BSCI105 Chapter 2- The Chemical Context of Life
BSCI105 Chapter 2- The Chemical Context of Life Bsci105
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This 6 page Class Notes was uploaded by clcindy.lin on Tuesday January 26, 2016. The Class Notes belongs to Bsci105 at University of Maryland taught by Dr. Alewall in Summer 2015. Since its upload, it has received 41 views. For similar materials see Intro to biological sciences in Biological Sciences at University of Maryland.
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Date Created: 01/26/16
BSCI105 Chapter 2: The Chemical Context of Life 1/26/16 7:00 PM 2.1: Matter Consist of Chemical Elements on Pure Form & in Combinations called Compounds • organisms are composed of matter – anything that takes up space & has mass • Elements & Compound: o Elements: a substance that can't be broken down to other substances by chemical reactions (92 elements) o Compound: substance consisting of 2 or more different elements combined in a fixed ratio. • Elements of Life: o Essential elements: 20-25% of the 92 elements are needed for organism to live a healthy life and reproduce, but there are some variations; humans need 25 elements and plants 17. o 4 elements (oxygen, Carbon, Hydrogen, and Nitrogen) make up 96% of living matters and calcium, phosphorus, potassium, sulfur and a few other account for the rest of the 4% . o Trace elements: are required by an organism in only minute quantities and are all needed by all forms of life. (Fe (Iron) and iodine (I) are the most important.) 2.2: An element’s properties depend on the structure of its atoms • atom: is the smallest unit of matter that still retains the properties of an element. • Subatomic particles: o Neutrons: electrically neutral (mass around 1.7 x 10^-24 g) o Protons: 1 unit of positive charge (mass around 1.7 x 10^ -24 g) o Electrons: 1 unit of negative charge (1/2000 of the mas above) o Atomic nucleus: where protons and neutrons are packed together (why the nucleus has a positive charged) o Orbital: where rapidly moving electrons are located around the nucleus 90% of the time. It is the attraction between opposite charges that keeps the electrons in the vicinity of the nucleus. o Dalton: unit of measurement used for atoms and subatomic particles. (same as the atomic number or amu) o Because the mass of an electron is 1/2000 of a neutron or proton we can ignore the electrons when computing mass. • Atomic number and atomic mass: o All atoms of a particular element have the same # of protons o Atomic number: number of protons (unique to that element) is written as a subscript to the left of the symbol for the element (ex2 He shows that there are 2 protons i n helium) ▯ An atom is neutral in electrical charge, which me ans that its protons must be balanced by electrons. And at this state the numbers of protons and electrons are the same, therefore the atomic number will also tell us how many electrons there are. o Mass number: the sum of protons and neutrons (the number above the atomic number) o Atomic mass: the mass # is an approximation of the total mass of an atom. • Isotopes: different atomic forms of the same element o Most common one is c arbon-12, which has 6 neutrons; carbon-13, which has 7 neurons. o Radioactive isotopes : “unstable” – the nucleus decays spontaneously, giving off particles and energy. ▯ When radioactive isotope decay it leads to a change in the number of protons, therefore it transform the atom to an atom if a different element. (ex: carbon -14 decays to Nitrogen-14) o Radioactive Tracers: ▯ The radioactive isotopes are incorporated into biologically active molecules, which are then used as tracers to track atoms during metabolism, the chemical process of organisms. ▯ Radioactive tracers are also used in combination with sophisticated imaging instruments, such as PET scanners that can monitor growth & metabolism of cancer in the body. ▯ Although radioactive isotopes are very useful in bio. Research & medic ine, it can be hazardous to life by damaging cellular molecules. o Radioactive dating ▯ Half-life: the time it takes for 50% of the parent isotope to decay. ▯ Radioactive dating: scientists measure the ratio of different isotopes & calculate how many half -lives (in years) have passed since an organism was fossilized or a rock has formed. • Energy Levels of Electrons: o Only electrons are directly involved in chemical reactions o An atom’s electron vary in the amount of energy possessed. o Energy: capacity to cause change by doing work o Potential energy: energy that matter possesses because of its location or structure. o The electrons have potential energy due to their distance from the nucleus. It takes work to move an electron that is far away from the nucleus, so the more distant an electron is from the nucleus, the greater its potential energy. o Electron shells: where the electrons are found in, each with a characteristic average distance and energy level. o An electron can exist only at certain energy levels, not b etween them o When the electron gains energy, it moves to a shell further away from the nucleus and vice versa. o Electrons in the outermost energy level (valence electrons) are the chemically important electIf the outer electron shell is full, then th e atom is non-reactive. If the outer shell is not full, then the atom reacts with other atoms to share their electrons, lose the electrons or steal them away from another