Acidity and pH
Acidity refers to the hydronium ion concentration of a solution. It is measured by pH. In pure water, the hydronium ion concentration is very low. Only about one in ten million water molecules naturally dissociates to form a hydronium ion in pure water. This gives water a pH of 7. The hydronium ions in pure water are also balanced by hydroxide ions, so pure water is neutral (neither an acid nor a base).
Because pure water is neutral, any other solution with the same hydronium ion concentration and pH is also considered to be neutral. If a solution has a higher concentration of hydronium ions and lower pH than pure water, it is called an acid. If a solution has a lower concentration of hydronium ions and higher pH than pure water, it is called a base. Several acids and bases and their pH values are identified on the pH scale, which ranges from 0 to 14, in Figure below.
Figure 2.25
Water has a pH of 7, so this is the point of neutrality on the pH scale. Acids have a pH less than 7, and bases have a pH greater than 7.
The pH scale is a negative logarithmic scale. Because the scale is negative, as the ion concentration increases, the pH value decreases. In other words, the more acidic the solution, the lower the pH value. Because the scale is logarithmic, each one-point change in pH reflects a ten-fold change in the hydronium ion concentration and acidity. For example, a solution with a pH of 6 is ten times as acidic as pure water with a pH of 7.
Acids
An acid can be defined as a hydrogen ion donor. The hydrogen ions bond with water molecules, leading to a higher concentration of hydronium ions than in pure water. For example, when hydrochloric acid (HCl) dissolves in pure water, it donates hydrogen ions (H+) to water molecules, forming hydronium ions (H3O+) and chloride ions (Cl-). This is represented by the chemical equation:
HCl + H2O → Cl- + H3O+.
Strong acids can be harmful to organisms and damaging to materials. Acids have a sour taste and may sting or burn the skin. Testing solutions with litmus paper is an easy way to identify acids. Acids turn blue litmus paper red.
Bases
A base can be defined as a hydrogen ion acceptor. It accepts hydrogen ions from hydronium ions, leading to a lower concentration of hydronium ions than in pure water. For example, when the base ammonia (NH3) dissolves in pure water, it accepts hydrogen ions (H+) from hydronium ions (H3O+) to form ammonium ions (NH4+) and hydroxide ions (OH-). This is represented by the chemical equation:
NH3 + H2O → NH4+ + OH-.
Like strong acids, strong bases can be harmful to organisms and damaging to materials. Bases have a bitter taste and feel slimy to the touch. They can also burn the skin. Bases, like acids, can be identified with litmus paper. Bases turn red litmus paper blue.
Neutralization
What do you think would happen if you mixed an acid and a base? If you think the acid and base would “cancel each other out,” you are right. When an acid and base react, they form a neutral solution of water and a salt (a molecule composed of a positive and negative ion). This type of reaction is called a neutralization reaction. For example, when the base sodium hydroxide (NaOH) and hydrochloric acid (HCl) react, they form a neutral solution of water and the salt sodium chloride (NaCl). This reaction is represented by the chemical equation:
NaOH + HCl → NaCl + H2O.
In this reaction, hydroxide ions (OH-) from the base combine with hydrogen ions (H+) from the acid to form water. The other ions in the solution (Na+) and (Cl-) combine to form sodium chloride.
Acids and Bases in Organisms
Enzymes are needed to speed up biochemical reactions. Most enzymes require a specific range of pH in order to do their job. For example, the enzyme pepsin, which helps break down proteins in the human stomach, requires a very acidic environment in order to function. Strong acid is secreted into the stomach, allowing pepsin to work. Once the contents of the stomach enter the small intestine, where most digestion occurs, the acid must be neutralized. This is because enzymes that work in the small intestine need a basic environment. An organ near the small intestine, called the pancreas, secretes bicarbonate ions (HCO3-) into the small intestine to neutralize the stomach acid.
Bicarbonate ions play an important role in neutralizing acids throughout the body. Bicarbonate ions are especially important for protecting tissues of the central nervous system from changes in pH. The central nervous system includes the brain, which is the body’s control center. If pH deviates too far from normal, the central nervous system cannot function properly. This can have a drastic effect on the rest of the body.
Water and Life
Humans are composed of about 70 percent water (not counting water in body fat). This water is crucial for normal functioning of the body. Water’s ability to dissolve most biologically significant compounds—from inorganic salts to large organic molecules—makes it a vital solvent inside organisms and cells.
Water is an essential part of most metabolic processes within organisms. Metabolism is the sum total of all body reactions, including those that build up molecules (anabolic reactions) and those that break down molecules (catabolic reactions). In anabolic reactions, water is generally removed from small molecules in order to make larger molecules. In catabolic reactions, water is used to break bonds in larger molecules in order to make smaller molecules.
Water is central to two related, fundamental metabolic reactions in organisms: photosynthesis (Photosynthesis chapter) and respiration (Cellular Respiration chapter). All organisms depend directly or indirectly on these two reactions.
