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Photosynthesis: C2, C3, and C4 Cycles

This is long and slightly technical review of the C2, C3, and C4 cycles. If you want a brief over of Photosynthesis and how it developed click here.

As you read this page your cells respire, green plants are photosynthesizing, and the sun is shining constantly. The grass our clumsy feet so proudly march on every day gives it’s portion of life without thinking for a picosecond why. Not even a cockroach would survive if photosynthesis was no longer carried out by plants and other organisms. The varied methods of photosynthesis have a structured order, and serve a necessary purpose for advanced life on earth.

Peziza Fungus (40x)
Peziza Fungus (40x)

Photosynthesis is defined as a process that uses “light” to make “something”. In chlorophyll containing organisms, like herbaceous dicotyledons (grass), it is the process by which light energy along with carbon dioxide and water is used to make a carbohydrate like glucose(Mauseth).

Photosynthesis is a necessary bridge between the sun and most organisms on earth. When light travels from the sun, the energy it contains is in packets called photons. The photons travel through the atmosphere and are absorbed by a light harvesting complex that is found in chlorophyll containing organisms or photoautotrophs. The energy is transferred adjacently by a process called resonance energy transfer.

In the presence of water and carbon dioxide, photosynthesis will ultimately produce oxygen and glucose that other organisms can use(Brooker). Neither humans nor any other mammals are capable of producing energy from sunlight. We must rely on photosynthesis in photoautotrophs. Since there is an abundance of photoautotrophs compared to other organism there is always an abundance of oxygen. I can easily understand why movie producers might portray advances lifeforms from other planets as green creatures in the movies. It would be beneficial if human skin contained chlorophyll and could carry out photosynthesis.

The Light and Dark Cycles of Photosynthesis

Photosynthesis contains two cycles. The light cycle and the dark (Calvin/Benson) cycle. The light cycle, as it name implies, occurs when a light source is available. Light energy and water molecules are used in the light reaction to produce NADPH.

The oxygen from the water is released as a waste. The hydrogen from water molecules along with hydrogen from the stroma are used to create concentration of hydrogen protons inside the lumen. This electrochemical gradient is then used to power the phosphorylation of ADP to make ATP(Brooker). Then, that newly created ATP and NADPH are used along with carbon dioxide from the environment in the Calvin Cycle also known as the dark reaction.

Note: Plants usually store materials produced during the daytime for use during the dark cycle.

The Calvin-Benson Cycle

While the light reaction occurs along the thylakoid membrane, the Calvin cycle(C3 cycle) occurs outside of the thylakoids in the stroma. The Calvin cycle consist of the following steps:

carbon fixation–>reduction—-> carbohydrate production—>RuBP regeneration.[insert image]

 

When the initial molecule(carbon fixation), ribulose1,5-bisphosphate (RuBP), reacts with carbon dioxide with the aid of the RuBP carboxylase enzyme(RUBISCO), the first stable molecules produced are two 3-carbon molecule called 3-phosphoglycerate(PGA). There is a non stable 6-carbon molecule that precedes PGA but it immediate breaks into 2 PGA molecules(Brooker). Once PGA is made ATP donates a phosphate group to PGA to form 1,3-bisphosphoglycerate. It now has 2 phosphate groups, one each on the first and third carbons. Now in the reduction and carbohydrate production stage 1,3-bisphosphate takes the H from an NADPH being reduced and releases its phosphate group to become 3-phosphoglceraldehyde(PGAL). PGAL can now be used to create carbohydrates for energy and regeneration of the original RuBP molecule.

The enzyme RUBISCO is such a pivotal factor in the Calvin cycle yet it is very large and unique. The amino acid sequence of the enzyme RUBISCO is identical in all species of plants(Mauseth)! For the hundreds of millions of years plants have existed every mutation, no matter how minor, must have affected the activation site and have been disadvantageous. So it can be inferred that RUBISCO must have been spontaneously or very quickly formed by natural means at the same time as all the other enzyme in the Calvin cycle. This can also be evidence for some type of intelligent design(Behe).
As complete as the C3 cycle may seem it is actually far from perfect. With the abundance of oxygen and RUBISCO’s low affinity for carbon dioxide, a phenomena known as photorespiration occurs. Oxygen is attached instead carbon dioxide. This produces 1 PGA like normal, but one irregular molecule called phosphoglycolate. The organism must take steps to convert this potentially harmful molecule into useful amino acids and CO2. The problem with photorespiration is the organism looses energy trying to convert the phosphoglycolate back into useful material.

Since for some reason the RUBISCO enzyme was unable to be evolutionarily modified to have a higher affinity for carbon dioxide, some organism have developed a separate compartment that has another enzyme called (PEP) phosphophenolpyruvate carboxylase. PEP has a high affinity for CO2 and none for oxygen. So in this compartment CO2 is rapidly picked up, and used along with PEP to form oxaloacetate. As more CO2 flows down the carbon dioxide gradient, the oxaloacetate powers the reduction of NADPH while oxaloacetate is being decarboxylated and moved into a new compartment. The decarboxylation of the molecule brings carbon dioxide into the compartment where RUBISCO is located so that only carbon dioxide is near RUBISCO(Mauseth). It would be a great advancement if PEP carboxylase could use carbon dioxide and PEP to make PGA. There would be no need for Kranz anatomy if RUBISCO could be modified to have a high affinity for carbon dioxide and none for oxygen like PEP carboxylase.

The basis of life on earth is energy in the form of light that arrives to the earth in the form of photons. The mechanism to harvest this energy is a highly ordered system and serves a highly specific purpose: conversion of light energy into a stable, transferable form of energy. While photosynthesis appears to be an irreducibly complex system(Behe), it ultimately serves it’s purpose of producing enough molecular oxygen and carbohydrates for life to survive. Photosynthesis is a complex system that it is not perfect due to oxygen binding which starts the process of photorespiration. C2 photosynthesis or photorespiration may benefit an organism in other unverified ways, but for now it is considered disadvantageous because photosynthesis occurs at a much slower rate than C3 or C4 photosynthesis. There is much debate whether photosynthesis and other complex systems are a product of design because the ordered, and interacting components serve a purpose, and can not be broken down into any less complex yet functioning system(Behe). Maybe bacteria by chance developed photosynthesis by random mutations, horizontal gene transfer, and natural selection millions of years ago and were absorbed in a symbiotic relationship by eukaryotic cells(Dawkins). “The search for truth is more precious than its possession.” -Albert Einstein

Literature Cited in this essay

Balbach, M. , Bliss,L.1991. A Laboratory Manual For Botany. 7th ed. Brooks/Cole-Thomas Learning. Pacific Grove, CA. 59-80

Behe, M. 2006. Darwin’s Black Box: the biochemical challenge to evolution. 10th anv. ed. Free Press- Simon and Schuster, New York, NY 187-202

Brooker,R. ,Widmaier,E. , Graham,L., Stiling,P. 2008. Biology 1st ed. McGraw-Hill Higher Education, New York, NY 151-168.

Dawkins, R. 2009. The Greatest Show on Earth: the evidence for evolution. Free Press-Simon and Schuster, New York, NY 375-389.

Mauseth, James.2009. Botany an Introduction to Plant Biology. 4th ed. Jones and Bartlett Publishers, Sudbury, MA. 217-241.

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