Cells and Tissues

The invention of microscope helped in the discovery of the cells. Robert Hooke discovered the cell in 1665. Soon a cell theory was formulated.

The cell theory states that 

• Cell is the structural and functional unit of all living beings and bodies of all organisms are composed of cells.
• All new cells arise by division of pre-existing cells.
• The functions of an organism are an outcome of the combined activities and the interactions of the cells that make the organism.

A cell may be defined as the structural and functional unit of living organisms which is capable of independent existence.

Prokaryotic are Eukaryotic Cells

All cells have three basic parts:

1. Cell membrane which limits the cell and gives it shape.
2. DNA which may be contained in a nucleus.
3. Fluid called cytoplasm filling in the space within the cell.

Whether DNA of the cell lies in the cytoplasm or is enclosed within a nuclear membrane, cells are termed prokaryotic are eukaryotic. 

Prokaryotic Cell (Pro-before; karyon-nucleus)

These cells do not have a well-organized nucleus. The genetic material is a single molecule of DNA lying in the cytoplasm. Cell organelles like mitochondria, lysosomes, endoplasmic reticulum, chloroplast, nucleolus, etc. are also not present in prokaryotic cells. Examples: Bacteria and blue-green algae.

Eukaryotic cell (Eu-true; karyon-nucleus)

DNA is enclosed in a nuclear membrane forming a nucleus. The genetic material is made of two or more DNA molecules, which are present as a network of chromatin fibres when the cell is not dividing. Membrane-bound organelles, such as mitochondria, endoplasmic reticulum, lysosome, chloroplast, nucleolus, etc. are present within the cytoplasm. Examples: Cells of plants, fungi, protozoa and animals.

Structure of Eukaryotic Cell

Cells within the body of a multi-cellular organism differ in shape, size and function, but have three basic parts - cell membrane, cytoplasm and nucleus.

Cell Membrane (Plasma Membrane)

It is a thin delicate membrane enclosing the cell. It forms outermost covering in animal cell and inner to cell wall in plant cell. Cell membrane is selectively permeable, so allows only selected substances to pass in and out of the cell. It protects cell from injury and maintains shape of cell.

Cytoplasm

It is translucent, homogeneous, colloidal semi fluid filling the space between plasma (cell) membrane and nucleus. Cell organelles are present in it. Cytoplasm helps in manufacture and distribution of substances within the cell and in exchange of materials between different cell organelles.

Nucleus

It is small, located in or near the centre of the cytoplasm. Nucleus is bound by a nuclear membrane. Network of chromosomes is present as chromatin. One or more rounded nucleolus are present in the nucleus. Nucleus coordinates the activities of the entire cell and contains the genetic material or DNA.

Endoplasmic reticulum (ER)

It is irregular network of double membranes in the cytoplasm. Ribosomes may be present on endoplasmic reticulum. It gives rigidity to the cell and helps in the synthesis and transport of various proteins and fats within the cell to the outside.

Ribosomes

Ribosomes are granules either scattered freely in the cytoplasm or attached to the endoplasmic reticulum. They are sites for protein synthesis.

Mitochondria

They are minute sausage shaped or rod shaped granular bodies scattered in the cytoplasm. They carry out cellular respiration. They are called powerhouse of cell because energy gets released and stored in them during respiration.

Golgi Bodies 

Stacks of flattened sacs or small vesicles generally located near the nucleus. Similar structures in a plant cell are called dictyosomes. They help in the secretion and storage of substances such as enzymes, hormones, etc.

Lysosomes

Lysosomes are small vesicles or sacs containing digestive enzymes, which destroy and digest the worn out cell parts. They help to rapidly destroy and digest damaged cells and their parts, hence these also known as suicide bags. They clean up the cell debris.

Vacuoles

These are fluid filled membrane-bound spaces. Large-sized vacuoles are in plants and smaller and fewer ones in animals. They help in storage of water and other substances. 

Granules

These are small particles, crystals or droplets. Granules containing starch, fat, etc. serve as food for the cell.

The vacuoles and granules are the non-living parts of a cell.

Cell wall (plant cell only)

It is outer, rigid, protective, supportive and semi-transparent covering of a plant cell made of cellulose. It provides a definite shape and rigidity to the cell. It protects the plasma membrane and internal structures.

Plastids (Plant cell only)

Plastids are of three types - chloroplasts, chromoplasts and leucoplasts. Chloroplasts are green. They possess photosynthetic pigment - chlorophyll and carotenoids. Chromoplasts contain yellow, orange or red coloured pigment. Leucoplasts are colourless plastids.

Chloroplasts help in photosynthesis. Chromoplasts provide colour to the flowers and the fruits. Leucoplasts help in the storage of food.

Centrosome (Animal cell only)

It is a small body lying above the nucleus. It consists of two small granules called centrioles. It participates in cell division and help in spindle formation during cell division.

Difference Between Plant and Animal Cell

Cell Division

New cells are required for replacement of worn out cells, for repair of cuts and injuries, and for growth and for reproduction. New cells are obtained through cell division. There are two types of cell division:

1. Mitosis: In mitosis, a cell gives rise to two identical daughter cells. Mitosis is needed for growth, and repair of worn out parts.
2. Meiosis: Cell division involved in production of sex cells which give rise to the egg in female and sperm in male.

Mitosis

Mitosis is an equational division in which the two daughter cells are identical to each other and to their parent cell.

• The chromosomal material (chromatin network) inside the nucleus condenses to form chromosomes. The nuclear membrane disappears.
• The centrosome (in animal cell) divides into two equal parts called centrioles, each of which migrates to opposite sides to orient the spindle which forms in the cytoplasm. A spindle of fibres appears between the centrioles.
• Each chromosome consists of two chromatids which are held by a centromere. The chromosomes arrange in the middle or equator of the spindle.
• Centromere splits. The chromatids (daughter chromosomes) of each chromosome now have their own centromere. The chromatids, now termed chromosomes separate from each other and subsequently, move to the opposite poles of the spindle.
• Nuclear membrane reappears around each of the two new clusters of the chromatin material, formed at the poles.
• In the middle of the cell, at the two sides a furrows appear in the cell membrane. The furrows deepen to divide the parent cell into two new identical daughter cells.

Meiosis

Meiosis is necessary for sexual reproduction. In animals, meiosis takes place in reproductive organs, such as the testis and the ovary, that produce eggs and sperms; and also in flowering plants it occurs in the anthers and ovary to produce pollen grains and the ovule, respectively.

Tissues

A tissue is defined as a group of cells similar in size, shape, performing the same function and having a common origin.

Plants are able to produce new tissues throughout their life. Animals can replace only some tissues under certain conditions. Muscles of heart and nervous tissue can never be formed again if damaged.

Plant Tissues

Plant tissues are of two types:

1. Meristematic tissue
2. Permanent tissue

Meristematic Tissue

It is found at the growing points of a plant such as at the tips of the roots, stems and branches.

Permanent Tissue

It is made up of cells, which have lost their ability to multiply. The permanent tissues are of three types:

• Protective tissue
• Supporting tissue
• Conducting tissue

Animal Tissues

Animal tissues are grouped under four main categories:

1. Epithelial tissue
2. Connective tissue
3. Muscular tissue
4. Nervous tissue