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Introduction
Signs of Life
Growth and Reproduction

Growth

Many nonliving things are capable of growth. Stalactites in caves, crystals of certain chemicals, layers of sediments at the bottom of rivers, and even piles of old newspapers all grow larger and larger with the passage of time.

This type of growth enlarges an object by accretion: the simple addition of more of the same material on top of the old. Living organisms, however, are capable of much more elaborate growth mechanisms than simple accumulation of matter.

A seed planted in moist earth will take in water, air, and various inorganic materials, such as magnesium, potassium, ion and sulfur, and incorporate them into its structure to build new leaves, roots, flowers and a longer stem. The final result is a lot more than a disorganized, gas-containing, mineral solution. It is a highly organized structure we call a plant.

Living things, such as plants, are capable of converting nonliving materials (inorganic substances) into a whole series of organic materials that are then incorporated into the body of the organism or used in its metabolism.

Ability to synthesize complex building materials from much simpler components and to organize them into larger and larger structures (growth and repair) is an important sign of life.

Reproduction

Eventually the plant, which started the growth process as a seed, produces seeds of its own. Some of these seeds will go on to produce new plants, which will grow and eventually produce more seeds, and so on.

In a never ending cycle of reproduction, the process whereby plants and animals give rise to new offspring, new individuals, eventually become independent of their parents. These new individuals are formed when the parental organism copies a set of growth and operating instructions and places these instructions in a specialized cell that then develops into an embryo. Eventually these new embryos grow and develop, break away from their parents, and become individuals.

Reproduction can be asexual, whereby only a single parent is needed, and the offspring are genetically identical to each other and their original parent.

Sexual reproduction, however, usually requires two parents, each parent contributing half of the growth instructions. Offspring are not genetically the same as each other or as either parent. In most cases, however, they resemble their parents strongly.

Growth, the process of enlargement and conversion of simple building materials into typical structural parts, and reproduction, the ability to produce offspring are two vital signs of life.


BIOdotEDU
© 2003, Professor John Blamire