Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 2 The Chemical Context of Life Organisms are composed of matter Matter is anything that takes up space and has mass

Sodium Chlorine Sodium chloride Element - substance that cannot be broken down to other substances by chemical reactions Compound - substance consisting of two or more elements in a fixed ratio

Table 2-1 About 25 of the 92 elements are essential to life Trace elements - required by an organism in minute quantities

(a) Nitrogen deficiency Fig. 2-4 (b) Iodine deficiency

Cloud of negative charge (2 electrons) Nucleus Electrons Each element consists of unique atoms, the smallest unit of matter that still retains the properties of an element Atomic number - # of protons in its nucleus Mass number - sum of protons plus neutrons in the nucleus Atomic mass, the atom’s total mass, can be approximated by the mass number

Fig. 2-7 Cancerous throat tissue Isotopes - two atoms of an element that differ in number of neutrons Radioactive isotopes decay spontaneously, giving off particles and energy dating fossils, tracing atoms through metabolic processes and diagnosing medical disorders

( a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell (highest energy level) Second shell (higher energy level) Energy absorbed First shell (lowest energy level) Atomic nucleus (b) Energy lost Energy - capacity to cause change Potential energy - energy that matter has because of its location or structure An electron’s state of potential energy is called its energy level, or electron shell

Electron Distribution and Chemical Properties Valence electrons - those in the outermost (valence shell) – determines chemical behavior – incomplete valence shells can share (covalent = STRONGEST) or transfer (ionic) valence electrons These interactions usually result in atoms staying close together, held by attractions called chemical bonds Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Fig Hydrogen atoms (2 H) Hydrogen molecule (H 2 )

Molecule consists of two or more atoms held together by covalent bonds The notation used to represent atoms and bonding is called a structural formula – For example, H–H This can be abbreviated further with a molecular formula – For example, H 2 Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Hydrogen Bonds Hydrogen bond - a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom – In living cells usually oxygen or nitrogen atoms Van der Waals interactions - attractions between molecules that are close together as a result of these charges Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

Fig (a) Structures of endorphin and morphine (b) Binding to endorphin receptors Natural endorphin Endorphin receptors Morphine Brain cell Morphine Natural endorphin Key Carbon Hydrogen Nitrogen Sulfur Oxygen Biological molecules recognize & interact with each other based on molecular shape

ReactantsReactionProducts 2 H 2 O2O2 2 H 2 O Chemical reactions - making and breaking of chemical bonds Some chemical reactions go to completion All chemical reactions are reversible Chemical equilibrium - reached when the forward and reverse reaction rates are equal