The Chemistry

of Life

Water and Diffusion

Objectives Relate water’s unique features to polarity

Identify how the process of diffusion occurs and why it is important to cells

Read section 6.2

Water’s Importance

Most life processes occur only when molecules and ions are free to move and collide with one another When does this condition exist?

Transport Ex/ blood and plant sap

Makes up 70% - 95% of most organisms

Water’s Importance

Water is polar

Water resists temperature change

Water expands when it freezes

Water is Polar Sometimes, when atoms form covalent

bonds they do not share the electrons equally. This is a polar bond.

Water is Polar A polar molecule has an unequal

distribution of charge

A polar molecule has a positive and and a negative end

A polar solvent (like water) dissolves ionic compounds and other polar molecules

Like dissolves like

Water is Polar

Water is Polar The weak bond formed between the

positively charged hydrogen atom and the negatively charged oxygen atom is called a hydrogen bond

Hydrogen bonds help hold biomolecules such as protein together

Water is Polar

Because water is polar, it has a couple more unique properties Capillary action Surface tension

Water resists temperature

change Water requires more heat to increase its

temperature than most other common liquids

Thus, water also loses a lot of heat when it cools

Why is this beneficial to an organism?

Water expands when it freezes

Thus, ice is less dense than water

Why does water expand when in cools if almost all other substances contract? Hydrogen bonds are stabilized!

Diffusion

Brownian motion: The random motion of atoms and molecules Characteristic of gases, liquids and some

solids

Diffusion The net movement of particles from an area

of high concentration to an area of low concentration due to the random movement of particles (Brownian motion) is called diffusion

3 factors affect diffusion rate: Concentration Temperature Pressure

The results of diffusion

Dynamic equilibrium: Materials move into

and out of the cell at equal rates

No net change in concentration inside or outside the cell

Diffusion in living systems

The difference in concentration of a substance across space is called a concentration gradient

Ions and molecules diffuse from an area of high concentration to an area of lower concentration The ions and molecules are said to be

‘moving with the gradient’ If no other processes interfere, diffusion will

continue until there is no concentration gradient (dynamic equilibrium)