April Weast ​ ​
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Unit 5 Assignment
Part 1: Overview of Photosynthesis
Plant cells convert light energy into chemical energy through photosynthesis. The
two reactions that make up photosynthesis are the Light Reaction and the Calvin Cycle.
In the Light Reaction, sunlight is absorbed by a pigment called ‘chlorophyll,’ which
reflects green light and is located in the thylakoids of a chloroplast. Each molecule of
the pigment absorbs a photon, which causes an electron in the chlorophyll to become
excited and leave the chlorophyll to enter the Electron Transport Chain. To replace this
electron, a molecule of water is split into oxygen and hydrogen ions. Oxygen is dispelled
into the atmosphere, but the hydrogen ions and electrons are used to create an
electrochemical gradient in the thylakoids that produces ATP, a form of energy. The last
part of this reaction uses the remaining electrons to push the hydrogen ions to react
with NADP+ to form NADPH, another form of energy that plants can use for the Calvin
Cycle. (Fowler et al., 2025).
After the Light Reaction, ATP and NADPH are now ready to be used for the Calvin
Cycle. There are three steps: fixation, reduction, and regeneration. During carbon
fixation, an enzyme, RuBisCO, catalyzes a reaction between carbon dioxide and an
organic molecule called RuBP (Ribulose bisphosphate) to create 3-PGA. This reaction
‘fixes’ the CO2 into this three-carbon compound. Reduction refers to when ATP and
NADPH use their energy to reduce electrons from 3-PGA into a compound called G3P.
This results in ADP and NAD+ that can be recycled in the Light Reaction. The G3P
molecule exits the Calvin Cycle to form glucose. Since glucose has six carbon
molecules, the Cycle needs to be repeated six times. Each new cycle creates molecules
required during fixation, allowing the Cycle to be repeated. (Fowler et al., 2025).
Part 2: Limitations of Photosynthesis
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However, the process of photosynthesis is not effective 100 percent of the time.
Many times, “less than five percent of that light energy [is converted] into biomass”
(Ehrenberg, 2017). There is a process called ‘photorespiration’ that happens when RuBP
binds with oxygen instead of carbon dioxide, creating a toxic compound called glycolate
that must be broken down by the plant. The enzyme that binds carbon dioxide with
RuBP, RiBisCO, is the same enzyme that is responsible for this mistake that costs a 40
percent decrease in photosynthesis efficiency. The only reason RuBisCO has remained
effective is because it has rarely had to worry about taking in oxygen; billions of years
ago, there was way more CO2 in the air than oxygen. Scientists are attempting to make
photosynthesis more efficient in order to feed the ever-growing human population more
effectively.
Part 3: Genetic and Biotechnological Innovations:
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Some solutions that have been proposed involve speeding up the RuBisCO
enzyme, or making it less likely to make mistakes. Neither have worked yet. What has
had some positive results is reducing the time it takes to break down glycolate. When
photosynthesis makes the mistake of taking in oxygen instead of CO2, the process of
breaking down glycolate takes time and energy that could be used more efficiently.
Therefore, if scientists could reduce these factors, then plant growth can have a
significant increase.
The Ort Lab at the University of Illinois has found a way to shorten the “recovery
pathway, conserving energy and resources that the plant can reinvest to increase crop
productivity by as much as 40 percent” (RIPE). This research has yielded results that
show a 30-41 percent increase in biomass for plants. Basically, larger plants. The
implications of this are promising. Now, research is being done to take these results to
crops that are heavily consumed, such as potatoes and soybeans. If plant biomass is
increased, that means more food overall can be distributed to consumers.
References
Ehrenberg, R. (2017). The photosynthesis fix. RIPE.
https://ripe.illinois.edu/news/the-photosynthesis-fix
Fowler, S., Roush, R., & Wise, J. (2025). Concepts of biology. Samantha Fowler. OpenStax.
https://openstax.org/books/concepts-biology/pages/5-introduction
Liu, A. (2024). Improving photosynthesis to fight climate change. Innovative Genomics Institute
(IGI). https://innovativegenomics.org/news/photosynthesis-climate-change-2023/
Our story. RIPE. (n.d.). https://ripe.illinois.edu/objectives/our-story