Cell Structure And Functions - Class 8 Biology - Extra Questions

A cell is the smallest unit of life which has a definite structure and a specific function.

Unicellular	Multicellular
1. Organisms made of a single cell are said to be unicellular.	1. Organisms made of more than one cell/cell types are said to be multicellular.
2. All the functions are performed by a single cell.	2. Different cells are specialized to perform specific functions.
3. They are usually microscopic (small) and are less complex.	3. They are large and more complex.
E.g: amoeba	E.g: plants, animals etc.

• The average cell cycle span for a mammalian cell is about 24 hours.
• The cells remains in the G1 phase for about 10 hours.
• The duration of S phase or the Synthetic phase varies with respect to the total DNA present in a cell.

Eukaryotic

The cytoplasm is a jelly-like substance present between the cell membrane and the nucleus. It holds organelles and provides a site for various biochemical reactions.

The nucleus is generally spherical and located in the centre of the cell.

Each living organism is made up of cells. All the life processes take place inside the cell. A group of cells form a tissue to perform specific functions, these tissues combine together to form organs. Different organs together make up an organism. 

PLANT CELL	ANIMAL CELL
A.      Have a cell wall.	A. Have no cell Have wall.
B.      Nucleus is on side of cell.	B. Nucleus is in center on cell.
C.      Have large vacuoles.	C. Have only small vacuoles.
D.      Have regular in shape.	D. Often irregular in shape.
E.       Has plasmodesmata.	E. Has no plasmodesmata.

Answer-

• The human body is composed of trillions of cells. 
• Their function in our body can be appreciated by the fact that provides structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions.
•  Cells also contain the body's hereditary material and can make copies of themselves.

A cell is the structural and functional unit of life. Cytoplasm is the fluid portion of the cell that contains cell organelles. Cell organelles are membrane-bound compartments that perform a specialized function. 

In unicellular organisms, the body is made up of a single cell which can undergo cell division to regenerate. The cell has different organelles to carry out different functions. Being a single cell it is exposed to the environment on all sides. There is no division of labor. The cell body cannot attain large size due to a limited surface to volume ratio and injury to the cell can cause the death of an organism.

Cells are the $$\underline {basic}$$ and $$\underline {fundamental}$$ unit of life

The nucleus of the bacterial cell is not well-organised like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without 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 well-organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes.

1. Cell wall is present only in plant cells.
2. Plant cells have a central large vacuole. Animal cells have numerous small vacuoles.

Prokaryotic cell, characteristic of bacteria and cyanobacteria is a cell with genetic material not covered by a membrane and lacks all membrane bound organelles. Eukaryotic cell, characteristic of all organisms other than bacteria and cyanobacteria, is a cell with a membrane bound nucleus and a number of organelles.

Explanation:

Cells are so little, so they can maximize their ratio of surface area to volume. Smaller cells have a higher ratio which allows more molecules and ions to move across the cell membrane per unit of cytoplasmic volume. Cells are so small because they need to be able to get the nutrients in and the waste out quickly.

• The cell is the structural and functional unit of life. All living things are made up of tiny cells.
•  Life begins with a single cell. All the life activities take place within the cell. A cell performs all the biochemical processes that a body requires. 
• It performs in respiration, digestion, producing energy, reproduction, etc., hence it is called the structural unit of life. Each cell has its specialization in an organism.

The cell is the smallest structural and functional unit of living organisms, which can exist on its own. Therefore, it is sometimes called the building block of life. Some organisms, such as bacteria or yeast, are unicellular consisting only of a single cell while others, for instance, mammalians, are multicellular.

A cell is a fundamental structural and functional unit of life. Thus a cell is capable of existing independently and performing all the vital functions. The structure of cell consists of following structures, namely-

The cell was first discovered and named by Robert Hooke in 1665. He observed slices of cork under a simple magnifying device. In his magnifying device, he saw compartment-like structures similar to small rooms or cells of the monastery that he once lived. Hence, the term 'cell' was coined.

Complete the following table by putting Yes or No :

Since cell 1 does not have a nucleus it is a prokaryote, a photosynthetic bacteria.

 All living organisms are made up of cells

Cell organelles are the structure which can be found in the cytoplasm.

Cells make up the smallest level of a living organism such as yourself and other living things. The cellular level of an organism is where the metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental of life.

Cytoplasm is the protoplasm enclosed by the plasma membrane of cell, excluding the nucleus in eukaryotic cell and cellular DNA in prokaryotic cell.

Oxygen is carried in the blood in two forms: (1) dissolved in plasma and RBC water (about 2% of the total) and (2) reversibly bound to hemoglobin (about 98% of the total).

Single-celled organisms are called unicellular organisms. 'Uni-' means 'one,' so the name 'unicellular' literally means 'one cell.' An example of a unicellular organism would be certain types of algae such as Euglena, green algae, as well as any prokaryotic organism such as most bacteria.

Largest cell in the female body is the egg cell (ovum).

The nucleus of the bacterial cell is not well-organised like the cells of multicellular organisms. There is no nuclear membrane. The cells having nuclear material without 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 well-organised nucleus with a nuclear membrane are designated as eukaryotic cells. All organisms other than bacteria and blue green algae are called eukaryotes.

