Revision: Organisation of Tissues Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

A group of similar cells, along with intercellular substances which perform a specific function, is called a tissue.

• Cells that perform a particular function always live in a group. This group of cells is called a tissue.
• For example, blood, phloem, muscle, etc. are examples of tissues.

A group of similar cells which perform a specific function.
example: Muscular tissue in animals.

A tissue is a group of cells that are similar in structure and are organized together to perform a specific task.

The tissue is a group of cells of similar structure and function.

Vascular tissue is the complex plant tissue in higher plants that are composed of xylem and phloem and is concerned with conducting water, minerals, and organic food throughout the plant body.

A tissue, in biology, is defined as a group of cells that have a similar structure and perform a specific function. The word tissue originates from French, which means "to weave."

Meristematic tissue is a group of cells that constantly divide and produce cells indefinitely throughout the life of the plant.

Permanent tissue refers to a group of cells which temporarily or permanently cease to divide and thus assume permanent form and function

The population of all plant and animal species living in a particular area constitutes a biotic community.

The zone on the earth in which all living beings exist is termed as the biosphere.

All the members or individuals of a particular species living in a particular area constitute its population.

A group of living organisms that can breed among themselves constitute a species.

The epithelial cells are connected to each other laterally as well as to the basement membrane by junctional complexes called cell junctions.

The tissue which connects various tissues together in any organ is called connective tissue.

An organ is a group of tissues that work together to perform a specific function or set of functions in the body.

The heart, lungs, liver, and kidneys are examples, each playing a vital role in maintaining the body's overall health and function.

The cells that constitute nervous tissue are called neurons or nerve cells.

The centrosome (in animal cell) splits into two along with the simultaneous duplication of the centrioles contained in it. The daughter centrioles move apart and occupy opposite "poles" of the cell. Each centriole is surrounded by radiating rays and is termed an aster (aster : star).

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

• Anatomy = Study of Internal Structure - Plant anatomy is the study of the internal structure of plants, which includes the organisation and structure of tissues.
• Basic Unit = Cell - The basic unit of plants is the cell. Cells are organised into Tissues → Organs (organisational hierarchy).
• Tissue Definition - A tissue is a group of similar cells having a common origin that perform a specific function together.
• Monocots vs. Dicots - Anatomical (internal structural) differences exist between monocots and dicots, so it's important to know them separately.
• Internal Structure Adapts - The internal structures of plants adapt according to their environment (e.g., water availability, climate), and structural similarities exist in both external and internal morphology of organisms.

• Meristematic tissue consists of cells that can divide and is restricted to specialised regions of the plant.
• Apical meristem is found at the root and shoot tips, is a primary meristem and increases the length of the plant.
• Intercalary meristem is found between mature tissues, is a primary meristem and helps form branches and flowers.
• Lateral meristem is found in mature regions, is a secondary meristem and is responsible for secondary growth (e.g. vascular cambium, cork cambium).
• Axillary bud is present in the axils of leaves and is responsible for forming branches or flowers.

• Parenchyma has thin-walled cells with intercellular spaces; the cell wall is made of cellulose. It performs photosynthesis, storage and secretion.
• Collenchyma is made of living, closely packed cells that are thickened at corners due to the deposition of cellulose and pectin. It provides mechanical support to young stems and petioles.
• Sclerenchyma is made of dead cells with thick, lignified walls and has two types of cells: fibres and sclereids.

Xylem

• Xylem consists of tracheids, vessels, xylem fibres and xylem parenchyma. It conducts water and minerals from roots to other parts of the plant.
• In stems (endarch), protoxylem is towards the centre and metaxylem towards the periphery. In roots (exarch), protoxylem is towards the periphery and metaxylem towards the centre.

Phloem

• Phloem consists of sieve tube elements, companion cells, phloem fibres and phloem parenchyma. It transports food from leaves to various parts of the plant.
• Mature sieve tube elements lack a nucleus, so their functions are controlled by the nucleus of companion cells.
• Protophloem has narrow sieve tubes (first formed), and metaphloem has bigger sieve tubes (later formed).

• Epithelial tissue has a free surface that faces either body fluids or the external environment, and its cells are tightly packed with very little intercellular space.
• Simple epithelium consists of a single layer of cells and mainly performs functions like lining, secretion, and absorption.
• Squamous epithelium is made of thin, flat cells and helps in diffusion, while cuboidal epithelium has cube-shaped cells and is involved in secretion and absorption.
• Columnar epithelium consists of tall cells that help in secretion and absorption, and ciliated epithelium contains cilia that move substances like mucus in a specific direction.
• Glandular epithelium is specialised for secretion and may be unicellular (goblet cells) or multicellular (such as salivary glands).
• Exocrine glands release their products through ducts, whereas endocrine glands secrete hormones directly into body fluids.
• Compound epithelium has multiple layers of cells and mainly provides protection, while cell junctions (tight, adhering, and gap junctions) help in preventing leakage, holding cells together, and communication between cells.

