In humans and other animals, the transport of substances occurs due to circulatory system. In unicellular organisms, the transport within the cell occurs by cytoplasmic movements or by diffusion. But, transportation in plants is quite different. They have no circulatory system yet, the substances have to move to distant places. Water, plant growth regulators, minerals, organic nutrients are the substances which need to move within the plant.
The transport usually occurs in two forms
- Short distance transport occurs by diffusion and cytoplasmic streaming.
- Long distance transport occurs by xylem and phloem (Translocation).
Sugars formed at the leaves have to move to the root tips, branches and other areas of plant. This is multi directional movement.
Similarly, water from root tips have to move to all the upper parts of the plant body. This movement is unidirectional and occurs only upwards from the roots.
The movement of substances into the cells and in between the cells is of short distance.
Methods of Transportation in plants
This is a process of movement from the regions of higher concentration to lower concentration. Here no energy expenditure occurs.
It help in transport of substance from one part of the cell to another, one cell to another cells, in between tissue spaces to outside etc.
Diffusion is suited to movement of gases, liquids and also dissolved solids. This is very crucial to plants as it helps in gases movement within the plant. This diffusion is controlled by the concentration, temperature, cell membrane permeability and pressure.
This is similar to simple diffusion in the it does not require ATP energy for transport. But, the process varies in that substances move through the membranes through pores or channels. These channels are formed by membrane proteins.
Of these porin channels some act as symports and others act as antiports. Symports allow movement of two molecules across the membrane simultaneously in the same direction. While the antiport channels allow simultaneous movement in opposite direction.
Certain proteins, ions and other water soluble substance are transported by this method.
Active transport: Here the molecules move against the concentration gradient. That is they move from a region of lower concentration to higher concentration with help of ATP energy.
Some membrane channels act as pumps. These pumps are made of proteins which use energy to move substances.
Both facilitated diffusion and active transport reaches saturation when all the pores are engaged. Further, they are very specific in what molecules travel through them across the membrane.
Water movement upwards involves
a) Capillary action or root pressure
b) Transpiration Pull
Root pressure is the pressure developed on water by root to move upwards. This pressure helps movement of water to a short distances.
This root pressure can be seen as oozing out of water from plant surfaces in the early mornings. This process account only for minute amount of water movement upwards. The transpiration pull plays a major role in movement of water upwards.
The transpiration by leaves leads to evaporation of water in the areal surface. This creates a water gradient for the water to move up into leaves from the xylem below. This process is higher during day and less at night.
So, the movement of water from roots to the leaves is facilitated by capillary action, water gradient and transpiration.
Movement of organic nutrients, proteins, growth modulators.
This occurs by a system of communicating tissue called as phloem. The movement occurs in three directions like upwards, downward and radial movement.
This is thought to occur by mass flow hypothesis. When the sugars are formed in leaves by photosynthesis. Then, they are moved by leaf veins into the sieve tube cells. The loading of sugar into the sieve tubes, occurs by active transport using H+/ ATP pump. This phloem loading increases osmotic pressure to draw water from xylem. As water accumulates inside, the sap moves into regions of low pressure. The sieve tubes form long columns with holes at their ends called as sieve plates. These plates connect with cytoplasmic strands to help move sap into the cells. This loading of sap into cells is also active process which needs energy. As the sugars move out of sieve tubes, the osmotic pressure reduces and water moves back into phloem.