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...
Bent
“repels” the two H’s off to the sides at an angle
Highly polar
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