atom to fill the shell. ▯ First energy level (electron shell) can hold two electrons. ▯ The second and third electron shells hold 8 electrons each . • Electron Distribution & Chemical Properties: o Rows (periods): correspond to the number of electron shells. The left -to-right sequence of elements in each row corresponds to the sequential addition of electrons & protons. o Valence electrons: the outer electrons. o Valence shells: outmost electron shell. ▯ Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. ▯ Atoms with complete valance shell is unreactive o r inert (it will not interact readily with other atoms.) • Electron Orbitals: o Orbital: the 3-D space where an electron is found 90% of the time. Each electron shell obtains electrons at a particular energy level, distributed among a specific number of orbita ls of distinctive shapes and orientations. No more than 2 electrons can occupy a single orbital. ▯ The 1 electron shell has only one spherical (s) orbital (called 1s). ▯ The 2 shell has 4 orbitals: 1 large spherical s orbital (called 2s) and 3 dumbbell-shaped p orbitals (called 2p). The 3 2p lie at right angles to one another along imaginary x -, y-, z-axes of the atom. 2.3: The formation and function of molecules depend on chemical nodding between atoms • Chemical bonds: interactions usually result in atoms staying close together. o Covalent bonds: is the sharing of a pair of valence electrons by 2 atoms. (ex: hydrogen has 1 valence electron in the first shell, but the shell can hold 2 electrons, so when 2 hydrogen atoms come close enough, their 1 s orbital will overlap and share their electrons.) ▯ When 2 or more atoms held together by covalent bonds constitutes a molecule. ▯ Molecular formula : indicates how many atoms are consisted in the molecule. (ex: H ) 2 ▯ Lewis dot structure: element symbols are surrounded by dots that represents the valence electrons ▯ Structural formula: where the line represents a single bond (a pair of shared electrons). ▯ Double bond: sharing 2 pairs of valence electrons. (2 lines in structural formula) ▯ Valence: the number of unpaired electrons required to complete the atom’s outmost (valence) shell. (hydrogen is 1, oxygen is 2) ▯ Electronegativity: attraction of a particular atom for the electrons of a covalent bond. The more electronegative an atom is, the most strongly it pulls shared elect rons toward itself. ▯ Nonpolar covalent bond : the electrons are shared equally because the 2 atoms have the same electronegativity. ▯ Polar covalent bond: the electrons of the bond are not shared equally. (ex: H O2is polar because oxygen is more electronegat ive than hydrogen shared electrons are pulled more toward oxygen (making that side negative charge and the hydrogen side positive charge.) o Ionic Bonds: ▯ 2 atoms are so unequal in their attraction for valence electrons that the more electronegative atom str ips an electron completely away from its partner. ▯ The 2 resulting oppositely charged atoms (or molecules) are called Ions. ▯ Cation: positively charged ▯ Anion: negatively charged ▯ Because of their opposite charges, cations and anions attract each other: ionic bond ▯ Ionic compound or salt: compounds formed by ionic bonds ▯ Each salt crystal is an aggregate of vast numbers of cations and anions bonded by their electrical attraction and arranged in a 3 -D lattice. ▯ Ionic compound, unlike covalent compound, does not consist of molecules. o Weak Chemical Bonds: ▯ Ionic Bond in water ▯ Hydrogen Bonds: attraction between a hydrogen and an electronegative atom. It is when a hydrogen atom is covalently bonded to an electronegative atom, the hydrogen atom has a partial positive charge that allows it to be attracted to a different electronegative atom nearby. ▯ Van der Waals Interactions : occurs only when atoms and molecules are very close together. Created by ever-changing regions of positive and negative charge that enable all a toms and molecules to stick to one another. • Molecular Shape and Function o The shape of the molecule is determined by the positions of the atoms orbitals. ▯ Ex: The single s and 3 p orbitals of a valence shell involved in covalent bonding combine to form 4 teardrop-shaped hybrid orbitals. These orbitals extend to 4 corners of an imaginary tetrahedron. o The shape of the molecule determines how biological molecules recognize & respond to one another with specificity. 2.4: Chemical Reactions make and beak chemi cal bonds: • Chemical reactions: the making and breaking of chemical bond, leading to changes in the composition of matter. o (Ex: 2H +2O ▯ 22 O) 2 o write a chemical reaction with an arrow to indicate the conversion starting with the reactants (starting material) leading to the product. o All chemical reactions are reversible and the opposite-headed arrows indicate this type of reaction. o One of the factors affecting the rate of a reaction is the concentration or reactants. The greater the concentration of reactant molecules, the more frequently they collide with one another and have an opportunity to react and from products. o Chemical Equilibrium: the point at which the reactions offset one another. It means that their concentrations have stabilized at a particular ratio. Eventually the forward and reverse reactions occur at the same rate, and the relative concentrations of products and reactants stop changing. 1/26/16 7:00 PM 1/26/16 7:00 PM
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