1. Water and Diffusion
Chapter 6
Section 2

2. Section 2 Objectives – page 152
Relate water’s unique features to polarity.
Identify how the process of diffusion occurs and why it is important to cells.
Section 2 Objectives – page 152

3. Summary Section 2 – pages 152-156
Water and Its Importance
Water is perhaps the most important compound in living organisms.
Water makes up 70 to 95 percent of most organisms.
Summary Section 2 – pages

4. Summary Section 2 – pages 152-156
Water is Polar
Sometimes, when atoms form covalent bonds they do not share the electrons equally. This is called a polar bond.
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5. Summary Section 2 – pages 152-156
Water is Polar
A polar molecule is a molecule with an unequal distribution of charge; that is, each molecule has a positive end and a negative end.
Water is an example of a polar molecule.
Water can dissolve many ionic compounds, such as salt, and many other polar molecules, such as sugar.
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6. Summary Section 2 – pages 152-156
Water is Polar
Water molecules also attract other water molecules.
Hydrogen atom
Weak hydrogen bonds are formed between positively charged hydrogen atoms and negatively charged oxygen atoms.
Hydrogen atom
Oxygen atom
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7. Summary Section 2 – pages 152-156
Water resists temperature changes
Water resists changes in temperature. Therefore, water requires more heat to increase its temperature than do most other common liquids.
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8. Summary Section 2 – pages 152-156
Water expands when it freezes
Water is one of the few substances that expands when it freezes.
Ice is less dense than liquid water so it floats as it forms in a body of water.
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9. Summary Section 2 – pages 152-156
Early observations: Brownian motion
In 1827, Scottish scientist Robert Brown used a microscope to observe pollen grains suspended in water. He noticed that the grains moved constantly in little jerks, as if being struck by invisible objects.
This motion is now called Brownian motion.
Today we know that Brown was observing evidence of the random motion of atoms and molecules.
Summary Section 2 – pages

10. Summary Section 2 – pages 152-156
The process of diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
Diffusion results because of the random movement of particles (Brownian motion).
Three key factors—concentration, temperature, and pressure—affect the rate of diffusion.
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11. Summary Section 2 – pages 152-156
The results of diffusion
When a cell is in dynamic equilibrium with its environment, materials move into and out of the cell at equal rates. As a result, there is no net change in concentration inside or outside the cell.
Material moving out of cell equals material moving into cell
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12. Summary Section 2 – pages 152-156
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 higher concentration to an area of lower concentration, moving with the gradient.
Dynamic equilibrium occurs when there is no longer a concentration gradient.
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13. Question 1
Explain why water is important to living organisms.
Answer
Living organisms must have water for life processes, because critical molecules and ions must be free to move and collide, which only happens when they are dissolved in water. Water also transports materials in living organisms, such as in blood or sap.
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14. Question 2 +
Positively charged end
How does water's chemical structure impact its role in living organisms? +
Negatively charged end ―
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15. Because water is polar, it can dissolve many ionic compounds and polar molecules. Water has the property of capillary action that enables plants to get water from the ground. Water also resists temperature changes, which allows cells to maintain homeostasis.
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16. Question 3 A. slow process resulting from random movement of particles
Which of the following best describes diffusion?
A. slow process resulting from random movement of particles
B. net movement of particles from area of low concentration to area of high concentration
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17. Question 3
Which of the following best describes diffusion?
C. rapid process that is unaffected by increases in temperature
D. net movement of particles from high to low concentrations that accelerates when pressure decreases
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18. The answer is A. Diffusion is a slow process resulting from the random movement of particles, and is the net movement of particles from areas of high concentration to areas of lower concentration.
Section 2 Check