Lipids and Fatty Acids
Lipids are water-insoluble molecules that dissolve in organic solvents. They serve various biochemical roles such as:
Energy storage (e.g., lipids store energy).
Structural components of membranes.
Roles in signal transduction pathways.
Unlike carbohydrates, amino acids, and nucleic acids, lipids do not form polymers.
Classes of Lipids
Free Fatty Acids
Simplest type of lipid, used as fuel.Vary in hydrocarbon chain length which affects their function.
Triacylglycerols
Storage form of fatty acids, composed of three fatty acid chains esterified to glycerol.Phospholipids
Composed of fatty acids attached to a glycerol or sphingosine backbone and include a phosphate group, leading to a polar head and hydrophobic tail.Glycolipids
Lipids bound to carbohydrates, found in cellular membranes.Steroids
Polycyclic hydrocarbons that function as hormones (e.g., cholesterol is a major component of membranes and a precursor to other steroids).
Chemical Properties of Fatty Acids
Fatty acids have chains of hydrogen-bearing carbon atoms terminating with a carboxylic acid group. They are commonly classified as:
Saturated (only single bonds, e.g., stearic acid).
Unsaturated (one or more double/cross bonds, e.g., oleic acid).
Notation systems:
Example: $18:1$ indicates a fatty acid with 18 carbons and one double bond.
Biological importance of chain length and saturation:
Shorter chains and unsaturation increase fluidity and lower melting points.
Fatty Acid Properties and Health Implications
Unsaturated fatty acids have lower melting points compared to saturated fatty acids.
High saturated and trans fats correlate with health risks, including cardiovascular disease.
Essential fatty acids (like omega-3) must be obtained from the diet and have protective effects against heart disease.
Triacylglycerols and Energy Storage
Triacylglycerols serve as a major energy reservoir due to their hydrophobic nature, storing energy in a nearly anhydrous form.
Use of triacylglycerols vs. glycogen (4:1 in energy storage efficiency per gram).
Stored primarily in adipose tissue (fat cells) and provide energy during fasting.
Membrane Lipids
Three major types of membrane lipids:
Phospholipids: Major membrane components, formed of glycerol, fatty acids, phosphate, and an alcohol.
Glycolipids: Sugar-containing lipids important for cell-cell interactions.
Cholesterol: Modulates membrane fluidity and is a steroid-derived compound.
Amphipathic nature of membrane lipids drives the formation of lipid bilayers in aqueous solutions, with hydrophobic tails inward.
Membrane Fluidity
Determined by fatty acid composition and the presence of cholesterol.
Transition from solid-like to fluid-like states occurs at characteristic phase transition temperatures.
Membrane Proteins and Their Functions
Membrane proteins facilitate specific transport functions and are integral to membrane dynamics.
Classifications:
Integral membrane proteins: Span the bilayer and interact with hydrophobic regions.
Peripheral membrane proteins: Associate non-covalently with the surface of the membrane or integral proteins.
Transport proteins include pumps (active transport) and channels (facilitated diffusion) that regulate ion and molecule movement across membranes.
Active vs. Passive Transport
Passive Transport: Molecules move down concentration gradients without energy input.
Active Transport: Requires energy to move substances against their concentration gradients, such as via ATP-powered pumps.
Example: Na+/K+ ATPase maintains ion gradients essential for cell function.
Summary of Key Terms
Fatty acid: Carbon chain with a carboxylic acid group.
Triacylglycerol: Fat storage molecule made of glycerol and three fatty acids.
Phospholipid: Major class of membrane lipid with polar and non-polar regions.
Sphingosine: A component of certain lipids like sphingomyelin.
Cholesterol: Steroid molecule important for membrane structure and fluidity.
Amphipathic molecule: A molecule with both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.