ORIGIN OF THE ORGANIC SOUP

For inquiries contact Arthur Stern, Professor Emeritus, Biology Department, University of Massachusetts Amherst

Photosynthesis

Introduction

Photosynthesis evolved over three billion years ago, shortly after the appearance of the first living organisms. The food we eat and the oxygen we breathe are both formed by plants (including algae) through photosynthesis. The power to drive this reaction comes from sunlight absorbed by chlorophyll in the chloroplasts of plants. At the present time, no known chemical system can be made to serve as a substitute for this process. It has been calculated that each CO₂ molecule in the atmosphere is incorporated into a plant structure every 200 years and that all the O₂ in air is renewed by plants every 2000 years. All life depends directly or indirectly on the sun's energy, and only plants are capable of capturing and converting this energy into chemical energy in the form of sugar and other organic compounds. Thus, if plants should suddenly disappear from the earth, so would we.

Our geological heritage of coal, oil, and gas also originated directly or indirectly from photosynthesis, since these fossil fuels were all derived from the remains of living organisms. Our stake in photosynthesis is, therefore, great, since we are not only dependent upon it for the food we eat, but also for many of the goods and most of the energy we use.

The Site of Photosynthesis in Vascular Plants

Leaves are the major organs of photosynthesis in vascular plants. Chloroplasts are found mainly in the mesophyll cells, the green tissue in the interior of the leaf

Diagram of a Leaf Cross Section and Structure of the Chloroplast

Chloroplasts are ellipsoidal or disc-like in shape and are between 5 - 7 micrometers in diameter and 1 - 2 micrometers thick (1 micrometer = one millionth of a meter, 1 meter = 39 inches). There are about 36 chloroplasts in each mesophyll cell. The chloroplast is bounded by a double membrane that encloses the stroma, a dense aqueous solution that contains DNA, RNA, metabolites, and the enzymes associated with the conversion of CO₂ into organic matter. Membranes of the thylakoid system separate the stroma from the thylakoids. Thylakoids are concentrated in stacks called grana. Thylakoids contain the pigments chlorophyll a and b, carotenoid, and the enzymes associated with the oxidation (splitting) of water (H₂O) and the production of oxygen.

The Chemistry of Photosynthesis

Photosynthesis takes place in the chloroplasts of plant cells and consists of Light-Dependent and Light-Independent reactions. The Light-Dependent reaction occurs in the thylakoids and converts light energy into ATP and NADPH₂. During this process water is split (oxidized) and oxygen is given off.

Overview of Photosynthesis.

Light-Independent reactions (the Calvin Cycle) incorporate CO₂ into sugar, the basic food source for all organisms. Thylakoid membranes are the sites of the Light-Dependent reactions, whereas the Calvin cycle occurs in the stroma. These reactions can be summarized by the following equations:

A Design Flaw in Photosynthesis - Photorespiration

Since plants first moved onto land about 425 million years ago, they have been adapting to the problems of terrestrial life, particularly the problem of dehydration. The solutions often involve tradeoffs. An important example is the compromise between photosynthesis and the prevention of excessive water loss from the plant. The CO₂ needed for photosynthesis enters a leaf via microscopic pores called stomata. However, the stomata are also the main avenues of transpiration, the evaporation of water from leaves. On hot, dry days, most plants close their stomata in order to conserve water. This response limits access to CO₂, thereby reducing photosynthetic yield. Under these conditions, CO₂ concentrations in the air spaces within the leaf begin to decrease and the concentration of oxygen released from photosynthesis begins to increase. This favors what appears to be a wasteful process within the leaf called photorespiration.

In most plants (about 800,000 species), CO₂ is initially fixed via rubisco of the Calvin cycle into ribulose bisphosphate. Because the product of this reaction is a three-carbon compound, 3-phosphoglycerate, such plants are called C₃ plants. Rice, wheat, and soybeans are among the C₃ plants that are important in agriculture. On hot, dry days when their stomata close, these plants produce less food as CO₂. levels within the leaf decline. Making matters worse, rubisco can use oxygen in place of CO₂. As O₂ increases, rubisco adds oxygen to RuBP instead of CO₂. The product splits, and one piece, a two-carbon compound (glycolate), is exported from the chloroplast. Other organelles (mitochondria and peroxisomes) then break down glycolate back to CO₂. This process is called photorespiration because it occurs in the light (photo) and consumes oxygen (respiration). However, unlike normal cellular respiration, photorespiration generates no ATP. And unlike photosynthesis, photorespiration produces no food. Photorespiration is probably a metabolic relic from a much earlier time when the atmosphere had less O₂ and more CO₂ than it does today. When rubisco first evolved, the inability of the enzyme's active site to exclude O₂ would have made little difference. Now, it is considered to be wasteful, since photorespiration drains away as much as 50% of the carbon fixed by the Calvin cycle. If photorespiration could be reduced in certain plant species, without affecting photosynthetic productivity, crop yields and food supplies would increase.

