CK-12 Life Science

by CK-12 Foundation

  Figure 2.4

  All living organisms are made of one or more cells.

  Figure 2.5

  This paramecium is a one-celled organism.

  All cells share at least some structures. But there are thousands of kinds of cells with different structures. The cells of plants and mushrooms have a cell wall, while the cells of animals do not. The cells of most organisms have a special membrane around the DNA, but bacterial cells do not. Although the cells of different organisms are built differently, they all function much the same way. Every cell must get energy from food, be able to grow and reproduce, and respond to its environment.

  Living Things Need Resources and Energy

  In order to grow, reproduce, and maintain homeostasis, living things need energy. The work you do each day, from walking to writing and thinking, is fueled by energy in your cells. But where does this energy come from?

  The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat. All animals must eat plants or other animals in order to obtain energy and building materials. Plants themselves don’t eat; they use the energy of the sun to make their “food” through the process of photosynthesis (see Cell Functions chapter). Like animals, mushrooms and other fungi obtain energy from other organisms. That’s why you often see fungi growing on a fallen tree; the rotting tree is their source of energy (Figure below). Although the means of getting energy might be different, all organisms need some source of energy. And since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of living things ultimately comes from the sun.

  Figure 2.6

  Fungi obtain energy from breaking down dead organisms, such as this rotting log.

  Besides obtaining energy from the foods you eat, you also need the chemical building blocks in food to build and maintain your body. For example, you get calcium for building bones from eating dairy products or leafy greens. Plants obtain nutrients from the soil. Nutrients will be discussed in the next lesson.

  Lesson Summary

  All living things grow, reproduce, and maintain a stable internal environment.

  All organisms are made of cells.

  All living things need energy and resources to survive.

  Review Questions

  Define the word organism.

  Give two examples of processes that help organisms achieve homeostasis.

  What are three characteristics of living things?

  What are a few ways organisms can get the energy they require?

  What is a cell?

  Further Reading / Supplemental Links

  http://publications.nigms.nih.gov/thenewgenetics/thenewgenetics.pdf

  http://en.wikipedia.org

  Vocabulary

  cell

  The smallest living unit of life; the smallest unit of structure of living organisms.

  DNA

  Deoxyribonucleic acid; the heredity material; carries the genetic information of the cell.

  heredity

  The passing of traits or a tendency to certain traits to the next generation through units of inheritance called genes.

  homeostasis

  Maintaining a stable internal environment despite changes in the environment.

  organism

  A living thing.

  reproduction

  The process by which an organism makes a new organism with at least some of its own genes.

  Points to Consider

  DNA is considered the “instructions” for the cell. What do you think this means?

  What kinds of chemicals do you think are necessary for life?

  Do you expect that the same chemicals can be in non-living and living things?

  Lesson 2.2: Chemicals of Life

  Lesson Objectives

  Distinguish between an element and a compound.

  Explain how elements are organized on the periodic table.

  Explain the function of enzymes.

  Name the four main classes of organic molecules that are building blocks of life.

  Check Your Understanding

  What are the main properties of all living things?

  What is homeostasis?

  Introduction

  Physical science and biology are two different subjects in school, so you might see them as two unrelated sciences. However, understanding physical science is essential for understanding biology. Living things are subject to the same physical laws of the universe as non-living things. The rules that apply to chemical reactions in a test tube also apply to the chemical reactions that take place inside your body. To understand how living things function, we must have a little knowledge of physics and chemistry. This includes knowing what elements are and how different molecules come together to form the components of life.

  The Elements

  Rocks, animals, flowers, and even your body, are made up of matter. Matter is anything that takes up space and has mass. Matter makes up everything, living and nonliving.

  Matter is composed of a mixture of elements. Elements are substances that cannot be broken down into simpler substances with different properties. Even chemical reactions or physical processes, like heating or crushing, cannot break it down to release a simpler substance. There are more than 100 known elements, and 92 occur naturally around us. The others have been made only in the laboratory.

  Elements are made up of identical atoms. An atom is the simplest and smallest particle of matter that still retains the chemical properties of the element. Atoms are so tiny that only the most powerful microscopes can detect them. Atoms are the building block of all elements, and of all matter. Each element has a different type of atom, and is represented with a one or two letter symbol. For example, the symbol for oxygen is O and the symbol for carbon is C.

  Atoms themselves are composed of even smaller particles, including: the positively charged protons, the uncharged neutrons, and the negatively charged electrons. Protons and neutrons are located in the center of the atom, or the nucleus, and the electrons move around the nucleus. How many protons an atom has determines what element it is. For example, Helium (He) always has two protons (Figure below), while Sodium (Na) always has 11. To restate this, all the atoms of a particular element have the exact same number of protons, and the number of protons is that element's atomic number.

