Why are plants green and how did chlorophyll take over the world? (conversion of light energy into chemical energy) (2023)

learning goals

1. Describe the properties of light as energy.
2. Distinguish between phototrophy in some archaea and photosynthesis in cyanobacteria and chloroplasts
3. Distinguish the capabilities of Photosystem I and Photosystem II.
4. Describe the innovation that led to oxygenated photosynthesis in cyanobacteria.
5. Compare photophosphorylation with oxidative phosphorylation
6. Track the flow of electrons in the light reactions of oxygenated photosynthesis.

Photosynthesis produces organic carbon.

Photosynthesis is the source of most of the organic carbon on Earth, as well as the oxygen in the air. The general chemical equation foroxygenated photosynthesises:

6 CO2 + 12 H2Oh-> C6H12O6 + 6Oh2 + 6 H2O

We will present photosynthesis in two parts: This page discusses the reactions that convert light energy into chemical energy in the form of ATP and reduced electron carriers (NADH or NADPH). Both are necessary for the carbon fixation reactions (the reduction of inorganic carbon to organic carbon molecules) presented on the next page. An important by-product of light reactions is the production of oxygen gas. In the chemical equation above, the oxygen atoms in the water are highlighted in red to indicate that they are the source of the oxygen atoms in the oxygen gas. Oxygenated photosynthesis evolved to take electrons from water to create oxygen gas and eventually pass the electrons to carbon dioxide to form organic (reduced) carbon molecules (food molecules) - the opposite of aerobic respiration, which takes electrons from organic carbon molecules and eventually converted to oxygen gas to produce water.

Light reactions provide both ATP and reducing power (NADH or NADPH) necessary for carbon fixation via the Calvin cycle.

There are two different general components of photosynthesis: the light reactions and the Calvin cycle. Light reactions require light (photons) and water as inputs and produce ATP, NADH or NADPH and oxygen. Oxygen is a waste product, but ATP and NADH/NADPH are essential for the next step, the Calvin cycle. The Calvin cycle is what actually fixes carbon (produces sugars) using the products of light reactions (ATP, NADH/NADPH) and carbon dioxide. The product of the Calvin cycle is solid carbon or sugar.

In this section we focus on the first part, the light reactions (need light, production of ATP, NADH/NADPH and oxygen). The second part, the Calvin cycle (which requires products of light reactions in the form of ATP and NADH/NADPH to make sugars from carbon dioxide), is discussed on the next page.

light as energy

Visible light is part of the electromagnetic (EM) spectrum with wavelengths ranging from about 400 nm to just over 700 nm.

Light has a wave-particle duality and a quantum of light energy is aPhotos. The energy of a photon is inversely proportional to its wavelength:
WoHis Planck's constant,Cis the speed of light andLambdais the wavelength. Therefore, photons with a shorter wavelength (blue-violet) have more energy than photons with a longer wavelength (red).

biological pigments

Biological pigments are molecules that preferentially absorb light of certain wavelengths. Organisms that capture light energy and convert it into chemical energy show evolutionary and phylogenetic differences in the pigments they use.

Phototrophism vs Photosynthesis

PhototrophOrganisms convert light energy into chemical energy in the form of ATP. The use of light energy to produce ATP is calledfotophosphorylering.

Photophosphorylation is similar to oxidative phosphorylation in that both use a proton gradient across a membrane to drive similar ATP synthase enzyme complexes.PhotosyntheticOrganisms (photoautotrophs) use light energy to produce ATP and reduced electron carriers (NADH or NADPH). The reduction of carbon dioxide (CO2) to carbohydrates (CH2O) requires both ATP and reducing power in the form of NADPH or NADH (NADP+ is NAD+ with an added phosphate group and is used by chloroplasts instead of NAD+). The first phototrophic and photosynthetic organisms were prokaryotes with unique photosystems that did not produce oxygen. Two different types of photosystems emerged, which were combined in cyanobacteria. One of the two photosystems of cyanobacteria evolved the ability to oxidize water molecules as an electron source, releasing O2.

Side light: phototrophic archaea

None of the archaea found this way actually engage in photosynthesis.Halobacteriën, which is an archaic species despite its nameof bacteriorhodopsine, a purple membrane protein, acts as a light-driven proton pump to create a proton gradient across the plasma membrane and enhance chemiosmotic ATP synthesis. ThereforeHalobacteriënThey are phototrophic but not photosynthetic because they do not use light energy to fix carbon dioxide into organic carbon. Phototrophic organisms still depend on organic food molecules to build their own biomass.

photosynthetic bacteria

Photosynthetic bacteria and chlorophyll granules use variants of chlorophyll.

Chlorophyll absorbs blue and red light.

Here are two great short videos on why chlorophyll is green:

Two kinds of photosystems.

photo systemThey are membrane complexes of proteins and chlorophyll molecules. Chlorophyll molecules absorb photons and conduct the energy to a chlorophyll reaction center, which oxidizes (loses electrons).

All oxygenated photosynthesizers (which produce gaseous oxygen as a by-product; cyanobacteria and chloroplasts) have two different types of photosystems linked together. In contrast, all photosynthetic, anoxygenic (non-oxygen gas-producing) bacteria have only one type of photosystem. The earliest phototrophs were probably anoxygenic.

