Photosynthesis

Photosynthesis

The ultimate source of energy for life on earth is the sun. Light is a form of kinetic energy. It can do work but most living things cannot trap and directly use its energy. The process of photosynthesis, however, is an exception.

Physicists recognize that light travels in waves. Not all of these waves are alike. Just like waves in the ocean, the closer the waves are together, the more frequently they strike the shore in a given period of time. If the energy of an ocean wave is released as it strikes the shore (the wave does “work” on the beach), the closer the waves are together, the more “work” is being performed in a period of time, say an hour. Scientists measure this interval as the distance from the top of one wave to the top of another wave. This is known as the wavelength.

Light behaves in a similar fashion. Light consists of the energy of particles termed photons. Photons vary from one another with respect to their wavelength and the energy that they carry. The relationship between wavelength and energy can be described using the following analogy:

QUESTION

1. What happens to wavelength as waves get closer together?

2. As the wavelength becomes shorter, would the number of waves striking the shore in a given period of time increase or decrease?

3. Would the amount of work being done increase, decrease, or stay the same?

1. The wavelengths get shorter.

2. It would increase, the shorter the distance between waves, the more waves arrive in a given amount of time.

3. The amount of work would increase…each wave would release energy as it strikes the beach; the more waves, the more energy.

So, here is the important question…if you look at all of the waves of energy coming from the sun (the electromagnetic spectrum) depicted in the figure below (Figure 6.3), which has the highest frequency (shortest wavelength)?

Which would transfer more energy in a one minute period, gamma waves or radio waves?

Obviously gamma waves. This is why radio waves are safe to use in the house, they carry little energy and, therefore, do not cause damage to biological tissue. On the other hand, gamma rays pack so much energy that they can actually change the basic structure of molecules. This can result in serious disorders and even cell death. Look at the wavelength of X-rays in the figure above…do you see why the X-ray technician leaves the immediate vicinity of the patient when an X-ray is taken?

The figure above presents the visible spectrum of light. This spectrum displays the various types of visible light based upon their different wavelengths.

Visible light is the portion of the electromagnetic spectrum that we actually see. Note that the visible spectrum of light ranges from violet at 400 nm to red at 700 nm. Note that these represent the colors of the rainbow. Rainwater acts as a prism to break visible light up into its constituent wavelengths, each having an associated color.

Please note from our discussion above that shorter wavelengths carry more energy. With this in mind we can see that the shorter the wavelength of light, the greater the amount of energy it carries. As a result violet and ultraviolet light have more energy, and are more dangerous to living cells, than red or infrared light.

Color is associated with the concept of light absorption. Different molecules absorb different wavelengths of light. As animals, we perceive the portions of the visible spectrum not absorbed by pigments. For example, a red flower contains pigments that absorb all of the colors of the visible spectrum with the exception of red. So, why are plants green? Plants are green due to the photosynthetic pigment chlorophyll. Chlorophyll serves to reflect green and yellow wavelengths of light and absorb all the others. Since we perceive primarily reflected colors with our eyes, plant tend to look green (refer to Figure 6.4 below).

Using a spectrophotometer, it is possible to see how much light chlorophyll will absorb at various wavelengths. The resulting absorption spectrum indicates the wavelengths of light absorbed by the pigment. Note the absorption spectrum presented in the figure below (Figure 6.6).

Chlorophyll exists in two forms, chlorophyll a (the primary photosynthetic pigment) and chlorophyll b. Please make note in the figure that both chlorophyll a and chlorophyll b absorb primarily blue and red wavelengths of light. Such a situation indicates that plants use primarily blue and red wavelengths of light to facilitate the overall process of photosynthesis.

You should note that the overall process of photosynthesis is not entirely reflective of the absorption spectrum of chlorophyll a. This is due to the presence of accessory pigments such as chlorophyll b, the carotenoids, phycoerythrin and phycocyanin. Accessory pigments serve to capture light energy and transfer this energy to chlorophyll. Once absorbed the energy associated with the photon is used to begin the process of photosynthesis.

Thought Question 1

Based upon the information described within this chapter concept, can you explain why leaves appear to change color during different times of the year?

Photosynthetic Pigments

Pigments are colorful compounds.

Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear “colorful”. Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths.

Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs –organisms which make their own food using photosynthesis. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun’s energy.

There are three basic classes of pigments.

 Chlorophylls are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll “captures” the energy of sunlight.

