Overview of Photosynthesis - OpenStax CNX8… · Overview of Photosynthesis * OpenStax This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License

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Overview of Photosynthesis*

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Abstract

By the end of this section, you will be able to:

• Explain the relevance of photosynthesis to other living things• Describe the main structures involved in photosynthesis• Identify the substrates and products of photosynthesis• Summarize the process of photosynthesis

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the onlybiological process that can capture energy that originates in outer space (sunlight) and convert it intochemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energyof sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugarmolecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burningof coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200million years ago.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performingphotosynthesis (Figure 1). Because they use light to manufacture their own food, they are called pho-toautotrophs (literally, �self-feeders using light�). Other organisms, such as animals, fungi, and most otherbacteria, are termed heterotrophs (�other feeders�), because they must rely on the sugars produced by pho-tosynthetic 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, theyare referred to as chemoautotrophs.

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Figure 1: Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their or-ganic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonicalgae can grow over enormous areas in water, at times completely covering the surface. In a (d) deepsea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic com-pounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array ofanimals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria. (credit a:modi�cation of work by Steve Hillebrand, U.S. Fish and Wildlife Service; credit b: modi�cation of workby "eutrophication&hypoxia"/Flickr; credit c: modi�cation of work by NASA; credit d: University ofWashington, NOAA; credit e: modi�cation of work by Mark Amend, West Coast and Polar RegionsUndersea Research Center, UAF, NOAA)

The importance of photosynthesis is not just that it can capture sunlight's energy. A lizard sunningitself on a cold day can use the sun's energy to warm up. Photosynthesis is vital because it evolved as away to store the energy in solar radiation (the �photo-� part) as high-energy electrons in the carbon-carbonbonds of carbohydrate molecules (the �-synthesis� part). Those carbohydrates are the energy source thatheterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percentof Earth's ecosystems. When a top predator, such as a wolf, preys on a deer (Figure 2), the wolf is at theend of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis,to vegetation, to deer, and �nally to wolf.



Figure 2: The energy stored in carbohydrate molecules from photosynthesis passes through the foodchain. The predator that eats these deer receives a portion of the energy that originated in the photo-synthetic vegetation that the deer consumed. (credit: modi�cation of work by Steve VanRiper, U.S. Fishand Wildlife Service)

1 Main Structures and Summary of Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), andwater as substrates (Figure 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 beconverted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules containenergy and the energized carbon that all living things need to survive.



Figure 3: Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing car-bohydrates. Oxygen is generated as a waste product of photosynthesis.



The following is the chemical equation for photosynthesis (Figure 4):

Figure 4: The basic equation for photosynthesis is deceptively simple. In reality, the process takesplace in many steps involving intermediate reactants and products. Glucose, the primary energy sourcein cells, is made from two three-carbon GA3Ps.

Although the equation looks simple, the many steps that take place during photosynthesis are actuallyquite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is importantto become familiar with the structures involved.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. Theprocess of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbondioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which alsoplay roles in the regulation of gas exchange and water balance. The stomata are typically located on theunderside of the leaf, which helps to minimize water loss. Each stoma is �anked by guard cells that regulatethe 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. Forplants, 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-shapedstructures called thylakoids. Embedded in the thylakoid membrane is chlorophyll, a pigment (moleculethat absorbs light) responsible for the initial interaction between light and plant material, and numerousproteins that make up the electron transport chain. The thylakoid membrane encloses an internal spacecalled the thylakoid lumen. As shown in Figure 5, a stack of thylakoids is called a granum, and theliquid-�lled space surrounding the granum is called stroma or �bed� (not to be confused with stoma or�mouth,� an opening on the leaf epidermis).

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Figure 5: Photosynthesis takes place in chloroplasts, which have an outer membrane and an innermembrane. 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 onphotosynthesis?

2 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 thatenergy is converted into stored chemical energy. In the light-independent reactions, the chemical energyharvested 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 productsof the light-dependent reactions to function. In addition, several enzymes of the light-independent reactionsare activated by light. The light-dependent reactions utilize certain molecules to temporarily store theenergy: These are referred to as energy carriers. The energy carriers that move energy from light-dependentreactions to light-independent reactions can be thought of as �full� because they are rich in energy. After theenergy is released, the �empty� energy carriers return to the light-dependent reaction to obtain more energy.Figure 6 illustrates the components inside the chloroplast where the light-dependent and light-independentreactions take place.



Figure 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 ATPand NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these com-pounds to make GA3P from CO2.



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Click the link1 to learn more about photosynthesis.

