A body is said to be permeable to a substance, if it allows the passage of the substance through it. This property is termed as permeability. The plant cell wall is permeable because it allows both solvent and solute molecules to pass through it. On the other hand, cuticle layer is impermeable because it does not allow both the solvent and
the solute particles to pass through it. However, there are certain membranes, viz. ‘Vapour’ membrane, which allow penetration of only solvent molecules but not the solute particles. Such membranes are known as semipermeable. The biological membranes are not perfectly semipermeable. They allow to pass solute molecules only up to a certain extent and are, therefore, selective in nature. They are known as selectively or differentially permeable membranes.
Differences between semipermeable and differentially permeable membranes are given in table
TABLE 1.1. Differences between semipermeable and differentially permeable
|Semipermeable Membrane||permeable membranes|
|1. A partition which per- mits the passage of pure solvent molecules to pass through it and not the solute particles, is called semiperme- able.||1. The membrane which allows some substance to pass through it more readily than others is known as selectively or differentially perme able.|
|2. Examples of such membranes are cellophane, collodion etc||.2. Examples are all living biological membranes.|
|3. These membranes are used as partitions between solution and solvent in osmotic measurements and dialysis.||3. These membranes help in water and salt uptake by plants.|
ATMOMETER: It is an apparatus for demonstrating and measuring pull caused by evaporation of water from apot.
BAROMETER: It is an instrument which is used for measuring atmospheric pressure. A recording of barometer is called a barograph.
MANOMETER: It is an instrument which is used for measurement ofpressure (such as Root-pressure).
OSMOMETER: It is a device that measures osmosis. A device that demonstrates osmosis is called osmoscope.
POROMETER: It is an apparatus used for knowing the relative sizes of stomata.
POTOMETER: It is an apparatus used for measuring the rate of transpiration.
PSYCHROMETER: It is an instrument used for measuring both the relative humidity and transpiration
HYGROMETER: It is an instrument for measuring the relative humidity of the atmosphere. A recording of hydrometer is called a hygrograph.
HYDROMETER: It is an instrument used for measuring the density or specific gravity of a fluid.
TENSIOMETER: It is an instrument used for measuring soil-water-tension.
Higher green plants are, infact, the most abundant and prominent part of the natural green landscape. They play a very important role in the existence and survival of the mankind on this planet. They provide food, clothings, shelter, furnishings, medicines and almost all our biological needs including the fresh air we need for breathing. They are so much intimately associated with our lives that it becomes our binding to find out means and ways to improve their growth, development, lifestyles and productivity(permeability)
In this regard, science of plant physiology comes to our help which makes us aware of the functional aspects of plant processes, tells us the needs and requirements of plants for their life activities and provides thorough knowledge of their vital activities.Plant physiology is concerned with the study of life activities of plants which include the functional aspects of its processes both at cellular as well as sub-cellular level.(permeability)
These processes are under the direct control of the inherent competencies in response to the environmental conditions. A plant process is defined as a continuous systematic series of biophysical and biochemical reactions that complete an act of the plant activity.The major topics included in physiology of plants are:
(1) Water relations.
Water plays a paramount role in plants. All physiological processes occur in the watery medium. Land plants absorb water mainly from the soil. Soil water occurs in the form of solutions in which many solute particles remain dissolved and many larger particles remain suspended. Entry of water into the plant body from external environment is called absorption of water (or water uptake).(permeability)
It involves various physical processes such as imbibition, diffusion, osmosis, water potential, permeability and plasmolysis. Transport of water from root hair to xylem, in roots, is called lateral movement of water. Water moves from roots to all aerial parts of plant body mainly via stem. The upward movement of water is called ascent of sap. (permeability)
Various theories have been put forward to explain the upward movement of water. These are-root pressure theory, vital theories and transpiration pull and cohesion of water theory. Practically most of the water absorbed by plants is ultimately lost into the atmosphere by the process of transpiration. (permeability)
Transpiration is the loss of water in the form of vapours from the living tissues of aerial parts of the plant.
(i) Mineral nutrition.
Green plants synthesize their own organic matter but absorb inorganic nutrients from their surroundings. Absorption and assimilation of inorganic nutrients by plants is called mineral nutrition. Essential elements, required by plants, are categorized into two groups – major elements and trace elements. The deficiency of these elements causes deficiency symptoms in plants. Green plants cannot utilize atmospheric free nitrogen directly. It is fixed into usable forms by various agencies, mainly by biological nitrogen fixation.(permeability)
(ii) Translocation of solutes.
Transport of organic solutes from one place to another in higher plants is referred to as translocation of solutes. It mainly occurs through phloem.(permeability)
It is an anabolic process that occurs in all the green plants, particularly inside the chloroplasts. During this process green plants synthesize complex carbohydrates and evolve molecular oxygen from simple substances like carbon dioxide and water with the help of light energy.
The process completes in two phases – photochemical (Light reaction) and biosynthetic (Dark reaction). Photochemical act involves electron transport system and photophosphorylation. It occurs inside the thylakoids of chloroplasts. The biosynthetic dark reaction involves Calvin cycle (C, cycle). It occurs inside the stroma of chloroplasts.(permeability)
(vi) Mode of nutrition.
There are mainly two types of nutrition autotrophic and heterotrophic. The heterotrophic nutrition may be saprophytic or parasitic. Some organisms are chemosynthetic. Some insectivorous plants supplement their nitrogen requirement from captured animal prey.(permeability)
It is an enzyme-controlled catabolic process in which the respiratory substrates are oxidised to evolve carbon dioxide and release energy in the form of ATP. This energy is used by living organisms in various metabolic processes. Respiration may be aerobic or anaerobic occurring with or without oxygen respectively. The initial reactions constituting glycolysis, are common in both forms of respiration. Krebs cycle occurs inside the mitochondria during aerobic breakdown of respiratory substrates.(permeability)