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Types of Photosynthesis: C3, C4 and CAM

By Martin Schweig

My first interest in succulent plants developed because of their unique physical differences to most other botanical species. What I did not realize was how different they were in many other aspects of their existence.

Their basic biochemical process is somewhat different from the chemistry of most other plants. To survive in a dry environment with irregular or little rainfall, succulent plants must store water in their leaves, stems or roots. These plants often show specific adaptations in their metabolism.

As we know, plants produce food by photosynthesis, which is the bonding together of carbon dioxide with water to make sugar and oxygen using the sun's energy. Sugar contains the stored energy and serves as the raw material from which other compounds are made.

What I was not aware of is that there are at least three different pathways in which photosynthesis can occur to achieve the same results. They are known as C3, C4 and CAM, because the first chemical made by the plant is a three- or four-chain molecule.

C3 (normal conditions)
C4 (high temperature/high water/high light availability)
CAM (high temperature/low water availability)

CAM stands for crassulacean acid metabolism, after the plant family in which it was first discovered. It is essentially a means of isolating in time the carbon dioxide intake from sunlight-fueled photosynthesis. Acid is stored at night within the plant so that during the day it can be turned into sugars by photosynthesis.

All plants can use C3 photosynthesis, and some are able to use all three types. However, C4 and CAM do not exist in the same plant. It is interesting to note that the only cacti to use C3 photosynthesis is the primitive pereskia.

C4 and CAM photosynthesis are both adaptations to arid conditions, because they are more efficient in the conservation of water. CAM plants are also able to "idle," thus saving energy and water during periods of harsh conditions. CAM plants include many succulents such as Cactaceae, Agavacea, Crassulaceae, Euphorbiaceae, Liliaceae, Vitaceae (grapes), Orchidaceae and bromeliads.

CAM plants take in carbon dioxide during the night hours, fixing it within the plant as an organic acid with the help of an enzyme. During the daylight hours, CAM plants can have normal C3 metabolism, converting carbon dioxide directly into sugars or storing it for the next day's metabolism for use in the evening.

With the sun's energy during daylight, the stored organic acid is broken down internally with the help of enzymes to release carbon dioxide within the plant to make sugars. The stomata (pores) can be open during the evening when the temperature is lower and humidity relatively higher.

During the day, the stomata can remain closed, using the internally released carbon dioxide and thus sealing the plant off from the outside environment. This is probably a six to 10 times more efficient way to prevent water loss compared to normal plant respiration. This modified effect seems to work best when there is a considerable difference between daytime and nighttime temperatures.

C4 plants can photosynthesize faster under a desert's extreme heat than C3 plants, because they use extra biochemical pathways and anatomy to reduce photorespiration. Photorespiration basically occurs when the enzyme (rubisco) that grabs carbon dioxide for photosynthesis grabs oxygen instead, causing respiration that blocks photosynthesis and thus causes a slowing of the production of sugars.

The majority of plants fall into the C3 category and are best adapted to rather cool, moist temperatures and normal light conditions. Their stomata are usually open during the day.

When conditions are extremely arid, CAM plants can just leave their stoma closed night and day, and the organic cycle is fed by internal recycling of the nocturnally fixed respiratory carbon dioxide. Of course, this is somewhat like a perpetual motion machine, and because there are costs in running this machinery, the plant cannot CAM-idle for very long. This idling does, however, allow the plants to survive dry spells and recover quickly when water is again available. This is quite unlike plants that drop their leaves and go dormant during dry spells.

The following comparison of photosynthesis and respiration may be helpful.

The light-independent reactions of photosynthesis

Despite much diversity in life form and biochemical process, all of the photosynthetic pathways focus upon a single enzyme which is by far the most abundant protein on earth, namely ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco (Figure 2.1a). Localised in the stroma of chloroplasts, this enzyme enables the primary catalytic step in photosynthetic carbon reduction (or PCR cycle) in all green plants and algae. Although Rubisco has been highly conserved throughout evolutionary history, this enzyme is surprisingly inefficient with a slow catalytic turnover (Vcmax), a poor specificity for CO2 as opposed to O2 (Sc/o), and a propensity for catalytic misfiring resulting in the production of catalytic inhibitors. This combination severely restricts photosynthetic performance of C3 plants under current ambient conditions of 20% O2 and 0.039% CO2 (390 μL L-1). Furthermore, Rubisco has a requirement for its own activating enzyme, Rubisco activase, which removes inhibitors from the catalytic sites to allow further catalysis. Accordingly, and in response to CO2 limitation, C4, C3-C4 intermediate, CAM and SAM variants have evolved with metabolic concentrating devices which enhance Rubisco performance (Section 2.2).

