LSM2106 Chap.10 Lipids, Membranes and Nucleic Acids
Lipids, Membranes and Nucleic Acids
A class defined by behaviour, and the structure that behaviour forces
The eleventh and twelfth lectures cover lipids with the cellular membrane, and nucleic acid structure and function. They belong together because both answer the same question: how does the chemistry of a monomer force a structure that can do a job.
Lipids are grouped by limited solubility in water rather than by a shared bond, and that single property is enough to build the compartments every other chapter has assumed.
Saturation changes a physical property rather than a chemical one. A fully saturated hydrocarbon chain is straight, packs closely and makes many van der Waals contacts, so the material melts higher.
One double bond in the usual biological geometry puts a permanent kink in the chain, which prevents close packing and lowers the melting point. That is the whole explanation for why one fat is solid and another liquid, and it is how an organism adapts its membranes to cold.
The phospholipid is distinctive because both preferences live in one molecule: a charged head that hydrates readily and two hydrocarbon chains that water excludes.
Self-assembly, selectivity and a copyable polymer
Placed in water, phospholipids arrange themselves with heads facing the water on both sides and chains meeting in the middle. No enzyme builds this; it is the arrangement that costs water least entropy.
Two consequences follow. The layer is self-sealing, because exposing the chains is the unfavourable state, so a puncture closes. And it is selectively permeable: the interior is hydrocarbon, so small uncharged molecules cross readily while ions and polar solutes do not, however small.
Charge rather than size is the dominant barrier, which makes transport proteins a requirement rather than an optimisation, since nearly everything a cell must move is charged or polar.
A nucleotide has three parts: a nitrogenous base, a five-carbon sugar and a phosphate. The phosphate joins one sugar to the next, so the backbone carries a negative charge at every link and the bases project from it.
Each base pairs with exactly one partner, because only those combinations place donors opposite acceptors and only those give pairs of equal width, so the duplex has a constant diameter and each strand specifies the other completely. The bases absorb near 260 nanometres and absorb more strongly unstacked than stacked, so separation can be watched as a rise in absorbance.
The two strands run in opposite directions, which is why the ends of a duplex are not equivalent.
What this chapter covers
- 01
Lipid classes and the property that defines the group
- 02
Saturation, chain packing and melting behaviour
- 03
Amphipathicity, and why a bilayer assembles without an enzyme
- 04
Self-sealing and selective permeability, with charge as the dominant barrier
- 05
Two kinds of transport protein and what each can and cannot do
- 06
Nucleotide anatomy and the charged backbone
- 07
Base pairing, constant duplex width and antiparallel strand direction
- 08
Absorbance at 260 nanometres as a way to watch strands separate
Predicting which duplex separates first
- 3Compare the number of hydrogen bonds per unit length in the two duplexes.
- 3Describe the absorbance measurement that detects separation.
- 2Name the variable that must be matched between the samples.
Key terms
- Amphipathic
- Carrying both a water-attracting and a water-excluding region in the same molecule, which is the property that drives phospholipids into a bilayer.
- Bilayer
- Two opposed leaflets of phospholipid with head groups on the aqueous faces and hydrocarbon chains meeting in the interior, an arrangement that forms spontaneously.
- Selective permeability
- The property of a bilayer that small uncharged molecules cross it readily while ions and polar solutes do not, so charge rather than size is the main barrier.
- Membrane fluidity
- The ease with which components diffuse within a leaflet, adjusted by the proportion of unsaturated chains and by rigid ring systems inserted between them.
- Phosphodiester backbone
- The alternating sugar and phosphate chain of a nucleic acid, negatively charged at every link and therefore uniformly charged per unit length.
- Complementary base pairing
- The rule that each base pairs with one specific partner, which gives the duplex a constant width and lets each strand specify the other.
- Antiparallel
- Running in opposite directions, describing the two strands of a duplex and the reason the two ends of the molecule are not equivalent.
Lipids, Membranes and Nucleic Acids FAQ
Why does an organism grown in the cold make more unsaturated membrane lipids?
Because cooling slows molecular motion and would leave a membrane of straight, closely packed chains too rigid for transport and signalling to work. Kinked unsaturated chains pack less closely, so a membrane containing more of them stays fluid at a lower temperature. The organism is holding fluidity roughly constant by changing composition rather than letting the temperature dictate it.
Why do ions need transport proteins when much larger uncharged molecules do not?
Because the interior of the bilayer is hydrocarbon. Moving an ion through it means stripping away the shell of water that stabilises the charge and then placing that charge in a non-polar environment, which is energetically very costly regardless of how small the ion is. Size matters much less than charge, which is why the rule surprises people the first time they meet it.
What does an absorbance ratio near 1.8 at 260 over 280 nanometres indicate?
That the dominant absorber is nucleic acid rather than protein. Protein absorbs more strongly at 280 and gives a much lower ratio, so a high ratio in a protein preparation means the 280 reading contains a large contribution from something that is not protein, and any direct ultraviolet protein estimate from it will be too high.
Why must ionic strength be matched when comparing duplex stability?
Because both strands carry a negative charge at every link and therefore repel one another. Counter-ions screen that repulsion, so a duplex in dilute salt separates more readily than the same sequence in higher salt. Comparing two sequences at different ionic strengths measures the salt difference as much as the sequence difference.
Exam move
Practise the one argument that spans both halves of this chapter: more weak interactions per unit length means more energy needed to disrupt them. Apply it to chain saturation and to base composition in turn, and write down the variable that must be held constant in each case. Then sketch a bilayer with a channel and a surface protein and label which species can cross unaided.
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