A cell is called as the fundamental structural and functional unit of life because they form the building blocks (bricks) of the main organs of organisms. The cells are the basic parts of all organs. These cells in group forms tissues that have specialized function and these tissues together form the organs. 

The given structure describes the level of organization of the body. Cells are minute structures and are the fundamental unit of life. A group of similar cells makes a tissue. An organ is made up of a group of tissues. These organs constitute an organ system. An organism is made of a group of organ systems.

A. Chloroplast - (iii) site of photosynthesis 
B. Cell membrane - (iv) controls the movement of materials into and out of cells. 
C. Nucleus - (ii) controls the activities of cells
D. Chromosome - (i) carries hereditary characters 

In somatic cells, mitosis-cell division takes place. 

Eukaryotic cell - has a nucleus and membrane bound cell organelles.

A unicellular organism has a single cell carrying out all life process. This is unable to exhibit a wide range of different functions and easily susceptible to damage, that can lead to the death of the organism.
For example: Amoeba, Chlamydomonas etc.
A multicellular organism has different cells carrying out different functions. They are more efficient and carry out many activities. They have a greater capacity of survival and they are continuously replaced.
For example: Cat, Dot, Human being etc. 

Chromosomes are located in nucleus and genes are located on the chromosomes. 

Differences between plant cell and animal cell.

	Plant cell	Animal cell
1.	Presence of cell wall and large central vacuole.	Cell wall absent, vacuole may or may not be present.
2	Dictyosomes present.	Golgi apparatus present.
3	Centrioles absent	Centrioles present.
4	Plastids present.	Plastids absent.

The body or structure of any living organism is made up of cells. These cells are also responsible for carrying out all the metabolic and regulatory functions in an organism. Due to these attributes, the cell is called the structural and functional unit of life.

The size of the cell does not dependent on the size of the body. This can be understood by taking the example of rat and elephant. The cells of the elephant won't be bigger than the size of the cells of the rat. The size of a cell only depends on its function. The nerve system of elephant and rat has the same function of receiving and transferring messages from body to brain. The nerve system helps to co-ordinate the functions of different parts of body. 

Cell	Tissue	Organ
RBC	Blood	Blood vessels
WBC	Muscle	Brain
Nerve cell		Heart
		Hand

A cell must have the following structural and functional attributes to be called a living cell: 

• Plasma Membrane: A cell should have a plasma membrane. Without the plasma membrane, the contents of the cell cannot get a container and the cell won't be able to maintain its independent existence.
• Functional Attributes of Plasma Membrane: The plasma membrane should be selectively permeable so as to facilitate the exchange of materials between the cell and its environment.
• Protoplasm: A cell must have a protoplasm to be called a living cell. It is the protoplasm which contains all the materials necessary for life to continue. If protoplasm dries up, the cell becomes dead; as in the case of sclerenchymatous cells.
• Endoplasmic Reticulum: Endoplasmic reticulum is necessary for the exchange of materials between nucleoplasm and cytoplasm. It is also necessary for the synthesis of various materials; like protein and lipid. Thus, a cell must have an ER to be considered a living cell. 
• Mitochondria are not mandatory for all living cells. For example; Red Blood Cells do not have mitochondria yet they are living cells. The prokaryotes do not have mitochondria but they are living cells. 
• The nucleus is not mandatory for all living cells. For example; mitochondria lack a nucleus. Similarly. prokaryotic cells lack a nucleus.
• Another important aspect of cells is that a cell is always formed from a pre-existing cell; as proposed in the Cell Theory. 

All living organisms are made up of cells that have different shapes and sizes. It is not the size of cells that brings such a big difference between an ant and an elephant but, it is the number of cells that make much difference in their body size.

Mast cells are oval-shaped cells of areolar connective tissue, having dense granules in their cytoplasm.
These secrete matrix of connective tissue, heparin and histamine. 

$$\begin{array}{|l|l|}\hline  {\text { Cell }} &  {\text { Tissue }} \\\hline \begin{array}{l}\text { Structural and functional units of living } \\\text { entities. They are the building blocks of } \\\text { life }\end{array} & \begin{array}{l}\text { Cluster of cells performing a specific } \\\text { function }\end{array} \\\hline \text { Example - Nerve cell } & \text { Example - Connective tissue } \\\hline\end{array}$$

Eukaryotes possess a true nucleus with nuclear membrane, whereas prokaryotes lack a true membrane-bound nucleus.

• All other organisms are multicellular, they consist of multiple cells and are more complex.
  
• The vast majority of plants and animals are multicellular.
• Being multicellular allows the organism to be larger and to have cell differentiation so different types of cells to carry out different tasks.

Unicellular Organisms : These are the living organisms that are made up of only one cell.

Answer:

PLANT CELL	ANIMAL CELL
A plant cell is surrounded by a rigid cell wall.	An animal cell does not have a cell wall.
Presence of a large central vacuole is seen in plant cells.	Animal cells show the presence of smaller vacuoles.
Chloroplast is present.	Chloroplast is absent.

• Although an elephant is larger in size than a rat, it doesn't mean that the elephant would have larger cells.
• The cells in the elephant's body are more in number than the rat.