Tissues in Action shows how cells of similar structure group into tissues and how each tissue’s structure suits its function. In plants it covers the apical, lateral and intercalary meristems, differentiation into permanent tissues, and the protective, supporting and conducting tissues. In animals it covers epithelial, connective, muscular and nervous tissues, then the musculoskeletal system and four types of joints. School exams test it through comparisons, assertion-reason items and case questions on onion root tips, tree rings and tissue culture, and it prepares you for Class 10 biology.
Key Concepts
1. Tissues, Levels of Organisation and Division of Labour
Life begins when a single cell divides many times. The cells formed give rise to skin (protection), muscles (movement), bones (support), nerves (control and coordination) and all other organs.
A tissue is a group of cells, similar in structure, that work together to perform a specific function. In a unicellular organism such as amoeba, one cell performs all the functions of life. In multicellular organisms like plants and animals, different groups of cells perform different functions.
Hierarchy of organisation in a multicellular organism:
Cell → Tissue → Organ → Organ system → Organism
- Cells of a similar type performing a similar function form a tissue.
- More than one type of tissue forms an organ.
- Different organs form an organ system.
- Organ systems together form the organism.
Division of labour: when different tissues take on different jobs, the body works more efficiently and can carry out complex life processes. In animals, muscle tissue enables movement and nervous tissue carries messages to different parts of the body. In plants, the conducting tissue xylem transports water and minerals, while phloem transports food.
Why are plant and animal tissues different?
| Feature | Plants | Animals |
|---|---|---|
| Movement | Most are fixed in one place and need support to stay firm and upright | Most can move from place to place (a few, such as sponges, are immobile) |
| Cell wall | Rigid cell wall gives rigidity and strength | No rigid cell wall, so cells can change shape easily, which suits locomotion |
| Nutrition | Tissues use solar energy to make food by photosynthesis | Tissues digest food obtained from different food sources |
The idea running through the chapter: the structure of a tissue matches its function. A thin layer suits exchange, a thick wall suits support, a long fibre suits carrying messages.
2. Meristematic Tissues: How Plants Grow
Plants grow in three ways:
- increase in length (height of stem and depth of roots),
- increase in girth (thickness of stem), and
- regrowth after branches are cut or after grazing.
All three need actively dividing cells. A tissue made of such cells is called a meristematic tissue (meristem). There are three types.
(a) Apical meristem: growth in length
Activity 3.1 (onion roots): two onion bulbs are placed in two jars of water, Jar A and Jar B. Root lengths are measured on days 1, 2 and 3. On day 3, the root tips of the bulb in Jar B are cut by about 1 cm. Root lengths in both jars are then recorded for four more days.
Observation: roots in Jar A keep growing in length. Roots in Jar B stop growing after their tips are cut.
Conclusion: roots grow only from their tips, because the tips contain cells that divide continuously. Shoot tips also contain actively dividing cells. These growth zones at the tips of roots and shoots are the apical meristems, and they make the plant grow in length.
(b) Lateral meristem: growth in girth
Stems of dicot plants grow in diameter over time. The cut trunk of a tree shows ring-like patterns called annual growth rings. Some rings are wide and some are narrow, which reflects favourable or unfavourable growth conditions in a particular year. By counting the rings, scientists estimate the age of a tree and learn about the climate it grew in.
The increase in girth is caused by actively dividing cells arranged in a ring in the stem. These cells divide and produce new cells inside and outside in a concentric manner, so the diameter of the stem increases. This tissue is the lateral meristem, located along the circumference of the stem.
Going further (cork cambium): a young plant is covered by a single-layered epidermis. As the plant grows older, some cells below the epidermis of the stem gain the ability to divide, act as lateral meristematic cells and form the cork cambium. Its cells divide to form cork cells, which are dead, compactly arranged and contain a substance that makes them impermeable to water and gases. Cork forms the bark of the tree.
(c) Intercalary meristem: regrowth after cutting
If the tip of a young stem is cut, the stem stops growing in length, but new branches arise from the nodes. A node is the point on a stem where branches or leaves arise. The part of the stem between two nodes is the internode. A trimmed garden hedge becomes bushy, and mowed or grazed grass grows back. This happens because of the intercalary meristem, located at the base of the internode or just above the node.
| Meristem | Location | Result |
|---|---|---|
| Apical | Tips of roots and shoots | Increase in length |
| Lateral | Along the circumference of stems (a ring in the stem) | Increase in girth |
| Intercalary | Base of the internode or just above the node (for example, in grasses) | Regrowth after cutting or grazing |
Features of meristematic cells
- Small cells with thin cell walls.
- A large, prominent nucleus.
- Dense cytoplasm with many organelles.
- Vacuoles are generally absent.
- Tightly packed, with little or no intercellular space.
These features allow continuous and rapid cell division.
Differentiation
Meristems keep adding new cells. Some of the new cells stay meristematic. Others lose the ability to divide, change in structure and function, and become permanent tissues specialised for support, transport or storage. The process by which meristematic tissue becomes specialised to perform specific functions is called differentiation.
3. Permanent Tissues in Plants
A T.S. of a sunflower stem under a microscope shows many kinds of cells, each group specialised for a particular job. These are permanent tissues. They are of two kinds:
- Simple permanent tissue: made of only one type of cell (parenchyma, collenchyma, sclerenchyma).
- Complex permanent tissue: made of more than one type of cell working together (xylem, phloem).
The textbook groups plant tissues by function into protective, supporting and conducting tissues.
(i) Protective tissue: epidermis
The epidermis is the outermost layer of the plant body and protects all parts of the plant. It is a tightly packed, single layer of flat, rectangular cells.
- Cuticle: epidermal cells are covered by a waxy layer of cutin called the cuticle. Plants of very dry habitats may have a thick cuticle, which reduces water loss by transpiration through stomata. The cuticle also protects against mechanical injury and invasion by parasites.
- Root hairs: hair-like projections of epidermal cells in roots increase the surface area for absorbing water and minerals from the soil.
- Stomata: pores in the epidermis of leaves. Apart from gaseous exchange, they help in transpiration, the evaporation of water vapour through stomata. Transpiration creates a transpiration pull in the xylem, which helps move water up the plant, and it also helps remove wastes from the plant body.
