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You are what you eat

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Lipids And Membranes

Contents

• Lipids.
• Smooth endoplasmic reticulum.
• Structures and functions of membranes
• Historical development of membrane models.
• How membrane systems are constructed.
• Eukaryote membranes.
• Bacterial membranes.
• Archaeal membranes.
• Transport and diffusion through membranes.
• Facilitated diffusion.
• Active transport.

Lipids

Lipids are in many ways the most diverse of the biological macromolecules, since they are something of a rag-tag bunch of leftovers. Lipids are pretty much everything in the cell that isn't very water soluble, and chemically they don't have a great deal in common with one another. The best known lipids are probably the fatty acids, so that is where we shall start.

The fatty acids are long chain carboxylic acids synthesised by the condensation and reduction of acetyl coenzyme-A units by fatty acid synthase. The more important ones have nonsystematic names in wide use. Lauric, myristic, palmitic and stearic acids are saturated (no multiple bonds), oleic and linoleic acids are monounsaturated (with 1 to 4 double bonds), and γ-linolenic and arachidonic acids are polyunsaturated. Note that all these acids (indeed, all common fatty acids) are cis (E) fatty acids. Because of the kinks in the chain caused by the double bonds, the unsaturated fatty acids tend to be liquids at room temperature (they are less easy to pack together to form a solid). Bacteria and plants (which cannot thermoregulate) will use more unsaturated acids in their cell membranes when they are exposed to cold: this helps to maintain membrane fluidity.

Molecular structure	Name
	Lauric acid: saturated C12
	Myristic acid: saturated C14
	Palmitic acid: saturated C16
	Stearic acid: saturated C18
	Oleic acid: monounsaturated C18
	Linoleic acid: diunsaturated C18
	γ-Linolenic acid: triunsaturated C18
	α-Linolenic acid: triunsaturated C18
	Arachidonic acid: tetraunsaturated C20

The nomenclature of these acids is rather complicated. There are at least five systems in use. Here are some of the above in the different systems. The delta system numbers the double bonds from the carboxyl group (the α carbon), whereas the omega system indicates where the first double bond is counting from the other end of the molecule (the ω carbon).

Trivial	Systematic	Colon	Delta	Omega
Stearic acid	Octadecanoic acid	18:0	Octadecanoic acid	-
Palmitic acid	Hexadecanoic acid	16:0	Hexadecanoic acid	-
Oleic acid	E-Octadec-9-enoic acid	18:1n9	cis-Δ9-octadecenoic acid	ω−9
Linoleic acid	9E, 12E-Octadeca-9, 12-dienoic acid	18:2n9	cis, cis-Δ9, 12-octadecadienoic acid	ω−6
γ-Linolenic acid	6E, 9E, 12E-Octadeca-6, 9, 12-trienoic acid	18:3n6	cis, cis, cis -Δ6,9,12- octadecatrienoic acid	ω−6
α-Linolenic acid	9E, 12E, 15E-Octadeca-9, 12, 15-trienoic acid	18:3n3	cis, cis, cis -Δ9,12,15- octadecatrienoic acid	ω−3

Modification of the polyunsaturated fatty acid arachidonic acid (5E, 8E, 11E, 14E-dodeca-5, 8, 11, 14-tetraenoic acid) form the prostaglandins, which are the inflammatory hormones that aspirin interferes with. The fatty acids may be combined with other compounds to form the lipids found in membranes, and as energy storage compounds. Triacylglycerols are the typical energy storage compound: they combine fatty acids with glycerol via and ester linkage. Diacyl and monoacyl compounds are also possible.

However, the most important use for lipids in the cell is in the formation of membranes. Membranes contain amphipathic molecules, i.e. ones with a hydrophobic end and a hydrophilic end.

Some of these lipids contain two fatty acids esterified to a glycerol molecule. These are glycerolipids. The fatty acids esterified to the glycerol molecule form a hydrophobic "tail" to the molecule, whilst the third position is taken up by a polar head group containing phosphate and another compound. This other compound can be choline (as below) forming phosphatidyl choline, or serine (phosphatidyl serine), glycerol, inositol, ethanolamine, hydrogen (phosphatidic acid) or even an entire other diacylglycerol unit (forming a cardiolipin).

There are also several kinds of polar head groups. The following molecules show the various headgroups esterified to a simple distearyl glycerophosphate:

Molecular structure	Name of head group
	Proton - phosphatidic acid.
	Choline - phosphatidyl choline.
	Ethanolamine - phosphatidyl ethanolamine.
	Serine - phosphatidyl serine
	Inositol - phosphatidyl inositol.
	Glycerol - phosphatidyl glycerol.

