Part of the cell · Membrane-lipid biosynthesis and maintenance
Building and maintaining the membrane
On the cell picture: Cell envelope
The oily envelope that holds the cell together, keeps the inside in, and holds the voltage that energy production depends on.
Proteins in the model: 51
Part of the model's 1051 proteins. Each group below is a book group, counted in the working count after the repeats and non-first-cell groups were removed. The proteins are not yet matched gene by gene.
| Group | Proteins |
|---|---|
| Fatty acid synthesis initiation | 3 |
| Fatty acid synthesis elongation | 5 |
| Fatty acid synthesis termination | 3 |
| Fatty Acid Elongation | 1 |
| Phospholipid biosynthesis | 2 |
| Glycerophospholipid biosynthesis | 3 |
| Alternative glycerophospholipid biosynthesis | 3 |
| Phospholipid transport | 1 |
| Phospholipid degradation | 4 |
| Lipid reuse and recycling | 1 |
| CDP-diacylglycerol pathway | 3 |
| Mevalonate pathway | 6 |
| Non-mevalonate pathway | 7 |
| Peptidoglycan biosynthesis | 7 |
| Peptidoglycan cross-linking | 2 |
Its pathways
- Glycerolipid assemblyatlas pathway8
- Lipid A and outer envelopeatlas pathway5
- Fatty-acid elongationatlas pathway4
- Substrate recognitionatlas pathway3
- Terpenoid backbone biosynthesisKEGG map8
- Fatty acid biosynthesisKEGG map3
- Lipopolysaccharide biosynthesisKEGG map2
- Biosynthesis of various nucleotide sugarsKEGG map2
- Not yet placed in a pathwayto sort17
If it fails
The cell cannot grow its envelope to divide, and the barrier that holds its energy gradient leaks away.
Dies at once · within the current generation
What it depends on
- Building carbon into moleculesAcetyl-CoA and other carbon intermediates feed fatty acid and membrane lipid synthesis.Book pp. 500–501
- Manufacturing cofactors and vitaminsCoA, biotin in selected pathways, and redox cofactors support lipid biosynthesis.Book pp. 498–501
- Turning hydrogen into usable energyATP and reducing power support membrane-component synthesis.Book pp. 499–501
What it supplies
- Turning hydrogen into usable energyA sufficiently ion-tight membrane supports electrochemical gradients and houses respiratory and ATP-synthase complexes.Book pp. 499–501
- Pulling nutrients in from outsideA membrane provides the physical environment in which transport channels and carriers operate.Book pp. 500–503
- Dividing into two cellsDaughter-cell formation requires expansion and partitioning of the cell envelope.Book pp. 500–501, 504
- Holding the internal balanceSelective membrane permeability allows internal conditions to differ from the surroundings.Book pp. 500–501, 504
Dependencies are curated from the book at the level of the map’s systems and are not measured protein contacts. See all systems together.
How it is built and controlled
Counts from UniProt annotations of this group’s proteins.
Its place in the control layer
None of this group’s proteins appear in the information, signaling, regulation, machine-signal or assembly lists.
From the control-layer pages. Which genes a regulator controls is mostly inferred from related bacteria, because Aquifex has no genetic system for testing it directly.
Origins
What needs to be explained
How did amphiphile production, compartment stability and compatible chemistry become coupled before modern lipid enzymes?
What needs explaining. Explain a growing boundary that can admit resources, retain useful components and support compatible membrane proteins and gradients.
