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There are some physical principles and biological processes that explain the transport of materials across the plant body. They are as follows:
It is a physical process that involves the movement of solute particles from the region of their higher concentration to the region of their lower concentration. Transport of materials within the cell is by diffusion. The entry of carbon dioxide into the stomata is by diffusion.
It is a physical process in which the solvent (water) moves from the region of its higher concentration to the region of its lower concentration across a semi-permeable membrane. This is to say that water moves from a dilute to a concentrated solution. Water is absorbed into the root cells by osmosis.
The transport of materials across the cell membranes with the help of energy is called active transport. Both diffusion and osmosis are passive transport mechanisms, not requiring energy. Active transport helps in the transport of ions across the membranes of the cells. The minerals are present in the vascular tissue as ions from where they have to be absorbed by the cells.
The pressure built up inside the cells as a result of the entry of water is called osmotic pressure. The entry of water into the cells is regulated by the solutes in the cell. Less concentration of solutes means more concentration of water. Positive osmotic pressure means more water inside the cell and negative osmotic pressure means less water inside the cell.
The positive osmotic pressure developed inside a cell as a result of entry of water is called turgor pressure. When the cells are full of water, they are called turgid and when they lose water, they are called flaccid. The root cells become turgid after the absorption of water and this develops a positive potential in them.
It is the potential energy associated with water. For example, water at a height has more water potential and will, therefore, flow down. When water is present in a dilute solution it has higher potential and will therefore move towards the concentrated solution.
The difference in the potential energy between two regions or across the cell membrane is called potential gradient.
The difference in the concentrations of the ions across a membrane is called ionic gradient. Since these ions are charged chemical molecules, ionic gradient is also called electrochemical gradient.
The attraction between two unlike molecules is called adhesion. For example, between mercury and glass which helps the rise of mercury in the glass tube of the thermometer.
The attraction between two like molecules is called cohesion. For example, between the mercury molecules that helps the mercury to move up a thin glass tube as a continuous column.
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