Name: lipids, fats, and cell membranes
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 3 min 38 s
Channel: JoVE

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From Molecules to Organisms: Structures and Processes

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Structure of Lipids

I lipidi sono un gruppo di molecole idrofobiche che includono trigliceridi, usati per immagazzinare energia, e fosfolipidi, che sono i principali componenti strutturali delle membrane cellulari. I trigliceridi e la maggior parte dei fosfolipidi sono costituiti da glicerolo e catene di acidi grassi. Gli acidi grassi possiedono gruppi metilici ad un'estremità della catena, e gruppi carbossilici all'estremità opposta.

Il carbonio collegato al gruppo carbossilico è detto carbonio alfa e il carbonio del gruppo metilico è noto come carbonio omega. Gli acidi grassi variano a seconda della lunghezza e della presenza di doppi legami nella catena degli idrocarburi. Quelli con doppi legami sono acidi grassi insaturi, e quelli con solo legami singoli sono acidi grassi saturi perché contengono il maggior numero possibile di atomi di idrogeno.

Gli acidi grassi hanno spesso nomi comuni, ma possono essere sistematicamente denominati in base al numero di atomi di carbonio e al numero e alla posizione dei doppi legami nella catena del carbonio. Esistono diversi metodi di numerazione comuni. Il sistema di riferimento carbossilico conta la posizione di tutti i doppi legami a partire dal carbonio carbossilico, che è numerato come 1.

Il sistema di riferimento omega conta la posizione del doppio legame più vicino al carbonio omega, con il carbonio omega numerato come 1. Per esempio, l'acido linolenico, un acido grasso omega 3, ha 18 atomi di carbonio e doppi legami nelle posizioni 9, 12 e 15 dal conteggio dall'estremità carbossilica. Questo sarà chiamato 18:3 Δ9, 12, 15 secondo il sistema di riferimento carbossilico e 18:3 secondo il sistema di riferimento omega.

Gli acidi grassi insaturi possono presentarsi in due configurazioni:cis e trans. Nella configurazione cis, gli idrogeni sugli atomi di carbonio coinvolti nel doppio legame sono sullo stesso lato del legame, mentre nella configurazione trans, gli idrogeni si trovano sui lati opposti. Strutturalmente, gli acidi grassi cis hanno catene piegate, mentre gli acidi grassi trans hanno catene diritte.

È stato scoperto che il consumo di grassi trans è responsabile di varie malattie cardiovascolari, mentre il consumo di acidi grassi cis, come omega-3 e omega-6, è noto per essere benefico per la salute. I trigliceridi, comunemente noti come grassi, sono principalmente usati per l'immagazzinamento di energia. Sono composti da tre acidi grassi legati al glicerolo tramite legami esteri fra le estremità idrossiliche del glicerolo e le estremità carbossiliche degli acidi grassi con conseguente formazione di una molecola non polare.

Se tutti e tre gli acidi grassi sono dello stesso tipo, sono noti come trigliceridi semplici. Se i tre acidi grassi differiscono, sono detti trigliceridi misti. Un altro tipo di lipidi, i fosfolipidi, rappresenta un'importante caratteristica strutturale delle membrane biologiche.

Essi hanno teste idrofile, costituite da gruppi fosfato modificati con un gruppo alcolico idrofilo, e code di acidi grassi idrofobici. La composizione di queste teste e code può variare, risultando in diversi tipi di lipidi di membrana. I glicerofosfolipidi sono un tipo comune di fosfolipidi costituito da due acidi grassi e un gruppo altamente polare, il quale è attaccato a ciascun carbonio di glicerolo tramite legami esteri e fosfodiesteri, rispettivamente.

struttura dei lipidi

I lipidi comprendono un gruppo eterogeneo di composti che sono in gran parte di natura non polare. Questo perché sono idrocarburi che includono per lo più legami carbonio-carbonio o carbonio-idrogeno non polari. Le molecole non polari sono idrofobiche ("temono l'acqua") o insolubili in acqua. I lipidi svolgono molte funzioni diverse in una cellula. Le cellule immagazzinano energia per un uso a lungo termine sotto forma di grassi. I lipidi forniscono anche isolamento dall’ambiente per piante e animali. Ad esempio, aiutano a mantenere asciutti gli uccelli acquatici e i mammiferi quando formano uno strato protettivo sulla pelliccia o sulle piume a causa della loro natura idrorepellente e idrofobica. I lipidi sono anche gli elementi costitutivi di molti ormoni e sono un costituente importante di tutte le membrane cellulari. I lipidi comprendono grassi, oli, cere, fosfolipidi e steroidi.