• Connective tissue is the most abundant tissue in the animal body, linking and supporting other tissues. Its cells (except in blood) secrete collagen or elastin fibres for strength, elasticity and flexibility.
• Loose connective tissue includes areolar tissue (supports epithelium; has fibroblasts, macrophages and mast cells) and adipose tissue (stores fat); both are found beneath the skin.
• Dense regular tissue has parallel collagen fibres; tendons connect muscle to bone, and ligaments connect bone to bone.
• Dense irregular tissue has collagen fibres oriented differently and is present in the skin.
• Specialised connective tissue includes cartilage (chondrocytes + collagen fibres) and bone (calcium salts + collagen fibres with osteocytes in lacunae).

• Muscle fibres are composed of fine fibrils called myofibrils and bring about movement and locomotion.
• Skeletal muscles are long, cylindrical, multinucleated, striated fibres closely attached to skeletal bones (voluntary).
• Smooth muscles are spindle-shaped, uninucleated, non-striated fibres found in the walls of internal organs like blood vessels, stomach and intestine (involuntary).
• Cardiac muscles are short, cylindrical, uninucleated fibres found only in the heart wall, with intercalated discs for communication between cells.

• Cell division is a vital process for growth, repair, and the formation of new organisms, helping maintain life in all living beings.
• It occurs in two forms: mitosis (in somatic and stem cells) for producing diploid identical cells, and meiosis (in germ cells) for forming haploid gametes.
• Mitosis supports body growth and tissue repair, while meiosis ensures genetic variation and maintains chromosome number in reproduction.
• Before division, the cell’s chromosome number doubles (e.g., from 2n to 4n) to ensure accurate distribution during mitosis or meiosis.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

• Cytokinesis is the division of cytoplasm into two daughter cells after karyokinesis (nuclear division).
• In animal cells, a furrow appears in the plasma membrane, deepens, and joins in the centre to divide the cytoplasm.
• In plant cells, a cell plate forms in the centre and grows outward to meet the lateral walls, forming a new cell wall. The cell plate represents the middle lamella between two adjacent cells.
• During cytokinesis, organelles like mitochondria and plastids are distributed between the two daughter cells.
• In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).

• Mitosis is an equational division that produces genetically identical daughter cells, maintaining chromosome number.
• It is responsible for growth in multicellular organisms by increasing the number of cells.
• Mitosis helps in repair, regeneration, and wound healing by replacing damaged or lost cells.
• It maintains the nucleo-cytoplasmic (surface/volume) ratio, which gets disturbed during cell growth.
• Mitosis also enables asexual reproduction in unicellular organisms and continuous cell replacement (e.g., skin, blood, plant meristems).

• Meiosis is a specialised reduction division that produces four haploid daughter cells with half the chromosome number.
• It involves two sequential divisions (Meiosis I and II) but only one round of DNA replication during interphase.
• Occurs during gametogenesis — in humans (testes and ovaries) and in plants (anthers and ovaries).
• It includes pairing of homologous chromosomes and crossing over, leading to genetic recombination.
• Maintains chromosome number across generations by alternating haploid and diploid phases; fertilisation restores the diploid (2n) condition.

• Meiosis I is a reductional division, where the chromosome number reduces from diploid to haploid.
• Prophase I is the longest and most complex stage, divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• In Leptotene, chromosomes become visible and start condensing.
• In Zygotene, homologous chromosomes pair (synapsis), forming bivalents/tetrads with the help of the synaptonemal complex.
• In Pachytene, crossing over occurs between non-sister chromatids, causing genetic recombination.
• In Diplotene and Diakinesis, homologous chromosomes begin to separate; chiasmata appear, chromosomes fully condense, and the nuclear membrane breaks down.
• During Metaphase I, Anaphase I, and Telophase I:
  • Chromosomes align at the equator (Metaphase I)
  • Homologous chromosomes separate (Anaphase I)
  • Two haploid cells (dyads) are formed after Telophase I and cytokinesis

• Formation of Gametes - Meiosis is responsible for the formation of gametes in sexually reproducing organisms.
• Reduces Chromosome Number - Meiosis reduces the chromosome number by half in gametes, preventing doubling of chromosomes in each generation.
• Restores Chromosome Number - The original chromosome number is restored when gametes fuse during fertilisation.
• Conserves Species-Specific Number - Meiosis ensures that each species maintains its specific chromosome number across generations.
• Genetic Variability - Meiosis increases genetic variability in the population from one generation to the next, which is crucial for the process of evolution.