Endoplasmic Reticulum

It was discovered by Porter (1945) and Thompson (1945). The name was given by Porter in 1953. Endoplasmic reticulum is a 3-dimensional, complicated and interconnected system of membrane-lined channels that run through the cytoplasm. The endoplasmic reticulum is found in all eukaryotic cells except the R.B.Cs of mammals. eggs and embryonic cells. In adipose tissue it is poorly developed. It is extensively developed in the cells which are actively engaged in metabolic activities and protein synthesis e.g. liver cells, cells of pancreas etc.
                                                               
Types:- Depending upon the nature of its membrane, endoplasmic reticulum is of two types, smooth and rough. Endoplasmic reticulum may develop from pre-existing E.R, plasmalemma or nuclear envelop.
Smooth Endoplasmic Reticulum:- It has a smooth membrane which do not bear ribosomes. It is, therfore, also called agranular endoplasmic reticulum. This type of ER is found in cells engaged in the synthesis and storage of glycogen e.g. glycogen storing liver cells, adipose cells, interstitial cells etc. Sphaerosomes are believed to originated from SER.
Rough Endoplasmic Reticulum:- This type of ER bears ribosomes attached to the outer surface of its membrane. RER is, therfore, also called granular endoplasmic reticulum. On account of the presence of ribosomes, the RER engaged in the protein synthesis and secretory activity e.g. plasma cells, pancreatic acinus cells etc.

Structure:- Endoplasmic reticulum consists of membrane lined channels or spaces. The channels contain a fluid called endoplasmic matrix. Endoplasmic reticulum can exist in three forms- Cisternae, Vesicles and Tubules.

Cisternae:- They are flat interconnected sac-like parts of ER which are 40-50 nm in diameter. The cisternae are found in bundles. They occur in the cells actively involved in synthetic activity.
Vesicles:- They are oval and round sacs of 25-500 nm in diameter. They remain isolated in the cytoplasm. The vesicles are also called microsomes. 
Tubules:- They are tube like extensions which may be connected with cisternae or vesicles to form a reticular system. The tubules can be regular or irregular with diameter of 50-100nm.

Functions:-
i) It provides a large surface inside the cell for various physiological activities.
ii) It functions as cytoskeleton by providing mechanical support to cytoplasmic matrix.
iii) Endoplasmic reticulum keeps the various organelles in their position.
iv) Endoplasmis reticulum controls the movement of materials between two adjecent protoplast through plasmodesmata.
v) It provide membranes to nuclear envelop after telophase.
vi) The membranes of ER contains large number of enzymes.
Functions of Rough Endoplasmic Reticulum:- 
i) RER provides a large surface area to ribosomes.
ii) It bears enzymes in the region of pores for modifying polypeptides synthesised by attached ribosomes.e.g. glycosylation.
iii) It provides enzymes precursors for the formation of lysosomes by Golgi Complex.

Plasmolysis

Shrinkage of the protoplast of a cell from its cell wall under the influence of a hypertonic solution is called plasmolysis. Hypertonic solution causes exosmosis Or withdrawal of water from cytoplasm and the the central vacuole of the cell. The size of the cytoplasm as well as central vacuole and hence protoplast becomes reduced. The pressure on the wall is reduced. This is first stage of plasmolysis is called limiting plasmolysis. 
                                          The extra hypertonic solution continues to withdraw water from the central vacuole by exosmosis. Initially the protoplast withdraws itself from the corners. This stage is known as incipient plasmolysis. The hypertonic solution now enters the cell in between the protoplast and the cell wall. Due to continued exosmosis, protoplast shrinks further and withdraws from the cell wall except one or few points. It is known as evident plasmolysis.

Deplasmolysis:- The swelling up of a plasmolysed protoplast under the influence of hypotonic solution or water is called deplasmolysis. It is due to endosmosis. Deplasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged. 

Importance of Plasmolysis;- i) Plasmolysis proves the cell membrane is semipermeable.

ii) It shows that cell wall is elastic as well as permeable.

iii) Osmotic pressure of a cell can be measured by plasmolysis. It will be roughly equivalent to the osmotic pressure of solution.

iv) Plasmolysis can be shown only by living cells.

v) By salting tennis lawns, the weeds can be killed due to permanent plasmolysis.

vi) Salting of pickles, meat and fish kill the spores of fungi and bacteria.

Turgor Pressure

Turgor pressure is the pressure exerted by protoplast upon the cell wall. It occurs due to the osmotic entry of water by endosmosis.
                                               As action and reaction are equal and opposite, therefore, cell wall exerts pressure on protoplast which is called wall pressure. It is denoted by W.P. and is equal to turgor pressure.

