Living things make up the world as we know it. Living things are involved in our
life constantly, seeing that we are alive. There are five characteristics that
are common to all living things. Living things are made up of one or more cells.
Each cell is made up of living matter and is separated by a barrier that
encloses the cell from its surroundings. However, there are many different kinds
of cells that make up living things. A single cell can be one organism. These
organisms are known as unicellular. Most of the organisms that we know best such
as people, trees, and dogs are all made up of more than one cell. Organisms made
of more than one cell are said to be multicellular. Another characteristic that
living things share is that they reproduce. They reproduce, or make new
organisms of the same sort. In order for a species to survive, it is a necessity
for them to reproduce because all organisms die eventually. There are two ways
living things reproduce, sexually and asexually. Sexual reproduction needs two
cells from two different organisms to merge and form the first cell of a new
organism. Asexual reproduction is when only one organism can reproduce without
the assistance of another. The third characteristic of living things, is that
all living things need to grow and develop. When an organism is growing, most go
through a cycle called development. The single cell that starts the cell divides
over and over again to make all the cells that the organism has when in
adulthood. As the cycle continues the organism ages. Aging is when the organism
becomes less efficient in the process of life. The organism will not be able to
reproduce, and death comes as finally too. The fourth characteristic of a living
thing is the ability to obtain and use energy. Living things obtain energy from
their environment or their surroundings. All living things require energy to
live and build their cells. This process is anabolism. Anabolism is the process
in a living thing that involves putting together complex substances from simpler
substances. Plants get their energy from the sunlight through a process called
photosynthesis. Animals get their energy from food that is eaten. The food is
then broken down through digestion, resulting in a release of energy called
catabolism. Living things practice anabolism and catabolism through the whole
time they are living. The balance of anabolism and catabolism is called
metabolism. The fifth and final characteristic that all living things share is
that all living things respond to their environment. Response to their
environment can be sudden, through behavior, or gradual, in metabolic process or
growth. Stimulus is anything in the environment that causes a living thing to
react. Stimuli include light, temperature, odor, gravity, sound, water, and
pressure. Plants generally act to stimuli slower than animals. The process in
which living things respond to stimuli in ways to keep conditions in their body
suitable for life is homeostasis. These five characteristics of living things
are just the basics to knowing what makes up living things. Atomic Structure of
Living Things The basic unit of matter is the atom. Atoms are extremely small,
in fact, if you placed 100 million atoms in a row one after the other, it would
be one centimeter long. Even though the atom is small it consists of even
smaller particles, called subatomic particles. Scientists believe that there is
at least 200 subatomic particles. The three main subatomic particles are the
neutron, proton, and electron. In the middle of the atom there is a nucleus. The
nucleus makes up 99.9 percent of the atoms weight even though it is a hundred
times smaller than the atom itself. The nucleus contains two different kind of
subatomic particles, the neutron and the proton. The proton has a positive
charge and the neutron is a electrically neutral subatomic particle. Both are
almost equal in mass, 1 amu (atomic mass unit). Another subatomic particle in
the atom is the electron. It is negatively charged and it's mass is about 2000
times smaller than that of a neutron or electron. Usually the number or electron
in an atom is the same as the number of protons. So, usually the atoms have not
a positive nor negative charge, but they are neutral. Electron are not in the
nucleus like the protons and neutrons are. They travel at very high speeds
throughout the atom in energy levels. The energy levels are like orbits that
surround the nucleus. The number of protons that are in the nucleus of an atom
is called the atomic number. The atomic number identifies the atom because no
two atoms have the same number of protons in there nucleus. For example,
hydrogen has the atomic number of 1, that means all atoms that have one proton
in its nucleus is hydrogen. The atomic mass number is the number of neutrons and
protons in the nucleus. To find the number on neutrons in an atom, you must
round the atomic mass number to the nearest whole number and then subtract the
atomic number. Remember the atomic number is the same as the number of protons
in the atom. To find out the number or electrons an atom contains, you just need
to know the atomic number because there is the same number of protons as there
is electrons in atoms. For example, in sodium, the atomic number is 11, and the
atomic mass number is 22.98977. This means that sodium contains 11 protons, 11
electron, and 12 neutrons. (See Figure 1) Substances known as elements are made
up of solely on type of atom. Scientists have discovered 109 elements, 90 were
found in nature, and 19 were artificially made in laboratories by scientists.