Plain phosphatidic acid is negatively charged because of the phosphate group, which is ionised (-PO₄²⁻) at physiological pH. Phosphatidylserine is also negatively charged (it has a -PO₄⁻- group, and at physiological pH, the charges on its carboxylate and amine groups cancel). Phosphatidylcholine has no net charge, because the positive charge on choline cancels the negative on phosphate (-PO₄⁻-CH₂CH₂N⁺(CH ₃)₃), and phosphatidylethanolamine is neutral for similar reasons.

Cardiolipin is phosphatidylglycerol esterified to phosphatidic acid to form a sort-of double lipid. It is abundant in the mitochondrial inner membrane, where it acts to insulate the electric gradient across the membrane.

Glycerophospholipids are not merely structural: phosphatidylinositol phosphates  are involved in many aspects of cell-signalling^[2]. PI(4,5)P₂ (phosphatidylinositol-4,5-bis-phosphate) is a target of the enzyme phospholipase C (PLC), which breaks it down to diacylglycerol (DAG) and IP₃ (inositol tris-phosphate) which are secondary messengers in cell-signalling cascades. Phosphatidylinositol-3-phosphate is also involved in marking membranes. PI(3)P is recognised by the retromer complex that carries M6P receptors back from the endosome to the Golgi.

Not all membrane lipids are formed from a backbone of glycerol however. Others use sphingosine instead, which is formed from serine and a fatty acid:

The sphingolipids are every bit as diverse as the glycerolipids. Sphingosine with a single acyl group attached via an amide linkage is called a ceramide. Ceramides can be further derivatised by the addition of groups to the terminal OH group. This head group may be any of those things that a glycerolipid can take: sphingomyelin is the sphingosine version of phosphatidyl choline, with a phosphate-choline residue connected to the terminal hydroxyl group by an ester bond.

The cerebrosides have a glucose or galactose unit connected to the terminal OH group by a glycosidic bond, and the gangliosides have an entire oligosaccharide connected here. This oligosaccharide contains at least one acid sugar derivative. These latter carbohydrate containing compounds are collectively called glycolipids, and one set of them is responsible for the ABO blood group system of erythrocyte membranes.

Sphingolipids form lipid rafts which are involved in cell signalling

Sphingomyelin is the sphingosine analogue of phosphatidylcholine. It is a very common component of eukaryote plasmalemma. Like all lipids, it is synthesised in the SER.

Fatty acids are not always covalently modified: waxes are simple mixtures of long chain fatty acids with long chain alcohols, and serve a waterproofing and protective function e.g. in the plant cuticle.

Smooth endoplasmic reticulum

The SER is formed of tubular cisternae, and is the main site of lipid synthesis in eukaryotic cells. Both the smooth and rough endoplasmic reticula are held together by the cytoskeleton, and their entire lumenal contents are contiguous with each other. The endoplasmic reticula comprise about half the mass of membranes in a typical eukaryotic cell.

The main role of the SER is the synthesis of lipids; although calcium storage (in the sarcoplasmic reticulum, a specialised ER found in muscle cells) is also important.

The synthesis of conventional phosphatidyl phospholipids occurs on the cytosolic leaflet of the SER. The synthetic enzymes responsible are membrane-bound. Acyl transferase adds two fatty acyl CoA molecules to glycerol-3-phosphate to form: phosphatidic acid. A phosphatase then removes the phosphate group to form diacylglycerol (DAG). The head group is then added to DAG from e.g. CDP-choline in the case of phosphatidylcholine.

There is much faster diffusion from one leaflet of a naturally occurring to the other leaflet than is observed in artificial membranes composed only of phospholipids. An enzyme called scramblase is responsible for this flip-flopping of lipids from one leaflet to the other. (The diagram is diagrammatic: scramblase allows lipids to equilibrate across the leaflets, not necessarily swapping lipids in a 1 for 1 fashion).

However, the cell membrane is still highly asymmetric. How can this be? The cell membrane also contains flippase, which specifically flips phosphatidylserine and phosphatidylethanolamine from lumenal to cytosolic leaflet. Hence, the inner leaflet of the plasma membrane has more of these lipids than phosphatidylcholine, which appears mostly on the outside.

Flippase discriminates between these lipids bases on the fact that phosphatidylcholine has a trimethylammonium head group, whilst phosphatidylserine and phosphatidylethanolamine have a free amine head group.

