Let’s try to define a Minimum Cell Model (MCM) for a cell in general and for the cell of a single cellular organism in particular. We will construct the model by identifying the minimum component types that make up the cell together with their basic role and function described in the context of their interactions as part of a self-sufficient self-replication entity.
We proceed from a starting proposition (assumption) that a cell has two fundamental properties:
P1. Has an enclosure (E)
P2. Has the ability to self-replicate accurately and this property is passed without degradation to its offspring daughter cells.
The property P2 of the cell to self-replicate means that it can be seen as a mechanism with certain interacting parts that together produce two daughter cells from the original mother cell. We can safely characterize the self-replication process as a development through which the cell:
• ingests through the gateway (G) placed on its enclosure (E) certain materials from its environment ( see Figure 1 below)
• grows by changing its volume and geometry during a “cloning” phase. During this cloning phase it is assumed that certain components of the cell are replicated (or cloned) through some internal ability of component/part fabrication.
• starts and completes a sub-process of separation of the now grown cell volume into two separate daughter volumes as part of the “division” phase. At the end of the division phase the two separated volumes become self-sufficient cells capable themselves to self-replicate.
One trivial consequence of this manifest property P2 of the cell to self-replicate is that we must find that each of the two daughter cell must have the same composition in components (parts) as the original mother cell and each one is a “replica” mechanism (a well-coordinated set of interacting parts) of the original mother cell mechanism.
The Component Types of the Minimum Cell Model (MCM)
We can infer thus that the cell contains at a minimum the following type of components (or parts)
The enclosure E has the role to isolate the internal volume of the cell from its environment and to protect its internal components and processes (see Figure 1 below).
There is at least one gateway G component on the enclosure. The gateway role is to detect certain materials in the cell environment and to open in order to allow them to enter the cell enclosure. The gateway will stay closed otherwise.
There is a power producer P component within the cell. This component is responsible to use certain materials accepted through the gateway to produce energy. The energy produced is used by other cell components to execute their functions, for example – as we can see below - to fabricate parts or to transport materials or parts within the cell.
There is a transporter and assembler T component that is responsible to transport various materials and parts during the cloning and division phases of the cell replication. It has also the ability to somehow manipulate the transported materials and parts and to put them together (assemble) when needed.
There is a fabricator F component that is responsible to fabricate, construct or manufacture needed parts and components during the cloning and division phase. The fabricator uses materials accepted through the gateway as input to fabrication process. The fabricator must have the capability to fabricate all cell component types and their parts. For example the fabricator is able to fabricate the enclosure and its elements and to fabricate a fabricator component and its parts.
There is a construction planner CP component that is responsible to store in certain ways detailed construction plans of all component types of the cell and their parts as well as the construction plan of the whole cell. This component is also responsible to use the information in its construction plan to coordinate the activities of various component types in the cell and to coordinate the overall progress of the cloning and the division phases of the cell self-replication.
Figure 1 The Minimum Cell Model (MCM) and its component types: E = enclosure, G = gateway, P = power generator, T = transporter and assembler, F = fabricator, CP = construction planner
Rationale for the definition of the Minimum Cell Model (MCM) and its Component Types
1. There is at least one component in the cell MCM for each component type
2. It is possible and accepted by the MCM that multiple instances of each component type (excluding E) exist in the MCM cell.
3. The six component types have been selected (identified) such they have distinct function and there is certainty that the associated function type is present in the actual cells.
4. The six component types have been also selected for a strong correspondence to the actual element types in the prokaryotic and eukaryotic cells.
5. There might be additional component types and associated functions that – for the simplicity and minimal characteristic of the MCM were not included. For example there might be a cell scaffolding component type or a material transformation function, or a material ejection function
6. Each of the six component types may actually represent a sub-family of components that have small variations between them but they play similar role in the MCM.
