Tissue | ICSE Class 7 Biology Notes

This note covers tissues and levels of organisation, growing and permanent plant tissues, simple and complex tissues, animal tissue groups, tissue structure and location, transport, support, movement, and communication between body parts.

What is a tissue, and why do organisms need different tissues?

Definition: A tissue is a group of cells working together to perform a specific function. Cells in a simple tissue are similar; a complex tissue contains different kinds of cells working together.

A cell is the basic unit of life. An organism is a living thing. A unicellular organism consists of one cell, whereas a multicellular organism consists of many cells. In a unicellular organism such as amoeba, one cell performs all the functions needed for life.

In multicellular plants and animals, different groups of cells perform different jobs. This sharing of work is called division of labour. It increases efficiency and allows the body to carry out complex life processes. Cells specialised for movement have a different job from cells specialised for transport.

How do cells form larger working units?

Several tissues form an organ, a body part with a particular role. Different organs work together as an organ system. Organ systems together make up an organism. Thus, cells, tissues, organs, organ systems and the organism represent successively larger levels of organisation.

Structure means how a tissue is built; location means where it occurs; function means what it does. These three features belong together. Muscle tissue produces movement, while nerve tissue carries messages. Their different jobs depend on their different structures.

Most plants are fixed in one place. Their cells have a cell wall, a rigid outer covering that provides strength. In general, animals can move, although some, such as sponges, are immobile. Animal cells lack a rigid cell wall and can change shape easily.

Plant and animal tissues therefore meet different needs for support, growth, movement and transport. However, both show the same underlying organisation: groups of cells work together, and different tissues cooperate within the living body.

How do meristematic tissues help a plant grow?

Meristematic tissue consists of actively dividing cells that add new cells to a plant. A region containing this tissue is called a meristem. Different meristems account for growth in length, increase in thickness and regrowth after cutting.

What are the cells like?

The cells are small and have thin cell walls. They contain a large, prominent nucleus, the cell structure that controls cell activities, and dense cytoplasm, the semi-fluid, jelly-like material inside the cell. These features accompany continuous, rapid cell division.

Vacuoles, storage sacs surrounded by their own thin covering within cells, are generally absent. The cells are tightly packed with little or no intercellular space, meaning space between neighbouring cells. Thin walls and a prominent nucleus are two useful characteristics for recognising meristematic cells.

Where are the different meristems located?

A node is a point on a stem where a leaf or branch arises. An internode is the part of the stem between two nodes. These location terms help distinguish intercalary growth from growth at a root or shoot tip.

The apical meristem supplies dividing cells at the growing tip. The lateral meristem adds cells that increase the stem's diameter. The intercalary meristem helps explain why grass can grow again after mowing or grazing.

Note: Meristematic tissue is not confined to the tips of roots and shoots. Growth in thickness and regrowth in grasses involve meristems at other locations.

When identifying a meristem, connect its name to both its position and the change it produces. A root becoming longer and a stem becoming thicker are both growth, but they point to different meristems.

How can an onion-root investigation show where growth occurs?

An onion-root investigation compares roots with intact tips and roots whose tips have been cut. The question is whether growth continues in both. Measuring length before and after cutting helps relate a visible change to the location of actively dividing cells.

A centimetre is a unit of length, written as cm. The letters A and B below label the two jars.

What is the procedure?

1. Fill two glass jars with water and place one onion bulb in each. Call the jars A and B so that observations from the two bulbs remain separate.
2. Observe the roots for a few days. Measure and record their lengths on days 1, 2 and 3.
3. On day 3, cut about 1 centimetre from the root tips of the bulb in jar B. Leave the root tips of the bulb in jar A intact.
4. Observe and measure the roots for four more days, beginning on day 4. Compare the continuing growth in jar A with the roots in jar B.

Measurements must refer to the same feature: the length of each root from the base of the bulb. Record each jar separately so that the roots with intact tips can be compared with those whose tips have been removed.

What does the comparison show?

The roots in jar A continue to grow in length. The roots in jar B stop growing after their tips are cut. This comparison locates the growth zone at the root tip, where the apical meristem contains actively dividing cells.

