“Structure and function are interlinked in all of biology / biochemistry.
Choose a biological structure (for example a tissue or molecule) and describe how
you would change its structure to enhance its function.”
Chlorophyll is an extremely important biological molecule and exists in 4 different
forms, but for the purpose of this essay I will focus on chlorophyll a, as it is the main
factor in photosynthesis. Without this molecule complex eukaryotic organisms would
not be able to survive as aerobic respiration would be made impossible, due to a
lack of oxygen. The reason why chlorophyll a is so important is due to its structural
features. The two main features I will talk about in this essay are the porphyrin ring,
which is what chelates the central magnesium ion and is also the reason why the
molecule is able to absorb photons, and the phytol tail, which is the component
responsible for the arrangement of the molecule in photosystems.
Figure 1: Kimball, J. Structure of chlorophyll, available at 3.18: Chlorophylls and
Carotenoids - Biology LibreTexts
From figure 1 we can see that the porphyrin ring contains alternating double and
single bonds, thus making it a conjugated system (Clayden, Greeves and
Warren,2012). Because the ring is conjugated, electrons are not contained to any
one atom and are free to move about the ring (Libretexts (2022)). This is important to
the function of chlorophyll because it enables the molecule to absorb photons of
light. Once a photon hits a chlorophyll contained within a light- harvesting antenna, it
causes electrons to become excited and move to a higher energy orbital, therefore
allowing resonance energy transfers to take place (Alberts (2017)). Resonance
energy transfers occur when an excited electron relaxes, moves to a grounded state,
causing energy to be released (something) which in turn means that an electron in a
neighbouring chlorophyll molecule can be excited. This process continues until the
excitation reaches the reaction centre (Alberts (2017)). The reaction centre is a
chlorophyll-protein complex consisting of a special pair of chlorophylls (Alberts
(2017)). They are a special pair because of their orientation, as the central
magnesium ion in the porphyrin ring is co-ordinately bonded to proteins contained
within the reaction centre. This special orientation means that when an electron in
one of the chlorophylls is excited it dissociates and can finally be passed on, via a
carrier, to the electron transport chain (Alberts (2017)).
The porphyrin ring is so important because it is what enables the excitation and
subsequent transfer of electrons. Once a chlorophyll in a reaction centre has been
ionized it becomes a very strong oxidant, which results in the oxidation of a molecule
of water to regain its electrons. Once water has been oxidised molecular oxygen is
released into the atmosphere. This then permits the existence of complex eukaryotic
life, as without the production of oxygen from plants aerobic respiration would not be
possible.
The other essential structure is the phytol tail, the reason why this structure is
important is because this is what anchors the molecule to the thylakoid
membrane (Durrett, T.P. and Welti, R., 2021). This Is important to the function
because it allows the molecules to be arranged in such a way that efficient energy
transfers can take place. Another important function of the phytol tail, which is
interlinked to its structure is the fact that upon chlorophylls degradation the phytol tail
can be synthesised into vitamin E (Durrett, T.P. and Welti, R., 2021). The function of
this vitamin E is to help the plant when it undergoes abiotic stresses such as high
temperatures. The way in which the plant does this is by scavenging singlet oxygen
radicals therefore maintaining the stability of the chloroplast (Niu, Y., Zhang, Q.,
Wang, J., Li, Y., Wang, X. and Bao, Y., 2022).
A potential modification that could be made to chlorophyll is the addition of a
lycopene chain to the phytol tail. The reason why this would enhance the function of
chlorophyll a is because lycopene is also a conjugated system, like the porphyrin
ring, and has an absorbance around 450-500 nm (Lycopene - Molecule of the Month
- January 2020) therefore meaning that the modified chlorophyll molecule would be
able to absorb green light. The advantage of being able to absorb green light would
be that plants would be able to undergo the light dependant stage of photosynthesis
in more shaded conditions, as green light penetrates deeper than other wavelengths
(Horne, 2021). This would also offer a competitive advantage to plants with this
modified chlorophyll as they would also be able to photosynthesise in crowded
conditions where the leaves of other plants create shade. However, the addition of
the lycopene chain to the phytol tail may jeopardise the arrangement of chlorophylls
in the photosystems, because it would potentially alter the way in which the phytol
tail anchors the molecule to the thylakoid membrane, thus resulting in a lower
efficiency of photosynthesis. Although the benefits of this modification, if
successful, would outweigh the negatives. With the continually growing
population, demand for food is becoming ever higher, so if photosynthesis was made
more efficient this could result in higher crop yields and thereby mitigate the effects
of potential food shortages.
To conclude, both the porphyrin ring and the phytol tail are integral to the function of
chlorophyll a as their unique structure allow electrons to be pass to the electron
transport chain, which results in plants being able to fix and utilise carbon, and
also to release molecular oxygen into the atmosphere. This exchange of molecules
underpins much of life on earth, creating a suitable environment for aerobically
respiring organisms to live in. New developments in technology, resulting in the
modification proposed becoming possible. An increase of photosynthesis would
absorb carbon dioxide, provide more food and oxygen and greatly help the growing
population of the world.
References
Alberts, B. (2017) Molecular biology of the cell. Garland Science
Clayden, J., Greeves, N. and Warren, S. (2012) Organic chemistry. Oxford University
press.
Durrett, T.P. and Welti, R., 2021. The tail of chlorophyll: fates for phytol. Journal of
Biological Chemistry, 296.
Kimball, J. Structure of chlorophyll, available at 3.18: Chlorophylls and Carotenoids Biology LibreTexts
Libretexts (2022) 3.18: Chlorophylls and carotenoids.
https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Biology_(Ki
mball)/03%3A_The_Cellular_Basis_of_Life/3.18%3A_Chlorophylls_and_Carotenoids
Lycopene - Molecule of the Month - January 2020
https://www.chm.bris.ac.uk/motm/lycopene/lycopeneh.htm
Niu, Y., Zhang, Q., Wang, J., Li, Y., Wang, X. and Bao, Y., 2022. Vitamin E synthesis
and response in plants. Frontiers in plant science, 13, p.994058.