Each element is represented by a chemical symbol. Each symbol is made up of one
or two letters, usually taken from the name of the element. The symbol of oxygen
is O, the symbol for phosphorus is P, and the symbol for Nitrogen is N. Most
chemical elements are solid, like gold, iron, bronze, and silver to name a few.
They are on the left 3/4 of the periodic table. Some elements are gases, like
oxygen and carbon. They are on the right 1/4 of the periodic table. Only a few
elements are liquids, mercury and bromine are the most common. The noble gases
are located all the way to the right on the periodic table. The atomic number of
an element is always the same, this means that an element will always have the
same number of protons. However, the number of neutrons in the nucleus may
differ from one atom to the next. For example, the typical hydrogen atom
contains one proton and no neutrons inside the nucleus. Another form of hydrogen
is called deuterium. It contains one proton and one neutron in the nucleus. The
third form of hydrogen is sometimes referred to as tritium. Tritium has on
proton and two neutrons in the nucleus of the atom. Even though the atomic mass
number may change the atomic number of hydrogen will be 1, and it will still
have one proton and one electron. An isotope is an atom with the same number of
protons and electrons but a different number of neutrons from the same element.
Isotopes are represented by putting a number in front of the atomic symbol of
that atom. The number represents the atomic mass. Regular hydrogen is written
1H, deuterium is 2H, and tritium is 3H. Compounds and Molecules When elements
combine to form substances of consisting of two or more atoms, chemical
compounds are produced. A chemical compound deals with the combination of two or
more atoms in definite proportions. Most materials in living things happen to be
compounds, so they are very important to us. Chemical compounds are represented
just as elements are with chemical symbols. A chemical formula is made up of the
chemical symbols that make the chemical compound. For example, water contains
two hydrogen atoms and one oxygen atom. The chemical formula would be H2O. Table
salt is made from one sodium atom and one chlorine atom, so the chemical formula
is NaCl. Chemical compounds are formed by the interaction of atoms. Chemical
bonding is the process in which atoms interact and combine. An important factor
in chemical bonding is the number of electrons in an atom's outermost energy
level. Each energy level can only hold a certain number of electrons. The
innermost energy level, or first energy level can hold only two electrons. The
second energy level can hold eight electrons. The third holds eighteen
electrons, the fourth and fifth energy levels hold up to thirty-two electrons.
The sixth energy level can bear eighteen electron, the seventh energy level can
hold eight electrons. The eighth and outermost energy level can accommodate for
a mere two electrons. In order for there to be electrons in outer energy levels,
the inner energy levels must be full. There can't be 1 electron on the first
energy level and five on the second. It would have to be two on the first energy
level and four electrons on the second. When the electrons of an atom fill the
outermost energy level they are said to be stable, or unreactive. These atoms
will not bond with other atoms to form chemical bonds. In order for an atom to
become stable, it will either have to lose or gain electrons to make it's
outermost energy level complete. There is one other way an atom can be stable.
It will be stable if it's outermost energy level contains eight electrons. One
type of bond to make atoms stable is called an ionic bond. An ionic bond is a
bond that involves the transfer of electrons. The name comes from the word ion.
Ion means charged particles. Ions are produced when ionic bonds occur. For
example, sodium has only one electron on its outermost energy level and chlorine
has seven on its outermost energy level. These two atoms want to bond in order
to become stable. That means it wants to get rid of it to become stable. The
loss of the one electron makes a sodium ion (Na+), which is positively charged.
