Unit 2: Lipids - Subjective Questions

Fatty acids are organic acids containing a hydrocarbon chain and a terminal carboxyl group. Their general structure is represented as .

• The carboxyl group is polar and acidic.
• The hydrocarbon chain is nonpolar and hydrophobic.
• Fatty acids may be saturated, containing no carbon-carbon double bonds, or unsaturated, containing one or more double bonds.
• Increasing chain length generally increases melting point and hydrophobicity.
• Double bonds, especially in the cis configuration, introduce bends into the chain and lower the melting point.
• Therefore, short-chain and unsaturated fatty acids are usually more fluid than long-chain and saturated fatty acids.

Saturated fatty acids:

• Contain no carbon-carbon double bonds.
• Their chains are relatively straight and pack closely.
• They are commonly solid at room temperature.
• Example: palmitic acid.

Monounsaturated fatty acids:

• Contain one carbon-carbon double bond.
• Example: oleic acid.
• The double bond usually produces a bend that increases membrane fluidity.

Polyunsaturated fatty acids:

• Contain two or more double bonds.
• Examples include linoleic acid and alpha-linolenic acid.
• They contribute greatly to membrane fluidity and serve as precursors of signaling molecules.

The degree of unsaturation affects membrane properties, storage-fat consistency, susceptibility to oxidation, and the formation of eicosanoids.

Essential fatty acids are fatty acids that the human body cannot synthesize in sufficient amounts and must obtain from the diet. The major essential fatty acids are linoleic acid and alpha-linolenic acid.

• Linoleic acid belongs to the omega-6 family.
• Alpha-linolenic acid belongs to the omega-3 family.
• They are needed for membrane structure, growth, skin integrity, and synthesis of eicosanoids.
• They can be converted into longer-chain polyunsaturated fatty acids, although this conversion is limited.
• Dietary sources include vegetable oils, nuts, seeds, legumes, and fatty fish.

Deficiency may cause dry and scaly skin, impaired growth, poor wound healing, hair loss, reduced immunity, and reproductive abnormalities.

The omega classification identifies the position of the first double bond when counting from the methyl end of a fatty acid.

• In an omega-3 fatty acid, the first double bond is located at the third carbon from the methyl end.
• In an omega-6 fatty acid, the first double bond is located at the sixth carbon from the methyl end.

Omega-3 examples: alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid.

Omega-6 examples: linoleic acid and arachidonic acid.

Both families are important for membrane structure and cell signaling. They produce different eicosanoid patterns and must be obtained in balanced amounts because excessive intake of one family may influence the metabolism and actions of the other.

Triacylglycerols, also called triglycerides, are esters of glycerol and three fatty acids. Each fatty acid is joined to a hydroxyl group of glycerol by an ester bond.

Structure:

• One glycerol molecule forms the backbone.
• Three fatty acid chains are esterified to glycerol.
• The fatty acids may be identical or different.

Properties:

• They are nonpolar and hydrophobic.
• They are insoluble in water.
• They are stored as lipid droplets in adipose tissue.
• They provide more energy per gram than carbohydrates or proteins.

Functions:

• Long-term energy storage.
• Thermal insulation.
• Mechanical protection of organs.
• Source of metabolic water and fatty acids during fasting.

They are hydrolyzed by lipases to release glycerol and fatty acids.

A triacylglycerol is formed by an esterification reaction between glycerol and three fatty acids.

• Each hydroxyl group of glycerol reacts with the carboxyl group of a fatty acid.
• One ester bond is formed during each reaction.
• Three molecules of water are released when one glycerol molecule combines with three fatty acids.

The overall reaction may be represented as:

During hydrolysis, the ester bonds are cleaved by water. Enzymes called lipases catalyze this process, producing glycerol, monoacylglycerols, and free fatty acids. Complete hydrolysis releases one glycerol molecule and three fatty acid molecules. This reaction is important during digestion and mobilization of stored fat.

Phospholipids are amphipathic lipids containing both hydrophilic and hydrophobic regions. A common glycerophospholipid consists of:

• A glycerol backbone.
• Two fatty acid chains attached by ester bonds.
• A phosphate group attached to the third carbon.
• An additional polar alcohol or nitrogen-containing head group.

The fatty acid chains form the hydrophobic region, while the phosphate-containing head is hydrophilic. In an aqueous environment, phospholipids spontaneously arrange into a bilayer with the hydrophobic tails facing inward and the polar heads facing outward.

Their functions include:

• Forming the basic structure of cell membranes.
• Providing a selective permeability barrier.
• Supporting membrane proteins.
• Participating in cell signaling.
• Acting as reservoirs for fatty acids used to synthesize eicosanoids.