In photosynthesis, cells use the energy in sunlight to change water and carbon dioxide into glucose and oxygen. This is an anabolic reaction, represented by the chemical equation:
6 CO2 + 6 H2O + energy → C6H12O6, + 6 O2.
In cellular respiration, cells break down glucose in the presence of oxygen and release energy, water, and carbon dioxide. This is a catabolic reaction, represented by the chemical equation:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
Two other types of reactions that occur in organisms and involve water are dehydration and hydration reactions. A dehydration reaction occurs when molecules combine to form a single, larger molecule and also a molecule of water. (If some other small molecule is formed instead of water, the reaction is called by the more general term, condensation reaction.) It is a type of catabolic reaction. An example of a dehydration reaction is the formation of peptide bonds between amino acids in a polypeptide chain. When two amino acids bond together, a molecule of water is lost. This is shown in Figure below.
Figure 2.26
In this dehydration reaction, two amino acids form a peptide bond. A water molecule also forms.
A hydration reaction is the opposite of a dehydration reaction. A hydration reaction adds water to an organic molecule and breaks the large molecule into smaller molecules. Hydration reactions occur in an acidic water solution. An example of hydration reaction is the breaking of peptide bonds in polypeptides. A hydroxide ion (OH-) and a hydrogen ion (H+) (both from a water molecule) bond to the carbon atoms that formed the peptide bond. This breaks the peptide bond and results in two amino acids.
Water is essential for all of these important chemical reactions in organisms. As a result, virtually all life processes depend on water. Clearly, without water, life as we know it could not exist.
Lesson Summary
Most of Earth’s water is salt water located on the planet’s surface. Water is constantly recycled through the water cycle.
Water molecules are polar, so they form hydrogen bonds. This gives water unique properties, such as a relatively high boiling point.
A solution is a homogeneous mixture in which a solute dissolves in a solvent. Water is a very common solvent, especially in organisms.
The ion concentration of neutral, pure water gives water a pH of 7 and sets the standard for defining acids and bases. Acids have a pH lower than 7, and bases have a pH higher than 7.
Water is essen
tial for most life processes, including photosynthesis, cellular respiration, and other important chemical reactions that occur in organisms.
Review Questions
Where is most of Earth’s water?
What is polarity, and why is water polar?
Define solution, and give an example of a solution.
What is the pH of a neutral solution? Why?
Draw a circle diagram to represent the water cycle. Identify the states of water and the processes in which water changes state throughout the cycle.
What type of reaction is represented by the chemical equation below? Defend your answer. KOH + HCl → KCl + H2O
Explain how hydrogen bonds cause molecules of liquid water to stick together.
Summarize how metabolism in organisms depends on water.
Further Reading / Supplemental Links
Philip Ball, Life’s Matrix: A Biography of Water. University of California Press, 2001.
Robert A. Copeland, Enzymes: A Practical Introduction to Structure, Mechanisms, and Data Analysis. Wiley, 2000.
Peter Swanson, Water: The Drop of Life. Cowles Creative Publishing, 2001.
www.infoplease.com/cig/biology/organic-chemistry.html
http://en.wikibooks.org/wiki/Organic_Chemistry/Introduction_to_reactions/Alkyne_hydration
http://en.wikipedia.org
Vocabulary
acid
Solution with a higher hydronium ion concentration than pure water and a pH lower than 7.
acidity
Hydronium ion concentration of a solution.
base
Solution with a lower hydronium ion concentration than pure water and a pH higher than 7.
condensation
Process in which water vapor changes to water droplets, forming clouds or fog.
evaporation
Process in which liquid water changes into water vapor.
hydrogen bond
Bond that forms between a hydrogen atom in one molecule and a different atom in another molecule.
ion
Electrically charged atom or molecule.
metabolism
Sum total of all body reactions, including those that build up molecules (anabolic reactions) and those that break down molecules (catabolic reactions).
neutralization
Chemical reaction in which an acid and a base react to form a neutral solution of water and a salt.
pH
Measure of the acidity, or hydronium ion concentration, of a solution.
polarity
Difference in electrical charge between different parts of a molecule.
precipitation
Rain, snow, sleet, or other type of moisture that falls from clouds.
solubility
Ability of a solute to dissolve in a particular solvent.
solute
Substance in a solution that is dissolved by the other substance (the solvent).
solution
Homogeneous mixture in which one substance is dissolved in another.
solvent
Substance in a solution that dissolves the other substance (the solute).
sublimation
Process in which snow or ice changes directly into water vapor.
transpiration
Process in which plants give off water, most of which evaporates.
Points to Consider
Most life processes take place within cells. You probably know that cells are the microscopic building blocks of organisms.
What do you think you would see if you could look inside a cell?
What structures might you see?
What processes might you observe?
