Transportation is an important phenomenon which will take place in all the higher organisms. In plants, materials of transport mainly include gases, water, hormones, minerals, organic material etc. through diffusion, active transport etc. Let us take a look.
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Transport in Plants
1. Transport of Water
Water gets absorbed by the root hair and gets transported inwards by different pathways till it reaches the xylem vessels. This continuous uptake of water creates a root pressure which pushes the water upwards. The pull created due to transpiration of water through the leaves causes the rising of water in the tall trees.
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2. Transport of Minerals
Minerals are also absorbed by the roots and transported upwards poet through the xylem along with water.
3. Translocation of Organic Solutes
Organic compounds synthesized during the process of photosynthesis gets transported through the phloem sieve tubes. Such a transport of organic solute is called translocation.
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Means of Transport
1. Diffusion
Movement of molecules and ions in solid, liquid and gaseous state from a region of their higher concentration region of lower concentration is called diffusion. This happens because, in a region of high concentration, partial pressure is higher as compared to the region where the concentration is low as the particles have inner and kinetic energy.
This process will continue till the concentration of molecules become same. Also, different substances can diffuse at the same time. The direction and rate of diffusion of one substance will be independent of the direction and rate of diffusion of another substance
Diffusion Pressure
The molecules while moving hit each other which creates a diffusion pressure. It can be defined as the potential ability of molecules to diffuse from an area of higher concentration to an area of lower concentration.
Factors affecting diffusion
- Temperature: More temperature causes increased kinetic energy, hence the rate of diffusion is more.
- The density of diffusing substance: Rate of diffusing substances is inversely proportional to the density of diffusing substance.
- Medium in which diffusion occurs: Gas diffuses more rapidly through a vacuum than air.
2. Facilitated Diffusion
It is a spontaneous passage of molecules and ions across a membrane through specific carrier proteins without involving energy. Hence facilitated diffusion is also a type of passive transport. This also occurs from an area of more concentration gradient to an area of less concentration gradient.
Importance of facilitated diffusion
Biological membranes basically are hydrophobic in nature. Water soluble molecules like glucose, sodium ions and chloride ions cannot pass through the membrane as the lipids make it hydrophobic in nature. Transport of such substances is facilitated by transport proteins. These proteins are embedded in the bilipid layer. They provide the binding site for molecules to be transported.
After binding the transport proteins to change their shape and the molecule is carried to the other side of the membrane where it is released. There are some large transport proteins which form huge pores in the membranes.
These are called porins. They provide passage to large molecules and ions.
Sometimes two different types of molecules move across the membrane simultaneously by the process of facilitated diffusion. When both kinds of molecules move in the same direction outside to inside or inside to outside, it is called symport.
When they move in opposite direction it is called antiport. And when a particular kind of molecules moves across the membrane independent to other molecules, the type of diffusion is called uniport.
3. Active Transport
When the transport of materials are not spontaneous but involves an expenditure of energy, the type of transport is called active transport. This transport generally takes place against the concentration gradient. Active transport always leads to accumulation of molecules are ions towards one side of the membrane.
Active transport is carried out by membrane proteins. The protein uses the energy to carry the substance across the membrane. Certain pumps are generated which can transport the substance uphill from lower concentration to higher concentration. The transport rate is maximum when all the protein transporters are saturated. The carrier proteins are generally specific in action. They are also sensitive to an inhibitor.
Comparison of Different Types of Transport
|
Diffusion |
Facilitated diffusion |
Active transport |
| Occurs along the concentration gradient | It occurs along the concentration gradient | Occurs against the concentration gradient |
| Does not require carrier molecules | Require special transport proteins | Require special transport proteins |
| Process is not sensitive to Inhibitors. | This process is sensitive to Inhibitors | Process is sensitive to Inhibitors |
| Energy is not required | The energy is not required | Energy is required |
| Saturation of transport does not occur | Transportation saturates when all the carrier proteins are being used | Transportation saturates when all the carrier proteins are being used |
Solved Questions for You
Question: Identify a type of molecular movement which is highly selective and requires special membrane proteins but does not require energy.
Answer: The type of transport is facilitated transport.
Question: How is it that the intracellular levels of K+ are higher than extracellular levels in animal cells?
Answer: This will happen through active transport. Active transport occurs from lower concentration to higher concentration and leads to accumulation of materials on one side of the membrane.
