How Chemicals Move in Plants
The movement of solutes through membranes is not yet fully understood and there are different theories to explain the physical process. Nevertheless, it is understood that transport across membranes is a function of what is known as osmotic potential – the tendency for a solution with a lower concentration of solutes (low osmotic potential) to move through a diferentially permeable membrane towards a solution with a higher concentration of solutes (high osmotic potential). In other words, a weak solution will move across a membrane towards a strong solution, causing a dilution of the stronger solution. The greater the difference between the two solutions, the faster the process occurs. This process is also described as diffusion, or non-mediated transport. However, many molecules and ions successfully pass through membranes towards a solution with lower osmotic potential. This occurs with the assistance of carrier molecules, a process known as mediated transport.
There are many different terms used to discuss the movement of molecules which you must become familiar with before your can fully understand the processes of movement:
- Passive Transport can be either simple diffusion of molecules through the cell membrane, through specific channels or pores, or require the molecules to be carried through the cell membrane by a carrier molecule (mediated diffusion) but does not require energy.
- Active Transport is the term used to refer to the movement of molecules when energy is required. The energy is in the form ATP (ATP is the abbreviated name for Adenosine-tri-phosphate, the form of energy which can be used by cells to perform cellular activity).
Mediated and non-mediated transport
All living orgasms are made up of compartments (e.g. cells, or groups of cells called tissues). Communication or movement between these compartments is facilitated by transport proteins. Well defined mechanisms exist that enable movement of everything from small ions (e.g. Na+, K+, Cl) to larger molecules (e.g. glucose, nucleotides, amino acids,). There are in fact, two types of transport processes that occur in living organisms:
- Mediated, and
Mediated transport is more complex, only occurring in living matter, and involving the facilitation of movement by specific carriers, such as Permeases, Porters, Translocases, Translocators and Transporters.
Glucose is a large molecule which must enter the cells in order to make energy, it is the molecule which is broken down to make energy in a form the cell can use (i.e. glucose is broken down to make ATP). As glucose is vital, it must be transported into cells at high speed and continuously.
Passive-Mediated Glucose Transport is the name given to one way in which glucose is ‘carried’ into cells. Passive means no energy is required for this process (there is no energy required when the molecule is being passed along the concentration gradient from an area of high concentration to an area of low concentration) and mediated means the glucose molecule is carried through the cell membrane. This process is somewhat complex, in the simplest terms; there a number of large proteins found at intervals along the cell membrane on which the glucose molecules to be transported bind to. The shape of the protein then changes shape encouraging the movement of the glucose from one side of the protein to the other, thus resulting in the glucose entering the cell.
However, there are two types of transport proteins – there are channels which open and close in response to concentration of ions or molecules, the channels are simply pores surrounded by protein sub-units; and there are carrier proteins on which the molecule or ion binds to then the protein changes it’s shape or confirmation thus ‘forcing’ the movement is molecule across the membrane. Some channels only allow the passing of one type of molecule, whereas others allow movement of more than one molecule and in both directions (in and out of the cell). Carrier proteins are specific i.e. they bind to and transfer only one type of molecule.
In addition, molecules can be transported through cell membranes passively, although by carrier molecules. Passive, means no energy is required by the cell to perform the transport. Active transport of molecules is the term used to refer to the ATP (energy) expenditure during molecule transfer.
Nonmediated transport involves simple diffusion, where chemicals move through a semi-permeable membrane, from an area of high concentration on one side, to an area of low concentration on the other. This type of transport is simple and will occur in even non-living situations.
When molecules are transported against the concentration gradient (i.e. from low concentrations to high concentrations), we say active transport is occurring.
Active Transport can be either ATP driven or Ion gradient driven. Two of the most common ions found in almost all animal cells are potassium (K+) and sodium (Na+) and so these can be used as a good example of ions which are transported as a result of ATP driven active transport or as Ion gradient active transport as a result of their concentration.
Ion Gradient Driven Active Transport
An ion gradient refers to the concentration gradient of ions. The protein used in the transport of sodium and potassium ions in to and out of cells is referred to as the sodium-potassium pump. The movement of sodium and potassium is an example of ion driven active transport. There is a much greater concentration of sodium ions found outside cells in the extracellular fluid than inside cells. Conversely, there is a much greater concentration of potassium inside cells than outside in the extracellular fluid. As per the laws of diffusion, sodium ions must then move from outside (where there is a high concentration) to inside the cells (area of lower concentration) and potassium ions must move from inside to outside of the cell. The transport of potassium out of the cell drives the transport of sodium into the cell. Energy (ATP) is required to move the ions from one side of the cell membrane to another; however it is the concentration of the ions which determines their transport.
Active transport occurs when molecules are transported against the concentrating gradient (i.e. from low concentrations to high concentrations).
ATP Driven Active Transport
This is an endergonic process which is coupled with the hydrolysis of ATP. One of the best known active transport systems is the Na+-K+ ATPase (sodium-potassium adenosine triphosphatase enzyme) of plasma membranes. This protein comprises the alpha subunits and the beta subunits. Each of the alpha subunits has cardiotonic binding sites where the molecules attach. This system is simply referred to as the NA-K pump, when a Na ion gets pumped out, a K ion gets pumped into the cell and an intracellular ATP molecule gets hydrolysed in the process. It is an electrogenic antiport, 3+ve charges exit the cell for every 2+ve charges that enter. This system utilises the free energy from the ATP hydrolysis to create an electrochemical potential gradient across membranes. It is worthwhile to note that a cell uses a very large fraction of the ATP it produces to maintain the required concentrations of Na and K ions in the cytosol.
The ATP phosphorylates the Na+-K+ ATPase in the presence of Na ions while hydrolysis occurs in the presence of K ions.
Mediated transport is categorised according to stoichiometry.
- A uniport involves the movement of a single molecule at a time. The erythrocyte glucose transporter is an example.
- A symport simultaneously transports two different molecules in the same direction.
- An antiport simultaneously transports two different molecules in opposite directions.
Ionophores are substances that increase permeability of membranes to particular ions. There are two types:
- Carriers. These are temperature sensitive, each specific carrier binding only to certain specific ions, then diffusing through a membrane (e.g. cell wall), and releasing the bound ion on the other side.
- Channel formers. These are not affected by temperature. They create channels through a membrane through which their specific ions can diffuse.
Ion transport may be either:
- Electrically neutral. After the transport, the number of electrons on both sides of the membrane remains unchanged; or
- Electrogenic. After the transport, one side of the membrane is left with more electrons, and the other side less, than before the transport.
It was assumed due the large proportion of water in biological systems that water simply moved through membranes via osmosis. However, in 1992 a protein that is highly specific for transporting water across membranes was discovered. These proteins were named aquaporins. In plants there are approximately 50 aquaporins known so far.
Each aquaporin subunit contains a pore that is approximately 3Ǻ across (water is 2.8Ǻ in diameter). This pore is hydrophobic apart from two side chains. This is important because it prevents the transport of Protons. When transporting a chain of water molecules the two side chains interrupt the water molecule chain by forming temporary hydrogen bonds to the molecule as it passes through.
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