Alternative Strategies of Carbon Fixation - C₄ and CAM C₄ Plants

The environmental conditions that promote photorespiration are hot, bright, dry days. In these climates, alternate modes of carbon fixation have evolved to minimize photorespiration. The two most important of these photosynthetic adaptations are exhibited by C₄ and CAM C₄ plants. C₄ plants are so named because they form a four-carbon compound as the first product of the nonlight requiring reactions of photosynthesis. Several thousand species in at least 19 families use the C₄ pathway. Agriculturally important C₄ plants are sugarcane and corn, members of the grass family.

Leaves of C₄ plants contain two distinct types of photosynthetic cells: a cylinder of bundle-sheath cells surrounding the vein, and mesophyll cells located outside the bundle sheath. CO₂ is initially fixed in mesophyll cells by the enzyme PEP carboxylase. A four-carbon compound is formed (malate in this case) which conveys the fixed CO₂ via plasmodesmata (protoplasmic connections) into a bundle sheath cell where the enzymes of the Calvin cycle are located.

C₄ Leaf Anatomy

In the bundle sheath cell, the malate is converted into pyruvate and CO₂; the latter is now used by rubisco and the Calvin cycle to make sugar. Compared to rubisco, the enzyme PEP carboxylase has a much higher affinity for CO₂. Thus, PEP carboxylase can fix CO₂ efficiently when it is hot and dry and stomata are partially closed. Also, by pumping CO₂ from the mesophyll cells into the bundle sheath, this keeps the CO₂ concentration high enough for rubisco to accept CO₂ rather than O₂. In this way, C₄ photosynthesis minimizes photorespiration and enhances sugar production. This adaptation is especially advantageous in hot climates with intense sunlight and it is where C₄ plants evolved and thrive today.

A second photosynthetic adaptation to arid conditions (as found in deserts) has evolved in succulent (water-storing) plants (including ice plants), many cacti, and representatives of other plant families. These plants close their stomata in the day and open them during the night, just the reverse of other plants. Closing the stomata during the day helps desert plants conserve water, but it also prevents CO₂ from entering the leaves. At night, when the stomata are open, these plants take up CO₂ and initially fix it into four-carbon compounds like malate. This mode of carbon fixation is called crassulacean acid metabolism, or CAM, after the plant family Crassulaceae, the succulents in which the process was first discovered. The photosynthetic cells of CAM plants store the malate formed in the night in their vacuoles until morning, when the stomata close. In the daytime, when the light reactions can make ATP and NADPH₂ for the Calvin cycle, CO₂ is released from the malate made the night before to become fixed into sugar in the chloroplasts.

The above diagram compares C₄ and CAM C₄ photosynthesis. Both adaptations are characterized by initial fixation of CO₂ into an organic acid such as malate followed by transfer of the CO₂ to the Calvin cycle. In C₄ plants, such as sugarcane, these two steps are separated spatially; the two steps take place in two cell types. In CAM C₄ plants, such as pineapple, the two steps are separated temporally (time); carbon fixation into malate occurs at night, and the Calvin cycle functions during the day. Both C₄ and CAM C₄ are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days. However, it should be noted, that in all plants, the Calvin cycle is used to make sugar from carbon dioxide. On a global scale, this represents a prodigious amount of sugar, about 160 billion metric tons of carbohydrate per year.

A List of C₄ Plants Found in the Connecticut River Valley

Cyperaceae (sedge family)

Cyperus esclentus L.

Eragrostoideae (Cloridoideae)

Cynodon dactylon (L.) Pers., Bermuda grass

Eragrostis pilosa (L.) Beauv., India love grass

Panicoideae

Andropogon scoparius Michx., little bluestem

Digitaria sangunalis(L.) Scop., crab grass

Echinochloa crus-galli(L.) Beauv., barnyard grass

Panicum capillare L., common witch grass

Setaria italica (L.) Beauv., foxtail millet

Amaranthaceae (amaranth family)

Amaranthus albus L., white pigweed

Amaranthus retroflexus L., redroot pigweed

Froelichia gracilis (Hook) Mog., froelichia

Euphorbiaceae

Euphorbia maculata L., spotted euphorbia

Portulacaceae (portulaca family)

Portulaca oleracea L., common purslane

References

Campbell, N.A. Biology, 4th ed., Menlo Park, CA: Benjamin/Cummings, 1996. Chapter 10.

Purves, W.K., Orians, G.H., Heller, H.C. and Sadava, D. Life, The Science of Biology, 5th ed.,

Sunderland, MA: Sinauer Associates, 1998. Chapter 8.