Photosystem I,When oxidized by absorbing light energy, it transfers electrons to a protein called ferredoxin, which in turn reduces NADP+ to NADPH. NADP+ is an electron carrier, a phosphorylated form of NAD+; It may be helpful to think of P as photosynthesis.

photosystem II,It absorbs light energy and transfers electrons to an electron transport chain in the membrane, which pumps protons to create an electrochemical gradient for chemiosmotic ATP synthesis. In cyanobacteria and chloroplasts, the oxidized photosystem II splits (oxidizes) water molecules to reclaim electrons, producing oxygen gas.

How did such a complicated system with two different photosystems come about? The clues come from the observation that some phototrophic and photosynthetic bacteria have only one photosystem. And they have a type I or type II photosystem.

Bacteria that only have a type II photosystem (PSII), such as purple bacteria, arefototroop:They use light energy to produce ATP through photophosphorylation, but do not use light energy to fix carbon dioxide. Photophosphorylation is very similar to oxidative phosphorylation. In both cases, a membrane-localized ATP synthase complex uses the energy of a proton gradient to produce ATP. Light energy oxidizes chlorophyll in the reaction center and transfers electrons to an electron transport chain that creates a proton gradient across the photosynthetic membrane. The terminal electron acceptors in purple bacteria are the chlorophylls of the oxidized reaction center; Electrons flow in a cycle from the PSII along the ETC back to the PSII. Pure PSII purple sulfur bacteria cannot use light energy to produce NADH or NADPH, but can use other pathways to oxidize hydrogen sulfide as an electron source to produce reduced electron carriers. In other words, while purple sulfur bacteria can capture carbon dioxide, they cannot do so through their photosystem II; The Photosystem II can only be used to produce ATP.

Bacteria with only one type I photosystem (PSI), such as sulfur green bacteria, could be truephotoautotroph:They use light energy both to produce ATP and to sequester (reduce) carbon dioxide. Light energy oxidizes chlorophyll in the reaction center and reduces the NAD+ electron carrier to generate NADH. The chlorophyll in the oxidized reaction center must then be reduced by electrons from a chemical electron donor such as hydrogen sulfide (H2S). Chlorophyll in the oxidized reaction center pulls electrons from H2S into the photosynthetic electron transport chain, creating a proton gradient to produce ATP. For example, green sulfur bacteria use light energy to produce ATP and reduce power; both are necessary for carbon fixation (reduction of CO2 to carbohydrates). However, they are limited by the availability of a suitable electron donor such as H2S.

Oxygen photosynthesis and non-cyclic electron flow.

About 2.5 to 2.7 billion years ago, cyanobacteria evolved a scheme that linked both types of photosystems. In the noncyclic electron flow scheme (often referred to as the Z scheme), light-activated PSII donates its electrons to the electron transport chain to drive photophosphorylation. At the same time, the light-activated PSI donates its electrons to reduce NADP+ to NADPH. The two systems are linked because oxidized PSI is reduced through the electron transport chain (an electron is transferred from ETC to PSI). The oxidized PSII recovers electrons from the oxidized water molecules to produce oxygen gas. Therefore, in cyanobacteria (and chloroplasts)The electron flow goes from water to PSII, then down the electron transport chain to PSI and finally to NADP+ to produce NADPH.(Cyanobacteria and chloroplasts use NADP+/NADPH instead of NAD+/NADH.) Cyanobacteria's ability to extract electrons from water gave them a huge evolutionary advantage over green sulfur bacteria, which were restricted to places where hydrogen sulfide or other suitable electron donors were present goods.

Photosynthesis in chloroplasts is essentially the same as photosynthesis in cyanobacteria. Both use a noncyclic flow of electrons to produce ATP, NADPH and O2. The figure below illustrates the non-cyclic electron flow during photosynthesis in chlorophyll grains.

Cyclic electron flow in cyanobacteria and chloroplasts.

When the metabolic demands on the chloroplast require supplemental ATP, but not supplemental NADPH, the cyclic flow of electrons from the PSI through the electron transport chain and back to the PSI can increase the proton gradient and hence the proton gradient. Photophosphorylation (light-driven ATP synthesis). Thus, PSI is versatile in the sense that oxidized PSI can donate electrons to NADP+ (usually) or ETC (if needed to make additional ATP).

Photosynthetic membranes

The membranes of photosynthetic bacteria are tightly coiled due to the folding of the plasma membrane, increasing the surface area for light absorption and photosynthesis. These folded membranes are calledThylakoïdeand are also found in chloroplasts evolved from endosymbiotic cyanobacteria. The thylakoid lumen corresponds to the extracellular or periplasmic space of cyanobacteria.

This 5 minute video provides a very nice animation of the light reactions of photosynthesis: Basic biology students don't need to remember the details of the electron transport chain or the name of the enzyme that reduces NADP+ to NADPH.

And here's a short video lecture on the topic of light reactions:

PowerPoint slides used for the above video screencasts:Reactions_Light_Photosynthesis

references

Blankenship RE, 2010. Early evolution of photosynthesis.Physiol vegetables. 154:434-438.doi: 10.1104/pp.110.161687

XiongJ., W.M. Fischer, K. Inoue, M. Nakahara, CE Bauer, 2000. Molecular evidence for the early development of photosynthesis.Science 289:1724-1730.DOI: 10.1126/science.289.5485.1724

UN Sustainable Development Goal (SDG)13: Climate Protection –Studying historical changes and biological innovations such as oxygen photosynthesis that led to large-scale ecological changes can help us better understand how organisms respond to climate change. By understanding how chemical processes are supported by the physiological properties of organisms, we can better predict which organisms are most susceptible to change and try to mitigate the negative consequences, such as those of large-scale changes in gas resources.