Struttura generale dei lipidi

Una molecola di grasso è costituita da due componenti principali- glicerolo e acidi grassi. Il glicerolo è un composto organico (alcol) con tre atomi di carbonio, cinque idrogeni e tre gruppi ossidrile (OH). Gli acidi grassi hanno una lunga catena di idrocarburi a cui è attaccato un gruppo carbossilico, da qui il nome “acido grasso”. Il numero di atomi di carbonio nell'acido grasso può variare da 4 a 36. I più comuni sono quelli contenenti 12-18 atomi di carbonio. In una molecola di grasso, gli acidi grassi si attaccano a ciascuno dei tre atomi di carbonio della molecola di glicerolo con un legame estere attraverso un atomo di ossigeno. Unendo tre acidi grassi a uno scheletro di glicerolo in una reazione di disidratazione si forma il triacilglicerolo. I tre acidi grassi nel triacilglicerolo possono essere simili o dissimili.

Un fosfolipide è un altro tipo comune di lipide. È una molecola anfipatica, ovvero ha una parte idrofoba e una idrofila. Le catene degli acidi grassi sono idrofobiche e non possono interagire con l'acqua; mentre il gruppo contenente fosfato è idrofilo e interagisce con l'acqua. Le teste idrofile dei fosfolipidi sono rivolte verso la soluzione acquosa. Le code idrofobe sono sequestrate al centro del doppio strato.

Acidi grassi

Gli acidi grassi possono essere saturi o insaturi. In una catena di acidi grassi, se sono presenti solo legami singoli tra gli atomi di carbonio vicini nella catena idrocarburica, l'acido grasso è saturo. L'acido stearico è un esempio di acido grasso saturo.

Quando la catena idrocarburica contiene un doppio legame, l'acido grasso è insaturo. L'acido oleico è un esempio di acido grasso insaturo. La maggior parte dei grassi insaturi sono liquidi a temperatura ambiente e sono chiamati oli. Se è presente un doppio legame nella molecola, allora si tratta di un grasso monoinsaturo (ad esempio, l'olio d'oliva), mentre se è presente più di un doppio legame, allora è un grasso polinsaturo (ad esempio, l'olio di canola). Gli acidi grassi lunghi e lineari con legami singoli generalmente si compattano strettamente e sono solidi a temperatura ambiente. I grassi animali con acido stearico e acido palmitico (comuni nella carne) e il grasso con acido butirrico (comune nel burro) sono esempi di grassi saturi.

Gli acidi grassi possono inoltre essere classificati in Cis e trans. Cis e trans indicano la configurazione della molecola attorno al doppio legame. Se gli idrogeni sono presenti sullo stesso piano è un grasso cis. Se gli atomi di idrogeno si trovano su due piani diversi, si tratta di un grasso trans. Il doppio legame cis provoca una piega o una “piega” che impedisce agli acidi grassi di compattarsi saldamente, mantenendoli liquidi a temperatura ambiente. L'olio d'oliva, l'olio di mais, l'olio di canola e l'olio di fegato di merluzzo sono esempi di grassi insaturi. I grassi insaturi aiutano ad abbassare i livelli di colesterolo nel sangue; mentre i grassi saturi contribuiscono alla formazione di placche nelle arterie.

Grassi trans

L’industria alimentare idrogena artificialmente gli oli per renderli semisolidi e di consistenza desiderabile per molti prodotti alimentari trasformati. Durante questo processo, i doppi legami della conformazione cis– nella catena idrocarburica possono convertirsi in doppi legami nella conformazione trans–.

La margarina, alcuni tipi di burro di arachidi e il burro sono esempi di grassi trans idrogenati artificialmente. Studi recenti hanno dimostrato che un aumento dei grassi trans nella dieta umana può portare a livelli più elevati di lipoproteine a bassa densità (LDL), o colesterolo “cattivo”, che a sua volta può portare alla deposizione di placche nelle arterie, con conseguenti malattie cardiache.

Acidi grassi Omega

Gli acidi grassi essenziali sono quelli che il corpo umano richiede ma non sintetizza. Di conseguenza, devono essere integrati tramite l'ingestione tramite la dieta. Gli acidi grassi Omega-3 rientrano in questa categoria e sono uno degli unici due conosciuti per l'uomo (l'altro è l'acido grasso omega-6). Questi sono acidi grassi polinsaturi e sono omega-3 perché un doppio legame collega il terzo carbonio dall’estremità della catena idrocarburica al carbonio vicino.

L'acido alfa-linolenico è un esempio di acido grasso omega-3. Ha tre doppi legami cis e, di conseguenza, una forma curva. Salmone, trota e tonno sono buone fonti di acidi grassi omega-3. La ricerca indica che gli acidi grassi omega-3 riducono il rischio di morte improvvisa dovuta ad attacchi cardiaci, abbassano i trigliceridi nel sangue, diminuiscono la pressione sanguigna e prevengono la trombosi inibendo la coagulazione del sangue. Riducono anche l’infiammazione e possono aiutare a ridurre il rischio di alcuni tumori negli animali.

Questo testo è adattato da <a href="https://openstax.org/books/biology-2e/pages/3-3-lipids">Openstax, Biology 2e, Chapter 3.3: Lipids</a>.

Struttura dei lipidi

I lipidi comprendono un gruppo eterogeneo di composti che sono in gran parte di natura non polare. Questo perché sono idrocarburi che includono per lo più ...

