could occur.
Membrane transport
To stay alive and function optimally, each cell has to take up useful molecules (such as oxygen, water, salts, sugars and amino acids), and eliminate waste products such as carbon dioxide, urea and uric acid. Movement of materials across the plasma membrane is called membrane transport.
Simple diffusion
Since the plasma membrane is a fluid structure consisting predominantly of phospholipid, molecules that are fat-soluble are able to dissolve in the phospholipid bilayer and pass rapidly across by a process called simple diffusion.
Simple diffusion can be defined as:
The passive movement of molecules from a region of high concentration to a region of low concentration until an even distribution of molecules (equilibrium) is achieved.
Gases such as oxygen and carbon dioxide and lipid-based hormones such as steroids including testosterone and oestrogen are highly soluble in the fluid phospholipid bilayer and pass readily into and out of cells via simple diffusion. Simple diffusion also occurs rapidly in the lungs with oxygen inspired at high concentration from the atmosphere before diffusing rapidly across the alveolar air sacs into the blood. Conversely, carbon dioxide is at high concentration in the blood and passes across into the alveoli by simple diffusion before being eliminated during expiration. Concepts such as diffusion are often very abstract in nature, so to help consolidate your understanding of this process, attempt Activity 1.2.
Activity 1.2 Team working
Add a small amount of perfume, aftershave or nail varnish remover (acetone) to a piece of tissue and place it in the centre of the room.
What do you notice?
Now that you have had the opportunity of exploring simple diffusion via the diffusion of odours, we can examine a variant of diffusion.
Facilitated diffusion
Since many of the molecules required by cells are water soluble and not particularly soluble in lipid, they cannot pass across the plasma membrane by simple diffusion. Facilitated diffusion makes use of channel proteins which function as physical passageways to carry molecules across the plasma membrane.
Facilitated diffusion can be defined as:
The passive movement of molecules across the plasma membrane from a region of high concentration to a region of low concentration aided (facilitated) by membrane channel proteins.
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
