Water

General Facts Water makes up 70% to 95% of most organisms. Water is used to transfer materials in organisms by dissolving ions or polar molecules.

Polar Bond Occurs when atoms in a covalent bond do not share electrons equally. In water, the oxygen atom attracts more electrons than the two hydrogen atoms.

Polar Molecule Molecule with an unequal distribution of charge. Each molecule has a positive and a negative end. Because of the charges, polar molecules attract ions and other polar molecules.

Water as a Polar Molecule In water molecules, the oxygen atom is slightly negative and the two hydrogen atoms are slightly positive. Because of the opposing charges, water can dissolve many ions, such as salt, as well as many other polar molecules, such as sugar. Polarity also gives water surface tension and allows it to move through capillary action.

Hydrogen Bond Water molecules also attract other water molecules. The oxygen of one water molecule will form a weak bond, called an hydrogen bond, with the hydrogen of another water molecule. Many biomolecules, such as proteins, are held together with these weak hydrogen bonds.

More Properties of Water Acts as an insulator by resisting changes in temperature. Expands when it freezes. Ice is less dense than liquid water and will float.

Brownian Motion Random motion of atoms and molecules. Observed in 1827 by Robert Brown. Notes: All particles in motion have kinetic energy. All particles move in a straight line until acted upon by a force (such as colliding with another particle).

Diffusion Net movement of particles from an areas of higher concentration to an area of lower concentration. Result of Brownian Motion. Slow process. One method by which cells move substances in and out of the cell. 3 key factors affect the rate of diffusion: Concentration, Temperature, and Pressure.

Affect of Concentration on Diffusion Primary controlling factor on rate of diffusion. A higher concentration results in a quicker diffusion rate. Result of the increase is collisions between particles.

Affect of Temperature on Diffusion Higher temperatures result in higher rates of diffusion. Result of more rapid particle motion.

Affect of Pressure on Diffusion Higher pressure results in higher rates of diffusion. Result of acceleration of particle motion.

Dynamic Equilibrium Occurs when there is continuous movement of particles but no overall change in concentration. Occurs when particles are evenly distributed.

Concentration Gradient Difference in concentration of a substance across space. When substances move from areas of higher concentration to areas of lower concentration, the substance moves with the gradient. Substances will continue to move with the gradient until the substance is evenly distributed (dynamic equilibrium) and there is no longer a gradient.