We tend to treat plants like passive objects that can ornament a home or yard, although perhaps requiring a bit more care than, say, a vase. But plants are in fact complex organisms that can interact with their environment, sense smells and sounds, communicate with each other and with insects, and even process information.
Plants may not be thinking in a way that we’d recognize, but they are certainly doing a lot more than sitting around splitting water molecules. Here is one thing plants do that look pretty damn smart — even to those of us over here in the Kingdom Animalia.
Plants make their own food out of inorganic compounds through a process called photosynthesis that takes place in the leaves. The ingredients necessary for photosynthesis are light, water and carbon dioxide.
Among terrestrial and aquatic autotrophs, there are three photosynthetic pathways. Three photosynthetic pathways exist among terrestrial plants: C3, C4, and crassulacean acid metabolism (CAM) photosynthesis.
In C3 photosynthesis, the compound to which CO2 is integrated first is a 3-carbon compound. The “enzyme” necessary for this process is known as RuBisCO. C3 photosynthesis is more efficient than C4 or CAM photosynthesis when the environment is cool and moist and when light is plentiful. However, since C3 uses more water than the other two types of photosynthesis, it is not as useful to organisms living in hot, arid environments.
C3 photosynthesis is the typical photosynthesis that most plants use. C4 and CAM photosynthesis are both adaptations to arid conditions because they result in better water use efficiency. In addition, CAM plants can “idle,” saving precious energy and water during harsh times, and C4 plants can photosynthesize faster under the desert’s high heat and light conditions than C3 plants because they use an extra biochemical pathway and special anatomy to reduce photorespiration.
C3 photosynthesis is the ancestral pathway for carbon ﬁxation and occurs in all taxonomic plant groups. The term C3 photosynthesis is based on the observation that the ﬁrst product of photosynthesis is a 3-carbon molecule. In C4 photosynthesis, the initial photosynthetic product is a 4-carbon molecule. C4 photosynthesis occurs in the more advanced plant taxa and is especially common among monocots, such as grasses and sedges, but not very common among dicots (most trees and shrubs).
CAM photosynthesis, in honor of the plant family in which this pathway was ﬁrst documented, occurs in many epiphytes and succulents from very arid regions.
Below are the details.
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.
- 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.
- Called C4 because the CO2 is first incorporated into a 4-carbon compound.
- Stomata are open during the day.
- 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 photsynthesis.
- Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
- 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.
- 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:, corn, and many of summer annual plants.
CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism
Shortly after the discovery of the C4 cycle the crassulacean acid metabolism (CAM) pathway was dissected and elucidated as a C4 variant. This pathway is found in desert succulents and epiphytes. In the typical C4 cycle, the fixation reaction occurred in a mesophyll cell and the decarboxylation reaction occurred in a bundle sheath cell, in CAM plants the two reactions are separated temporally rather than spatially. The CAM cycle is shown below…notice how all the reactions occur in the same cell but at different times of the day.
The fixation step occurs at night with the guard cells open to receive carbon dioxide during the cool night. This process is driven by use of starch to make the PEP required for PEP carboxylase activity. The malic acid is transported to the vacuole and accumulates there at night. There is a strong pH change in the vacuole at night!
- 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.).
- 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. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis. 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
In the desert, water is a precious resource, so desert plants must minimize water loss in order to stay alive. One way plants conserve water involves their ability to close thestomata in their leaves. Now thats what I call smart
What scientists in the field are now attempting is to engineer plants to be able to close their pores sooner so that precious water is not lost due to drought conditions, which can cause plants to suffer or to often die completely.
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