Hurdles
- H-MEM-1Lipid supplyThe problem. Cell membranes are made of phospholipids (two fatty-acid chains, glycerol and a phosphate head), produced by about twenty enzymes.Why it is hard. Prebiotic syntheses yield mostly short fatty acids at low concentration, while stable vesicles need longer chains at concentrations of tens of millimolar.Chemistry before lifeStage 1: Ingredients made under plausible early-Earth conditionsShown: Fatty acids form in proposed prebiotic syntheses and are found in meteorites.In the books: 36 open problems on lipid and membrane metabolism ↗
- H-MEM-2Salt and pH toleranceThe problem. The membrane must stay intact in the salt, magnesium and pH of the cell's environment.Why it is hard. Fatty-acid vesicles fall apart in sea-water salt and with magnesium or calcium, and are stable only in a narrow pH range.Chemistry before lifeStage 2: One step shown on its ownShown: Mixtures of different amphiphiles make vesicles more tolerant of salt and pH.In the books: 36 open problems on lipid and membrane metabolism ↗
- H-MEM-3Permeability trade-offThe problem. The membrane must keep proteins, RNA and metabolites inside while letting nutrients and waste through.Why it is hard. Leaky membranes lose contents; tight membranes block nutrients. Cells resolve this with transport proteins, which need the membrane to exist first.Molecular machinesStage 2: One step shown on its ownShown: Fatty-acid vesicles let some nucleotides in while keeping larger molecules inside.In the books: 36 open problems on lipid and membrane metabolism ↗
- H-MEM-4Growth and divisionThe problem. Cells grow their membrane by inserting newly made lipids and divide it with a dedicated machinery (FtsZ and partner proteins).Why it is hard. Growth and division must keep the internal network intact; in the laboratory vesicles divide only when pushed by external forces.The integrated cellStage 3: Several steps shown working togetherShown: Vesicles grow and divide in repeated cycles in the laboratory, though without an internal metabolism (Zhu & Szostak 2009).In the books: 36 open problems on lipid and membrane metabolism ↗
- H-MEM-5Cellular membrane compositionThe problem. A cell membrane needs lipid-synthesis enzymes, which sit in the membrane they make, plus transport and energy-coupling proteins.Why it is hard. A phospholipid membrane blocks the nutrient entry that simple vesicles allow, so it is workable only when transporters are present at the same time.The integrated cellStage 0: Not yet addressed by experimentShown: not yet addressed by experiment.In the books: 36 open problems on lipid and membrane metabolism ↗
Scope. Spontaneous lipid assembly is an established physical process. The open problem concerns the origin and sustained integration of boundary, metabolism, and heredity.
Experimental progress. Manipulating fatty-acid elongation in bacteria changes the chain-length distribution of products, illustrating how enzyme activity controls lipid building blocks.
What would strengthen a natural account. Steps shown to work together under plausible early-Earth conditions, without a chemist choosing the reagents, purifying the products or changing conditions between steps, would strengthen a natural account of origins in proportion to what they explain. Loss of a claimed dependency or discovery of functional alternatives would weaken the corresponding design argument. Gaps in the record do not supply a numerical likelihood for either explanation.
Observations. The membrane co-locates reactions and provides a functional setting for membrane proteins. Ion-gradient storage requires a sufficiently selective and intact boundary.
Hallmarks that apply.
- H28 Co-location of components: The membrane co-locates reactions and provides a functional setting for membrane proteins.
- H22 Gradient-based energy storage: Ion-gradient storage requires a sufficiently selective and intact boundary.
- H40 Cross-domain constraint satisfaction: Lipid production, growth, transport and energy conservation must remain compatible.
The author’s conclusion. The coordinated features give the author a positive reason to consider design within the wider cellular architecture. For this local system alone, they do not establish that design is uniquely the best historical explanation. The conservative inference is that the system contributes to a cumulative design case, with its weight conditional on the comparative evidence and on how much plausible natural pathways explain.
From the book article Lipid and Membrane Metabolism, with its scientific references.
How it was discovered
- 1925Gorter and Grendel conclude that cell membranes are a double layer of lipids.Gorter & Grendel, Journal of Experimental Medicine 41:439–443 (1925)
- 1965Alec Bangham shows that lipids spontaneously form closed vesicles in water (liposomes).Bangham, Standish & Watkins, Journal of Molecular Biology 13:238–252 (1965)
- 1972Singer and Nicolson describe the membrane as a fluid lipid layer with proteins floating in it.Singer & Nicolson, Science 175:720–731 (1972)
- 1978Hargreaves and Deamer show that simple fatty acids, not only modern phospholipids, can form vesicles.Hargreaves & Deamer, Biochemistry 17:3759–3768 (1978)
- 2003Hanczyc, Fujikawa and Szostak find that clay speeds up the formation of fatty-acid vesicles, which can grow and divide.Hanczyc, Fujikawa & Szostak, Science 302:618–622 (2003)
- 2009Zhu and Szostak show fatty-acid vesicles growing and dividing in repeated cycles while keeping most of their contents.Zhu & Szostak, Journal of the American Chemical Society 131:5705–5713 (2009)
Each hurdle above is sorted by kind and by how far experiments have got (see “How hurdles are sorted” at the top).
All parts of the cell side by side: origins hub. The book articles:
- Lipid and Membrane Metabolism: system page · full description · origins