B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

Specialized Animal Cells and the Four Animal Tissues

B2.4 B3.3 B2.1 A1.11About 70 minutes

Learning goals

I can describe how the structure of a specialized cell (nerve, muscle, blood, bone, epithelial) fits its function.
I can name the four main types of animal tissue and give examples and functions of each.
I can tell skeletal, smooth and cardiac muscle apart and draw labelled diagrams of specialized cells.
I can identify tissues in a real sample (such as a chicken wing) and communicate what I observed.

Words to know

tissue
A group of similar cells that work together to do a job.

epithelial tissue
Tissue that covers the body and lines organs and tubes; it protects and can absorb or secrete.

connective tissue
Tissue that supports, connects and protects other tissues: bone, cartilage, tendons, ligaments, fat and blood.

muscle tissue
Tissue made of cells that contract (shorten) to cause movement: skeletal, smooth or cardiac.

nervous tissue
Tissue made of neurons that carry electrical signals, plus supporting cells.

neuron
A nerve cell, specialized to carry signals.

axon
The long extension of a neuron that carries signals away from the cell body.

striated
Having a striped pattern under the microscope (skeletal and cardiac muscle).

matrix
The material between cells in connective tissue, such as the hard mineral part of bone or the liquid plasma of blood.

Built for the job

A hockey team doesn't use five goalies. Each player has a role, and the equipment fits the role: goalies wear huge pads, forwards wear lighter gear and carry sticks with curved blades. Cells are the same. In biology there is one big idea that explains almost every specialized cell: structure fits function. When you see an unusual cell shape, ask, How does this shape help the cell do its job?

Remember from Grade 8

In Grade 8 you learned the four main kinds of animal tissue (epithelial, connective, muscle, nervous) and met the three types of muscle. In Grade 10 you will look at the cells inside each tissue in much more detail, explain the link between their structure and function, and draw them as a biologist would.

Nerve cells (neurons)

Neurons carry electrical signals through the body, sometimes over long distances.

Branching dendrites receive signals from other cells.
The cell body contains the nucleus and most organelles.
A single long axon carries the signal away from the cell body. Some axons run from the spinal cord all the way down the leg.
Many axons are wrapped in a fatty myelin sheath, which insulates them and makes signals travel much faster.
Axon terminals pass the signal to the next cell (another neuron, a muscle cell or a gland).

A motor neuron (not to scale). Its long, thin shape is perfect for carrying a signal from one place to a distant one, like a cable. The arrow shows the direction of the signal: in at the dendrites, out at the axon terminals.

Muscle cells

Muscle cells are packed with protein fibres that slide past each other to contract (shorten). They contain many mitochondria to supply the energy. There are three types:

Skeletal muscle is attached to bones and moves them. Its cells (called muscle fibres) are very long, have many nuclei and look striated (striped). You control it: it is voluntary.
Smooth muscle is found in the walls of the stomach, intestines, bladder and blood vessels. Its cells are spindle-shaped with one nucleus and no stripes. It works without your control: involuntary.
Cardiac muscle is found only in the heart. Its cells are striated and branched, and are joined end to end by special connections (intercalated discs) that pass the signal to contract quickly from cell to cell, so the heart beats as a unit. It is involuntary.

The three types of muscle tissue as they look under a microscope (simplified). Look for stripes, the number of nuclei, and the shape of the cells to tell them apart.

Blood cells

Blood is a connective tissue: cells floating in a liquid matrix called plasma.

Red blood cells carry oxygen. They are small (about 7.5 µm), flexible discs that are thinner in the middle (biconcave), which gives a large surface area for oxygen to diffuse in and out and lets them squeeze through narrow capillaries. Mature human red blood cells have no nucleus, which leaves more room for haemoglobin, the oxygen-carrying protein.
White blood cells defend the body. Some, called phagocytes, can change shape to wrap around and engulf bacteria; others make antibodies.
Platelets are cell fragments that help blood clot.

Bone cells

Bone is a hard connective tissue. Its matrix is made of protein fibres hardened with calcium phosphate minerals. Bone cells (osteocytes) live in small spaces in the matrix, arranged in rings around central canals that carry blood vessels and nerves. Fine channels connect the cells so they can get nutrients. Bone is living tissue: it is constantly rebuilt, and it repairs itself when broken.

Epithelial cells

Epithelial cells sit tightly packed in sheets, so they make good barriers. Their shape matches their job:

Three shapes of epithelial cells. Flat cells make thin linings that substances can diffuse across. Cube-shaped cells secrete and absorb. Tall, ciliated cells in the airways have hair-like cilia that beat to sweep mucus away from the lungs.

Tissue type	Main jobs	Examples
Epithelial	Covers, lines, protects; absorbs and secretes	Outer layer of skin (epidermis); lining of mouth, stomach, intestines, airways, blood vessels
Connective	Supports, connects, protects, stores, transports	Bone, cartilage, tendons (muscle to bone), ligaments (bone to bone), fat (adipose tissue), blood
Muscle	Contracts to cause movement	Skeletal muscle, smooth muscle in the gut and blood vessels, cardiac muscle in the heart
Nervous	Carries signals; coordinates the body	Brain, spinal cord, nerves

The four animal tissue types. Most organs contain all four.

