Amoeba and Paramecium are examples of unicellular organisms.
The largest cell is the egg of an ostrich. It is 170mm x 130mm in size.
Each organ is made up of smaller parts called tissues. A tissue is a group of similar cells that perform a specific function.
Nerve cells carry messages from the brain to different parts of the body and vice-versa. Thus, they control the working of different parts of the body.
The ‘cell’ is the smallest structural unit of living organisms that can function independently. A single cell can be a complete organism like bacteria, amoeba, paramecium etc. Such a unicellular organism can perform all the functions of a living organism. It captures, makes and digests its food, respires, excretes, grows and reproduces. Some cells such as a plant cell can also make its own food. In case of multicellular organisms, such as higher plants and animals, these functions are performed by groups of specialized cells which are organised into tissues and organs. Hence, the ‘cell’ is known as the basic structural and functional unit of life.
Cells have a variety of shapes. Some cells have a definite shape while others keep changing their shape. For example, Amoeba or White Blood Cells (WBCs) present in our bodies keep changing their shape. However, most cells maintain a constant shape, which is related to the specific functions that they perform. For example, blood cells are spherical, muscle cells have spindle shape and nerve cells are long and branched to conduct nervous impulses.
When a cell is formed, it is small. It grows after absorbing and digesting the food particles. However, its growth is limited to a certain size. Once it reaches a certain size, it divides to form two cells which are known as daughter cells. These new cells are required for the growth of organisms and to replace the cells that die.
Thus, we can say that in an organism, growth occurs by increase in the size of cells, as well as, by increase in their number.
Yes, a group of cells that form a tissue have identical shapes and perform similar function. For example, the epithelial tissue is a group of similar cells that act as a protective layer for the other cells lying underneath.
An amoeba changes its shape by the formation of finger- like projections called pseudopodia, which facilitates movement and helps it in capturing food.
White blood cell present in the human blood is a single cell which can change its shape.
Cell wall and chloroplast
The entire content of a living cell is known as protoplasm. It includes cytoplasm and the nucleus.
Prokaryote: Prokaryotic cells do not have a nucleus or nuclear membrane, but they can still perform all the basic functions. Examples are bacteria and blue green algae.
Eukaryote: Eukaryotic cells have a well-defined nucleus and nuclear membrane. These include all multicellular plants and animals.
The cytoplasm contains various organelles such as nucleus, vacuole, ribosomes, mitochondria etc. It is a jelly-like structure that lies between the plasma membrane and the nucleus.
Chromosomes are found in the nucleus of a cell. They are clearly visible only at the time of cell division. Their function is to carry characteristic features of parent cells to the daughter cell i.e., from the parent to the offspring.
Chloroplasts are found only in plant cells because they contain a green coloured pigment called chlorophyll which helps in the process of photosynthesis in green plants. Chlorophyll captures solar energy and utilizes it to manufacture food for the plant.
(a) Gene: Gene is a unit of inheritance in living organisms. The nucleus of a cell contains thread-like structures called chromosomes that carry genes in them.
(b) Chromosomes: These are the microscopic, thread-like structures that can be seen in the nucleus of a cell at the time of cell division. They are responsible for transmission of hereditary characteristics from parents to the offspring, in the form of genes.
(c) Organelles: The tiny components of a cell present in the cytoplasm are known as organelles. These include mitochondria, golgi bodies, ribosome etc.
(d) Vacuole: It is a clear space in the cytoplasm that is bound by a membrane. A large vacuole is found in the plant cell whereas in animal cells they are very small. In amoeba, vacuoles are structures performing many functions such as digestion, excretion etc.
(e) Plastids: Plastids are found in the plant cells but are absent in animal cells. They are found scattered in the cytoplasm of the leaf cells. Green coloured plastids are called chloroplasts. They contain chlorophyll, which is necessary for photosynthesis.
Eukaryotes are made up of eukaryotic cells, which, like onion cells and cheek cells have a well-organised nucleus with a nuclear membrane. All organisms other than bacteria and blue green algae are called eukaryotes.
Prokaryotes are made up of prokaryotic cells, in which nuclear material exists without nuclear membrane. The organisms with these kinds of cells are called prokaryotes. Examples are bacteria and blue green algae.
The differences between animal and plant cells are given below:
• They do not have cell wall.
• They do not contain chloroplasts.
• They are smaller in size.
• They have rigid cell walls.
• They consist of chloroplasts.
• They are larger in size.
(a) Cytoplasm: Cytoplasm is the jelly-like substance that lies between the cell membrane and the nucleus. It contains many important structures called organelles e.g. mitochondria, chloroplast, etc.
(b) Nucleus of a cell: It is an important component of the living cell and functions as the control centre of the cell. It is generally spherical and located in the centre of the cell. It is separated from the cytoplasm by a membrane called the nuclear membrane, which bears the nuclear pores for exchange of materials between the cytoplasm and the nucleus.
In addition, the nucleus contains thread-like structures called chromosome that carry genes and help in the transfer of characteristics from the parent cell to the offspring. The chromosomes can be seen only when the cell divides.
Cell ‘A’ is prokaryotic cell because it has a nucleus, whereas cell ‘B’ is eukaryotic cell because it does not have a nucleus.
A bacterium is a prokaryotic cell, while animal or plant cells are eukaryotic.
A cell membrane is semipermeable in nature. It regulates which chemicals can pass through and which chemicals cannot pass through, thereby allowing the movement of water, minerals and other substances in and out from the cell.