Content Standard #1 Summary

The first knowledge of cells came from the work of an English scientist, Robert Hooke, who in 1665 used a primitive microscope to study thin sections of cork and called the boxlike cavities he saw "cells." Antony van Leeuwenhoek later observed one-celled "animalcules" in pond water, but not until the 1830s did Theodor Schwann view cartilage tissue in which he discovered cells resembling plant cells. He published the theory that cells are the basic unit of life. Rudolf Virchow used the work of Schwann and Matthias Schleiden to advance the cell theory, presenting the concept that plants and animals are made of cells that contain fluid and nuclei and arise from preexisting cells.

After the cell theory was established, detailed study of cell structure and function depended on the improvement of microscopes and on techniques for preparing specimens for observation. It is now understood that cells in plants and animals contain genes to control chemical reactions needed for survival and organelles to perform those reactions. Living organisms may consist of one cell, as in bacteria, or of many cells acting in a coordinated and cooperative manner, as in plants, animals, and fungi. All cells have at least three structures in common: genetic material, a cell or plasma membrane, and cytoplasm.

Chapter and section Numbers

(For Biology: The Dynamics of Life, 2002, Glencoe/McGraw-Hill)

Cell Biology (15% of CST, 9 items) - Q 1, 2, 3

[9 items] 1. Fundamental life processes of plants and animals depend on a variety of chemical reactions that are carried out in specialized areas of the organism's cells. As a basis for understanding this concept, students know:

- Q 2

a. -cells are enclosed within semi-permeable membranes that regulate their interaction with their surroundings.

• Chapter 7.1-2
• Chapter 8.1
• BioDigest 251-252
• MiniLab 204

The plasma membrane consists of two layers of lipid molecules (lipid bilayer) organized with the polar (globular) heads of the molecules forming the outside of the membrane and the nonpolar (straight) tails forming the interior of the membrane. Protein molecules embedded within the membrane move about relative to one another in a fluid fashion. Because of its dynamic nature the membrane is sometimes referred to as the fluid mosaic model of membrane structure. Cell membranes have three major ways of taking in or of regulating the passage of materials into and out of the cell: simple diffusion, carrier-facilitated diffusion, and active transport. Osmosis of water is a form of diffusion. Simple diffusion and carrier-facilitated diffusion do not require the expenditure of chemical bond energy, and the net movement of materials reflects a concentration gradient or a voltage gradient or both. Active transport requires free energy (from ATP), in the form of either chemical bond energy or a coupled concentration gradient, and permits the net transport or "pumping" of materials against a concentration gradient.

- Q 2 - 3e

b. -enzymes are proteins and catalyze biochemical reactions without altering the reaction equilibrium. The activity of enzymes depends on the temperature, ionic conditions and pH of the surroundings.

• Chapter 6.3
• Inside Story 166
• Design Your Own BioLab 168-169

Almost all enzymes are protein catalysts made by living organisms. Enzymes speed up favorable (spontaneous) reactions by reducing the activation energy required for the reaction, but they are not consumed in the reactions they promote. To demonstrate the action of enzymes on a substrate, the teacher can use liver homogenate or yeast as a source of the enzyme catalase and hydrogen peroxide as the substrate. The effect of various environmental factors, such as pH, temperature, and substrate concentration, on the rate of reaction can be investigated. These investigations should encourage student observation, recording of qualitative and quantitative data, and graphing and interpretation of data.

- Q 1 - 2

c. -how prokaryotic cells, eukaryotic cells (including those from plants and animals), and viruses differ in complexity and general structure.

• Chapter 7.1
• Chapter 18.1-2
• Inside Story 192
• Investigate BioLab 194
• MiniLab 215
• Focus On 498-499

All living cells are divided into one of two groups according to their cellular structure. Prokaryotes have no membrane-bound organelles and are represented by the Kingdom Monera, which in modern nomenclature is subdivided into the

Eubacteria and Archaea. Eukaryotes have a complex internal structure that allows thousands of chemical reactions to proceed simultaneously in various organelles. Viruses are not cells; they consist of only a protein coat surrounding a strand of genetic material, either RNA or DNA.

- Q 2 - 3

d. -the central dogma of molecular biology outlines the flow of information from transcription of RNA in the nucleus to translation of proteins on ribosomes in the cytoplasm.