Phospholipids are suited for membrane formation because they possess both hydrophilic and hydrophobic regions. Triacylglycerols are better energy-storage molecules because they are highly reduced, compact, and stored without associated water.

Sphingolipids are lipids built on a sphingosine backbone rather than a glycerol backbone. Their basic structural unit is ceramide, which consists of sphingosine linked to a fatty acid by an amide bond.

Major features include:

• A long-chain amino alcohol called sphingosine.
• A fatty acid attached through an amide linkage.
• An optional polar head group attached to the terminal hydroxyl group.

Examples include sphingomyelin, cerebrosides, and gangliosides.

Functions of sphingolipids include:

• Forming part of cell membranes.
• Contributing to the myelin sheath of nerve fibers.
• Participating in cell recognition and adhesion.
• Acting in signal transduction and programmed cell death.
• Forming specialized membrane regions called lipid rafts.

Abnormal sphingolipid metabolism can result in lysosomal storage diseases.

Sphingomyelin:

• Contains ceramide, phosphate, and a polar group such as choline.
• It is a phosphosphingolipid.
• It is abundant in nerve-cell membranes and myelin.

Cerebrosides:

• Contain ceramide and one simple sugar, usually glucose or galactose.
• They do not contain phosphate.
• They are important components of neuronal membranes.

Gangliosides:

• Contain ceramide and a complex oligosaccharide chain that includes sialic acid.
• They are concentrated in the outer surface of nerve-cell membranes.
• They participate in cell recognition, receptor activity, and signal transmission.

Thus, the main differences involve the type of polar head group and the complexity of the carbohydrate portion.

Glycolipids are lipids that contain one or more carbohydrate groups. In animal cells, most glycolipids are based on ceramide and are therefore classified as glycosphingolipids.

Classification:

• Cerebrosides contain one sugar residue.
• Globosides contain several neutral sugar residues.
• Gangliosides contain oligosaccharides with one or more sialic acid residues.

The lipid portion anchors the molecule in the outer layer of the plasma membrane, while the carbohydrate portion projects into the extracellular environment.

Functions include:

• Cell-cell recognition.
• Tissue and organ development.
• Cell adhesion.
• Immune-system interactions.
• Acting as receptors for certain hormones, toxins, viruses, and bacteria.

Changes in glycolipid composition may affect nervous-system function and disease development.

Cholesterol is a sterol containing a four-ring steroid nucleus, a hydroxyl group, a hydrocarbon side chain, and a small polar region.

Properties:

• It is amphipathic because of its hydroxyl group and hydrophobic ring and side-chain regions.
• It is poorly soluble in water.
• It is transported in the blood within lipoproteins.

Functions:

• Modulates membrane fluidity and permeability.
• Prevents membranes from becoming excessively rigid at low temperatures or excessively fluid at high temperatures.
• Serves as a precursor for bile acids and bile salts.
• Serves as a precursor for steroid hormones.
• Serves as a precursor for vitamin D.
• Contributes to the structure of myelin and other membranes.

Cholesterol is synthesized in the body and is also obtained from animal-derived foods.

Cholesterol is inserted between phospholipid molecules in biological membranes. Its effects depend on temperature and membrane composition.

• At high temperatures, cholesterol restrains phospholipid movement and decreases membrane fluidity.
• At low temperatures, it prevents phospholipid chains from packing too closely and helps maintain fluidity.
• Its hydrophobic rings interact with fatty acid chains, while its hydroxyl group is positioned near the polar head groups.
• Cholesterol reduces the permeability of the membrane to small water-soluble molecules and ions.
• It also helps organize membrane microdomains called lipid rafts, which contain selected proteins and signaling molecules.

Thus, cholesterol acts as a membrane fluidity buffer and contributes to membrane stability.

Eicosanoids are biologically active, oxygenated derivatives of twenty-carbon polyunsaturated fatty acids, especially arachidonic acid.

Synthesis:

• A stimulus activates phospholipase A2.
• Arachidonic acid is released from membrane phospholipids.
• It is metabolized through cyclooxygenase, lipoxygenase, or related pathways.

Major types:

• Prostaglandins.
• Thromboxanes.
• Leukotrienes.
• Lipoxins.

Physiological actions:

• Regulation of inflammation and pain.
• Control of blood-vessel diameter.
• Regulation of platelet aggregation.
• Contraction of bronchial and intestinal smooth muscle.
• Regulation of uterine contraction.
• Participation in fever and immune responses.

Eicosanoids act locally, are produced on demand, and usually have short half-lives.

