NEET Biology Notes Transport in Plants
Movement of Water, Gases and Nutrients
In a flowering plant, the substances that need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators. Over small distances, substances move by diffusion and by cytoplasmic streaming supplemented by active transport. Transport over long distances proceeds through the vascular system (xylem and phloem) and is called translocation.
In rooted plants, transport in xylem (water and minerals) is unidirectional from roots of the stems. Organic nutrients undergo multidirectional transport. Organic compounds synthesised in the photosynthetic leaves are exported to all the storage organs of plants and later are re-exported.
Transport of water, minerals and all other substances in plants take up either of the means of transport, described below :
It is the movement of particles or molecules or ions from a region of higher concentration to the region of lower concentration. Diffusion is a passive process, which requires no expenditure of energy and remains unaffected by the concentration gradient of the other molecules.
- Exchange of gases i.e. C02 and 02 during photosynthesis and respiration.
- Absorption of ions during passive salt uptake.
- Stomatal transpiration where diffusion of water vapour takes place.
- Aroma of flower is the result of diffusion of volatile aromatic compounds.
It is the force or pressure developed by the movement of the diffusible particles of a substance, as they move from higher concentration to lower concentration. Diffusion pressure of pure water is maximum.
The diffusion rate depends on the size of the substances and on its solubility in lipids, the major constituent of the membrane. Substances that find difficult to pass through the membrane, their movement has to be facilitated by proteins. Such movement is called facilitated diffusion. However, a concentration gradient must be there for molecules to diffuse.
Facilitated diffusion is very specific, it allows the cells to select substances for uptake. The proteins called porins form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
The transport proteins releases the molecule inside the cell. Some transport proteins allow diffusion only when two molecules move together such as in a symport, both molecules cross the membrane in the same direction. In an antiport, they move in opposite directions. When a molecule moves across a membrane independent of other molecules, the process is called uniport.
It uses energy to pump molecules against a concentration gradient. It is carried out by the membrane-proteins. Pumps are proteins that use energy to carry substances across the cell membrane
Water is essential for all physiological activities of the plant and plays a very important role in all living organisms. Distribution of water within a plant varies, i.e. woody parts have relatively very little water, while soft parts mostly contain water.
Water potentialof pure water is zero and addition of solute in it decreases its y (i.e. negative value). The water moves from a higher water potential (lower DPD) to lower water potential (higher DPD). As water potential is measured as a negative value therefore, water moves from less negative water potential to more negative water potential.
Water potential is the sum of the osmotic potential and the pressure potential.
The pressure that develops in a cell due to the osmotic diffusion is called Turgor Pressure (TP), as cell wall is rigid, so it will exert equal and opposite pressure to that of turgor pressure, which is termed as wall pressure.
TP = WP
Diffusion Pressure Deficit (DPD) or Suction Pressure (SP) is the measure of index or sucking power. It is the difference between diffusion pressure of pure water and diffusion pressure or solution.
PD = SP
The relation between Diffusion Pressure Deficit (DPD), Suction Pressure (SP), Osmotic Pressure (OP), Turgor Pressure (TP) and Wall Pressure (WP) is as follows: