Chapter Notes: Tissues in Action

Table of Contents
1. Introduction
2. 3.1 Why are Plant and Animal Tissues Different?
3. 3.2 Tissues for Growth in Plants
4. 3.3 Animal Tissues
5. 3.4 The Musculoskeletal System
6. 3.5 Types of Joints
7. 3.6 Skeletal System
8. Think as a Scientist
9. Scientists Spotlight
10. AT A GLANCE - Summary
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Introduction

Life begins when a single cell divides several times to form many cells. These cells gradually become different organs like skin, muscles, bones, and nerves. In multicellular organisms, there is a hierarchy - cells form tissues, tissues form organs, organs form organ systems, and organ systems form the organism.

Tissue: A group of cells similar in structure that work together to perform a specific function.

Division of labour in multicellular organisms increases efficiency and enables complex life processes. For example, muscle tissue enables movement and nervous tissue carries messages.

3.1 Why are Plant and Animal Tissues Different?

Plants and animals differ greatly in structure, lifestyle and nutrition, which is why their tissues are also different.

3.2 Tissues for Growth in Plants

Plants grow in three ways:

• Increase in length (height of stem, depth of roots) - by apical meristem
• Increase in girth (thickness of stem) - by lateral meristem
• Regrowth after cutting or grazing - by intercalary meristem

All these growth processes require actively dividing cells, which together form meristematic tissue.

3.2.1 Apical Meristem - How do plants grow in length?

Plants have growth zones at the tips of their roots and shoots. These regions are called apical meristems. The experiment with onion bulbs (Activity 3.1) shows that roots grow only from their tips - when tips are cut, roots stop growing. This confirms that apical meristems contain actively dividing cells responsible for increase in length.

3.2.2 Lateral Meristem - How do plants grow in girth?

The stems of dicot plants increase in diameter over time. This is due to the lateral meristem - a ring of actively dividing cells in the stem. These cells divide and produce new cells inward and outward in a concentric manner, increasing the diameter. Annual growth rings visible in a cut tree trunk are formed by the lateral meristem activity. By counting these rings, scientists can estimate the age of a tree.

3.2.3 Intercalary Meristem - How do plants grow after being cut?

When the tip of a young stem is cut, the stem stops growing in length but new branches arise from the nodes. The intercalary meristem is located at the base of the internode or just above the node. This is why:

• Grass grows back after mowing or grazing by animals
• Hedges become bushy after cutting

Key Terms:

• Node: Point on stem where branches/leaves arise.
• Internode: Part of stem between two nodes.

3.2.4 Characteristics of Meristematic Tissue Cells

Meristematic cells are specially adapted for continuous, rapid cell division:

• Small in size
• Thin cell walls
• Large and prominent nucleus
• Dense cytoplasm with many organelles
• Vacuoles absent (vacuoles are absent because they would use up space needed for dividing)
• Tightly packed - no intercellular spaces

3.2.5 Permanent Tissues

Due to continuous cell division, meristematic tissue adds new cells to the plant body. Some newly formed cells remain meristematic; others lose the ability to divide. These cells undergo changes and become permanent tissues - specialised to perform specific functions like support, transport or storage. The process by which meristematic tissue becomes specialised is called differentiation.

Permanent tissues are of two types:

• Simple permanent tissues - made of only one type of cell
• Complex permanent tissues - made of more than one type of cell

Internal Structure of a sunflower stem

(i) Protective Tissue - Epidermis

The epidermis is the outermost protective layer of the plant body. It is a tightly packed, single layer of flat and rectangular cells.

• Cells are covered by a waxy layer called the cuticle - reduces water loss and provides protection
• In dry habitat plants, the cuticle may be very thick
• In roots, hair-like projections from epidermal cells are called root hair - they increase surface area for absorption of water and minerals
• In leaves, the epidermis has pores called stomata - help in gaseous exchange, transpiration, and elimination of wastes
• Transpiration creates a transpiration pull in xylem, which helps water move upward

(ii) Supporting Tissue - Simple Permanent Tissues

Three types of simple permanent tissues provide support to plants:

(iii) Conducting Tissues - Complex Permanent Tissues

Plants have specialised conducting tissues called xylem and phloem, together called complex permanent tissues (made of more than one type of cell).

