How do substances manage to cross the plasma membrane?

Not all particles can actually pass through a plasma membrane unaided. This is because of the largely lipid nature of the membrane. To pass through the plasma membrane by simple diffusion particles must be small, lipid soluble and non-charged. This excludes particles such as ions (they are charged), sugars and amino acids (they are not lipid soluble and are not small particles) and any of the really large particles, such as proteins.

We can group the processes by which substances cross plasma membranes into two main types:

• passive processes – these processes rely only on the kinetic energy of the particles of the substances and on concentration gradients; they need no extra energy from the cell’s metabolism

• active processes – these require energy from the cell’s metabolism in the form of ATP to drive the transport.

Passive processes

Simple diffusion

In fluids – liquids and gases – the particles that make up the fluid are free to move around. This kinetic energy is what drives diffusion. If particles are, for some reason, concentrated in a small area, they will move in such a way that the particles ‘spread out’ and occupy all the space that is available to them. This is a result of random particular motion. Diffusion need not involve a membrane. When particles diffuse across a plasma membrane, there must be a concentration difference between the two sides of the membrane (a concentration gradient) to drive the process. As diffusion proceeds, the high concentration will decrease and the low concentration will increase until the two concentrations are the same. At this point there will be no further net diffusion. This means that although particles will still move across the membrane, they will move equally in both directions, so there will be no overall effect. We say that the concentrations are in equilibrium.

Th rate at which diffusion across a membrane takes place is influenced by:

• the concentration gradient – a bigger difference in concentration results in faster diffusion than a smaller gradient

• the thickness of the membrane – as all plasma membranes are the same thickness, this is not really an issue when considering diffusion into and out of cells, but for other situations where particles must cross some kind of barrier, a shorter distance results in faster diffusion

• the surface area of the membrane – clearly if there is more membrane where diffusion can take place, diffusion will happen faster

Facilitated diffusion

Facilitated diffusion is essentially the same process as diffusion, in that it depends on a concentration gradient to allow particles to cross the membrane. However, it differs in that the particles must be helped to diffuse across the membrane (their diffusion must be ‘facilitated’) by a carrier protein or a channel protein with an ion pore. Note in both cases that the particles are moving from a high concentration to a low concentration (as with simple diffusion).

However, also note that whilst the ions can simply move straight through the ion pore of a channel protein, the carrier protein must undergo a conformational change (change in shape) to move particles through the membrane. Th rate of facilitated diffusion is affected by the same factors that affect simple diffusion with the exception that it is not the actual surface area of the membrane that determines the rate, but the number of carrier proteins (or channel proteins) present.

Osmosis

Osmosis is the process by which water moves across a partially permeable membrane. It is, effectively, the diffusion of water. However, we do not refer to the concentration of water molecules, but to water potential. We can say that osmosis is the movement of water from a system with a high water potential to a system with a low water potential across a partially permeable membrane. The symbol for water potential is the Greek letter Ψ (psi). Water potential is measured in units of pressure – pascals (Pa), kilo-pascals (kPa) or mega-pascals (MPa). Pure, liquid water has a higher water potential than any other system. It is defied as zero: Ψ (pure water) = 0 Pa All other systems (cells, solutions and suspensions) have a water potential that is lower than that of water. Therefore, their water potential values must be negative. So we can define osmosis more accurately as follows: Osmosis is the movement of water from a system with a high (less negative) water potential to one with a lower (more negative) water potential, across a partially permeable membrane.

Active processes

Active transport

Sometimes, substances must be moved against a concentration gradient – from a low concentration to a higher one. This cannot happen by diffusion, since it would tend to concentrate particles rather than spread them out. It can only happen if metabolic energy is used to drive the process. In living organisms, this energy is released from the ATP produced in respiration. When the energy is released from ATP, it is broken down into ADP and P i (inorganic phosphate). Th proteins used to actively transport substances across plasma membranes are called pumps. Endocytosis In this process, large particles are engulfed by a cell. There are several ways in which it can happen, but, essentially, part of the plasma membrane surrounds the particles to form a vesicle which is then processed by the cell. All of them require ATP to move the membrane around the particles to form the vesicle.

Phagocytosis

This involves the creation of pseudopodia (extensions of the plasma membrane) to enclose large particles or even whole organisms outside the cell. Once enclosed by the pseudopodia, they form an internal vesicle which is then moved further inside the cell.

Pinocytosis

This differs from phagocytosis only in scale. It involves the ingestion of smaller particles (but particles that are still too large to cross the membrane by other methods) and does not require the formation of large pseudopodia to engulf the particles.

Receptor-mediated endocytosis

The membrane unfolds to form vesicles only in regions where particles have bound to specific receptors. Th binding stimulates the unfolding.

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