21/08/2022
Definitions:
Osmosis: This is a movement of water molecules from a region of higher water potential or lower solute potential to a region of lower water potential or higher solute potential through a deferentially permeable membrane.
Diffusion: This is a movement of materials from a region of higher concentration to a region of lower concentration.
Active transport: This is a transportation of materials against concentration gradient. Due to this, the process involves the consumption of energy. Any part of the body where active transport occurs is characterized by;
a) Presence of numerous mitochondria.
b) High rate of metabolism.
c) High concentration of ATP.
Since active transport involves the use of energy, the materials transported actively move faster than those transported passively.
Significance of transportation system
The system of transportation of materials is important for:-
Distribution of food materials in the body.
Carriage of excretory wastes from their sites of synthesis.
Carriage of hormones from their respective glands to their target organs.
Distribution of antibodies.
Carriage of respiratory gases.
(I) TRANSPORT IN PLANTS
The movement of substances through the conducting or vascular tissues of plants is called Translocation.
Important application of the study of Translocation:
It is useful to know how herbicides, fungicides, growth regulators and nutrients enter plants and the routes that they take through plants, in order to know how best to apply them and to judge possible effects that they might have.
Plant pathogens are sometimes translocated, and such knowledge could influence treatment or preventive measures.
Terms used:
(i) Water potential, symbol Ψ, Greek latter psi.
The term is used to describe water movement through membranes. It can be described as the tendency of water molecules to move from one place to another. The higher (less negative) the water potential, the greater tendency to leave a system.
Factors affecting water potential of plant cells are:-
Solute concentration and
Pressure generated when water enters and inflates plant cells.
They are expressed in terms of Solute and Pressure Potentials respectively.
NOTE
Pure water has maximum water potential (zero).
Water always moves from a region of higher Ψ to a region of lower Ψ.
All solutions have lower Ψ than pure water, therefore negative values of Ψ
(ii) Solute potential, Ψs
The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential.
Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules.
(iii) Pressure potential, Ψp
If pressure is applied to pure water or a solution, its water potential increases. This is because the pressure tends to force water from one place to another.
Ψ = Ψs + Ψp
(iv) Plasmolysis and Turgidity
If a plant cell is in contact with a solution of lower water potential than its own contents, then water leaves the cell by osmosis through the cell surface membrane. Consequently, the protoplast shrinks and eventually pulls away from the cell wall. The process is called plasmolysis and the cell is said to be plasmolysed.
The point at which plasmolysis is just to happen is called Incipient plasmolysis. At this point, the protoplast has just ceased to exert any pressure against the cell wall, so the cell is Flaccid. Water will continue to leave the protoplast until its contents have the same Ψ as the external solution. No further shrinkage then occurs.
If a plasmolysed cell is placed in pure water or a solution of lower solute potential or higher water potential than the contents of the cell, water enters the cell by osmosis. As the volume of protoplast increases, it begins to exert pressure against the cell wall and stretches it.
The pressure inside the cell rises rapidly, the pressure is called the Ψp. As the Ψp of the cell increases due to water entering by osmosis, the cell becomes turgid.
Animal cells have no cell wall and the cell surface membrane is too delicate to prevent the cell expanding and bursting in a solution of higher Ψ. They are therefore protected by Osmoregulation.
Question.
What occupies a space between the cell wall and the shrunken protoplasts in plasmolysed cells?
What is the Ψp of a flaccid cell?
Answer;
The external solution, since the cell wall is freely permeable to solutions.
Zero. The protoplast is not exerting pressure against the cell wall.
In higher plants, the materials are transported by the vascular tissues. Which are of two types:-
The xylem and
The phloem.
(i) The xylem tissue
This is a plant vascular tissue which is mainly concerned with transportation of water and dissolved mineral salts through the plant.
Structure of the xylem
The histology of the xylem tissue reveals the presence of four types of cells.
Note: The only conducting cells are the vessels and tracheids.
1. TRACHEIDS
Structural features:
-They are more or less elongated cells with tapering ends.
-They have secondary thickened or lignified walls with a variety of pits (simple or bordered).
-They are not perforated.
-They are dead at maturity ie: they lose all the protoplasmic contents leaving an empty lumen.
NOTE:
Tracheids are present in all vascular plants, but in the coniferophytes they are only xylem conducting cells
