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Photosynthetic Cells
Cells get nutrients from their environment, but where do those nutrients come from? Virtually
all organic material on Earth has been produced by cells that convert energy from the Sun
into energy-containing macromolecules
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Figure 1: Photosynthetic plants synthesize carbon-based energy molecules from the energy in sunlight
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Plants exist in a wide variety of shapes and sizes
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E
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(B) Chara (Charophyceae) gametophyte; magnification x 1
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Feist)
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N
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(D) Anthoceros (hornwort) gametophyte showing unbranched sporophytes; magnification x 2
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N
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(E) Mnium (moss) gametophyte showing unbranched sporophytes with terminal sporangia
(capsule); magnification x 4
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Burger)
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8
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08
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4
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4
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05 (photograph courtesy of W
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What Is Photosynthesis? Why Is it Important?
Most living things depend on photosynthetic cells to manufacture the complex organic
molecules they require as a source of energy
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During the process of
photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules
and oxygen
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Then, via respiration processes, cells use oxygen and

glucose to synthesize energy-rich carrier molecules, such as ATP, and carbon dioxide is
produced as a waste product
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Figure 2

The building and breaking of carbon-based material — from carbon dioxide to complex
organic molecules (photosynthesis) then back to carbon dioxide (respiration) — is part of
what is commonly called the global carbon cycle
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The carbon cycle would not be possible without
photosynthesis, because this process accounts for the "building" portion of the cycle (Figure
2)
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Interestingly, although green plants contribute much of the
oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are
thought to produce between one-third and one-half of atmospheric oxygen on Earth
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Different pigments
respond to different wavelengths of visible light
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In plants,
photosynthesis takes place in chloroplasts, which contain the chlorophyll
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In electron micrographs, thylakoid
membranes look like stacks of coins, although the compartments they form are connected like
a maze of chambers
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Chlorophyll A is the major pigment used in photosynthesis, but there are several types of
chlorophyll and numerous other pigments that respond to light, including red, brown, and
blue pigments
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For example, the photosynthetic protists called
dinoflagellates, which are responsible for the "red tides" that often-prompt warnings against
eating shellfish, contain a variety of light-sensitive pigments, including both chlorophyll and
the red pigments responsible for their dramatic coloration
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Other features of the cell
include the nucleus (N), mitochondrion (M), and plasma membrane (PM)
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Note the relationship between the granal and stromal membranes
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In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the
aforementioned chlorophyll pigments reside
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Every step in the electron transport chain
then brings each electron to a lower energy state and harnesses its energy by producing ATP
and NADPH
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Figure 5: The light and dark reactions in the chloroplast
The chloroplast is involved in both stages of photosynthesis
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There, water (H2O) is
oxidized, and oxygen (O2) is released
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The dark
reactions then occur outside the thylakoid
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The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism
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Once the light reactions have occurred, the light-independent or "dark" reactions take place in
the chloroplast stroma
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Cells then use G3P to build a wide variety of other sugars
(such as glucose) and organic molecules
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The products of these reactions
are then transported to other parts of the cell, including the mitochondria, where they are
broken down to make more energy carrier molecules to satisfy the metabolic demands of the
cell
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Conclusion
Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar
energy
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These cells not only drive the global carbon
cycle, but th

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