The other main sort of eukaryotic lipid, sphingolipids, are synthesised exclusively on the lumenal face of the SER. Many of these lipids are glycosylated, and are therefore antigenic (the ABO blood grouping rely partly on glycolipids of this sort).

Structures and functions of membranes

Membranes are important in many aspects of cellular structure and function. Early studies showed that membranes formed the boundary between the cell and the environment. Erythrocyte ghosts (empty red blood cells) are good examples of cellular (plasma) membranes, and are used as a model. From these studies, it has become clear that membranes are much more complex than mere boundaries between the cytoplasm an extracellular environment.

There are three main functions of membranes:

1. Compartmentation of organelles and enzymes.
2. Regulation of transport.
3. Detection and transmission of signals within and between cells.

The cell membrane has long been known to exchange of material with its environment. This is well illustrated in the processes of exocytosis and endocytosis, where solids are expelled from or taken into cells.

In 1894, Overton observed that lipophilic (hydrophobic) molecules could enter root hairs, but hydrophilic molecules could not. He concluded that the cells were coated with two lipids: lecithin (a phospholipid, also called phosphatidyl choline) and cholesterol, which accounted for the observed permeability. Modern research shows that phospholipids and cholesterol do make up a large proportion of membrane structure, but that there are also many other components.

Models of membranes

Many models have been proposed for biological membranes. This is an (approximate) timeline of these ideas.

Langmuir's monolayers (1920)

Langmuir showed that if phospholipids are dissolved in benzene they could be dispersed as a monolayer on the surface of water in a Langmuir trough.

Gortner & Grendel's bilayers (1925)

These researchers extracted the lipid from the plasma membrane of RBCs and applied them to a Langmuir trough. They covered twice the area of the original membrane showing that natural membranes are bilayers.

The composition of membranes reflects their specialisations: nerve cell membranes contain non-conductive lipid, mitochondria contain significant amounts of protein (respiratory complexes).

Source	Protein (%w/w)	Lipid (%w/w)	Carbohydrate (%w/w)
Myelin nerve sheath	18	79	3
Erythrocyte	49	43	8
Chloroplast	70	30	0
Mitochondrion (inner)	76	24	0

Davison & Danielli's sandwich (1935)

The earliest true model of membranes proposed a phospholipid bilayer covered in a globular protein coat.

Robertson's unit membrane

Under the electron micrograph, this model appears acceptable: 7 nm thick, with lipid in the middle (white) and protein on outside (black).

But freeze-fracture scanning electron micrographs showed things inconsistent with the unit membrane, such as pores and pits.

This micrograph was prepared by freezing a cell, then fracturing it with a sharp blow. The forces holding the leaflets of a membrane together are quite weak, so freeze fracture often pulls the two leaflets apart, allowing you to see the proteins that span the membrane very clearly.

Singer-Nicholson fluid mosaic (1972)

The fluid mosaic model pictures the membrane as a phospholipid bilayer with many proteins, some integral to the membrane, others attached more loosely. Note the many other components, such as cholesterol; and the attachement sites for the extracellular environment (via glycoproteins) and intracellular cytoskeleton.

Membrane construction

Many proteins participate in the structure and function of membranes. They may be broadly classified as those that are tightly bound (integral) and those that are loosely bound (peripheral).

Fatty acids and proteins are able to diffuse in the membrane, either laterally (parallel to the membrane), or transversely (flip-flopping from one leaflet of the membrane to the other). Lateral diffusion of phospholipids is far faster than transverse 'flip-flop'. Proteins can also migrate laterally. If we fuse a mouse and a human cell, the membrane proteins are thoroughly mixed within 1 hour.

Membrane fluidity is affected by several factors. Membranes are more fluid when warm, because the lipid molecules have more kinetic energy. They are also more fluid when full of kinky unsaturated fatty acids, which prevent the lipids from packing closely together.

Nerve transmission relies on ion pumping: when membranes are very fluid, this interferes with ion channels (transmembrane proteins), and prevents rapid passage of ions. This is how local anaesthetics such as lignocaine work.