• Chapter 11
• Lesson 2
• MiniLab 299
• Investigate BioLab 308-309
• BioDigest 370-371

DNA, which is found in the nucleus of eukaryotes, contains the genetic information for encoding proteins. The DNA sequence specifying a specific protein is copied (transcribed) into messenger RNA (mRNA), which then carries this message out of the nucleus to the ribosomes located in the cytoplasm. The mRNA message is then translated, or converted, into the protein originally coded for by the DNA.

- Q 2

e. the role of the endoplasmic reticulum and Golgi apparatus in secretion of proteins.

• Chapter 7.3

There are two types (rough and smooth) of endoplasmic reticulum (ER), both of which are systems of folded sacs and interconnected channels. Rough ER synthesizes proteins, and smooth ER modifies or detoxifies lipids. Rough ER produces new proteins, including membrane proteins. The proteins to be exported from the cell are moved to the Golgi apparatus for modification, packaged in vesicles, and transported to the plasma membrane for secretion.

- Q 1 - 2L

f. -usable energy is captured from sunlight by chloroplasts, and stored via the synthesis of sugar from carbon dioxide.

• Chapter 7.3
• Chapter 9.2
• Inside Story 235
• Internet BioLab 244-245
• BioDigest 254

Photosynthesis is a complex process in which visible sunlight is converted into chemical energy in carbohydrate molecules. This process occurs within chloroplasts and specifically within the thylakoid membrane (light-dependent reaction) and the stroma (light-independent reaction). During the light-dependent reaction, water is oxidized and light energy is converted into chemical bond energy generating ATP, NADPH + H+, and oxygen gas. During the light-independent reaction (Calvin cycle), carbon dioxide, ATP, and NADPH + H+ react, forming phosphoglyceraldehyde (PGAL) which is then converted into sugars. By using a microscope with appropriate magnification, students can see the chloroplasts in plant cells (e.g., lettuce, onion) and photosynthetic protists (e.g., euglena).

Students can prepare slides of these cells themselves, an activity that provides a good opportunity to see the necessity for well-made thin sections of specimens and for correct staining procedures. Commercially prepared slides are also available. By observing prepared cross sections of a leaf under a microscope, students can see how a leaf is organized structurally and think about the access of cells to light and carbon dioxide during photosynthesis. The production of oxygen from photosynthesis can be demonstrated and measured quantitatively with a volumeter, which can collect oxygen gas from the illuminated leaves of an aquatic plant, such as elodea. By varying the distance between the light source and the plant, teachers can demonstrate intensities of the effects of various illumination. To eliminate heat as a factor, the teacher can place a heat sink, such as a flat-sided bottle of water, between the plant and light source to absorb or dissipate unwanted heat.

- Q 2

g. -the role of the mitochondria in making stored chemical bond energy available to cells by completing the breakdown of glucose to carbon dioxide.

• Chapter 7.3
• Chapter 9.3
• Inside Story 239
• BioDigest 254

Mitochondria consist of a matrix where three-carbon fragments originating from carbohydrates are broken down (to CO₂ and water) and of the cristae where ATP is produced. Cell respiration occurs in a series of reactions in which fats, proteins, and carbohydrates, mostly glucose, are broken down to produce carbon dioxide, water, and energy. Most of the energy from cell respiration is converted into ATP, a substance that powers most cell activities.

- Q 1 - 2

h. most macromolecules (polysaccharides, nucleic acids, proteins, lipids) in cells and organisms are synthesized from a small collection of simple precursors.

• Chapter 6.3
• Chapter 11.1
• BioDigest 251

Many of the large carbon compound molecules necessary for life (e.g., polysaccharides, nucleic acids, proteins, and lipids) are polymers of smaller monomers. Polysaccharides are composed of monosaccharides; proteins are composed of amino acids; lipids are composed of fatty acids, glycerol, and other components; and nucleic acids are composed of nucleotides.

NE- Q 1 - 2

i.* how chemiosmotic gradients in the mitochondria and chloroplast store energy for ATP production.

• Chapter 9.2-3
• BioDigest 254

Enzymes called ATP synthase, located within the thylakoid membranes in chloroplasts and cristae membranes in mitochondria, synthesize most ATP within cells. The thylakoid and cristae membranes are impermeable to protons (H⁺) except at pores that are coupled with the ATP synthase. The potential energy of the proton concentration gradient drives ATP synthesis as the protons move through the ATP synthase pores. The proton gradient is established by energy furnished by a flow of electrons (e^-) passing through the electron transport system located within these membranes.

