Cell Activities

 Introduction

In unit 2 you learnt about cell structure. You learnt that every organism is made up of cells. You learnt that there are some organisms that are single celled while others are multicellular. You saw a cell under the microscope, you saw how tiny it was. You were not able to see a cell with your naked eyes. Despite the fact that you were not able to see a cell with your naked eyes, the cell carries out the
process that make the organism a living thing. In this unit, you will study cell activities. Cells undergo cell division Osmosis, i.e. passage of water through the cell membrane and plasmolysis. In this unit you will look at these three aspects of cell activities.

Cell Activities

You have seen earlier that cells can exist singly and collectively as in advanced organism like plant. For the organism to become tall or fat or big, series of cell activities must have taken place. This is called cell processes. Each cell formed becomes a permanent structure of a plant. The cell may die, as long as it has not been broken off it is part of the plant.


Cells are formed from the meristem of a plant, and they are formed by cell division. The terminal end of the stem, root, (shoot and root) are regions where cell division take place. As cell divide, the meristem is pushed ahead, adding increase (both in the shoot and root) and the diameter also increases.

 Cell Division

You have learnt that cell give birth to cell. A cell must be existing to give birth to another cell. Biologist refer to that s `Cell coming from pre-existing cell.' Cells multiply to keep the organism growing (increase). Even where the organism has substantially stopped growing, cells still multiply to renew old and dead cells. Cell division is very active at the tip of a typical plant (Apex).
Two processes are involved, (a) the nuclei content becomes twin inside and divide. They divide from each other, the DNA of the cell also separate (as the nuclei divide) the DNA materials also divide so that, as the nuclei is separating the divided contents also separate. The biologists call this Mitosis. (b) This is the separation of the cytoplasm and biologists call it cytokinesis. Now the cytoplasm also divide and separate. There are two cells now, each has its own cell wall.

 Osmosis

You must have heard the word Osmosis. In most cases, the word is used wrongly. When the biologist says Osmosis, he is simply saying that water passes some membrane. What are membranes? Instead of defining membranes, let us illustrate it. for instance, if you have solution of a kind and want to separate it from another solution by `something in between' (what we call membrane now),
say solution of sugar is to be separated from ordinary water. The membrane is the structure that allows the molecules to pass from one solution to the other. The membrane allows the passage of the smaller molecules to pass through freely but does not allow the bigger molecule to pass through easily. The membrane is said to be selective. When a membrane allows selective passage, it is -
semi permeable or differentially permeable. Examples of semi-permeable membranes are fish or animal bladder, egg membranes.

In plant cell, the ectoplasm (not the cell wall) act as the differentially permeable membrane. When weak and strong solutions are separated by such a membrane, there is a net transfer of the solvent from the weaker solution to the stronger solution. Osmosis is the process of selective transmission of a liquid in preference to another or a solvent in preference to the solute through a membrane . You will learn more about the exact process, but for now know that semi-permeable this process is made possible by the cell.

 Plasm olysis

The knowledge you have acquired from the cell activity in Osmosis will allow you to explain many processes in biology. For instance if you put a fruit that has dried up inside water, after many hours the fruit would have swollen up. What has happened is that (as you learnt earlier) water has moved through the cytoplasm to swell the fruit. You can also put a normal healthy fruit into concentrated solution of salt and leave it for hours, on examination you will discover that water has
moved from the fruit into the concentrated solution. The fruit now will look placid, and squeezed. We are talking about a whole fruit.


If we take a cell and observe it under a microscope under the two conditions, i.e.
condition (a) - cell in a very strong solution.

1. Put the healthy, normal cell in a concentrated solution and observe it, it will loose water to the surrounding environment (i.e. the concentrated solution) and look shrinked, contracted, the protoplasm with the nucleus and the plastids will shrink from the cell wall, forming a rounded irregular mass in the centre. The space between the cell wall and the protoplasmic mass will be filled with sugar solution. The cell wall is the one that is freely permeable to the solution. The protoplasmic membrane is selectively or indifferentially permeable to the solution. Why did the protoplasm shrink? The reason is that the concentrated solution has a greater osmotic value than the cell-sap. The cell looses water by outward osmosis. This process of the cell protoplasm shrinking from the cell-wall under the action of concentrated solution is called plasmolysis 
2. Condition b is that, if this same cell is placed in a pure water solution, the reverse will happen, i.e. the protoplasm will return to its normal position. The reverse is deplasmolysis. 

 Advantages of Plasm olysis

1. Explains Osmosis 
2. Shows permeability of the cell-wall and semi-permeability of the outer layer of the protoplasm(cytoplasm) to the entrance of certain substances. 
3.  Shows that the protoplasm can retain the osmotically active substance of the sap. 

Conclusion

In this unit, you have learnt that the cell as tiny as it is, carries out various activities like cell division Osmosis and plasmolysis which come into the cell or go out of the cell depending on the condition of the surrounding environment of the cell.

 Sum mary

Cell activities include cell division. Cell division leads to cell increase or growth. There are two types of cell division: Mitosis and Meiosis. Osmosis and plasmolysis are part of cell activities. Plasmolysis has some advantages explaining cell activity like Osmosis.