Function of Turgor presure:- i) The opening and closing of stomatal pores is regulated by the turgidity of the guard cells.
ii) Turgor pressure provides the mechanical support to the non-woody stems.
iii) It helps in keeping leaves erect and fully expanded. In case of loss of turgidity, the shoots droop down and the leaves show wilting. Wilting is observed in  many plants during summer noon.In wilting the cells of leaves and other softer parts become flaccid due to loss of water. Plants gain their turgidity during night because continued absorption of water from soil, recovery may be partial or it may not occur at all. The latter condition is known as permanent wilting.
iv) Turgor pressure controls the turgor movements, like in the leaf of Mimosa pudica.

v) It helps in cell enlargement.

Diffusion Pressure Deficit

The reduction in the diffusion pressure of water in a solution over its pure state is called diffusion pressure deficit or DPD. It is an older term that was used in place of water potential. Pure water has the maximum diffusion pressure. Diffusion pressure deficit is also called suction pressure. 
                                                                            The term diffusion pressure deficit was given by Meyer (1938). Originally it was described as suction pressure and presently it is replaced by the term water potential. Its value is equal to the osmotic pressure of the solution in a cell minus the wall pressure (= turgor pressure) which opposes the entry of water into it provided the external water is pure.

Interrelationship of OP, TP and DPD:- In a plasmolysed cell the net force with which water enters the cell is equal to its osmotic pressure. As the water enters the cell its turgor pressure increases which acts aganist the osmotic entry of water in a cell.
                                                                                 The net force with which water enters the cell would be equal to the difference of OP and TP.


                                         i.e.            DPD = OP - WP
                                         as               WP = TP
                                     Therfore       DPD = OP - TP

In a flaccid cell
                                             DPD = OP ( as TP = 0 )
In a turgid cell
                                              DPD = 0 ( as OP = TP )

Plastids

The term plastid was introduced by E. Haeckel in 1886. Plastids are semi-autonomous organelles having DNA and double membrane envelope which store or synthesise various types of organic compounds. Plastids develop from colourless precursors called proplastids. The plastids are large cytoplasmic organelles found in eukaryotic cells. They are absent in all prokaryotes and eukaryotic animal except Euglena and Volvox. On the basis of colour, plastids are of three main types:-

i) Leucoplasts
ii) Chromoplasts
iii) Chloroplasts

i) Leucoplasts:- They are the colourless plastids and do not contain any pigments. They usually perform the function of storage of reserve food material. There are three types of spherical leucoplasts. a) Amyloplasts. They are the starch contaibg leucoplasts. An amyloplast is several times larger than  the original size of leucoplast, e.g. potato tuber, rice, wheat.   b) Elaioplasts. The colourless plastids store fat, e.g. Tube rose.   c) Aleuroplasts, Proteoplasts or Proteinoplasts. The plastids contain protein in the amorphous or crystalloid state, e.g. endosperm cells of Castor.

ii) Chromoplast:- the plastids are yellow or reddish in colour because of the presence of carotenoid pigment. Chlorophylls are absent. Chromoplasts are formed either from leucoplasts or chloroplasts. Change of colour from green to reddish during the ripening of tomato and chilli is due to transformation of chloroplasts to chromoplasts. The orange colour of carrot roots are due to chromoplasts. Some important functions related to chromoplasts are;
a) Chromoplasts provide colours to many flowers for attracting pollinating agents.
b) They provide bright red or orange colour to fruits for attracting animals for dispersal.
c) They are also the site of synthesis of membrane lipids.

iii) Choroplasts:- They are greenish plastids which posses photosynthetic pigments, chlorophylls and carotenoids, and take part in the synthesis of food from the inorganic raw material in the presence of sun light.
             The number of chloroplasts per cell of algae is usually fixed for a species. However different species of different genus may have different number of chloroplasts, e.g. 1 in Spirogyra indica and 16 in S. rectospora. A photosynthetic leaf chlorenchyma cell has 20-40 chloroplasts.
                                                                                           Usually chlororplasts are spherical, oval or discoidal in shape. They are cup-shaped in chlamydomonas and ribbon-shaped in in spirogyra.

Ultrastructure:- A chloroplast has three parts- envelop, matrix and thylakoids.