  Figure 2.7

  An atom of Helium (He) contains two positively charged protons (red), two uncharged neutrons (green), and two negatively charged electrons (yellow).

  The Periodic Table

  Each element also has unique properties, such as density, boiling point, and how well it dissolves ("solubility"). Density is the mass of the substance per unit of volume. That means that if you take an equal volume of different elements, each different sample will weigh a different amount. For example, a liter of the metal mercury weighs 13 times as much as a liter of water. The boiling point is the temperature at which an element will change from a liquid to a gas. For example, the boiling point of water is 100 degrees Celsius. Once you heat water to this temperature, you see bubbles form as the water turns into vapor. Each element has a different boiling point. Solubility is how well a substance will dissolve in water. You can dissolve more sugar in a liter of water than salt, because sugar is more soluble than salt. Density, boiling point, and solubility have unchanging values for each element.

  In 1869, Dmitri Mendeleev constructed the periodic table, organizing all the elements according to their atomic number, density, boiling point, solubility, and other values. As mentioned above, each element has a one or two letter symbol. For example, H stands for hydrogen and Au for gold. The vertical columns in the periodic table are known as groups and elements in groups tend to have very similar properties. The table is also divided into rows, known as periods.

  Group 1 (see Figure below ) contains the highly reactive metals, such as sodium (Na) and lithium (Li). Just a small amount of these metals will explode into
flames when put into water. Another group are the less-reactive metals, such as gold (Au) and platinum (Pt). Since they will not react readily with air and tarnish, these metals are highly valued for making jewelry. There are also highly reactive nonmetals, such as chlorine and oxygen, and some nonreactive gases, such as helium (He) and neon (N), which you might recognize from helium balloons and neon signs.

  Figure 2.8

  The periodic table groups the elements based on their properties.

  Chemical Reactions

  A molecule is any combination of two or more atoms. The oxygen in the air we breathe is two oxygen atoms connected by a chemical bond to form O2, or molecular oxygen. A carbon dioxide molecule is a combination of one carbon atom and two oxygen atoms. Because carbon dioxide includes two different elements it is a "compound" as well as a molecule.

  A compound is any combination of two or more elements. A compound usually has very different properties from the elements that it contains. Elements and combinations of elements make up all the diverse types of matter in the universe.

  The process by which two different elements come together to form a compound is one example of a chemical reaction. For example, hydrogen and oxygen together form water. Water has the properties of a liquid, not the properties of the gases hydrogen and oxygen. Water is the product, or end result, of the chemical reaction while hydrogen and oxygen are the reactants, or “ingredients” necessary for the chemical reaction.

  One important chemical reaction in your everyday life is oxidation, or the combination of oxygen and another element. Examples of oxidation are burning and rusting. When oxygen combines with gas on your stove top, the reaction releases heat that you can use to cook with. (In fact, since fires need oxygen to burn, most fire extinguishers are composed of heavier gasses that will displace the oxygen, smothering the fire.) Rust is formed when oxygen combines with iron (Figure below). These are a few examples of chemical reactions.

  Figure 2.9

  Rust is the result of a chemical reaction between iron and oxygen.

  Organic Compounds

  The chemical components of living things are known as organic compounds, which means they contain the element carbon (C). Living things are made up of compounds that are quite large. These large compounds molecules, known as macromolecules, are made of smaller molecules. You might recognize some of these organic molecules as parts of the food you eat (Figure below). Through eating food, we obtain the organic molecules we need to grow and be healthy.

  The Four Main Classes of Organic Molecules Proteins Carbohydrates Lipids Nucleic Acids

  Elements C,H,O,N,S C,H,O C,H,O,P C,H,O,P,N

  Examples Enzymes, muscle fibers, antibodies Sugar, Starch, Glycogen, Cellulose Phospholipids in membranes, fats, oils, waxes, steroids DNA, RNA, ATP

  Monomer (small building block molecule) Amino acids Sugars Often include fatty acids Nucleotides

  Organic compounds all contain the elements carbon (C) and hydrogen (H). The chain of carbon and hydrogen in organic compounds is sometimes called the “backbone” of organic compounds since they make up the core center structure. What makes organic compounds different from one another is the functional groups, groups of atoms that have unique chemical properties. The addition of a functional group vastly changes the properties of the carbon-hydrogen backbone of organic compounds. Each organic compound is therefore suited to its unique role in living things.

