Biology by Bradford
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The subtopic in the middle and five tasks round it: label two diagrams, complete a table and fill 17 gaps. Every box rests on a learning objective of the topic in the AP Biology course and exam description. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.
Oxygen pulls the shared electrons harder than hydrogen does, so each water molecule has a slightly negative end (at the oxygen) and slightly positive ends (at the hydrogens). A molecule with uneven charge like this is . The positive hydrogen of one molecule is attracted to the negative oxygen of the next, forming a weak . Each bond is weak, but there are huge numbers of them, and they keep breaking and re-forming in liquid water.
| Property | Cause | Why a living thing needs it |
|---|---|---|
| water molecules hydrogen bond to each other | unbroken columns of water pulled up xylem | |
| water hydrogen bonds to other polar surfaces, such as cell walls | water climbs the walls of plant vessels | |
| Surface tension | molecules at the surface are pulled inward by their neighbors | insects such as water striders can walk on a pond |
| High | heat must first break hydrogen bonds before molecules speed up | keeps cell and lake temperatures steady |
| Solvent for and ionic substances | water shells its partial charges around them | reactions and transport happen in solution |
Water dissolves ions and polar molecules because they are : water molecules cluster round them and keep them apart. Oil and the tails of membrane lipids are nonpolar, so water cannot hydrogen bond with them and they are . Water crowds nonpolar groups together, which is one reason a cell membrane assembles itself and a protein folds with its nonpolar side chains tucked away from the surrounding water. Ice floats because the hydrogen bonds in a crystal hold molecules in a lattice that is dense than liquid water, so lakes freeze from the top down.
Oxygen pulls the shared electrons harder than hydrogen does, so each water molecule has a slightly negative end (at the oxygen) and slightly positive ends (at the hydrogens). A molecule with uneven charge like this is polar. The positive hydrogen of one molecule is attracted to the negative oxygen of the next, forming a weak hydrogen bond. Each bond is weak, but there are huge numbers of them, and they keep breaking and re-forming in liquid water.
| Property | Cause | Why a living thing needs it |
|---|---|---|
| Cohesion | water molecules hydrogen bond to each other | unbroken columns of water pulled up xylem |
| Adhesion | water hydrogen bonds to other polar surfaces, such as cell walls | water climbs the walls of plant vessels |
| Surface tension | molecules at the surface are pulled inward by their neighbors | insects such as water striders can walk on a pond |
| High specific heat | heat must first break hydrogen bonds before molecules speed up | keeps cell and lake temperatures steady |
| Solvent for polar and ionic substances | water shells its partial charges around them | reactions and transport happen in solution |
Water dissolves ions and polar molecules because they are hydrophilic: water molecules cluster round them and keep them apart. Oil and the tails of membrane lipids are nonpolar, so water cannot hydrogen bond with them and they are hydrophobic. Water crowds nonpolar groups together, which is one reason a cell membrane assembles itself and a protein folds with its nonpolar side chains tucked away from the surrounding water. Ice floats because the hydrogen bonds in a crystal hold molecules in a lattice that is less dense than liquid water, so lakes freeze from the top down.