Which statement describes the difference between photosynthesis in chloroplasts

Photosynthesis and chemosynthesis are both processes by which organisms produce food; photosynthesis is powered by sunlight while chemosynthesis runs on chemical energy.

To view this video please enable JavaScript, and consider upgrading to a web browser that supports HTML5 video 

The majority of life on the planet is based in a food chain which revolves around sunlight, as plants make food via photosynthesis. In the deep ocean, however, there is no light and thus there are no plants; so instead of sunlight being the primary form of energy, chemical energy is produced via chemosynthesis. Places with chemosynthetic organisms can become oases of life in an environment often otherwise depleted of food. Video courtesy of NOAA Ocean Exploration, Gulf of Mexico 2017. Download larger version (mp4, 108.2 MB).

Ecosystems depend upon the ability of some organisms to convert inorganic compounds into food that other organisms can then exploit (or eat!). The majority of life on Earth is based on a food chain which revolves around the Sun, as plants use sunlight to make food via photosynthesis. However, in environments where there is no sunlight and thus no plants, organisms instead rely on primary production through a process called chemosynthesis, which runs on chemical energy. Together, photosynthesis and chemosynthesis fuel all life on Earth.

Photosynthesis occurs in plants and some bacteria, wherever there is sufficient sunlight, be it on land, in shallow water, or even inside and below clear ice. All photosynthetic organisms use solar energy to turn carbon dioxide and water into sugar (food) and oxygen: CO2 + 6H2O -> C6H12O6 + 6O2.

Chemosynthesis occurs in bacteria and other organisms and involves the use of energy released by inorganic chemical reactions to produce food. All chemosynthetic organisms use energy released by chemical reactions to make a sugar, but different species use different pathways. For example, at hydrothermal vents, vent bacteria oxidize hydrogen sulfide, add carbon dioxide and oxygen, and produce sugar, sulfur, and water: CO2 + 4H2S + O2 -> CH20 + 4S + 3H2O. Other bacteria make organic matter by reducing sulfide or oxidizing methane.

Our knowledge of chemosynthetic communities is relatively new, brought to light by ocean exploration when humans first observed a vent on the deep-ocean floor in 1977 and found a thriving community where there was no sunlight. Since then, chemosynthetic bacterial communities have been found in hot springs on land and on the seafloor around hydrothermal vents, cold seeps, whale carcasses, and even sunken ships. No one had ever thought to look for them, but these communities were there all along.

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.

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. 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.

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. Indeed, the fossil fuels we use to power our world today are the ancient remains of once-living organisms, and they provide a dramatic example of this cycle at work. The carbon cycle would not be possible without photosynthesis, because this process accounts for the "building" portion of the cycle (Figure 2).

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.

What Cells and Organelles Are Involved in Photosynthesis?

Figure 3: Structure of a chloroplast

Photosynthetic cells contain special pigments that absorb light energy. Different pigments respond to different wavelengths of visible light. Chlorophyll, the primary pigment used in photosynthesis, reflects green light and absorbs red and blue light most strongly. In plants, photosynthesis takes place in chloroplasts, which contain the chlorophyll. Chloroplasts are surrounded by a double membrane and contain a third inner membrane, called the thylakoid membrane, that forms long folds within the organelle. In electron micrographs, thylakoid membranes look like stacks of coins, although the compartments they form are connected like a maze of chambers. The green pigment chlorophyll is located within the thylakoid membrane, and the space between the thylakoid and the chloroplast membranes is called the stroma (Figure 3, Figure 4).

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.

What Are the Steps of Photosynthesis?

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.

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.

Conclusion

Photosynthetic cells contain chlorophyll and other light-sensitive pigments that capture solar energy. In the presence of carbon dioxide, such cells are able to convert this solar energy into energy-rich organic molecules, such as glucose. These cells not only drive the global carbon cycle, but they also produce much of the oxygen present in atmosphere of the Earth. Essentially, nonphotosynthetic cells use the products of photosynthesis to do the opposite of photosynthesis: break down glucose and release carbon dioxide.

Which statement describes the difference between photosynthesis and chloroplasts in cellular respiration in the mitochondria?

How would you describe the differences between photosynthesis?

What is the main function of chloroplast in photosynthesis?

How would you describe the differences between photosynthesis and respiration?