Before instruction, students often believe and can:
Students who view the world in this way believe and can:
Students who fully understand this topic believe and can:
Structures and function of important biological molecules
Students will likely have heard of terms like protein, fat, and sugar as part of their diet.1 Students are also more likely to think about the role of these substances as food – as simply anything useful that that is ingested (AAAS, 1993).
Students typically have difficulty distinguishing the difference between cells, molecules and atoms. They often confuse the different entities and have difficulty representing or identifying where they are located. (CPRE, 2009). Some students are unlikely to recognize that living organism or cells are made of molecules.
[Naive and early ideas about DNA or genes are described in genetics]
Structures and functions of important biological molecules2 Relate and distinguish the size and relationship between cells, molecules, and atoms. Recognize that living cells and all of the subcomponents are comprised of molecules. [Link to matter in chemistry] Recognize that carbon is a common element among all biological molecules found in cells.
Recognize that there are simple or small molecules, like water and glucose inside of cells, and much larger and complex molecules like proteins and DNA.
[Link to location of genes in genetics] Proteins: Describe proteins as little machines that carry out the work of cells and realize proteins are molecules that have specific shapes (e.g., like a tiny glove that catches a ball, or a tiny channel through which only certain shaped objects fit). Describe proteins as a long chains folded on itself to form a particular shape.3 DNA (one of the 2 nucleic acids): Describe DNA as a long chain molecule4 packed inside of cells that have a role in influencing traits. Realize DNA/genes provide information to build proteins.5
[Link to gene function in genetics] Carbohydrates: Identify carbon as a key element in carbohydrates and explain that carbohydrates are a food substance because they provide a source of energy and building material for the cell. Recognize that smaller sugar molecules can be linked together to form long chains of sugars and locate places in cells where such sugars can be found.
Lipids or fats: Recognize that fats can be burned (i.e. broken down) to release energy and that lipids can be found in cell membranes. Therefore they can be considered a source of energy and building material for the cell and satisfy their scientific classification as food for living organisms. Identify long strings of carbon atoms as a distinguishing characteristic of this type of molecule.6 Recognize that DNA, proteins, and some sugars are long “chains” that can be broken down into their small subunits (or “chain links”). Conversely recognize that these molecules can be reassembled from smaller subunits or molecules.
[Link to digestion and metabolism in anatomy and physiology] Possible misconception:
Scale and size issues of cells and molecules continue to pose problems here. Students are most likely to associate proteins with nutrition and not as a molecule inside of cells that carry out work. (Duncan, n.d.). Given that students often do not see the break down and reassembly of organic molecules in food chains (AAAS, 1993) it is likely they likely are not going to see this occurring in organisms either.
Structures and functions of important biological molecules7
Students should know that cellular structures are made of biologically important molecules: nucleic acids (DNA, RNA), proteins, carbohydrates, and lipids.
Each of these has a characteristic structure through which it interacts with other molecules; its functioning emerges from these interactions.
Each is composed primarily of atoms of carbon, nitrogen, oxygen, hydrogen, phosphorous, and sulphur; but they require interactions with minerals such as iron and calcium to accomplish their functions.
Each molecule can be assembled and disassembled through biochemical reactions.
Explain that biological organisms are comprised primarily of very few elements, with the six most common being C, H, N, O, P, and S. Explain that carbon is an especially important element and serves as a major atomic building block in important biological molecules such as nucleic acids (DNA and RNA), proteins (e.g. enzymes), carbohydrates (sugars), and lipids (fats).
Describe the basic molecular structures and primary functions of the four major categories of organic molecules (carbohydrates, lipids, proteins, nucleic acids).8 Describe proteins as being comprised of smaller carbon-based subunits, amino acids, that are chained together, and nucleic acids (DNA and RNA) as being comprised of smaller units that are carbon-based nucleotides. Finally, describe lipids as being comprised of long chain of carbon atoms.