It's positively charged because it lost one of it's negatively charged
electrons. Thus, the electrons and protons don't balance, because now there is
one more proton than electron, so the ion has a positive charge. The addition of
one electron makes a negatively charged chlorine ion (Cl-). The two ions are
oppositely charged and now have an intense attraction to each other. The
attraction is caused by the transfer of electrons that holds the ions together
in an ionic bond. (See Figure 2) A different type of bond is called a covalent
bond. A covalent bond is formed when atoms share electrons in order to become
stable. The shared electrons are located in the outermost energy levels of both
atoms. This forms a strong bond that is in many living things. Covalent bonds
can be in the form of single bonds, double, or triple. The bond between two
hydrogen atoms and oxygen atom (H2O), forms a single bond. A single pair of
electron is shared between the two hydrogen atoms and the oxygen atoms. (See
Figure 2) On the other hand, the compound that forms carbon dioxide (CO2), forms
a double bond. The carbon atom shares two pairs of electrons, four total with
the two oxygen atoms. In covalent bonds the combination of atoms that are caused
from sharing form molecules. A molecule is the smallest particle of a covalently
bonded compound. Besides water and carbon dioxide that were already mentioned,
sugar (C6H12O6) and ammonia (NH3) are compounds. Organic compounds are compounds
that contain carbon. Carbon is a unique element because of its ability to form
covalent bonds that are exceptionally strong and stable. The carbon atom has two
electrons in the first energy level and four in the second energy level. There
are four open positions in carbon's outermost energy level, allowing it to form
four single covalent bonds. Carbon can easily bond with hydrogen, oxygen,
nitrogen, phosphorus, and sulfur atoms. Carbon also has the extraordinary
ability to form long chains with other carbon atoms. The bonds between carbon
can be single, double, or triple covalent bonds. No other element has this rare
ability. (See Figure 2) Cell Structures Cells from a living thing come in many
different sizes and shapes. Even though cells differ in size and shape, certain
parts of the cells are the same. The cells of animals, plants, and other
organisms have three major but basic structures in common: the cell membrane,
the nucleus, and the cytoplasm. The cell membrane acts as the cell's outer wall
and protects it from it's surroundings. It also moderates what goes in, and what
comes out of the cell. The cell membrane is made up of several different types
of molecules. The most important of these is lipids. Most of the cell membrane
is made up of a double layer of lipids. The cell membrane is also made up of
proteins and carbohydrates. In plants the cell membrane is surrounded by the
cell wall of the plant. The cell wall helps protect and support the plant. The
cell wall lets water, oxygen, and carbon dioxide pass through easily. The cell
wall is made up of three layers which are extremely porous. In the majority of
cells there is a dark structure we know as the nucleus. Not all cells have
nuclei though. Bacteria and other small unicellular organisms don't have a
nucleus. These are said to be prokaryotes, or cells without nuclei. Cells that
do have a nucleus are called eukaryotes. The nucleus is very important to the
cell, it is the information center and contains DNA. DNA stores genetic
information that is passed to one generation to the next. The DNA in a cell is
attached to special proteins. These proteins are called chromosomes. Chromosomes
contain genetic information that is passed through generations. The nucleus of a
cell tend to be about two to five micrometers in diameter. Surrounding the
nucleus there are two membranes called the nuclear envelope. The nuclear
envelope contains dozens of small pores, through which molecules move in and out
of the nucleus. In most nuclei, there is a small region called the nucleolus. It
is made up of RNA and proteins. In the nucleolus, ribosomes are made. Ribosomes
are important because they help out with the productions or proteins in a cell.
The space inside of a cell can be divided into two parts, the nucleus and the
cytoplasm. The cytoplasm is the area between the nucleus and the cell membrane.
The cytoplasm contains other important structures in the cell. Structures inside
the cell are called organelles. An organelle is a tiny structure in the cell
that preforms a special function within the cell. The mitochondria is greatly
important to the cell. In animals, the mitochondria changes the stored chemical
energy from food into more useful energy for the cell. In plants, an organelle
called the chloroplast changes energy from sunlight to energy that can be used
by the cell. The mitochondria is found in both the cells of plants and animals,
where as the chloroplast is only found in plants. Ribosomes are the structures
in which proteins are produced. They are made out of protein and RNA. Some
ribosomes in a cell are attached to membranes, while some are free in the
cytoplasm. Ribosomes are one of the smallest organelles in a cell. Many cells
are filled with a network of channels we call the endoplasmic reticulum. The
endoplasmic reticulum transports through the inside of the cell. There happens
to be two different types of endoplasmic reticulums. The smooth endoplasmic
reticulum has channels that are smooth. In some cells special enzymes and
chemicals are stored within the smooth endoplasmic reticulum. The other type of
endoplasmic reticulum is called the rough endoplasmic reticulum. It is called
rough because it has ribosomes that are attached to the surface making it look
rough. Many proteins that are released are transported from the cell in the
rough endoplasmic reticulum. The newly formed proteins are often first moved
into special compartments known as the Golgi apparatus. In the Golgi apparatus
the proteins are modified and then releases it. The Golgi apparatus' function is
to modify, collect, package, and finally distribute molecules made in one
location to another location. When foreign materials that are too big to move in
the cell get into the cell, the cell membrane forms a pocket around it. Then the
lysosomes come in and digest, then break down the particle. Lysosomes are small
structures that contain chemicals and enzymes that help break down and digest
foreign particles in the cell. Lysosomes are made in the Golgi apparatus, and
plants don't have lysosomes. Vacuoles are sac-like structures in a cell that
store water, salts, proteins, and carbohydrates. Plants also have a structure
besides the vacuole called the plastid. The plastid also stores food as well as
pigments for the plant. The cytoskeleton in a cell is the frame work that holds
the cell together and gives it their shape. The cytoskeleton is made from
filaments and fibers. One of the main parts in a cytoskeleton is a component
called microtubules. They are made out of hollow tubules made from proteins.
