Carleton
College
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Carleton
College
|
Photosynthetic adaptations: The C4 and CAM alternative pathways
Outline:
I. Overview
II. C4 photosynthesis
III. CAM photosynthesis
Recommended Readings
Terms you should be able to explain to a friend:
Overview
We have talked a lot so far about the C3 photosynthetic pathway--so named because the first stable product after carboxylation (the addition of CO2 to Rubp by rubisco) is a 3 carbon compound-- 3-P-glycerate.
We have also talked some about photosynthetic responses to high light, when we looked at protective accessory pigments and antioxidants in alpine plants.
Here's something cool to think about: High light environments are often characterized by arid conditions and periods of drought. Deserts and grasslands are two examples. This puts a serious constraint on photosynthesis. How can a plant keep its stomates open long enough to let in CO2 without losing a ton of water due to evaporation from the moist intercellular leaf spaces?
In addition to the adaptations of having protective pigments and antioxidants, some plant have evolved new photosynthetic pathways to help them cope with this carbon uptake vs. water loss in dry/high light conditions. These pathways are called the
(1) C-4 or Hatch-Slack Pathway. This path is common to grasses
(2) Crassulacean Acid Metabolism (CAM). This pathway is common to desert succulents (e.g. cacti) and epiphytes (e.g. orchids)
IMPORTANT POINT 1: We will see that these plants concentrate CO2 inside their leaf cells so that they don't have to keep their stomates open as long as C3 plants.
IMPORTANT POINT 2: As we walk through each of these pathways, note that the biochemistry of C-4 and CAM pathways is remarkably similar to that of C-3 plants. In fact, C4 and CAm plants use the C3 Calvin cycle, but they have added a few additional biochemical steps that enable them to concentrate CO2 quickly in leaves and then to shut stomates. In addition, there is temporal differentiation between C-4 and CAM: C4 plants concentrate during the day whereas CAM concentarte only at night.
**The bottom line is that very subtle evolutionary changes in (1) the leaf anatomy and (2) biochemical pathways of plants can lead to radically different ways of acquiring CO2. This has profound impacts on the ecological distribution of these species:
- C3 usually temparate, tropical, and boreal
- C4 usually grassland
- CAM usually desert (some tropical).
C4 photosynthesis
Let's start with differences in leaf anatomy between C3 and C4 species. In C3 plants, the vascular tissue is more or less embedded in the spongy mesophyll cells. In C4 plants, however, the vascular tissue is surrounded by an additional ring of bundle sheath cells that are closely connected with the spongy mesophyll (Taiz, Figs 8.9 and 8.10). This is sometimes referred to as Kranz anatomy, because "kranz" is German for wreath, and Germans were big into plant anatomy in the 1800's. It's important to note that all of the machinery for C3 dark reactions (RUBISCO, etc.) are concentrated in the bundle sheath cells. So what's the purpose of the mesophyll cells? Let's look at C4 photosynthesis as a 4-step process.
4 Stages of CO2 fixation (see fig 8.11, the reactions are in table 8.3):
Why is this cool and ecologically significant?
IMPORTANT POINT 1: The PEP carboxylase in the mesophyll cells have a high affinity for CO2. By rapidly converting CO2 into a 4-C molecule, this reduces the internal concentration of CO2 and creates a steep gradient of high CO2 outside the leaf and low CO2 inside the mesophyll cell. Having a steep concentration gradient allows the stomates to remain closed more tightly and still maintain a CO2 flux into the leaf. Let's check out a little math to drive home this point. Remember the universal flux equation:
Flux = conductivity * gradient
well, in this case it's
CO2 flux into leaf = k * gradient in CO2
CO2 flux into leaf = k * (Ca - Ci)
where k is the conductivity (= 1/resistance)
Ca = atmospheric CO2 concentration
Ci = internal CO2 concentration.
Now, consider how a C4 and C3 leaf can have the same flux of CO2 into the leaf.
So in the end, both plants may photosynthesize at the same rate, but the C4 plant does it with much tighter stomates, preventing water loss. We say that this kind of photosynthesis has a high water use efficiency:
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WUE =
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carbon uptake
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water loss
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and this is a beneficial strategy where water is limited (like grasslands and the desert). See Fig. 42 in Lambers Chapter 2.
IMPORTANT POINT 2: The bundle sheath cells accumulate CO2 at a much higher concentration than you would expect if they were in direct contact with the atmosphere. Higher levels of CO2 means that Rubisco spends more of its time fixing CO2 rather than O2, and photorespiration is reduced. Remember that photorespiration increases dramatically in C3 plants as temperature is increased. This doesn't happen in C4 plants, so C4 plants are often more competitive in environments with higher temperature (because C3 plants lose a lot of C to photorespiration). See Fig. 43 in Lambers Chapter 2.
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Why did this pathway evolve in the first place?
Until the last 50 million years ago, Earth's atmosphere was loaded with CO2--levels between 5-10 times higher than today's atmosphere. From 540MYA to about 260MYA the earth's plates were moving together to forming more-or-less one giant continent called "Pangea." (this is a cool map animation--scroll down a bit to the little map on the left; drag your mouse sideways across the map and watch the continents come together). Over the last 200 MY, the continental plates have been moving apart, separating the continents with wide oceans. Around 50 million years ago, the Indian subcontinent slammed into Asia as a result of plate techtonics, initiating the uplift of the Himalayan Mountains.