NE- Q 1 - 2

j* how eukaryotic cells are given shape and internal organization by a cytoskeleton and/or cell wall.

• Chapter 7.3
• BioDigest 252

The cytoskeleton, which gives shape to and organizes eukaryotic cells, is composed of fine protein threads called microfilaments and thin protein tubes called microtubules. Cilia and flagella are composed of microtubules arranged in the 9 + 2 arrangement, in which nine pairs of microtubules surround two single microtubules. The rapid assembly and disassembly of microtubules and microfilaments and their capacity to slide past one another enable cells to move, as observed in white blood cells and amoebae, and also account for movement of organelles within the cell. Students can observe prepared slides of plant mitosis in an onion root tip to see the microtubules that make up the spindle apparatus. Prepared slides of white fish blastula reveal animal spindle apparatus and centrioles, both of which are composed of microtubules.

Content Standard #2 Summary

Students should know that organisms reproduce offspring of their own kind (specific biogenesis) and that organisms of the same species resemble each other. Students have been introduced to the idea that some characteristics can be passed from parents to offspring and that individual variations appear among offspring and in the broader population. Understanding genetic variation requires mastery of the fundamentals of sex cell formation and the steps to reorganize and redistribute genetic material during defined stages in the cell cycle. Students should understand the difference between asexual cell reproduction (mitosis) and the formation of male or female gamete cells (meiosis). Sexual reproduction initially requires the production of haploid eggs and haploid sperm, a process occurring in humans within the female ovary and the male testis. These haploid cells unite in fertilization and produce the diploid zygote, or fertilized cell.

The mechanisms involved in synapsis and movement of chromosomes during meiosis bring about the halving of the chromosome numbers for the production of the haploid male or female gamete cells from the original diploid parent cell and different combinations of parental genes. The exchange of chromosomal segments between homologous chromosomes (crossing over) revises the association of genes on the chromosomes and contributes to increased diversity. Any change in genetic constitution through mutation, crossing over, or chromosome assortment during meiosis promotes genetic variation in a population.

Chapter and section Numbers

(For Biology: The Dynamics of Life, 2002, Glencoe/McGraw-Hill)

Genetics (30% of CST, 18 items) - Q 2, 3 [ 6 items] 2. Mutation and sexual reproduction lead to genetic variation in a population. As a basis for understanding this concept, students know:

- Q 2L - 3

a. meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce gametes containing one chromosome of each type

Chapter 10.2

BioDigest 369

Haploid gamete production through meiosis involves two cell divisions. During meiosis prophase I, the homologous chromosomes are paired, a process that abets the exchange of chromosome parts through breakage and reunion. The second meiotic division parallels the mechanics of mitosis except that this division is not preceded by a round of DNA replication; therefore, the cells end up with the haploid number of chromosomes. (The nucleus in a haploid cell contains one set of chromosomes.) Four haploid nuclei are produced from the two divisions that characterize meiosis, and each of the four resulting cells has different chromosomal constituents (components). In the male all four become sperm cells. In the female only one becomes an egg, while the other three remain small degenerate polar bodies and cannot be fertilized. Chromosome models can be constructed and used to illustrate the segregation taking place during the phases of mitosis (covered initially in Standard 1.e for grade seven in Chapter 4) and meiosis. Commercially available optical microscope slides also show cells captured in mitosis (onion root tip) or meiosis (Ascaris blastocyst cells), and computer and video animations are also available.

- Q 2L - 3E

b. - only certain cells in a multicellular organism undergo meiosis

• Chapter 10.1-2
• Chapter 38.1
• Inside Story 1033

Only special diploid cells, called spermatogonia in the testis of the male and oogonia in the female ovary, undergo meiotic divisions to produce the haploid sperm and haploid eggs.

- Q 2L - 3E

c. -how random chromosome segregation explains the probability that a particular allele will be in a gamete.

• Chapter 10.1-2
• BioDigest 369

The steps in meiosis involve random chromosome segregation, a process that accounts for the probability that a particular allele will be packaged in any given gamete. This process allows for genetic predictions based on laws of probability that pertain to genetic sortings. Students can create a genetic chart (Punnett Square(s)) and mark alternate traits on chromosomes, one expression coming from the mother and another expression coming from the father. Students can be shown that partitions of the chromosomes are controlled by chance (are random) and that separation (segregation) of chromosomes during karyokinesis (division of the nucleus) leads to the random sequestering of different combinations of chromosomes.

