PHOTOSYNTHESIS: What we have learned so far?

Editor's Notes

1. No matter how complex or advanced a machine, such as the latest cellular phone, the device cannot function without energy. Living things, similar to machines, have many complex components; they too cannot do anything without energy, which is why humans and all other organisms must “eat” in some form or another. That may be common knowledge, but how many people realize that every bite of every meal ingested depends on the process of photosynthesis?
2. Plants absorb water through their roots. What part of the plant takes in water? The water then travels from the roots up the stem to the leaves. Twenty-percent of soil is made up of water that is stored between the particles of weathered rock. The plant roots absorb this water. The bottom part of a plant’s leaves has holes called stomata. Carbon dioxide enters the leaf through these stomata The green leaves of a plant absorb light energy from the Sun. Plant cells have cell structures called chloroplasts which contain chlorophyll, a green substance that absorbs light energy. Chlorophyll is what gives plant leaves their green color
3. Not all cells of a leaf carry out photosynthesis. Cells within the middle layer of a leaf have chloroplasts, which contain the photosynthetic apparatus
4. The process of photosynthesis has a theoretical efficiency of 30% (i.e., the maximum amount of chemical energy output would be only 30% of the solar energy input), but in reality the efficiency is much lower. It is only about 3% on cloudy days. Why is so much solar energy lost? There are a number of factors contributing to this energy loss, and one metabolic pathway that contributes to this low efficiency is photorespiration. During photorespiration, the key photosynthetic enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) uses O2 as a substrate instead of CO2. This process uses up a considerable amount of energy without making sugars (Figure). When a plant has its stomata open (CO2 is diffusing in while O2 and water are diffusing out), photorespiration is minimized because Rubisco has a higher affinity for CO2 than for O2 when air temperatures are below 30°C (86°F). However, when a plant closes its stomata during times of water stress and O2 from photosynthesis builds up inside the cell, the rate of photorespiration increases because O2 is now more abundant inside the mesophyll. So, there is a tradeoff. Plants can leave the stomata open and risk drying out, or they can close the stomata, thereby reducing the uptake of CO2, and decreasing the efficiency of photosynthesis. In addition, Rubisco has a higher affinity for O2 when temperatures increase, which means that C3 plants use more energy (ATP) for photorespiration at higher temperatures. Evolutionarily speaking, why is photorespiration still around? One hypothesis is that it is evolutionary baggage from a time when the atmosphere had a lower O2 concentration than it does today. In other words, when Rubisco first evolved millions of years ago, the O2concentration was so low that excluding O2 from its binding site had little or no influence on the efficiency of photosynthesis. The modern Rubisco retains some of its ancestral affinity for O2, which leads to the energy costs associated with photorespiration. However, plant cell physiologists are discovering that there might be some metabolic benefits associated with photorespiration, which would help explain why this seemingly wasteful pathway is still found in plants. Adding to the dilemma is the fact that when plant geneticists “knock out” Rubisco’s ability to fix O2, Rubisco also loses its ability to fix CO2. It is possible that the active site of this enzyme cannot be engineered, by artificial or natural selection, so that it exclusively binds CO2 and not O2. 
5. C4 Pathway In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells and the Calvin cycle occurring in special cells that surround the veins in the leaves. These cells are called bundle-sheath cells. How does this work? Atmospheric CO2 is fixed in the mesophyll cells as a simple 4-carbon organic acid (malate) by an enzyme that has no affinity for O2. Malate is then transported to the bundle-sheath cells. Inside the bundle sheath, malate is oxidized to a 3-C organic acid, and in the process, 1 molecule of CO2 is produced from every malate molecule (Figure). The CO2 is then fixed by Rubisco into sugars, via the Calvin cycle, exactly as in C3 photosynthesis. There is an additional cost of two ATPs associated with moving the three-carbon “ferry” molecule from the bundle sheath cell back to the mesophyll to pick up another molecule of atmospheric CO2. Since the spatial separation in bundle-sheath cells minimizes O2 concentrations in the locations where Rubisco is located, photorespiration is minimized (Figure). This arrangement of cells reduces photorespiration and increases the efficiency of photosynthesis for C4 plants. In addition, C4 plants require about half as much water as a C3 plant. The reason C4 plants require less water is due to the fact that the physical shape of the stomata and leaf structure of C4 plants helps reduce water loss by developing a large CO2 concentration gradient between the outside of the leaf (400 ppm) and the mesophyll cells (10 ppm). The large CO2concentration gradient reduces water loss via transpiration through the stomata. The C4 pathway is used in about 3% of all vascular plants; some examples are crabgrass, sugarcane and corn. C4 plants are common in habitats that are hot, but are less abundant in areas that are cooler, because the enzyme that fixes the CO2 in the mesophyll is less efficient at lower temperature. One hypothesis for the abundance of C4 plants in hot habitats is that the benefits of reduced photorespiration and water loss exceeds the ATP cost of moving the the CO2 from the mesophyll cell to bundle-sheath cell.