This large amount of exposed rock surface was subject to weathering by carbonic acid, produced when atmospheric H20 and CO2 combine to form H2CO3. The weathering process of silicate rocks can be represented generally as
CaSiO3 + H2CO3 --> HCO3- (bicarbonate ion) + Ca2+ (calcium ion) + SiO2 (silicate)
These materials flow in rivers to the ocean where marine organisms use them to build shells:
Ca2+ + HCO3- -> CaCO3 (calcium carbonate shells)
Thus, this overall rock weathering process scrubs CO2 out of the atmosphere and buries the C in the deep ocean. Over the last 50 million years, this weathering process has lowered the atmospheric CO2 to levels between 180-400 ppm. Therefore, CO2 in the last 5-7 million years has become a limiting resource for plants, and natural selection would favor innovative biochemical pathways that allow plants to concentrate CO2.
IMPORTANT POINT: Plants subject to lower CO2 are subject to relatively higher rates of O2 and photorespiration, leading to carbon loss. C-4 photosynthesis evades photorespiration by nearly saturating Rubisco with CO2.
Why doesn't every plant do this?
C4 plants represent only about 3% of the world's flora (C3 are 87%; CAM are 10%), suggesting that not may plants use the C4 path. Table 8.3 indicates that C4 plants need an additional 2 ATP--one to decarboxylate (reaction 5, table 8.3) and one to regenerate PEP (reaction 7, table 8.3).
As you might expect, this is an extra energy expense. However, think about what environments this added expense is truly costly. If the light reactions make ATP, then you might expect a couple of extra ATP would be easy to make in high light environments. This is not as simple for shaded plants or plants that grow in cloudy conditions.
Is this a difficult pathway to evolve from C3 photosynthesis?
Not really. You see it in plants that live all over the world, and it has evolved independently many times.
Figure 8.13 indicates that there are three common variants of the pathway, indicatin that the basic CO2 pump is fairly easy to evolve using different biochemical constituents for the 3 and 4-C organic acids.
Is it easy to determine the extent to which plants use this pathway?
Carbon isotopes are an incredibly cool way to estimate WUE among the C3, C4, and CAM pathways.
Carbon occurrs naturally in three forms, or isotopes, depending on the number of neutrons: 12C, 13C, and 14C
As you can see, 14C is the heaviest, 13C next heaviest, and 12C the lightest form of C. Here's why this is useful:
**Therefore, biochemical pathways of plants are a major factor determining which carbon isotopes are incorporated into plant matter. For example, the atmosphere has a 13C content of -8‰ relative to standard (Pee Dee bellamite--a limestone from North Carolina). In contrast, plants have lower 13C content because discriminate against 13C. C3 plants under well watered conditions have 13C = -30 to -35. Therefore autotrophic organisms always have more negative delta 13C ratios, because they discriminate against 13C. So far as we know, only living autotrophic organisms can impart this isotopic fractionation. Thus, you can grab a chunk of organic matter, figure out the C isotopes and determine if an organism produced it. This is how
Here's where things get interesting for physiological ecology:
C3 plants under well watered conditions have 13C = -30 to -35 (see Lambers chapter 2, figure 44)
C4 plants: Pep carboxylase shows virtually no discrimination between 12C and 13C (see Lambers chapter 2, figure 44)
Summary:
C isotopes are useful because 1) they give an integrated measure of WUE during
time carbon was being fixed, and 2) you can use them to trace source of carbon
found in soil or higher trophic levels.
Crassulacean Acid Metabolism (CAM) photosynthesis
CAM photosynthesis is a second evolutionary strategy employing a "CO2 pump" to accumulate CO2. It is technically also a C4 pathway, because CO2 is fixed first into a 4-C organic acid like oxaloacetate.
This path occurs in a wide variety of plant species, mainly in arid and tropical regions
As Taiz mentions on p. 217, to make the C4 "CO2 pump" happen, a plant needs spatial separation between the sites of initial CO2 fixation (mesophyll) and decarboxylation (bundle sheath). The specialized Kranz anatomy allows this to happen.
IMPORTANT POINT: CAM plants achieve the CO2 accumulating mechanism, not by a spatial separation of CO2 uptake and unloading, but, rather, a temporal separation of these two events. Check out figure 8.12 in Taiz or figure 45 in Lambers to get a sense of the following main steps of CAM photosynthesis in a mesophyll cell:
The net result is that the plant cell has a lot of CO2 in the mesophyll during daylight hours when it can be fixed by Rubisco in the Calvin cycle. the cool point, however, is that it loaded up all of the CO2 without ever having to open the stomates during the daylight hours! At night, the relative humidity of the air is higher, and there is less water lost from the plant.
This is an extremely inefficient process for several reasons:
If you measure each of these biochemical constituents, in a CAM leaf cell, you can see several distinct phases corresponding to the CAM cycle. Check out page 75 in Lambers--they identify 4 stages of the CAM cycle, and fig. 46 shows how CO2 fixation, malic acid, and glucal (PGA, or triose phosphates) change during the day (midnight = 24.00 hour).
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Interesting points about CAM photosynthesis:
1. During the wet seasons, such as the late summer monsoons in the desert Southwest, many succulents switch to exclusively C3 photosynthesis. That is, the open their stomates during the day, and the CO2 that enters the leaf is immediately fixed by Rubisco in the calvin cycle.
2. CAM plants exhibit a very high WUE. There is often a tradeoff, however, between efficiency and rate of process.
3. CAM idling --this is a cool phenomenon that happens with plants that shut their stomates for a long period of time.
Something similar to CAM idling occurs in some desert C3 species that keep their stomates shut for extended periods of time, such as the ocotillo bush with its small leaves.
4. CAM plants have 13C of about -12 to -20
