SSC Biology Cells and Tissues / कोशिका और ऊतक Cells and Tissues

Introduction

• Cell is a basic structural and functional unit of life.
• Robert Hooke in 1665 coined the word 'cell'.
• Anton von Leeuwenhoek first saw and described a live cell.
• Robert Brown later had discovered the nucleus.
• Cell theory was proposed by Schleiden and Schwann in 1855 to explain the concept of the cellular nature of living organism.

• Prokcaryotic Cells
• Prokaryotic cells are morphologically most primitive.
• Prokaryotic cells are devoid of membrane bound organelles like plastids, mitochondria and advanced (9+2) flagella.
• Prokaryotic cells are represented by bacteria, cyanobacteria (blue green algae) mycoplasma and PPLO (pleuropneumonia like organisms).

• Eukaryotic Cells:

A eukaryotic cell consists of the following components:

• Cell Wall
• The cell wall is a non-living, semi-rigid, external protective covering of the cell.
• Cell wall is entirely lacking in animals.
• It is made up of cellulose secreted by the cell itself.

• Cell Mebrane
• The cell membrane is a living, thin, elastic and semi- permeable membranous covering of plant and animal cells.

• Fluid mosaic model of plasma membrane
• J. Singer and G. Nicolson in 1972 proposed the most accepted model of membrane structure. The plasma membrane is a lipid-bilayer with proteins embedded in it.
• Lipids are amphipathic, i.e., they are structurally asymmetric with polar hydrophilic and non-polar hydrophobic group.
• One of the most important function of plasma membrane is the transport of the molecules across it.

• Endoplasmic Reticulum (ER)

There are two types of endoplasmic reticulum i.e.,

• Smooth or agranular ER - They do not have attached ribosomes on their surface.
• Rough or granular ER - They bear ribosomes on their surface, for protein synthesis.

• Golgi apparatus
• Golgi apparatus or Golgi complex is a stack of flattened, membrane bounded, parallely arranged organelles that occur in the association of endoplasmic reticulum in the cytoplasmic matrix.
• The Golgi apparatus principally performs the function of packaging materials to be delivered either to the intra-cellular targets or secreted outside the cell.

• Lysosomes
• Lysosomes are popularly called "suicide bags".

• Vacuoles
• In plant cells, the vacuoles can occupy up to 90 percent of the volume of the cell. The vacuole is bound by a single membrane called tonoplast. They are responsible for maintenance of turgour pressure.

• Mitochondria
• Mitochondria are also called as powerhouse of cells.

• Plastids
• Plastids are found in plants and few protists Euglena.

• Ribosomes
• Ribosomes are smallest cell organelles. They are protein synthesising factories. There are two types of ribosomes viz., Prokaryotic or 70S ribosomes - Eukaryotic or 80S ribosomes

Nucleus

• Nucleus is centrally located, spherical and largest component of the all eukaryotic cell. It contains the genetic material of the cell.
• Structure of Nucleus

A typical nucleus consists of four structures: (i) nuclear membrane, (ii) nucleoplasm (iii) chromatin and (iv) the nucleolus.

• Nucleolus
• It is present inside the nucleus.
• It is the site of active ribosomal RNA synthesis.

Cell Cycle

• Cell cycle consists of two basic phases. There is a long non-dividing growing I-phase (Inter-phase) and a short- dividing M-phase.

• Phases of Cell Cycle

Interphase:

It has following three sub-phases:

• \[{{G}_{1}}\]Phase: It (G stands for gap) includes the synthesis of substrate and enzyme necessary for DNA synthesis.
• S phase: During this phase the amount of DNA per cell doubles.
• \[{{G}_{2}}\] Phase: Proteins are synthesized in preparation for mitosis while cell growth continues.
• \[{{G}_{0}}\] Phase: Cells that do not divide farther exit \[{{G}_{1}}\] phase enter an inactive stage known as\[{{G}_{0}}\].

• M Phase (Mitosis Phase)

It is also called as equational division as the number of chromosomes in the parent and progeny cells is the same. The 4 Stages of M Phase are: (usually divided into several stages or phases)

• Prophase: Initiation of assembly of spindle formation begins.
• Metaphase: Chromosomes align at the equatorial plate of cells.
• Anaphase: The two daughter chromatids, now free of each other, move towards their respective poles.
• Telophase: The nucleolus condense and reappear. The spindle fibres disperse. The nuclear envelope is assembled around the chromatin mass endoplasmic reticulum reform again.
• Cytokinesis: Karyokinesis (division of nucleus into two) is followed by cytokinesis ie., division of cytoplasm into two daughter cells.

Meiosis

 It occurs in reproductive cells and has two parts:

• Meiosis I
• Prophase I: It is the longest stage and includes 5 stges: -
• Leptotene: The chromosomes become gradually visible under the light microscope.
• Zygotene: The pairing of homologous chromosomes takes place.
• Pachytene: Crossing over is the exchange of genetic material between two homologous chromosomes.
• Diplotene: The participating chromatids of the paired homologous chromosomes physically joined at one or more discrete points having X-shaped structure called chiasmata.
• Diakinesis: During diakinesis, the terminalisation of chiasmata take place.
• Metaphase I: Spindle fibres attach themselves to chromosomes and chromosomes align at the equator.
• Anaphase I: Homologous chromosome with its two chromatids moves towards the opposite poles of the cell and separate from each other.
• Telophase I: The nuclear membrane and nucleolus reappear.

• Meiosis II
• Prophase II: The nuclear membrane and the nucleolus disappear. The chromosomes condense further.
• Metaphase II: The chromosomes get arranged on the equator of the spindle.
• Anaphase II: The daughter chromosomes move towards the opposite poles.
• Telophase II: Cytoplasm divides and 4 haploid daughter cells arise.

Tissues

A group of structurally similar or dissimilar cells that perform a common function and have a common origin is called a tissue.

• Simple Permanent Plant Tissue

These tissues are of 3 types:

• Parenchyma: The cell wail is thin and made up of cellulose. It helps in storage of food, conduction of substances, provides turgidity to softer parts of plants.
• Collenchyma: It provides mechanical support to the organs and resists bending in wind.
• Sclerenchyma: These are dead, mechanical and act as skeleton in plants. It is hardest plant tissue, madeup of highly thick walled cells with no nucleus and no cytoplasm. This uniform thicknening is made up of mainly lignin and cellulose or both.

Conducting tissue in plants

• Xylem cells conduct water and minerals from roots to shoots.
• Phloem cells transport food or photosynthates from leaves to all parts of plants.

• Animal Tissues

The structure of the cells vary according to their function. Therefore, the tissues are different and are broadly classified into four types:

• Epithelial Tissue

• Connective Tissue

• Muscle Tissue

(i) Skeletal muscle

(ii) Cardiac muscle

(iii) Smooth muscle