Explain that biological molecules have properties and structures that are key to their function in cells. In particular, describe proteins as essentially chains that fold into a multitude of shapes to carry out an infinite variety of functions inside of cells. Realize that amino acids linked together have particular properties that influence how the amino acids interact with one another influencing how the proteins fold (e.g. hydrophobic, hydrophilic, positively charged, negatively charged, form bends or bridges).9 DNA or RNA. Realize the nucleotide sequence determines the sequence of amino acids in proteins.
[Link to genetics]
Lipids. Describe lipids as being long carbon-hydrogen chains are hydrophobic, which means they repel water and tend to stick together. This allows the cell to control the flow of substances into and out of the cell, in particular, water and substances dissolved in water.
Centrality of protein function Possible misconception:
Student likely to think about proteins merely as a dietary need. (Duncan, n.d.) Students are likely to have heard of the term protein, but in the context of food they take in or specific foods that are high in protein like meat, milk, or beans.10
Centrality of protein function11 Recognize that proteins carry out work insides of cells.
Describe or predict a few types of basic cell functions that are mediated by proteins12 including transport of molecules into or out of cells through channel proteins (e.g. the transport of glucose into cells), enzymes that break down or assemble other cellular molecules during growth (e.g. the break down of glucose inside of cells), and breaking down simple sugars inside of cells during cellular respiration. Explain that such activities are mediated by proteins.13Recognize that proteins are found in both plant and animal cells.
There is the possibility that if students focus on proteins as carrying out work inside of cells (like little machines) and also maintain their strong association with protein as a dietary substance, they may think proteins are taken in and used by cells “as is.”14
Centrality of protein function15 Students know that proteins play a central role in the structure and function of cells.
Explain the central role that proteins play in carrying out many of the basic functions that cells must undergo to survive16 (replication—requiring biosynthesis of cellular molecules for growth and repair; breaking down food—to extract energy and building material; getting rid of waste; responding to the environment and communicating with other cells) and provide examples of some basic types of proteins (e.g. enzymes that facilitate chemical reactions, membrane bound channel proteins that allow molecules to flow through membranes, ligands that bind to membrane-bound receptors and facilitate inter-cellular communication, and cytoskeletal proteins that provide structure) and describe their basic activities or functions.17 Students realize proteins taken in through diet are broken down into smaller units during digestion and reassembled into proteins during cellular metabolism.
Students realize proteins are built inside of cells and perform critical functions inside of cells. Students realize proteins are found in all cell types in all organisms.
Recognize one major function of proteins is to act as an enzyme. Explain that enzymes enable chemical reactions to occur that normally would take much, much longer to occur without assistance. Explain the structure of an enzyme is critical to its function and that if the structure is changed, due to changes in the environment like pH, it can not perform its function.18
Early on many students focus on movement as a basic characteristic of all life (Driver, 1994). Later they may include notions related to requirements for food and its role in providing energy (but not necessarily cellular building materials) (CPRE, 2008). However, cells typically do not become part of students’ conceptions of essential attributes of life early on (Driver, 1994)
Students likely believe plants are living.
However, children may over-generalize or anthropomorphize plants to think that plants eat, sleep, etc. like humans. (NRC, 2007) Student often do not appreciate that food is a source for BOTH energy and building material. Water, vitamins, and minerals are often mistaken as food – simply because anything ingested is considered food. (AAAS, 1993). A particularly strong misconception is that plants take in food (from the soil) as opposed to generating their own food through photosynthesis (NRC, 2007).
Cellular reproduction: Most students know that parents produce babies and that reproduction appears to be common to all animals – but may not apply reproduction to all organisms.
Cells as a central structural and functional unit Explain that individual cells carry out all the basic functions of any living thing.
Explain that all organisms are composed of cells, and that many organisms are single-celled (unicellular, e.g., bacteria). Relate through models or drawings subatomic structures such as DNA or proteins, cells, tissues, organs, and systems in plants and animals.19 Possible misconceptions:
Students may not believe that plants or fungi are composed of cells. Even if they believe animals or plants are made of cells they will have difficulty identifying what is composed of cells and what is not in an organism. They will not be able to move between different levels of biological organization easily – e.g. cell to tissue to organ to system to whole organism (CPRE, 2008). Early on children may see cells as inanimate objects (NRC, 2007).
