Chapter 10: How Lipids Work

Introduction

• Lipids are a group of molecules that don't mix well with water.

• They have three main jobs in living things:

  • Storing energy for later use.

  • Building the walls (membranes) of cells.

  • Sending signals, helping with chemical reactions, and giving color.

• The way a lipid is built (its structure) determines what it does.

1. Lipids for Storing Energy

• Fatty Acids: These are like basic fuel sticks for the cell.

  • They are made of a long chain of carbon and hydrogen atoms with a carboxyl group (acid part) at one end. This long chain is hydrophobic (doesn't like water), while the acid part is a bit polar (likes water a little).

  • Fatty acid chains can be saturated (all single bonds between carbons, making them straight) or unsaturated (have double bonds, which create kinks in the chain).

  • Cells can break down fatty acids to get a lot of energy. This is because the carbon atoms in fatty acids hold many electrons (they are highly reduced).

  • Fatty acids are stored as triacylglycerols.

• Triacylglycerols (Triglycerides): These are the main way the body stores fat.

  • Imagine a glycerol molecule (a small molecule with three alcohol groups) with three fatty acids attached to it through ester linkages (a type of chemical bond).

  • Triacylglycerols are very hydrophobic and don't hold onto water, which makes them a very efficient way to store energy without adding extra weight from water.

  • They are stored in fat cells (adipocytes) in animals and in seeds of plants.

  • Because fatty acids have more stored energy than sugars, breaking down triacylglycerols gives more energy.

  • Triacylglycerols also act as insulation in animals living in cold environments.

• Trans Fatty Acids: These are unhealthy fats formed during food processing. They can mess with cholesterol levels and increase the risk of heart disease.

2. Lipids for Building Cell Membranes

• Membrane Lipids: These special lipids have one end that likes water (hydrophilic head) and another end that doesn't (hydrophobic tail). This amphipathic nature allows them to form the double-layered structure of cell membranes.

• Phospholipids: The main type of lipid in cell membranes.

  • They have a glycerol or sphingosine backbone.

  • Glycerophospholipids (Phosphoglycerides): These have a glycerol backbone with two fatty acids attached and a phosphate group linked to another polar molecule (the head group). Some have an ether linkage instead of an ester linkage for one of the fatty acids.

  • Sphingolipids: These have a sphingosine backbone instead of glycerol. A fatty acid is attached to sphingosine to form a ceramide, which is the base for all sphingolipids. Different head groups attached to the ceramide create different types, like sphingomyelins (with a phosphate-containing head) and glycosphingolipids (with sugar head groups).

• Glycolipids: These are membrane lipids with sugar molecules attached to their polar heads. They are found on the outer surface of cell membranes and are important for cell recognition. Blood types are determined by the sugar groups on glycosphingolipids.

• Sterols: Lipids with a rigid structure of four fused rings.

  • Cholesterol is the main sterol in animal cells. It's also amphipathic and helps to regulate the fluidity of cell membranes.

3. Lipids for Signaling, Helping Reactions, and Giving Color

• Signaling Lipids: Some lipids act as messengers within or between cells.

  • Eicosanoids: These are made from a fatty acid called arachidonic acid and include prostaglandins (involved in inflammation and other processes).

  • Ceramide and Sphingomyelin: These membrane lipids can be broken down to produce signaling molecules that regulate processes like cell growth and death.

  • Steroid Hormones: These are made from cholesterol. They travel through the bloodstream and bind to receptors inside cells, affecting which genes are turned on or off.

  • Volatile Signals in Plants: Plants produce various lipid-like molecules that can attract pollinators, repel pests, or communicate with other plants.

• Lipid Cofactors and Electron Carriers: Some lipids help enzymes work or carry electrons in important cellular processes.

  • Vitamins A and D: These fat-soluble vitamins are made from isoprenoid precursors and act like hormones, important for vision, bone growth, and other processes.

  • Vitamin E (Tocopherols): These act as antioxidants, protecting cell membranes from damage.

  • Vitamin K: Helps with blood clotting.

  • Ubiquinone (Coenzyme Q) and Plastoquinone: These isoprenoids carry electrons during the process of making ATP (cellular energy) in mitochondria and chloroplasts.

• Lipid Pigments: Some lipids have colors because they can absorb certain wavelengths of light.

  • These pigments often have conjugated dienes, which are chains of carbons with alternating single and double bonds that allow electrons to move around and absorb light. Beta-carotene, which gives carrots their color, can be converted to Vitamin A.