Transport across Plasma Membranes

TRANSPORT ACROSS PLASMA MEMBRANES

By Johnny McKinstry

A phospholipid bilayer makes a good barrier, particularly against the movement of water-soluble molecules. The water contents of the cell are stopped from escaping. However, some exchange between the cell and its environment is essential.

Diffusion & Facilitated Diffusion

If you open a bottle of perfume in a room, it is not long before molecules of its scent have spread to all parts of the room. This will happen, even in still air, by the process of diffusion. Diffusion is the movement of molecules from a region of their higher concentration to a region of their lower concentration. The molecules move down a concentration gradient. It happens because of the natural kinetic energy possessed by molecules or ions, which make them move about at random. As a result of diffusion, molecules tend to reach an equilibrium situation where they are equally spread within a given volume of space. Some substances have molecules or ions that are able to pass through cell membranes by diffusion. The rate at which a substance diffuses across a cell membrane depends on a number of factors, including:

the steepness of the concentration gradient, that is the difference in the concentration of the substance on the two sides of the surface. If there are, for example, many more molecules on one side of a membrane than on the other, then at any one moment more molecules will be moving from this side than from the other side. The greater the difference in concentration, then the greater the difference in the number of molecules passing in the two directions, and the faster the net rate of diffusion.

Temperature. At high temperatures, molecules have more kinetic energy than at low temperatures. They move around faster, and thus diffusion takes place faster.

The surface area across which diffusion is taking place. The greater the surface area, then the more molecules can cross it at any one moment, and therefore the faster diffusion can occur.

The size of the molecules or ions. Large molecules require more energy to get them moving than small ones do, so substances with large molecules tend to diffuse more slowly than ones with small molecules.

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The surface area across which diffusion is taking place. The greater the surface area, then the more molecules can cross it at any one moment, and therefore the faster diffusion can occur.

The size of the molecules or ions. Large molecules require more energy to get them moving than small ones do, so substances with large molecules tend to diffuse more slowly than ones with small molecules.

Water molecules can diffuse rapidly across the phospholipid bilayer because they are small enough. However, large molecules, such as glucose and amino acids, cannot diffuse through the phospholipid bilayer. These can only cross the membrane by passing through hydrophilic channels created by protein molecules. Diffusion that takes place through these channels is called facilitated diffusion. Plasma membranes contain many different types of protein channel, each type allowing only one kind of molecule or ion to pass through it. The movement of the molecules or ions is passive, just as in ordinary diffusion, and movement into or out of the cell will only take place down a concentration gradient from a high concentration to a low concentration. However, the rate at which this diffusion takes place depends on how many appropriate channels there are in the membrane, and on whether they are open or not.

Osmosis

Osmosis is best regarded as a special type of diffusion involving water molecules only. It is the diffusion of water particles from a dilute solution (lots of solvent particles per unit volume) into a more concentrated solution (less solvent molecules per unit volume) through a selectively permeable membrane.

Water Potential & Solute Potential

It is useful to be able to measure the tendency of water molecules to move from one place to another. This tendency is known as water potential. The symbol for water potential is the Greek letter psi. Water always moves from a region of high water potential to a region of lower water potential. It therefore moves down a water potential gradient. Equilibrium is reached when the water potential in one region is the same as in the other. There will be no net movement of water molecules. We can now define osmosis as the movement of water molecules from a region of higher water potential to one of lower water potential through a partially permeable membrane.

By convention the water potential of pure water is set at zero. Since solutes make water potential lower, they make the water potential of solutions less than zero, thus negative. The more solute, the more negative (lower) the water potential becomes. The amount that the solute molecules lower the water potential of a solution by, is called the solute potential. Solute Potential is therefore always negative.

Osmosis in Plant Cells

Pressure potential is especially important in plant cells. Unlike animal cells, plant cells are surrounded by cell walls that are very strong and rigid. Imagine a plant cell being placed in pure water or a dilute solution. The water or solution has a higher water potential than the plant cell and the water therefore enters the plant cell through its partially permeable plasma membrane by osmosis.

The pressure potential increases the water potential of the cell until the water potential outside the cell equals the water potential inside the cell, and equilibrium is reached. The cell wall is so inelastic that it takes very little water to enter the cell to achieve this. The cell wall prevents the cell from bursting, unlike the situation when an animal cell is placed in pure water or a dilute solution. When a plant cell is fully inflated with water it is described as turgid. For plant cells then, the water potential is a combination of pressure potential and solute potential.

Active Transport

If the concentration of particular ions inside cells is measured it is often found that they are 10-20 times more concentrated inside than outside the cell. In other words a concentration gradient exists with a lower concentration outside the cell and a higher concentration inside. Since the ions inside the cell originally came from the external solution, diffusion cannot be responsible for this gradient because, as we have already seen, ions diffuse from a high concentration to a low concentration. The ions must therefore accumulate against a concentration gradient.

The process responsible for this is called Active Transport. Like facilitated diffusion it is achieved by special transport proteins, each of which is specific for a particular type of molecule or ion. However, unlike facilitated diffusion, active transport requires energy because movement occurs up a concentration gradient. The molecule that is produced during respiration inside the cell supplies the energy. The energy is used to make the transport protein change its shape, transferring the molecules or ions across the membrane in the process.

Active transport can therefore be defined as the energy-consuming transport of molecules or ions across a membrane against a concentration gradient (from a lower to a higher concentration) made possible by transferring energy from respiration. It can occur either into or out of the cell, depending on the particular molecules or ions and transport protein involved. Active transport is important in re-absorption in the kidneys where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules. In plants, active transport is used to load sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant, and to load inorganic ions from the soil into root hairs.

Bulk Transport

So far we have been looking at ways in which individual molecules or ions cross membranes. Mechanisms also exist for the bulk transport of large quantities of materials into cells (endocytosis) or out of cells (exocytosis).

Endocytosis involves the engulfing of the material by the plasma membrane to form a small sac, or 'endocytotic vacuole'. It takes two forms:

Phagocytosis or 'cell eating' - this is the bulk uptake of solid material. Cells specialising in this are called phagocytes. The process is called phagocytosis and the vacuoles phagocytic vacuoles. An example is the engulfing of bacteria by certain white blood - this is the bulk uptake of liquid.
Exocytosis is the reverse of endocytosis and is the process by which materials are removed from cells. It happens, for example, in the secretion of digestive enzymes from cells of the pancreas. Secretory vesicles carry the enzymes to the cell surface and release their contents. Plant cells use exocytosis to get their cell wall building materials to the outside of the plasma membrane.

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