(ii) Supporting tissues: simple permanent tissues
| Tissue | Cells | Wall | Functions and examples |
|---|---|---|---|
| Parenchyma | Living, loosely packed with intercellular spaces | Thin | Mainly stores food; photosynthesis in green parts; specialised parenchyma forms air spaces that help aquatic plants float |
| Collenchyma | Living | Unevenly thickened at the corners due to deposition of pectin (a chemical that gives rubber-like flexibility) | Support with flexibility, so stems and tendrils bend without breaking |
| Sclerenchyma | Mostly dead | Thick, due to deposition of lignin, making them hard and strong | Forms the woody structure; found in stems, leaf veins and hard coverings of seeds and nuts, such as coconut husk and walnut shell |
(iii) Conducting tissues: complex permanent tissues
Xylem transports water and minerals from the roots to other parts of the plant and also gives the plant strength. It has four components:
- Tracheids and vessels: tubular and thick-walled.
- Xylem parenchyma: the only living component of xylem.
- Xylem fibres.
Tracheids, vessels and xylem fibres are primarily sclerenchymatous.
Phloem transports food from the leaves to other parts of the plant. Unlike xylem, phloem is mostly made of living cells. It has four components:
- Sieve tubes: long, tubular cells joined end to end by perforated walls. They carry food.
- Companion cells: specialised parenchyma cells that regulate the cellular functions of sieve tube cells. Their main function is to monitor the loading and unloading of sugars in sieve tubes.
- Phloem parenchyma: stores food materials, and resin, tannins and latex.
- Phloem fibres: primarily sclerenchymatous; they support the sieve tubes and give strength.
| Point | Xylem | Phloem |
|---|---|---|
| Carries | Water and minerals | Food |
| Direction | From roots to other parts | From leaves to other parts |
| Living or dead | Mostly sclerenchymatous; only xylem parenchyma is living | Mostly living cells |
| Components | Tracheids, vessels, xylem parenchyma, xylem fibres | Sieve tubes, companion cells, phloem parenchyma, phloem fibres |
Plant tissue systems
Plant tissues are organised into three tissue systems:
- Dermal tissue system: the outer covering of the plant. It protects inner parts and reduces water loss.
- Ground tissue system: the main body of the plant between the dermal and conducting tissues. It includes parenchyma, collenchyma and sclerenchyma.
- Vascular tissue system: the conducting tissues, xylem and phloem.
4. Epithelial Tissues: Coverings and Linings
Animal tissues are mainly of four types: epithelial, connective, muscular and nervous.
Epithelial tissue forms the outer covering of the body (skin) and lines internal organs such as the mouth, lungs, blood vessels and intestine. Its cells are closely packed with very little space between them. This prevents the entry of germs, reduces water loss, and helps in absorption, secretion and movement of substances.
Different epithelia are built for different jobs (textbook Table 3.2):
| Function | Structure | Location |
|---|---|---|
| Exchange: rapid diffusion of liquids and gases | Single layer of thin, flat cells | Lining of blood vessels and lungs |
| Protection: against mechanical injury, friction and entry of microbes | Many layers of cells; outer cells flat and tightly packed | Skin, mouth and oesophagus |
| Secretion: mucus, enzymes, hormones, sweat, saliva | Cells specialised to produce and release substances; may be cuboidal or columnar | Salivary glands, sweat glands, stomach lining |
| Sensory: smell, taste, sound and balance | Specialised receptor cells with hair-like cilia | Nostrils, taste buds, inner ear |
| Absorption: efficient uptake of nutrients and water | Single layer of tall, pillar-like cells, often with hair-like structures | Lining of the small intestine |
Structure and function: where materials must pass quickly (lungs, blood vessels), the lining is one thin layer, so the distance is short. Where wear and tear is high (skin, mouth, oesophagus), there are many layers, so damaged outer cells can be lost without exposing the tissue below. Where food must be absorbed (small intestine), tall cells with hair-like structures give a large surface.
5. Connective Tissues: Blood, Bone, Cartilage, Tendons and Ligaments
A tissue that connects and supports other tissues is a connective tissue. Blood connects body parts by transporting nutrients, gases and hormones. Bones connect and support the body from head to toe. Both are connective tissues, yet blood is fluid and bone is hard. The difference lies in the matrix: watery, soft and jelly-like in blood, but hard, solid and rigid in bone.
Blood
Blood has a fluid part, plasma (about 55% of total volume), and formed elements (about 45% of total volume): red blood cells, white blood cells and platelets. Activity 3.2 links daily experiences to blood:
| Experience | Observation | Explanation |
|---|---|---|
| A small cut on the skin | Red blood oozes out; a clot forms after some time | The red colour is due to haemoglobin, an iron-rich protein in RBCs. Platelets help blood clot at the site of injury. |
| A skin infection | The area turns red and may swell; there may be fever | White blood cells collect at infected areas, causing inflammation (redness and swelling) and possible pus formation. |
| Exercise or running | Faster breathing; face may turn red | Muscles need more oxygen, so breathing becomes faster and blood flow increases. |
RBCs live for about 4 months and are replaced regularly.
Bone, cartilage, tendon and ligament (Activity 3.3)
| Action | Experience | Function | Tissue |
|---|---|---|---|
| Touch your elbow gently | Hard, rigid structure | Strength, support and protection | Bone |
| Press and fold your ear, or press your nose and stop | Soft, flexible, returns to shape | Flexibility; cushions ends of bones for shock absorption | Cartilage |
| Touch forearm muscles and wiggle your fingers | Movement felt in the forearm though fingers are far away | Connects muscle to bone and so brings about movement | Tendon |
| Sit and raise your leg as far as the knee allows | The joint does not go beyond a limit | Connects bone to bone; gives stability, limits movement, helps prevent dislocation | Ligament |
- Bone: rigid matrix with calcium and phosphorus compounds, giving strength and rigidity.
- Cartilage: soft, jelly-like matrix; gives flexibility and cushioning.
- Tendon: connects muscle to bone.
- Ligament: connects bone to bone and prevents excessive movement.
Going further (stem cells): stem cells in the bone marrow can divide and make new cells. In a bone marrow transplant, stem cells from a healthy person are given to patients with blood cancers like leukemia or disorders such as thalassemia.
