What actually defines a living cell?
A living cell is an incredibly complex, self-sustaining system. It's not just a bag of chemicals; it's a highly organized, compartmentalized structure where thousands of different biomolecules and metabolites interact in intricate cycles and reaction networks [2]. This complexity is what makes a cell 'alive' — it can grow, replicate, respond to its environment, and maintain itself. A synthetic cell, by contrast, is a human-made assembly of non-living molecules designed to mimic one or a few of these functions. The boundary, then, isn't a single switch but a gradient of complexity: the more life-like functions a synthetic system can perform, the closer it gets to the 'living' side of the line.
How close have synthetic cells gotten to being 'alive'?
Synthetic cells have made impressive strides, but they are still simplified models. For instance, researchers have built synthetic cells that can carry out basic metabolism and even grow [2], and they have engineered chemical communication channels between synthetic cells and living bacteria [3]. This means a synthetic cell can send and receive signals, a hallmark of living systems. However, these synthetic cells often lack a truly crowded, dynamic interior (cytoplasm) that is essential for the complex biochemistry of life [2]. They are more like a simple engine than a whole car — they can perform a specific task, but they don't have the full, integrated system of a living organism.
The real boundary: self-sustaining complexity vs. simplified mimicry
The core difference is that a living organism is a self-sustaining system that can maintain itself and reproduce, while a synthetic cell is a human-designed tool that performs a limited set of tasks. Even the most sophisticated synthetic cells, such as those that can communicate with living cells [3] or carry out in vitro transcription and translation (making proteins from genes) [2], are not self-sustaining. They rely on a supply of nutrients or energy from the outside, and they cannot repair themselves or evolve in the same way a natural cell can. The boundary, therefore, is not a single trait but a threshold of complexity and autonomy. A synthetic cell that could independently grow, divide, and evolve would be considered a living organism, but no such system exists yet.
About These Sources
This answer is built on 3 peer-reviewed studies — published from 2021 to 2023, 3 in Q1 journals, collectively cited 339 times — selected as the most relevant from 3 studies that passed quality screening, drawn from 43 papers retrieved from a database of over 500 million.
Sources used in this answer
Synthetic chromosomes, genomes, viruses, and cells
This review describes the construction of viruses and bacteria with synthetic genomes, including the creation of a minimal bacterial cell and recoded E. coli strains, showing that synthetic genomes can be 'booted up' to create living cells.
Complex Coacervate Materials as Artificial Cells
This paper reviews artificial cell research, noting that living cells are self-sustaining systems with thousands of interacting biomolecules, while synthetic cells (like coacervates) are simplified models that can mimic functions like metabolism and growth but lack full complexity.
Chemical communication at the synthetic cell/living cell interface
This review discusses chemical communication between synthetic cell models (e.g., vesicles, coacervates) and living cells, identifying three modes of communication and highlighting that this remains a key challenge in synthetic biology.