- Q 2L - 3E

d. -new combinations of alleles may be generated in a zygote through fusion of male and female gametes (fertilization).

• Chapter 10.1-2
• Chapter 24.3
• Chapter 38.2

Once gametes are formed, the second half of sexual reproduction can take place. In this process a diploid organism is reconstituted from two haploid parts. When a sperm is coupled with an egg, a fertilized egg (zygote) is produced that contains the combined genotypes of the parents to produce a new allelic composition for the progeny. Genetic charts can be used to illustrate how new combinations of alleles may be present in a zygote through the events of meiosis and the chance union of gametes. Students should be able to read the genetic diploid karyotype, or chromosomal makeup, of a fertilized egg and compare the allelic composition of progeny with the genotypes and phenotypes of the parents.

- Q 2L - 3E

e. -why approximately half of an individual's DNA sequence comes from each parent.

• Chapter 10.2
• Chapter 11.1

Chromosomes are composed of a single, very long molecule of double-stranded DNA and proteins. Genes are defined as segments of DNA that code for polypeptides (proteins). During fertilization half the DNA of the progeny comes from the gamete of one parent, and the other half comes from the gamete of the other parent.

- Q 2L

f. -the role of chromosomes in determining an individual's sex.

• Chapter 12.2

The normal human somatic cell contains 46 chromosomes, of which 44 are pairs of homologous chromosomes and 2 are sex chromosomes. Females usually carry two X chromosomes, and males possess one X and a smaller Y chromosome. Combinations of these two sex chromosomes determine the sex of the progeny.

- Q 3E

g. -how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents.

• Chapter 10.1
• Chapter 12.1
• Problem-Solving Lab 317

When the genetic makeups of potential parents are known, the possible assortments of alleles in their gametes can be determined for each genetic locus. Two parental gametes will fuse during fertilization, and with all pair-wise combinations

of their gametes considered, the possible genetic makeups of progeny can then be predicted.

Content Standard #3 Summary

Breeding of plants and animals has been an active technology for thousands of years, but the science of heredity is linked to the genetics pioneer Gregor Mendel. He studied phenotypic traits of various plants, especially those of peas. (A phenotypic trait is the physical appearance of a trait in an organism). From the appearance of these traits in different generations of growth, he was able to infer their genotypes (the genetic makeup of an organism with respect to a trait) and to speculate about the genetic makeup and method of transfer of the hereditary units from one generation to the next. (Probability analysis is now used to predict probable progeny phenotypes from various parental genetic crosses.) The genetic basis for Mendel's laws of segregation and independent assortment is apparent from genetic outcomes of crosses.

Chapter and

Section Numbers

(For Biology: The Dynamics of Life, 2002, Glencoe/McGraw-Hill)

- Q 2, 3 [ 3 items] 3. A multicellular organism develops from a single zygote, and its phenotype depends on its genotype, which is established at fertilization. As a basis for understanding this concept, students know:

- Q 3

a. - how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance (autosomal or X-linked, dominant or recessive).

• Chapter 10.1
• Chapter 12.1-2
• Internet BioLab 281
• MiniLab 316
• Design Your Own BioLab 336-337

Monohybrid crosses, including autosomal dominant alleles, autosomal recessive alleles, incomplete dominant alleles, and X-linked alleles, can be used to indicate the parental genotypes and phenotypes. The possible gametes derived from each parent are based on genotypic ratios and can be used to predict possible progeny. The predictive (probabilistic) methods for determining the outcome of genotypes and phenotypes in a genetic cross can be introduced by using Punnett Squares and probability mathematics.

Teachers should review the process of writing genotypes and help students translate genotypes into phenotypes. Teachers should emphasize dominant, recessive, and incomplete dominance as the students advance to an explanation of monohybrid crosses illustrating human conditions characterized by autosomal recessive alleles, such as albinism, cystic fibrosis, Tay-Sachs, and phenylketonuria (PKU). These disorders can be contrasted with those produced by possession of just one autosomal dominant allele, conditions such as Huntington disease, dwarfism, and neurofibromatosis. This basic introduction can be followed with examples of incomplete dominance, such as seen in the comparisons of straight, curly, and wavy hair or in the expression of intermediate flower colors in snapdragon plants. Sex-linked characteristics that are found only on the X chromosome should also be considered, and students should reflect on how this mode of transmission can cause the exclusive or near exclusive appearance in males of color blindness, hemophilia, fragile-X syndrome, and sex-linked muscular dystrophy.

