Learning objectives
By the end of this section, you will be able to do the following:
- Explain the significance of photosynthesis to other living organisms
- Describe the main structures involved in photosynthesis
- Identify the substrates and products of photosynthesis
Photosynthesis is essential to almost all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates from sunlight and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. It is also a source of oxygen necessary for many living organisms. In brief, the energy of sunlight is “captured” to energize electrons, whose energy is then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis 350 to 200 million years ago during the Carboniferous Period.
Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (fig-ch08_01_01). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds. For this reason, they are referred to as chemoautotrophs.
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Original figure description: Photo a shows a fern leaf. Photo b shows thick, green algae growing on water. Micrograph c shows cyanobacteria, which are green rods about 10 microns long. Photo D shows black smoke pouring out of a deep sea vent covered with red worms. Micrograph E shows rod-shaped bacteria about 1.5 microns long.
The importance of photosynthesis is not just that it can capture sunlight’s energy. After all, a lizard sunning itself on a cold day can use the sun’s energy to warm up in a process called behavioral thermoregulation. In contrast, photosynthesis is vital because it evolved as a way to store the energy from solar radiation (the “photo-” part) to energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (fig-ch08_01_02), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to visible light, to photosynthesis, to vegetation, to deer, and finally to the wolf.
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Original figure description: A photo shows deer running through tall grass beside a forest.
Main Structures and Summary of Photosynthesis
Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide (which is low in energy), and water as substrates (fig-ch08_01_03). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (G3P), as well as simple carbohydrate molecules (high in energy) that can then be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.
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Original figure description: Photo of a tree with labels shows photosynthesis. Arrows indicate that the tree uses carbon dioxide, water, and sunlight to make sugars and oxygen. Water is absorbed through the tree's roots; sunlight is absorbed through the tree's leaves; the tree also absorbs carbon dioxide, and releases oxygen.
The following is the chemical equation for photosynthesis (fig-ch08_01_04):
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Original figure description: The photosynthesis equation is shown. According to this equation, six carbon dioxide and six water molecules produce one sugar molecule and six oxygen molecules. The sugar molecule is made of six carbons, twelve hydrogens, and six oxygens. Sunlight is used as an energy source.
Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.
Basic Photosynthetic Structures
In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss due to high temperatures on the upper surface of the leaf. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.
In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants, chloroplast-containing cells exist mostly in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane), and are ancestrally derived from ancient free-living cyanobacteria. Within the chloroplast are stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen. As shown in fig-ch08_01_05, a stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).
The Two Parts of Photosynthesis
Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, however, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. fig-ch08_01_06 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.
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Original figure description: This illustration shows a chloroplast with an outer membrane, an inner membrane, and stacks of membranes inside the inner membrane called thylakoids. The entire stack is called a granum. In the light reactions, energy from sunlight is converted into chemical energy in the form of A T P and N A D P H. In the process, water is used and oxygen is produced. Energy from A T P and N A D P H are used to power the Calvin cycle, which produces G 3 P from carbon dioxide. A T P is broken down to A D P and Pi, and N A D P H is oxidized to N A D P superscript plus sign baseline. The cycle is completed when the light reactions convert these molecules back into A T P and N A D P H.
Section Summary
The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, the evolution of photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.
Only certain organisms (photoautotrophs), can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain wavelengths of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a byproduct into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.
Visual Connection Questions
fig-ch08_01_05 On a hot, dry day, the guard cells of plants close their stomata to conserve water. What impact will this have on photosynthesis?
fig-ch08_01_05 Levels of carbon dioxide (a necessary photosynthetic substrate) will immediately fall. As a result, the rate of photosynthesis will be inhibited.
Review Questions
Which of the following components is not used by both plants and cyanobacteria to carry out photosynthesis?
- chloroplasts
- chlorophyll
- carbon dioxide
- water
A
What two main products result from photosynthesis?
- oxygen and carbon dioxide
- chlorophyll and oxygen
- sugars/carbohydrates and oxygen
- sugars/carbohydrates and carbon dioxide
C
In which compartment of the plant cell do the light-independent reactions of photosynthesis take place?
- thylakoid
- stroma
- outer membrane
- mesophyll
B
Which statement about thylakoids in eukaryotes is not correct?
- Thylakoids are assembled into stacks.
- Thylakoids exist as a maze of folded membranes.
- The space surrounding thylakoids is called stroma.
- Thylakoids contain chlorophyll.
B
Predict the end result if a chloroplast’s light-independent enzymes developed a mutation that prevented them from activating in response to light.
- G3P accumulation
- ATP and NADPH accumulation
- Water accumulation
- Carbon dioxide depletion
B
How are the NADPH and G3P molecules made during photosynthesis similar?
- They are both end products of photosynthesis.
- They are both substrates for photosynthesis.
- They are both produced from carbon dioxide.
- They both store energy in chemical bonds.
D
Critical Thinking Questions
What is the overall outcome of the light reactions in photosynthesis?
The outcome of light reactions in photosynthesis is the conversion of solar energy into chemical energy that the chloroplasts can use to do work (mostly anabolic production of carbohydrates from carbon dioxide).
Why are carnivores, such as lions, dependent on photosynthesis to survive?
Because lions eat animals that eat plants.
Why are energy carriers thought of as either “full” or “empty”?
The energy carriers that move from the light-dependent reaction to the light-independent one are “full” because they bring energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. There is not much actual movement involved. Both ATP and NADPH are produced in the stroma where they are also used and reconverted into ADP, Pi, and NADP+.
Describe how the grey wolf population would be impacted by a volcanic eruption that spewed a dense ash cloud that blocked sunlight in a section of Yellowstone National Park.
The grey wolves are apex predators in their food web, meaning they consume smaller prey animals and are not the prey of any other animal. Blocking sunlight would prevent the plants at the bottom of the food web from performing photosynthesis. This would kill many of the plants, reducing the food sources available to smaller animals in Yellowstone. A smaller prey animal population means that fewer wolves can survive in the area, and the population of grey wolves will decrease.
How does the closing of the stomata limit photosynthesis?
The stomata regulate the exchange of gases and water vapor between a leaf and its surrounding environment. When the stomata are closed, the water molecules cannot escape the leaf, but the leaf also cannot acquire new carbon dioxide molecules from the environment. This limits the light-independent reactions to only continuing until the carbon dioxide stores in the leaf are depleted.
Glossary
- chemoautotroph
- organism that can build organic molecules using energy derived from inorganic chemicals instead of sunlight
- chloroplast
- organelle in which photosynthesis takes place
- granum
- stack of thylakoids located inside a chloroplast
- heterotroph
- organism that consumes organic substances or other organisms for food
- light-dependent reaction
- first stage of photosynthesis where certain wavelengths of the visible light are absorbed to form two energy-carrying molecules (ATP and NADPH)
- light-independent reaction
- second stage of photosynthesis, through which carbon dioxide is used to build carbohydrate molecules using energy from ATP and NADPH
- mesophyll
- middle layer of chlorophyll-rich cells in a leaf
- photoautotroph
- organism capable of producing its own organic compounds from sunlight
- pigment
- molecule that is capable of absorbing certain wavelengths of light and reflecting others (which accounts for its color)
- stoma
- opening that regulates gas exchange and water evaporation between leaves and the environment, typically situated on the underside of leaves
- stroma
- fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of photosynthesis take place
- thylakoid
- disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of photosynthesis take place; stacks of thylakoids are called grana
- thylakoid lumen
- aqueous space bound by a thylakoid membrane where protons accumulate during light-driven electron transport