AP Biology 2.7 Tonicity and Osmoregulation Study Notes - New Syllabus Effective 2025
AP Biology 2.7 Tonicity and Osmoregulation Study Notes- New syllabus
AP Biology 2.7 Tonicity and Osmoregulation Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
2.7.A : Explain how the structure of biological membranes influences selective permeability.
2.7.B : Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.
Key Concepts:
- Tonicity and Osmoregulation
2.7.A.1 – Hypotonic and hypertonic regions of cells
In healthcare settings and biology labs, it’s often helpful to think about how solutions will affect water movement into and out of cells. The ability of an extracellular solution to make water move into or out of a cell by osmosis is known as its tonicity. Tonicity is a bit different from osmolarity because it takes into account both relative solute concentrations and the cell membrane’s permeability to those solutes.
Three terms—hypertonic, hypotonic, and isotonic—are used to describe whether a solution will cause water to move into or out of a cell:
If a cell is placed in a hypertonic solution, there will be a net flow of water out of the cell, and the cell will lose volume. A solution will be hypertonic to a cell if its solute concentration is higher than that inside the cell, and the solutes cannot cross the membrane.
If a cell is placed in a hypotonic solution, there will be a net flow of water into the cell, and the cell will gain volume. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.
If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell, and the cell’s volume will remain stable. If the solute concentration outside the cell is the same as inside the cell, and the solutes cannot cross the membrane, then that solution is isotonic to the cell.
In the case of a plant cell, however, a hypotonic extracellular solution is actually ideal. The plasma membrane can only expand to the limit of the rigid cell wall, so the cell won’t burst, or lyse. In fact, the cytoplasm in plants is generally a bit hypertonic to the cellular environment, and water will enter a cell until its internal pressure—turgor pressure—prevents further influx.
Maintaining this balance of water and solutes is very important to the health of the plant. If a plant is not watered, the extracellular fluid will become isotonic or hypertonic, causing water to leave the plant’s cells. This results in a loss of turgor pressure, which you have likely seen as wilting. Under hypertonic conditions, the cell membrane may actually detach from the wall and constrict the cytoplasm, a state called plasmolysis (left panel below).
Tonicity is a concern for all living things, particularly those that lack rigid cell walls and live in hyper- or hypotonic environments. For example, paramecia—pictured below—and amoebas, which are protists that lack cell walls, may have specialized structures called contractile vacuoles. A contractile vacuole collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its hypotonic environment.
2.7.B.1 – Growth and homeostasis
Homeostasis is a dynamic process that drives the function of the human body. Homeostasis depends mainly on the collaboration between the nervous system, hormones secreted by endocrine glands and the immune system. Together, they form the intercommunication loop between the mind and the body. Therefore, homeostasis is influenced by mental, physical and chemical factors including the emotional state of the individual. In this review, we will focus on laughter as a positive emotion that can regulate homeostasis and may, to some extent, alleviate the abnormal homeostatic state associated with type 2 diabetes mellitus (T2DM). The therapeutic potential of laughter medicine in delaying the complications of T2DM will be explored.
2.7.B.2 – Osmoregulation
Osmoregulation is a fundamental process of living systems, equivalent in importance to respiration, digestion, or reproduction. Osmoregulatory processes are those that enable a fish to maintain its cellular fluid composition and volume. This is of critical importance because the protein-based processes of cells are sensitive to cytoplasmic ionic concentrations, and cell membranes tolerate relatively small deviations in cell volume (they will either collapse or explode). While terrestrial organisms are continually faced with the problem of dehydration, fishes are surrounded by water, and the water can tend to either dehydrate (in seawater, SW) or hydrate them (in freshwater, FW).
The differences in the ionic and total osmotic concentrations of the water and those of the fish body fluids determine the magnitude and the direction of ion (diffusion) and water (osmosis) movements across the fish gill epithelium. Fishes can limit ion and water exchanges by creating barriers at the skin surface (e.g., scales and mucus layers) that limit permeability. However, they still possess gills that are specialized for gas exchange (large surface area, thin, highly vascularized), which remains an excellent site for osmotic movement of ions and water. In fact, when oxygen and carbon dioxide diffusion across the gills increases during activity, ion and water diffusion increases too. This creates a problem for a fish called the osmorespiratory compromise.
Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. A cell placed in water tends to swell due to gain of water from the hypotonic or “low salt” environment. A cell placed in a solution with higher salt concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or “high salt” environment. Isotonic cells have an equal concentration of solutes inside and outside the cell; this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.
