Tissues in Action

Why This Matters

Think about a tiny seed. You drop it in the soil, water it, and wait. Slowly it pushes out a root and a shoot. The shoot becomes a stem. Leaves open. Years later, that same seed has grown into a tall mango tree that gives you fruit and shade.

Now think about you. You began life as a single cell. That one cell divided again and again. The new cells did not all stay the same. Some became your skin, some your muscles, some your bones, some your nerves. From one cell, a whole working human body was built.

How does this happen? How does a body know to make skin in one place and bone in another? The answer is tissues.

Your body is not just a heap of cells. The cells are sorted into teams. Each team does one special job. One team carries water in a plant. Another team makes you move. Another team carries messages in your body at lightning speed. When you blink, breathe, or pull your hand back from a hot pan, different tissues are quietly doing their work.

In this chapter you will meet these teams — in plants and in animals. You will see how the shape of a tissue is perfectly matched to its job. Once you see this, the whole body starts to make sense.

The Big Idea

The Big Idea: A tissue is a group of similar cells that work together to do one specific job. Big bodies split their work between many tissues. This split is called division of labour, and it lets each job be done really well. Plants and animals have different tissues because they live in different ways — plants stay fixed and make their own food, while animals move and eat food. The clever part is that each tissue’s structure is shaped to fit exactly what it has to do.

Let’s Break It Down

Before we meet the tissues, let us be clear about a word from your last chapter — the cell. Everything here builds on it.

From one cell to a whole organism

A single-celled creature like an amoeba is just one cell. That one cell must do everything on its own — move, eat, breathe, get rid of waste. It is a one-person shop where the same person sweeps, cooks, serves and bills.

Big living things — plants and animals — are multicellular. They have billions of cells. These cells are not loose. They are organised in levels, one inside the next. Figure 3.1 shows this ladder of organisation.

We saw the ladder is built in clear steps. Let us picture each rung.

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So the order is: cell → tissue → organ → organ system → organism. A tissue sits at the second rung — it is the first team that forms when similar cells join up.

Why splitting the work helps — division of labour

Here is a question NCERT states as a fact but does not fully explain: why is it better to have many tissues each doing one job, instead of every cell doing everything?

Think of a small tea shop run by one person. He boils tea, fries samosas, takes money and washes cups. He is busy all day but slow, because he keeps switching jobs. Now think of a big restaurant. One person only cooks. One only serves. One only handles the cash. Each person becomes an expert at their single task. The whole restaurant runs much faster.

The body works the same way. When a group of cells does only one job, those cells can become really good at it. Their shape and parts get fine-tuned for that one task. This sharing out of jobs is called division of labour. It makes the whole body more efficient, and lets it do complex things a single cell never could.

Why plant and animal tissues are different

Plants and animals live in very different ways, so they ended up with very different tissues. Two big differences explain most of it.

1. Movement and support. Most plants stay fixed in one spot. They cannot run, so they need to stand firm and upright on their own. Their cells have a stiff cell wall that gives strength and rigidity, like a frame holding them up. Most animals move from place to place. Animal cells have no rigid cell wall, so they can change shape easily. That flexibility is exactly what lets an animal body bend, run and crawl.

2. Getting food. Plants make their own food from sunlight, through photosynthesis. So they have tissues built to catch sunlight and to carry food and water around the plant. Animals cannot make food. They eat it. So animals have tissues built to digest food and to carry it around the body.

Because the jobs differ, the tissues differ. We will look at plant tissues first, then animal tissues.

Tissues for Growth in Plants

You have seen a seedling grow into a tall tree, roots dig deeper, and grass come back after it is cut. Growth like this needs cells that keep dividing to make new cells. The tissue that does this is called meristematic tissue (you can call it a meristem). “Meristem” cells are young cells whose only job is to divide and make more cells.

Plants grow in three ways, and there is a meristem for each one:

• growing longer (taller stems, deeper roots),
• growing thicker (more girth in the stem), and
• growing back after being cut or grazed.

Figure 3.2 shows where each of the three meristems sits on a plant. Keep referring back to it as we go.

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Apical meristem — growing in length

The apical meristem sits at the very tip of every root and every shoot. (“Apical” comes from apex, meaning tip.) Its cells divide non-stop, so the tip keeps adding new cells and the plant grows longer.

How do we know growth happens only at the tip? Try the onion experiment NCERT describes. Put two onions in water and let their roots grow. Then snip the root tips off one onion. The cut onion’s roots stop growing. The other onion’s roots keep getting longer. The growing part was the tip all along. Cut off the tip, and you cut off the apical meristem.

This is the same dividing tissue you saw under the microscope in Chapter 2, where onion root-tip cells were caught in the act of dividing (mitosis).

