Lipids, Membranes, and the First Cells

1. Lipids, Membranes, and the First Cells 6

2. Key Concepts Plasma membranes are made up of selectively permeable bilayers of phospholipids. Phospholipids are amphipathic lipid molecules – they have hydrophobic and hydrophilic regions. Ions and molecules diffuse spontaneously from regions of higher concentration to regions of lower concentration. Movement of water across a plasma membrane is called osmosis. In cells, membrane proteins are responsible for the passage of insoluble substances that can’t cross the membrane on their own.

3. The Importance of Membranes The plasma membrane, or cell membrane, separates life from nonlife. The plasma membrane separates the cell’s interior from the external environment. Membranes function to: Keep damaging materials out of the cell Allow entry of materials needed by the cell Facilitate the chemical reactions necessary for life

4. Lipids: What Is a Lipid? Lipids are carbon-containing compounds that are found in organisms and that are largely nonpolar and hydrophobic. Hydrocarbons are nonpolar molecules that contain only carbon and hydrogen. Lipids do not dissolve in water because they have a major hydrocarbon component called a fatty acid. A fatty acid is a hydrocarbon chain bonded to a carboxyl (—COOH) functional group. Fatty acids and isoprene are the key building blocks of lipids.

8. Three Types of Lipids Found in Cells Lipid structure varies widely. The three most important types of lipids found in cells: Fats are composed of three fatty acids linked to glycerol. Also called triacylglycerols or triglycerides Steroids are a family of lipids with a distinctive four-ring structure. Cholesterol is an important steroid in mammals. Phospholipids consist of a glycerol linked to a phosphate group (PO42–) and to either two chains of isoprene or two fatty acids.

12. The Structure of Membrane Lipids Membrane-forming lipids contain both a polar, hydrophilic region and a nonpolar, hydrophobic region. Phospholipids are amphipathic: The “head” region, consisting of a glycerol, a phosphate, and a charged group, contains highly polar covalent bonds. The “tail” region is comprised of two nonpolar fatty acid or isoprene chains. When placed in solution, the phospholipid heads interact with water while the tails do not, allowing these lipids to form membranes.

16. Phospholipids and Water Phospholipids do not dissolve when they are placed in water. Water molecules interact with the hydrophilic heads but not with the hydrophobic tails. This drives the hydrophobic tails together. Upon contact with water phospholipids form either: Micelles Heads face the water and tails face each other. Phospholipid bilayers (lipid bilayers)

17. Phospholipid Bilayers Phospholipid bilayers form when two sheets of phospholipid molecules align. The hydrophilic heads in each layer face a surrounding solution, while the hydrophobic tails face one another inside the bilayer. Phospholipid bilayers form spontaneously, with no outside input of energy required.

21. Selective Permeability of Lipid Bilayers The permeability of a structure is its tendency to allow a given substance to pass across it. Phospholipid bilayers have selective permeability. Small or nonpolar molecules move across phospholipid bilayers quickly. Charged or large polar substances cross slowly, if at all.

23. Many Factors Affect Membrane Permeability Many factors influence the behavior of the membrane: Number of double bonds between the carbons in the phospholipid’s hydrophobic tail Length of the tail Number of cholesterol molecules in the membrane Temperature

24. Bond Saturation and Membrane Permeability Double bonds between carbons in a hydrocarbon chain can cause a “kink” in the hydrocarbon chain, preventing the close packing of hydrocarbon tails, and reducing hydrophobic interactions. Unsaturated hydrocarbon chains have at least one double bond. Hydrocarbon chains without double bonds are termed saturated. Saturated fats have more chemical energy than unsaturated fats. Membranes with unsaturated phospholipid tails are much more permeable than those formed by phospholipids with saturated tails.

26. Other Factors That Affect Permeability Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length. Membranes containing phospholipids with longer tails have reduced permeability. Adding cholesterol to membranes increases the density of the hydrophobic section. Cholesterol decreases membrane permeability. Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly. Decreased membrane fluidity causes decreased permeability.

35. Fluidity of the Membrane Individual phospholipids can move laterally throughout the lipid bilayer. They rarely flip between layers. How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer.

37. Solute Movement across Lipid Bilayers Materials can move across the cell membrane in different ways. Passive transport does not require an input of energy. Active transport requires energy to move substances across the membrane. Small molecules and ions in solution are called solutes, have thermal energy, and are in constant, random motion. This random movement is called diffusion. Diffusion is a form of passive transport.

38. Diffusion along a Concentration Gradient A difference in solute concentrations creates a concentration gradient. Molecules and ions move randomly when a concentration gradient exists, but there is a net movement from high- concentration regions to low-concentration regions. Diffusion along a concentration gradient increases entropy and is thus spontaneous. Equilibrium is established once the molecules or ions are randomly distributed throughout a solution. Molecules are still moving randomly but there is no more net movement.

40. Diffusion BLAST Animation: Diffusion

41. Osmosis Water moves quickly across lipid bilayers. The movement of water is a special case of diffusion called osmosis. Water moves from regions of low solute concentration to regions of high solute concentration. This movement dilutes the higher concentration, thus equalizing the concentration on both sides of the bilayer. Osmosis only occurs across a selectively permeable membrane.

43. Osmosis and Relative Solute Concentration The concentration of a solution outside a cell may differ from the concentration inside the cell. An outside solution with a higher concentration is said to be hypertonic to the inside of a cell. A solution with a lower concentration is hypotonic to the cell. If solute concentrations are equal on the outside and inside of a cell, solutions are isotonic to each other.

44. Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions In a hypertonic solution, water will move out of the cell by osmosis and the cell will shrink. In a hypotonic solution, water will move into the cell by osmosis and the cell will swell. In an isotonic solution, there will be no net water movement and the cell size will remain the same.

46. Diffusion and Osmosis Web Activity: Diffusion and Osmosis

47. The Fluid-Mosaic Model of Membrane Structure Although phospholipids provide the basic membrane structure, plasma membranes contain as much protein as phospholipids. The fluid-mosaic model of membrane structure suggests that some proteins are inserted into the lipid bilayer, making the membrane a fluid, dynamic mosaic of phospholipids and proteins.