Diffusion and Facilitated Diffusion
Diffusion can be defined as the net 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 (energy of movement) possessed by molecules or ions, which makes them move about at random. As a result of diffusion, molecules tend to reach an equilibrium situation where they are evenly spread within given volume of space.
Some substances have molecules or ion that are able to pass through cell membranes by diffusion. The rate at which a substance diffuses across a membrane depends on a number of factors, including the 'steepness' of the concentration gradient, temperature, the surface area across which diffusion is taking place, and the nature of the molecules or ions.
The respiratory gases, oxygen and carbon dioxide, cross membranes by diffusion. They are uncharged and non-polar, and so can cross through the phospholipid bilayer directly between the phospholipid molecules. water molecules, despite being very polar, can diffuse rapidly across the phospholipid bilayer because they too are small enough. However, larger polar molecules cannot diffuse through the phospholipid bilayer. Nor can ions. 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. Facilitated means 'make easy' or 'make possible', and this is what the protein channels do.
Some substances have molecules or ion that are able to pass through cell membranes by diffusion. The rate at which a substance diffuses across a membrane depends on a number of factors, including the 'steepness' of the concentration gradient, temperature, the surface area across which diffusion is taking place, and the nature of the molecules or ions.
The respiratory gases, oxygen and carbon dioxide, cross membranes by diffusion. They are uncharged and non-polar, and so can cross through the phospholipid bilayer directly between the phospholipid molecules. water molecules, despite being very polar, can diffuse rapidly across the phospholipid bilayer because they too are small enough. However, larger polar molecules cannot diffuse through the phospholipid bilayer. Nor can ions. 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. Facilitated means 'make easy' or 'make possible', and this is what the protein channels do.
Osmosis
Osmosis is best regarded as a special type of diffusion involving water molecules. It is useful to be able to measure the tendency of water molecules to move from one place to another. This tendency id known as water potential. Water always moves from a region of higher 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 then be no net movement of water molecules. Osmosis can be defined as the movement of water molecules from a region of higher water potential to a region lower water potential through a partially permeable membrane. The water potential of pure water is set at zero. The more solute, the more negative (lower) the water potential becomes. The amount that the solute molecules lowers the water potential of a solution is called the solute potential. The contribution made by pressure to water potential is known as pressure potential. The pressure potential makes the water potential less negative and is therefore positive.
An example of the latter would be a concentrated sucrose solution. In such a solution water will leave the cell by osmosis. As it does so, the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. At this point the pressure potential is zero, so the water potential of the cell is equal to its solute potential. As the protoplast continues to shrink it begins to pull away from the cell wall. This process is called plasmolysis, and a cell in which it has happened is said to be plasmolysed.
An example of the latter would be a concentrated sucrose solution. In such a solution water will leave the cell by osmosis. As it does so, the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. At this point the pressure potential is zero, so the water potential of the cell is equal to its solute potential. As the protoplast continues to shrink it begins to pull away from the cell wall. This process is called plasmolysis, and a cell in which it has happened is said to be plasmolysed.
Active Transport
A concentration gradient exists with a lower concentration outside and a higher concentration inside the cell. Since the ions inside the cell originally came from the external solution, diffusion cannot be responsible for the gradient because ions diffuse from high concentration to low concentration. The ions must therefore accumulate against a concentration gradient. The process responsible is called active transport. Like facilitated diffusion, each of which os specific for a particular type of molecule or ion. However active transport requires energy because movement occurs up a concentration gradient. The energy is supplied by the molecules ATP which is produced during respiration i side the cell. The energy is used to make the transport protein (carrier protein) change its 3D shape, transferring the molecules or ions across the membrane in the process. Therefore active transport can 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.
Bulk Transport
Mechanisms exist for the bulk transport of large quantities of material into cells or our of cells.
Endocytosis involves engulfing of the material by plasma membrane to form a small sac or 'endocytotic vacuole'. It takes two forms:
Endocytosis involves engulfing of the material by 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 specializing in this are called phagocytes. The process is called phagocytosis and the vacuoles phagocytic vacuoles.
- pinocytosis or 'cell drinking' - the is the bulk uptake of liquid. The small vacuoles formed are often extremely small, in which case the process is called mircopinocytosis.