Xylem: Transports water and minerals from roots to other parts of the plant. Also provides strength. Consists of:

• Tracheids - tubular, thick-walled, dead cells
• Vessels - tubular, thick-walled, dead cells
• Xylem parenchyma - the only living component of xylem
• Xylem fibres - primarily sclerenchymatous, dead cells

Phloem: Transports food (mainly sugars) from leaves to other parts. Mostly made of living cells. Consists of:

• Sieve tubes - long tubular cells joined end to end with perforated walls; transport food
• Companion cells - specialised parenchyma cells; regulate cellular functions of sieve tubes; monitor loading/unloading of sugars
• Phloem parenchyma - stores food materials like resin, tannins, latex
• Phloem fibres - primarily sclerenchymatous; provide strength

Vascular tissue: (a) xylem, and (b) phloem

3.2.6 Tissue Systems in Plants

Plant tissues are organised into three tissue systems:

• Dermal tissue system - forms outer covering; protects inner parts and reduces water loss (epidermis)
• Ground tissue system - forms the main body between dermal and conducting tissues; includes parenchyma, collenchyma, sclerenchyma
• Vascular tissue system - consists of conducting tissues xylem and phloem

Ready to Go Beyond

In young plants, the outer protective layer is a single layer. As the plant matures, some cells beneath the epidermis of the stem gain the ability to divide. These cells act as lateral meristematic cells and form the cork cambium. When cork cambium cells divide, they produce cork cells. 

•  Cork cells are dead, tightly packed, and contain a substance that makes them impermeable to water and gases. 
•  This layer of cork cells eventually forms the bark of the tree.

3.3 Animal Tissues

Like plants, animal cells also group together and specialise to perform different functions. Animal tissues are mainly of four types: epithelial, connective, muscular and nervous tissues.

3.3.1 Epithelial Tissues - Structure and Functions

Epithelial tissue forms the outer covering of the body (skin) and also lines internal organs such as the mouth, lungs, blood vessels and intestine. It is composed of closely packed cells with very little space between them. This structure prevents entry of germs, reduces water loss, and helps in absorption, secretion, and movement of substances.

3.3.2 Connective Tissues - How are various parts connected in our body?

A tissue that connects and supports other tissues is called a connective tissue. Both blood and bones are connective tissues. The key difference is the matrix - watery, soft and jelly-like in blood but hard, solid and rigid in bones.

Types of connective tissues

3.3.3 Muscular Tissues - Can we control movement in our body?

Muscular tissues produce movement. There are three types of muscles:

3.3.4 Nervous Tissue - How does the body sense, communicate and respond?

Nervous tissue forms the body's control and coordination network. The brain acts as the control centre, coordinating activities, memory and responses across the body.

The cells of nervous tissue are called neurons (nerve cells). Each neuron has three main parts:

• Cell body - contains the nucleus; controls cell activities
• Dendrites - branch-like projections that receive signals from other neurons
• Axon - a long fibre that carries messages from the cell body and ends at axon terminals, which transmit messages to other cells

3.4 The Musculoskeletal System

The musculoskeletal system is made up of bones, muscles, joints, cartilage, tendons and ligaments. It helps us stand upright, move, maintain posture and protect delicate organs.

• Muscles pull on bones to produce movement
• They are attached to bones by strong, flexible bands called tendons
• The adult human skeleton makes up about 12-15% of body weight

Mucoskeletal System

3.5 Types of Joints

A joint is a junction between two or more bones. The type of joint determines the range of movement possible.

Types of Joints

Ready to Go Beyond

Stem cells in the bone marrow are special cells that can divide and make new cells. In a bone marrow transplant, stem cells from a healthy person are given to patients who have blood cancers like Leukemia or disorders, such as Thalassemia.

3.6 Skeletal System

The skeletal system is a framework of bones that provides strength and protects delicate internal organs. It includes the skull, vertebral column and rib cage.

• Backbone (vertebral column/spine) - made up of a series of small bones called vertebrae; supports the body; helps us stand upright; between each vertebra is a cartilage disc acting as a cushion, allowing flexibility without injuring the spinal cord
• Rib cage - 12 pairs of ribs; protects vital organs like heart and lungs; joined to spine at back and sternum (breastbone) at front by flexible cartilage; flexibility allows the rib cage to expand and contract during breathing

Bridging Science and Society

Yoga and Health

• Yoga, as described in ancient Indian texts, involves physical postures, breathing exercises, and meditation.
• Research indicates that yoga enhances flexibility, posture, and breathing, while also reducing stress and helping to prevent lifestyle-related diseases.
• To promote the role of yoga in health and well-being, International Yoga Day is observed on June 21st each year.
• Maintaining correct posture, proper nutrition, regular exercise, and practicing yoga contributes to strong bones, fit muscles, flexible joints, and protects the body from stiffness.

Agrobacterium and Plant Genetic Engineering

• Plant pathologists have observed a condition in plants called crown gall disease, characterized by tumor-like swellings on the stems due to rapid and uncontrolled cell division.
• This disease is caused by a bacterium known as Agrobacterium tumefaciens.
• Rather than merely attempting to cure this disease, scientists studied how this bacterium transfers its genetic material into plant cells.
• This understanding was later applied in plant tissue culture and genetic engineering.
• Today, Agrobacterium is utilized as a tool to introduce beneficial genes into plants, leading to the production of valuable phytochemicals, improved crops, and disease-resistant varieties. 

