Exudation or bleeding in plants occurs due to

(i) Plants obtain a variety of inorganic elements(ions) and salts from their surroundings, especially from water and soil.

(ii) The movement of these nutrients from the environment into the plant and from one plant cell to another essentially involves movement across a cell membrane.

(iii) Transport across cell membranes can be through diffusion, facilitated transport or active transport.

(iv) In facilitated diffusion, special proteins called porins help in transport and are called carriers.

(v) Porins are proteins that form huge pores in the outer membranes of plastids, mitochondria, and some bacteria. They allow the passage of molecules up to the size of small proteins.

(vi) Passive transport (diffusion, osmosis) and active transport are the two modes of nutrient transport across cell membranes in living organisms.

(vii) In passive transport, nutrients move across the membrane by diffusion, without any use of energy as it is always down the concentration gradient and hence entropy-driven.

(vii) In plants, the diffusion process is evident in gaseous exchange and transpiration.

(viii) In active transport, energy in the form of ATP is utilised to pump molecules against a concentration gradient across membranes.

2. Plant-Water Relation:

(i) Water potential is the potential energy of water that helps in the movement of water. Water potential is determined by solute potential and pressure potential.

(ii) Osmosis is water movement across a semipermeable membrane which depends on the pressure gradient and concentration gradient.

(iii) The behaviour of the cell depends on the external solution, which is normally hypotonic or hypertonic.

(iv) Hypotonic solution causes endosmosis and hypertonic causes exosmosis.

(v) The highly concentrated external solution leads to plasmolysis through excessive exosmosis in which shrinkage of protoplasm occurs.

(vi) When the cells are placed in a hypotonic solution, water diffuses into the cell. As a result, the cytoplasm builds up a pressure against the wall. It is called turgor pressure. It can also be defined as the pressure exerted by the incoming water on the walls of the cell.

(vi) Imbibition is a special type of diffusion when solid colloids absorb water, causing them to increase in volume.

3. Long-Distance Transport of Water:

(i) Transport over long distances takes place through the vascular system and is called translocation.

(ii) In higher plants, there is a vascular system, xylem, and phloem, responsible for translocation.

(iii) Water and minerals absorbed by roots are transported by the xylem and the organic material synthesised in the leaves is transported to other parts of the plant through the phloem.

(iv) Water, minerals and food cannot be moved within the body of a plant by diffusion alone.

(v) These are therefore transported by a mass flow system - movement of substance in bulk from one point to another as a result of pressure differences between the two points.

(vi) Water absorbed by root hairs moves deeper into the root by two distinct pathways, i.e., apoplast and symplast.

4. Forces Responsible for upward movement of water in Plants:

(i) Passive absorption is actually mass flow involving apoplast and the force responsible is transpiration pull.

(ii) The transpiration pull also facilitates ascent of sap and passive mineral absorption

(iii) Transpiration pull develops as a result of rapid transpiration causing more negative water potential or ‘tension’ in root xylem.

(iv) Transpiration pull is maintained due to cohesion and adhesion of water molecules in the conducting elements.

(v) Transpiration is loss of internal water from the plants in the form of water vapours from high vapour pressure to low vapour pressure.

(vi) Atmospheric humidity is the main factor regulating the opening and closing of stomata.

(vii) Temperature, light, humidity, wind speed and number of stomata affect the rate of transpiration.

5. Guttation:

(i) Guttation is a process of exudation of liquid drops from uninjured parts such as the tip and margin of leaves.

(ii) It occurs due to root pressure under the influence of reduced transpiration.

(iii) It can take place through hydathodes.

6. Translocation of food:

(i) Phloem is responsible for transport of food (primarily) sucrose from the source to the sink.

(ii) The translocation in phloem is bidirectional.