy
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Organisms can be classified by how they obtain energy. There are 2 ways
o Autotrophs are organisms that use energy from sunlight or from chemical bonds
in inorganic substances to make organic compounds
 Photosynthesis process used by most autotrophs where light energy
from the sun is converted into chemical energy (glucose)
 Chemosynthesis process where carbohydrates are produced using
inorganic compounds rather that sun light…some bacteria do this
o Heterotrophs are organisms that must get energy from food they ingest

Photosynthesis can be broken down in to 2 parts:
o The Light Reactions- Light energy absorbed from the sun is converted to
chemical energy, which is temporarily stored in ATP and the energy carrier
molecule NADPH
o Calvin Cycle- Organic compounds are formed using CO2 and the chemical energy
stored in ATP and NADPH
Photosynthesis can be summarized by the following equation:
o 6CO2 + 6H2O + light energy ---------> C6H12O6 + 6O2
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Capturing Light Energy
The light reactions begin with absorption of light in the chloroplasts, an organelle found
in the cells of plants and algae
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Chloroplasts are surrounded by a pair of membranes
Inside the inner membrane are a series of flattened sacs called thylakoids
The thylakoids are connected and layered to form stacks called grana
Surrounding the grana is a solution called stroma
o Think of this as a stack of pancakes with syrup…the thylakoids are the
individual pancakes, the stack of pancakes are the grana and the syrup is
the stroma
o Inside the chloroplasts are pigment molecules which are embedded in the
membrane of the thylakoid
o In green plants, the main pigment is chlorophyll.
o There are several types of chlorophyll, chlorophyll a and chlorophyll b are
the most common types
 Chlorophyll a absorbs less blue light but more red light than
chlorophyll b does.
 They both reflect green light and that is why most plants are
green
o Only chlorophyll is directly involved in the light reactions.
o Chlorophyll assists chlorophyll a in capturing light energy and therefore
chlorophyll b is called an accessory pigments
 other pigment molecules, such as carotenoids, are the yellow,
brown, and orange pigments, are also present and absorb some
green light so the plant can maximize absorption