Biology and Society: Nuclear Medicine

Radioactivity and Its Biological Impact

Radioactivity, the emission of high-energy particles from unstable atomic nuclei, has significant effects on living organisms and is also harnessed for medical purposes.

• Radioactivity can damage DNA and kill cells, making it both harmful and useful in medicine.

• Radiation therapy targets specific body areas with controlled doses to treat cancers, such as prostate cancer.

• Understanding the effects and uses of radiation requires a foundation in chemistry.

Matter: Elements and Compounds

Basic Chemical Concepts in Biology

All biological systems are composed of matter, which is made up of elements and compounds.

• Matter: Anything that occupies space and has mass.

• Mass: The measure of the amount of material in an object.

• Element: A substance that cannot be broken down into other substances by chemical reactions.

• There are 92 naturally occurring elements; 25 are essential for human life.

• Four elements (oxygen, carbon, hydrogen, nitrogen) make up about 96% of the human body’s weight.

• Trace elements are required in very small amounts but are vital for life (e.g., iron, iodine).

• Compound: A substance containing two or more elements in a fixed ratio (e.g., H2O, NaCl).

Atoms

Structure and Properties of Atoms

Atoms are the fundamental units of elements and the building blocks of matter.

• Each element consists of one kind of atom.

• Atom: The smallest unit of matter that retains the properties of an element.

• Atoms are composed of subatomic particles:

  • Proton: Positively charged

  • Neutron: Electrically neutral

  • Electron: Negatively charged

• The atomic number is the number of protons in an atom and defines the element.

• The mass number is the sum of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioactive isotopes: Nuclei decay spontaneously, emitting radiation.

Chemical Bonding and Molecules

Types of Chemical Bonds

Chemical bonds form when atoms interact to achieve stable electron configurations, resulting in molecules and compounds.

• Only electrons are directly involved in chemical reactions.

• The number of electrons determines an atom’s chemical properties.

• Chemical bonds are attractions that hold atoms together in molecules or compounds.

Ionic Bonds

• Formed when atoms transfer electrons, resulting in charged particles called ions.

• Ionic bond: Attraction between oppositely charged ions (e.g., Na+ and Cl- in NaCl).

• Ionic compounds are held together by ionic bonds.

Covalent Bonds

• Formed when two atoms share one or more pairs of electrons.

• Covalent bonds are the strongest type of chemical bond and hold atoms together in a molecule (e.g., H2O, O2).

Hydrogen Bonds

• Occur between polar molecules, such as water.

• In water, electrons are shared unequally, making it a polar molecule with partial charges.

• Hydrogen bonds are weak attractions between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another.

Chemical Reactions

Rearrangement of Matter

Chemical reactions involve the making and breaking of chemical bonds, leading to changes in the composition of matter.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of the reaction.

• Chemical reactions rearrange matter but do not create or destroy it (Law of Conservation of Mass).

Example: Predicting the formula for the product of SO3 and H2O:

• SO3 + H2O → H2SO4

• No atoms are gained or lost; they are rearranged to form sulfuric acid.

Major Themes in Biology

Connecting Chemistry to Biological Principles

• Cells rearrange molecules by breaking and forming chemical bonds, illustrating pathways that transform energy and matter.

• The polarity of water molecules supports life, demonstrating the relationship of structure to function.

• Environmental changes, such as CO2 release, affect distant ecosystems, showing interactions within biological systems.

Water and Life

Properties of Water Essential for Life

Water’s unique chemical and physical properties are crucial for supporting life on Earth.

• Life originated in water and remains dependent on it; human cells are 70–95% water.

• Water’s abundance is a key factor in Earth’s habitability.

• The polarity of water and resulting hydrogen bonds explain its life-supporting properties.

Cohesion and Surface Tension

• Cohesion: Water molecules stick together due to hydrogen bonding, stronger than in most liquids.

• Essential for water transport in plants.

• Surface tension: Water has high surface tension, making its surface difficult to break.

Temperature Moderation

• Water resists temperature changes due to hydrogen bonding.

• Heat is absorbed to break hydrogen bonds, so water warms slowly.

• When cooling, hydrogen bonds form and release heat.

• Evaporative cooling: As water evaporates, the surface cools (e.g., sweating).

Ice Floating

• As water freezes, molecules move apart, making ice less dense than liquid water.

• Floating ice insulates water below, allowing aquatic life to survive in winter.

• If ice sank, bodies of water would freeze solid, threatening life.

Water as a Solvent

• Solution: Homogeneous mixture of substances.

• Solvent: The dissolving agent (water in aqueous solutions).

• Solute: The substance dissolved.

• Water’s polarity makes it an excellent solvent for many substances.

Acids, Bases, and pH

Regulation of Hydrogen Ion Concentration

The pH scale measures the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity.

• Acid: Releases H+ ions into solution.

• Base: Accepts H+ ions, reducing their concentration.

• pH scale: Ranges from 0 (most acidic) to 14 (most basic); 7 is neutral.

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.

Equation for pH:

Evolution Connection: Radioactivity as an Evolutionary Clock

Using Radioactive Isotopes to Date Fossils

Radioactive decay provides a method for determining the age of fossils and reconstructing the evolutionary history of life.

• Radioactive isotopes decay at predictable rates, measured as half-lives.

• By measuring the ratio of isotopes in fossils, scientists estimate their age.

• This technique helps build the fossil record, an ordered sequence of life’s history.

Additional info: The half-life is the time required for half of the radioactive atoms in a sample to decay. This property allows scientists to estimate the age of materials containing radioactive isotopes.