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.