Lateral meristem — growing in girth

Look at the trunk of a big tree. It is thick. A young plant’s stem was thin. What made it grow wider over the years? The lateral meristem.

The lateral meristem is a ring of dividing cells running along the inside of the stem. It makes new cells towards the inside and the outside of the ring. Slowly the stem gets fatter, year after year.

This is also why a cut tree trunk shows annual rings. Each year the lateral meristem adds a new ring of wood. A wide ring means a good year with plenty of water; a narrow ring means a hard year. So by counting the rings, scientists can tell a tree’s age — and even read the weather of long ago.

Intercalary meristem — growing back after a cut

Have you ever wondered why grass keeps coming back, no matter how often it is mowed or eaten by cows? If the only growth tissue were at the tips, cutting the tips would stop growth for good. But grass has a third meristem.

The intercalary meristem sits at the nodes of the stem. A node is the point on a stem where a leaf or branch joins. The gap between two nodes is an internode. Because the intercalary meristem is lower down at the nodes — not just at the very tip — the plant can grow back even after its top is cut. That is why a trimmed hedge soon turns bushy again, and why a mowed lawn turns green within days.

Let us put a typical “where does which growth happen” question to the test.

Why meristematic cells are built to divide fast

Look closely at a meristem cell and you see a special design. The cells are small, with thin cell walls. Each has a large, clear nucleus and dense cytoplasm packed with organelles. They have little or no vacuole, and they sit tightly packed with almost no gaps between them.

Why no big vacuole? Here is the “why” NCERT leaves hanging. A vacuole is mostly a water-filled bag for storage. A cell that is about to divide does not want to be busy storing water — it wants to be busy making new cell parts. A big vacuole would also push the working bits of the cell to the side. So a dividing cell keeps its vacuoles tiny and fills itself with active cytoplasm instead. Everything about a meristem cell is tuned for one thing: divide, divide, divide.

From meristem to permanent tissue — differentiation

A meristem cell divides into two. Sometimes both stay young and keep dividing. But often one of the new cells stops dividing and changes. It grows a special shape and takes up a special job — storing food, giving support, or carrying water.

This change, where a young dividing cell turns into a specialised, non-dividing cell, is called differentiation. The cells made this way are called permanent tissues, because they have settled into their final form and no longer divide.

So the journey is: meristematic tissue → (differentiation) → permanent tissue.

Permanent Tissues in Plants

Permanent tissues are the worker tissues of a plant. Each kind has a fixed shape and a fixed job. They come in two broad groups:

• Simple permanent tissue — made of just one type of cell.
• Complex permanent tissue — made of more than one type of cell working as a team.

Before we list them all, Figure 3.3 lays out the full family tree of plant tissues. Use it as your map for this whole section.

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The protective tissue — epidermis

What stops a plant from drying out, getting hurt, or being invaded by germs? Its outer layer, the epidermis.

The epidermis is the outermost layer of the plant. It is a single layer of flat, rectangular cells packed tightly with no gaps. It often has a waxy coat called the cuticle on the outside. This wax is waterproof, so it cuts down water loss — very useful for desert plants, which have a thick cuticle.

The epidermis also grows useful extras:

• In roots, some epidermal cells stretch out into root hairs. These give the root a huge surface to soak up water and minerals from the soil.
• In leaves, the epidermis has tiny pores called stomata. Stomata let gases in and out, and let water vapour escape (this water loss is called transpiration).

The three simple support tissues

What keeps a plant standing? Why does a fresh green twig bend while a dry twig snaps? Why is a coconut shell so hard? These come down to three simple tissues — parenchyma, collenchyma and sclerenchyma. They differ mainly in how thick and how hard their cell walls are. Figure 3.4 puts the three side by side.

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Parenchyma is the simplest. Its cells are alive, with thin walls, and they sit loosely with gaps between them. Its main job is to store food. In green parts of the plant, parenchyma also does photosynthesis. In water plants, special parenchyma has air spaces inside that help the plant float.

Collenchyma is the flexible one. Its living cells have walls that are thickened only at the corners. This makes the tissue strong but still bendy — it can bend without breaking. That is why a coriander stalk or a young stem can sway in the wind and spring back. The flexibility comes from a substance called pectin, the same gummy stuff that sets jam.

Sclerenchyma is the hard one. Its cells have very thick walls all the way round, stiffened by a tough material called lignin. These cells are mostly dead. This makes sclerenchyma hard and strong. It forms the woody, fibrous parts — the husk of a coconut, the shell of a walnut, the hard veins in a leaf.

Here is a “why” worth pausing on: why are the coconut husk fibres so much harder than a soft coriander stalk? Because the husk is sclerenchyma — thick lignified walls, dead cells, built for pure strength. The coriander stalk is collenchyma — thinner walls, living cells, built for flexible support. Same family, opposite jobs.

