Joints, the skeleton, and tissues that regrow

In Table 3.5 your shoulder moved every which way, your elbow only bent, and your neck turned. The shape of the bone ends at each joint decides what it can do. Pick a joint and move it; push it as far as it will go.

At the shoulder, the upper arm bone ends in a rounded head, and that head rests in a shallow cup on the shoulder bone: a ball in a socket. With so little to block it, the arm can move forwards, backwards, sideways and round in circles.

The shoulder girdle, made of the shoulder plus the collarbone, is the frame that fixes the arm to the rest of the skeleton.

Your elbow folds and opens along a single direction, the way a door swings on its hinge. That is a hinge joint. The knee is a hinge joint too, and in front of it sits a small bone, the kneecap, that protects the joint.

Shake your head for "no" with your fingers resting on the back of your neck. You feel a smooth turning. Turning your head is the work of a pivot joint where the skull sits on top of the backbone: one part turns around another, much as a doorknob turns in a door.

The skull is built from flat bones that fit together edge to edge into a hard box around the brain, eyes and ears. The joints between them are fixed joints: the bones cannot move at all. That is exactly what you want here, because the brain stays safe even while the rest of the body is running and jumping.

India's classical and folk dances are joint workouts set to music. A deep bent-knee stance, arms flung wide, a quick turn of the head, a hand curled into a gesture: every pose uses particular joints in particular ways. Pick a pose (our own drawings) and spot the joints at work.

The skeletal system is the body's framework of bones: it gives strength and protects delicate organs inside. Its three main parts are worth learning by name: skull, vertebral column (backbone) and rib cage.

Running down from the base of the skull is the backbone, also called the vertebral column or spine. It is a stack of small bones, the vertebrae, and it holds the body up. Each vertebra is separated from the next by a disc of cartilage. The discs work like shock absorbers and let the spine bend, so you can stoop and twist while the spinal cord running inside stays unharmed.

Press your chest and you feel your ribs: 12 pairs of them, forming the rib cage around the heart and lungs. Breathe with it below.

A cage of bone that moves? At the back every rib is fixed to the spine; at the front it reaches the breastbone (sternum), and flexible cartilage makes the joins. That give lets the rib cage swing out and in, which makes more and then less room in the chest, so air flows into and out of the lungs. It also explains why every breath hurts after a rib injury.

Bones and muscles stay healthy when they are used well. Four habits do most of the work: a correct posture when you sit and stand, nutritious food, regular exercise and yoga. Between them they build strong bones, fit muscles and supple joints, and stop the body from stiffening.

Surya Namaskar is a sequence of twelve poses that flow into each other (Fig. 3.18 of your book shows one version). Play it below and watch which joints bend at each step.

Here is a bold question: does a mature plant cell, already busy with one special job, still hold everything needed to build a whole plant? In 1958 F. C. Steward became the first to show the answer is yes. He cut tiny bits of phloem out of a carrot root and kept them in a broth of simple sugars and hormones. Single cells came loose and started dividing into clumps, and the clumps went on to grow roots, a shoot and finally a full carrot plant. Step through it.

Two big changes happened on the way. Backwards first: the phloem cells dropped their special job and turned back into plain cells that could divide, a step called dedifferentiation. Then forwards again: with food and growth chemicals in the broth, those plain cells took on new jobs as root and shoot tissue, which is redifferentiation. A mature plant cell that can make this round trip is totipotent, and the ability is totipotency. A zygote does something similar when it grows into a complete organism.

Steward also changed the conditions (Table 3.6 in your book). Only the combination with light, air and a liquid medium with nutrients raised the fresh weight, by 20 per cent; without air (solid medium) or without light it fell. Ask yourself what each missing condition took away from the cells, and what growers could do with this.

Plant pathologists noticed tumour-like swellings on the stems of some plants, a disease called crown gall. Behind each swelling is a burst of rapid, uncontrolled cell division, and behind that is a bacterium, Agrobacterium tumefaciens.

Scientists did more than look for a cure. They asked how the bacterium manages to slip pieces of its own genes into a plant cell, and found out. Plant tissue culture and genetic engineering both borrowed what they learned. So a germ that once only caused swellings now works as a delivery van: scientists load it with useful genes and let it carry them into crop plants, to make valuable plant chemicals, better yields and varieties that resist disease.