They help move organelles throughout the cell. (See Figure 3) The Cell As a
Living Thing Living things are made up of cells and they grow in size. In most
instances, a living thing grows because it produces more and more cells. Cells
in an adult human are no bigger that cells in a human baby, there is just more
of them. In a cell, water, oxygen, and food enter the cell through the cell
membrane, and waste products exit the cell. The time it takes to exchange these
materials depends on the surface area of the cell. How quickly food and oxygen
is used, and how quickly waste products are produced depends on the volume of
the cell. As a cell gets bigger, the volume increases faster than the rate of
its surface area. This can be a problem for the cell. If the diameter of a cell
increases 5 times, the surface area would increase 25 times, and the volume
would increase 125 times. The bigger the cell is the harder time it has getting
the nutrients and oxygen it needs in order to support it's massive volume. Cell
growth is controlled in multicellular organisms. Cells in parts of the body like
the heart and liver rarely divide. These cells are unlike skin cells that divide
rapidly through a person's lifetime. Controls on cell growth can be turned on
and off like a light switch. If a bone or skin is broken, cells divide in order
it repair the damage that needs to be fixed. Uncontrolled cell growth can be
very harmful to multicellular organisms. Cancer is a disorder when cells have
lost the ability to control their growth. Cancer cells keep growing and growing
until the supply of nutrients shuts off. Cancer is a very serious disease that
shows the importance of controls on cell growth. Eukaryote cells divide in order
to slow down cell growth. Cell division is the process in which a cell divides
to form two daughter cells. The first stage of cell division is called mitosis.
Mitosis is the process when the nucleus of a cell is divided into two nuclei,
and both have the same number and type of chromosomes as the parent cell.
Mitosis can be split into four parts. Interpahse occurs before mitosis can
begin. It is the period in between cell division and is the longest part of the
cell cycle. The cell cycle is the process when a cell grows, prepares for
division, divides, and begins a new cell cycle. Interphase itself is divided
into three phases: G1, S, and G2. G1, called growth 1, or gap 1, is the stage in
which a cell grows. The S stage is called the DNA synthesis stage. During this
stage of interphase the DNA is replicated in DNA replication. Proteins are also
synthesized in the S phase. G2, or growth 2, takes place when the S stage is
finished. During G2 the synthesis or organelles and other materials happens
furthermore preparing the cell for division. While interphase is taking place
the nucleus is busy in synthesizing messenger RNA to direct all the steps. The
first phase in mitosis is called prophase. Prophase takes the longest time in
mitosis, consuming 50-60% of the time it takes mitosis to occur. In prophase the
chromosomes in a cell condense and coil up, making them more visible. The
centrioles separate and go to opposite sides of the cell. Centrioles are small
structures in the cytoplasm that contain tubulin, a microtubule protein. Plant
cells don't contain centrioles. The condensed chromosomes become attached to
fibers in the spindle. The spindle is a mesh-like structure that helps move the
chromosomes apart. At the end of prophase the chromosomes condense tighter, the
nucleolus disappears, and the nuclear envelope begins to break down. Metaphase
is the second phase of mitosis, and is the shortest as well. During this phase
the chromosomes line up across the center of the cell. Anaphase is the next
phase in mitosis. It begins when the sister chromatids split. Chromatids are the
identical parts that form the chromosome. The chromatids become individual
chromosomes and continue to split until they reach the opposite poles. Anaphase
ends when the new chromosomes stop moving. Telophase is the fourth and final
stage of mitosis. The chromosomes begin o uncoil into a tangle of chromatin.