The complex conducting tissues — xylem and phloem

How does water get from the roots to the topmost leaf of a tall tree? How does the food made in leaves reach the roots underground? Through two complex tissues — xylem and phloem. They are “complex” because each is a team of several different cell types working together. Figure 3.5 shows both.

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Xylem carries water and minerals from the roots upward, to every part of the plant. It also gives the plant strength. Xylem is a team of four cell types:

• Tracheids and vessels — tube-shaped, thick-walled, and dead. These are the actual water pipes.
• Xylem fibres — give extra strength.
• Xylem parenchyma — the only living part of xylem; it stores food.

Now the famous “why”: why are the water-carrying cells of xylem dead? This puzzles many students, because we expect living things to be made of living cells. Picture the cell as a room. A living cell is a furnished room — full of jelly (cytoplasm), a nucleus, vacuoles. All that stuff would get in the way of water trying to rush through. A xylem vessel solves this by emptying itself out. The cell dies, its insides clear away, and even the walls between stacked cells dissolve. What is left is a hollow, thick-walled tube — like joining many small pipes into one long open pipe. Water can then shoot straight up with nothing to slow it. So being dead is not a flaw. It is the whole point: a clear, open pipe.

Phloem carries food (sugar made in the leaves) to the rest of the plant. Unlike xylem, phloem is mostly alive. Its team is:

• Sieve tubes — long living cells joined end to end. Their joining walls have tiny holes (like a sieve), called sieve plates, so food can pass from one cell to the next. These tubes carry the food.
• Companion cells — special living cells beside each sieve tube. The sieve tube has lost its nucleus, so it cannot fully run itself. The companion cell controls it and manages the loading and unloading of sugar.
• Phloem parenchyma — stores food and other substances.
• Phloem fibres — give strength (these are sclerenchyma-like).

A quick worked question on telling them apart:

Plant tissue systems

In a real plant these tissues do not sit alone. They are grouped into three larger tissue systems:

1. Dermal tissue system — the outer covering (epidermis). It protects and reduces water loss.
2. Ground tissue system — the main bulk of the plant between the skin and the conducting tissue. It is made of parenchyma, collenchyma and sclerenchyma.
3. Vascular tissue system — the conducting tissues, xylem and phloem.

Animal Tissues

Now to animals. Like plants, animal cells also group into tissues, each with one job. Try these little actions: blink your eyes, clench your fist, take a deep breath, touch something warm. Each one used a different tissue.

There are four main animal tissues:

• Epithelial — covering and lining tissue.
• Connective — joining and supporting tissue.
• Muscular — movement tissue.
• Nervous — message-carrying tissue.

Let us meet each one.

Epithelial tissue — the covering and lining

Epithelial tissue forms the outer covering of the body (your skin) and lines the inside of organs — the mouth, the lungs, the blood vessels, the intestine. Its cells are packed very closely with almost no gaps. This tight packing makes a barrier that keeps germs out and water in.

But epithelium does not only cover. Depending on the job, its cells take different shapes. Figure 3.6 shows five types, each shaped for its task.

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Notice the clever matching of shape to job:

• Thin, flat cells let gases and liquids cross fast — perfect for the lungs, where oxygen must move quickly from air to blood. This answers a common exam point: the lung lining is one cell thick on purpose, so the gas has the shortest possible distance to cross.
• Many layers make a thick, tough shield — perfect for skin, which takes rubbing and knocks.
• Tall pillar cells in the small intestine stretch up to grab as many nutrients as possible.
• Gland cells are built to make and pour out useful liquids like saliva, sweat and stomach juice.
• Cilia (tiny hairs) help us sense the world — in the nose, ears and taste buds.

Connective tissue — joining and supporting

A connective tissue connects and supports the other tissues and organs of the body. Two surprising members of this group are blood and bone. They look nothing alike — blood is a flowing liquid, bone is rock-hard. Yet both are connective tissues. What makes them so different is their matrix, the material the cells sit in. Blood’s matrix is a watery liquid, so blood flows. Bone’s matrix is hard and solid, so bone is rigid. Figure 3.7 shows both.

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Blood is a fluid connective tissue. Its cells float in a watery matrix called plasma. The cells are:

• Red blood cells (RBCs) — carry oxygen. Their red colour comes from haemoglobin, an iron-rich protein. RBCs last about 4 months, then are replaced.
• White blood cells (WBCs) — fight germs. They gather at an infection, which causes redness, swelling and sometimes pus.
• Platelets — help blood clot to seal a cut.

Bone is a hard connective tissue. Its matrix is packed with calcium and phosphorus compounds, which make it strong and rigid. Bones give the body strength, support and protection.

