PHOTOSYNTHESIS

Introduction

The processes in all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating, that is, by ingesting other organisms. But where does the stored energy in food originate? All of this energy can be traced back to photosynthesis.

Overview of Photosynthesis

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are 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 almost 200 million years ago.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (Figure 1.1). 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; hence, they are referred to as chemoautotrophs.

Figure  1.1 Photoautotrophs including (a) plants,  (b) algae, and (c) cyanobacteria synthesize their organic  compounds via photosynthesis using  sunlight  as an energy source. Cyanobacteria and  planktonic  algae can  grow over enormous areas in water,  at  times  completely covering  the  surface. In a  (d) deep sea vent,  chemoautotrophs, such  as these (e) thermophilic  bacteria, capture energy from inorganic  compounds to produce organic  compounds. The ecosystem surrounding the  vents   has   a  diverse array  of  animals, such   as tubeworms, crustaceans, and  octopi  that  derive energy from the bacteria. (credit a: modification  of work by Steve Hillebrand,  U.S. Fish and  Wildlife Service; credit b: modification  of work by "eutrophication&hypoxia"/Flickr; credit c: modification  of work by NASA; credit d: University of Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research Center, UAF, NOAA)

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo-” part) as high-energy electrons 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 (Figure 1.2), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

Figure  1.2  The  energy stored in carbohydrate molecules from photosynthesis passes  through  the  food  chain.  The predator that eats these deer  receives a portion of the energy that originated in the photosynthetic vegetation that the deer  consumed. (credit: modification of work by Steve VanRiper,  U.S. Fish and Wildlife Service)

Main Structures and Summary of Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 1.3). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in energy) that can subsequently 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.

Figure  1.3  Photosynthesis uses solar  energy, carbon dioxide,  and  water  to produce energy-storing carbohydrates. Oxygen  is generated as a waste product  of photosynthesis.

The following is the chemical equation for photosynthesis (Figure 1.4):

Figure  1.4  The  basic  equation for photosynthesis is deceptively simple.  In reality,  the  process takes place  in many steps involving intermediate reactants and  products. Glucose, the  primary  energy source in cells,  is made from two three-carbon GA3Ps.

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.

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. 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 in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). 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 Figure 1.5, 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).

  

Figure  1.5 Photosynthesis takes place  in chloroplasts, which have  an outer  membrane and  an inner membrane. Stacks of thylakoids  called grana form a third membrane layer.

On a hot, dry  day, plants close their stomata to conserve water. What impact will this have on photosynthesis?

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 drive 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, 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. Figure 1.6 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Figure  1.6 Photosynthesis takes place  in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take  place  in the thylakoid membrane, use  light energy to make  ATP and  NADPH. The Calvin cycle, which takes place  in the stroma, uses energy derived  from these compounds to make  GA3P from CO2.