6. Muscular and Nervous Tissues
Muscular tissue
Movements under our conscious control, such as running, writing or lifting objects, are voluntary movements. Movements that happen automatically, such as food moving through the intestine or the heart beating, are involuntary movements. Three kinds of muscle carry out these movements.
| Feature | Skeletal muscle | Smooth muscle | Cardiac muscle |
|---|---|---|---|
| Control | Voluntary | Involuntary | Involuntary |
| Location | Attached to the skeleton | Organs like the stomach and intestines | Only in the heart |
| Cell shape | Long, cylindrical fibres, unbranched | Spindle-shaped | Cylindrical and branched |
| Nuclei | Many (multinucleate) | Single | Single |
| Striations | Striated (light and dark bands) | No striations | Faint striations |
| Typical work | Running, writing, lifting | Slow, continuous movements like digestion | Rhythmic beating throughout life without fatigue |
Skeletal muscle is made of bundles of muscle fibres.
Nervous tissue
Pulling your hand away from something hot is controlled by nervous tissue, the body’s control and coordination network. The brain is the control centre, coordinating activities, memory and responses. Muscles, voluntary and involuntary, cannot work on their own; they receive instructions from nervous tissue. During exercise, for example, the brain signals the heart to beat faster to meet the higher demand for oxygen.
The cells of nervous tissue are neurons (nerve cells), specialised to receive, process and transmit messages. A neuron has three main parts:
- Cell body: contains the nucleus and controls the cell’s activities.
- Dendrites: receive signals from other neurons.
- Axon: a long fibre that carries messages away from the cell body and ends in axon terminals, which pass the message to other cells.
Direction of a message: dendrites → cell body → axon → axon terminals → next cell. The long axon suits carrying messages over long distances in the body.
7. The Musculoskeletal System and Joints
The musculoskeletal system is made of bones, muscles, joints, cartilage, tendons and ligaments. It helps us stand upright, move, maintain posture and protect delicate organs. It works under the control of the nervous system.
How movement happens: muscles pull on bones. They are attached to bones by strong, flexible bands called tendons. When a muscle contracts, the tendon transmits this force to the bone, and movement occurs at a joint. A joint is a junction between two or more bones. Joints allow movement but cannot move bones on their own; muscles do the pulling.
Body weight from bones and muscles: on average, the adult human skeleton makes up about 12 to 15 per cent of body weight, though this varies with age, gender and body composition. The textbook gives average muscle as about 40 to 50 per cent for adult males and about 30 to 40 per cent for adult females.
Types of joints
| Joint | Example | How it is built | Movement allowed |
|---|---|---|---|
| Ball and socket | Shoulder | Rounded top of the upper arm bone fits into a shallow hollow of the shoulder bone | Forward, backward, sideways and circular movements |
| Hinge | Elbow, knee | Bends and straightens like a door hinge; a small bone, the kneecap, protects the knee joint | In one direction only |
| Pivot | Skull on the backbone (neck) | Allows the head to turn like a doorknob in its socket | Side to side turning of the head, as in shaking it for “no” |
| Fixed | Bones of the skull | Flat bones joined firmly together | No movement; keeps the brain safe even when the body moves |
The shoulder, together with the collarbone, forms the shoulder girdle, which connects the arm to the skeleton.
The skeletal system
The skeletal system is a framework of bones that gives strength and protects delicate internal organs. It includes the skull, vertebral column and rib cage.
- Vertebral column (backbone, spine): a flexible column from the base of the skull, made of small bones called vertebrae. It supports the body and helps us stand upright. A cartilage disc between vertebrae acts as a cushion and allows us to bend and twist without injuring the spinal cord.
- Rib cage: formed by 12 pairs of ribs. It protects vital organs such as the heart and lungs. The ribs are attached to the spine at the back and to the breastbone (sternum) in front, and are joined by flexible cartilage. This lets the rib cage expand and contract during breathing, changing the space in the chest so air moves in and out of the lungs. Injury to the ribs makes breathing painful and difficult.
8. Totipotency, Tissue Culture and Applications
In 1958, F. C. Steward showed that single cells from the phloem of carrot can regenerate whole plants. His team grew phloem cells in a nutrient medium with simple sugars and hormones under suitable conditions. The cells divided to form a mass of cells, which gradually divided and differentiated into a complete plant.
- Dedifferentiation: the specialised phloem cells regained the ability to divide and formed an undifferentiated mass of unspecialised cells.
- Redifferentiation: under suitable conditions, with nutrients and growth chemicals, this mass divided further and formed roots, shoot and finally a complete plant.
- Totipotency: the ability of some mature plant cells to undifferentiate, divide and redifferentiate into a new plant under specific conditions. Such cells are totipotent. This is similar to the way a zygote divides and differentiates into a whole organism.
Steward also compared combinations of light, air and nutrient medium (textbook Table 3.6). With a solid medium and nutrients the fresh weight reduced. Two combinations used a liquid medium with nutrients: one gave a 20% increase in fresh weight and the other reduced it. Since the medium was the same in these two rows, the light and air conditions made the difference.
Crown gall disease: tumour-like swellings form on stems due to rapid, uncontrolled cell division. It is caused by the bacterium Agrobacterium tumefaciens. Scientists studied how this bacterium transfers its genetic material into plant cells and now use it to introduce useful genes into plants, to produce valuable phytochemicals, improved crops and disease-resistant varieties.
Key Terms Sheet
The chapter has no formulas, so this sheet collects its key terms and figures.
| Term or fact | What to write |
|---|---|
| Meristems | Apical (length), lateral (girth), intercalary (regrowth) |
| Wall materials | Cuticle: cutin; collenchyma corners: pectin; sclerenchyma: lignin |
| Blood composition | Plasma about 55%, formed elements about 45% of total volume |
| RBC life span | About 4 months |
| Ribs | 12 pairs, attached to spine and sternum, joined by flexible cartilage |
| Skeleton share of body weight | About 12 to 15 per cent in adults |
| Bone or muscle mass estimate | Estimated mass = body weight × percentage / 100 |
| Totipotency | F. C. Steward, 1958, carrot phloem cells |
| Crown gall | Agrobacterium tumefaciens |
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Important Definitions
| Term | Definition |
|---|---|
| Tissue | A group of cells, similar in structure, that work together to perform a specific function. |
| Meristematic tissue | A tissue made of actively dividing cells, responsible for growth in plants. |
| Differentiation | The process by which meristematic tissue becomes specialised to perform specific functions, forming permanent tissue. |
| Permanent tissue | Tissue formed from cells that have lost the ability to divide and are specialised for a function. Simple: one type of cell. Complex: more than one type of cell. |
| Epithelial tissue | Tissue of closely packed cells forming the outer covering of the body and lining internal organs. |
| Connective tissue | Tissue that connects and supports other tissues, such as blood and bone. |
| Tendon | Connective tissue that connects muscle to bone. |
| Ligament | Connective tissue that connects bone to bone and limits movement. |
| Neuron | A cell of nervous tissue, specialised to receive, process and transmit messages. |
| Totipotency | The ability of some mature plant cells to undifferentiate, divide and redifferentiate into a new plant under specific conditions. |
Solved Examples (NCERT-Based)
Example 1: Cutting the onion root tips (NCERT Activity 3.1)
Question: Two onion bulbs grow roots in Jars A and B. On day 3 the root tips in Jar B are cut by about 1 cm. Predict the growth of roots in both jars from day 4 onwards and explain.
