Why Are Plants Green? or Why Aren't Plant Black?

If I was hired as an engineer to design a machine whose job was to convert light energy into chemical energy I probably would not choose to use a green pigment. Instead, I would choose to use a black pigment because black pigments would absorb more energy because they would absorb all wavelengths of light. If you look at a field of plants you will notice that they are green (OK this doesn't work too well around Lubbock in the winter)and we have learned that chlorophyll, a green pigment, is the dominant photosynthetic pigment. What is going on?

Here is one theory about why chlorophyll is the dominant photosynthetic pigment in plants today. Early on there were photosynthetic bacteria with purple pigments (purple is a combination of red and violet). These aquatic bacteria had a very simple sort of cyclic electron flow that was able to convert light energy into energy in ATP (they didn't have non-cyclic flow or the Calvin Cycle).

Origin of chlorophyll- The purple pigment absorbed all wavelengths of light except for the reds and violets. Thus, any bacteria using purple pigments that lived deeper in the water than the purple bacteria on the surface would have no light to use because it had all been absorbed by the surface bacteris (exploitative competition). Because red and violet wavelengths pass through to deeper water, bacteria that contained a pigment that was able to absorb these wavelengths would be able to coexist with the purple bacteria. This was the origin of chlorophyll.

Competition purple and green photosynthetic pigments. Over time there was competition between organisms with purple photosynthetic pigments and green photosynthetic pigments. Obviously, the green photosynthetic pigments won this competition because chlorophyll is the dominant photosynthetic pigment today (there are still examples of photosynthetic bacteria with purple pigments, but they are limited to very harsh environments). Interestingly, chlorophyll came to dominate, not because it was a better at absorbing light energy, but rather because the cyclic flow machinery associated with chlorophyll was more efficient at producing ATP than the machinery associated with the purple pigment was. Thus, it is an evolutionary accident that modern plants are green.

Black Plants

It would be possible for modern plants to be black if they had enough accessory pigments to allow them to absorb all wavelengths of light. In fact, some red algae that live deep below the surface where light levels are low are basically black. Because the amount of light is not the factor that limits the rate of photosynthesis in most terrestrial plants, it is not worth the cost of producing extra accessory pigments. However, deep in the ocean where light levels are low, plants benefit from being able to absorb all wavelengths of light so deep marine algae have invested in extra accessory pigments.

Expected Learning Outcomes

By the end of this course a fully engaged student should be able to

- discuss why terrestrial plants to not invest in the accessory pigments required to make them black

Carbon Fixation

Technically, arbon fixation is defined as the first chemical reaction that incorporates carbon dioxide into an organic molecule (a molecule with more than one carbon atom).

In C3 photosynthesis the following step is considered to be carbon fixation-

carbon dioxide + RuBP ==> PGA

In CAM photosynthesis the following is considered to be carbon fixation-

carbon dioxide ===> malate

Note: CAM plants also have the reaction- carbon dioxide + RuBP ===> PGA, but in this case this step is not considered to be carbon fixation.

Sometimes people will loosely use the term carbon fixation to mean the production of glucose by photosynthesis. Be sure that you are aware of how different authors are using the term and you should attempt to use the term as precisely as possible in your own work.

Expected Learning Outcomes

By the end of this course a fully engaged student should be able to

- define carbon fixation
- identify carbon fixation in C3 and CAM photosynthesis

CAM Photosynthesis

CAM photosynthesis is an alternative mode of photosynthesis found in some plants adapted to living in desert environments. CAM plants are able to separate the timing of carbon uptake from the timing of the use of carbon dioxide in the Calvin Cycle which reduces the water loss of these plants.

Critical Point- All of the nuts and bolts of photosynthesis (e.g., photosystems, light dependent reactions, and Calvin Cycle) take place the same in C3 and CAM plants.

Epected Learning Outcomes

By the end of this course a fully engaged student should be able to

- discus the timing of carbon uptake and glucose production in CAM plants
- discuss the advantages and disadvantages of CAM photosynthesis
- explain why CAM photosynthesis is limited to plants growing in arid environments
- compare and contrast photosynthesis if C3 and CAM plants

Leaf Structure

In most plants, leaves are the major sites of photosynthesis. Thus, we can think of leaves as "photosynthesis machines" and use our knowledge of natural selection to try to understand aspects of leaf structure.

Further Reading

http://micro.magnet.fsu.edu/cells/leaftissue/leaftissue.html

Expected Learning Outcomes

By the end of this course a fully engaged student should be able to

- discuss important differences between animals and plants in gas uptake
- diagram the cross section of a leaf
- explain the adaptive basis of leaf structure

Global Carbon Cycle Revisited

Here is some more information that I would like you to know about the global carbon cycle

Mauna Loa Curve

The best data that we have examining is from Mauana Loa in Hawaii (see Mauna Loa Curve- http://www.eoearth.org/article/Mauna_Loa_curve). This curve shows two things. First, that the concentration of carbon dioxide in the atmosphere has indeed increased over time. Second, that there is seasonal variation in the concentration of carbon dioxide in the environment. Carbon dioxide is most abundant in the atmosphere in the North American winter and lowest in the North American summer. This pattern is caused by seasonal variation in the amount of photosynthesis. In the summer, when photosynthetic rates are the highest, carbon dioxide is removed from the atmosphere at a high rate which reduces the amount of carbon in the atmosphere. Because there is more land mass in the Northern Hemispere and most photosynthesis happens on land, the global pattern is determined by seasons in the Northern Hemispere (this is truly a global cycle, carbon dioxide move so quickly though the environment that the conentration is virtually the same all over the world)

Expected Learning Outcomes

By the the of this class a fully engaged student should be able to

- diagram how carbon dioxide concentration in the atmosphere varies seasonall.
- discuss the causes of this pattern

What is Happening to the Rate Carbon Dioxide is Being Added to the Atmosphere?