[This has relevance to anatomy and physiology as well] Realize that all living things need food – including plants – and that food includes only proteins, carbohydrates, and fats. Explain that food (glucose being one type) is used as both a source of energy and as building materials in organisms.20 Realize that plants produce their own food (glucose/simple sugar) and animals must take it in by eating other organisms (like a plant or another animal).
[Link to ecosystems] Explain that both animal and plant cells must extract energy from food – and that both animal and plant cells break down glucose into smaller molecules and that energy is released during this process. Students should describe the mitochondrion as a cell structure where energy is released and transformed into chemical energy that the cell can use later.21 In this process carbon dioxide and water is released.
[Link to digestive and circulatory systems 22] Identify mitochondria in both plant and animal cells, and chloroplasts in plants cells. Explain why chloroplasts are in plant cells and not animal cells.
Photosynthesis & respiration: 23Understand that plants take in water and carbon dioxide from their environment and capture sunlight and use it as energy to produce food (glucose) and oxygen. The glucose is stored in the plant while the oxygen is released.
Recognize that photosynthesis and cellular respiration are chemical reactions where principles of conservation of matter apply.
Track and identify energy types and transformations during processes that involve chemical reactions such as photosynthesis and respiration.24
[Link to chemistry and physics strands] Possible misconceptions:
Students confuse the process of photosynthesis and respiration. They often do not appreciate that plants also carry out respiration. Students sometimes conflate breathing (exchange of gases carried out by lungs) with cellular respiration (Anderson, n.d.; CPRE , 2008; Driver et al. 1984). Some students may over-generalize functions of cells (NRC, 2007). Cellular reproduction: Explain that all organisms need to reproduce (replicate), including single-celled organisms like bacteria. Explain that animals and plants, which are multi-cellular, reproduce by producing specialized cells (e.g. sperm and egg in animals) and these sperm and egg must fuse together to produce a new organism. After fusion, growth occurs as a result of multiple cell divisions, generating more and more cells.
Recognize that some organisms have two sexes and that when they reproduce the offspring are not identical to the parents, but instead there is variation among the offspring and the offspring are a mix of both parents. In contrast some organisms come in only one form and when they reproduce the progeny are completely identical to the parents.25 Possible misconceptions:
Students confuse sexual reproduction with copulation and therefore may not associate meiosis with sexual reproduction (CPRE, 2008).
Cells as a central structural and functional unit Students know that the cell is the fundamental structural and functional unit of living things, which either as a single cell or as a system of cells, accomplish functions such as
maintain an internal environment
growth and repair, and
Describe that all organisms are composed of cells.26
Explain that in all individual cells proteins facilitate all the basic functions that living cells need to carry out, even if they are a cell that constitutes a single-celled organism like a bacterium. Also explain that in all organisms’ genes in DNA provide instructions for the assembly of proteins and influence what proteins are present and how they function.
[Link to proteins in cells and genetics] Realize (or predict) that for all the basic functions of life that all cells must carry out, one can find similar, or even the same proteins inside of the cell.27 Predict (or draw) what you would find if looked at different tissue samples from plants and animals under the microscope (e.g. tissue from intestine, stomach, lung, brain, muscle, skin, or tissue from leaf, root, or stalk).28 In multi-cellular organisms like plants and animals, be able to compare and contrast two different cell types (e.g. brain cell vs. muscle cell in animals or leaf cell vs. root cell in plants).
Draw models of what sub-cellular structures one could find inside of the following types of organisms: a plant cell, an animal cell, a bacterium, a fungi cell, and a protist. Predict if one would find proteins for cellular respiration, assembly of DNA molecules, assembly of protein molecules, and photosynthesis (generated glucose using light energy) in a plant cell or an animal cell. Identify where these proteins might be located.