The conclusion concerns the roots being observed. It connects removal of their tips with loss of further length growth. It does not mean that every growing region of a plant is a root tip: shoot tips, lateral meristems and intercalary meristems have their own locations.

Keep the observations separate from the explanation. “The cut roots stopped growing” records the result; “the dividing cells were at the tips” explains the result through the position of the meristem.

How do permanent tissues differ from meristematic tissues?

Permanent tissues consist of cells that have become specialised for particular functions after losing the ability to divide. Their work includes storage, support and transport. They arise from cells produced by meristematic tissue.

The change into specialised cells is called differentiation. It involves changes in both structure and function. Producing more cells and giving those cells particular jobs are related parts of plant development, but they describe different events.

How does differentiation occur?

1. Cells in meristematic tissue divide continuously, adding new cells to the plant body.
2. Some newly formed cells remain meristematic and continue their role in producing more cells.
3. Other newly formed cells lose the ability to divide and undergo changes in structure and function.
4. These cells become specialised for jobs such as support, transport or storage, forming permanent tissues.

Simple permanent tissue contains one type of cell. Parenchyma, collenchyma and sclerenchyma belong to this group. Complex permanent tissue contains more than one type of cell working together. Xylem and phloem are examples.

The word “complex” describes the mixture of cell types, not whether the tissue has an important job. Simple tissues also perform essential functions. Likewise, “permanent” does not mean “dead”: parenchyma and collenchyma contain living cells.

How do the three simple permanent tissues compare?

What does parenchyma do?

Parenchyma is a simple permanent tissue made of living, thin-walled cells. The cells are loosely packed, leaving intercellular spaces. It occurs in plant parts such as stems and green regions. Its main function is food storage.

In green parts, parenchyma also carries out photosynthesis, the use of solar energy to make food. In aquatic plants, meaning plants living in water, specialised parenchyma forms air spaces that help the plants float.

These functions show why a tissue should not be matched to just one rigid description. Parenchyma mainly stores food, but its role also depends on its location and specialisation. Food storage, food making and floating support are different functions associated with it.

How does collenchyma give flexible support?

Collenchyma consists of living cells whose walls are unevenly thickened at the corners. The thickening contains pectin, a substance that gives flexibility. This tissue supports plant parts while allowing them to bend without breaking.

Collenchyma occurs in parts such as stems and tendrils. A tendril is a slender plant structure used in climbing. In the sunflower stem, collenchyma lies near the outside, beneath the outer covering. Its structure combines support with flexibility.

How does sclerenchyma provide strength?

Sclerenchyma has thick cell walls strengthened by lignin, a substance that makes the walls hard and strong. Most of these cells are dead. It occurs in stems, leaf veins and the hard coverings of seeds and nuts.

Coconut husk and walnut shell are examples of structures containing sclerenchyma. Their toughness is linked to thick, strengthened walls. Collenchyma provides flexible support, whereas sclerenchyma provides hardness and strength.

Three panels show parenchyma, collenchyma and sclerenchyma. The labels identify thin walls, thick walls and thick lignified walls respectively. Compare the wall thickness and the spaces visible between cells.

See Fig. 3.8 in your NCERT textbook

For identification, use the cell walls together with the function. Thin walls suggest parenchyma; uneven corner thickening suggests collenchyma; thick lignified walls suggest sclerenchyma. The location provides another check rather than replacing the structural evidence.

How do xylem and phloem transport substances?

Xylem is the complex permanent tissue that transports water and minerals from roots to other plant parts. Minerals are inorganic nutrients taken up by the plant. Xylem also provides strength. It contains living cells as well as dead conducting and supporting cells.

Phloem is the complex permanent tissue that transports food from leaves to other parts of the plant. It is mostly made up of living cells. The word conducting means carrying substances from one region to another.

Why are both called complex tissues?

Each contains different kinds of cells that work together. Their classification depends on this cooperation between cell types. Although xylem and phloem both transport materials, they carry different materials and differ in the nature of their cells.

A plant needs both kinds of transport. Roots take in water and minerals, while leaves make food through photosynthesis. Materials must reach the parts that use them. This explains why conducting tissues are necessary alongside tissues that store food or provide support.