- Q 3

b. -the genetic basis for Mendel's laws of segregation and independent assortment.

• Chapter 10
• Lessons 1, 2
• Chapter 12.1
• Problem-Solving Lab 270

Mendel deduced that for each characteristic, an organism inherits two genes, one from each parent. When the two alleles differ, the dominant allele is expressed, and the recessive allele remains hidden. Two genes or alleles separate (segregate) during gamete production in meiosis, resulting in the sorting of alleles into separate gametes (the law of segregation). Students can be shown how to diagram Mendel's explanation for how a trait present in the parental generation can appear to vanish in the first filial (F1) generation of a monohybrid cross and then reappear in the following second filial (F2) generation.

Students should be told that alternate versions of a gene at a single locus are called alleles. Students should understand Mendel's deduction that for each character, an organism inherits two genes, one from each parent. From this point students should realize that if the two alleles differ, the dominant allele, if there is one, is expressed, and the recessive allele remains hidden. Students should recall that the two genes, or alleles, separate (segregate) during gamete production in meiosis and that this sorting of alleles into separate gametes is the basis for the law of segregation. This law applies most accurately when genes reside on separate chromosomes that segregate out at random, and it often does not apply or is a poor predictor for combinations and frequencies of genes that reside on the same chromosome. Students can study various resources that describe Mendel's logic and build models to illustrate the laws of segregation and independent assortment.

NE- Q 3

c.* -how to predict the probable mode of inheritance from a pedigree diagram showing phenotypes.

• Chapter 12.1
• MiniLab 316
• Problem-Solving Lab 324, 332
• Social Studies Connection 338

Students should be taught how to use a pedigree diagram showing phenotypes to predict the mode of inheritance.

NE-Not addressed

d.* - how to use data on frequency of recombination at meiosis to estimate genetic distances between loci, and to interpret genetic maps of chromosomes.

• Chapter 13.1

Students should be able to interpret genetic maps of chromosomes and manipulate genetic data by using standard techniques to relate recombination at meiosis to estimate genetic distances between loci.

Content Standard #4 Summary

All cells contain DNA as their genetic material. The role of DNA in organisms is twofold: first, to store and transfer genetic information from one generation to the next, and second, to express that genetic information in the synthesis of proteins. By controlling protein synthesis, DNA controls the structure and function of all cells. The complexity of an organism determines whether it may have several hundred to more than twenty thousand proteins as a part of its makeup. Proteins are composed of a sequence of amino acids linked by peptide bonds (see Standard 10.c for chemistry). The identity, number, and sequence of the amino acids in a protein give each protein its unique structure and function. Twenty types of amino acids are commonly employed in proteins, and each can appear many times in a single protein molecule. The proper sequence of amino acids in a protein is translated from an RNA sequence that is itself encoded in the DNA

Chapter and Section Numbers

(For Biology: The Dynamics of Life, 2002, Glencoe/McGraw-Hill)

- Q 2, 3 [ 5 items] 4. Genes are a set of instructions, encoded in the DNA sequence of each organism that specify the sequence of amino acids in proteins characteristic of that organism. As a basis for understanding this concept, students know:

- Q 1 - 3

a. - the general pathway by which ribosomes synthesize proteins, using tRNAs to translate genetic information in mRNA.

• Chapter 11.2
• Problem-Solving Lab 297
• MiniLab 299
• BioDigest 370-371

DNA does not leave the cell nucleus, but messenger RNA (mRNA), complementary to DNA, carries encoded information from DNA to the ribosomes (transcription) in the cytoplasm. (The ribosomes translate mRNAs to make protein.) Freely floating amino acids within the cytoplasm are bonded to specific transfer RNAs (tRNAs) that then transport the amino acid to the mRNA now located on the ribosome. As a ribosome moves along the mRNA strand, each mRNA codon, or sequence of three nucleotides (triplet) specifying the insertion of a particular amino acid, is paired in sequence with the anticodon of the tRNA that recognizes the sequence.