Answer:
- Roots in Jar A keep increasing in length because their tips are intact.
- Roots in Jar B stop growing in length after the tips are cut.
- Reason: root tips contain the apical meristem, whose cells divide continuously. Removing the tip removes the dividing cells.
- Conclusion: roots grow in length only from their tips. Jar A is the control; Jar B is the test.
Example 2: Teak tree growth data (NCERT Exercise 7)
Data: age (years), diameter at breast height (cm), annual rings: 5, 4, 5; 10, 8, 10; 20, 24, 20; 25, 28, 25; 30, 32, 30; 40, 40, 40.
(i) Analyse the graph in terms of the diameter of the stem over time.
- The diameter rises at every age in the table, so the stem keeps thickening over all 40 years.
- Increase per year: 5 to 10 years, 4/5 = 0.8 cm; 10 to 20 years, 16/10 = 1.6 cm; 20 to 25, 25 to 30 and 30 to 40 years, 0.8 cm each.
- Growth in girth was fastest between 10 and 20 years; in the other intervals it was steady at 0.8 cm per year.
(ii) What is the relation between the diameter of the teak tree to the annual rings formed? The number of annual rings equals the age of the tree (one ring per year). As rings increase, the diameter also increases, so both rise together with age. The diameter does not increase by the same amount per ring, because ring width depends on growth conditions in that year.
(iii) Which specialised tissue is responsible for the girth of the stem and where is it located? The lateral meristem, a ring of actively dividing cells along the circumference of the stem.
Example 3: Estimating bone and muscle mass (NCERT Activity 3.4)
Question: An adult man weighs 70 kg. Using the textbook averages (bone about 12 to 15 per cent, adult male muscle about 40 to 50 per cent), estimate his bone and muscle mass.
Solution:
- Bone: 70 × 12/100 = 8.4 kg; 70 × 15/100 = 10.5 kg. Estimate: 8.4 kg to 10.5 kg.
- Muscle: 70 × 40/100 = 28 kg; 70 × 50/100 = 35 kg. Estimate: 28 kg to 35 kg.
Actual values vary with age, gender and body composition.
Example 4: Coconut husk and coriander stalk (NCERT Pause and Ponder 1)
Question: You may have noticed that fibres of coconut husk are hard and brittle, whereas the leaf stalks of coriander are soft and flexible. Find out the reason.
Answer: Coconut husk fibres are made of sclerenchyma. Their cells have thick walls with lignin and are mostly dead, so they are hard and strong but cannot bend. Coriander leaf stalks have collenchyma, which is living, with walls thickened only at the corners by pectin. Pectin gives rubber-like flexibility, so the stalk bends without breaking.
Example 5: Thick cuticle in desert and water plants (NCERT Pause and Ponder 2)
Question: Why do you think that a thick cuticle on the outer wall of epidermis is advantageous for a plant living in the desert but disadvantageous for a plant living underwater?
Answer: The cuticle is a waxy layer of cutin that reduces water loss. In the desert, water is scarce, so a thick cuticle reduces loss by transpiration and helps the plant survive. Underwater, there is no shortage of water, and a thick waxy layer would only hinder the exchange of gases and substances with the surrounding water, so it gives no benefit and becomes a disadvantage.
Example 6: Assertion-Reason set (NCERT Exercise 6)
Options: (i) Both (A) and (R) are true, and (R) is the correct explanation of (A). (ii) Both (A) and (R) are true, but (R) is not the correct explanation of (A). (iii) (A) is true, but (R) is false. (iv) (A) is false, but (R) is true.
- A: Epithelium is well-suited for gas exchange in the lungs. R: It consists of multiple layers of tall cells that slow down diffusion. Answer: (iii). The exchange epithelium of the lungs is a single layer of thin, flat cells for rapid diffusion, so R is false.
- A: Cardiac muscle can contract continuously without fatigue. R: Cardiac muscle cells have a high number of mitochondria and an abundant blood supply. Expected answer: (i). A is stated in this chapter (cardiac muscle works without fatigue). This chapter does not cover mitochondria or the heart’s blood supply; R relies on Chapter 2, where mitochondria provide energy to the cell. A steady energy supply lets the muscle keep contracting, so R explains A.
- A: Tendons connect bone to bone and allow joint movement. R: Tendons are made of tough connective tissue that transmits force from muscle to bone. Answer: (iv). Tendons connect muscle to bone; ligaments connect bone to bone.
- A: In a hinge joint, movement occurs primarily in one plane. R: The bone ends are shaped to allow sliding in all directions. Answer: (iii). A hinge joint, like a door hinge, bends and straightens in one direction only, so R is false.
Example 7: The mango sapling in monsoon winds (NCERT Exercise 9)
Question: Aamrapali observed that a young mango sapling’s stem bends flexibly during monsoon winds and does not break. Which tissue is responsible for this flexibility? Predict and provide your explanation of the impact if the existing tissue was replaced by sclerenchyma.
Answer: Collenchyma gives the flexibility. Its cells are living and their walls are thickened unevenly at the corners by pectin, which gives support with rubber-like flexibility. If collenchyma were replaced by sclerenchyma, the stem would become hard and rigid because sclerenchyma walls are thick with lignin and the cells are mostly dead. A rigid young stem could not bend with strong winds, so it would be more likely to snap.
Example 8: Sugarcane cuttings (NCERT Exercise 10)
Question: Sohan grew sugarcane from two types of cuttings, A and B. After a few weeks, type B cuttings sprouted and grew into plants, while type A did not. (i) Why were the type ‘B’ cuttings able to grow as sugarcane but type ‘A’ could not? (ii) What difference was present in type ‘B’ compared to type ‘A’? (iii) What observation or measurement was made to determine whether this change had an effect? (iv) What parameters should be kept the same for both types of cuttings to ensure a fair comparison?