What can coloured water reveal?

A twig of petunia bearing white flowers can be kept in a beaker containing coloured water. After a few hours, the flowers become coloured. This observation makes the movement of water into the flowers visible and connects with the conducting role of xylem.

In a transport question, first identify what is moving. Water and minerals point to xylem; food points to phloem. Calling both “transport tissue” gives their shared role, but does not distinguish their functions.

What are the animal tissue groups, and what does epithelium do?

Animal tissues are grouped into epithelial, connective, muscular and nervous tissues. Epithelial tissue covers and lines surfaces. Connective tissue connects and supports body parts. Muscular tissue produces movement, and nervous tissue receives and carries messages.

Where is epithelial tissue found?

Epithelium is another name for epithelial tissue. It forms the outer covering of the body and lines internal parts such as the mouth, lungs, blood vessels and intestine. Its cells are closely packed, with very little space between them.

This close arrangement helps form a protective covering. Epithelial tissue helps prevent the entry of germs and reduces water loss. Its function depends on its location: a covering exposed to the outside has different needs from a lining involved in taking in substances.

How do its functions depend on location?

In the intestine, epithelial tissue helps with absorption, the taking in of substances such as nutrients and water. Nutrients are substances needed for nourishment. In glands, organs that make and release substances, epithelial tissue participates in secretion, their production and release.

Epithelial tissue in the lungs and blood vessels also helps the exchange of substances. The general functions of epithelium therefore include protection, absorption, secretion and exchange. These are functions of a tissue group whose cells form coverings and linings.

Location makes a function more precise. “Epithelium protects” is useful, but “epithelium forms a protective outer covering” connects the function to its position. Similarly, linking the intestinal lining with absorption shows how a tissue contributes to the work of an organ.

How do areolar tissue, adipose tissue and blood support the body?

Connective tissue connects, supports or binds other tissues and body parts. It includes areolar tissue, adipose tissue, blood, bone, cartilage, tendons and ligaments. These tissues do not all have the same firmness or the same function.

The matrix is the material between the cells of a connective tissue. Differences in the matrix help explain why blood is fluid while bone is hard. A tissue can connect body parts by transporting materials as well as by physically holding structures together.

What are areolar and adipose tissues?

Areolar tissue is a loose connective tissue found between the skin and muscles, around blood vessels and nerves, and in bone marrow, the soft tissue within bones. It fills spaces, supports internal organs and helps repair tissues.

Adipose tissue is connective tissue specialised for storing fat. It occurs beneath the skin and around internal organs. Stored fat provides an energy reserve, and adipose tissue helps reduce heat loss from the body.

Areolar and adipose tissues therefore have different principal roles. Areolar tissue acts as packing and support, while adipose tissue stores fat and provides insulation, meaning resistance to heat loss. Their locations help connect these functions to the surrounding body parts.

Why is blood a connective tissue?

Blood is a fluid connective tissue that transports nutrients and gases between body parts. It circulates through blood vessels, the tubes that carry blood. Its transport role links distant parts of the body rather than fastening one part directly to another.

Blood and areolar tissue illustrate why “connective” does not simply mean hard or string-like. Transport, packing, support and storage are all represented within this broad group. To identify a particular connective tissue, use its location and its specific function together.

How do bone, cartilage, tendons and ligaments differ?

What gives support and cushioning?

Bone is a hard connective tissue that forms the skeleton, the body's supporting framework. Its rigid matrix contains compounds of calcium and phosphorus, substances that give strength and rigidity. Bones support the body and protect internal organs.

Cartilage is a flexible connective tissue with a soft, jelly-like matrix. It is found in the ear, nose and at the ends of bones. It provides flexibility and cushions bone ends, helping absorb shocks.

A joint is a junction between two or more bones. At a joint, cartilage provides cushioning, while other connective tissues hold the parts together or transmit the pull of muscles. These tissues cooperate but have distinct jobs.

What attaches muscles and bones?

A tendon connects a muscle to a bone. When the muscle contracts, meaning that it shortens and produces a pull, the tendon transmits the force to the bone. This helps bring about movement at a joint.

