Some substances have to be transported from the extracellular fluid to the inside of a cell. Vice versa, substances from inside a cell must be able to be transported to the extracellular fluid.

Some proteins in the cellular membrane create openings or canals in the membrane. Through these canals some hormones and ions can be transported. These substances are recognized by a receptor molecule in the cellular membrane or are attached to a carrier: a molecule that can pass through the membrane. Since the cellular membrane is quite specific as to which substances are allowed to pass, it is called selective. Permeable means that most substances can easily pass through the membrane, while semi-permeable means that only some substances can pass.

For the transport of substances through the membrane it might be required that a part of the cell’s energy is used. When this is the case, the transport is called active. When no energy is required, it is called passive transport.

Active transport

Sometimes a molecule is simply too large to pass through the cellular membrane. In such a case the cell has to spent some energy to help the molecule in or out of the cell. Some of the protein molecules in the membrane will act as carriers and attach themselves to a molecule. After this the carrier ‘carries’ the molecule through the membrane. In order to do this, energy is required and the transport is called active.

Passive transport

There are three ways in which a molecule can pass the cellular membrane passively.

• Diffusion: transporting molecules from an area with a high concentration of this molecule to an adjacent area with a low concentration, is called diffusion. This happens, for example, in the lungs, where capillaries with a low oxygen concentration are found close to the alveoli with a high oxygen concentration. The oxygen thus passively moves from the alveoli to the capillaries.
• Osmosis: this term is used when water molecules pass through a membrane through diffusion. The concentration of substances in the water is also important with osmosis. An isotonic solution means that the concentration of these substances is equal on both sides of the membrane. The water will not pass through the membrane. When a solution is hypotonic there is a lower concentration of substances and hypertonic means that there is a higher concentration. For example, in the blood salts are found. The normal concentration is isotonic towards the fluid in the blood cells. When the concentration of salts rises, the blood becomes hypertonic, which means that water from inside the blood cells will be transported to the extracellular blood, causing the blood cells to shrink.
• Filtration: this form of passive transport is utilized mostly in the capillaries. These are so thin that diffusion is very easy here. The blood pressure results in the fact that water and molecules can be ‘pushed’ through the membrane of the capillaries, which will act as a filter.