Comprehensive Study Notes — Lipids Flashcards

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Energy storage

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Lipids serve as significant energy reserves, providing more than double the energy per gram compared to carbohydrates. They are stored as triglycerides in adipose tissue and mobilized when metabolic energy is needed.

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Structural components

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Certain lipids, notably phospholipids and cholesterol, are essential structural components of cell membranes. They help maintain membrane architecture and control membrane fluidity and permeability.

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Insulation

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Lipids form insulating layers of fat beneath the skin that help maintain body temperature and protect against temperature extremes. This thermal insulation reduces heat loss and contributes to homeostasis.

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Protection

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Stored lipids provide a protective cushion around vital organs, absorbing mechanical shocks and reducing injury. This physical buffering is an important mechanical function of fat depots.

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Lipid

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A lipid is a carbon-containing biological compound that is largely hydrophobic and found in all organisms. Lipids include a diverse set of molecules such as fatty acids, triglycerides, phospholipids, and steroids with roles in energy storage, membrane structure, and signaling.

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Hydrocarbon

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Hydrocarbons are molecules composed solely of carbon ($C$) and hydrogen ($H$) atoms, often forming nonpolar chains or rings. In lipids, long hydrocarbon tails determine hydrophobicity and influence packing and fluidity.

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Isoprene

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Isoprene is a small hydrocarbon unit with formula $C_5H_8$ that can polymerize or link end-to-end to form isoprenoids. Isoprenoid chains provide the backbone for many lipid types, especially in archaeal membranes.

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Functional groups

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Functional groups are specific arrangements of atoms (often containing $H$, $N$, $O$, $P$, and $S$) that confer chemical properties to organic molecules. In lipids, different functional groups (e.g., carboxyl, phosphate) determine polarity, reactivity, and biological role.

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Amino group

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An amino group contains a nitrogen atom bonded to hydrogens (commonly written as $-NH_2$) and can accept a proton, acting as a base. Amino groups are important in proteins and certain lipid-associated head groups.

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Carboxyl group

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A carboxyl group is written as $-COOH$ and can donate a proton, making it acidic. Fatty acids have a terminal carboxyl group that confers polarity to one end of the hydrocarbon chain.

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Carbonyl group

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A carbonyl group ($C=O$) is a polar functional group where a carbon is double-bonded to oxygen and can serve as a site for covalent linkage. Carbonyls are common in lipid backbones and intermediates involved in synthesis and breakdown.

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Hydroxyl group

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A hydroxyl group is $-OH$ and behaves as a weak acid with polar character that increases solubility in water. Hydroxyls occur in glycerol backbones and many lipid head groups, affecting interactions with aqueous environments.

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Phosphate group

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A phosphate group typically carries two negative charges (often written as $PO_4^{2-}$) and is highly polar. Phosphate-containing head groups make phospholipids amphipathic and enable interactions with water and ions.

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Sulfhydryl group

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A sulfhydryl group is written as $-SH$ and can form disulfide bonds ($-S-S-$) that stabilize molecular structures. In lipid-associated proteins, sulfhydryl chemistry can affect folding and membrane association.

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Fatty acid

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A fatty acid is a hydrocarbon chain terminating in a carboxyl group ($-COOH$), typically containing 14–20 carbons. Fatty acids can be saturated or unsaturated and serve as energy sources and building blocks for complex lipids.

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Saturated fatty acid

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A saturated fatty acid has only single bonds between carbon atoms in its hydrocarbon chain and thus the maximum number of hydrogens. These straight chains pack tightly, favoring dense, less fluid lipid phases and are common in animal fats.

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Unsaturated fatty acid

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An unsaturated fatty acid contains one or more carbon–carbon double bonds ($C=C$) that remove hydrogen atoms and introduce kinks in the chain. Those kinks prevent tight packing, increasing membrane fluidity and lowering melting point.

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Monounsaturated

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A monounsaturated fatty acid contains exactly one double bond in its hydrocarbon chain. These fats (e.g., oleic acid in olive oil) increase membrane fluidity relative to saturated fats but less than polyunsaturated fats.

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Polyunsaturated

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A polyunsaturated fatty acid contains two or more $C=C$ double bonds in its hydrocarbon chain. Multiple double bonds create multiple kinks, markedly increasing fluidity and including essential fatty acids like omega fatty acids.

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Omega notation

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Omega notation labels the position of the terminal double bond counting from the methyl end of the fatty acid; for example $\omega$-3 indicates a double bond three carbons from that end. This convention is important for classifying essential fatty acids and their biological roles.

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Hydrophobic interactions

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Hydrophobic interactions arise when nonpolar molecules or groups cluster together in water to minimize disruption of the hydrogen-bonding network of water. These interactions are not covalent bonds but are critical for lipid tail aggregation and membrane formation.

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Van der Waals forces

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Van der Waals forces are weak intermolecular attractions caused by temporary or permanent uneven electron distributions between neighboring molecules. In lipids, these forces stabilize packing between long hydrocarbon tails and influence membrane rigidity.

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Lipid fluidity

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Fluidity describes how easily lipid fatty acid chains move and slide past one another within a membrane or lipid phase. Fluidity influences membrane permeability, protein mobility, and cellular processes and depends on tail length, saturation, cholesterol content, and temperature.

