Water, lipids, and cell membrane

• Biological molecules are the basis of cellular life

  • They form the structural basis of cells, store information, and drive all cellular processes
• Molecules are formed when atoms become attached, or bond, to each other
• The type of bonds that hold a molecule together determines how that molecule will behave around other molecules in the cell
• The type of bond an atom forms depends on how selfish it is w/ electrons (Electronegativity) compared to the other atom in the bond
• When two atoms have similar electronegativities, they will share their electrons equally between them, forming a nonpolar covalent bond
• When two atoms have pretty different electronegativities, they will share their electrons, but not equally, forming a polar covalent bond
• In a polar covalent bond, the more electronegative atom will hog the shared electrons

  • This gives the more electronegative atom a partial negative charge, while the less electronegative partner ends up with a partial positive charge
• Finally, when two atoms have extremely different electronegativities, one will steal the electrons from the other, forming an ionic bond
• In an ionic bond, the more electronegative atom completely takes one or more electrons from the less electronegative one
• Hydrogen bonds are weak and temporary interactions caused by the attraction between opposite partial charges on two molecules with polar covalent bonds
• Hydrogen bonds govern (affect) interactions between water molecules, and between water and other molecules
• Water is the “Medium of Life”

  • Cells: the fundamental unit of life – are 70-80% water
• Water is...

  1. Bent
  2. “repels” the two H’s off to the sides at an angle
  3. Highly polar
  4. hogs the electrons it shares with each H
• water readily forms H-bonds with:

  • other water molecules

  • other polar molecules

  • ions
• Due to it’s partial positive and negative charges, H2O can easily dissociate (dissolve) ionic compounds, such as salt
• Adhesion: water molecules stick to something else (as long as it also has partial charges)
• cohesion: Water molecules sticks to each other
• H-bonds make water a great temperature buffer

  • High specific heat capacity
  • Takes a lot of heat to raise the temp of water
• A lot of heat goes into breaking the H-bonds– THERFORE –Not as much goes into increasing molecular motion(which is what heats substances up)
• H-bonds enable ice to float

  • Cold temps cause water molecules to move around less, allowing more H-bonds to form
  • Increased H-bonding forces water molecules into a crystal lattice structure that separates each molecule from the other, making it less dense
• Because water is so central to cell biology, molecules are classified by how they interact with water: hydrophilic vs. hydrophobic
• Polar and charged molecules (ions) interact readily with water through attraction between opposite charges (partial and/or full charges)
• Because they readily interact with water, these molecules are known as hydrophilic (“water-loving”)
• • Nonpolar molecules do not have partial or full charges.
  • Because they do not readily interact with water, these molecules are known as hydrophobic (“water-fearing”).
• Lipids are the building block of cell membranes

  • Lipid molecules are defined by having a large nonpolar, hydrophobic region
• Some lipids have nonpolar (hydrophobic) and polar (hydrophilic) regions

  • These are referred to as amphipathic lipids
• Cell membranes are composed of a type of amphipathic lipid called a phospholipid
• Because the hydrophilic (polar) “heads” readily interact with water and the hydrophobic (nonpolar) “tails” do not...

  • phospholipids will spontaneously form a double layer (“bilayer”) in water
  • Hydrophobic heads are “protected” from H2O by facing inward and hanging with fellow hydrophobes
• B/c of hydrophobic interior of the bilayer, polar and charged (hydrophilic) molecules will not readily pass across through membrane
• The cell’s phospholipid bilayer is fluid
• The more fluid a membrane is, the more freely proteins and other molecules embedded in it can move around
• Factors like temperature affect the fluidity/permeability of cell membranes.
• Membrane fatty acids exist in saturated or unsaturated forms
• Saturation and length of tails affects membrane permeability and fluidity
• Saturated (straight) and long fatty-acid tails of phospholipids will pack more tightly, leading to a more solid, less fluid and permeable membrane
• Unsaturated (bent) and short fatty-acid tails of phospholipids will not pack tightly, leading to a more fluid and permeable membrane
• In response to environmental conditions cells can change:

  • Length of the fatty-acid tails

  • Saturation of the fatty-acid tails

  • Cholesterol content (for animal cells)
• Cells can compensate for (counteract) environmental effects by:

  • adding a higher % of phospholipids with unsaturated tails and/or removing carbons from existing tails to keep the membrane from getting too stiff
• Some of the organisms that have been shown change the lipid composition of their bilayer as a response to decreasing temperature
• Like a phospholipid, cholesterol is amphipathic. Thus, hydrophilic head interacts with phospholipid heads and large hydrophobic region interacts with phospholipid tails