Here is an article from the Associated Press entitled- Climate Warming Gasses Rising Faster than Expected that was published yesterday. Chris Field, one of the scientists quoted in this article, was one of my professors at University of Utah.

CHICAGO (AP) -- Despite widespread concern over global warming, humans are adding carbon to the atmosphere even faster than in the 1990s, researchers warned Saturday. Carbon dioxide and other gases added to the air by industrial and other activities have been blamed for rising temperatures, increasing worries about possible major changes in weather and climate.

Carbon emissions have been growing at 3.5 percent per year since 2000, up sharply from the 0.9 percent per year in the 1990s, Christopher Field of the Carnegie Institution for Science told the annual meeting of the American Association for the Advancement of Science.

''It is now outside the entire envelope of possibilities'' considered in the 2007 report of the International Panel on Climate Change, he said. The IPCC and former vice president Al Gore received the Nobel Prize for drawing attention to the dangers of climate change.

The largest factor in this increase is the widespread adoption of coal as an energy source, Field said, ''and without aggressive attention societies will continue to focus on the energy sources that are cheapest, and that means coal.''

Past projections for declines in the emissions of greenhouse gases were too optimistic, he added. No part of the world had a decline in emissions from 2000 to 2008.

Anny Cazenave of France's National Center for Space Studies told the meeting that improved satellite measurements show that sea levels are rising faster than had been expected. Rising oceans can pose a threat to low level areas such as South Florida, New York and other coastal areas as the ocean warms and expands and as water is added from melting ice sheets.

And the rise is uneven, with the fastest rising areas at about 1 centimeter -- 0.39 inch -- per year in parts of the North Atlantic, western Pacific and the Southern Ocean surrounding Antarctica, she said.

Also, highly promoted efforts to curb carbon emissions through the use of biofuels may even backfire, other researchers said. Demand for biologically based fuels has led to the growing of more corn in the United States, but that means fields were switched from soybeans to corn, explained Michael Coe of the Woods Hole Research Center. But there was no decline in the demand for soy, he said, meaning other countries, such as Brazil, increased their soy crops to make up for the deficit.
In turn, Brazil created more soy fields by destroying tropical forests, which tend to soak up carbon dioxide. Instead the forests were burned, releasing the gasses into the air. The increased emissions from Brazil swamp any declines recorded by the United States, he said.

Holly Gibbs of Stanford University said that if crops like sugar and oil palm are planted after tropical forests are burned, the extra carbon released may be balanced by lower emissions from biofuel in 40 to 120 years, but for crops such as corn and cassava it can take hundreds of years to break equal.

''If we run our cars on biofuels produced in the tropics, chances will be good that we are effectively burning rainforests in our gas tanks,'' she said.

However, there could be benefits from planting crops for biofuels on degraded land, such as fields that are not offering low productivity due to salinity, soil erosion or nutrient leaching.

''In a sense that would be restoring land to a higher potential,'' she said. But there would be costs in fertilizer and improved farming practices.

In some cases simply allowing the degraded land to return to forest might be the best answer, she said.

BAC Results Retest

The answer key for the retest has been corrected.

Here are the results of the meeting with the BAC on Wednesday. The committee brought me four questions to look at.

Question 12.

The correct answer for this question is (b). A mutualism is an interaction between two species in which both species gain a benefit. This is true when there is reciprocal altruism between different species of birds.

Correct answer (b)

Question 27.

The correct answer is (c). Some peopole argued that a similar question was on a past test that showed answer (a) as correct. The purpose of showing you old exams is to give you some idea of the form that the questions will take. This question was not on any information that I supplied you this year.

Correct answer (c)


Question 29.

The correct answer is (b). Some people tried to argue that (a) is also correct so ansswer (d) should be a suitable answer. However, by definition dN/dt = rN any time that r is not equal to zero, dN/dt must depend on population size.

Correct answer (b)

Question 30.

I wrote this question thinking that males fertilizing a female in pines was an example of exploitative competition (when an egg is fertilized it is effectively "consumed" because so other male can fertilize it). However, the way I wrote the question the same logic could be used to choose (b) as a correct answer. this confusion caused several students to choose answer (e). Thus, there is no best answer. I will throw out this question and everyone will receive credit for it.

Photosynthesis- Summary

One you understand the light dependent reactions, the light independent reactions, and how these two sets of interactions are linked together then you understand the basics of photosynthesis. The pattern of photosythesis that we have discussed so far is known as "C3 photosynthesis" (it is named this becasue the first stable product of the Calvin Cycle is a molecule that contains three carbon atoms). C3 photosynthesis is considered to be the basic mechanism of photosynthesis.

As I have mentioned in class the best way to assure that you understand what is going on is that you should be able to describe photosynthesis at different levels of detail.

1) the one sentence answer
2) the one paragraph answer
3) the full detail answer

In my experience, students quickly get lost by worrying about the details. Thus, I suggest that you start at the one sentence answer, then move to the one paragraph answer, and finally finish up with the full detail answer. You will find that by thinking about the one sentence and the one paragraph answers you will have already figured out how to organize your full detail answer.

I suggest that you try writing out the answers to these questions at the three levels.

1) What is photosynthesis?
2) What are the light dependent reactions of photosynthesis?
3) What are the light independent reactions of photosynthesis?
4) What is a photosystem?

I encourage you to post your answers here and then to provide critical feedback to your classmates about their answers.