chapter 2 essential chemistry for biology. matter: elements and compounds matter is anything that...
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CHAPTER 2CHAPTER 2
Essential Chemistry for Biology
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Matter: Elements and Compounds
• Matter is anything that occupies space and has mass.
• Matter is found on the Earth in three physical states:
– Solid
– Liquid
– Gas
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• Matter is composed of chemical elements.
– Elements are substances that cannot be broken down into other substances.
– There are 92 naturally occurring elements on Earth.
• All the elements are listed in the periodic table.
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Figure 2.2
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• Twenty-five elements are essential to life.
– Four of these make up about 96% of the weight of the human body.
– YOU NEED TO KNOW NHCOPS
– Trace elements occur in smaller amounts.
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Figure 2.3
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• Trace elements are essential for life.
– An iodine deficiency causes goiter.
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• Elements can combine to form compounds.
– These are substances that contain two or more elements in a fixed ratio.
– Example: NaCl (salt)
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Atoms
• Each element consists of one kind of atom.
– An atom is the smallest unit of matter that still retains the properties of an element.
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The Structure of Atoms
• Atoms are composed of subatomic particles.
– A proton is positively charged.
– An electron is negatively charged.
– A neutron is electrically neutral.
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• Elements differ in the number of subatomic particles in their atoms.
– The number of protons, the atomic number, determines which element it is.
– An atom’s mass number is the sum of the number of protons and neutrons.
– Mass is a measure of the amount of matter in an object.
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Isotopes
• Isotopes are alternate mass forms of an element.
– They have the same number of protons and electrons.
– But they have a different number of neutrons.
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• In radioactive isotopes,
– The nucleus decays, giving off particles and energy.
• Radioactive isotopes have many uses in research and medicine like PET scans
• Used in dating ancient fossils
• Uncontrolled exposure to radioactive isotopes can harm living organisms by damaging DNA.
– Example: the 1999 Chernobyl nuclear accident
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Electron Arrangement and the Chemical Properties of Atoms
• Electrons determine how an atom behaves when it encounters other atoms.
• Electrons orbit the nucleus of an atom in specific electron shells.
– The number of electrons in the outermost shell determines the chemical properties of an atom.
– Magic number is 8
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Figure 2.7
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Chemical Bonding and Molecules
• Chemical reactions enable atoms to give up or acquire electrons in order to complete their outer shells.
– These interactions usually result in atoms staying close together.
– The atoms are held together by chemical bonds.
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Ionic Bonds
• When an atom loses or gains electrons, it becomes electrically charged.
– Charged atoms are called ions.
– Ionic bonds are formed between oppositely charged ions.
Ionic Bonds
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Covalent BondsA covalent bond forms when two atoms share one or more pairs of
outer-shell electrons.
Covalent Bonds
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Hydrogen Bonds
• Water is a compound in which the electrons in its covalent bonds are shared unequally.
– This causes it to be a polar molecule, one with opposite charges on opposite ends.
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• The polarity of water results in weak electrical attractions between neighboring water molecules.
– These interactions are called hydrogen bonds.
– (Video)
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Chemical Reactions
• Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones.
– Such changes in the chemical composition of matter are called chemical reactions.
• Chemical reactions include:
– Reactants, the starting materials
– Products, the end materials
• Chemical reactions cannot create or destroy matter,
– They only rearrange it.
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Water and Life
• Life on Earth began in water and evolved there for 3 billion years.
– Modern life still remains tied to water.
– Your cells are composed of 70%–95% water.
• The abundance of water is a major reason Earth is habitable.
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Water’s Life-Supporting Properties
• The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties:
– Water’s cohesive nature
– Water’s ability to moderate temperature
– Floating ice
– Versatility of water as a solvent
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The Cohesion of Water
• Water molecules stick together as a result of hydrogen bonding.
– This is called cohesion.
– Cohesion is vital for water transport in plants.
– Adhesion occurs when water sticks to another surface
Water Transport
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Figure 2.12
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• Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid.
– Hydrogen bonds give water an unusually high surface tension.
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How Water Moderates Temperature
• Because of hydrogen bonding, water has a strong resistance to temperature change.
• Water can moderate temperatures.
– Earth’s giant water supply causes temperatures to stay within limits that permit life.
– Evaporative cooling removes heat from the Earth and from organisms.
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The Biological Significance of Ice Floating
• When water molecules get cold, they move apart, forming ice.
– A chunk of ice has fewer molecules than an equal volume of liquid water.
• The density of ice is lower than liquid water.
– This is why ice floats.
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Figure 2.15
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• Since ice floats, ponds, lakes, and even the oceans do not freeze solid.
– Marine life could not survive if bodies of water froze solid.
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Water as the Solvent of Life
• A solution is a liquid consisting of two or more substances evenly mixed.
– The dissolving agent is called the solvent.
– The dissolved substance is called the solute.
• When water is the solvent, the result is an aqueous solution.
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Figure 2.16
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Acids, Bases, and pH
• Acid
– A chemical compound that donates H+ ions to solutions.
• Base
– A compound that accepts H+ ions and removes them from solution.
• To describe the acidity of a solution, we use the pH scale.
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Figure 2.17
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• Buffers are substances that resist pH change.
– They accept H+ ions when they are in excess.
– They donate H+ ions when they are depleted.
• Buffering is not foolproof.
– Example: acid precipitation
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Figure 2.18