CTS Guide: Cells and Biomolecules: Structure and Function pp. 96-97- Research Summaries
Cell Concept
Research indicates it may be easier for students to understand that the cell is the basic unit of structure than the cell is the basic unit of function. This may be because the former is observable whereas the latter needs to be inferred from investigations (AAAS 2009).
Dreyfus and Jungwirth (1989): Their research acknowledged that the cell is, when first introduced, an abstract concept. Students were also found to ascribe behaviors and functions of organs to parts of cells. For example, some students tend to see the nucleus of a cell more as a miniature brain that commands the cell rather than a cellular part that stores the information used by a cell.
Dreyfus and Jungwirth (1988): Some students merely see cells as structural units and fail to recognize that they also carry out life processes.
Arnold (1983): Arnold’s classic study showed students ages 11-15 confused the concept of cell and molecule. When 11-year-old students were asked to draw molecules, most of their drawings resembled cells and they seemed to have a concept of very small units of living things which Arnold described as a “molecell.” Students ages 14-15 thought things associated with living things were made of cells.
Animal and Plant Cells
Clément (2007): This researcher noted that “the cell concept is generally introduced by two juxtaposed drawings, a plant cell and an animal cell.” The drawings show common, labeled features of animal and plant cells. The plant cell is generally depicted as polygonal and attached to other cells while the animal cell is more rounded in shape and isolated. If students are not presented with a greater variety of images of cells it could introduce or reinforce the misunderstanding that all animals’ cells and all plants’ cells have the same shape and structures as these two typical depictions; Clément found this misunderstanding persisting in students all the way up to undergraduate level. It was suggested that it may be helpful for students to understand that the typical textbook representations of animal and plant cells are models. Not all animal and plant cells have the same shape or structures as those depicted in the models; but the models are a useful description of the common features of animal and plant cells to help us understand there are differences. Also, the study suggested that limiting students’ experiences with cells to just animal and plant cells (typically epidermal cells from onion and human cheek) that they typically observe through microscopy or cell imagery can introduce or reinforce the misunderstanding that there are only these two kinds of cells.
Flores, Tovar, and Gallegos (2003): Despite instruction, some students fail to recognize that plants are made of cells and that all tissues are made of cells. Some students thought only a small part of tissues were made of cells.
Cell Shape and Size
Haşiloğlu and Eminoğlu (2017): When students were asked to draw cells, the drawings revealed a number of misunderstandings held about what cells look like, their size, and the relationship between cells and organisms. The study showed that combining drawing cells with observing cells through a microscope rather than primarily looking at photos and representations of cells was effective in helping students overcome some of their misunderstandings.
Vijapurkar, Kawalkar, and Nambiar (2014): Research has shown that students at age 11-14 resist accepting that cells are three-dimensional objects. They tend to view cells as being flat. It is suggested that students build 3-D models of cells, rather than just drawings, to overcome this.
Clement (2007): Students tend to see the cell as a “fried egg” model- 2 concentric circles with one circle being the cell membrane and the other the nucleus, often lacking other structures and emphasizing these two.
Stavy and Tirosh (2000) asked students in grades 7–12 to compare muscle cells of a mouse to muscle cells of an elephant. The majority of students, especially in grades 7 and 8, thought that larger animals have larger cells. The common justification was that “according to the dimensions of the elephants and those of the mice, it is obvious that the muscle cells of the mice are smaller than those of the elephants” (p. 30). This is an example of the intuitive rule “more A, more B.” Most of the younger students who answered correctly explained the equality in terms of the cells having the same function and therefore being the same size. Most of the high school students who responded correctly used formal biological knowledge of cells and also described the elephant as having more cells.
Driver et al. (1994): Several researchers have reported that children aged 11-16 lack an appreciation of size and scale, and that this impacts their understanding of the relative sizes of cells and other biological structures. Difficulties students have in comprehending scale leads to confusion about the relationships between atoms, molecules, and cells. Some students also believed that there are organisms too small to be made up of cells.
Studies have shown that students have difficulties with orders of magnitude. In a study of 16-year-old Israeli students (Dreyfus and Jungwirth 1988, 1989), students thought that molecules of protein were bigger than the size of a cell.
Arnold (1983): Some children aged 11-16 believe that atoms, molecules and cells are all the same size.
Levels of Organization
Rogat, Hug, and Duncan (2017): Reasoning across levels of organization may be counterintuitive to students as structure and function at one level may look entirely different at a higher level. The related understanding that small changes at one organization level, such as a protein’s structure, can result in very large changes at the organism level may also be counterintuitive.
Students have difficulty conceptualizing which parts of living organisms are made of cells. They may think that only some tissues, such as brain tissue or the blood, are made of cells.