6. Many plants such as cacti and pineapples, which are adapted to arid environments, use a different energy and water saving pathway called crassulacean acid metabolism (CAM). This name comes from the family of plants (Crassulaceae) in which scientists first discovered the pathway. Instead of separating the light-dependent reactions and the use of CO2 in the Calvin cycle spatially, CAM plants separate these processes temporally (Figure). At night, CAM plants open their stomata, and an enzyme in the mesophyll cells fix the CO2 as an organic acid and store the organic acid in vacuoles until morning. During the day the light-dependent reactions supply the ATP and NADPH necessary for the Calvin cycle to function, and the CO2 is released from those organic acids and used to make sugars. Plant species using CAM photosynthesis are the most water efficient of all; the stomata are only open at night when humidity is typically higher and the temperatures are much cooler (which serves to lower the diffusive gradient driving water loss from leaves). The CAM pathway is primarily an adaptation to water-limited environments; the fact that this pathway also stops photorespiration is an added benefit.
7. Plants absorb water through their roots. What part of the plant takes in water? The water then travels from the roots up the stem to the leaves. Twenty-percent of soil is made up of water that is stored between the particles of weathered rock. The plant roots absorb this water. The bottom part of a plant’s leaves has holes called stomata. Carbon dioxide enters the leaf through these stomata The green leaves of a plant absorb light energy from the Sun. Plant cells have cell structures called chloroplasts which contain chlorophyll, a green substance that absorbs light energy. Chlorophyll is what gives plant leaves their green color
8. Global food production will need to increase more than 50% before 2050 to satisfy the food and fuel demands of an increasing population. Despite the fact that more than 90% of crop biomass is derived from photosynthetic products, increasing photosynthetic capacity and/or efficiency has not yet been addressed by breeding. Thus, photosynthetic improvements are being considered as a way to increase crop yields.
9. Yamori, W. (2013). Improving photosynthesis to increase food and fuel production by biotechnological strategies in crops. Journal of Plant Biochemistry & Physiology.
10. Tanaka, Y., Sugano, S. S., Shimada, T., & Hara‐Nishimura, I. (2013). Enhancement of leaf photosynthetic capacity through increased stomatal density in Arabidopsis. New Phytologist, 198(3), 757-764.
11. Kirschbaum, M. U. (2011). Does enhanced photosynthesis enhance growth? Lessons learned from CO 2 enrichment studies. Plant physiology, 155(1), 117-124.
12. Chloroplast genomes defied the laws of Mendelian inheritance at the dawn of plant genetics, and continue to defy the mainstream approach to biotechnology, leading the field in an environmentally friendly direction. Recent success in engineering the chloroplast genome for resistance to herbicides, insects, disease and drought, and for production of biopharmaceuticals, has opened the door to a new era in biotechnology. The successful engineering of tomato chromoplasts for high-level transgene expression in fruits, coupled to hyper-expression of vaccine antigens, and the use of plant-derived antibiotic-free selectable markers, augur well for oral delivery of edible vaccines and biopharmaceuticals that are currently beyond the reach of those who need them most. Daniell, H., Khan, M. S., & Allison, L. (2002). Milestones in chloroplast genetic engineering: an environmentally friendly era in biotechnology. Trends in plant science, 7(2), 84-91.
13. Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight,water, and carbon dioxide into carbohydrates
14. Kanan, M. W., & Nocera, D. G. (2008). In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+. Science, 321(5892), 1072-1075.
15. Marshall, J. (2014). Springtime for the artificial leaf. Nature, 510(7503), 22.