CBSE Class 8 Science Cell and Tissue Notes

Lateral meristematic tissue occurs along the sides of the cental (longitudinal) axis of the plant. It gives rise to v ascular tissues.

Intercalary meristem occurs at the base of leaves or internodes. These cells grow fast and soon change into permanent tissues.

Permanent Tissue

The div ision and differentiation of the cells of meristematic tissues given rise to permanent tissues. Cell division is the formation of two or more daughter cells from one mother cell. Simple permanant tissue consist of similar permanent cells that perform the same function or a similar set of functions. Parenchyma, collencyma and sclerenchyma are three types of simple permanent tissues. Complex permanent tissue are a group of different types of cells that perform a common function. Xylem and phloem are two types of complex permanent tissue.

Parenchyma

This tissue is composed of large, thin-walled cells which are generally oval or spherical. The cells are not packed closely, i.e., there are intercellular space. These living cells with a nucleus and a v acuole are found in the soft parts of the plant. They store food, fill up spaces between other tissue and provide temporary support to the plant. W hen they contain chloroplasts, as in leaves, they help manufacture food.

Collenchyma

This tissue is composed of cells that are elogated and thickened with cellulose at the corners. There is no intercellular space. Collenchyma provides mechnical support to plant organs and is found in leaf stalks and below the epidermis of stems. It helps leav es and stems bend without breaking. It prov ides support, protection and flexibility ot plant organs. It is generally absent in roots.

Sclerenchyma

This tissue is composes of long, narrow cells whose walls are ev enly thickened with lignin. Lignin is a chemical that acts like cement, sticking fibres and hardening them. Sclerenchyma cells are dead. They are packed together closely, and prov ide strength and flexibility to plant parts. They are present in stems, v eins of leav es, the hard covering of seeds and nuts, and the husk of coconut. Fibre-yielding plants like jute and flax contain this tissue in abundance.

Xylem

Xylem, or wood, as it is often called, is a complex tissue. The cells are thick-walled, tubular and often dead. This tissue has four types of cells– tracheids, v essels, xylem parenchyma and xylem fibres. Of these only tracheids and vessels transport sap.

Phloem

Phloem too is a complex tissue made up of four types of cells, or elements–siev e tubes, companion cells, phloem fibres and phloem parenchyma. It is not necessary for the phloem to contain all four types of cells. Phloem to contain all four types of cells. Phloem cells are living cells (except phloem fibres) which help transport food from leav es to the storage organs and growing regions of the plant.

Animal Tissues

While doing the activities in this chapter, you have come across two types of animal tissue, the cheek cells are a type of epithelial tissue, while blood is a kind of connective tissue. There are two other types of animal tissue–muscular and nerv ous.

Epithelial Tissue:

This tissue covers the surface of the body and lines the internal organs. Its main function is protection. The cells that form the different types of epithelial tissue differ in shape. Some are thin and flat, some cubelike, while others are columnar.

Connective Tissue:

Blood, bones cartilages, tendons (which connect muscles with bones) and ligaments (which tie bones together) are diferent types of connectiv e tissue. Adipose tissue, or what is generally known as fat, is also a kind of connective tissue. Though different in structure, connective tissues have one thing in common-the cells are suspended or embedded in a matrix. In blood, the matrix is liquid and is called the plasma.

Muscular Tissue:

Muscular tissue is also of different types. However, the different types of muscular tissue (or muscles) have the same basic functoin. They contract and relax to make different parts of the body mov e. The muscles in our arms, legs thighs, back and so on help us move. The muscles in the heart help it pump blood. The muscles in the alimentary canal help the passage of food. The muscles in the blood v essels help them dilate and get constricted.

Nervous Tissue:

Nerve cells make up nerv ous tissue. A nerve cell has a long tail and short branches coming out of it.The tail too has branches. These help to carry message from one cell to the other. The brain and spinal cord are make up of nervous tissue.

¶ Plant Tissues

Plant tissue are basically of two types-meristematic and permanent. This differentiation is based on the ability of the mature cells of the tissue to divide and produce new cells. Meristematic tissue cells are capable of dividing, while permanent tissue cells are not.

¶ Meristematic Tissue

This tissue consists of actively dividing cells and is present in the growing regions of plants, e.g., the tips of roots and stems. The cells can be round, oval, polygonal or rectangular, but there are a few things they have in common. They are packed closely without intercellular spaces, have thin cellulose walls, dense cytoplasm and prominent nuclei. Vacuoles are almost absent in such cells because they are completely filled with sap. Depending on the region of the plant where it is present, meristematic tissue can be of three types- apical, lateral and intercalary.

Apical (from apex) meristem, as the name suggests, is present at the growing tips of stems and roots. Apical meristem is primary meristem.

Lateral meristematic tissue occurs along the sides of the cental (longitudinal) axis of the plant. It gives rise to vascular tissues.

Intercalary meristem occurs at the base of leaves or internodes. These cells grow fast and soon change into permanent tissues.

¶ Permanent Tissue

The division and differentiation of the cells of meristematic tissues given rise to permanent tissues. Cell division is the formation of two or more daughter cells from one mother cell. Simple permanant tissue consist of similar permanent cells that perform the same function or a similar set of functions. Parenchyma, collencyma and sclerenchyma are three types of simple permanent tissues. Complex permanent tissue are a group of different types of cells that perform a common function. Xylem and phloem are two types of complex permanent tissue.

