1	Isotopes, half life, and Energy Biological reactions acquire materials and energy from the environment and transform the materials with energy to make more individuals.
2	Reading Assignment I have asked you to read chapter 2. The lecture emphasis is on isotopes which are important in environmental research & on measuring energy in an ecological context. We already made some comments on ‘what is life’ and ideas about the origin of life..
3	Isotopes The number of protons in an atom determines its chemical properties and its element name (=type). Most elements have more than one isotope (always with different weights) Nitrogen has ¹⁴N and ¹⁵N isotopes.
4	Stable versus Radioactive Isotopes that do not decay spontaneously are referred to as stable. Isotopes that transform into other elements spontaneously are known as radioactive. Detectable particles are emitted during the radioactive decay event. Particles colliding with atoms is the main way new isotopes are created.
5	Radioactive Isotopes The ability to detect a single decay particle made radioactive isotopes extremely useful in studying biological reactions. Radioactive isotopes may be very unstable (half life in hours) or close to stable (half life in billions of years). The half life is a measure of the stability.
6	Radioactive Decay Rates The probability that an unstable isotope will break down is independent of the age of the atom. Each isotope may be characterized by a time period (called the half life) in which half the original atoms of the isotope undergo decay.
7	Half Lives vary greatly Half lives of some important isotopes ²³⁸U = 4.56x10⁹ years ²²²Rn = 3.8 days ¹⁴C = 5600 years Obviously, isotopes with short half lives must be generated by current processes. ¹⁴C is generated in the upper atmosphere by cosmic rays. The amount of ¹⁴C in the atmosphere stays approximately constant.
8	Half Life Formulas Proportion remaining = 2^{-(time elapsed)/half life} Application: After 1000 years the proportion of ¹⁴C remaining is 0.88 or 88% Amount = Initial amount•e^{-(0.693/half life)•t} Application: 88% = 100%•e^{-(.693/5600)•1000} ΔN/Δt = -c•N, the change in the number per unit time is proportional to the number.
9	Age of the Earth Astrophysicists estimate the universe to be about 1 x 10¹⁰, ten billion years old. Geologists estimate the earth’s age as 4.6x10⁹ (=billion) years and the age of the moon as 4.5 billion years. Based on the presence of the element carbon in sedimentary rock, life is thought to have begun 3.8 billion years ago. Multicellular life began 10⁹ years ago.
10	No Aging in radioactive decay If the probability of death (decay) is constant independent of the age of the isotope, the amount remaining follows a pattern called exponential decay (Fig. 2.3). Exponential decay means individuals in the population do NOT age. In humans the probability of death rises greatly with age (for ages >30 years).
11	Ratios of Stable Isotopes Because isotopes of the same element have the same chemical properties but different weights, all isotopes are found in the same compounds, but heavier isotopes are often slightly enriched or depleted in the products of the reaction. The slight differences in the proportions of isotopes can tell us about conditions in past.
12	Using Stable Isotopes By enriching a particular nitrogen containing compound with ¹⁵N, one can trace the path of transformations of the compound in the environment. Stable isotopes can be separated and measured with the mass spectrometer. Besides nitrogen, isotopes of hydrogen (deuterium), carbon and oxygen are widely used in ecological research.
13	Molecules Atoms are typically linked together by covalent bonds into structures called compounds or molecules. Molecules can react with other molecules to form new compounds (releasing or absorbing energy). Many biologically important molecules are built from similar subunits. Most are very large and are called macromolecules.
14	Macromolecule Types Fats or lipids are the most reduced and most hydrophobic. Biological molecules all contain oxygen, but fats have a lot of CH₂ per oxygen. Carbohydrates (sugar) all have a chemical formula close to a multiple of CH₂O. Proteins always have nitrogen and usually sulfur in addition to C, H and O. Nucleic acids always have nitrogen and phosphorous in addition to C, H and O.
15	Reactions create new molecules Compounds (reactants) interact to produce new compounds (products). This transformation is known as a chemical reaction. Reactions normally absorb or release energy. Reactions that release energy can occur spontaneously and are expected to happen. Reactions go to equilibrium. (At equilibrium the concentrations of reactants and products stay the same, i.e., are constant thru time.)
16	Respiration reactions Reactions that generate energy to run the body involve oxidation. Organic compounds are oxidized to Carbon dioxide (CO₂) and water. Other reactions use that energy to build the macromolecules needed to control the flow of materials and energy in a cell or the body.
17	Source of energy is sun
18	Measures of Energy A primary measure of heat is the calorie. The Joule is the SI measure of energy = 1 kg•m²sec^-2. 1 calorie = 4.184 Joules. In nutrition, the Calorie is 1000 calories, which biologists abbreviate as kcal = kilocalorie. Energy is a measure of the ability to do work.
19	Productivity, a measure of energy The increase in the biomass (growth and reproduction) of autotrophs (=plants) is called net primary productivity, abbreviated NPP. Though it could be measured in energy units, NPP is measured in grams (dry weight) of biomass per unit area per year. The highest NPP values, 2400 g•m^-2•y^-1, are for coral reefs, while the lowest, 100 g•m^-2•y^-1. are associated with open ocean.
20	How can energy be measured in dry weight (=mass)?
21	Energy in Food More reduced organic compounds have more energy per unit weight than less reduced ones. Lipids and fats have about 9 kcal•g^-1 = 9 Cal per gram The figure of 4 kcal•g^-1 is used for protein and carbohydrate (dry weight).
22	Problem If the half life of an isotope is 1000 years, how many years will have elapsed when only 10% of the original amount remains? Answer the same question for 1%.
23	Vocabulary calorie Kilocalorie = Calorie Energy Productivity NPP Radioactive decay Stable isotopes Exponential decay Half life Equilibrium Oxidation Aging