Eukaryote membranes

The Eukarya have highly compartmentalised cells, and almost certainly represent the products of symbioses between bacteria-like cells some 3 500 000 years ago. The Eukarya have an anaerobic Archaea-like substratum (the Archaea or Archaebacteria are a group of 'bacteria', many of which live e.g. in boiling water, at pH 1, under conditions of extreme salinity, etc), as evidenced by their protein-bound DNA and multiple RNA polymerase types. Chloroplasts are closely related to the cyanobacteria (blue-green algae), particularly Prochloron. Mitochondria are closely related to the proteobacteria (which includes Escherichia coli), possibly descendents of a bacterium with a similar life-style to the intracellular parasite Bdellovibrio. The is some small evidence that the cytoskeleton may also be an endosymbiont, but the evidence is far less convincing.

The eukaryotic endomembrane system consists of several distinct structures:

• Plasmalemma. Surrounds the cell.
• Endoplasmic reticulum. Site of protein and lipid synthesis.
• Golgi body. Site of protein modification.
• Lysosomes and vacuoles. Site of intracellular digestion and storage.
• Peroxisomes. Site of hydrogen peroxide degradation.
• Nuclear envelope. Surrounds the nucleus.
• Endosymbionts. Vacuolar membranes surround the internal membranes of plastids and mitochondria.

There is great variation in protein and lipid content of different eukaryotic membranes. Some of this variation is genetically controlled, some is influenced by the environment. Plants acclimatised to cold conditions (hardened-off) become more resistant to cold. This is due to unsaturated membrane lipids accumulating and allowing fluidity at low temperatures, which is controlled by the van der Waals forces between the lipids.

Van der Waals forces are caused by attraction between dipoles (i.e.  molecules with positive and negative 'ends'). They are stronger if molecules are big, can be closely packed and are unbranched. There are three sorts (although A-level chemistry syllabuses often erroneously equate van der Waals forces with London dispersion forces only).

Bacteria use unsaturated lipids to make their membranes more fluid, but they also make extensive use of shorter lipids, which have lower melting points, for the same purpose.

The myelin sheath of a nerve cell contains only 18% protein. The 82% of lipid provides good electrical insulation. The inner membrane of a mitochondrion contains 76% protein. This high content of protein complexes is required for oxidative phosphorylation and to delimit a proton-gradient.

Cholesterol has conflicting effects on membrane fluidity: the steroid rings help stiffen the membrane, making it less fluid (remember that clathrin is needed to bend the plasmalemma during endocytosis?), but they also prevent the other lipids from packing tightly, so prevent membranes from freeing (cholesterol 'abolishes phase transitions').

Cholesterol is made by epoxidation of squalene ('shark oil'), which requires molecular oxygen. This means the evolution of cholesterol must post-date photosynthesis (which is probably why only eukaryotes have it).

Eukaryotes often have glycolipids in the outer leaflet of their plasmalemma. These come in several forms: cerebrosides are ceramides with a single sugar head group. Gangliosides have acidic oligosaccharide head groups and are involved insulation in neurone membranes. Some glycolipids are antigenic, e.g. ABO antigen system.

Cells need to communicate with each other, and with the extracellular matrix (ECM). This is mostly achieved by interactions between proteins in the membranes of cells. These are synthesised on the RER and glycosylated in the Golgi body.

Reversible phosphorylation of integrins (the trapezium-shaped things) regulates attachment through the membrane between fibronectin and actin.

Bacterial membranes

The Bacteria are simpler in construction than the Eukarya. Bacteria have very few internal membranes, and no double-membrane bound organelles. Their lipid and protein components are different to those of the Eukarya.

• Plasmalemma. Surrounds the cell.
• Periplasmic membrane. Surrounds the plasmalemma in Gram negative cells.
• Thylakoids. Invaginations of the plasmalemma in cyanobacteria.

The infamous Gram stain depends on the fact that Gram negative and Gram positive bacteria have very different cell architectures. Gram positive bacteria have much thicker cell walls, whereas Gram negatives have thin cell walls and an extra periplasmic membranes exterior to their plasmalemma.

DNA databases are often poor for environmental bacteria, and quick biochemical tests for bacteria (like the API kits) are biased to clinical isolates. Fatty acid methyl ester analysis (FAME) provides a lipid fingerprint for bacteria and is therefore useful for identifying some bacteria.

Archaeal membranes

Archaea (Archaebacteria) are bacteria on the surface, but eukaryotes under the hood. Many are extremophiles, living in conditions of extreme heat (100°C), acidity (pH 1.5) or salinity (1.5 M). How do their membranes cope?

Archaeal membrane lipids are very odd. They are joined by ether linkages (-O-), not ester linkages (-COO-). The 'fatty acids' are terpenoids (polyisoprenes), not real fatty acids, and they are generally diglycerides with no head groups. The glycerol in the backbone is also of the opposite chiral configuration to that in bacteria and eukaryotes.