Plant Energy Transformations-Photosynthesis - …

Photosynthesis - Plant Structure (BOT315)

which depends on the maximal Rubisco activity and provided the quantitative framework for comparing rates of CO2 assimilations with the amount of Rubisco present in leaves (von Caemmerer and Farquhar 1981). Difference in CO2 assimilation rates observed under different growth conditions could then be explained according to variations in the amount of Rubisco present in leaves. In Figure 2 the dotted line shows a CO2 response curve modelled by Equation 7. Chloroplast CO2 partial pressure was then assumed to be similar to that in the intercellular airspaces. Using on-line discrimination between 13CO2 and 12CO2, and deriving an estimate of CO2 partial pressure at fixation sites within chloroplasts, we subsequently learned that a further draw down can occur, but the general applicability of Equation 7 was not compromised. As an aside, these equations became basic to most photosynthetic models long before the order of the reaction mechanism of Rubisco had been unequivocally established. Had CO2 and O2 bound to Rubisco before RuBP, or the reaction not been ordered, our equations would have been much more complex with both m(CO2) and m(O2) dependent upon RuBP concentration.

View Ways are C4 photosynthesis and CAM photosynthesis similar: They both use PEP carboxylase in the fixation.C3 Photosynthesis Plants which use The limits are placed by the fact that rubisco begins to fix oxygen rather than CO 2, In the CAM strategy.(photochemical stage) and carbon fixation (thermochemical) stages.

Plant Structure (BOT315) Search ..

important enzyme called RuBisCO ..

Rubisco first evolved when the earth’s atmosphere was rich in CO2, but virtually devoid of O2. With the advent of oxygen-producing photosynthesis by land plants, and the resulting increases in atmospheric O2, one key deficiency of this enzyme became apparent. Rubisco would not only catalyse fixation of CO2 but would also permit incorporation of O2 into RuBP to produce, instead of two molecules of 3-PGA, just one molecule of 3-PGA with one molecule of a two-carbon compound, 2-phosphoglycolate (Section 2.3). Indeed, CO2 and O2 compete directly for access to the active sites of Rubisco. So feeble is Rubisco’s ability to distinguish between these two substrates that in air (20% O2) approximately one molecule of O2 is fixed for every three molecules of CO2.

Crassulacean acid metabolism (CAM) is a water-conserving mode of photosynthesis that, like C4 photosynthesis, is a modification of the C3 photosynthetic pathway fitted with a CO2 concentrating mechanism (CCM) that can increase the [CO2] around ribulose bisphosphate carboxylase/oxygenase (Rubisco) by more than 10-fold and suppress photorespiration. The overall energy demand of the CAM pathway is only about 10% more than that of C3 photosynthesis, as costs of the CCM machinery are partially offset by reducing photorespiration.

cam photosynthesis limits co2 fixation - …
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C 2 photosynthesis; CAM photosynthesis; ..

C3: Rice, soybean, wheat, rye, oats,
millet, barley, potato
C4: Maize (corn), sorghum, pearl millet
and sugarcane
CAM: Pineapple, Cactus
The C4 plant will have the highest rate of photosynthesis over 12 hours of daylight compared to CAM and C3 plants because C4 plants can thrive in direct sunlight and it uses Pep carboxylase instead of Rubisco.
Experimental Design
There will be one plant of each kind; a soybean plant is a C3 plant, a sugarcane plant is a C4 plant, and a cactus is a Cam plant.
There will be 3 chambers with intense sunlight(50 watts)
shining into all of the chambers equally.

Life Happens: Rubisco - YouTube

Photosynthetic carbon fixation in air is constrained by the kinetic properties of Rubisco. Form I Rubisco in higher plants is a large protein (approximately 550 kDa) comprised of eight large (approx. 50-55 kDa) and eight small subunits (approx. 13-18 kDa) to form an L8S8 hexadecamer. Rubisco synthesis and assembly in higher plants is a complex process whereby the large subunit gene (rbcL) is encoded in the chloroplast genome, while the small subunit genes (rbcS) are encoded as a multi-gene family in the nucleus. The Rubisco small subunits are translated as precursors in thc cytosol and are equipped with a transit-peptide to target them to the chloroplast. Upon import in the chloroplast the transit-peptide is cleaved by a stromal peptidase and the N-terminus modified by methylation of the n-terminal methionine. The large subunits are synthesised within the chloroplast and also post-translationally modified through the removal of the the N-terminal methionine and serine amino acids and the subsequent acetylation of proline at the N-terminus and the methylation of lysine at position 14. The assembly of large and small subunits into functional hexadecameric Rubisco is reliant on the coordination of chloroplast-localised chaperones.

14/11/2010 · Life Happens: Rubisco Life Happens

In summary there are three photosynthetic-types commonly recognized; C3, C4, and CAM. C3 plants fix atmospheric CO2 directly onto RuBP and thus into glucose. C4 plants first fix atmospheric CO2 into 4-carbon acids in the mesophyll of the leaf and decarboxylate the 4-carbon acids in the bundle sheath cells where the CO2 is then fixed by RuBP carboxylase (all of this takes place in the day). CAM plants first fix atmospheric CO2 into malic acid and other 4C-acids at night. During the day, malic acid is decarboxylated and the CO2 released is then fixed by Rubisco (all of this takes place in the same cell).

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