Worked example

Let's explain a cell's structure

Explain how three features of a red blood cell help it carry oxygen.

Biconcave disc shape: a large surface area compared with its volume, so oxygen can diffuse in and out quickly; no part of the cell is far from the surface.
No nucleus (in mature human red blood cells): more room for haemoglobin, so each cell can carry more oxygen.
Small and flexible: it can bend to squeeze through capillaries that are about as narrow as the cell itself, bringing oxygen right up to body cells.

Shape (surface area), no nucleus (more haemoglobin) and flexibility (fits through capillaries) all help red blood cells carry and deliver oxygen.

Worked example

Let's identify a tissue from a microscope description

A student looks at a prepared slide and records: "Long cells, side by side. Clear stripes across them. Several nuclei near the edge of each cell. No branching." What tissue is this, and where in the body might it come from?

Long cells that contract suggest muscle tissue.
Stripes mean skeletal or cardiac muscle (not smooth).
Several nuclei and no branching point to skeletal muscle (cardiac cells are branched with usually one nucleus).
Skeletal muscle is attached to bones, for example in the arm (biceps) or leg (quadriceps).

Skeletal muscle, for example from the biceps of the arm.

Just for fun

Funny but true: some of your nerve cells are among the longest cells in your body. A single motor neuron can reach from your lower spinal cord down to your foot, about a metre in a tall adult. If a neuron's cell body were the size of a tennis ball, its axon could be hundreds of metres long.

Hands-on activity

Tissues in a chicken wing

You need: one raw chicken wing from the grocery store, disposable gloves, a plate or cutting board, kitchen scissors or a knife used by a grown-up, paper towel, soap and a disinfecting cleaner. Grown-up note: raw chicken can carry Salmonella bacteria. Wear gloves, keep everything away from other food, then throw out the wing, clean all surfaces and wash hands well with soap. Do not eat this wing.

Look at the skin: the outer layer is epithelial tissue. Notice the yellowish fat (connective tissue) under it.
Have the grown-up cut the skin away. The pink skeletal muscle underneath is the meat. Notice that it is made of bundles of fibres.
Find the shiny white tendons that attach the muscles to bones. Pull gently on a muscle: what moves?
Bend the joint. Find the smooth, white, slippery cartilage covering the ends of the bones (connective tissue).
Look for thin white threads (nerves) and small tubes (blood vessels).
Make a labelled sketch and a table: structure, tissue type, function.

What to look for: at least five structures and all four tissue types (epithelial: skin; connective: fat, tendon, cartilage, bone, blood; muscle: the meat; nervous: nerves). Pulling a muscle should bend the joint, showing how muscle, tendon and bone work together.

Your turn

Name the four main types of animal tissue and give one example of each.

How does the long axon of a neuron fit its function?

Give two differences between skeletal and smooth muscle.

Why is blood classified as a connective tissue?

Why are muscle cells packed with mitochondria?

What is the function of cilia on the cells lining the trachea?

What connects muscle to bone? What connects bone to bone?

More practice: warm-up

What does "structure fits function" mean?

Which type of muscle is found only in the heart?

What protein in red blood cells carries oxygen?

What part of a neuron receives signals?

Which tissue type covers the body and lines organs?

More practice: on your own

A slide shows branched, striped cells joined end to end by dark lines. Identify the tissue and explain the dark lines.

Explain why the cells lining the air sacs of the lungs are flat (squamous).

How does the myelin sheath help a neuron?

Bone feels hard and dead, but it is a living tissue. Give two pieces of evidence.

Draw (or describe) a labelled diagram of a neuron. What labels must it include?

Challenge

People with a certain disease lose myelin from their neurons. Predict the effect on the body and explain using structure and function.

A red blood cell has no nucleus. Using what you learned about the cell cycle, explain why red blood cells cannot divide, and how the body still keeps up its supply.

The stomach contains all four tissue types. Suggest a job for each one in the stomach.

Check your understanding

Quiz

Which tissue type includes bone, blood and fat?

A cell has a large surface area, no nucleus and is very flexible. What is it?

Which muscle type is striated and voluntary?

What is the job of the myelin sheath?

Which epithelial cells sweep mucus out of the airways?

What connects a muscle to a bone?

For parents and teachers

Big idea: expectation B2.4 asks students to investigate specialized cells (such as bone and muscle cells) and draw labelled diagrams showing their structural differences; B3.3 links specialized cells to tissues, organs and systems. Plant cells (leaf and root) come later in this unit.

How to help: The chicken wing activity is memorable but needs care with raw poultry: gloves, a separate board, and thorough cleaning. If that isn't possible, a cooked drumstick shows muscle, tendon, cartilage and bone too. Ask your teen to explain one cell's structure and function to you as if you were the student.

Common difficulties:

Forgetting that blood and bone are connective tissues.
Mixing up tendons and ligaments.
Mixing up the three muscle types; a quick table (stripes? nuclei? voluntary? where?) helps.