Scientists construct artificial photosynthetic cells: New system paves the way to construct energetically-independent artificial cells

National Geographic Society

Most life on Earth depends on photosynthesis.The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O2) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores .

The process

During photosynthesis , plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose . The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts, which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent reactions vs. light-independent reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH. The light-independent stage, also known as the Calvin Cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis , including C3 photosynthesis and C4 photosynthesis . C3 photosynthesis is used by the majority of plants. It involves producing a three- carbon compound called 3-phosphoglyceric acid during the Calvin Cycle , which goes on to become glucose . C4 photosynthesis , on the other hand, produces a four- carbon intermediate compound, which splits into carbon dioxide and a three- carbon compound during the Calvin Cycle . A benefit of C4 photosynthesis is that by producing higher levels of carbon , it allows plants to thrive in environments without much light or water.

Where are most photosynthetic cells in plants found?

Question:

Where are most photosynthetic cells in plants found?

Chloroplasts:

Chloroplasts are organelles only found in plants that are in charge of the photosynthesis process. It consists of a double membrane and an inner membrane known as thylakoid, in which the photosynthesis process will take place. The space between the thylakoid and the outer membrane is known as the stroma. This organelle mostly contains green pigments known as Chlorophyll, which will mostly reflect green light, and absorb other wavelengths of light.

Answer and Explanation: 1

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Scientists construct artificial photosynthetic cells: New system paves the way to construct energetically-independent artificial cells

A team led by associate professor Yutetsu Kuruma of the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology has constructed simple artificial cells that can produce chemical energy that helps synthesize parts of the cells themselves. This work marks an important milestone in constructing fully photosynthetic artificial cells, and may shed light on how primordial cells used sunlight as an energy source early in life's history.

Scientists build artificial cells as models of primitive cells, as well as to understand how modern cells function. Many sub-cellular systems have now been built by simply mixing cell components together. However, real living cells construct and organize their own components. It has also been a long time goal of research to build artificial cells that can also synthesize their own constituents using the energy available in the environment.

The Tokyo Tech team combined a cell-free protein synthesis system, which consisted of various biological macromolecules harvested from living cells, and small protein-lipids aggregates called proteoliposomes, which contained the proteins ATP synthase and bacteriorhodopsin, also purified from living cells, inside giant synthetic vesicles. ATP synthase is a biological protein complex that uses the potential energy difference between the liquid inside a cell and the liquid in the cell's environment to make the molecule adenosine triphosphate (ATP), which is the energy currency of the cell. Bacteriorhodopsin is a light-harvesting protein from primitive microbes that uses light energy to transport hydrogen ions outside of the cell, thus generating a potential energy difference to help ATP synthase operate. Thus, these artificial cells would be able to use light to make a hydrogen ion gradient that would help make the fuel cells use to run their sub-cellular systems, including making more protein.

Just as the scientists hoped, the photosynthesized ATP was consumed as a substrate for transcription, the process by which biology makes messenger RNA (mRNA) from DNA, and as an energy for translation, the process by which biology makes protein from mRNA. By also including the genes for parts of the ATP synthase and the light-harvesting bacteriorhodopsin, these processes also eventually drive the synthesis of more bacteriorhodopsin and the constituent proteins of ATP synthase, a few copies of which were included to "jump-start" the proteoliposome. The newly formed bacteriorhodopsin and ATP synthase parts then spontaneously integrated into the artificial photosynthetic organelles and further enhanced ATP photosynthesis activity.

As professor Kuruma states "I have been trying for a long time to construct a living artificial cell, especially focusing on membranes. In this work, our artificial cells were wrapped in lipid membranes, and small membrane structures were encapsulated inside them. In this way, the cell membrane is the most important aspect of forming a cell, and I wanted to show the importance of this point in the study of artificial cell and feedback in origins of life studies."

Kuruma thinks the most impact point of this work is that artificial cells can produce energy to synthesize the parts of the cell itself. This means that the artificial cells could be made to be energetically independent and then it would be possible to construct self-sustaining cells, just like actual biological ones cells. "The most challenging thing in this work was the photosynthesis of the bacteriorhodopsin and the ATP synthase parts, which are membrane proteins. We tried to photosynthesize a full ATP synthase, which has 8 kinds of component proteins, but we could not because of the low productivity of the cell-free protein synthesis system. But, if it was upgraded, we may photosynthesize the whole 8 kinds component proteins."

Nevertheless, this work demonstrates that a simple biologically inspired system including two kinds of membrane protein is able to supply energy to drive gene expression inside a microcompartment. Thus, primordial cells using sunlight as a primal energy source could have existed early in life's evolution before modern autotrophic cells arose. The team believes attempts to construct living artificial cells will help understand the transition from non-living to living matter that took place on early Earth and, help develop biology-based devices that can sense light and drive biochemical reactions. These artificial photosynthetic cell systems also help pave the way to constructing energetically independent artificial cells.