Answer: the cuttings are shown in textbook Fig. 3.23, not reproduced here, so (i) and (ii) are reasoned from the result. Check them against the figure.
- The chapter says new branches arise from the nodes of a stem, and the intercalary meristem lies at the base of the internode or just above the node. The most likely explanation is that type B had a node with dividing meristematic cells, so it could sprout.
- The likely difference is that type B cuttings included a node and type A cuttings did not. Confirm this from the figure before writing it in an exam.
- Whether each cutting sprouted and developed into a plant after a few weeks (sprouting or no sprouting).
- Same length and thickness of cutting, same variety, soil, water, light and temperature, and the same observation period.
Competency-Based Questions (with answers)
1. Case-based: The school lawn
Mowed grass on the school lawn grows back within days. A neem sapling whose top shoot was eaten by a goat stopped growing taller but developed two new side branches.
(a) Which meristem helps the grass grow back? (b) Why did the sapling stop growing taller? (c) From where did the new branches arise?
Answer: (a) The intercalary meristem at the nodes of the grass stem. (b) The goat removed the shoot tip, which carries the apical meristem responsible for growth in length. (c) From the nodes of the stem, where intercalary meristem is present.
2. Case-based: Injury on the football field
Riya twisted her knee. The band joining her thigh bone to her shin bone was torn; the band joining her calf muscle to her heel bone was fine.
(a) Name the two bands. (b) Which type of joint is the knee? (c) Why might her knee now move beyond its normal limit?
Answer: (a) The torn band is a ligament (bone to bone); the healthy band is a tendon (muscle to bone). (b) Hinge joint. (c) Ligaments give stability, limit movement and help prevent dislocation. With the ligament torn, that limit is lost.
3. Source-based: Steward’s carrot
“He observed that these phloem cells divided to form a mass of cells, which gradually divided and differentiated into a complete plant.”
(a) What property of the carrot cells does this show? (b) Name the two steps the cells passed through. (c) Which natural cell shows a similar ability?
Answer: (a) Totipotency. (b) Dedifferentiation (regaining the ability to divide and forming an unspecialised mass) followed by redifferentiation (forming roots, shoot and a complete plant). (c) The zygote, which divides and differentiates into an entire organism.
4. Assertion-Reason
Assertion (A): Xylem parenchyma is the only living component of xylem. Reason (R): Tracheids, vessels and xylem fibres are primarily sclerenchymatous.
Answer: Both A and R are true, and R is the correct explanation of A. The other three components are sclerenchymatous, which leaves xylem parenchyma as the living part.
5. Assertion-Reason
Assertion (A): Meristematic cells divide continuously and rapidly. Reason (R): Meristematic cells have thick walls and large vacuoles.
Answer: A is true, but R is false. Meristematic cells have thin walls, a large prominent nucleus, dense cytoplasm, and vacuoles are generally absent.
6. Assertion-Reason
Assertion (A): The ribs can move during breathing. Reason (R): The ribs are joined by flexible cartilage.
Answer: Both A and R are true, and R is the correct explanation of A. The flexible cartilage lets the rib cage expand and contract, changing the space in the chest so air moves in and out.
7. Case-based: The debarked tree (NCERT Exercise 8)
An elephant severely debarked a tree in a forest to eat the nutrient-rich bark.
(i) Which function(s) of the tree is/are hampered by debarking? (ii) Which plant tissue would be affected by further damage to the tree trunk even after debarking? (iii) Which function of the tree would be hampered if the tissues beneath the bark were severely damaged? (iv) What assumptions are you making to answer the questions above? How would the answer change if your assumptions are also changed?
Answer: (i) Protection: bark is made of cork cells that are impermeable to water and gases, so its loss exposes the trunk to water loss, injury and parasites. (ii) The tissues inside the trunk: the conducting tissues (phloem and xylem) and the lateral meristem. The chapter’s text does not give their exact order beneath the bark, so name them as the tissues that would be damaged. (iii) Transport of food from the leaves (phloem), transport of water and minerals from the roots (xylem), and growth in girth (lateral meristem). (iv) Assumption: only the outer bark was removed and the tissues inside were intact, and these tissues lie beneath the bark. If the damage went deeper all round the trunk, phloem could no longer carry food from the leaves to the roots, and the harm would be far greater.
8. Competency MCQ: Identify the muscle
A tissue sample under the microscope shows cylindrical, branched cells, each with a single nucleus and faint striations. Which organ did it come from? (a) Biceps (b) Stomach (c) Heart (d) Intestine
Answer: (c) Heart. Branched cells with one nucleus and faint striations are cardiac muscle, found only in the heart.
Important Questions for Exams
1-Mark Questions
- If a plant is unable to transport food from leaves to roots which tissue is malfunctioning? (i) Xylem (ii) Phloem (iii) Epidermis (iv) Sclerenchyma (ii) Phloem.
- Meristematic tissues divide repeatedly. What property of their cells allows them to do this? (i) They have thick walls for protection. (ii) They contain large vacuoles that store nutrients. (iii) They have thin walls, dense cytoplasm and large prominent nucleus. (iv) They are functionally differentiated cells. (iii) They have thin walls, dense cytoplasm and large prominent nucleus.
- Why are the epithelial tissues that line an animal’s internal organs usually only one or a few cells thick? (i) To store food efficiently. (ii) To provide maximum strength. (iii) To allow quick exchange of materials across them. (iv) To reduce friction. (iii) To allow quick exchange of materials across them.
- Which type of joint is involved when you bend your knees and ankles? (i) Ball and socket (ii) Hinge (iii) Pivot (ii) Hinge.
- Name the chemical deposited in the walls of sclerenchyma cells. Lignin.
2-Mark Questions
- Differentiate between a tendon and a ligament. A tendon connects muscle to bone and transmits the force of muscle contraction to the bone. A ligament connects bone to bone, gives stability, limits movement and helps prevent dislocation.
- Write two functions each of stomata and root hairs. Stomata: gaseous exchange; transpiration, which creates a transpiration pull in xylem and helps eliminate wastes. Root hairs: increase surface area; absorb water and minerals from the soil.
- A plant cell and an animal cell are of the same size. (i) Which cell will have a larger vacuole? Give reasons. (ii) What assumptions are you making to answer the question above? (i) The plant cell. As learnt in Chapter 2, a mature plant cell usually has one large central vacuole that stores water, minerals, sugars and wastes; in animal cells vacuoles are only sometimes present and are much smaller. (ii) That both are mature, non-dividing cells. A meristematic plant cell generally lacks vacuoles, so the answer would change.