Methanopyrus lives in black smokers (which are very hot). The ester bonds in conventional lipids would be hydrolysed, yielding free fatty acids, phosphate, choline and glycerol. The ether bonds of Methanopyrus resist hydrolysis and it has no head group to lose!

Halobacterium lives in 1.5 M NaCl. The terpenoids chains make membrane less fluid. This helps regulate salt uptake by making the membrane extremely impermeable.

Thermoplasma has no cell wall and lives at pH 2, 100°C. Its lipids are crosslinked across the membrane, to form a stiff, gel-like monolayer.

Ether linkages are less prone to hydrolysis, and membrane-spanning phytanyl groups make the membrane very rigid and impermeable. This stops salt getting in, and stops the membrane becoming too fluid at high temperatures. This means that for many Archaea, the membrane is not a fluid mosaic, but a more solid gel mosaic.

Bacteriorhodopsin is a trimeric light-dependent proton pump with a central channel, which some Archaea use to generate a proton gradient for ATP synthesis. It was one of the earliest photosynthetic apparatuses to evolve. It is well understood because it is not denatured when removed carefully from the membrane, and is therefore easy to purify for X-ray crystallography.

Transport and diffusion

Membranes are (approximately) thin layers of grease, so their cores are very hydrophobic. So, how do water-soluble things get through?

Chemicals can cross in three main ways.

• Unmediated (simple) diffusion: chemical species cross the hydrophobic core by simple diffusion down a chemical gradient.
• Facilitated diffusion: chemical species cross the hydrophobic core with help from proteins in the membrane, down a concentration gradient.
• Active transport: chemical species cross the hydrophobic core with help from proteins in the membrane, often against a concentration gradient.

The latter two types are mediated movements: they require a protein to help.

Much of the research on membrane conduction is done using patch clamping. Patch clamps are thin glass needles, which can be used to suck off a piece of membrane for study. Differences in the buffer outside the needle, and that contained within the needle can be used to study the electrical properties of the membrane, or the kinetics of transport of particular chemical species. You can also puncture the cell and manipulate the whole cell by e.g. injecting ions or ATP.

Simple unmediated diffusion is limited to low molecular weight (<150 Da), uncharged species, going down their concentration gradient, such as gases and small organic chemicals.

• O₂
• CO₂
• Alcohol
• Anaesthetics
• Pesticides

Solubility vs. diffusion graph:

Water does not fit nicely on this line. Although it is quite insoluble in oil, it diffuses much more readily across a membrane than expected. What is going on?

Facilitated diffusion

Water's diffusion across the membrane is facilitated by aquaporin pore proteins, which form hydrophilic channels through the membrane. These are size-selective for water (one of the smallest molecules that cells deal with), so other metabolites, like alcohol, do not pass through.

Facilitated diffusion differs from simple diffusion:

• It involves specific protein molecules.
• It can be specific for the molecule translocated.
• It is much more rapid and shows saturation kinetics.
• It can be regulated (gated).

Charged molecules diffuse at a negligibly small rate, and do not obey Fick's law, because they respond to electrical gradients as well as to gradients of concentration. An electrical potential exists across most plasma membranes, the potential usually being negative on the inside. e.g. negatively charged phosphatidylserine is commoner on the cytosolic leaflet. This potential is smaller in animal cells −50 mV, than in most plant cells −200 mV. Ions diffuse down their electrochemical gradient, usually through pores called ion channels.

Ion channels can be highly selective for the chemical species they let through. Sodium's diffusion across the membrane is facilitated by an ion channel. It is selective for Na⁺ by the size of the pore in the channel and the charges on amino acids inside the pore. K⁺ is too big to pass through; Cl⁻ is too negative. Li⁺ slips through though, as it is even smaller than sodium. Lithium is used as a treatment for manic depression, which is caused by sodium/potassium channel problems.

Like an enzyme, protein-mediated transport (whether active or passive) shows saturation kinetics. This diagram shows the saturation kinetics displayed by the erythrocytic GluT1 glucose transporter.

Channels may also be ligand- or electrically- gated. The acetylcholine receptor is a gated sodium/potassium channel in synaptic membranes. The change in membrane potential caused by the potassium/sodium concentrations propagates to the sarcoplasmic reticulum, where it opens voltage-gated calcium channels, which release calcium ions into the cytoplasm, causing muscle contraction.