Chapter 3: Cell Structure and Function
Introduction to Cells
Lesson Objectives
Identify the scientists that first observed cells.
Outline the importance of microscopes in the discovery of cells.
Summarize what the cell theory proposes.
Identify the limitations on cell size.
Identify the three parts common to all cells.
Compare prokaryotic and eukaryotic cells.
Introduction
Knowing the make up of cells and how cells work is necessary to all of the biological sciences. Learning about the similarities and differences between cell types is particularly important to the fields of cell biology and molecular biology. The importance of the similarities and differences between cell types is a unifying theme in biology. They allow the principles learned from studying one cell type to be applied when learning about other cell types. For example, learning about how single-celled animals or bacteria work can help us understand more about how human cells work. Research in cell biology is closely linked to genetics, biochemistry, molecular biology, and developmental biology.
Discovery of Cells
A cell is the smallest unit that can carry out the processes of life. It is the basic unit of all living things, and all organisms are made up of one or more cells. In addition to having the same basic structure, all cells carry out similar life processes. These include transport of materials, obtaining and using energy, waste disposal, replication, and responding to their environment.
If you look at living organisms under a microscope you will see they are made up of cells. The word cell was first used by Robert Hooke, a British biologist and early microscopist. Hooke looked at thin slices of cork under a microscope. The structure he saw looked like a honeycomb as it was made up of many tiny units. Hooke’s drawing is shown in Figure below. In 1665 Hooke published his book Micrographia, in which he wrote:
... I could exceedingly plainly perceive it to be all perforated and porous, much like a Honey-comb, but that the pores of it were not regular.... these pores, or cells, ... were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this...
Figure 3.1
Drawing of the structure of cork from as it appeared under the microscope to Robert Hooke. The first scientific use of the word appears in this book.
During the 1670s, the Dutch tradesman Antony van Leeuwenhoek, shown in Figure below, used microscopes to observe many microbes and body cells. Leeuwenhoek developed an interest in microscopy and ground his own lenses to make simple microscopes. Compound microscopes, which are microscopes that use more than one lens, had been invented around 1595. Several people, including Robert Hooke, had built compound microscopes and were making important discoveries with them during Leeuwenhoek’s time. These compound microscopes were very similar to the microscopes in use today. However, Leeuwenhoek was so good at making lenses that his simple microscopes were able to magnify much more clearly than the compound microscopes of his day. His microscope's increased ability to magnify over 200 times is comparable to a modern compound light microscope.
Leeuwenhoek was also very curious, and he took great care in writing detailed reports of what he saw under his microscope. He was the first person to report observations of many microscopic organisms. Some of his discoveries included tiny animals such as ciliates, foraminifera, roundworms, and rotifers, shown in Figure below. He discovered blood cells and was the first person to see living sperm cells. In 1683, Leeuwenhoek wrote to the Royal Society of London about his observations on the plaque between his own teeth, "a little white matter, which is as thick as if 'twere batter." He called the creatures he saw in the plaque animacules, or tiny animals. This report was among the first observations on living bacteria ever recorded.
Figure 3.2
Antony van Leeuwenhoek (1632-1723). His carefully crafted microscopes and insightful observations of microbes led to the title the "Father of Microscopy."
Figure 3.3
Rotifers, similar to the type that Leeuwenhoek saw under his microscope.
Microscopes
Hooke’s and Leeuwenhoek’s studies and observations filled people with wonder because their studies were
of life forms that were everywhere, but too small to see with the naked eye. Just think how amazed you would be if you were to read about the first accounts of a newly discovered microorganism from the moon or Mars. Your first thought might be "Things can live there?!" which was probably the first thought of the people who read Hooke’s and Leeuwenhoek’s accounts. The microscope literally opened up an amazing new dimension in the natural sciences, and became a critical tool in the progress of biology.
Magnifying glasses had been in use since the 1300s, but the use of lenses to see very tiny objects was a slowly-developing technology. The magnification power of early microscopes was very limited by the glass quality used in the lenses and the amount of light reflected off the object. These early light microscopes had poor resolution and a magnification power of about 10 times. Compare this to the over 200 times magnification that Leeuwenhoek was able to achieve by carefully grinding his own lenses. However, in time the quality of microscopes was much improved with better lighting and resolution. It was through the use of light microscopes that the first discoveries about the cell and the cell theory (1839) were developed.
However, by the end of the 19th century, light microscopes had begun to hit resolution limits. Resolution is a measure of the clarity of an image; it is the minimum distance that two points can be separated by and still be distinguished as two separate points. Because light beams have a physical size, it is difficult to see an object that is about the same size as the wavelength of light. Objects smaller than about 0.2 micrometers appear fuzzy, and objects below that size just cannot be seen. Light microscopes were still useful, but most of the organelles and tiny cell structures discussed in later lessons were invisible to the light microscope.
CK-12 Biology I - Honors Page 14