What is this life stuff any way

Introduction.

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What makes living thingsd different from non living things? In earlier times many people thought that there was a vital life force that living things had and nonliving things did not. Most people don't believe that any more, but the remnants of this idea persist. For instance the chemistry of carbon compounds is called organic chemistry because scientists once thought these compounds could only be made by living things. The belief that there is a vital force that is responsible for life and distinguishes living from non living things is called vitalism. We know now that there is no evidence for a vital force, and yet lving things are clearly different from non-living things. It turns out there is no single characteristic that clearly distinguishes living things from non living things but rather a suite of characteristics that need to be taken together.
To add to the diffculty sometimes a living thing may not be showing all of these characteristics. For instance, certain animals such as brine shrimp have eggs or other resistant stages in their life cycle that can dry out, show no signs of life, and remain viable for many years. Just add water and presto instant organisms!
The image at the top of this page shows a desert plant called Lithrops, literally stone. These plants are so slow growing and so perfectly blend in with the desert that they could easily be mistaken for, well, rocks. Yet these critters show all the characteristics of life viewed as a process.

What then makes living things different?

Living things are highly organised systems. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Consider your body. The body is made up of organ systems groups of organs that work together, for instance the digestive system. These in turn consist of organs, for instance the pancreas. Organs of similar cells that work together. In turn cells, consist of specialized parts called organelles. Scientisists then recognize recognize various levels of organization and you should be familiar with these.
Living things harvest and use energy to maintain themselves. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Again you take in food (complex organic molecules) which supplies you with energy for your daily activites and for growth. Organisms that obtain energy from complex organic molecules are called heterotrophs.  Our cells harvest energy from complex organic molecules by the process of cellular respiration. Plants take in energy by absorbing sun light. This energy is converted to chemical energy by a process called photosynthesis. The plant converts carbon dioxide and water to complex organic molecules within which the energy is stored. We take advantage of the products of photosynthesis as the utlimate source of our food. Plants and other critters that carry out photosynthesis are called photosynthetic autotrophs.
Living things exhibt metabolism. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Metabolism is the sum total of all the energy using chemical reactions and the energy yielding chemical reactions in an organism. When you eat food digestion breaks to the food down into simple compounds which may serve as a source of energy to power the building of other compounds. In a sense living things are complex chemical machines. This slide shows a section of rat intestine courtesy of Nanoworld.
Living things are made of one or more cells.  VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Cells are complex organic structures surrounded by a membrane. Cells contain genetic material and can interact with their environment. Cells are typically considered the basic unit of life. Thus viruses are not considered alive by most biologists because they are not made of cells, do not have independent metabolism and do not take in energy.  All cells have the following three things in common: DNA Cell membrane Cytoplasm This image is of human blood containing trypanosomes,single celled members of the kingdom Protista that are parasites in humans and that cause sleeping sickness. The trypansomes are the purplish eleongate cells in the center. They are surrounded by the host's red blood cells.
Living things contain genetic material and can reproduce themselves. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page The genetic material in all known living things consists of a nucleic acid called DNA. This molecule is a polymer of nucleotides and contains the information necessary for cell replication and maintenance.
Living things respond to their environment. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Obviously we respond to our environment. So don't plants for instance when they turn toward light as they grow. Even bacteria often can change their behavior in response to chemicals in their surroundings. Simple celled critters such as Euglena can detect and swim toward light.  The ability to respond to the environment means that organisms of all types have ways of receiving information about the environment. This bright looking photograph shows chemoreceptors found on the feet of house flies. What do flies use these receptors for? Why to taste of course. This is a false color image of a scanning electron micrograph courtesy of Nanoland. (False colored by Dr. Paul)
Living things are the product of evolution. VBS Home page,VBS Course Navigator, What is this life stuff anyway? Previous Page, Next Page, top of page Populationsof living things can evolve via natural selection in response to changes in the environment. If one looks at an organism, the organism will be seen to have many structures and behaviors that enable the organism to survive in its environment. For example, our wonderful hands probably arose as an adaptation to an arboreal environment. The long neck of this wading bird is clearly suited to this bird's fish and frog eating habits. A structure that has evolved in response to the demands that a particular environment places on a population is called an adaptation. Adaptations arise by one of the main mechanisms of evolution (2), namely natural selection.  The basic mechanism, natural selection by which adaptations evolve was laid out by Charles Darwin and Alfred Wallace in 1859. This same mechanism is believed to be involved in the evolution of new species from preexisting species. What evolution suggests is that all the cells in our body and all the other organisms on this planet have a long evolutionary history stretching back to the beginning of life on this planet. Scientists today believe that life arose perhaps four billion years ago and some argue that life or at least the biochemical precursors of life may have arisen within 500 million years of the earth's formation.  What ever the the time frame, evolution is important because it helps explain scientifically an important feature of the living world when viewed as a whole: namely there are milions of species, each with adaptations for particular environments, yet the cells of these species are very similar in terms of their biochemical bases. For instance, all cells use the same or very similar genetic code for protein synthesis. All cells also seem to use the same starting pathway for releasing energy from glucose (2) and all cells use the same basic energy carrier, namely ATP. Finally, all cells have then same molecule for carrying genetic information, namely DNA.