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There are a variety of ways in which substances can get across the membrane. These are all important throughout A-level biology.

  • Diffusion
  • Osmosis
  • Facilitated diffusion
  • Active transport

Lipid soluble and very small molecules can diffuse down their concentration gradients across the membrane. This is because their molecules can pass between the phospholipid molecules. Substance which move across the membrane by diffusion include steroids (lipid soluble) and the respiratory gases (O2 and CO2) (very small molecules). The energy used is the molecules own kinetic energy.

Figure 3 : Diffusion
Diffusion

Osmosis is the diffusion of water across a plasma membrane. As water molecules are small they are free to diffuse across the membrane. However osmosis is a little different to simple diffusion, as the most important factor affecting the water molecules is their ability to move, not their concentration.

The water molecules present in distilled water at a set temperature have a certain amount of kinetic energy. If solutes are added to the water they dissolve. The dissolved substances attract water molecules to their surface, and each solute particle is surrounded by water molecules. The water molecules that are associated to the solute molecules are less able to move freely. The average kinetic energy of water molecules is therefore reduced. If a plasma membrane separates some distilled water from a strong solution (solution containing a lot of solute), then water molecules will tend to move by osmosis from the strong solution to the distilled water. This is shown in the figure 4 below.

Figure 4
Movement across membranes

Even though the majority of water molecules are moving in one direction there will always be some individual molecules that move in the opposite direction. That is why it is most accurate to use the term net osmosis.

The ability of the water molecules to move can be quantified as their water potential. This is a measure of the potential the water molecules have to move. The water potential of distilled water is zero. As distilled water has molecules which are the most able to move, all other solutions must have negative water potentials. The more solute present the more negative the water potential. Water potential is measured in units of pressure (kPa).

Water always moves by net osmosis from regions of less negative water potential to regions of more negative water potential, across plasma membranes.   Exam Advice   Exam Advice

Sometimes the water potential of the cytoplasm of a cell is referred to as its' osmotic potential.

Isotonic, hypotonic and hypertonic are terms that describe the concentration of solution relative to cells placed in the solutions. Remember the greater the concentration of a solution the more negative its water potential will be.

Isotonic = the same concentration as

hypotonic = less concentrated than

hypertonic = more concentrated than

The effect of placing cells in different solutions is shown in figure 5 below.

Figure 5
different solutions

Some molecules, which are too large to fit between the phospholipids, can be transported across the membrane through specific carrier proteins. These proteins do not provide energy to move the molecule, they just provide a pathway that it can move through. There are two main types of carrier protein, which are shown in figure 6.

Figure 6 : Two main types of carrier protein
Carrier protein
  1. Channel Proteins these form a pore through which the molecules can pass.
  2. Carrier Proteins which act a little like turnstiles at a football ground. The molecule actually interacts with the protein as it passes through.

Often other molecules are involved in opening and closing or blocking carrier proteins, allowing the cell control over the transport of substances by facilitated diffusion.

Active transport involves moving molecules across the membrane through carrier proteins. The key feature of active transport is that the molecules move against their concentration gradient. ATP is required by the process, it provides the energy required to change the shape of the protein and move the molecule across the membrane. A different type of protein is required for each substance transported, so for example the protein that transports calcium ions can not also transport glucose.

The most common active transport protein is the one that transports sodium and potassium ions. It's action is shown in figure 7.

Figure 7
The most common active transport protein is the one that transports sodium and potassium ions

Larger molecules (e.g. polysaccharides) cannot cross the membrane by any of the methods already covered. They can enter the cell by endocytosis.

Endocytosis is where a vesicle forms around the substance to be transported, and the substance enters the cell within the vesicle. This is shown in figure 8.

Figure 8 : Endocytosis
Endocytosis

The reverse process by which large molecules can leave the cell is called exocytosis.

Summary of Membrane Transport



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