Think as a Scientist

From One Cell to an Organism: Totipotency

In 1958, F. C. Steward made a groundbreaking discovery by showing that individual cells from the vascular phloem of carrots have the remarkable ability to regenerate entire plants. He conducted an experiment using phloem cells from carrots, growing them in a nutrient medium rich in simple sugars and hormones under suitable conditions. To his amazement, these phloem cells began to divide, forming a mass of cells that eventually differentiated into a complete plant.

Steward's experiment demonstrated that phloem cells initially dedifferentiated, meaning they regained the ability to divide and formed an undifferentiated mass of unspecialized cells. Under the right conditions, these cells further divided and redifferentiated into specialized structures such as roots and shoots, ultimately developing into an entire plant.

This phenomenon, known as totipotency, indicates that some mature plant cells have the potential to undifferentiate, divide, and redifferentiate into a new plant when provided with specific conditions. Totipotent cells are similar to zygotes, which have the ability to divide and differentiate into an entire organism.

In his experiments, F. C. Steward tested various combinations of nutrients and environmental factors on phloem cells and observed different outcomes in terms of cell growth and weight increase.

Based on his findings, he concluded that the phloem cells of carrots have the capacity to grow and differentiate under the right conditions, and that factors such as light, air, and the composition of the nutrient medium play a crucial role in their growth.

Steward's research laid the foundation for understanding totipotency and regeneration in plants, highlighting the remarkable potential of plant cells to develop into entire organisms under suitable conditions.

F. C. Steward's Experiment on Phloem Cells of Carrot: Effects of Nutrient Medium on Growth

Composition of Nutrient Medium	Increase in Fresh Weight (mg) of Cells	Conditions
Solid Medium + Nutrients	Reduced	Light: Yes, Air: No
Liquid Medium + Nutrients	20% Increase	Light: Yes, Air: Yes
Liquid Medium + Nutrients	Reduced	Light: No, Air: Yes

(a) Characteristics of Phloem Cells of Carrot: Based on the experiment, it can be concluded that phloem cells of carrot have the ability to grow and increase in weight under specific conditions. The presence of light and air, along with an appropriate nutrient medium, positively impacts their growth. 

(b) Highest and Lowest Biomass Combinations: The combination that would likely yield the highest biomass is the liquid medium with nutrients, as it showed a 20% increase in fresh weight. The lowest biomass could be observed in the solid medium with nutrients, where growth was reduced. This variation could be attributed to the differences in nutrient availability and environmental conditions in each combination. 

(c) Culturing Animal Cells vs. Carrot Cells: It is unlikely that the same results would be obtained if animal cells were cultured instead of carrot cells. Animal cells have different growth requirements and may not exhibit the same level of totipotency or responsiveness to the experimental conditions used for carrot phloem cells.

(d) Commercial Applications of Totipotency Study: Two potential commercial applications of the study on totipotency in carrot phloem cells could include:

• Mass Propagation of Plants: Utilizing totipotent cells for large-scale production of specific plant varieties, such as fruits, vegetables, or ornamental plants.
• Genetic Engineering of Plants: Employing totipotent cells to introduce desired genes into plants, enhancing traits like disease resistance, yield, or nutritional content. 

Scientists Spotlight

• B. G. L. 29 Swamy was a renowned Indian botanist known for his contributions to the area of plant morphology and anatomy. His book, Hasuru Honnu, is a popular book in Kannada language. It is a blend of science, satire and culture. The book describes the botanical excursion in the forests of Western Ghats and is a treasure of plant descriptions, utilisation, conservation practices, botanical folklores and myths. The book won the Kendra Sahitya Akademi Award in 1978.
• Sipra Guha Mukherjee in collaboration with S. C. Maheshwari made a breakthrough discovery in the area of plant tissue culture. Their research led to the development of a complete plant through anther culture, using an artificial nutrient medium under controlled laboratory conditions. This pioneering work greatly contributed to crop improvement and brought significant progress in modern agriculture.

AT A GLANCE - Summary

• Tissues are groups of similar cells that work together to perform specific functions
• Plant tissues are classified into meristematic (actively dividing) and permanent tissues
• Meristematic tissues: Apical (length), Lateral (girth), Intercalary (regrowth after cutting)
• Simple permanent tissues: Parenchyma, Collenchyma, Sclerenchyma
• Complex permanent tissues: Xylem (water/minerals) and Phloem (food)
• Animal tissues: Epithelial, Connective, Muscular, Nervous
• Musculoskeletal system: bones + muscles + joints + cartilage + tendons + ligaments
• Movement occurs by coordination of muscles and bones under control of the nervous system