A ligament connects a bone to another bone. It provides stability, limits movement and helps prevent dislocation, which means displacement of bones from their normal position at a joint. Tendons and ligaments differ in what they connect.

The drawing shows bones meeting at a joint, muscle above them, and labels for tendon, cartilage and ligament. Follow each label to distinguish the muscle attachment, cushioning tissue and connection between bones.

See Fig. 3.12c in your NCERT textbook

The key distinction is the relationship between the structures. A tendon carries a muscle's pull to bone; a ligament connects bones and stabilises their joint. Cartilage cushions, while bone gives the supporting framework on which movement acts.

How do the three kinds of muscular tissue produce movement?

Muscular tissue produces movement through contraction. Its cells are called muscle fibres. Some movements are voluntary, meaning under conscious control; others are involuntary, meaning they occur without conscious control.

Which muscles move the skeleton?

Striated muscles, also called skeletal or voluntary muscles, are attached to the skeleton. “Striated” means showing light and dark bands. Their long, cylindrical fibres are unbranched and contain many nuclei, the plural of nucleus.

Running, writing and lifting objects involve voluntary movement. Skeletal muscles pull on bones, with tendons transmitting the force. This links the function of muscular tissue with the supporting and connecting functions of the skeleton and connective tissues.

Which muscles work inside organs?

Unstriated muscles, also called smooth muscles, occur in organs such as the stomach and intestines. Their fibres taper towards both ends, a shape called spindle-shaped. They have a single nucleus and lack striations.

Smooth muscles act involuntarily. Their movements help move food through the digestive organs, the organs involved in breaking down food and taking in nutrients. This movement occurs without the conscious control used when deciding to write or lift an object.

What is special about cardiac muscle?

Cardiac muscle is the specialised muscle found only in the heart. Its fibres are cylindrical and branched, with a single nucleus and faint striations. It contracts rhythmically, producing the heartbeat throughout life.

Cardiac muscle acts involuntarily even though it has striations. The presence of bands therefore does not by itself show that a muscle is under conscious control. The tissue's location and function must also be considered.

When comparing muscles, connect each name to a location and a movement. The skeleton, digestive organs and heart need different patterns of movement, so muscular tissue is specialised for these different roles.

How does nerve tissue carry messages and coordinate movement?

Nerve tissue, also called nervous tissue, receives and transmits messages. It forms the body's communication and coordination network. Coordination means organising the activities of different body parts so that they work together.

The brain acts as a control centre, coordinating activities, memory and responses. A neuron is a nerve cell specialised to receive, process and transmit messages. Its structure includes a cell body, receiving branches and a long outgoing fibre.

What are the main parts of a neuron?

The cell body contains the nucleus and controls cell activities. A dendron is a receiving process extending from the cell body; its smaller branches are called dendrites. Dendrites receive signals from other neurons.

The axon is a long fibre that carries messages away from the cell body. It ends in axon terminals, branches that pass messages to other cells. A nervous message is also called a nerve impulse.

The drawing shows a branched region around the cell body, a long axon, and branching axon terminals. Labels identify the cell body, nucleus, dendrites, axon and axon terminals. Trace the receiving branches and the long outgoing fibre separately.

See Fig. 3.14 in your NCERT textbook

How do different tissues cooperate in movement?

1. Nervous tissue carries instructions involved in controlling muscular activity.
2. A skeletal muscle contracts and produces a pulling force.
3. The tendon connecting that muscle to a bone transmits the force to the bone.
4. The bone moves at a joint, producing movement through the combined work of nervous, muscular and connective tissues.

This sequence shows why no single tissue accounts for the whole action. The nerve tissue communicates; the muscle produces the pull; the tendon transmits it; and the bone moves. Ligaments help stabilise the joint, while cartilage cushions the bone ends.

A muscle fibre and a neuron are therefore both specialised cells, but their roles differ. One is adapted for contraction and the other for communication. Their cooperation connects the idea of division of labour with the visible movements of the body.