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Chain length (fluidity)

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Chain length affects fluidity because shorter hydrocarbon tails have weaker van der Waals attractions and therefore increase fluidity. Conversely, longer tails strengthen hydrophobic interactions, promoting tighter packing and reduced fluidity.

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Saturation (fluidity)

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The degree of saturation influences packing: double bonds introduce kinks that prevent close packing and thus increase fluidity. Saturated tails are straight and pack tightly, decreasing fluidity and making membranes denser.

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Temperature (fluidity)

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Temperature modulates membrane fluidity by changing kinetic energy; higher temperatures increase molecular motion and fluidity, while lower temperatures slow movement and promote tight packing. Temperature shifts can therefore alter membrane permeability and function.

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Types of lipids

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Major lipid categories include fatty acids, triglycerides, phospholipids, steroids, glycolipids, waxes, sphingolipids, and lipoproteins. Each type has distinct structural features and functions ranging from energy storage to membrane structure and signaling.

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Triglyceride

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A triglyceride (fat) consists of glycerol linked by ester bonds to three fatty acids ($3$ fatty acids + glycerol). Triglycerides are formed by condensation (dehydration) reactions and primarily function in long-term energy storage and insulation.

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Ester bond

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An ester bond is a covalent linkage formed when a carboxyl group ($-COOH$) reacts with a hydroxyl group ($-OH$) releasing water. In triglycerides, ester bonds connect each fatty acid to the glycerol backbone.

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Condensation reaction

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A condensation (dehydration) reaction joins two molecules by removing a water molecule, forming a new covalent bond. Lipid synthesis often uses condensation chemistry to assemble esters and other linkages.

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Steroid

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Steroids are lipids with a characteristic core of four fused carbon rings, and they differ by functional groups attached to the rings. Examples include cholesterol and steroid hormones like estrogen and testosterone, which function in membrane structure and signaling.

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Phospholipid

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A phospholipid has a glycerol backbone linked to two hydrocarbon tails and a phosphate-containing head group, making it amphipathic. Phospholipids are the primary components of cell membranes and self-assemble into bilayers in water.

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Amphipathic

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An amphipathic molecule contains both hydrophilic (polar) and hydrophobic (nonpolar) regions. Phospholipids are amphipathic, enabling the spontaneous formation of micelles and bilayers when exposed to water.

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Micelle

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A micelle is a spherical aggregate of amphipathic molecules with hydrophobic tails inward and hydrophilic heads outward interacting with water. Micelles form from single-tailed amphipathic lipids and are common for detergents and fatty acid assemblies.

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Phospholipid bilayer

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A phospholipid bilayer is formed when two sheets of phospholipids align tail-to-tail so hydrophilic heads face the aqueous environments on either side. Bilayers form spontaneously, create a semipermeable membrane, and serve as the structural basis of cell plasma membranes.

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Plasma membrane

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The plasma membrane separates the cell interior from the external environment, providing fluidity, flexibility, and selective transport. It contains proteins for transport and signaling and acts as a semipermeable barrier controlling molecular traffic.

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Selective permeability

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Selective permeability means that a membrane allows some substances to cross more easily than others; small nonpolar molecules cross rapidly, while large polar or charged molecules cross slowly or not at all. This property underpins cellular control of solute distribution and homeostasis.

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Membrane permeability factors

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Membrane permeability is influenced by hydrocarbon tail length, degree of saturation, cholesterol content, and temperature. These factors alter packing density and fluidity, thereby modulating how readily molecules traverse the bilayer.

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Cholesterol

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Cholesterol is a sterol lipid present in animal membranes that fills gaps between phospholipids, increasing hydrophobic density and reducing membrane permeability. It stabilizes membranes by restricting phospholipid movement and buffering fluidity across temperatures.

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Artificial membranes

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Artificial membranes like liposomes and planar bilayers are experimental models used to study membrane properties and permeability. Liposomes are spherical vesicles enclosing an aqueous compartment, while planar bilayers are flat membranes stretched across an opening for quantitative assays.

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Liposome

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A liposome is a spherical vesicle formed when phospholipids self-assemble into a bilayer around an aqueous core. Liposomes mimic cell membranes, are used experimentally to study transport, and serve as drug-delivery vehicles.

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Planar bilayer

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A planar bilayer is an artificial flat phospholipid membrane constructed across a small aperture separating two aqueous compartments. It provides a two-dimensional model to measure membrane permeability and protein-mediated transport under controlled conditions.

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Permeability (definition)

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Permeability is a property describing how readily a substance crosses a membrane or barrier, typically quantified as rate of movement across. Experimentally, permeability is measured by whether a substance appears on the other side of an artificial membrane and how fast it crosses.

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Liposomal nanoparticles

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Liposomal nanoparticles (LNPs) are lipid-based vesicles engineered to encapsulate and deliver therapeutic agents to target tissues. LNPs can accumulate in damaged vasculature such as tumors, release payloads by fusion or leakage, and are widely used for drug delivery applications.

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Membrane repairability

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Membrane repairability refers to the ability of lipids to reorganize and reseal after damage because they are not covalently bonded to each other. This property allows membranes to restore continuity and maintain cellular integrity after minor disruptions.

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Membrane expandability

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Membrane expandability denotes how cells can increase surface area by incorporating additional lipids into the plasma membrane. Because lipids assemble noncovalently, adding more molecules enables growth and changes in cell shape without complex synthesis constraints.