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lipids, fats, and cell membranes

Lipids are nonpolar molecules that do not mix well with water. They are hydrophobic, which means “water fearing,” because they contain mostly nonpolar carbon-carbon and carbon-hydrogen bonds. In cells, lipids store long-term energy as fats, help insulate plants and animals, and form protective water-repellent layers on feathers and fur. They are also part of cell membranes and serve as building blocks for many hormones. Common lipids include fats, oils, waxes, phospholipids, and steroids.

A fat molecule is built from glycerol and fatty acids. Glycerol is an alcohol with three carbons, five hydrogens, and three hydroxyl groups. Fatty acids have a long hydrocarbon chain with a carboxyl group attached. Their chains can range from 4 to 36 carbons, but 12 to 18 carbons is most common. In a fat, three fatty acids join to one glycerol backbone through ester bonds in a dehydration reaction. This forms a triacylglycerol. The three fatty acids may be the same or different.

Phospholipids are another major lipid type. They are amphipathic, which means they have both a hydrophobic part and a hydrophilic part. The fatty acid tails repel water, while the phosphate-containing head attracts water. In water, the heads face outward toward the solution and the tails gather in the middle of the bilayer.

Fatty acids can be saturated or unsaturated. Saturated fatty acids have only single bonds between carbon atoms. Stearic acid is one example. Unsaturated fatty acids contain one or more double bonds. Oleic acid is an example, and most unsaturated fats are liquid at room temperature and are called oils. Monounsaturated fats, such as olive oil, have one double bond. Polyunsaturated fats, such as canola oil, have more than one. Straight saturated fatty acids pack tightly, so they are usually solid at room temperature. Animal fats with stearic acid and palmitic acid, as well as butter with butyric acid, are examples.

Unsaturated fatty acids can also be cis or trans. These terms describe the arrangement around a double bond. In cis fats, the hydrogens are on the same plane. In trans fats, they are on different planes. The cis double bond creates a kink that prevents tight packing, which helps keep the fat liquid. Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats tend to lower blood cholesterol levels, while saturated fats can contribute to plaque formation in arteries.

Trans fats are often made by hydrogenating oils to make them more semi-solid for processed foods. During this process, some cis double bonds can change to trans double bonds. Margarine, some peanut butter, and shortening are examples of artificially hydrogenated trans fats. Studies show that more trans fats in the diet can raise low-density lipoproteins, or LDL, the “bad” cholesterol. This can increase plaque buildup in arteries and raise the risk of heart disease.

Omega-3 fatty acids are essential fatty acids, which means the body needs them but does not make them. They must come from the diet. Omega-3s are one of only two known essential fatty acid groups for humans, along with omega-6 fatty acids. They are polyunsaturated, and the name omega-3 means the third carbon from the end is part of a double bond. Alpha-linolenic acid is an example. It has three cis double bonds and a curved shape. Salmon, trout, and tuna are good sources. Research suggests omega-3 fatty acids can reduce the risk of sudden death from heart attacks, lower blood triglycerides, decrease blood pressure, prevent blood clotting, and reduce inflammation. They may also help lower the risk of some cancers in animals.

Lipids, Fats, and Cell Membranes

functional groups

Functional Groups in Organic Chemistry

Functional groups in organic chemistry are groups of atoms that give molecules specific properties. When these groups attach to a carbon skeleton, they can change how the molecule behaves. Some functional groups make a molecule hydrophilic, or water-attracting. Others make it hydrophobic, or water-repelling. Functional groups are a key part of organic chemistry and are also found in major biological molecules. They appear in carbohydrates, proteins, lipids, and nucleic acids. Each functional...

structure of lipids

Lipids are nonpolar molecules that do not mix well with water. They are hydrophobic, which means “water fearing,” because they contain mostly nonpolar carbon-carbon and carbon-hydrogen bonds. In cells, lipids store long-term energy as fats, help insulate plants and animals, and form protective water-repellent layers on feathers and fur. They are also part of cell membranes and serve as building blocks for many hormones. Common lipids include fats, oils, waxes, phospholipids, and steroids. A...

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Explore how lipids store energy, build cell membranes, and differ as saturated, unsaturated, trans, and omega-3 fats in high school biology.

Lipids, Fats, and Cell Membranes | JoVE K12

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Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins, which carry out the essential functions of life through systems of specialized cells. [Assessment Boundary: Assessment does not include identification of specific cell or tissue types, whole body systems, specific protein structures and functions, or the biochemistry of protein synthesis.]

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Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. [Clarification Statement: Emphasis is on functions at the organism system level such as nutrient uptake, water delivery, and organism movement in response to neural stimuli. An example of an interacting system could be an artery depending on the proper function of elastic tissue and smooth muscle to regulate and deliver the proper amount of blood within the circulatory system.] [Assessment Boundary: Assessment does not include interactions and functions at the molecular or chemical reaction level.]

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Lipids, Fats, and Cell Membranes

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