Characteristics of Life

All living things show the following characteristics:

1.  Living things are composed of organizational units called cells.

A cell consists of living material separated from the environment by a barrier called the cell membrane.  Most cells can perform all the functions we associate with life.  They use energy, reproduce, grow, and respond to changes in their external environment.  Cells are very diverse in form and function.  Look at some of the different cells found in the human body.

             Cheek Cell                     Nerve Cell                Red Blood Cell                       Sperm Cells

                                                                           (Conducts impulses)   (Transports oxygen)  (Transports genetic information)

The size of an organism is determined by the number of cells it houses.  Thus infants are smaller than adults because they have fewer cells.  An adult human is composed of about 10 trillion cells.  There are 200 or so varieties.

Ø    Unicellular organisms are composed of one cell!  They cannot be seen without the aid of a microscope.  Many pathogens (disease causing agents) are unicellular.  The pictures below show highly magnified clusters of bacteria that cause different diseases.

                (Gonorrhea)                                    (Tuberculosis)                            (Strep throat)

Ø    Multicellular refers to organisms composed of many cells.  This includes most organisms you are familiar with.  The picture below shows the numerous cells composing a very thin section of an onion.

2.  Living things reproduce to make more organisms of the same type.

This is obviously very important since all living things eventually die.

Ø    Reproduction can be asexual (a single organism reproducing without the aid of another)

                    Bacteria reproducing asexually.

Ø    Reproduction can be sexual (two cells from different individuals unite to form the first cell of a new organism)

                       Sperm surrounding egg.       Sperm entering egg.            Fusion of sperm and egg

                                                                                                        genetic material.

                            Division of fertilized egg.       Embryonic development                Fetus

3.  Living things grow and develop.

This seems pretty obvious.  Think about how much you have grown since birth.  We certainly can't forget that special type of development you are undergoing right now called PUUUUBERTY!! 

Ø    Growth: increase in size during an organism’s lifetime.

Ø    Development: change in form during an organism’s lifetime.

4.  Living things obtain and use energy.

All of the chemical reactions taking place in your body that keep you alive require energy.  Plants obtain this energy through a process know as photosynthesis.  Animals obtain their energy from food.  As food is broken down, energy is released.  Of course, some organisms, such as the beagle, are know take in more energy than is required to sustain them.  This excess energy gets stored on the form of fat.    

5.  Living things respond to their environment.

The term environment refers to an organism's surroundings.  Anything in the environment that causes an organism to react is called a stimulus.  The reaction that takes place as a result of the stimulus is called the response.  Responses can be short-term or long-term and they can be positive or negative.

Positive Response: causes the organism to move towards the stimulus.  For example, if I put chocolate chip cookies out on the lab stations tomorrow during class (dream on) most of you would have a positive response.  You would get up and move towards the stimulus.  Another example is plant stem growth.  It has a positive response to light.  You can see this in the picture below.

Stimulus: Sun

Response: Growth

Negative Response: causes the organism to move away from the stimulus.  For example, roots have a negative response to sunlight.  They grow away from it.  If you touch your hand to a hot pan, you will have a negative response.  (If you don't, you should have some serious concerns.)

Stimulus: Hot pan

Response: Remove hand

The ability to adjust and respond to stimuli is very important because it allows organisms to maintain a constant internal environment necessary for life.  This is called homeostasis.  The prefix homeo means same.  Stasis refers to stopping.  Your body runs a tight ship.  Conditions on the inside are tightly controlled.

 Homeostasis is similar to the heating and cooling system in the modern home.  Suppose you set your home thermostat at 73 degrees (homeostasis).  As the heat of the afternoon kicks in, the temperature inside the house will begin to increase.  However, before it even has the change to jump one degree, a sensor detects the change and sends a signal to the air conditioner to kick on.  As cool air flows out the vents, the house goes back to the set temperature (homeostasis).

If you think about it, your body works the same way.  Most people maintain a constant body temperature around 98.6 degrees.  That is your homeostasis.  When facing conditions such as heavy exercise or hot weather, your body has numerous mechanisms to keep your temperature at homeostasis.  Sweat is produced from glands in the dermis.  As the water in the sweat changes from liquid to gas (evaporation), heat is lost allowing you to feel cooler.  Your face gets red and flushed.  This is due to the dilation of blood vessels which brings heat to the surface of your skin where it is lost to the environment. 

There is no doubt about it.  You are a homeostatic machine!!!