Chloroplast Envelop:- A chloroplast envelop is made up of two smooth membranes. The outer membrane is more permeable than inner membrane. the outer membrane may be attached to endoplasmic reticulum. At places the inner membrane is connected to thylakoids.
Matrix:-  The ground substance of chloroplast is known as matrix or stroma. It is semifluid colloidal complex that is made up of 50% of soluble proteins.The remaining is DNA, RNA, ribosomes and enzymes. Chloroplast DNA is naked, circular or occasionally linear. DNA makes the chloroplast genetically autonomous because it can both replicate and transcribe to form RNA. Chloroplast ribosomes are 70 S. They resemble the ribosomes of prokaryotes. With the help of ribosomes chloroplast is able to synthesize most of the enzymes required by it. 
Thylakoids :- They are membrane lined flattened sacs which run through out the stroma or matrix of the chloroplast. Since, they take part in photosynthesis. Thylakoids  are thus the structural elements of the chloroplast. In the chloroplast of higher plants, thylakoids are stacked at places to form grana. 40-60 grana may occur in a chloroplast. Each granum has 2-100 thylakoids. Grana are absent in algal chloroplast. The latter are, therefore, agranal. 
                                                                Because of the presence of grana, thylakoids are differentiated into two- granal thylakoids and stroma thylakoids. thylakoids other than grana present in matrix or stroma are called stroma thylakoids. 

                                                                         Thylakoids membranes posses photosynthetic pigments and coupling factor ( involved in ATP synthesis). There are twp photosystems, I and II present on the thylakoid membranes. Photosystem II occurs in appressed parts of granal thylakoids while photosystem I is found in stromal thylakoids and nonappressed parts of granal thylakoids.

Functions:- 
i) Photosynthesis:- Chloroplasts are the centres of photosynthesis and formation of organic compounds from inorganic raw material.
ii) Energy Transduction:- Chloroplasts are able to trap sun energy and change it into chemical energy.
iii) Consumption of carbon Dioxide:- Chloroplasts pick up carbon dioxide and use the same in photosynthesis.
iv) Liberation of Oxygen:- Chloroplasts liberate oxygen which is passed into the atmosphere. This keeps the balance of oxygen constant in the atmosphere. 
v) Storage of starch:- They store starch either temporarily or permanently. 
vi) Photosenstivity:- Chloroplast of some algae provide photosenstivity because of the presence of stigma or eye spot.
vii) Storage of Lipids:- Chloroplast store fat in the form of plastoglobuli.
viii) Formation of chromoplast:- They can be change into the chromoplasts to provide colour to many flowers and fruits.  

Different types of solutions

There are three types of solutions:-

i) Isotonic solution
ii) Hypotonic solution
iii) Hypertonic solution

i) Isotonic solution:- A solution having a concentration such that it neither gain or loses water by osmosis when it is separated by semipermeable membrane from a specific solution eg. 0.9% sodium chloride solution and 5% glucose solution are isotonic.
ii) Hypertonic solution:- The solution having concentration such that it gains water solvent by osmosis across a semipermeable membrane from some other specified solution. When RBCs are placed in a hypertonic solution then exosmosis occurs and RBCs become smaller.
iii) Hypotonic solution:- A solution having a concentration such that it loses water or solvent by osmosis across a semipermeable membrane to some other specified solution. When RBCs placed in hypotonic solution, then endosmosis occurs and RBCs become swollen and spherical.

Osmosis

Osmosis is defined as the movement of water from its higher chemical potential to its lower chemical potential through semipermeable membrane. The chemical potential of water is also called water potential.                                                          
                                                                          OR

Osmosis is a special type of diffusion that occurs through a semipermeable membrane.

Osmosis is of two types:-     a) Exosmosis                   b) Endosmosis

a) Exosmosis:- It is the outward movement of water from a cell when it is placed in a solution of higher concentration .
b) Endosmosis:- Osmotic inflow of water into a cell when it is placed in a dilute solution having the solute concentration lesser than that of a cell.


Osmotic pressure:- It is the pressure which develops in a osmotically active solution when it is separated from its pure solvent by a semipermeable membrane. Osmotic pressure is also defined as the pressure required to completely stop the entry of water into an osmotically active solution across a semipermeable membrane. It is measured in atomspheres, bars or pascals. The instrument used to measure osmotic pressure is called osmometer.

Significance of osmosis:- i) Osmosis helps in the absorption of water and minerals by roots.
ii) Cell to cell movement of water occur by osmosis.
iii) Opening and closing of stomata are due to osmotic changes in guard cells.
iv) It provides turgidity to guard cells and therefor plant organ.
v) The plant movement like nyctinasty etc. occur due to changes in osmotic pressure.