CELL MEMBRANE
The basic components of a cell are (Fig. 8.7).
1. Cell membrane,
2. Cytoplasm and
3. Nucleus
Figure 8.7:
The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. The membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and allows the movement of substances or materials both inward and outward.
Activity 8.3:
* In order to observe the basic components of the cell, take an onion bulb.
* Remove the dry pink coverings (peels).
* You can easily separate these from the fleshy white layers of the bulb with the help of forceps or even with your hand.
* You can also break the onion bulb and separate out thin layers.
* Place a small piece of the thin onion peel in a drop of water on a glass slide.
* The thin layer can be cut into smaller pieces with the help of a blade or forceps.
* Add a drop of methylene blue solution to the layer and place a coverslip on it.
* While placing the coverslip ensure that there are no air bubbles under the coverslip.
* Observe the slide under the microscope. Draw and label.
* You may compare it with Fig. 8.5.
Figure 8.5: Cells observed in an onion peel
The boundary of the onion cell is the cell membrane covered by another thick covering called the cell wall. The central dense round body in the center is called the nucleus. The jelly-like substance between the nucleus and the cell membrane is called cytoplasm.
You have learned earlier that the cell membrane gives shape to the cell. In addition to the cell membrane, there is an outer thick layer in the cells of plants called the cell wall. This additional layer surrounding the cell membrane is required by plants for protection. Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture etc. They are exposed to these variations because they cannot move. Cells can be observed in the leaf peel of Tradescantia, Elodea or Rhoeo. You can prepare a slide as in the case of onion.
Paheli asks Boojho if he can also observe animal cells.
Activity 8.4:
* Take a clean toothpick or a matchstick with the tip broken.
* Scrape inside of your cheek without hurting it.
* Place it in a drop of water on a glass slide.
* Add a drop of iodine and place a coverslip over it.
* Alternatively, add 1-2 drops of methylene blue solution.
* Observe it under the microscope.
* You may notice several cells in the scraped material (Fig. 8.6).
* You can identify the cell membrane, the cytoplasm, and the nucleus.
* A cell wall is absent in animal cells.
Gene
Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. This means that your parents pass some of their characteristics on to you. If your father has brown eyes, you may also have brown eyes. If your mother has curly hair, you might also end up having curly hair. However, the different combinations of genes from parents result in different characteristics.
Figure 8.6: Human cheek cells
Source: This topic is taken from NCERT TEXTBOOK
CYTOPLASM AND NUCLEUS
Cytoplasm
It is the jelly-like substance present between the cell membrane and the nucleus. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, ribosomes, golgi bodies, etc. You will learn about them in later classes.
Nucleus
It is an important component of the living cell. It is generally spherical and located in the center of the cell. It can be stained and seen easily with the help of a microscope. The nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.
With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus. In addition, the nucleus contains thread-like structures called chromosomes. These carry genes and help in the inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.
The nucleus, in addition to its role in inheritance, acts as the control center of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.
While observing the onion cells under the microscope, did you notice any blank-looking structures in the cytoplasm? It is called a vacuole. It could be as single and big as in an onion cell. Cheek cells have smaller vacuoles. Large vacuoles are common in plant cells. Vacuoles in animal cells are much smaller.
You might have noticed several small colored bodies in the cytoplasm of the cells of Tradescantia leaf. They are scattered in the cytoplasm of the leaf cells. These are called plastids. They are of different colors. Some of them contain a green pigment called chlorophyll. Green-colored plastids are called chloroplasts. They provide green color to the leaves. You may recall that chlorophyll in the chloroplasts of leaves, is essential for photosynthesis.
The nucleus of the bacterial cell is not well-organized like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without a nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes (pro: primitive; karyon: nucleus). Examples are bacteria and blue-green algae. The cells, like onion cells and cheek cells having a well-organized nucleus with a nuclear membrane, are designated as eukaryotic cells. All organisms other than bacteria and blue-green algae are called eukaryotes. (eu : true; karyon: nucleus).
Source: This topic is taken from NCERT TEXTBOOK