Interactions between the Component Types in the MCM
We illustrate – without being exhaustive - the kind of functional interactions that exists between the six component types of the MCM. These interactions manifest during various steps of the cloning and division phases of the cell replication.
Interactions of the Enclosure E component type with the other component types
• Enclosure E hosts spatially the gateway(s) G component types
• Enclosure E elements are built by the Fabricator F as the volume of the cell grows during cloning and new sub-elements of the E need to be inserted
• Fabricated sub-elements of the enclosure are transported at the point of the insertion in E by the Transport T function using energy produced by the Power Producer P
• The Construction Planner CP function coordinates the change in geometry and 3D change and extension or shrinkage of the enclosure during the cloning and division phases.
Interactions of the Gateway G component type with the other component types
• Gateway component types are built by the Fabricator F.
• Gateway component types and their parts are built by the Fabricator according with the specific construction plans for the gateway stored by the Construction Planner CP component type.
• The new Gateway component types constructed for the daughter part of the enclosure are inserted geometrically in the growing envelope of the enclosure in a position directed by the Construction Planner CP.
• Gateway G interacts with the Transport and Assembler T component type to load for transport the materials accepted by the gateway from external cell environment
• The gateway component type is instructed by the Construction Planner CP component type when to open and accept inside the cell specific material types that are needed for fabrication of particular component types or parts. Similarly it is instructed when to stay closed in presence of other materials that are in full supply inside the cell.
Interactions of the Transport and Assembler T component type with the other component types
• The transport and assembler component and its parts are fabricated by the Fabricator F
• The transport and assembler component types are using energy generated by the Power Producer P to carry materials and parts and to assemble other components or their parts from basic elements.
• The transport component type picks materials from the Gateway G and carries them to various locations in the cell like the Fabricator component.
Interactions of the Fabricator F component type with the other component types
• The fabricator uses materials and parts transported to it by the Transport and Assembler T component type.
• The fabricator uses the Transport and Assembler T component type to assemble fabricated parts into higher level assemblies or components.
• The activity of the fabricator F is fully coordinated by the Construction Planner CP component type in respect to what needs to be fabricated and how to fabricate, i.e. what is the fabrication, construction and assemblage plan for the element. The Construction Planner CP component type coordinates also the end part of the fabrication by instructing the Transport and Assemblage T component type on where in the cell to transport the fabricated part.
• Although the Fabricator F acts always under the coordination of the Construction Planner (CP) by virtue of the rule that it must have the ability to fabricate any MCM component (and component elements) of the MCM, F have the ability to fabricate a Construction Planner (CP) for the daughter MCM during the cloning phase. Since CP cannot logically be a coordinator and planner without storing locally some kind of information, results by consequence that the Fabricator F has the ability to copy any information (set) from the CP component into the cloned CP component.
Figure 2 The interactions between the component types of the Minimum Cell Model (MCM) machinery: E = enclosure, G = gateway, P = power generator, T = transporter and assembler, F = fabricator, CP = construction planner
The Minimum Cell Model as a Mechanism and Irreducible Complex System.
The picture in figure 2 above illustrates schematically the complex interactions between the six component types of the Minimum Cell Model. At the same time the picture illustrates at the high level the Minimum Cell Model as a mechanism or machinery that has 6 parts and the mechanism works as intended only if all the parts are in place (present) and each part works as specified (or designed). Another way to express this is to state that each part of the mechanism must be present and fulfill its ‘contractual’ obligations enumerated by the interfaces and interactions with other components of the mechanism.