¶ Parenchyma

This tissue is composed of large, thin-walled cells which are generally oval or spherical. The cells are not packed closely, i.e., there are intercellular space. These living cells with a nucleus and a vacuole are found in the soft parts of the plant. They store food, fill up spaces between other tissue and provide temporary support to the plant. When they contain chloroplasts, as in leaves, they help manufacture food.

¶ Collenchyma

This tissue is composed of cells that are elogated and thickened with cellulose at the corners. There is no intercellular space. Collenchyma provides mechnical support to plant organs and is found in leaf stalks and below the epidermis of stems. It helps leaves and stems bend without breaking. It provides support, protection and flexibility ot plant organs. It is generally absent in roots.

¶ Sclerenchyma

This tissue is composes of long, narrow cells whose walls are evenly thickened with lignin. Lignin is a chemical that acts like cement, sticking fibres and hardening them. Sclerenchyma cells are dead. They are packed together closely, and provide strength and flexibility to plant parts. They are present in stems, veins of leaves, the hard covering of seeds and nuts, and the husk of coconut. Fibre-yielding plants like jute and flax contain this tissue in abundance.

• Xylem
Xylem, or wood, as it is often called, is a complex tissue. The cells are thick-walled, tubular and often dead. This tissue has four types of cells– tracheids, vessels, xylem parenchyma and xylem fibres. Of these only tracheids and vessels transport sap.

• Phloem
Phloem too is a complex tissue made up of four types of cells, or elements–sieve tubes, companion cells, phloem fibres and phloem parenchyma. It is not necessary for the phloem to
contain all four types of cells. Phloem to contain all four types of cells. Phloem cells are living cells (except phloem fibres) which help transport food from leaves to the storage organs and growing regions of the plant.

• Animal Tissues
While doing the activities in this chapter, you have come across two types of animal tissue, the cheek cells are a type of epithelial tissue, while blood is a kind of connective tissue. There are two other types of animal tissue–muscular and nervous.

• Epithelial Tissue:
This tissue covers the surface of the body and lines the internal organs. Its main function is protection. The cells that form the different types of epithelial tissue differ in shape. Some are thin and flat, some cubelike, while others are columnar.

• Connective Tissue:
Blood, bones cartilages, tendons (which connect muscles with bones) and ligaments (which tie bones together) are diferent types of connective tissue. Adipose tissue, or what is generally known as fat, is also a kind of connective tissue. Though different in structure, connective tissues have one thing in common-the cells are suspended or embedded in a matrix. In blood, the matrix is liquid and is called the plasma.

• Muscular Tissue:
Muscular tissue is also of different types. However, the different types of muscular tissue (or muscles) have the same basic functoin. They contract and relax to make different parts of the body move. The muscles in our arms, legs thighs, back and so on help us move. The muscles in the heart help it pump blood. The muscles in the alimentary canal help the passage of food. The muscles in the blood vessels help them dilate and get constricted.

• Nervous Tissue:
Nerve cells make up nervous tissue. A nerve cell has a long tail and short branches coming out of it.The tail too has branches. These help to carry message from one cell to the other. The brain and spinal cord are make up of nervous tissue.

Question. Make a sketch of the human nerve cell. What funtion do nerve cells perform? 

Answer. Functions of human nerve cell:
(i) Nerve cells receive message from different parts of body.
(ii) They further transfer these messages to brain and accordingly brain send commands for functioning of different organs of body.

Question. Write short notes on the following:
(i) Cytoplasm (ii) Nucleus of a cell
Answer. (i) Cytoplasm: Cytoplasm is a jelly like substance which is present between the cell membrane and the nucleus. Various other organelles of cells are present in the cytoplasm. Cytoplasm is made up of chemical substances like carbohydrates, proteins and water. These chemical substances are present in cells of all types and sizes. Cytoplasm contains many important tiny substances called Organelles.
(ii) Nucleus of a cell: Nulceus is the master of the cell. It commands all the functioning of the cell. It is generally located in the center of the cell and is spherical in shape. A membrane called nuclear membrane separates it from cytoplasm. It contains the genetic material DNA and RNA in it. This porous membrane allows the transfer of material in the nucleus and cytoplasm. Nucleus contains a dense body called Nucleolus which actually contains Chromosomes, the genetic material.

Question. Which part of the cell contains organelles?
Answer. Cytoplasm.

Question. State a difference between eukaryotes and prokaryotes.
Answer. Prokaryotes do not have a well designed nuclear membrane while, eukaryotes have a well designed nuclear membrane.

Question. Where are the chromosomes found in cell? State their functions?
Answer. Chromosomes are found in the nucleus of a cell. Their function is to carry characteristic features of parent cells to the daughter cell means, from parent to offspring.

Question. Cells are the basic structural units of living organism. Explain.
Answer. In Biology, the basic unit of which all living thins are composed is knows as cell. The cell is the smallest structural unit of living matter that is capable of functioning independently. A single cell can be a complete organism in itself, as in bacteria and protozoans. A unicellular organism also captures and digests food, respires, excretes, grows, and reproduces. Similar functions in multi-cellular organisms are carried out by groups of specialized cell which are organized into tissues and organs such as, the higher plants and animals. Hence, ‘cell’ is known as the basic structural and functional unit of life.

Question. Explain why chloroplast are found only in plant cells.
Answer. Chloroplasts are found only in plant cells because they are required for photosynthesis.

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