<P> A biological membrane is a form of lamellar phase lipid bilayer . The formation of lipid bilayers is an energetically preferred process when the glycerophospholipids described above are in an aqueous environment . This is known as the hydrophobic effect . In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a vesicle; depending on the concentration of the lipid, this biophysical interaction may result in the formation of micelles, liposomes, or lipid bilayers . Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behavior . Phase behavior is an area of study within biophysics and is the subject of current academic research . Micelles and bilayers form in the polar medium by a process known as the hydrophobic effect . When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form hydrogen bonds to the lipophilic areas of the amphiphile . So in an aqueous environment, the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule . </P> <P> The formation of lipids into protocell membranes represents a key step in models of abiogenesis, the origin of life . </P> <P> Triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants . They are a major source of energy because carbohydrates are fully reduced structures . In comparison to glycogen which would contribute only half of the energy per its pure mass, carbohydrate carbons are all bounded to hydrogens unlike in carbohydrates . The adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone - sensitive enzyme lipase . The complete oxidation of fatty acids provides high caloric content, about 38 kJ / g (9 kcal / g), compared with 17 kJ / g (4 kcal / g) for the breakdown of carbohydrates and proteins . Migratory birds that must fly long distances without eating use stored energy of triglycerides to fuel their flights . </P> <P> In recent years, evidence has emerged showing that lipid signaling is a vital part of the cell signaling . Lipid signaling may occur via activation of G protein - coupled or nuclear receptors, and members of several different lipid categories have been identified as signaling molecules and cellular messengers . These include sphingosine - 1 - phosphate, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization, cell growth, and apoptosis; diacylglycerol (DAG) and the phosphatidylinositol phosphates (PIPs), involved in calcium - mediated activation of protein kinase C; the prostaglandins, which are one type of fatty - acid derived eicosanoid involved in inflammation and immunity; the steroid hormones such as estrogen, testosterone and cortisol, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the oxysterols such as 25 - hydroxy - cholesterol that are liver X receptor agonists . Phosphatidylserine lipids are known to be involved in signaling for the phagocytosis of apoptotic cells or pieces of cells . They accomplish this by being exposed to the extracellular face of the cell membrane after the inactivation of flippases which place them exclusively on the cytosolic side and the activation of scramblases, which scramble the orientation of the phospholipids . After this occurs, other cells recognize the phosphatidylserines and phagocytosize the cells or cell fragments exposing them . </P>

Where does the energy in a lipid come from
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