For inquiries contact Arthur Stern, Professor
Emeritus, Biology Department, University of Massachusetts Amherst
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.
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
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.
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
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
A List of C4 Plants Found in the Connecticut River Valley
Cyperaceae (sedge family)Cyperus esclentus L.
Cynodon dactylon (L.) Pers., Bermuda grass
Eragrostis pilosa (L.) Beauv., India love grass
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
Euphorbia maculata L., spotted euphorbia
Portulacaceae (portulaca family)
Portulaca oleracea L., common purslane
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
Sunderland, MA: Sinauer Associates, 1998. Chapter 8.