Water and Diffusion Chapter 6 Section 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

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 152-156

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. Summary Section 2 – pages 152-156

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. Summary Section 2 – pages 152-156

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 Summary Section 2 – pages 152-156

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. Summary Section 2 – pages 152-156

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. Summary Section 2 – pages 152-156

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 152-156

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. Summary Section 2 – pages 152-156

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 Summary Section 2 – pages 152-156

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. Summary Section 2 – pages 152-156

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. Section 2 Check

Question 2 + Positively charged end How does water's chemical structure impact its role in living organisms? + Negatively charged end ― Section 2 Check

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. Section 2 Check

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 Section 2 Check

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 Section 2 Check

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