- Why does the skull have fixed joints while the shoulder has a ball and socket joint? The skull protects the brain, eyes and ears, so its flat bones are joined firmly and cannot move. The shoulder must move the arm freely forward, backward, sideways and in circles, which a rounded bone end in a shallow hollow allows.
3-Mark Questions
- Compare parenchyma, collenchyma and sclerenchyma on cell wall, living or dead, and function. Parenchyma: thin walls, living, stores food, photosynthesis in green parts, air spaces for floating in aquatic plants. Collenchyma: corners unevenly thickened with pectin, living, support with flexibility. Sclerenchyma: thick walls with lignin, mostly dead, hardness and strength (coconut husk, walnut shell).
- Coconut husk fibres are used for mats which are tough and fibrous. Which tissue has structural features suitable for providing this strength? Explain why living parenchyma couldn’t serve the same purpose. Sclerenchyma: thick walls with lignin make the cells hard and strong. Parenchyma has thin walls and loosely packed cells with intercellular spaces; it is suited to storage and photosynthesis, and its thin walls give no hardness, so mats made from it would be weak.
- Vibha claims to her friend Neha that, “Meristematic cells are located only at the root and shoot apices”. What do you think about this statement? What question can Neha ask Vibha to help her understand further if the statement is incorrect? The statement is incorrect. Apical meristems are at the tips, but the lateral meristem lies in a ring along the circumference of the stem, and the intercalary meristem at the base of the internode or just above the node. Neha can ask: “If meristems were only at the tips, how does a tree trunk get thicker, and how does grass grow back after mowing?”
- Draw up a table comparing skeletal, smooth and cardiac muscle on shape, nuclei and striations. Skeletal: long, cylindrical, unbranched; multinucleate; striated. Smooth: spindle-shaped; single nucleus; no striations. Cardiac: cylindrical, branched; single nucleus; faint striations.
5-Mark Questions
- Describe the three types of meristems with their location and role. List the features of meristematic cells and explain differentiation. Apical meristem at root and shoot tips: growth in length (onion root tips stop growing when cut). Lateral meristem, a ring of cells along the circumference of the stem: divides to give cells inside and outside, so girth increases; the annual growth rings in a cut trunk record this growth year by year. Intercalary meristem at the base of the internode or just above the node: regrowth after cutting, as in grass and hedges. Features: small cells, thin walls, large prominent nucleus, dense cytoplasm, vacuoles generally absent, tightly packed. Differentiation: some new cells lose the ability to divide, change in structure and function, and become permanent tissues specialised for support, transport or storage.
- Explain the structure and functions of xylem and phloem. Why are they called complex permanent tissues? Xylem: tracheids and vessels (tubular, thick-walled), xylem parenchyma (the only living part) and xylem fibres; carries water and minerals from roots to other parts and gives strength. Phloem: sieve tubes (long cells joined end to end by perforated walls, carry food), companion cells (specialised parenchyma that regulate sieve tube cells and monitor loading and unloading of sugars), phloem parenchyma (stores food, resin, tannins, latex) and phloem fibres (strength); mostly living; carries food from leaves to other parts. Both are complex because each has more than one type of cell, and permanent because their cells have differentiated.
- During the discussion in class, Rohan gives a statement that, “A tissue is a group of similar cells performing similar functions”. But Rajiv counter argues that, “this is true in case of simple tissues but little different in case of complex tissues”. Provide your explanation in view of the discussion in class. Simple tissues such as parenchyma are made of one type of cell doing one job, which fits Rohan’s statement. Complex tissues such as xylem contain different cell types (tracheids, vessels, parenchyma, fibres) with different structures, all working together for one overall function, conduction. So Rajiv is right: in complex tissues, the cells are not all similar, but they share a common function.
Common Mistakes and Examiner Tips
- Swapping tendon and ligament. Tendon: muscle to bone. Ligament: bone to bone. Write the connection in full each time.
- Saying meristems are only at the tips. There are three: apical (tips), lateral (ring in the stem) and intercalary (at the nodes).
- Giving meristematic cells large vacuoles or thick walls. They have thin walls, a large nucleus, dense cytoplasm and vacuoles are generally absent.
- Mixing up pectin and lignin. Pectin at the corners of collenchyma gives flexibility; lignin in sclerenchyma walls gives hardness.
- Calling sclerenchyma living. Most sclerenchyma cells are dead. Parenchyma and collenchyma are living.
- Calling all of xylem dead. Xylem parenchyma is its only living component. Phloem is mostly living.
- Reversing the direction of transport. Xylem: water and minerals from roots to other parts. Phloem: food from leaves to other parts.
- Writing “lungs have thick, many-layered epithelium”. Exchange needs a single layer of thin, flat cells. Many layers are for protection (skin, mouth, oesophagus).
- Muscle feature errors. Only skeletal muscle is multinucleate. Cardiac muscle is branched with faint striations. Smooth muscle has no striations.
- Neuron direction. Dendrites receive; the axon carries the message away and passes it on at the axon terminals.
- Joint examples. Shoulder: ball and socket. Elbow and knee: hinge. Neck (skull on backbone): pivot. Skull bones: fixed.
- Rushing assertion-reason items. Judge A and R separately as true or false first, then decide whether R explains A.
Quick Revision Points
- Cell → tissue → organ → organ system → organism; tissues bring division of labour and efficiency.
- Apical meristem (root and shoot tips): length. Lateral meristem (ring in the stem): girth. Intercalary meristem (at nodes): regrowth.
- Meristematic cells: thin walls, large nucleus, dense cytoplasm, vacuoles generally absent, tightly packed.
- Epidermis: single layer of flat cells with a cuticle of cutin; root hairs absorb, stomata exchange gases and transpire.
- Parenchyma: living, thin walls, stores food, photosynthesis, air spaces in aquatic plants.
- Collenchyma: living, corners thickened with pectin, flexibility.
- Sclerenchyma: thick lignified walls, mostly dead, hardness (coconut husk, walnut shell).
- Xylem: tracheids, vessels, xylem parenchyma (only living part), fibres; water and minerals up from roots.
- Phloem: sieve tubes, companion cells, phloem parenchyma, fibres; mostly living; food from leaves.
- Epithelium: closely packed cells; thin single layer for exchange, many layers for protection, tall cells for absorption.
