The part played by the movement of substances across cell membranes in the functioning of
different organs and organ systems.
The movement of substances across membranes is crucial for the nervous system to function.
At resting potential, when a neuron is not conducting an electrical impulse, a membrane
potential of -70 mV is maintained via a sodium-potassium pump. 3 sodium ions (Na+) are
actively transported out of the neuron through its membrane, and 2 potassium ions (K+) are
transported in. This established an electrochemical gradient in which there is a greater
movement of K+ into the neuron than Na+ out of the neuron by facilitated diffusion due to the
membrane being more permeable to K+. Upon detection of a stimulus, voltage-gated Na+
channels open, causing an influx of Na+ into the neuron. Once the threshold of +40 mV is
reached, depolarisation of the neuron occurs, and it now conducts an action potential.
Na+-gated channels close and K+-gated channels open to enable repolarisation. After a
refractory period of hyperpolarisation, the neuron returns to its resting potential of -70 mV.
Without the movement of Na+ and K+ across the neuron membrane, the nervous system would
not function, as it is this movement that enables depolarisation of the neuron and allows the
transmission of an action potential through it. It is important that the nervous system functions
properly to enable a rapid response to environmental stimuli and therefore, the survival of an
animal.
In the digestive system, it is important that the small, soluble products of digestion are able to
move across membranes into the blood to be used by cells. In the co-transport of glucose into
the blood, sodium ions (Na+) are actively pumped out of epithelial cells lining the ileum into the
bloodstream. This establishes a Na+ concentration gradient in which there is a higher Na+
concentration in the lumen than in the epithelial cell lining. Na+ and glucose can now enter the
epithelial cell down this concentration gradient by facilitated diffusion via a sodium-glucose
co-transporter protein in the plasma membrane. From the epithelial cells, glucose can simply
enter the bloodstream down a concentration gradient by facilitated diffusion. This movement of
substances across membranes is important, as otherwise the products of digestion would not
be absorbed from the small intestine and, as a result, could not be used by the body. In the case
of glucose, it is essential that this molecule can pass through membranes to enable both
aerobic and anaerobic respiration to occur in cells and produce ATP.
The gaseous exchange system in fish involves the gills. Here, oxygen and carbon dioxide readily
diffuse between the membranes of capillaries within the gill lamellae and water. In the gills,
blood and water flow in opposite directions; this countercurrent flow ensures a steep
concentration gradient is maintained across all points of the gill lamellae and absorbs as much
oxygen into the blood capillaries from the water, maximising the efficiency of the fish gas
exchange system. Oxygen diffuses from water into deoxygenated blood and carbon dioxide out,
each along their respective concentration gradients. A similar mechanism occurs at capillary
