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In order to function, cells are required to move materials in and out of their cytoplasm via their cell membranes. These membranes are semipermeable, meaning that certain molecules are allowed to pass through, but not others. This movement of molecules is mediated by the phospholipid bilayer and its embedded proteins, some of which act as transport channels for molecules that otherwise would not be able to pass through the membrane, such as ions and carbohydrates.
One reason cells are so small is the need to transport molecules into, throughout, and out of the cell. There is a geometrical constraint on cells due to the relationship between surface area and volume that limits the ability to bring in enough nutrients to support a larger cell size. The ratio between surface area and volume (SA:V) decreases as the cell increases in size due to the different scaling factors of surface area and volume. This means that as the cell grows larger, there is less membrane area able to supply nutrients to a greater cell volume.
Some ions are brought into the cell by diffusion, which is the net movement of particles from an area of high concentration to an area of lower concentration. This is known as moving “down” a concentration gradient. Diffusion is net directional; while the net movement of particles is down the concentration gradient, they are constantly moving in both directions due to the random motion of particles. This means that particles in solutions at equilibrium are still moving, but at a constant exchange rate so the solution remains evenly mixed. In an aqueous environment such as the cell, this process involves dissolved ions, known as solutes, moving through water, the solvent. It can take place in an open environment, such as dye spreading through a beaker, or across a cell membrane, such as ions moving through a protein channel.
Water moves across cell membranes by diffusion, in a process known as osmosis. Osmosis refers specifically to the movement of water across a semipermeable membrane, with the solvent (water, for example) moving from an area of low solute (dissolved material) concentration to an area of high solute concentration. In this case, the semipermeable membrane does not allow the solute to pass through. This can be thought of as water moving down its own concentration gradient and involves the same random process as diffusion.
Solutions that are separated by semipermeable membranes can be described as hypertonic, hypotonic, or isotonic depending on the relative solute concentrations in each. A solution that is hypertonic (hyper- meaning “above” in Greek) has a greater concentration of solutes than an adjacent solution, while a hypotonic (hypo- meaning “below” in Greek) solution has a lower concentration of solutes. In this situation, water will move from the hypotonic solution to the hypertonic solution until the solute concentrations are equal. Solutions that are isotonic (iso- meaning “equal” in Greek) have equal concentrations of solute, and therefore do not have a concentration gradient 1.
The capacity for water to move into cells is different between plant and animal cells due to the presence of a cell wall in plants. Cell walls are rigid and only permeable to very small molecules. As water moves into the cell, the membrane is pushed up against the cell wall, creating hydrostatic, or turgor, pressure. This pressure limits the rate and amount of water that can enter the cell. The likelihood of water moving into a cell is referred to as water potential, defined quantitatively as the pressure potential plus the solute potential. The pressure potential is dependent on the pressure inside the cell and the solute potential depends on the solute concentration in the cell.
Water potential can be observed in action in a living plant cell, such as Elodea, an aquatic plant. Under the microscope, a phenomenon called cytoplasmic streaming, or cyclosis, in which cytoplasm and organelles such as chloroplasts move throughout the cell, can be monitored. This process changes visibly when the cells are immersed in different solutions. Interestingly, this motion allows chloroplasts to function more efficiently in photosynthesis; they move in and out of the shadows, collecting photons when they re-enter the lighted regions of the cells3.
The process of osmosis is essential for the mechanism whereby plants get water from their roots to their leaves, even dozens of feet above ground level. In brief, plants transport sugars and other solutes to their roots in order to generate a gradient between the inside and outside of the root; water from the soil then moves in to the root by osmosis. From that point, a process called transpiration results in the water being pulled up tubes inside the plant called the xylem and evaporating out the leaves. Ideally, once this water column is established, it remains intact throughout the life of the plant.4
This naturally occurring phenomenon has been used to develop valuable technologies. One example is in water purification. Recently, NASA has begun to study using the process of forward osmosis to clean and reuse wastewater aboard the International Space Station, as well as for Earth-bound applications. 2 This process uses semi-permeable membranes to remove impurities from water, making it safe to drink. This technology was deployed recently to aid in relief efforts after a severe flood in Western Kenya5.