Each amino acid is added, in turn, to the growing polypeptide at the specified position. After learning about transcription and translation through careful study of expository texts, students can simulate the processes on paper or with representative models. Computer software and commercial videos are available that illustrate animated sequences of transcription and translation.

- Q 3

b. how to apply the genetic coding rules to predict the sequence of amino acids from a sequence of codons in RNA.

• Chapter 11
• Lessons 1, 2
• Problem-Solving Lab 297
• Investigate BioLab 308
• BioDigest 370-371

The sequence of amino acids in protein is provided by the genetic information found in DNA. In prokaryotes, mRNA transcripts of a coding sequence are copied from the DNA as a single contiguous sequence. In eukaryotes, the initial RNA

transcript, while in the nucleus, is composed of exons, sequences of nucleotides that carry useful information for protein synthesis, and introns, sequences that do not. Before leaving the nucleus, the initial transcript is processed to remove introns and splice exons together. The processed transcript, then properly called mRNA and carrying the appropriate codon sequence for a protein, is transported from the nucleus to the ribosome for translation.

Each mRNA has sequences, called codons, that are decoded three nucleotides at a time. Each codon specifies the addition of a single amino acid to a growing polypeptide chain. A start codon signals the beginning of the sequence of codons to be translated, and a stop codon ends the sequence to be translated into protein. Students can write out mRNA sequences with start and stop codons from a given DNA sequence and use a table of the genetic code to predict the primary sequences of proteins.

- Q 3

c. -how mutations in the DNA sequence of a gene may or may not affect the expression of the gene, or the sequence of amino acids in an encoded protein.

• Chapter 11.3
• Chapter 15.2
• Problem-Solving Lab 305
• MiniLab 306

Mutations are permanent changes in the sequence of nitrogen-containing bases in DNA (see Standard 5.a in this section for details on DNA structure and nitrogen bases). Mutations occur when base pairs are incorrectly matched (e.g., A bonded to C rather than A bonded to T) and can, but usually do not, improve the product coded by the gene. Inserting or deleting base pairs in an existing gene can cause a mutation by changing the codon reading frame used by a ribosome. Mutations that occur in somatic, or nongerm, cells are often not detected because they cannot be passed on to offspring. They may, however, give rise to cancer or other undesirable cellular changes. Mutations in the germline can produce functionally different proteins that cause such genetic diseases as Tay-Sachs, sickle cell anemia, and Duchenne muscular dystrophy.

d. -specialization of cells in multicellular organisms is usually due to different patterns of gene expression rather than to differences of the genes themselves.

• Chapter 8.2
• Chapter 25.1

Gene expression is a process in which a gene codes for a product, usually a protein, through transcription and translation. Nearly all cells in an organism contain the same DNA, but each cell transcribes only that portion of DNA containing the genetic information for proteins required at that specific time by that specific cell. The remainder of the DNA is not expressed. Specific types of cells may produce specific proteins unique to that type of cell.

- Q 3

e. -proteins can differ from one another in the number and sequence of amino acids.

• Chapter 6.3
• Chapter 11.2
• Problem-Solving Lab 297
• MiniLab 299

Protein molecules vary from about 50 to 3,000 amino acids in length. The types, sequences, and numbers of amino acids used determine the type of protein produced.

NE- Q 2L - 3E

f.* -why proteins having different amino acid sequences typically have different shapes and chemical properties.

• Chapter 6.3
• Chapter 11.2

The 20 different protein-making amino acids have the same basic structure: an amino group; an acidic (carboxyl) group; and an R, or radical group (Chemistry Standard 10, "Organic and Biochemistry," ). The protein is formed by the amino group of one amino acid linking to the carboxyl group of another amino acid. This bond, called the peptide bond, is repeated to form long molecular chains with the R groups attached along the polymer backbone.

The properties of these amino acids vary from one another because of both the order and the chemical properties of these R groups. Typically, the long protein molecule folds on itself, creating a three-dimensional structure related to its function. Structure, for example, may allow a protein to be a highly specific catalyst, or enzyme, able to position and hold other molecules. The R group of an amino acid consists of atoms that may include carbon, hydrogen, nitrogen, oxygen, and sulfur, depending on the amino acid. Amino acids containing sulfur sometimes play an important role of cross-linking and stabilizing polymer chains. Because of their various R groups, different amino acids vary in their chemical and physical properties, such as solubility in water, electrical charge, and size. These differences are reflected in the unique structure and function of each type of protein.