Like any other functioning mechanism or machinery it is natural that the Minimum Cell Model be viewed as an irreducible complex system that depends on the presence of all its designed parts and the good interlocking and coordination of these parts in order to work well and provide the expected behavior – in our case to produce exact copies of itself through replication. The removal or absence of any single component type of the Minimum Cell Model will compromise its working and will lead to failure of its ability to replicate – as explained by Michael Behe when discussing the irreducible complex systems (see Darwin's Black Box: The Biochemical Challenge to Evolution)
The Minimum Cell Model and the Origin of Life Problem
In the Origin of Life (OOL) literature there were historically three distinct avenues for the research trying to find ways that life originated as result of the laws of physics and chemistry in conjunction with pure natural happenings and circumstances. See: Signature in the Cell by Stephen C. Mayer
These three research avenues for the OOL research are briefly identified as:
1. Replicator – First
2. Metabolism – First
3. Membrane – First
The Minimum Cell Model defined in previous sections reveals though clearly that any rational attempt to identify any naturalistic OOL scenario must follow a totally different approach, for which we would give THREE quasi-equivalent names:
a. Full System – First
b. All-Integrated-Components – First
c. The Integral Irreducible Complex System – First
The Problem with Current Approaches in OOL Research
The justification for declaring that only such a Full System - First approach (as named alternatively as b. or c. above) must be considered by OOL research for a chance to succeed is quite straightforward: a Minimum Cell Model (MCM) CANNOT function as a true replicator in any of the following circumstances:
I. There is an internal component that has the ability to self-replicate itself but it is not able to also construct other indispensable components like the Enclosure (E), the Power Generator (P) or the Construction Planner (CP) components and also is not able to place and assemble these components in required functional relationships for an overall self-replication capability. This circumstance is characteristic for the Replicator – First approach in OOL research.
II. There is an internal component that has the ability to use external materials in order to fabricate one or only a few other components but is lacking the ability to construct other indispensable components like Enclosure (E), Transporter and Assembler (T) or the Construction Planner (CP) components and also is not able to place and assemble these components in required functional relationships for an overall self-replication capability. This circumstance is characteristic for the Metabolism – First approach in OOL research.
III. There is a component that has the ability to function as an Enclosure (E) but is lacking the ability to construct other indispensable components like the Power Generator (P), the Fabricator (F) or the Construction Planner (CP) components and also is not able to place and assemble these components in required functional relationships for an overall self-replication capability. This circumstance is characteristic for the Membrane– First approach in OOL research.
There are good grounds to estimate that the OOL research trying to identify naturalistic scenarios for the origin of life is facing extreme (insurmountable) difficulties – which we believe are made clear with the Minimum Cell Model. Here are a few more thoughts that can farther support this statement.
There is no debate that a cell (or any single cell organism) is a very complicated mechanism – at least for its undisputed ability to accurately self-replicate.
The Minimum Cell Model (MCM) is used to create a simplified but realistic model of the cell as a mechanism composed of a number of components each with specific functions and capabilities and integrated through defined interactions and interfaces into an overall machinery that manifests the ability to self-replicate. It can be argued that there is too much simplification in the MCM and other component types besides the six identified ones might be needed. But a more complex model of the cell will farther strengthen all this reasoning– rather than weaken it.
It is known that many components of the cell – and thus the components of the MCM - are quite complex with research being conducted continuously in order to get a better understanding of the inner workings of each such component. Experimental research on creating artificial, concrete, self-replicating objects, revealed an unimaginable level of complexity, and numerous technical barriers for any attempt to build fully autonomous, concrete self-replicators. See:
Freitas, R.A. Jr., Merkle, R., Kinematic Self-Replicating Machines
The Design of the Simplest Self-Replicator (youtube conference video)
The Design of the Simplest Self-Replicator (powerpoint slides: take 2 minutes to download)
Freitas, A.R. Jr., Gilbreath, W.P., Editors, Advanced Automation for Space Missions
There is no known success in the creation of concrete, autonomous self-replicators by engineers, technologists or scientists. In other words the design and construction of a material, fully autonomous self replicator is considered by the author beyond the current engineering and technological capabilities of the humankind. Then this state of affairs seems to be perfectly consistent with the previous statement expressing total skepticism regarding any possible success on OOL research for natural origins of life.