Prostaglandins:

• Formed mainly through the cyclooxygenase pathway.
• Regulate inflammation, pain, fever, smooth-muscle contraction, and blood flow.
• Some protect the stomach lining and support kidney function.

Thromboxanes:

• Also formed through the cyclooxygenase pathway.
• Produced especially by platelets.
• Promote platelet aggregation and vasoconstriction.

Leukotrienes:

• Formed mainly through the lipoxygenase pathway.
• Important mediators of inflammation.
• Cause bronchoconstriction and contribute to asthma and allergic responses.
• Promote leukocyte recruitment and vascular permeability.

These substances are locally acting eicosanoids derived from membrane fatty acids, but they differ in biosynthetic enzymes, sites of production, and physiological effects.

Membrane phospholipids contain polyunsaturated fatty acids, particularly arachidonic acid, at the second carbon of glycerol. When a cell is stimulated by injury, hormones, cytokines, or other signals, phospholipase A2 hydrolyzes the relevant ester bond and releases arachidonic acid.

The released arachidonic acid can then enter several pathways:

• The cyclooxygenase pathway produces prostaglandins and thromboxanes.
• The lipoxygenase pathway produces leukotrienes and lipoxins.
• Other enzymes produce additional signaling derivatives.

This process allows cells to produce eicosanoids rapidly at the site of stimulation. Because eicosanoids are not usually stored in large quantities, their synthesis is tightly controlled and their actions are generally local and short-lived.

Lipids perform several essential biological functions:

• Energy storage: Triacylglycerols store concentrated chemical energy in adipose tissue.
• Membrane structure: Phospholipids, cholesterol, and glycolipids form and stabilize biological membranes.
• Insulation: Stored fat reduces heat loss from the body.
• Protection: Adipose tissue cushions internal organs against mechanical injury.
• Signaling: Eicosanoids and other lipid-derived molecules regulate inflammation, blood flow, and immunity.
• Hormone production: Cholesterol is a precursor of steroid hormones.
• Vitamin absorption: Dietary lipids support absorption of fat-soluble vitamins A, D, E, and K.
• Bile-acid formation: Cholesterol is converted into bile acids that aid lipid digestion.
• Nerve function: Sphingolipids and cholesterol contribute to myelin formation and neuronal signaling.
• Cell recognition: Glycolipids help cells identify and interact with one another.

Lipids are efficient energy-storage molecules for several reasons:

• Fatty acids contain many reduced carbon-hydrogen bonds, which release substantial energy during oxidation.
• Lipids provide approximately twice as much energy per gram as carbohydrates and proteins.
• Triacylglycerols are stored in nearly anhydrous form, whereas glycogen is stored with a considerable amount of associated water.
• Lipids are stored compactly in specialized adipocytes.
• Their hydrophobic nature allows them to accumulate without significantly affecting cellular osmotic pressure.

Carbohydrates are more suitable for rapid energy release because they can be mobilized quickly and metabolized under anaerobic conditions. Lipids are better suited for long-term energy storage and sustained aerobic metabolism.

Membrane fluidity depends strongly on the chain length and saturation of membrane fatty acids.

• Shorter fatty acid chains have weaker hydrophobic interactions and increase fluidity.
• Longer chains interact more strongly and tend to decrease fluidity.
• Cis double bonds introduce bends that prevent tight packing and increase fluidity.
• Saturated fatty acids are relatively straight and pack closely, decreasing fluidity.
• A higher proportion of polyunsaturated fatty acids generally produces a more fluid membrane.
• Cholesterol modifies these effects by buffering fluidity across temperature changes.

Cells regulate lipid composition through changes in fatty acid synthesis, desaturation, and incorporation into phospholipids. This homeostatic regulation helps maintain suitable membrane function.

Saturated fatty acids:

• Occur naturally in many animal fats and some plant oils.
• Excessive intake may contribute to increased low-density lipoprotein cholesterol in susceptible individuals.

Unsaturated fatty acids:

• Include monounsaturated and polyunsaturated fatty acids.
• Common sources are nuts, seeds, vegetable oils, and fish.
• They support membrane function and may improve the dietary lipid profile when they replace some saturated fats.
• Essential polyunsaturated fatty acids are needed for normal growth and signaling.

Trans fatty acids:

• Contain at least one trans-configured double bond.
• May be produced during industrial hydrogenation or occur naturally in small amounts in some animal products.
• Excessive intake is associated with adverse effects on blood lipoproteins and cardiovascular health.

A balanced dietary pattern emphasizing unsaturated fats and limiting trans fats is generally recommended.