Model Answer:

An atom is made up of positively charged particles called protons, neutral particles called neutrons, and negatively charged particles called electrons. The dense central nucleus of an atom is made up of protons and neutrons. Electrons orbit around the nucleus, and the regions of space where they are most likely to be found are called orbitals.

Model Answer:

The periodic table of elements is organized by the increasing atomic number of each atom. The atomic number is the number of protons an atom has in its nucleus. Hydrogen appears first in the periodic table because it has an atomic number of 1. The elements in a column share similar chemical properties and each have the same number of electrons in their outermost orbital. Elements in a row have the same number of shells (energy levels), and thus the same number and types of orbitals.

Model Answer:

A covalent bond is when two atoms share their valence electrons (the electrons in the outermost orbital of an atom). Each shared pair of valence electrons make a covalent bond that is depicted by a single line connecting the two chemical symbols for the atoms. A polar covalent bond occurs when the valence electrons are not shared equally by the two atoms, thus giving areas of the molecule a positive or negative charge. A hydrogen bond forms when a hydrogen atom covalently bound to an electronegative atom (giving the hydrogen a partial positive charge) interacts with an electronegative atom of another molecule. A hydrogen bond is typically depicted by a dotted line. An ionic bond is formed between a molecule that has a positive charge (due to the loss of one electron) and a molecule that has a negative charge (due to the gain of one electron). The two molecules are not covalently bound, but they associate with each other due to their opposite charges.

Model Answer:

Water is conducive to life in the following ways: Water is a polar molecule and because of its regions of positive and negative charge, some molecules are attracted to water (hydrophilic) and some are repelled by it (hydrophobic). It is this property that allows things like lipid cellular membranes, and thus cells, to exist. The polar nature of water also makes it a good solvent—hydrophilic compounds dissolve readily in water. Water also has a neutral pH (around 7), the pH of most cells. Since many chemical reactions can only be carried out in a solution around a neutral pH, it is important that the cell remain in this range to function. Water resists temperature changes better than other substances due to its extensive network of hydrogen bonding. This is important for a variety of reasons. In the cell, this phenomenon allows chemical reactions, which we know produce heat as a byproduct, to occur inside, without changing the internal temperature. In a similar way, but on a global scale, the oceans act as a temperature regulator and keep the Earth in a temperature range that supports life. The network of hydrogen bonds that forms when water freezes makes ice less dense than liquid water. As a result, ice floats on water, which allows aquatic life to survive below the ice in the winter. Finally, the cohesive properties and surface tension of water facilitate water transportation in plants.

Model Answer:

The four most common elements in organic molecules in order of decreasing abundance are carbon, oxygen, hydrogen, and nitrogen.

Model Answer:

Features of carbon that allow it to form diverse structures are: Carbon behaves as if it has four unpaired electrons, allowing it to form covalent bonds with up to four different molecules. Each of these bonds can also rotate freely, giving the carbon-based molecule structural diversity. Carbon can bond with itself to form large carbon chains that branch or form rings, thus giving rise to a great diversity of structures. Carbon can also form double bonds (sharing two electrons). A double bond does not freely rotate, again giving the molecule different structural options.

Model Answer:

Proteins act as catalysts to facilitate chemical reactions and also provide structural support of the cell. Nucleic acids encode and transmit genetic information. Carbohydrates provide a source of energy and make up the cell wall in bacteria, plant, and algae cells. Lipids store energy, act as signaling molecules, and make up the membranes of the cell.

Model Answer:

Diversity is achieved in polymers through endless combinations of subunits. Think of a protein, which is a polymer of amino acid subunits. As we will discuss in Chapter 4, there are 20 different kinds of amino acids. Thus there are numerous combinations of subunits that could be made, each resulting in a different protein. In this way, polymers are capable of displaying virtually limitless diversity.

Model Answer:

Model Answer:

The principal molecules found in organisms are themselves made of simpler molecules joined together. So if we want to understand how proteins emerged on Earth, then we first have to understand the synthesis of amino acids. Stanley Miller conducted experiments that showed that when a mixture of gasses―which were thought to have been present in the early atmosphere―were ignited with a spark, amino acids were generated. Other scientists have subsequently shown that the other building blocks of life—sugars, bases, and lipids—can also be formed in laboratory conditions that simulate the early atmosphere. Analyses of meteorites that provide samples of the early solar system have also shown the presence of diverse amino acids, lipids, and other organic compounds, which supports Miller’s initial hypothesis. Building off of Miller’s work, Leslie Orgel performed experiments that showed how nucleotides would spontaneously join to synthesize nucleic acids. Many years later, John Sutherland and his colleagues were able to synthesize nucleotides themselves under conditions thought to be like those on the early Earth.