PHOTOSYNTHESIS

Photosynthesis: the jamming together of CO₂ (carbon dioxide) with H₂O (water) to make CH₂O (sugar) and O₂ (oxygen), using the sun's energy. The sugar contains the stored energy and serves as the raw material from which other compounds are made

Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight into chemical energy

TYPES OF PHOTOSYNTHESIS

There are two types of photosynthetic processes: oxygenic photosynthesis and anoxygenic photosynthesis. Oxygenic photosynthesis is the most common and is seen in plants, algae and cyanobacteria. 

During oxygenic photosynthesis, light energy transfers electrons from water (H₂O) to carbon dioxide (CO₂), which produces carbohydrates. In this transfer, the CO₂ is "reduced," or receives electrons, and the water becomes "oxidized," or loses electrons. Ultimately, oxygen is produced along with carbohydrates. 

Oxygenic photosynthesis functions as a counterbalance to respiration; it takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere.

Oxygenic photosynthesis is written as follows: 

6CO₂ + 12H₂O + Light Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

CO₂ + 2H₂A + Light Energy → [CH₂O] + 2A + H₂O

PHOTOSYNTHETIC PROCESS

Anoxygenic photosynthetic and oxygenic photosynthetic organisms use different electron donors for photosynthesis. Moreover, anoxygenic photosynthesis takes place in only one type of reaction center, while oxygenic photosynthesis takes place in two, each of which absorbs a different wavelength of light, according to Govindjee and Whitmarsh. However, the general principles of the two processes are similar. Below are the steps of photosynthesis, focusing on the process as it occurs in plants.

The reactions of plant photosynthesis are divided into those that require the presence of sunlight and those that do not. Both types of reactions take place in chloroplasts: light-dependent reactions in the thylakoid and light-independent reactions in the stroma. 

Light-dependent reactions (also called light reactions): When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron. “The trick to do useful work, is to prevent that electron from finding its way back to its original home,” Baum told LiveScience. “This is not easily avoided because the chlorophyll now has an “electron hole” that tends to pull on nearby electrons.” The released electron manages to escape by traveling through an electron transport chain, which generates the energy needed to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH. The “electron hole” in the original chlorophyll pigment is filled by taking an electron from water. As a result, oxygen is released into the atmosphere.

Light-independent reactions (also called dark reactions): ATP and NADPH are rich energy sources, which drive dark reactions. During this process carbon dioxide and water combine to form carbohydrates like glucose. This is known as carbon fixation.

VARIOUS FORMS OF PHOTOSYNTHESIS

C3 Photosynthesis : C3 plants.

• Called C3 because the CO₂ is first incorporated into a 3-carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO₂. Rubisco accounts for 16% of the protein content of the chloroplast and is likely the most abundant protein on Earth!
• Photosynthesis takes place throughout the leaf.
• Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy).
• Most plants are C3!

C4 Photosynthesis : C4 plants.

• Over 8000 species of angiosperms (flowering plants) scattered among 18 different families, have developed adaptations which minimize the losses to photorespiration (loss of H20).
• They all use a supplementary method of CO₂ uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C₄ plants. (Plants that have only the Calvin cycle are C₃ plants.)
• Uses PEP Carboxylase for the enzyme involved in the uptake of CO₂. This enzyme allows CO₂ to be taken into the plant very quickly, and then it "delivers" the CO₂ directly to RUBISCO for photosynthesis. By bringing the CO₂ (substrate) directly to the RUBISCO (enzyme), the enzyme-catalyzed reaction can happen much faster and more efficiently!
• Additional advantage is that sometimes RUBISCO binds with O₂ instead of CO₂ which is counterproductive. By having the PEP Carboxylase bring the CO₂ directly to the RUBISCO this problem is elimated!
• The trade-off for operating with this extra step that speeds-up photosynthetic rates is that on average, C4 plants need more energy to fix CO₂ than normal C3 plants so cannot grow in many shaded environments where C3 plants can.
• Adaptive Value:
• Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO₂ is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration (sometimes O₂ taken by mistake which is BAD).
• Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO₂ gain for photosynthesis.
• C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.
• So why don't all plants adopt the C4 process? Or, more correctly, why don't the C4 plants out-compete the C3 plants, which are inefficient? Well, notice that it takes ATP to bring the carbon dioxide to the Rubisco. In moderate temperatures, the energy burden that this puts on the plant outweighs the advantage of eliminating the one in five times that Rubisco binds oxygen instead of carbon dioxide. In warmer climates, however, the C4 plants win with their novel strategy.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism

• Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO₂ is stored in the form of an acid before use in photosynthesis.
• Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO₂ is converted to an acid and stored during the night. During the day, the acid is broken down and the CO₂ is released to RUBISCO for photosynthesis
• Adaptive Value:
• Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
• May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO₂ given off in respiration is used for photosynthesis (remember, plants do both!). This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).

CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

COMPARISON BETWEEN THE THREE FORMS OF PHOTOSYNTHESIS