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The stem is the plant’s support and the conduit between roots, leaves, and plants. It contains phloem, a tissue that transports food within the plant; and xylem, which principally transports water. It is the xylem that hardens to form the trunks of trees and shrubs.

The roots anchor the plant in the ground and absorb nutrients and water from the soil. A tap root system has a single main root; a fibrous system has—well, lots of fibers. In root vegetables, such as turnips and carrots, the vegetable part is, in fact, a swollen root. Adventitious roots are less common; the name means coming from the outside, and these roots grow in unusual places, such as from the stem.

☞ THE CARBON CYCLE

The process by which carbon (in the form of carbon dioxide) is absorbed from the atmosphere during photosynthesis and is then transferred from one organism to another and eventually released back into the atmosphere is known as the carbon cycle. For example, a plant takes in carbon dioxide; the plant is eaten by a herbivorous animal, which is in turn eaten by a carnivore; when the animal dies, its rotting body releases carbon dioxide. Alternatively, the herbivorous animal excretes its waste, which also degrades to give off carbon dioxide.

This provides a smooth transition from plants to the human body.

☞ CHROMOSOMES

A normal human body has 46 chromosomes composed of 22 matched pairs and two sex chromosomes. Half of each pair, along with a single sex chromosome, is found in the sperm. The other half is in the egg. Fusion of the two creates the human embryo. Sex chromosomes are of two types, called X (female) and Y (male). The egg always contains an X chromosome, so the sex of the embryo is determined by whether a sperm is carrying an X or Y chromosome. Other chromosomes dictate other genetic factors, such as hair and eye color.

Chromosomes are made up of DNA, RNA, and protein.

DNA stands for deoxyribonucleic acid and is fundamental to the organization and functioning of living cells. It consists of the famous “double helix” (identified by the scientists Crick and Watson in 1953), with two strands coiled around each other. When the strands of a helix separate, each provides a template for the synthesis of an identical strand, containing the same genetic information. This enables normal growth, cell repair, and the production of cells that will turn into the next generation—which is why humans produce babies rather than tiger cubs, and why tigers produce tiger cubs rather than roses.

RNA stands for ribonucleic acid, which occurs as a single strand and contains different sugars and bases but is otherwise structurally similar to DNA. It’s vital to the synthesis of…

Proteins, which fulfill many important roles in a living organism—they are involved in the makeup of tissue; the properties of muscles; and the functioning of hormones, the immune system, and the digestive system, to name a few. They are manufactured within cells using information conveyed by the DNA and RNA.

☞ THE SKELETAL SYSTEM

The human skeleton is made up of more than 200 bones, held together by fibrous tissue called ligaments, and linked at the joints. Joints allow varying degrees of movement from none (between the bones that make up the skull) through some (the hinge joints at the elbow and knee) to lots (the ball-and-socket joints at the hip and shoulder).

The principal bones of the body, starting at the top and working down, are:

• cranium: skull

• spine: made up of 26 smaller bones called vertebrae

• clavicle: collar bone

• scapula: shoulder blade

• humerus: upper arm

• radius and ulna: lower arm—the radius is the broader one on the thumb side, the ulna the narrower one on the little finger side

• carpus: a collective name for the bones of the wrist, individually known as carpals

• metacarpus: ditto for the five long bones of the hand

• phalanges: fingers

• sacrum: actually a fusion of five vertebrae attached to the

• hip