Studies of children’s ideas related to the organization of the human body reveal that between the ages of 8 and 10 children begin to understand that the body is made up of organs that work together to maintain life (Driver et al. 1994).
Dreyfus and Jungwirth (1988, 1989) Students tend to think the body “contains” cells rather than the body is made up of cells. They tend to view cells as being contained in some type of “sac.” They also found that 16-year-old Israeli students revealed confusion about levels of organi- zation in living things, including the idea that single-celled organisms contained organs such as intestines and lungs, even though they had been taught about cells in previous years.
Single Celled Organisms and Number of Cells
Driver et al. (1994): Some students believed that a single cell is not alive and that living things cannot be made up of only one cell.
Dreyfus and Jungwirth (1988, 1989): 16-year-old Israeli students thought single-celled organisms contained intestines and lungs. These students had been taught about cells in the previous year.
AAAS Project 2061 Assessment Study: In a large sample of middle and high school students, 37% thought that there are no single-celled organisms, believing instead that the smallest number of cells an organism could be made up of is “about 100”; a further 11% thought that “about 100” is the largest number of cells an organism could be made up of, and another 11% thought the maximum to be “about 1000” cells (AAAS Project 2061)
Biomolecules
The Project 2061 Assessment Project (BSCS Science Learning 2023) asked middle and high school students what the functions of proteins are in animals. 48% of middle school students correctly identified that protein molecules help cells carry out many of their functions and are part of body structures such as hair and nails. 61% of high school students answered the question correctly. Common misconceptions were that actions of protein molecules do not affect the basic functions of cells and that the actions of protein molecules do not affect an organism’s body structures.
A study by Thörne and Gericke (2014) found that teachers talked differently about both the importance and functions of proteins. As a result, students have widely differing opportunities to learn about the function of proteins.
Arnold (1983): Students who studied items such as proteins, carbohydrates, and water in a biology class thought they were made of cells. Many of these students also thought living organisms were not made of molecules, but heat and energy were. They seemed to confine the concept of molecules to chemistry and physics. Arnold’s study found students had difficulty differentiating between the concepts of cell and molecule. Students identified any materials encountered in biology class (carbohydrates and proteins) as being made up of smaller units called cells. Arnold coined the term molecell to describe the notion of organic molecules being considered as cells.
Dreyfus and Jungwirth (1988, 1989): 16-year-old Israeli students thought molecules of protein were larger than a cell.
Cell Diffusion, Osmosis, and Cell Membranes
Researchers have uncovered several beliefs of how students thought substances diffused through cell membranes. This includes the common belief, even at the university level, that substances only move in only one direction- from higher concentration to lower concentration, which is a failure to understand the random movement of particles and the concept of net movement (Stains and Sevian 2015, Otzas and Otzas 2016).
Sanger, Brecheisen, and Hynek (2001): These researchers concluded that explaining diffusion requires students to have an understanding of concepts from chemistry and physics, including the particulate nature of matter and the behavior of particles in solutions. Students may struggle to understand and explain diffusion because of the need for them to be able to visualize and think about the process at the particle level.
Odom and Barrow (1995): Results of the “Diffusion and Osmosis Diagnostic Test” revealed common misunderstandings about diffusion including that molecules move only in one direction, from an area of higher concentration to an area of lower concentration (a failure to understand the random movement of particles versus the concept of net movement);and movement of particles stops after the concentration gradient between two areas has been equalized by diffusion (possibly because students interpret “no net movement” to mean “no movement of particles”). These tests also revealed the common misunderstanding that diffusion across a cell membrane requires energy from a cell (or in some other way depends on the presence of a living cell), and would stop if the cell died. Odom also defined a list of pre-requisite knowledge statements required for understanding cell diffusion including: 1) all particles are in constant motion, 2) diffusion involves the movement of particles, and 3) diffusion results from the random motion and/or collisions of particles (ions or molecules), and 4) diffusion occurs in living and non-living systems. These tests have revealed common misunderstandings about diffusion amongst students, including that diffusion across a cell membrane requires energy from a cell (or in some other way depends on the presence of a living cell), and would stop if the cell died.
A study by Marek (1994) revealed that some students thought that a substance diffuses through water because the water is semi-permeable (or selectively permeable). These students most likely misapplied a term they heard in the context of diffusion across membranes and did not really understand what it means.
Dreyfus and Jungwirth (1988) found that some 16-year-olds had anthropomorphic views about cell membranes, such as intentionality, believing that cell membranes knew what kinds of things a cell needs and that it could decide what could pass in and out.
Cell Organelles
Cherif et al. (2016) suggests that getting students to role-play as cell organelles can help to engage them in deep learning about cell structures and promote conceptual change.