TISSUE CLASS 9

 Tissues

1. What is a Tissue?

• A tissue is a group of cells that are similar in structure and work together to carry out a specific function. 

• Studying tissues is called histology. 

• In multicellular organisms, cells are organized into tissues, organs, organ systems, etc. 

2. Differences Between Plant and Animal Tissues

Some key structural and functional differences:

• Plants are mostly stationary; animals can move. 

• Many plant tissues (especially supportive ones) are made up of dead cells, while most animal tissues are living. 

• In plants, growth happens only in certain regions (meristem), while in animals growth is more uniform. 

• Animal organs and systems tend to be more specialized and localized compared to plants. 

3. Plant Tissues

Plant tissues are broadly classified into meristematic tissue and permanent tissue. 

3.1 Meristematic Tissue

• These are actively dividing cells. 

• Characteristics of meristematic cells: they have dense cytoplasm, thin cell walls, prominent nucleus, and lack large vacuoles. 

• Types of Meristem (based on location):

  1. Apical meristem — at tips of roots and shoots; responsible for length growth. 

  2. Intercalary meristem — at the base of leaves or internodes; helps in elongation. 

  3. Lateral meristem (Cambium) — located on sides (e.g., vascular cambium); helps in girth (thickness) growth. 

3.2 Permanent Tissue

After dividing, meristematic cells can differentiate into permanent tissue. 
Permanent tissues are of two kinds: simple and complex. 

3.2.1 Simple Permanent Tissue

Consists of only one type of cell. Three main types:

1. Parenchyma

   • Living cells, thin walls, loosely arranged with intercellular spaces. 

   • Functions:

     • Storage of food. 

     • In some parenchyma, chlorophyll is present (called chlorenchyma) and helps in photosynthesis. 

     • In aquatic plants, parenchyma may have large air cavities (aerenchyma) to help float. 

2. Collenchyma

   • Living cells, elongated, with cell walls thickened at the corners. 

   • Very little intercellular space. 

   • Function: Provides mechanical support and flexibility (allows bending of stems/tendrils without breaking). 

3. Sclerenchyma

   • Cells are dead at maturity, with very thick, lignified cell walls. 

   • Function: Provides rigidity and strength. Found in seed coats, hard coverings, veins, etc. 

Protective Simple Permanent Tissue:

• Epidermis: Single layer of cells covering the plant surface. 

  • Functions: protection against injury, pathogens; in leaves, contains stomata for gas exchange; in roots, root hairs increase surface area for absorption. 

• Cork (in secondary growth): Cells are dead, compact, and have suberin in their walls making them water-resistant. 

3.2.2 Complex Permanent Tissue

• Made up of more than one type of cell. 

• Xylem and Phloem are the two major complex tissues.

Xylem:

• Functions: transport water and minerals, and provide support. 

• Components:

  1. Vessels – long tubes, dead cells, thick walls. 

  2. Tracheids – elongated dead cells, thick walls. 

  3. Xylem parenchyma – living cells, store food.

  4. Xylem fibres – provide support; dead, with thick walls. 

Phloem:

• Function: transports food (mainly sugars) from leaves to other parts. 

• Components:

  1. Sieve tubes – elongated cells with perforated walls (sieve plates), living but no nucleus in mature sieve elements.

  2. Companion cells – help the sieve tubes; living, with nucleus. 

  3. Phloem parenchyma – for storage. 

  4. Phloem fibres – provide mechanical strength; dead or living depending on species. 

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4. Animal Tissues

Animal tissues are classified into four main types: epithelial, connective, muscular, and nervous. 

4.1 Epithelial Tissue

• Covers body surfaces, lines cavities and organs. 

• Characteristics: tightly packed cells, very little intercellular space, form a continuous sheet. 

• There is also a basement membrane which separates epithelium from underlying tissues. 

• Functions: protection, absorption, secretion, and filtration (depending on location). 

4.2 Connective Tissue

• Most abundant tissue type in animals. It supports, connects, or separates other tissues or organs. 

• Types of connective tissue include: bone, cartilage, blood, adipose tissue, ligaments, tendons, etc. 

• Characteristics vary, but generally have lot of extracellular matrix (space outside cells) which may be solid (bone), semi-solid (cartilage), or fluid (blood). 

• Functions: support, protection, transport (blood), storage (fat), and more. 

4.3 Muscular Tissue

• Function: movement. Muscle cells (fibres) can contract. 

• Types of muscles in animals:

  1. Skeletal muscle – attached to bones, voluntary control.

  2. Smooth muscle – in walls of internal organs, involuntary.

  3. Cardiac muscle – in heart, involuntary, specialized.

4.4 Nervous Tissue

• Function: transmission of impulses (messages) throughout the body. 

• The basic unit is neuron (nerve cell).

• Structure of a neuron:

  • Cell body (soma): contains nucleus and cytoplasm. 

  • Dendrites: receive signals. 

  • Axon: transmits impulses away from the cell body. 

• Nervous tissue helps in coordination, sensory input, and responses. 

5. Importance of Tissues in Multicellular Organisms

• Division of Labour: Different tissues do different jobs, making the organism efficient.

• Specialization: Because cells are grouped into tissues, they can specialize, e.g., muscle cells contract, nerve cells transmit signals. 

• Support & Strength: In plants, sclerenchyma and collenchyma give structure; in animals, connective tissues (like bone) provide support.

• Transport: Vascular tissues (xylem and phloem) in plants transport water, minerals, and food; in animals, blood transports oxygen, nutrients, etc

• Protection: Epithelial tissue protects organs, prevents infection; in plants, epidermis protects against injury and water loss.

• Coordination: Nervous tissue helps the body respond to stimuli; muscles help in movement. 

6. Important Diagrams to Draw / Remember

For exams, it's helpful to be able to draw and label:

1. A meristematic cell (showing thin wall, dense cytoplasm)

2. Parenchyma, collenchyma, and sclerenchyma cells (structure differences)

3. A piece of xylem (showing vessels, tracheids, fibres, parenchyma)

4. A piece of phloem (sieve tube, companion cell, fibre, parenchyma)

5. Neuron (nerve cell) — with cell body, dendrites, axon

7. Key Definitions / Terms to Remember

• Tissue

• Histology

• Meristematic tissue

• Apical, intercalary, lateral meristem

• Differentiation

• Permanent tissue (simple vs complex)

• Parenchyma, Collenchyma, Sclerenchyma

• Epidermis, Cork

• Xylem (vessel, tracheid, xylem parenchyma, xylem fibre)

• Phloem (sieve tube, companion cell, phloem parenchyma, phloem fibre)

• Epithelial tissue

• Connective tissue

• Muscular tissue (types)

• Nervous tissue, Neuron