- Blood: plasma about 55%, formed elements about 45%; haemoglobin in RBCs, platelets clot, WBCs collect at infected areas; RBCs live about 4 months.
- Bone: rigid matrix of calcium and phosphorus compounds. Cartilage: soft, jelly-like, cushions.
- Skeletal muscle: voluntary, multinucleate, striated. Smooth: involuntary, spindle, no striations. Cardiac: involuntary, branched, faint striations, no fatigue.
- Neuron: cell body, dendrites, axon, axon terminals; the brain is the control centre.
- Joints: ball and socket (shoulder), hinge (elbow, knee), pivot (neck), fixed (skull).
- 12 pairs of ribs joined by flexible cartilage; cartilage discs between vertebrae; skeleton about 12 to 15 per cent of adult body weight.
- Totipotency: Steward (1958) grew whole carrot plants from phloem cells; Agrobacterium tumefaciens causes crown gall and is used to add genes to plants.
Weightage and Exam Pattern
Class 9 has no board exam. This chapter is assessed in school exams (periodic tests, half-yearly and annual papers), and the NCERT source gives no fixed mark allocation. Questions lean on definitions, comparisons and structure-function reasoning.
| Question type | What this chapter typically supplies |
|---|---|
| MCQ, one-word and assertion-reason | Identifying a tissue from its features, naming a joint, pectin versus lignin, tendon versus ligament |
| Short answer | Types of meristems, simple permanent tissues compared, muscle types compared, parts of a neuron |
| Long answer | Xylem and phloem in detail, meristems with differentiation, joints and the musculoskeletal system |
| Case-based and data-based | Onion root tips, teak tree rings, debarked trees, sugarcane cuttings, carrot tissue culture, sports injuries |
This chapter is the foundation for Class 10 biology. Xylem, phloem, stomata and transpiration return when you study transport in plants, and the neuron returns when you study the nervous system.
Class 9 Science · Chapter 3 – swipe through all 10 cards to understand the whole chapter.
What Is a Tissue
A group of similar cells working together for a specific function.
In amoeba one cell does every job; multicellular bodies share the work.
- Division of labour increases the efficiency of the body.
- Muscle tissue moves, nervous tissue carries messages.
- Xylem carries water and minerals, phloem carries food.
Three Meristems
Actively dividing cells in fixed zones make plants grow.
Cut onion root tips stop growing: roots grow only from their tips.
- Apical: root and shoot tips.
- Lateral: a ring of cells along the stem’s circumference.
- Intercalary: base of internode or just above the node, as in grass.
Meristem Cells and Differentiation
Meristem cells are built for fast, repeated division.
Differentiated cells lose the ability to divide and specialise.
- Thin walls, large prominent nucleus, dense cytoplasm.
- Vacuoles generally absent; tightly packed cells.
- Permanent tissues support, transport or store.
Simple Permanent Tissues
Parenchyma, collenchyma and sclerenchyma each support in their own way.
Coconut husk is sclerenchyma; a coriander stalk bends thanks to collenchyma.
- Parenchyma: living, thin walls, stores food, air spaces help floating.
- Collenchyma: living, thickened corners, stems and tendrils bend.
- Sclerenchyma: mostly dead, thick walls, seed coats and nut shells.
Xylem and Phloem
Complex permanent tissues made of several cell types working together.
Xylem parenchyma is the only living part of xylem; phloem is mostly living.
- Xylem: tracheids, vessels, xylem parenchyma, xylem fibres.
- Phloem: sieve tubes, companion cells, phloem parenchyma, fibres.
- Companion cells monitor loading and unloading of sugars.
Epidermis and Tissue Systems
A single layer of flat cells guards the whole plant.
A thick cuticle suits desert plants by cutting water loss.
- Cuticle of cutin reduces water loss, injury and parasite entry.
- Root hairs absorb; stomata exchange gases and transpire.
- Ground system holds parenchyma, collenchyma and sclerenchyma.
Epithelial Tissue
Closely packed cells cover the body and line internal organs.
Structure follows function: the lungs need a short diffusion path.
- Exchange: single thin, flat layer in lungs and blood vessels.
- Protection: many layers in skin, mouth and oesophagus.
- Absorption: tall, pillar-like cells in the small intestine.
Connective Tissues
Tissues that connect and support, differing in their matrix.
Blood has a fluid matrix; bone has a rigid one of calcium and phosphorus compounds.
- Blood: plasma about 55%, formed elements about 45%.
- Platelets clot blood; WBCs gather at infections; RBCs live about 4 months.
- Cartilage cushions bone ends and keeps the ear flexible.
Muscles and Neurons
Muscles move the body on instructions from nervous tissue.
Cardiac muscle beats rhythmically throughout life without fatigue.
- Skeletal: voluntary, multinucleate, striated.
- Smooth: involuntary, spindle-shaped, no striations.
- Cardiac: involuntary, branched, faint striations.
Musculoskeletal System and Joints
Muscles pull bones through tendons, and movement happens at joints.
12 pairs of ribs joined by flexible cartilage let the chest expand.
- Shoulder: ball and socket, forward, backward, sideways, circular.
- Elbow and knee: hinge, one direction only.
- Neck: pivot; skull bones: fixed, protecting the brain.
📝 Practice Tissues in Action - 10 questions
Concept-check questions · with answers & explanations
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Why B: A fixed plant must stand firm and upright, so its cells have a cell wall that gives rigidity and strength. A moving animal needs flexibility; without a rigid wall its cells can change shape easily, and this helps the body in locomotion.
Why not A: It reverses the facts: plant cells have cell walls and animal cells do not.
Why not C: Plant cells do have a nucleus; the nucleus is not what makes plants fixed.
Why not D: Animal cells have no cell wall at all, thin or thick.
Remember: Fixed life needs rigid walls; moving life needs flexible cells.
Why C: New branches arise from the nodes of a stem, and the region just above the node has meristematic tissue that can divide. A cutting with a node carries these dividing cells, so it can form new shoots; a piece from the middle of an internode has only permanent tissue and cannot.
Why not A: Stored food can support growth but cannot produce new organs without dividing cells; both types stored sugar.
Why not B: Nothing in the setup says the lengths differed, and length alone does not create new shoots.
Why not D: The only difference between P and Q was the node, and the result was consistent, so it is not chance.
Remember: No node, no new shoot.
