6.1 Lipid Structure and Function

• Molecules that contain only carbon and hydrogen are known as hydrocarbons.

• Fatty acid is a simple lipid consisting of a hydrocarbon chain bonded to a polar carboxyl functional group

• Hydrocarbon chains that consist of only single bonds between the carbons are called saturated.

  • If one or more double bonds exist in the hydrocarbon chains, then they are unsaturated.

  • Saturated lipids that have extremely long hydrocarbon tails, like waxes, form particularly stiff solids at room temperature

  • Highly unsaturated lipids are liquid at room temperature and called oils

• Steroids a family of lipids distinguished by the bulky, four-ring structure

• Fats are nonpolar molecules composed of three fatty acids that are linked to a three-carbon molecule called glycerol.

• In organisms, energy storage is the primary role of fats.

• The glycerol and fatty acid molecules become joined by what is called an ester linkage.

  • An ester linkage occurs when two atoms ( one of them carrying a double-bonded oxygen, often a carbonyl group) are linked together by an oxygen.

• Phospholipids consist of a glycerol that is linked to a phosphate group and two hydrocarbon chains of either isoprenoids or fatty acids.

• Substances that contain both hydrophilic and hydrophobic regions are amphipathic.

6.2 Phospholipid Bilayers

• A lipid bilayer is created when lipid molecules align in paired sheets.

• Micelles tend to form from free fatty acids or other simple amphipathic lipids with single hydrocarbon chains.

  • Micelles and phospholipid bilayers form spontaneously in water-no input of energy is required.

• Vesicles are small bubble-like structures consisting of lipid bilayers surrounding a small amount of aqueous solution.

• Artificially generated membrane-bound vesicles like these are called liposomes.

• Liposomes provide a three-dimensional model that mimics a membrane-bound cell.

• The permeability of a structure is its tendency to allow a given substance to pass through it.

• Selective permeability means that some substances cross a membrane more easily than other substances do.

  • A membrane’s permeability is closely related to its level of fluidity, which is a measure of molecular mobility. As temperature drops, molecules in a bilayer move more slowly and become less fluid.

6.3 How Substances Move across Lipid Bilayers: Diffusion and Osmosis

• Spontaneous movement of molecules and ions is known as diffusion.

• A difference in solute concentrations creates what is called a concentration gradient.

• Solutes move randomly in all directions, but when a concentration gradient exists, there is a net movement from regions of high concentration to regions of low concentration.

• When substances diffuse across a membrane in the absence of an outside energy source, it is known as passive transport.

• The movement of water is a special case of diffusion that is given its own name: osmosis.

• Osmosis occurs only when solutions are separated by a membrane that permits water to cross, but holds back some or all of the solutes-that is, a selectively permeable membrane

• If the solution outside the vesicle has a higher concentration of solutes than the interior has, the solution outside is said to be hypertonic relative to the inside of the vesicle.

• If the solution outside the vesicle has a lower concentration of solutes than the interior has, the outside solution is said to be hypotonic relative to the inside of the vesicle.

• If solute concentrations are equal on both sides of the membrane, the outside is said to be isotonic.

• Simple vesicle-like structures that harbor nucleic acids are referred to as protocells.

6.4 Proteins Alter Membrane Structure and Function

• Their hypothesis was called the fluid-mosaic model. Singer and Nicolson suggested that membranes are a dynamic and fluid mosaic of phospholipids and different types of proteins.

• The method is called freeze-fracture electron microscopy because the steps involve freezing and fracturing the membrane before examining it with a scanning electron microscope (SEM), which produces images of an object’s surface

• Some proteins span the membrane and have segments facing both the interior and the exterior of the cell which are called integral membrane proteins or transmembrane proteins.

• Proteins that bind to membrane lipids or integral membrane proteins without passing through it are called peripheral membrane proteins.

• In cells, ions routinely cross membranes by way of specialized transmembrane proteins called ion channels.

• When considered together, concentration and electrical gradients are called an electrochemical gradient.

• Cells have many different types of pore-like channel proteins in their membranes.

• Gated channels open or close in response to a signal, such as the binding of a particular substance or a change in the electrical voltage across the membrane

• When transmembrane proteins assist the passive transport of substances that otherwise would not cross a membrane readily, the process is called facilitated diffusion.

• The movement of water and K+ are examples of facilitated diffusion through channel proteins, but facilitated diffusion can also occur through specialized membrane proteins called carrier proteins.

• Transport against a gradient is called active transport.

  • A classic example of how structural changes can lead to active transport is provided in the sodium-potassium pump,

  • Gradients are crucial to the function of the cell, in part because they make it possible for cells to engage in secondary active transport-also known as cotransport.