Water, Solutions, and Membranes Roles of water in body functions Characteristics of water Solutions: composition, concentration, and pH Role of membranes in limiting permeability of cells

Roles of Water in Body Functions Water comprises the major bulk of the body ( %) Water plays essential roles in: -stabilizing body temperature -protection (lubrication) -chemical reactions (solvent) -mixing medium

Roles of Water in Body Functions Types of mixing media: - solution: contains dissolved material (salt water) - suspension: materials are not dissolved, and would settle out if not mixed (blood) - colloid: nondissolved materials in liquid; do not settle out (jelly-like)

Intake of water is essential for life Small amounts of water are produced by chemical reactions, or derived from food sources. The majority of water in humans must be taken in through drinking (1.5 to 3.0 liters/day)

What’s so special about water? Properties of water important in body functions: - Chemical structure: H 2 O, a polar molecule - H 2 O molecules stick together due to hydrogen bonding. -Due to this bonding, water has: -high heat of vaporization -high capacity to absorb and distribute heat -high surface tension

What Dissolves in Water? Due to its polarity, many substances dissolve in water. - Polar substances are molecules with charge. These dissolve readily in water. - Salts consist of a positively charged ion (like Na + ) together with a negatively charged ion (Cl - ). These are readily soluble in water. - Nonpolar substances (like oil) do NOT readily dissolve in water. All solutions consist of a liquid (solvent) in which some substance (the solute) is dissolved.

MINUTE QUIZ Nonpolar substances dissolve readily in water? True False

Solutions: Expressing the Concentration of Solute Concentration: How much solute is dissolved in a volume of solvent (usually water). Concentration can be expressed as: - Percent Concentration - Molarity - Normality - Osmolarity

Calculating Percent Concentration The percent concentration is defined as how much solute is dissolved in 100 milliliters (ml) of solvent. Example: 1.0 gram of NaCl dissolved in 100 ml of water is equal to a 1% solution. Example: 1.0 gram of NaCl dissolved in 10 ml of water is equal to a ___% solution. answer: 1) 0.1 2) 1 3) 10 4) 100 correct answer: 3) 1g/10 ml = 10g/100 ml

Calculating Molarity of a Solution The molarity of a solution is determined by how many moles of solute are dissolved in 1000 ml (1 liter) of solvent. (1 molar = 1 mole in 1000 ml) Example: The molecular weight of NaCl is 58 grams. A 1.0 molar solution has 58 g NaCl in 1000 ml Example: The molecular weight of CaCl 2 is 110 g. A 1.0 M solution has 110 grams of CaCl 2 in 1000 ml.

Calculating the Normality of a Solution The normality of a solution is based on both the molecular weight and the chemical composition of the solute. The molecular weight is divided by the numbers of electrons exchanged. Example: NaCl: MW = 58 g, 1 electron exchanged, so, 1.0 N = 58 g/1 electron in 1000 ml (same as molarity) Example: CaCl 2 : MW = 110 g, 2 electrons exchanged, so, 1.0 N = 110 g/2 electrons in 1000 ml = 55 g in 1000 ml (NOT the same as molarity)

Calculating the Osmolarity of a Solution The osmolarity of a solution is dependent upon the total number of particles produced when the solute dissolves and the molecular weight of the solute. Example: NaCl: In a 1 molar solution, forms: 1 mole of Na + and 1 mole of Cl - = 2 osmolar Example: CaCl 2 : 1 molar solution forms: 1 mole of calcium and two moles of chloride = 3 osmolar solution Correct osmolarity of cellular environment is critical!

Converting between types of Concentrations Suppose you have a 1% NaCl solution. What molarity is this solution? 1% NaCl = 1 g/100 ml = 10 g/1000 ml MW of NaCl = 58, so 1 M = 58 g/1000 ml So, divide [10g/1000ml] / [58 g/1000 ml] = 0.17 M NaCl

Role of Hydrogen ions: pH The concentration of hydrogen ions is very important. The concentration of hydrogen ions is usually very low: [H + ] in water = M = M [H + ] expressed in pH scale: pH = log [1/ [H + ] ] Thus, the pH of water = log[1/ ] = log[10 7 ] = 7.0 In water, H 2 O -----> H + + OH -, so [H + ] = [OH - ]. Thus, at pH = 7, a solution is said to be neutral.

Role of Hydrogen ions: pH Acids are defined as solutes which contribute H + ions to the solution, decreasing the pH below 7.0: HCl > H + + Cl - Bases are defined as solutes which contribute hydroxyl ions to the solution, joining with H + ions to form water, and thus increasing the pH higher than 7.0: NaOH > Na + + OH - H + + OH > H 2 O

Role of Hydrogen ions: pH Its very important to prevent wide fluctuations in pH. The pH of blood is between 7.35 and 7.45 This narrow pH range is maintained by buffers, which act to absorb or release H + ions, resisting changes in pH. Improper H + ion concentrations interfere with normal cellular function.

Membrane Permeability Recall that the cell membrane is important in maintaining the interior composition of the cell It selectively allows things to pass through (semipermeable membrane). The semipermeable nature of the membrane is due to its structure.

Structure of the Plasma Membrane phospholipid cholesterol Channel protein carbohydrate The plasma membrane is composed of: a lipid bilayer (45 to 50%) proteins anchored in the bilayer (45 to 50%) carbohydrates (4 to 8%)

Permeability of Materials Through the Plasma Membrane Whether or not a substance can pass through the membrane easily is determined by its charge: -Nonpolar (uncharged) particles pass easily (lipids) -Polar (charged) particles don’t pass easily (salts)

Next Lecture.... Permeability and Transport