This module introduces action potentials in the context of understanding nerve impulses. It is intended for an introductory biology audience.

Meiosis is well known for being a sticky topic that appears repeatedly in biology curricula. We observe that a typical undergraduate biology major cannot correctly identify haploid and diploid cells or explain how and why chromosomes pair before segregation. We published an interactive modeling lesson with socks to represent chromosomes and demonstrated that it could improve student understanding of ploidy (1). Here we present an improvement on that lesson, using DNA paper strips in place of socks to better demonstrate how and why crossing over facilitates proper segregation. During the lesson, student volunteers act out the roles of chromosomes while the whole class discusses key aspects of the steps. Strips of paper with DNA sequences are used to demonstrate the degrees of similarity between sister chromatids and homologous chromosomes and to prompt students to realize how and why homologous pairing must occur before cell division. We include an activity on Holliday Junctions that can be used during the main lesson, skipped, or taught as a stand-alone lesson.

The function of the kidneys is to help maintain a constant internal environment (homeostasis) by regulating the volume and chemical composition of the blood. This regulation occurs via three fundamental processes: filtration, secretion, and reabsorption. Because these three processes all concern transfers between the blood and the pre-urine, inexperienced biology students frequently confuse them with each other and with the related process of excretion. Such confusion impairs understanding of the kidney’s regulatory functions. For instance, the effects of H⁺ secretion and HCO₃^- reabsorption on plasma pH can only be predicted if one knows that secretion entails removal from the blood while reabsorption entails addition to the blood. The enclosed three-part lesson teaches these processes through the use of multiple related examples with clinical relevance. In Module A (“Simple Math”), students define the direction of transfer (blood to pre-urine or pre-urine to blood) for each process, create a simple equation to show how excretion rate depends on these three processes, and solve the equation for missing values. In Module B (“Simple Graphs”), students show qualitatively how the three processes affect the composition of the pre-urine and (by implication) the blood. In Module C (“GFR”), students examine the relationship between glomerular filtration rate (GFR) and plasma levels of solutes like creatinine. By presenting multiple related examples embedded in the framework of Test Question Templates (TQTs), this lesson promotes a solid understanding of filtration, secretion, reabsorption, and excretion that can be applied to any naturally occurring substance or drug.

Primary image: Four urinary system processes. This image visually summarizes the four processes covered in this lesson: filtration, secretion, reabsorption, and excretion.