Facilitated diffusion is particularly important for getting water-soluble molecules such as sugars, e.g. glucose, into cells. Indeed, as we will see in Chapter 5, when we consume sugar, the hormone insulin is released which increases the number of channel proteins in our plasma cell membranes, facilitating the movement of sugar from the blood into our cells where it can be used to release energy in the mitochondria.
Active transport
Diffusion only allows movement of molecules from a high to low concentration; sometimes it is necessary to move molecules against their natural concentration gradients, from a low to a high concentration. Moving material against a concentration gradient requires energy. Fortunately, as we have seen above, cells hold a steady stockpile of energy in the form of the energy storage molecule ATP. Many molecules are continually transported across membranes against their natural concentration gradients, including electrolytes such as sodium (Na+) and potassium (K+) and amino acids. Since this process utilises channel proteins, it can be regarded as an ATP-powered form of facilitated diffusion and is termed active transport.
Active transport can be defined as:
The active movement of molecules against their natural concentration gradients using channel proteins and powered by the energy storage molecule ATP.
Good examples of active transport are the dedicated ion pumps that maintain the correct balance of ions across cell membranes (Figure 1.5). These pumps play a key role in generating electrical signals termed action potentials which are essential to the functioning of the nervous system (Chapter 6).
Figure 1.5 Active transport: sodium, potassium and calcium ion pumps
Osmosis
Osmosis is the process by which water passes passively across the plasma membrane.
The classic experiment to help explain osmosis involves taking a vessel such as a beaker and dividing it into two using a semi-permeable material such as cellophane. Into one side of the beaker a solution of sugared water is added, and to the other side pure water is added. If the experiment is left at room temperature for an hour or so then the pure water will gradually move across the selectively permeable cellophane into the side of the beaker containing the sugared water, and the water level on this side of the beaker will begin to rise (Figure 1.6). The cellophane is referred to as being selectively permeable since it has pores that are just large enough to allow the water molecules to pass through but too small to allow the larger sugar molecules through. All human plasma membranes are selectively permeable and behave like the cellophane in this experiment.
Figure 1.6 The process of osmosis
Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/preface
Of all the mechanisms of membrane transport, osmosis causes most confusion among students. The reason much of this confusion arises is because there are two common definitions provided for osmosis in textbooks. Although these definitions are worded differently, they are effectively saying the same thing.
Osmosis can be defined as:
The movement of water from a region of low-solute concentration to a region of high-solute concentration across a selectively (semi-) permeable membrane.
Osmosis is also frequently defined as:
The movement of water from a region of high water concentration to a region of low water concentration across a selectively (semi-) permeable membrane.
While both definitions are accurate, the second definition is preferable since it highlights that osmosis is actually the diffusion of water through a selectively permeable membrane.
A nice, simple rule to help remember osmosis is that ‘water follows solutes’, or in plain English, ‘water follows sugar, salt or other dissolved material’.
Knowledge of osmosis is essential for nurses to understand how the kidneys function and to understand water balance. Now that you have an understanding of osmosis and diffusion, read through the therapeutic clinical application to develop your understanding of how this knowledge can be applied to a patient with significant kidney disease.
Therapeutic clinical application
Patients with renal failure may undergo peritoneal dialysis in which a catheter is implanted into the abdomen and glucose-rich fluid (known as the dialysate) is infused via the catheter into the peritoneal cavity. The peritoneum acts as a selectively permeable membrane through which excess fluid (and electrolytes and waste products) are drawn out of the blood and into the dialysis fluid. Peritoneal dialysis may be continuous ambulatory peritoneal dialysis (CAPD) in which the dialysate is infused into the abdomen and retained there for approximately eight hours before being allowed to drain. The process is then repeated two or three times a day. Alternatively, automated peritoneal dialysis (APD) may be used in which a machine is used to cycle the fluid into and out of the abdomen. This is usually done overnight.
Isotonic, hypertonic and hypotonic
The term isotonic has become more familiar to the general public with the introduction of isotonic sports drinks. Isotonic solutions are at the same or close to the same concentration as the fluid found in human cells. Nurses routinely use isotonic saline solutions to help keep patients hydrated, particularly when they are confined to bed, unconscious or unable to drink fluids normally. To help you understand the nature and composition of isotonic saline when you are on your next hospital placement, attempt Activity 1.3.
Activity 1.3 Evidence-based practice and research
On your next placement take a few minutes to examine the saline drip bags at the side of your patients’ beds; pay close attention to the chemical composition specified.
What do you notice?
Now that you have an understanding of the composition of isotonic saline, we can explore why these are routinely used in clinical practice.
Human cells are stable in isotonic solutions because the concentrations of dissolved materials (solutes) are equal on both sides of the plasma membrane and so no net movement of water is occurring.
In health, the blood is a near-perfect isotonic medium kept at the same concentration as the cytosol of our cells by a multitude of homeostatic mechanisms. However, in certain diseases human cells can be taken out of their isotonic comfort zone, which can cause damage and in some circumstances become life-threatening.
Dehydration
In patients with diabetes the presence of large amounts of sugar (hyperglycaemia) results in the blood becoming too concentrated. Highly concentrated blood is referred to as being hypertonic (too concentrated) to human cells. In hypertonic solutions water will leave the cells of the body by osmosis and move into the blood, and this can lead to progressive dehydration which is a common presenting symptom in patients with undiagnosed or poorly controlled diabetes.
Dehydration caused by not drinking enough fluids or by severe vomiting or diarrhoea will similarly lead to hypertonic blood and loss of water from cells. As cells lose water, their cell membranes become loose and flaccid and may take on a crinkled appearance; this phenomenon is referred to as crenation. Progressive loss of water from the intracellular compartments can lead to tissues of the body such as the skin becoming noticeably looser, and this can be detected in patients using skin-pinch tests. As