- Basic Terminology:-
- System-
- Part of the universe under investigation.
Open System: A system that can exchange both energy and matter with its surroundings.
Closed System: A system that permits passage of energy but not mass, across its boundary.
Isolated system: A system that can neither exchange energy nor matter with its surrounding.
Surroundings: Part of the universe other than the system, which can interact with it.
Boundary: Anything which separates the system from surroundings.
State variables:
=> The variables which are required to be defined in order to define state of any system i.e. pressure, volume, mass, temperature, surface area, etc.
- State Functions-
=> Property of the system which depends only on the state of the system and not on the path. Example: Pressure, volume, temperature, internal energy, enthalpy, entropy etc.
- Intensive properties-
=> Properties of a system that do not depend on the mass of the system i.e. temperature, pressure, density, concentration,
- Extensive properties-
=> Properties of a system that depend on the mass of the system i.e. volume, energy, enthalpy, entropy etc.
Process
The path along which state of a system changes.
- Isothermal process: Process which takes place at a constant temperature
- Isobaric process: Process which takes place at constant pressure
- Isochoric process: Process which takes place at constant volume.
- Adiabatic process: Process during which transfer of heat cannot take place between system and surrounding.
- Cyclic process: Process in which system comes back to its initial state after undergoing a series of changes.
- Reversible process: Process during which the system always departs infinitesimally from the state of equilibrium i.e. its direction can be reversed at any moment.
- Irreversible process: This type of process is fast and gets completed in a single step. This process cannot be reversed. All the natural processes are of this type.
Sexual Reproduction in Flowering Plants
(3) Entry of pollen tube into embryo sac:
The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule. The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through the filiform apparatus. The synergids direct the growth of the pollen tube by secreting some chemical substances (chemotropic secretion). The tip of pollen tube enters into one synergid. The penetrated synergid starts degenerating. After penetration, the tip of the pollen tube enlarges and ruptures releasing most of its contents including the two male gametes and the vegetative nucleus into the synergid.
(4) Double fertilization:
The nuclei of both the male gametes are released in the embryo sac. One male gamete fuses with the egg to form the diploid zygote. The process is called syngamy or generative fertilization. This syngamy was discovered by Strasburger (1884). The diploid zygote finally develops into embryo. The other male gamete fuses with the two polar nuclei (or secondary nucleus) to form the triploid primary endosperm nucleus. The process is called triple fusion or vegetative fertilization. These two acts of fertilizations constitute the process of double fertilization. The process was discovered by S.G. Nawaschin (1898) and Guignard in Lilium and Frittillaria. Double fertilization occurs in angiosperms only.