Regents Biology

2.1-2 – Chemistry and Water

Prepared by Kim Foglia. Adapted and modified by Nhan Pham.

Regents Biology

Objectives §  Discuss why we study chemistry in biology §  Review structure of an atom §  Explain the role of valence electrons in chemical

bonding §  Distinguish different types of bonds

Regents Biology

Why study chemistry in a biology class?

Regents Biology

Life requires ~25 chemical elements §  About 25 elements are essential for life §  4 elements make up 96% of living matter

•  carbon (C) •  hydrogen (H) •  oxygen (O) •  nitrogen (N)

§  Four elements make up most of remaining 4% •  phosphorus (P) •  calcium (Ca) •  sulfur (S) •  potassium (K)

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The Atom §  indivisible unit of matter §  100 million atoms side

by side ~ 1 cm

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The Octet Rule §  Electrons

determine the chemical behavior of atom

§  Valence electrons (on the outer most shell)

§  8 is the magic number!

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Bonding properties

How does this atom behave?

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–

–

–

Chemical reactivity Atoms tend to: §  complete a partially filled valence shell OR §  empty a partially filled valence shell

This tendency drives chemical reactions…

and creates bonds

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Sodium Metal in Water

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–

–

H2 (hydrogen gas)

Covalent bond

Bonds in Biology §  Weak bonds -  ionic -  hydrogen bonds -  hydrophobic and hydrophilic

interactions -  van der Waals interaction

§  Strong bonds -  covalent bonds

Hydrogen bond

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Bond Strength

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Ionic Bond §  1 or more electrons are

transferred from one atom to another atom

§  Ex: NaCl (table salt) §  Why?

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Covalent bonds §  Why are covalent bonds strong? -  two atoms share a pair of electrons -  both atoms “holding onto” the electrons -  very stable

§  forms molecules

–

–

H2 (hydrogen gas)

H — H

H2O (water)

H

H

Oxygen H

H O

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Multiple covalent bonds

§  2 atoms can share more than 1 pair of electrons §  double bonds §  triple bonds

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van der Waals §  slight attraction

between oppositely charged regions of nearby molecules

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Objectives §  Explain why water is polar §  Examine different properties of water and explain

how they are good for living things

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Station 1 – Surface Tension and Adhesion §  Water molecules

“stick” to each other

§  Cohesion §  Such “stickiness”

creates high surface tension

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Water is a Polar Molecule

§  Oxygen wants electrons a lot more than hydrogen à pulls electrons toward itself à becomes a slightly negative “pole”

§  The hydrogen atoms become positive “poles”

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Hydrogen Bond §  This is why water

molecules stick to each other like little magnets

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Station 2 – Climbing Property §  Water molecules not

only stick to each other, they also… stick to other molecules (adhesion)

§  climb up the piece of paper… and inside tree trunks

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How does H2O get to top of trees? Transpiration is built on cohesion and adhesion

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Station 3 – Polarity of Water §  Water is polar §  Oil is non-polar à They don’t MIX

§  How is this good for living things?

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Station 4 – Universal Solvent §  Solvent – substance that

dissolve, or break apart, another substance (solute)

§  Water dissolves many substances that are polar, ionic, and even non-polar

§  How is this good for living things?

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Water is the solvent of life §  What makes H2O a good solvent? POLARITY §  polar H2O molecules surround (+) and (–) ions

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Dissolved Oxygen in Blood

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Station 5 – High Specific heat §  It takes a long time for water

to change temperature à high specific heat

§  How is this good for living things?

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Specific heat §  H2O resists changes in temperature §  H2O moderates temperatures on Earth

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The special case of ice §  Most (all?) substances are more dense when they are

solid… but not water. §  Why is this property important?

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Ionization of water & pH §  Water ionizes

§  pH scale

H2O → H+ + OH–

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pH Scale 10–1

H+ Ion Concentration

Examples of Solutions

Stomach acid, Lemon juice 1

pH

100 Hydrochloric acid 0

10–2 2

10–3 Vinegar, cola, beer 3

10–4 Tomatoes 4

10–5 Black coffee, Rainwater 5

10–6 Urine, Saliva 6

10–7 Pure water, Blood 7

10–8 Seawater 8

10–9 Baking soda 9

10–10 Great Salt Lake 10

10–11 Household ammonia 11

10–12 Household bleach 12

10–13 Oven cleaner 13

10–14 Sodium hydroxide 14

tenfold change in H+ ions

pH1 → pH2 10-1 → 10-2

10 times less H+

pH8 → pH7 10-8 → 10-7

10 times more H+

pH10 → pH8 10-10 → 10-8

100 times more H+

Battery acid

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1

2

3

4

5

6

7

8

9

3

Amount of base added

Buffering range

4 5 2

pH

Buffers and Cellular Regulation §  pH of cells must be kept ~7. Why?

§  pH affects shape of molecules

§  shape of molecules affect function

à pH affects cellular function

§  Control pH by buffers

reservoir of H+ -  donate H+ when

[H+] falls -  absorb H+ when

[H+] rises