The Physics of Photosynthesis

Plants are some of the most important organisms on the planet; they form the foundation of numerous ecosystems and environments. Although they appear as relatively simple organisms, there are many complex processes occurring within all plants. Many of these processes involve the application of physical laws and principles. Plants rely on light absorption as well as electron excitation to produce chemical energy and use osmosis to transport the products.

A plant’s source of chemical energy is glucose formed through photosynthesis (Lambers, Chapin and Pons, 2008). Photosynthesis is carried out within chloroplasts, which are specialized organelles containing chlorophyll (Palta, 1990). There are two types of the pigment: chlorophyll a and b. Chlorophyll a is a blue-green colour, while chlorophyll b is a yellow-green colour. Chlorophyll a and b have characteristic wavelengths of 660 nm (red light) and 450 nm (blue light), respectively, while both transmitting green light (Palta, 1990). The basis of photosynthesis is connected with two photosystems within chloroplasts where chlorophyll is found (Lambers, Chapin and Pons, 2008). The pigments absorb incoming photons of light and become excited in the reaction center (Figure 1), (Krause and Weis, 1991). The excited chlorophyll molecules are then in an unstable state of higher energy. This energy from the photon then moves through the two photosystems in the form of electron movement. This movement occurs along an electron transport chain in order to develop an electrochemical gradient (Wege, 2020). This electrochemical gradient is used by the enzyme ATP synthase to produce ATP (Wege, 2020). ATP is then used as one of the sources of energy for further cellular processes to turn carbon dioxide in the atmosphere into glucose to be used by the plant as nutrition (Wege, 2020).

Figure 1: Schematic diagram of a photosystem absorbing an incoming photon. The photon (1) is absorbed by chlorophyll pigments (2) in the reaction center (3) within the photosystem (5) and an electron is excited (4). The excited electron is then moved through the electron transport chain (Photosystem, 2006).

The movement and distribution of substances within plants is accomplished by yet another set of physical mechanisms. The glucose formed using the energy from photosynthesis must be transported to other locations of the plant; this is done through bulk liquid flow via phloem (Jensen, 2018). Phloem is a type of vascular tissue that is responsible for the movement of materials and fluid dynamics throughout plants. Phloem tubes compose the system responsible for glucose transport and is driven by pressure differences (Jensen, 2018). Osmosis is responsible for generating the motile force of phloem pressure. The accumulation of glucose areas with high rates of photosynthesis causes an increase in turgor pressure (Jensen, 2018). Turgor pressure is a measure of the water volume and osmotic pressure of a cell (Sheng-Xu, Zhao-Hui and Stewart, 2013). A pressure of 1 megapascal is enough to allow the transport of glucose in small plants (Jensen, 2018). However, as the size of the organism increases, so does the required amount of pressure to force glucose transport. The glucose molecules are moved into the vascular tissue via membrane transporters (Jensen, 2018). This process requires energy since the phloem may have a higher glucose concentration than the photosynthetic cells. The glucose is then moved through the plant in the phloem tubes to be used by other cells as nutrients (Jensen, 2018).

The use of physical phenomenon, such as light absorption and osmosis, for natural purposes is extensively displayed in plants. The mechanics of energetically exciting chlorophyll in photosystems and osmotic transport of the resulting energy product is vital to a plant’s proper functioning. The multifaceted processes that plants make use of is a demonstration of their, at first unapparent, complexity.

References

Jensen, K.H., 2018. Phloem physics: mechanisms, constraints, and perspectives. Current Opinion in Plant Biology, 43, pp.96–100. https://doi.org/10.1016/j.pbi.2018.03.005.

Krause, G. and Weis, E., 1991. Chlorophyll Fluorescence and Photosynthesis: The Basics. Annual Review of Plant Physiology and Plant Molecular Biology, 42(1), pp.313–349. https://www.annualreviews.org/doi/abs/10.1146/annurev.pp.42.060191.001525.

Lambers, H., Chapin, F.S. and Pons, T.L., 2008. Photosynthesis. In: H. Lambers, F.S. Chapin and T.L. Pons, eds. Plant Physiological Ecology. [online] New York, NY: Springer. pp.11–99. https://doi.org/10.1007/978-0-387-78341-3_2.

Palta, J.P., 1990. Leaf chlorophyll content. Remote Sensing Reviews, 5(1), pp.207–213. https://doi.org/10.1080/02757259009532129.

Photosystem. 2006 [image online] Available at: <https://commons.wikimedia.org/wiki/File:Schema-photosysteme.svg> [Accessed 20 November 2022]

Sheng-Xu, L., Zhao-Hui, W. and Stewart, B., 2013. Responses of Crop Plants to Ammonium and Nitrate N. 118. In: Advances in Agronomy. pp.205–397. Available from> https://doi.org/10.1016/B978-0-12-405942-9.00005-0.

Sujatha, B., 2015. Photosynthesis. In: B. Bahadur, M. Venkat Rajam, L. Sahijram and K.V. Krishnamurthy, eds. Plant Biology and Biotechnology: Volume I: Plant Diversity, Organization, Function and Improvement. [online] New Delhi: Springer India. pp.569–591. https://doi.org/10.1007/978-81-322-2286-6_22.

Wege, S., 2020. Plants Increase Photosynthesis Efficiency by Lowering the Proton Gradient across the Thylakoid Membrane. Plant Physiology, 182(4), pp.1812–1813. https://doi.org/10.1104/pp.20.00273.


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