Transport across the cell membrane
Joanna George-Johnson, Contributor

The cell membrane separates the cell's internal environment from its external environment. This is necessary to prevent the contents of the cell from escaping. However, materials need to be exchanged between the two environments so that the life of the cell may be maintained.

Functions of transport across the cell membrane:

• To obtain nutrients
• Excretion of wastes
• Secretion of useful substances (e.g. hormones)
• To maintain optimum concentration of ions and suitable pH for the action of enzymes.

Solute + Solvent = Solution

To understand the concept of transport across the cell membrane these three terms must be grasped:

Solute: A substance which is dissolved
Solvent: A substance which can dissolve another
Solution: Substance formed when a solute is dissolved in a solvent

For example, in a solution of salt and water, salt is the solute, and water is the solvent.

The mechanisms of transport across the cell membrane are divided into two main groups: Passive transport and active transport. It must be stated that in both cases, there needs to be a difference in concentration between the intracellular and the extracellular environments. In other words, a concentration gradient must be set up.

Passive transport

Passive transport requires no energy as substances are moving from a region where they are in high concentration to one where they are in low concentration. There are two types of passive transport - osmosis and diffusion. The difference between the two exists in the fact that different substances are being transported.

Osmosis

There are two ways of looking at osmosis. Students often confuse these two perspectives, so we will take a look at both.

The first is in terms of the concentration of water molecules. From this perspective, osmosis is defined as the movement of solvent particles, usually water, from an area where they are in high concentration to one where they are in low concentration across a selectively permeable membrane.

The second is in terms of the concentration of the solution itself. A concentrated solution has a high solute concentration (thus lower water concentration), while a dilute solution has a low solute concentration (thus a higher water concentration). It takes little thought to realise that water will move from the dilute solution to the concentrated solution, that is, down its concentration gradient. Thus osmosis may also be defined as the movement of water from a dilute solution to a concentrated solution across a selectively permeable membrane.

Examples of osmosis in living systems:

• Water enters the root hairs of plants (try to identify the areas of high and low concentration)
  
  
• Water is absorbed from the colon into the bloodstream.

Diffusion

Diffusion is the movement of solute particles from an area where they are in high concentration to one where they are in low concentration (i.e. down the concentration gradient). Solutes may include gases, molecules and ions. Diffusion, though it may take place across one, does not require the presence of a membrane.

Examples of diffusion in living systems

• Carbon dioxide is taken into the leaf for photosynthesis through the stomata
• Gaseous exchange in the stomata CO2 diffuses out of the bloodstream
  O2 diffuses into the bloodstream.

Active Transport

Active transport is the movement of particles against their diffusion gradient (from low concentration to high concentration). This process requires energy in the form of Adenosine Triphosphate (ATP), thus only respiring cells carry out this process.

Examples of active transport:

• Ion carriers in the cell membrane of red blood cells actively pump sodium and potassium out of and into the cells respectively
  
  
• Companion cells load sucrose into the phloem tubes
  
  
• Ions are actively pumped into root hair cells.

Behaviour of Cells in Solutions

When plant and animal cells are placed in solutions, their behaviour is determined by the relative concentrations of the solutions inside the cells and that outside the cell. In the following cases, when we speak of concentration, we are speaking in terms of the concentration of the solution itself.

Dilute Extracellular Solution

Animal Cell

When an animal cell is placed in a dilute solution, water enters the cell. As more water enters, the cell membrane may rupture, causing cellular death.

Plant Cell

The cell wall of plant cells prevents osmotic bursting. However, the cytoplasm presses against the cell wall, causing turgidity. This is an important support system in herbaceous plants.

Concentrated Extracellular Solution

When an animal cell is placed into a solution that is more concentrated in solute than the solution inside the cell, water leaves the cell and the cell shrinks.

Water also leaves the cell in the case of a plant cell causing flaccidity. In this state, the cell contents no longer exert any pressure on the cell wall, so the cell loses its rigidity.

If water continues to leave the cell, the cell membrane separates from the cell wall, causing plasmolysis. Cell damage occurs, and cellular death is almost certain.

Isotonic Situation

It follows that if the concentration on both sides of the cell membrane is equal then there is no net movement of water. Both solutions are said to be isotonic to each other.

Joanna George-Johnson teaches at Ardenne High School. masterbio@gmail.com