Chromatin is the material that makes up chromosomes and itself is made from
protein and DNA. All of this takes place where the two new daughter cells are
taking shape. Two nuclear envelopes begin to reappear around the chromatin. The
spindle begin to break apart and the nucleolus forms around the nucleus of the
daughter cells. Mitosis is over but there is still one more step. Cytokenesis
follows quickly after mitosis is finished. In cytokenesis the cytoplasm of the
parent cell splits into two to form the daughter cells. In animals, the cell
membrane moves together and pinches the cells, giving making the daughter cells
have their own nucleus and organelles. In plants the cell plate appears and
forms a barrier between the two daughter cells. The cell plate then forms into a
cell membrane, then the cell wall develops. (See Figure 4) Tissues and Organs In
multicellular organisms, cells are organized in specialized groups, known as
tissues. A tissue is a group of similar cells that preform similar functions.
Different tissues form many different tasks. For example, a kind of tissue is
made up of cells that produce digestive enzymes in the pancreas, and the cells
in an eye respond to light. Most multicellular organisms have four main types of
tissues: muscle, epithelial, nerve, and connective. Some tasks in the body are
too complicated to be preformed by only one type of tissue. So, organs preform
these duties. An organ is a group of tissues that work together to preform a
specific function. Many types of tissues may be used to form one organ. For
example, a muscle in an organism is classified as an organ because not only
muscle tissue makes up the muscle. There is nerve tissue, connective tissue, as
well as a special tissue that connects the muscle with certain parts of the
body. All the tissues in an organ work together to preform one common function.
Sometimes not just one organ can complete one task, so an organ system is
needed. An organ system is a group of organs that work together to preform one
function. There are many organ systems in our body. We have a muscular system,
skeletal system, nervous system, and circulatory system. Multicellular Organisms
A multicellular organism is a living thing that is made up of more than one
cell. These organisms can contain hundreds, thousands, even billions of cells or
more. We see multicellular organisms everyday: people, plants, and house pets.
To describe a multicellular organism, we have to put them into levels of
organization. The levels of organization in multicellular organisms include
cells, tissues, organs, and organ systems. The first level is cells, the second
is tissues, next is the organs, and finally the fourth level is the organ
system. Multicellular organisms start off with one basic unit, the atom, and
build up to make bigger things. Atoms combine to form compounds which then form
organelles. Organelles then come together to make a cell. Cells then form
tissues, which could then make organs. After organs are formed, then organs can
be in an organ system. Eagle The eagle is sometimes referred to as the
"king of flight" because of the power it shows while in flight. The
eagle has been a symbol or strength and courage since ancient times. In 1782,
Congress chose the American bald eagle to be the symbol of our nation. The
national seal was the bird with its wings spread outward. It holds an olive
branch in one claw and arrows in the other. The eagle appears in many places
today in the United States. Only two species of eagles are found in North
America today: the American bald eagle, and the golden eagle. The bald eagle is
more common than the golden eagle. This extraordinary bird has white tail
feathers and white plumes on its head and neck. The bald eagle lives in open
areas, or forests, near water. The bald eagle is usually 35-40 inches in length,
and have a wingspan of 7.5 feet. The female bald eagle is more ferocious than
the male, and is a couple inches larger. A bald eagle migrates only if the water
it feeds in freezes in the winter months. It returns every year to the same nest
and the same mate. The nests are built in trees or on cliffs, and sometimes on
the ground. The eagle adds to it every year, making it bigger and bigger as time
goes on. The nests can weigh up to one thousand pounds. The nests are made from
sticks, weeds, and dirt. Bald eagles eat carrion, waterfowl, and especially
fish. The golden eagle was more common than the bald eagle when settlers first
came here, but this is not the case today. It's found in the western portion of
North America, from Alaska, south to Mexico. The golden eagle is about the same
size as a bald eagle. It's feathers are much darker than that of its famous
counterpart. There are feathers on the head and the neck of the bird that shine
like gold when they're in the sun. The toes and claws of the golden eagle are
feathered, where as the bald eagle has no feathers on its legs. With their
claws, golden eagles eat squirrels, prairie dogs, and rabbits. The golden eagle
is very brave and can attack large animals such as deer, but can't carry them
away. They build nests in trees and rocky cliffs with sticks. The golden eagle
has been known to defend its nest up to 75 square miles. As you can see, the two
types of eagles in North America are similar and different in many ways. Both of
the eagles are very powerful birds. One thing is for sure, the eagle is a very
beautiful bird that is extremely interesting.