The Minimum Cell Model just helps us to see and understand the Cell as a special composition of integrated functional parts:
The Cell is a Complex Irreducible System made up of a number of components that through their precise arrangements, interactions and coordination manifest the hard-to-achieve self-replication ability.
The Minimum Cell Model as well alows us to see with better clarity what problems need to be resolved by OOL research looking into a naturalistic explanation of the apparition of the cell (or life).
Expectations for Successfull OOL Research
Any OOL research that desires to provide a scientific, convincing explanation for a naturalistic origin of the life need to be aware that it must provide rigurous answers in all three areas below:
a. A naturalistic explanation of the origin of life (or self-replicating cell) requires a detailed, credible scenario that explains how each component type originated by the interplay of physical and chemical laws with natural circumstances and phenomena [the natural apparition of all needed component types],
b. A naturalistic explanation of the origin of life (or self-replicating cell) requires a detailed, credible scenario that explains how the collection of diverse component types of the cell may have happened to engage in the exact working interactions, interfaces, communication and coordination patterns that resulted in an overall self-replication scheme (the cell system (abstract) plan) [the natural apparition of an effective self-replicating system scheme],
c. A naturalistic explanation of the origin of life (or self-replicating cell) requires a detailed, credible scenario that explains how (and the probabilities that) all necessary component types in all necessary required numbers, with all necessary interactions, interfaces and communication and coordination patterns integrated as an overall self-replication capability happened to exist (be created) concomitantly and all within a small space and with the required spatial arrangements to function as an (apparently) designed system [the natural apparition precisely at the same time and at the same place of all needed component types in the required numbers and fortuituos precise assemblage and start-up of all these parts into an effective self-replicating machinery that appears to be designed for this purpose but it is not].
We defined the Minimum Cell Model as an empirical representation of the nature of a cell as a mechanism that has the capability to self-replicate by virtue of the nature, capabilities and interactions of its defined component types. The MCM fundamentally represents simply the cell as an irreducible complex system that provides its function (ability to create replicas of itself) as long as all its required components are present and work and interacts as expected.
The Minimum Cell Model is used as an adequate abstraction that allow us to reflect with clarity upon the question if such a machinery may have appeared spontaneously by the interplay of natural laws and natural phenomena and circumstances.
Using the paradign of the irreducible complex system that applies perfectly to the cell and our Minimum Cell Model we determined that:
A. Previous and current OOL efforts that engage on any of the Replicator - First, Metabolism - First or Membrane - First approaches is destined to overall failure since only a Full-System - First approach is rationally acceptable.
B. Any OOL effort with a chance of final success must provide coherent and credible answers and explanations on three distinct but related planes:
1. The natural origination of all required component types of the MCM (cell)
2. The natural origination of a self-replication scheme or machinery plan were self-replication is achieved through careful interactions and coordination of all components of the MCM (cell)
3. The natural putting together or assemblage of all required component types conforming to the machinery plan, within specific same-time, same-place apparition constraints and proper assemblage and engaging-in-action constraints
C. We determined and stated that the chances of a successful OOL effort on naturalistic basis are extremely small or inexistent based on following justifications:
1. The perceived extreme difficulty that any OOL effort may provide credible answers for all three logical planes identified above at B.
2. The ever increasing complexity revealed by scientific research of the only known entities able to self-replicate: the cell and all living world creatures. The micro biology scientists do not have a complete understanding of many details and mechanisms involved in the cell self-replication.
3. The independent studies and research into the design and creation of artificial, concrete self-replication machinery. In our view the design and creation of artificial, concrete, fully-autonomous self-replicating machinery is much beyond the current engineering and technological capabilities of the mankind. It is absolutely evident that believing that natural laws and random happenings may create machinery of such complexity that the most advanced scientific and engineering labs in the world cannot create has no rational and scientific basis.
The OOL research looking for a naturalistic explanation for the origin of the first cell is confronted with a sysiphic, hopeless task.