  Carbohydrates

  Essentially, carbohydrates are sugars or long chains of sugars. An important role of carbohydrates is to store energy. Glucose is a simple sugar molecule with the chemical formula C6H12O6. Sugar is one type of carbohydrate, but carbohydrates also include long chains of connected sugar molecules. These chains of sugar molecules can be used to store sugar for later use, such as in the form of starches or glycogen. Plants store sugar in long chains called starch, whereas animals store sugar in long chains called glycogen. Both storage molecules contain hundreds or thousands of linked glucose molecules. Chains of sugar molecules also can be used as structural molecules. For example, the hard skeletons of insects and lobsters are made of chitin, a type of carbohydrate. These long chains of sugar molecules are know as polysaccharides. You get the carbohydrates you need for energy from eating carbohydrate-rich foods, including fruits and vegetables, as well as grains such as bread, rice, or corn.

  The chemical formula C6H12O6 of glucose means that this molecule has 24 atoms: 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Carbohydrates have a general chemical formula consisting of twice as many hydrogen atoms as carbon and oxygen atoms. Glucose is a monomer, a single unit that when linked together with other monomers forms a long chain known as a polymer. Starch is an example of a polymer.

  Proteins

  Proteins have many different functions in living things. Enzymes are a type of protein. Antibodies that protect your body from disease are proteins, and your muscles are made of protein. All proteins are made of monomers (small building block molecules) called amino acids that line up to form long chains. There are only 20 common amino acids. These amino acids have the general chemical formula H2NCHRCOOH, where R is a "side group" which varies between amino acids. It is this side group that gives the amino acids its physical and chemical properties. These amino acids form in thousands of different combinations, generating up to 100,000 unique proteins. Proteins can differ in both the number and order of amino acids. Small proteins have just a few hundred amino acids, whereas the largest proteins have over 25,000 amino acids.

  Figure 2.10

  General Structure of Amino Acids. This model shows the general structure of all amino acids. Only the side chain, R, varies from one amino acid to another. For example, in the amino acid glycine, the side chain is simply hydrogen (H). In glutamic acid, in contrast, the side chain is CHCHCOOH. Variable side chains give amino acids acids different chemical properties. The order of amino acids, together with the properties of the amino acids, determines the shape of the protein, and the shape of the protein determines the function of the protein. KEY: H = hydrogen, N = nitrogen, C = carbon, O = oxygen, R = variable side chain

  After a cell makes a protein chain, the chain folds into a 3-dimensional structure (Figure below). Proteins fold based on the sequence and properties of the amino acids. The properties of amino acids can vary widely, so the position of each amino acid in a protein is important. Each folded protein has its own unique shape. It is this shape that gives the protein its function. The primary structure of a protein is the linear sequence of amino acids. The amino acids appear as "beads on a string," as shown in the figure below. The folding of the protein into the 3-dimensional working molecule is based on the initial primary sequence of amino acids.

  Figure 2.11

  Proteins fold into unique 3-dimensional structures, starting with the linear "beads on a string," shown at the top, to the complex structure on the bottom.

  It’s important for you and other animals to eat food with protein because we cannot synthesize some of these amino acids ourselves. You can get proteins both from plant sources such as beans and from animal sources, like milk or meat. When you eat food with protein, your body breaks the proteins down into individual amino acids and uses them to build new proteins. Therefore, you really are what you eat!

  Lipids

  The lipids - the fats, oils, and waxes - are a diverse group of organic compounds. Lipids are not soluble in water. (As you probably know, oil and water don’t mix.) The most common lipids in your diet are probably fats and oils. Fats are solid at room temperature, whereas oils are fluid. Animals use fats for long-term energy storage and insulation. Plants use oils for long-term energy storage. When preparing food, we often use animal fats, such as lard and butter, or plant oils, such as olive oil or canola oil.

  There are many more type of lipids that are important to life. One of the most important are the phospholipids (see the chapter titled Cell Functions) that make up the membranes that surround all cells. Steroids are
the basis for the hormones like testosterone and estrogen. Waxes are useful lipids for plants and animals since they are waterproof. Plants coat their leaves in a waxy covering to prevent water loss, while bees use wax to make their honeycombs.

  Nucleic acids

  Nucleic acids are long chains of nucleotides, which are units composed of a sugar, a nitrogen-containing base, and a phosphate group. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two main nucleic acids. DNA is the molecule that stores our genetic information and RNA is involved in making proteins. Nucleotides also make up the high-energy molecule Adenosine Triphosphate (ATP). ATP is the energy currency of the cell. Every time you think a thought or move a muscle, you are using the energy stored in ATP.

  The following series of Figures A, B, C and D show examples.

  Figure 2.12

  (A) A molecule of glucose (a carbohydrate).

  Figure 2.13

  (B) Muscle fibers (protein).

  Figure 2.14

  (C) Phospholipids in a membrane (lipid).