: Photosynthesis at the Grocery Store

1http://openstaxcollege.org/l/photosynthesis



Figure 7: Foods that humans consume originate from photosynthesis. (credit: Associação Brasileira deSupermercados)

Major grocery stores in the United States are organized into departments, such as dairy, meats,produce, bread, cereals, and so forth. Each aisle (Figure 7) contains hundreds, if not thousands, ofdi�erent products for customers to buy and consume.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link,because the animals were fed plant-based foods. The breads, cereals, and pastas come largelyfrom starchy grains, which are the seeds of photosynthesis-dependent plants. What about dessertsand drinks? All of these products contain sugar�sucrose is a plant product, a disaccharide, acarbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are lessobviously derived from plants: For instance, paper goods are generally plant products, and manyplastics (abundant as products and packaging) are derived from algae. Virtually every spice and�avoring in the spice aisle was produced by a plant as a leaf, root, bark, �ower, fruit, or stem.Ultimately, photosynthesis connects to every meal and every food a person consumes.

3 Section Summary

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesisevolved to allow living things access to enormous amounts of energy. Because of photosynthesis, living thingsgained access to su�cient energy that allowed them to build new structures and achieve the biodiversityevident today.

Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presenceof chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture



energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules andrelease oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, haveorganelles 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 theplasma membrane, and in the cytoplasm.

4 Art Connections

Exercise 1 (Solution on p. 12.)

Figure 5 On a hot, dry day, plants close their stomata to conserve water. What impact will thishave on photosynthesis?

5 Review Questions


Which of the following components is not used by both plants and cyanobacteria to carry outphotosynthesis?

a. chloroplastsb. chlorophyllc. carbon dioxided. water


What two main products result from photosynthesis?

a. oxygen and carbon dioxideb. chlorophyll and oxygenc. sugars/carbohydrates and oxygend. sugars/carbohydrates and carbon dioxide


In which compartment of the plant cell do the light-independent reactions of photosynthesis takeplace?

a. thylakoidb. stromac. outer membraned. mesophyll


Which statement about thylakoids in eukaryotes is not correct?

a. Thylakoids are assembled into stacks.b. Thylakoids exist as a maze of folded membranes.c. The space surrounding thylakoids is called stroma.d. Thylakoids contain chlorophyll.



6 Free Response


What is the overall outcome of the light reactions in photosynthesis?


Why are carnivores, such as lions, dependent on photosynthesis to survive?


Why are energy carriers thought of as either �full� or �empty�?



Solutions to Exercises in this Module

to Exercise (p. 10)Figure 5 Levels of carbon dioxide (a necessary photosynthetic substrate) will immediately fall. As a result,the rate of photosynthesis will be inhibited.to Exercise (p. 10)Ato Exercise (p. 10)Cto Exercise (p. 10)Bto Exercise (p. 10)Bto Exercise (p. 11)The outcome of light reactions in photosynthesis is the conversion of solar energy into chemical energy thatthe chloroplasts can use to do work (mostly anabolic production of carbohydrates from carbon dioxide).to Exercise (p. 11)Because lions eat animals that eat plants.to Exercise (p. 11)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 andNADPH are produced in the stroma where they are also used and reconverted into ADP, Pi, and NADP+.

Glossary

De�nition 7: chemoautotrophorganism that can build organic molecules using energy derived from inorganic chemicals insteadof sunlight

De�nition 7: chloroplastorganelle in which photosynthesis takes place

De�nition 7: granumstack of thylakoids located inside a chloroplast

De�nition 7: heterotrophorganism that consumes organic substances or other organisms for food

De�nition 7: light-dependent reaction�rst stage of photosynthesis where certain wavelengths of the visible light are absorbed to form twoenergy-carrying molecules (ATP and NADPH)

De�nition 7: light-independent reactionsecond stage of photosynthesis, though which carbon dioxide is used to build carbohydrate moleculesusing energy from ATP and NADPH

De�nition 7: mesophyllmiddle layer of chlorophyll-rich cells in a leaf

De�nition 7: photoautotrophorganism capable of producing its own organic compounds from sunlight

De�nition 7: pigmentmolecule that is capable of absorbing certain wavelengths of light and re�ecting others (whichaccounts for its color)



De�nition 7: stomaopening that regulates gas exchange and water evaporation between leaves and the environment,typically situated on the underside of leaves

De�nition 7: stroma�uid-�lled space surrounding the grana inside a chloroplast where the light-independent reactionsof photosynthesis take place

De�nition 7: thylakoiddisc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions ofphotosynthesis take place; stacks of thylakoids are called grana

De�nition 7: thylakoid lumenaqueous space bound by a thylakoid membrane where protons accumulate during light-driven elec-tron transport


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Overview of Photosynthesis - OpenStax CNX8… · Overview of Photosynthesis * OpenStax This work is produced by OpenStax-CNX and licensed under the Creative Commons Attribution License