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. This process, called photosynthesis, is essential to the global carbon cycle and organisms that conduct photosynthesis represent the lowest level in most food chains (Figure 1).

Figure 1: Photosynthetic plants synthesize carbon-based energy molecules from the energy in sunlight. Consequently, they provide an abundance of energy for other organisms.

Plants exist in a wide variety of shapes and sizes. (A) Coleochaete orbicularis (Charophyceae) gametophyte; magnification x 75 (photograph courtesy of L. E. Graham). (B) Chara (Charophyceae) gametophyte; magnification x 1.5 (photograph courtesy of M. Feist). (C) Riccia (liverwort) gametophyte showing sporangia (black) embedded in the thallus; magnification x 5 (photograph courtesy of A. N. Drinnan). (D) Anthoceros (hornwort) gametophyte showing unbranched sporophytes; magnification x 2.5 (photograph courtesy of A. N. Drinnan). (E) Mnium (moss) gametophyte showing unbranched sporophytes with terminal sporangia (capsule); magnification x 4.5 (photograph courtesy of W. Burger). (F) Huperzia (clubmoss) sporophyte with leaves showing sessile yellow sporangia; magnification x 0.8. (G) Dicranopteris (fern) sporophyte showing leaves with circinate vernation; magnification x 0.08. (H) Psilotum (whisk fern) sporophyte with reduced leaves and spherical synangia (three fused sporangia); magnification x 0.4. (I) Equisetum (horsetail) sporophyte with whorled branches, reduced leaves, and a terminal cone; magnification x 0.4. (J) Cycas (seed plant) sporophyte showing leaves and terminal cone with seeds; magnification x 0.05 (photograph courtesy of W. Burger).

© 1993 Elsevier Part A: Graham, L. E. Origin of land plants. New York: J. Wiley and Sons, 1993. All rights reserved. Part B: courtesy of M. Feist, University of Montpellier. Parts C and D: courtesy of Andrew Drinnan, Univeristy of Melbourne, School of Botany. Parts E, F and J: Courtesy of William Burger, Field Museum, Chicago.

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Most living things depend on photosynthetic cells to manufacture the complex organic molecules they require as a source of energy. Photosynthetic cells are quite diverse and include cells found in green plants, phytoplankton, and cyanobacteria. During the process of photosynthesis, cells use carbon dioxide and energy from the Sun to make sugar molecules and oxygen. These sugar molecules are the basis for more complex molecules made by the photosynthetic cell, such as glucose. 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. Therefore, the synthesis of glucose and its breakdown by cells are opposing processes.

Figure 2

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However, photosynthesis doesn't just drive the carbon cycle — it also creates the oxygen necessary for respiring organisms. 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.

Figure 3: Structure of a chloroplast

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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. These other pigments may help channel light energy to chlorophyll A or protect the cell from photo-damage. 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.

Figure 4: Diagram of a chloroplast inside a cell, showing thylakoid stacks

Shown here is a chloroplast inside a cell, with the outer membrane (OE) and inner membrane (IE) labeled. Other features of the cell include the nucleus (N), mitochondrion (M), and plasma membrane (PM). At right and below are microscopic images of thylakoid stacks called grana. Note the relationship between the granal and stromal membranes.

© 2004 Nature Publishing Group Soll, J. & Schleiff, E. Protein import into chloroplasts. Nature Reviews Molecular Cell Biology 5, 198-208 (2004) doi:10.1038/nrm1333. All rights reserved.

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Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called "light" reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. 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. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

Figure 5: The light and dark reactions in the chloroplast

The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H₂O) is oxidized, and oxygen (O₂) is released. The electrons that freed from the water are transferred to ATP and NADPH. The dark reactions then occur outside the thylakoid. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO₂). The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma (the aqueous fluid surrounding the stacks of thylakoids) and in the cytoplasm.

© 2010 Nature Education All rights reserved.

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Once the light reactions have occurred, the light-independent or "dark" reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. 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. In plants, some sugar molecules are stored as sucrose or starch.