Other connective tissues you can feel right now:

• Cartilage — the soft, bendy part of your ear and the tip of your nose. Its matrix is a soft jelly. It is flexible and cushions the ends of bones so they do not grind.
• Tendon — connects a muscle to a bone. When the muscle pulls, the tendon passes the pull to the bone, and the bone moves.
• Ligament — connects a bone to a bone across a joint. It holds the joint steady and stops the bones moving too far.

Muscular tissue — making movement

Muscular tissue is what makes your body move. There are three kinds, and you can tell them apart by their stripes (called striations) and the number of nuclei in each cell. Figure 3.8 shows all three.

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Skeletal muscle moves your bones — it powers running, writing, lifting. You choose when to move it, so it is voluntary. Its cells are long cylinders called muscle fibres. They are unbranched, have many nuclei (multinucleate), and show clear stripes (striated).

Smooth muscle works without you telling it to — it is involuntary. It pushes food along your intestine and does other slow, steady jobs. Its cells are spindle-shaped (pointed ends), have a single nucleus, and have no stripes.

Cardiac muscle is found only in the heart. It is also involuntary. Its cells are branched with a single nucleus and faint stripes. The amazing thing: cardiac muscle beats all your life without ever getting tired. It works tirelessly, day and night, from before you are born until you die.

Nervous tissue — sensing and communicating

How do you pull your hand back from a hot pan in an instant? How do you still remember a song from years ago? That is nervous tissue at work. It is the body’s control-and-message network. The brain is the control centre, and nerves carry its messages all over the body.

The cells of nervous tissue are called neurons (nerve cells). A neuron is specially shaped to receive a message, carry it a long way, and pass it on. Figure 3.9 shows its parts.

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Each neuron has three main parts:

• Cell body — holds the nucleus; it controls the cell.
• Dendrites — short branches that receive signals from other neurons.
• Axon — one long fibre that carries the message away from the cell body, ending in axon terminals that pass it to the next cell.

So a message flows in one direction: into the dendrites, through the cell body, down the axon, and out of the terminals. Muscles cannot act on their own — they wait for instructions from nervous tissue. For example, during exercise the brain signals your heart (cardiac muscle) to beat faster.

The Skeleton and Joints

The bones, muscles, tendons, ligaments, cartilage and joints together form the musculoskeletal system. It lets you stand, move and keep your shape, and it protects soft organs. It works under the control of the nervous system. Muscles pull on bones through tendons, and movement happens at a joint.

A joint is the junction where two or more bones meet. But a joint cannot move bones by itself — the muscles do the pulling. The kind of joint decides how the bones can move. Figure 3.10 shows the four main types.

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• Ball and socket joint — the rounded end of one bone fits into a cup-shaped hollow of another. This allows movement in all directions. Found in the shoulder and hip.
• Hinge joint — bends and straightens in one direction only, like a door hinge. Found in the elbow and knee (the kneecap protects the knee).
• Pivot joint — lets a bone turn side to side. The skull joins the backbone with a pivot joint, so you can shake your head “no”.
• Fixed joint — the bones cannot move at all. The flat bones of the skull are joined this way, locking together to protect the brain.

The skeletal system is the framework of bones — the skull, the vertebral column (backbone, made of small bones called vertebrae), and the rib cage. Soft cartilage discs between the vertebrae cushion the spine and let you bend. The rib cage (12 pairs of ribs) guards the heart and lungs, and its flexible cartilage joints let it expand and shrink as you breathe.

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Summary

You can now explain:

• What a tissue is — a group of cells that work together for one specific function — and why big bodies use division of labour to do each job better.
• Why plant and animal tissues differ: plants stay fixed and make their own food (so they have stiff cell walls and food-making, conducting tissues), while animals move and eat food.
• The three meristems — apical (length, at tips), lateral (girth, a ring in the stem) and intercalary (regrowth, at nodes) — and how differentiation turns dividing cells into permanent tissues.
• The simple permanent tissues (parenchyma, collenchyma, sclerenchyma) by their walls and jobs, and the complex ones (xylem, phloem) — including why xylem cells are dead, empty pipes while phloem cells stay alive.
• The four animal tissues — epithelial (covering/lining), connective (joining/supporting, including blood and bone), muscular (skeletal, smooth, cardiac) and nervous (neurons) — and how each tissue’s shape fits its job.
• How joints (ball-and-socket, hinge, pivot, fixed) allow different movements, and how the musculoskeletal system lets the body move under the control of the nervous system.

What’s Next

You have now seen how living bodies are built and how their parts move. Next we step away from biology and into physics — but movement is still the theme. In Chapter 4 — Describing Motion, you will learn how to describe exactly how things move: how fast, how far, in which direction, and how their speed changes over time. Just as tissues gave structure to a body, the ideas of distance, speed and acceleration give structure to motion.

Chapter 4 — Describing Motion Around Us