Gating is extremely important. Muscle contraction is regulated by calcium fluxes through voltage gated channels. The opening of plant stomata is regulated by ligand (abscisin) gated channels. Channels can mediate very rapid ion movements and these can act as signals transferring information from cell to cell. This is important in nerve impulse transfer.

Pores are 'passive': they are basically holes that allow chemical species to travel through them. Gating merely (un)blocks the hole. Carrier proteins, on the other hand, change their conformation significantly when transporting species.

Ionophores are small carrier molecules (not usually proteins). Valinomycin is a cyclic peptide. It acts as an antibiotic ionophore that carries 10⁵ potassium ions per second across membranes.

Gramicidin is a short dimeric peptide. It acts as an antibiotic ionophore that carries 10⁷ potassium and sodium ions per second across membranes.

Active transport

The concentration of metabolites across e.g. an erythrocyte membrane is not in thermodynamic equilibrium. The cell expends energy (ATP) maintaining these gradients.

Ion	Cell (mM)	Blood (mM)
K+	139	4
Na+	12	145
Cl−	4	116
Ca2+	0.0002	1.8

Active transport differs from facilitated diffusion:

• It can actively pump molecules against a chemical or electrical gradient.
• It requires the expenditure of energy.
  • Light or ATP: primary active transport.
  • At the expense of another concentration gradient: secondary active transport.
• However, it is similar to facilitated diffusion in that it requires specific protein carriers.

The proteins involved in transport can be classified according to the following 'porter protein' scheme:

The calcium ATPase uniport (pump) is associated with the membrane of the sarcoplasmic reticulum. It regulates the levels of calcium in the muscle cells. High levels of calcium in the cytoplasm cause contraction and low levels cause relaxation.

The sodium/potassium (Na⁺/K⁺) ATPase exports 3 Na⁺ and imports 2 K⁺ through the plasmalemma per ATP. Both ions are moved against their electro-chemical gradients. This pump is the main source of the membrane potential in animal cells. It is usually classed as an antiport, although it might be better considered as a multifunction pump, because it is relying on primary active transport, rather than secondary: note that both species are transported against their electrochemical gradients.

The glucose/Na⁺ symport uses the energy stored in the Na⁺ gradient (produced by the Na⁺/K⁺ ATPase) to transport glucose against its concentration gradient.

The pumping of protons across a membrane by an energy-expending uniport generates a proton motive force, which can be exploited by an ATPase running 'in reverse' to generate ATP. This is how bacteriorhodopsin, oxidative phosphorylation and photosynthesis all work:

Test yourself

1. Compare and contrast sphingomyelin and phosphatidyl choline.
2. What features of phospholipids and proteins allow them to form membranes?
3. Why is the fluid mosaic the preferred model for membranes?
4. What are the major biochemical differences between the membrane lipids of the three domains?
5. What is it that oily fish, plants and other sources of 'healthy' unsaturated fat have in common?
6. Why does the erythrocyte cell membrane stimulate an immune response?
7. Complete the table below.

8. Transporter	Uni-, sym- or antiport?	Carrier or channel; any gating?	Form of energy required?
   Glucose/Na+ pump
   AChE receptor
   Fo/F1 proton ATPase
   Na+/K+ ATPase

Answers

Bibliography

• Taiz, L. and Zeiger, E. (2002). Plant Physiology. 3rd edition. Sinaur Associates Incorporated, Sunderland, Massachusetts. 283-308. "Secondary metabolites and plant defense"
• Vickery, M. L. and B., V. (1981). Secondary plant metabolism. Macmillan Press Limited, London
• Alberts, B., et al. (2002). Molecular biology of the cell. 4th edition. Garland Science, New York. 583-614. "Membrane structure"
• Alberts, B., et al. (2002). Molecular biology of the cell. 4th edition. Garland Science, New York. 616-631. "Principles of membrane transport"
• Alberts, B., et al. (2002). Molecular biology of the cell. 4th edition. Garland Science, New York. 631-656. "Ion channels and the electrical properties of membranes"
• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 326-350. "Lipids and cell membranes"
• Berg, J. M., Tymoczko, J. L. and Stryer, L. (2006). Biochemistry. 6th edition. W. H. Freeman and Company, New York. 351-380. "Membrane channels and pumps"
• Gabriel, J. L. and Chong, P. L. G. (2000). Molecular modeling of archaebacterial bipolar tetraether lipid membranes. Chemistry and Physics of Lipids 105:193-200. http://dx.doi.org/10.1016/S0009-3084(00)00126-2