Photosynthesis & respiration: Identify the reactants, products, and basic purposes of photosynthesis and cellular respiration.29 Explain the interrelated nature of photosynthesis and cellular respiration in the cells of photosynthetic organisms. Identify where cellular respiration and photosynthesis occur at the sub-cellular level.
Explain the important role that ATP serves in metabolism.30 Describe how ATP is generated when food substances are broken down (e.g. during cellular respiration) and is used by proteins to perform work inside of cells (e.g. when protein enzymes build cellular molecules like DNA during cellular reproduction).31
Cellular reproduction: Describe the cell cycle and the process of mitosis.32 Explain the role of mitosis in the formation of new cells, and its importance in maintaining chromosome number during asexual reproduction.
Describe how the process of meiosis results in the formation of haploid cells. Explain the importance of this process in sexual reproduction, and how gametes form diploid zygotes in the process of fertilization.
Compare and contrast mitosis and meiosis in terms of the number of chromosomes at the beginning and end of each cycle, and in what cell types from multi-cellular organisms these cellular processes occur.
Sub-cellular structures and diversity Students have some understanding that something inherent in living things makes them different from non-living things and they can identify many living things from non-living things at a very young age (NRC, 2007).
Some students will generally realize plants are living.
Some students will have heard the term germs – but may or may not associate them with bacteria or viruses.33 Students may not recognize that bacteria are cells or that they are living organisms (Driver et al., 1984) or that they have DNA (CPRE, 2008).
Sub-cellular structures and diversity 34 Draw models of a few basic sub-cellular structures in cells: namely the cell membrane, the nucleus, mitochondria, and the chloroplast.35 Possible misconceptions:
Students often confuse the relationship between a cell, a nucleus and biological molecule like proteins and DNA. Students often do not realize that plants undergo respiration like animals. (CPRE, 2008). Some students may anthropomorphize cells and over-generalize thinking that the nucleus directs all cell processes and that cells make decisions (NRC, 2007). Some students may even think that some cells have little lungs or little stomachs (CPRE, 2008). Students apply atomic molecular theory to living organisms and understand that cells and all their sub-components are composed of molecules too(NRC, 2007).
[Link to atomic molecular theory] Explain that bacteria must carry out all of the characteristics of life just like other living organisms, e.g. need to replicate (biosynthesize molecules), extract energy from food (cellular respiration), get rid of waste, and respond to the environment. Understand that bacteria have DNA.
Describe that viruses are smaller than bacteria and are not made of cells so they need a host cell to replicate themselves.36 Recognize that viruses do not carry out all the basic functions that cells do.
[Link to cell function above] Possible misconceptions:
Students often do not appreciate the small size of viruses compared to cells (or even tiny cells like bacteria). They may even confuse viruses and bacteria. (Rogat, n.d.)
Sub-cellular structures and diversity Students know that there are a variety of sub-cellular parts that have specific structures and do specific functions for the cell. Sometimes these sub-cellular structures come from outside the cell and use the sub cellular structures of the cell to carry out their own functions.
Draw models of structures one would find in a plant cell and an animal cell if looked under a powerful microscope: include sub-cellular parts/organelles (e.g. plasma membrane, nucleus, cytoplasm, mitochondrion, endoplasmic reticulum, Golgi apparatus, ribosome, vacuole, cell wall, chloroplast, cytoskeleton). Relate these structures to their functions.37 Explain the role of cell membranes as a highly selective barrier (diffusion, osmosis, facilitated diffusion, and active transport). Predict what would happen if one of these structures was removed or ablated.38 Compare and contrast, at the cellular level, the general structures and degrees of complexity of prokaryotes and eukaryotes. Predict what structures one would find if one looked under the microscope at organisms from these two different kingdoms. Compare and contrast different cell types from animal and plant cells.
Compare and contrast a virus and a cell (in terms of genetic material, structure and size, and reproduction, and other basic requirement of living organisms).39