Glossary

• Tissue — A group of cells working together to carry out a particular function in an organism.
• Division of labour — The sharing of different functions among specialised cells or tissues within a living organism.
• Meristematic tissue — Plant tissue containing actively dividing cells that add new cells for growth.
• Differentiation — The process through which cells change in structure and become specialised for particular functions.
• Permanent tissue — Plant tissue formed from cells that have lost the ability to divide and become specialised.
• Parenchyma — Simple permanent tissue of living, thin-walled cells that mainly stores food in plants.
• Collenchyma — Living plant tissue with unevenly thickened cell corners that provides support and flexibility.
• Sclerenchyma — Plant tissue with thick, lignified walls providing strength; most of its cells are dead.
• Xylem — Complex permanent tissue that carries water and minerals from roots to other plant parts.
• Phloem — Complex permanent tissue, mostly living, that transports food from leaves to other plant parts.
• Epithelium — Closely packed animal tissue forming the body's outer covering and lining internal parts.
• Connective tissue — Animal tissue that connects, supports or binds tissues and body parts in different ways.
• Tendon — Connective tissue joining muscle to bone and transmitting the muscle's pulling force.
• Ligament — Connective tissue joining bone to bone, helping stabilise joints and limit excessive movement.
• Neuron — A specialised nerve cell that receives, processes and transmits messages within the body.

Common errors and misconceptions

• Misconception: All tissues contain just one kind of cell. Correct: Simple tissues contain one kind; complex tissues contain different cell types working together.
• Misconception: All plant growth comes from root and shoot tips. Correct: Lateral meristems increase thickness, and intercalary meristems help regrowth in grasses.
• Misconception: Permanent tissues are all dead. Correct: Parenchyma and collenchyma contain living cells. Most sclerenchyma cells are dead.
• Misconception: Xylem transports food and phloem transports water. Correct: Xylem carries water and minerals; phloem transports food.
• Misconception: Tendons and ligaments make the same connection. Correct: Tendons connect muscles to bones, whereas ligaments connect bones to other bones.
• Misconception: Any muscle with striations is voluntary. Correct: Cardiac muscle has faint striations but works involuntarily in the heart.
• Misconception: Nerve cells produce the pulling force that moves a bone. Correct: Nervous tissue carries instructions, muscles contract, and tendons transmit their pull to bones.

Exam-style questions with model answers

Q1. Define a tissue and explain what division of labour means in a multicellular organism. Give one point for each. [2 marks]

1. A tissue is a group of cells working together to perform a particular function.
2. Division of labour means that different specialised groups of cells perform different jobs within the organism.

Q2. State the location and growth function of each of these meristems: apical, lateral and intercalary. Give one complete point for each. [3 marks]

1. Apical meristems occur at root and shoot tips. Their actively dividing cells help increase the length of roots and shoots.
2. Lateral meristems occur along the circumference of stems. Their production of new cells increases girth, meaning the thickness of the stem.
3. Intercalary meristems occur at the base of an internode or just above a node. They help grasses grow again after cutting or grazing.

Q3. Onion roots in jar A have intact tips and continue growing. Roots in jar B stop growing after about 1 cm is cut from their tips. Identify the growth tissue, state its location and explain the difference in growth. [3 marks]

1. The tissue responsible for this growth is meristematic tissue, specifically the apical meristem. Its cells divide actively and add new cells.
2. The root apical meristem is located at the root tip. Jar A retains these growing regions, so its roots continue increasing in length.
3. Cutting the tips in jar B removes the growing regions. The stated loss of further growth therefore connects root length growth with the tips.

Q4. Compare parenchyma, collenchyma and sclerenchyma. Give three points describing their cells and walls, then two points distinguishing their functions. Preserve the qualification about sclerenchyma cells. [5 marks]

1. Parenchyma consists of living cells with thin walls. Its cells are loosely packed, leaving intercellular spaces, meaning spaces between neighbouring cells.
2. Collenchyma consists of living cells with walls unevenly thickened at the corners. This arrangement is associated with support that allows bending.
3. Sclerenchyma has thick walls strengthened with lignin, which makes them hard and strong. Most of these cells are dead.
4. Parenchyma mainly stores food. In green plant parts it also performs photosynthesis, and specialised parenchyma in aquatic plants forms air spaces.
5. Collenchyma provides support with flexibility, while sclerenchyma provides hardness and strength. Coconut husk and walnut shell are examples of sclerenchyma-containing structures.