Why C: Stomata are pores in the epidermis that allow gaseous exchange and transpiration. Without them gases could not pass in and out through the leaf surface, and water vapour could not evaporate, so the transpiration pull in the xylem would be lost and water movement up the plant would weaken.
Why not A: Transpiration happens through stomata, so removing them would reduce it, not speed it up.
Why not B: Gas exchange depends on stomata, so it could not be normal; phloem food transport is not the direct effect.
Why not D: Nothing links missing stomata to cuticle thickness or root hairs.
Remember: No stomata, no transpiration pull.
Why C: Ligaments connect bone to bone, give stability, limit movement at a joint and help prevent dislocation. That is why the knee stops at a limit. If a ligament ruptures, the joint loses this restraint, becomes unstable and can move too far or dislocate.
Why not A: Cartilage cushions bone ends and gives flexibility; it is not what stops the knee, and rupture would not harden bones.
Why not B: Tendons connect muscle to bone and transmit pull; they do not set the joint limit.
Why not D: Bone gives strength and support, not the limit on joint movement.
Remember: Ligaments are the joint’s safety straps.
Why D: Cylindrical, branched fibres with a single nucleus and faint striations are the features of cardiac muscle, found only in the heart. It works tirelessly and rhythmically so the heart beats throughout life without fatigue.
Why not A: Skeletal muscle moves limbs; its fibres are unbranched and multinucleate.
Why not B: Smooth muscle in the stomach has spindle-shaped cells with no striations.
Why not C: Carrying messages is the job of neurons in nervous tissue, not muscle.
Remember: Branched + one nucleus + faint stripes = the tireless heart muscle.
Why A: Muscles pull on bones to produce movement. On a signal from the nervous system, the muscle contracts, the tendon transmits this force to the bone, and the bone moves at a joint; the elbow is a hinge joint that bends in one direction.
Why not B: A joint is only a junction between bones; it cannot contract.
Why not C: Ligaments join bone to bone and limit movement; they do not pull bones.
Why not D: Cartilage cushions bone ends and gives flexibility; it does not push bones to move.
Remember: Nerve signals, muscle pulls, tendon transmits, bone moves, joint allows.
Why A: The assertion is true: the chapter says tissue formation increases the efficiency of the body and enables complex life processes. The reason is true and is exactly why: when different groups of cells specialise in different jobs (division of labour), each job is done better.
Why not B: R is not a separate true fact here; it is the very mechanism that produces the efficiency in A.
Why not C: R is correct; tissues do take up different functions, such as muscle for movement and nervous tissue for messages.
Why not D: A is correct as stated in the chapter.
Remember: Specialise the jobs, and the whole body works better.
Why B: Mature phloem cells had stopped dividing. In the culture they first dedifferentiated (regained the ability to divide), formed an undifferentiated mass, and then redifferentiated into roots and shoot, finally forming a complete plant. This ability is totipotency.
Why not A: The order is reversed; a cell must first regain the ability to divide before it can form new organs.
Why not C: A single specialised cell cannot become a plant just by elongating; many new cells had to be produced by division.
Why not D: No fusion or fertilisation happened; single cells were cultured, which is why the result was remarkable.
Remember: Totipotency: undo the specialisation, divide, then specialise again.
Why B: Xylem is a complex permanent tissue. It contains several different cell types, some dead and one living, yet they work together for one function, carrying water and minerals. So a tissue can be made of more than one type of cell, which is Rajiv’s point.
Why not A: Parenchyma is a simple tissue with one type of cell, so it fits Rohan’s statement.
Why not C: Collenchyma is a simple tissue with one cell type.
Why not D: Sclerenchyma is also a simple tissue with one cell type.
Remember: Simple = one cell type; complex (xylem, phloem) = many cell types, one job.
Why D: The assertion is false, because bones are connected to bones by ligaments, not tendons. The reason is true: a tendon is tough connective tissue that transmits the pull of a contracting muscle to the bone.
Why not A: A cannot be explained by R when A itself is false.
Why not B: This option needs A to be true, and it is not.
Why not C: It marks the wrong statement as false; R is correct and A is the false one.
Remember: If the tendon is said to join bone to bone, the statement is false.
Chapter Navigation
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Frequently Asked Questions
A tissue is a group of cells, similar in structure, that work together to perform a specific function. In a multicellular organism different tissues take on different jobs, such as movement, conduction or protection. This division of labour increases the efficiency of the body and lets it carry out complex life processes, which a single cell like amoeba has to manage alone.
The apical meristem is at the tips of roots and shoots and increases length. The lateral meristem is a ring of dividing cells along the circumference of the stem and increases girth. The intercalary meristem is at the base of the internode or just above the node and helps plants such as grasses grow back after mowing or grazing.
Parenchyma has living, thin-walled, loosely packed cells that store food and can photosynthesise. Collenchyma has living cells whose corners are thickened with pectin, giving flexibility to stems and tendrils. Sclerenchyma has mostly dead cells with thick lignified walls, giving hardness, as in coconut husk and walnut shell.
Xylem carries water and minerals from the roots to other parts and gives strength. It has tracheids, vessels, xylem parenchyma and xylem fibres, and only xylem parenchyma is living. Phloem carries food from the leaves to other parts and is mostly living. It has sieve tubes, companion cells, phloem parenchyma and phloem fibres.
A tendon connects muscle to bone. When the muscle contracts, the tendon transmits the force to the bone and movement happens at a joint. A ligament connects bone to bone. It gives stability to a joint, limits its movement and helps prevent dislocation, which is why the knee does not bend beyond a certain limit.
Skeletal muscle is voluntary, attached to the skeleton, with long, unbranched, multinucleate and striated fibres. Smooth muscle is involuntary, found in the stomach and intestines, with spindle-shaped cells, one nucleus and no striations. Cardiac muscle is involuntary, found only in the heart, with branched cells, one nucleus and faint striations, and works without fatigue.
Totipotency is the ability of some mature plant cells to undifferentiate, divide and redifferentiate into a whole new plant under specific conditions. In 1958 F. C. Steward grew complete carrot plants from single phloem cells kept in a nutrient medium with sugars and hormones. The cells first formed an unspecialised mass and then formed roots, shoot and a plant.
The ball and socket joint at the shoulder allows forward, backward, sideways and circular movement. The hinge joint at the elbow and knee moves in one direction only. The pivot joint between the skull and backbone lets the head turn side to side. Fixed joints join the skull bones so they cannot move and keep the brain safe.