There are several kinds of chlorophyll, the most important being chlorophyll “a”. This is the molecule which makes photosynthesis possible, by passing its energized electrons on to molecules which will manufacture sugars. All plants, algae, and cyanobacteria which photosynthesize contain chlorophyll “a”. A second kind of chlorophyll is chlorophyll “b”, which occurs only in “green algae” and in the plants. A third form of chlorophyll which is common is (not surprisingly) called chlorophyll “c”, and is found only in the photosynthetic members of the Chromista as well as the dinoflagellates. The differences between the chlorophylls of these major groups was one of the first clues that they were not as closely related as previously thought.

 Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color. These compounds are composed of two small six-carbon rings connected by a “chain” of carbon atoms. As a result, they do not dissolve in water, and must be attached to membranes within the cell. Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms.

 Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta.

The picture at the right shows the two classes of phycobilins which may be extracted from these “algae”. The vial on the left contains the bluish pigment phycocyanin, which gives the Cyanobacteria their name. The vial on the right contains the reddish pigment phycoerythrin, which gives the red algae their common name.

Phycobilins are not only useful to the organisms which use them for soaking up light energy; they have also found use as research tools. Both pycocyanin and phycoerythrin fluoresce at a particular wavelength. That is, when they are exposed to strong light, they absorb the light energy, and release it by emitting light of a very narrow range of wavelengths. The light produced by this fluorescence is so distinctive and reliable, that phycobilins may be used as chemical “tags”. The pigments are chemically bonded to antibodies, which are then put into a solution of cells. When the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been “tagged” by the antibodies. This has found extensive use in cancer research, for “tagging” tumor cells.

 Pigment Type Colors Anthocyanins (subclass of flavonoids) blue/purple/red Anthoxanthins (subclass of flavonoids) yellow – ivory Betacyanins yellow – red/purple Carotenoids yellow – red Chlorophylls Chlorophyll a Chlorophyll b greens blue-green yellow-green Xanthophylls (a subclass of carotenoids) ivory – yellow

Photosynthesis

Review Questions

I. Plant Pigments

A. Pigment Chromatography Questions:

1. Why are the various pigments carried different distances up the chromatography paper by the solvent?

2. Based on your results, name the different pigments that appear to be involved in collecting light energy for spinach plants?

3. Why is it beneficial for a plant to have several different types of pigments?

4. Which pigments are polar? Which are non-polar?

B. Absorption Spectrum of Plant Pigments Questions

1. What color(s) of light do the spinach pigments absorb most poorly? Why?

2. If an absorption spectrum were done on a solution of water with red food coloring in it, what color(s) of light would you expect the solution would absorb most poorly? Why?

3. Suppose an experiment was done in which two ways spinach plants were kept in separate rooms. In one room, the plant was exposed only to 430 ηm light and in the other, the plant was exposed to only 550ηm light. Which plant would grow best? Why?

4. How would a third spinach plant grown under white light grow compared to those of the previous question? Why?

Self-Test Questions

1. Which of the following is released (produced) during the light-dependent reactions?

a) glucose, ATP, and carbon dioxide

b) ATP, oxygen, and carbon dioxide

c) NADPH, ATP and glucose

d) ATP, NADPH and oxygen

2. Which of the following is NOT a product of photosynthesis?

a) glucose

b) carbon dioxide

c) oxygen

d) All of the above are products of photosynthesis

3. Which of the following forms of visible light would have the most energy associated with it?

a) blue light

b) yellow light

c) green light

d) red light

4. Which of the following is the primary photosynthetic pigment?

a) chlorophyll a

b) chlorophyll b

c) b-carotene

d) chlorophyll c

5. In general, what two wavelengths of light are absorbed the most by chlorophyll?

a) green and yellow

b) red and orange

c) blue and green

d) blue and red

6. The electrons lost from Photosystem II are replaced by

a) Photosystem I

b) the electron transport chain

c) carbon dioxide

d) the splitting of water

7. __________ is the enzyme responsible for incorporating carbon into the Calvin cycle.

a) photosynthase

b) Rubisco

c) hydrolase

d) glucase

8. Glucose is produced directly from PGAL which was produced during the

a) Calvin cycle

b) light-dependent reactions

c) absorption of light by Photosystem I

d) splitting of water

9. Which of the following factors affect photosynthetic productivity?

a) water

b) light

c) carbon dioxide levels

d) All of the above affect photosynthetic productivity.

10. __________ are small pores located on the surface of the leaf that allow gas exchange.

a) hairs

b) stomata

c) stroma

d) thylakoids

11. The light-dependent reactions of photosynthesis occur on the surface of the

a) stroma

b) leaf

c) cell

d) thylakoid membrane

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