Q5. Give the main transport function of xylem and of phloem, and state the nature of the cells in each. Use one complete point for each tissue. [2 marks]

1. Xylem transports water and minerals from roots to other plant parts. It includes living cells and dead conducting and supporting cells.
2. Phloem transports food from leaves to other plant parts. It is mostly made up of living cells.

Q6. Give the location or connection and one function of each of the following connective tissues: areolar tissue, adipose tissue, cartilage, tendon and ligament. [5 marks]

1. Areolar tissue occurs between the skin and muscles and around blood vessels and nerves. It fills spaces and supports tissues and internal organs.
2. Adipose tissue occurs beneath the skin and around internal organs. It stores fat as an energy reserve and helps reduce heat loss.
3. Cartilage occurs in the ear, nose and at the ends of bones. It provides flexible support and cushioning at bone ends.
4. A tendon connects a muscle to a bone. It transmits the pull produced when the muscle contracts, helping to move the bone.
5. A ligament connects one bone to another. It helps stabilise the joint, limits excessive movement and helps prevent displacement of the bones.

Q7. For striated skeletal muscle, unstriated smooth muscle and cardiac muscle, state a location, whether control is voluntary or involuntary, and one function. [3 marks]

1. Striated skeletal muscle is attached to the skeleton. It acts under voluntary control and moves bones during actions such as writing or lifting objects.
2. Unstriated smooth muscle occurs in organs such as the stomach and intestines. It acts involuntarily and helps move food through these organs.
3. Cardiac muscle is found only in the heart. It acts involuntarily and produces the rhythmic contractions responsible for the heartbeat throughout life.

Q8. Describe the cell body, dendron with dendrites, and axon of a neuron, giving the function of each in one separate point. [3 marks]

1. The cell body contains the nucleus and controls the activities of the neuron. It is the central cell region from which processes extend.
2. A dendron is a receiving process extending from the cell body, with smaller branches called dendrites. Dendrites receive signals from other neurons.
3. The axon is a long fibre carrying messages away from the cell body. Its terminal branches pass messages on to other cells.

Key takeaways

• Tissues are groups of cells working together; different tissues share functions through division of labour in multicellular organisms.
• Apical, lateral and intercalary meristems support different patterns of plant growth because their actively dividing cells occupy different locations.
• Permanent tissues develop through differentiation and may be simple or complex, depending on the kinds of cells present.
• Parenchyma mainly stores food, collenchyma provides flexible support, and sclerenchyma provides strength through thick, lignified cell walls.
• Xylem carries water and minerals from roots, while phloem transports food from leaves to other plant parts.
• Animal tissues form four main groups: epithelial, connective, muscular and nervous, each contributing different functions to the body.
• Tendons connect muscle to bone; ligaments connect bone to bone; cartilage provides cushioning; bone provides support and protection.
• Nervous tissue communicates, muscular tissue contracts, and connective tissues help transmit force and support movement.

Test yourself

What does “complex” mean in complex permanent tissue?

It means the tissue contains different types of cells working together, as in xylem and phloem.

Which meristem increases the thickness of a stem?

The lateral meristem increases stem girth by producing new cells.

What happens during differentiation into permanent tissue?

Cells lose their ability to divide and change in structure and function, becoming specialised for particular jobs.

Which simple tissue gives flexible support, and how are its walls thickened?

Collenchyma provides flexible support. Its living cells have walls unevenly thickened at the corners.

Why can blood be classified as connective tissue?

Blood connects body parts by transporting substances such as nutrients and gases between them.

How is a tendon different from a ligament?

A tendon connects muscle to bone, while a ligament connects one bone to another.

Is cardiac muscle voluntary because it has striations?

No. Cardiac muscle has faint striations but acts involuntarily to produce the heartbeat.

Which neuron part carries messages away from the cell body?

The axon carries messages away, and its terminals transmit them to other cells.